Author Topic: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)  (Read 23136 times)

Online Chris Bergin

Third discussion thread for Elon's second BFR overview at the IAC. Yes, we are looking at a new section for all BFR, per it not being just about Mars.

Open discussion, but everyone has to be civil and posts need to be useful. No "You're concern trolling". No "Wowzers. LOLZ. Cool" <---true, but that doesn't add to anything. No posts with masses of smilies, or it just looks silly, especially when quoted several times.

Most of you know we're going for quality over quantity here, best we can. Bad posts will be removed. Don't quote bad posts (or your post will be removed too, obviously). Report to mod if you see a bad post (against the above rules) - some of you didn't take note of this on thread two and those posts were removed (you were warned!)


New Articles (more to follow) :

 [September 29, 2017] The Moon, Mars, & around the Earth - Musk updates BFR architecture, plan

 [October 4, 2017] Sputnik at 60: Ambition ties first satellite to SpaceX’s BFR, Mars plans

 Previous major discussion thread:
  IAC 2017 -- ITS (BFR) v0.2 Pre and During Speech

 Live Thread from the event:
  IAC 2017 --Elon Speech

Full video:



Additional NSF Articles Of High Relevance:

 [March. 7, 2014] SpaceX advances drive for Mars rocket via Raptor power
 [Aug. 29, 2014] Battle of the Heavyweight Rockets – SLS could face Exploration Class rival
 [Sept. 27, 2016]SpaceX reveals ITS Mars game changer via colonization plan
 [Jul. 24, 2017] Includes Subscale BFR on 39A

 Major NSF L2 Resources:

 L2 Level: Evaluations And Renderings - Thread 2 (Includes link to Thread 1)
 ITS Cargo Modules AIAA - by the Author
 Rocket and Spacecraft Traj Sim
 The Evolution of the Interplanetary Transport System Overview
 SpaceX McGregor, includes Raptor Testing and photos
 Master All SpaceX Pads Updates, Photos and Status, including for BFR


NSF Public Threads:
 
   Discussion before and during the 2017 presentation:

      IAC 2017 -- ITS v0.2
      9m ITS Development Updates and Discussion Thread

   
   Discussion after the 2016 presentation:
 
      ITS Propulsion – The evolution of the SpaceX Raptor engine
      ITS Development Updates and Discussion Thread

9m ITS Development Updates and Discussion Thread 2

Elon's presentation at IAC 2016:



SpaceX: ITS Video from 2016:



Offline jpo234

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #1 on: 10/06/2017 09:48 AM »
It's interesting how close Richard Heidmann came in his analysis:
http://planete-mars.com/mars-colonization-transport-main-findings-of-our-analysis/

BFR really looks almost exactly like he imagined (except for the horizontal landing.

« Last Edit: 10/06/2017 09:49 AM by jpo234 »
You want to be inspired by things. You want to wake up in the morning and think the future is going to be great. That's what being a spacefaring civilization is all about. It's about believing in the future and believing the future will be better than the past. And I can't think of anything more exciting than being out there among the stars.

Online Lar

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #2 on: 10/06/2017 03:27 PM »
Great first post to start tne new thread. But I'm not ready to give as much credit as you, the general outline was already known at the time of this study and the OML is going to be basically bullet shaped.... Also the study includes an "emergency capsule" which I think we know is out.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline MikeAtkinson

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #3 on: 10/06/2017 04:05 PM »
It's interesting how close Richard Heidmann came in his analysis:
http://planete-mars.com/mars-colonization-transport-main-findings-of-our-analysis/

Of the 13 points in his analysis, it looks like he got 9 (perhaps 10) wrong.

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #4 on: 10/06/2017 04:10 PM »
Great first post to start tne new thread. But I'm not ready to give as much credit as you, the general outline was already known at the time of this study and the OML is going to be basically bullet shaped.... Also the study includes an "emergency capsule" which I think we know is out.
The mentality for where the "emergency capsule" comes from is the idea that this is still an experimental vehicle with very high risks. Musk wants this vehicle to be like an airline not a combat aircraft.

Redundancies, both similar systems and use of dissimilar systems. The only problems for risk are the main engines of the SC and the prop tanks. LOX is an inherently dangerous liquid. To get the risks of a composite LOX tank down to that of an airliner will take a lot of testing both on the ground in destructive tests and many flight tests to get statistical data on behavior in the actual flight regimes to them correct deficiencies in the tank design.

Engines like for F9 would be isolated by ballistic barriers as well as protection of piping and tanks, which allows for a RUD of any specific engine to not cause a LOM or a LOC event. But since the engine redundancy does not use dissimilar redundancy but similar redundancy it like the tanks must be tested extensively on the ground and in flight looking for those design "errors" and software "bugs" in the engine and engine controllers that keep engine failures from being a design fault to being a somewhat random manufacture or wear fault.

If the reliability of those two systems can be made to be very high then the overall safety of the SC and even the Booster can be made so that large numbers of passengers can ride in relative low risk similar to that of an airliner.

A P2P system that has a significant in-flight emergency of about 2 times per year and a LOC event once every 2 or three years but at flight rates of 4 times a day per launch site with 200 launch sites gives a yearly flight rate of the P2P usage of ~300,000 flights/year and a LOC (crew and passengers) event of 1 in 1,000,000 flights.

The key here is that a highly fully reusable vehicle (1000 gas-n-go) is possible to achieve these reliability numbers but a expendable and a low level partial or even fully reusable vehicle (10 reuses) would not reach the reliability levels needed to be airline like safety levels. The expendable and partial reusable and low reusability fully reusability vehicle does not have the engineering safety margins to get high reliability. These vehicles would still be much like an experimental aircraft with the similar LOC number values requiring some sort of crew escape.

Online edkyle99

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #5 on: 10/06/2017 04:16 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle

Online envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #6 on: 10/06/2017 04:18 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.

Offline QuadmasterXLII

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #7 on: 10/06/2017 04:23 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #8 on: 10/06/2017 04:32 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
It puts the fully reusable payload to EXPD (expendable) payload at the 60% value. Something we have known for quite awhile for efficient fully reusable designs vs a same size vehicle expendable design as the best case possibility for a fully reusable vehicle vs that same vehicle used as an expendable.

In general not something from this graph, as the vehicle gets smaller this factor gets worse. For F9 it has been hypothesized that this value between a fully reusable F9 and a EXPD F9 would be 40%. So on a 22mt EXPD payload to LEO you would get only a max of 8mt of payload with a fully reusable F9 without any margins (margins here are mainly for engine out which reduces significantly the payload). With margins the payload could be as low as 30% that of EXPD F9.

Online Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #9 on: 10/06/2017 05:01 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle

Looks like expendable payload numbers to a low earth orbit. (It matches the known 22t to LEO for expendable F9)

An interesting comparison, but not that meaningful since the plan is to always reuse.

Online edkyle99

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #10 on: 10/06/2017 05:03 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Online Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #11 on: 10/06/2017 05:04 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Expending both is my assumption, otherwise the comparison to expendable F9/FH makes less sense.
« Last Edit: 10/06/2017 05:04 PM by Lars-J »

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #12 on: 10/06/2017 05:09 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Expending both is my assumption, otherwise the comparison to expendable F9/FH makes less sense.

Although in practise you could presumably refuel the ship in orbit and bring it back anyway.
Waiting for joy and raptor

Offline su27k

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #13 on: 10/06/2017 05:12 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.

Yes, but fortunately they don't need to fly people on BFR for quite a while yet, a dedicated cargo version and automated landing helps a lot.

Offline AncientU

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #14 on: 10/06/2017 05:13 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Expending both is my assumption, otherwise the comparison to expendable F9/FH makes less sense.

I think it is an attempt to allow apples-to-apples comparison with all other (expendable) rockets.  Believe that at SpaceX there is neither an interest in ever expending any stage(s), nor is the BFR architecture at all optimized for throw-away launches... that said, this vehicle is approximately equivalent to a pair of SLS Block II vehicles in IMLEO if one must compare throw-away throw weight.

By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
« Last Edit: 10/06/2017 05:23 PM by AncientU »
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Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #15 on: 10/06/2017 05:27 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.

Yes, but fortunately they don't need to fly people on BFR for quite a while yet, a dedicated cargo version and automated landing helps a lot.
A item would be to get the gas-n-go reuse rate to over 50 ideally to 100 or more. This creates a very mature design with a individual unit statistical database on failures and causes of failures. At lower reuse rates (5 to 20) the statistical data is less precise and can be full of assumptions and miss critical items. The other item is total flights of the system in the 100's before significant numbers of persons flying on any one flight. Number of passengers kept artificially low like a dozen for LEO, Lunar, and Mars initial manned flights. To get the high total flights in 100's could take as long as 10 years from the first successful flight of a Cargo/Tanker.

Online John Alan

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #16 on: 10/06/2017 05:29 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Expending both is my assumption, otherwise the comparison to expendable F9/FH makes less sense.

I think it is an attempt to allow apples-to-apples comparison with all other (expendable) rockets.  Believe that at SpaceX there is neither an interest in ever expending any stage(s), nor is the BFR architecture at all optimized for throw-away launches... that said, this vehicle is approximately equivalent to a pair of SLS Block II vehicles in IMLEO if one must compare throw-away throw weight.

By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
Using the above expendable figures and other 2017 presentation numbers...
Can we possibly deduct the empty weight and tank volume sizes of the Booster?...  ???
« Last Edit: 10/06/2017 05:31 PM by John Alan »

Offline AncientU

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #17 on: 10/06/2017 05:45 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.

The missing factoid is that the BFR/BFS will fly the same vehicle repeatedly to build up not only a statistical data base on the launch system's reliability, but also on that specific flight hardware.  Recall that shuttle flew for 30 years, and the only disasters were associated with the launch system -- not reusable vehicle related. 
"If we shared everything [we are working on] people would think we are insane!"
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Online edkyle99

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #18 on: 10/06/2017 06:31 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle


« Last Edit: 10/06/2017 06:40 PM by edkyle99 »

Offline 2552

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #19 on: 10/06/2017 06:48 PM »
Maybe the ship mass includes the mass of the cabins and other crew accommodations that wouldn't be there in the satellite launcher variant?

Online schaban

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #20 on: 10/06/2017 06:55 PM »
Can anyone explain this slide in Elon's presentation?  He skipped over it quickly and didn't discuss its meaning.

 - Ed Kyle
By expending the booster it can get 250 tonnes to LEO.
Booster and "Ship" both expended versus Booster and "Ship" both recovered?

 - Ed Kyle

Booster is expended (run to depletion) and I think there will be no "Ship" just big non-returnable structure, like space station, but could be big equipment to lend on moon/mars/etc.

edit:typos
« Last Edit: 10/06/2017 07:00 PM by schaban »

Online John Alan

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #21 on: 10/06/2017 06:56 PM »
Ref Ed Kyle posting up above

I'm starting to wonder if the stated 85 metric ton BFS dry weight (2017 presentation slides) was for the pictured Mars manned transport configuration same slide...

If the dry empty weight of a BFS configured to carry 250 tons was less... say 70 tonnes dry empty...
...would the Math work?...

On edit...
I think the 250 tonnes payload into orbit math now works...
(on a bone dry BFS that could be then be saved with a tanker flight BTW)...

The 150 tonnes reusable figures... something else assumed is off there maybe... 
« Last Edit: 10/06/2017 07:31 PM by Chris Bergin »

Online envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #22 on: 10/06/2017 07:00 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.

The missing factoid is that the BFR/BFS will fly the same vehicle repeatedly to build up not only a statistical data base on the launch system's reliability, but also on that specific flight hardware.  Recall that shuttle flew for 30 years, and the only disasters were associated with the launch system -- not reusable vehicle related.

The real lesson from STS is not "have an escape system". It's "don't normalize deviance" especially for flight-critical systems. If something isn't working as expected, figure out why and don't wait until it kills someone.

It's much harder to verify that this lesson is actually being followed.

Online envy887

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #23 on: 10/06/2017 07:17 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle

I agree that 235 tonnes seems a lot more realistic for maximum payload, not that it's ever likely to be used that way.

However, for reusable payload, the 2016 booster was only supposed to need 5% of it's prop load to RTLS and land. And the ship should need only 2-3% (about 800 m/s) to deorbit and land. It should be able to get 150 tonnes to LEO fully reuseable.

Offline RocketmanUS

Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.
Yes.
I personally think they need as escape system for lift off and landing for Earth, Lunar and Mars.
To much to risk without one.

New thread for escape system
https://forum.nasaspaceflight.com/index.php?topic=43923.new#new
Mars and beyond, human exploration
The grass is always greener on the other side. When you stand on top of the hill you see both sides!

Offline ncb1397

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #25 on: 10/06/2017 07:27 PM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.
« Last Edit: 10/06/2017 07:28 PM by ncb1397 »

Online Craig_VG

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #26 on: 10/06/2017 07:33 PM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

Just for comparison's sake, what PV efficiency did you use for the calculation?

Offline DJPledger

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #27 on: 10/06/2017 07:38 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.
No rocket will ever be as safe as an airliner so EM has made a huge mistake by not designing the BFR system with a LAS. Lets hope EM realizes this and incorporates a LAS in the next BFR design revision which may be announced at next year's IAC.

Also the engine no. on BFR booster needs to come down significantly to simplify the design and make it more reliable. Not to mention excessive maintenance costs that the 31 engines on booster will incur. EM gave BFR 31 engines because he is trying to dev. a nova class rocket with high performance engines as fast as possible on a shoestring budget. This may well bite him in the back in maintenance costs even if BFR never has an engine failure or mission failure.

If 1st FH fails then I will bet on EM reducing the engine no. on BFR in it's next design revision.
« Last Edit: 10/06/2017 07:57 PM by DJPledger »

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #28 on: 10/06/2017 07:40 PM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

That's impossible to judge unless you know how much insulation the cabin has, and how much insolation it receives (which depends on vehicle attitude).
Waiting for joy and raptor

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #29 on: 10/06/2017 07:50 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle

I think the 85 tonnes ship mass refers to the Spaceship version.

Recall in ITSv2016, the ship version weighed 150t, and the tanker version 90t. There was no mass given for a cargo version, but it probably would have been closer to the tanker in weight. Also, the payload was 300t for the spaceship variant and 380t for the tanker.

If I had to guess, the tanker/cargo version are probably 55,000-60,000 kg dry.


As I go over the numbers, I think the re-usable payload figures are for the spaceship version as well, meaning BFR's payload to LEO/GTO for the cargo variant is likely 25-30 tonnes higher... It's the only way I can reconcile the tanker variant's required ~210 LEO payload (of fuel).

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #30 on: 10/06/2017 08:01 PM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

That's impossible to judge unless you know how much insulation the cabin has, and how much insolation it receives (which depends on vehicle attitude).
In space the only way you lose heat is radiation. I didn't see any radiators, but they could have been hidden on the other side of the solar panels. Also, the heatshield on the bottom should be dark which will serve as a radiator. Waste heat from electronics, the life support system, chemical reactions powering human bodies, and whatever else needs energy on the ship should keep things plenty warm, they just have to keep the radiators sized and positioned correctly to balance the energy needs. If they need more heating, they just need to point the black side of the ship a bit more towards the sun.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #31 on: 10/06/2017 08:04 PM »

I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".

No, it isn't unrealistic. The first stage, despite being bigger, is a lot simpler than the second stage. This PMF doesn't seem unreasonable to me given how large the booster is, and that it uses engines likely to be very efficient weightwise and composite tanks.

Also, putting "ship" in quotes reads as if it is intended to cast aspersion. Ship is the correct term and scare quotes are not helpful.
« Last Edit: 10/06/2017 08:05 PM by Lar »
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #32 on: 10/06/2017 08:10 PM »
No rocket will ever be as safe as an airliner so ...
I understand the anxiety and lack of trust.  But what is your basis for this being permanent state?
gyatm . . . Fern

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #33 on: 10/06/2017 08:18 PM »
No rocket will ever be as safe as an airliner so ...
I understand the anxiety and lack of trust.  But what is your basis for this being permanent state?
Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. The Raptor engines on BFR will be running at even closer to chemistry and materials limits than many other rocket engines due to it's high Pc. Will take many decades before rocket engines approach the reliability level of commercial turbofans.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #34 on: 10/06/2017 08:35 PM »

I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".

No, it isn't unrealistic. The first stage, despite being bigger, is a lot simpler than the second stage. This PMF doesn't seem unreasonable to me given how large the booster is, and that it uses engines likely to be very efficient weightwise and composite tanks.

Also, putting "ship" in quotes reads as if it is intended to cast aspersion. Ship is the correct term and scare quotes are not helpful.
ITS first stage PMF was given as 0.96.  This rocket is going to be smaller, so I don't see how it could have a better ratio. 
Those 31 Raptor engines are going to weigh around 31 tonnes, likely more, all by themselves.  First stage engine mass probably accounts for only 1/4th of the total stage dry mass.  Those assumptions right there gets us close to 0.96.

"Aspersion"?  Don't be silly.  I was using quotes to identify the name "Ship" as given by Mr. Musk.

Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version).  The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation.  With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc.  I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t.

 - Ed Kyle
« Last Edit: 10/06/2017 08:48 PM by edkyle99 »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #35 on: 10/06/2017 08:47 PM »
No rocket will ever be as safe as an airliner so ...
I understand the anxiety and lack of trust.  But what is your basis for this being permanent state?
Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. The Raptor engines on BFR will be running at even closer to chemistry and materials limits than many other rocket engines due to it's high Pc. Will take many decades before rocket engines approach the reliability level of commercial turbofans.
Emphasis mine.  My point is never say never.  Much of engineering is about balancing limits to achieve both performance and reliability.  Having thinner margins and greater reliability is not an oxymoron.
gyatm . . . Fern

Online Lar

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #36 on: 10/06/2017 08:51 PM »

I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".

No, it isn't unrealistic. The first stage, despite being bigger, is a lot simpler than the second stage. This PMF doesn't seem unreasonable to me given how large the booster is, and that it uses engines likely to be very efficient weightwise and composite tanks.

Also, putting "ship" in quotes reads as if it is intended to cast aspersion. Ship is the correct term and scare quotes are not helpful.
ITS first stage PMF was given as 0.96.  This rocket is going to be smaller, so I don't see how it could have a better ratio. 
Those 31 Raptor engines are going to weigh around 31 tonnes, likely more, all by themselves.  First stage engine mass probably accounts for only 1/4th of the total stage dry mass.  Those assumptions right there gets us close to 0.96.

"Aspersion"?  Don't be silly.  I was using quotes to identify the name "Ship" as given by Mr. Musk.

Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version).  The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation.  With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc.  I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t.

 - Ed Kyle
Your landing propellant fraction is too high, I think, and so is your raptor weight.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online John Alan

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #37 on: 10/06/2017 09:06 PM »
Ed... glad to hear the numbers now match up to what was 2017 presented...  8)

I agree with your thought the tanker and cargo are in the 45 Tonnes dry weight range... (45~50)

I also agree with you on Raptor weight... I'm at 980kg each SL these days personal estimate...
« Last Edit: 10/06/2017 09:11 PM by John Alan »

Offline ZachF

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #38 on: 10/06/2017 09:09 PM »
ITS2016 required 7% of fuel to land its first stage per the slideshow, BFR1`17 stages at a lower velocity.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #39 on: 10/06/2017 09:13 PM »
I just realized, for BFR/BFS dont need a TEL. If I understand correctly, its a large part of the current launch pad structure of F9. I think it will be eliminated for BFR/BFS. because it doesnt need it. The first stage is put onto the launch mount by the tower crane seen in the animation. The bottom umbilicals and launch mounts are connected then. The second stage is vertically set on top by the tower crane again. All umbilicals for the BFS run through BFR. The rocket is structurally stable enough that it does not need a support from a TEL. I would even take the guess that all connections between first and second stage are actuated and connect automatically without manual interaction.

After the first flight, BFR will stay on the launch mount after each flight. It will not go back to the hanger. And if it needs to go back, it is craned to a transporter. This will need a lot of manual work and they will avoid that like the plaque.

It seems BFR and BFS are designed to minimize operation costs. The way to minimize operation costs and reduce turn around time is automation. Seeing the way the fuel pipes are designed in the presentation convinced me of that. I would guess that they will make all checkouts after integration of BFR and BFS automatically and only have an operator on a console. This will save a lot of costs compared to F9 launch processing.

Also, due to the mating procedure, vertical integration comes with it. There is no other way of doing it anyway. This will make a lot of people happy. Turns out, for BFR, vertical integration is simpler and far cheaper than horizontal. Its just differently done than anyone else does vertical integration right now.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #40 on: 10/06/2017 09:48 PM »
The BFS can’t really be expended unless they don’t bother to fill the header tanks. Since those are specifically for landing fuel and the only engines for landing are the two sea-level engines, I would guess that the header tanks will only be plumbed to feed fuel to the sea-level raptors and nowhere else. I suppose there could be plumbing to move fuel between tanks, but that would add weight and complexity. Do we have a sense of how much fuel the header tanks can hold? How much is mass to orbit reduced to have full header tanks so you can recover the ship. It’ll never be used expendable so it hardly matters but just curious.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #41 on: 10/06/2017 10:40 PM »
Quote
Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version).  The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation.  With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc.  I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t.

 - Ed Kyle

Using Ed's cargo solution numbers...
Where do we end up on the Tanker version?
How many tonnes of off loadable prop to LEO... the 220 tonnes (1/5 full) hinted in the 2017 presentation?...
And are the tanks likely a stretched 1250 tonnes prop volume as some have opinion'd?...Or something else?
With no need to support a payload in front of them... could the tanks be a lighter, less beefy version?
It's also thought the nose section is as light as possible with no openings beyond maybe a maintenance access hatch...
 ???
« Last Edit: 10/06/2017 10:59 PM by John Alan »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #42 on: 10/06/2017 11:28 PM »
Is anyone else spooked by all this talk of "no need for an escape system, we'll be safe like an airline?" The parallels with the shuttle program seem almost too obvious.
Yes.
I personally think they need as escape system for lift off and landing for Earth, Lunar and Mars.
To much to risk without one.

New thread for escape system
https://forum.nasaspaceflight.com/index.php?topic=43923.new#new

The following should not be considered an argument either for or against the need for an emergency escape system.

Considering the work that was done on both the B-58 Hustler and the XB-70 Valkyrie escape pod systems, with the additional work that was accomplished under project MOOSE, it would seem that an escape system would both be possible/practical.  This system could be usable from the pad all the way into orbit and during return from the moon or Mars reentry all the way to touchdown.

I am not stating that designing/building/testing such a system would be simple nor cheap just that it would be possible and practical.  As the earlier flights of the BFR would be limited in crew size to possibly 10 – 20 and the BFR has so much performance to spare that the performance hit would be relatively minor.  Even when the crew size gets up to the 100-200 the BFR has so much performance that it could still absorb the performance hit.

The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.
« Last Edit: 10/07/2017 01:55 AM by DAZ »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #43 on: 10/07/2017 01:18 AM »
No rocket will ever be as safe as an airliner so ...
I understand the anxiety and lack of trust.  But what is your basis for this being permanent state?
Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. ...

...like some of your other statements, that isn't actually true.

Modern turbofans are operating at higher and higher temperatures in order to get higher and higher efficiency, and their turbine blades actually have to interact with this hot flow. But in a rocket, only the turbopump's blades have to do that (and it can be designed for lower combustion temperature).

And there's one huge advantage for rocket engines over turbofans when it comes to reliability: turbofans will ingest anything in the air. Birds, insects, sand, volcanic ash, people, etc. That can and does cause catastrophic failure. Rocket engines bring their own air which can be carefully screened for contaminants, with actual screens being put in place to catch anything that might hurt the engine.
« Last Edit: 10/07/2017 01:40 AM by Robotbeat »
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Offline Nathan2go

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #44 on: 10/07/2017 02:32 AM »
No need for long duration storage of cryo-propellants?

Did you folks notice the very low speed at which the ship switches from aero-braking to retro-propulsion?  That means that the amount of propellant needed for landing is very low, so storable propellant could be practical.

On Mars, the retro-burn starts at Mach 2.3 = 620 m/s = 1380 mph (which is less than half of what JPL said a 100t, 13' diameter capsule did). Assuming 13% gravity loss (3 gees of thrust and .4 gees of gravity), 10% reserves, and Isp=300s, the landing burn will have a Mass Ratio (Mr) of 1.30.  So the landing propellant is 70t for 150t payload and 85t dry wt.

On return to Earth, aero-braking should get the terminal speed down by 1 order of magnitude, since the air density is 2 orders magnitude more (drag and lift are proportional to density*V^2), perhaps 140 mph.  So the landing propellant requirement is very low.

That means that the ship could use storable propellants for landing, even if all the landing propellant for the round trips is provided at Earth.  Maybe it would be Super-draco derived engines, presumably burning hydrazine and N2O4.

But it would also be possible to make storable propellant on Mars:  propane, which is also storable at room temp, can probably be made in the same chemical reactor that makes methane, if a separator is provided.  N2O4 could be made using Nitrogen from Mar air, but since it is only about 1% N2, maybe it would be easier to bring N2O from Earth (also an easily storable liquid), then react it with more O2 to form the desired N2O4

For on-orbit refilling, a full-tanker boil-off rate of 1% per week is a reasonable goal (for flights every 2 weeks, 10 weeks per Mars departure).  But this can be achieved with a specially insulated propellant depot, and need not impact the ship design.

Of course Musk's initial plan will be the simplest, with more complexity (like propellant depots or storable landing propellant) added when they starting to get deeper into the system design.
« Last Edit: 10/07/2017 02:56 AM by Nathan2go »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #45 on: 10/07/2017 02:42 AM »
Methane and oxygen are in a different class from hydrogen when it comes to cryogenics. Both methane and oxygen can be passively stored in space without boiloff. Practically speaking, hydrogen can't.

Hydrogen boiling point: 20K
Oxygen boiling point: 90K
Methane boiling point: 112K

Being MUCH higher above absolute zero means it's also much, MUCH easier and more efficient to actively cool both methane and oxygen than hydrogen. Multiple times less energy required to refrigerate them. That's if it's even necessary to refrigerate them at all.

SpaceX is NOT going to use storables (i.e. hypergols) for BFR. They don't use them for the F9 booster, and they sure as heck won't use them for BFR. The handling and regulatory headaches alone would increase the cost significantly, and they're also about an order of magnitude or two more expensive than methane and oxygen. And they're also WAY harder to make on Mars. Not happening.
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Offline Zed_Noir

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #46 on: 10/07/2017 08:22 AM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruise ship to do the job of a life boat.  :o
« Last Edit: 10/07/2017 09:07 AM by Zed_Noir »

Offline octavo

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #47 on: 10/07/2017 09:02 AM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.  :o
How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #48 on: 10/07/2017 09:26 AM »
ITS first stage PMF was given as 0.96.  This rocket is going to be smaller, so I don't see how it could have a better ratio. 
Those 31 Raptor engines are going to weigh around 31 tonnes, likely more, all by themselves.  First stage engine mass probably accounts for only 1/4th of the total stage dry mass.  Those assumptions right there gets us close to 0.96.
 - Ed Kyle
That gives a SL Raptor T/W ratio of about 174:1.

That sounds aggressive, until you discover the RD270 (only other complete FFSC actually built) had a T/W of 189:1.
[EDIT. However that was from a Russian site. Astronautix give T/W as 153.25, but running the numbers gives more like 143:1, although I think the Russians sometimes leave off the TVC mass, maximum +/-12deg gimbal) ]

It's also true that Hydrogen (used in the AJR Integrated Powerhead Demonstration) is a special case, both because its a deep cryogen and because it's the only propellant that's compressible at pressures achievable in rocket engines, so difficult to achieve as high a T/W ratio.

But Methane is not H2, it's a hydrocarbon.

It seems hard to believe SX could not achieve the T/W of an engine designed close to 1/2 a century ago, given the advances in design tools (huge) and materials (significant, but perhaps not as dramatic as people imagine given the environment they have to survive)  :( At 189:1 that's a saving of a bit less than 3 tonnes. I'll take a wild stab and guess Musk would target 200:1, because y'know, he's Musk.

[EDIT, and even given the Astronautix values, I'd still say 200:1, as Merlin is already around 180:1 and I'd say Musk is of the "You don't know what the limits are till you exceed them" school  :) ]

The joker in this pack is the weight of the piping to feed those 31 engines at their maximum flow rate. For the same lengths narrow bores should weight less but will cause more fluid hammer (faster fluid flow being stopped) and need higher tank pressures.

Sudden valve closure can give a pressure on the valve 3x driving pressure, enough to snap the pipe off the engine. It should not be underestimated.  :( So 1 tonne/engine (all inclusive) might be accurate.

Modern turbofans are operating at higher and higher temperatures in order to get higher and higher efficiency, and their turbine blades actually have to interact with this hot flow. But in a rocket, only the turbopump's blades have to do that (and it can be designed for lower combustion temperature).

And there's one huge advantage for rocket engines over turbofans when it comes to reliability: turbofans will ingest anything in the air. Birds, insects, sand, volcanic ash, people, etc. That can and does cause catastrophic failure. Rocket engines bring their own air which can be carefully screened for contaminants, with actual screens being put in place to catch anything that might hurt the engine.
There are 2 big issues with rocket engine turbines.

Historically they have run uncooled. In contrast gas turbine blades have typically (except for very small designs) used part of the airflow to do blade cooling, which is why they can run above the melting point of the raw alloys they are made of (and have done so for some time).

The other issue is the very fast startup process.  It turned out a lot of the blade damage in the SSME was caused by a temperature spike of (IIRC) 500F above normal operating temp in the preburner during the few seconds of startup. It, coupled with the LH2 temp  in the cooling channels, lead to "dog kenneling" of the channels (wall thinning leading to leaks) and cracks in the blades. IOW  what happens in those first 3-5 of engine caused damage out of all proportion to the loads during the rest of the time.

Obviously Raptor startup will have been modeled extensively (unlike SSME, which had no CFD models of its preburner built until much later) and AIUI the full flow SC means no inter-propellant seals between an X rich turbine driving a Y rich pump (where X & Y could be either the Oxidizer or the Fuel), which eliminates a major failure mode and also eliminates a shed load of purge gas.

The problem is that start transient is likely to remain short, so is likely to remain (by gas turbine standards) very stressful.
« Last Edit: 10/07/2017 10:03 AM by john smith 19 »
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Offline Zed_Noir

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #49 on: 10/07/2017 09:29 AM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.  :o
How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?

There should be SX Starlink satellites around if the BFS is there at the Moon or Mars.

Distress beacons and reflective panels  from the survivors's means of escape from a doomed BFS to tracked the trajectory to the landing site from orbital and static observation asserts.

Since both the Moon and Mars are extensively survey by orbital satellites. The BFS doing the rescue can use a terrain matching navigation system to go to the survivors once they are located by orbital asserts.


Offline DJPledger

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #50 on: 10/07/2017 11:37 AM »
No rocket will ever be as safe as an airliner so ...
I understand the anxiety and lack of trust.  But what is your basis for this being permanent state?
Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. ...

...like some of your other statements, that isn't actually true.

Modern turbofans are operating at higher and higher temperatures in order to get higher and higher efficiency, and their turbine blades actually have to interact with this hot flow. But in a rocket, only the turbopump's blades have to do that (and it can be designed for lower combustion temperature).

And there's one huge advantage for rocket engines over turbofans when it comes to reliability: turbofans will ingest anything in the air. Birds, insects, sand, volcanic ash, people, etc. That can and does cause catastrophic failure. Rocket engines bring their own air which can be carefully screened for contaminants, with actual screens being put in place to catch anything that might hurt the engine.
Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.

Offline jpo234

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #51 on: 10/07/2017 12:59 PM »


Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.

This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane.

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Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #52 on: 10/07/2017 01:19 PM »
Merlin was designed to ingest a nut.

"Fleck of paint" is, as usual, false.
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Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #53 on: 10/07/2017 01:21 PM »


Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.

This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane.



I'm unable to find the quote but I'm sure I remember reading about an engine where part of the design brief was to be able to ingest a loose nut into the turbopump without failure. Anyone else recall this?
(Sorry for slight thread drift...)
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #54 on: 10/07/2017 01:30 PM »


Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.

This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane.



I'm unable to find the quote but I'm sure I remember reading about an engine where part of the design brief was to be able to ingest a loose nut into the turbopump without failure. Anyone else recall this?
(Sorry for slight thread drift...)

Ok found it- it was the Merlin!
https://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?no-ist=&page=2

Although IMHO I'm wary of the accuracy of this account and would prefer a better source. It just sounds very unlikely to me that part of qualification testing involves chucking nuts and bolts into the fuel tanks...
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Online John Alan

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #55 on: 10/07/2017 02:15 PM »
Shifting subtopic gears here a minute away from FOD in the prop tanks...  ;)

Quote
Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version).  The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation.  With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc.  I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t.

 - Ed Kyle

Using Ed's cargo solution numbers...
Where do we end up on the Tanker version?
How many tonnes of off loadable prop to LEO... the 220 tonnes (1/5 full) hinted in the 2017 presentation?...
And are the tanks likely a stretched 1250 tonnes prop volume as some have opinion'd?...Or something else?
With no need to support a payload in front of them... could the tanks be a lighter, less beefy version?
It's also thought the nose section is as light as possible with no openings beyond maybe a maintenance access hatch...
 ???
I have been looking at Ed's solution and pondering it's various nuances...
Trying to Wrap my head around what SpX is doing here...  ???

One key thing that blew me away...
(on edit... I'm thinking the 1.25:1 take off thrust to weight working on Gravity Losses is key in this working)

250 tonnes payload (expendable) only has 850 tonnes of prop sloshing around in BFS's 1100 tonnes capacity tanks...
And between the BFR and BFS both running to empty...
It gains 9200+m/s (LEO) velocity...
...295 tonnes...
250 tonnes payload (plus the 45 tonnes empty BFS)...
...going around and around in orbit...
And the prop tanks were not full...  :o

Profound... and here is why I think so...
As the Raptor matures and chamber pressure inches toward the 300 bar goal.
They can instantly take advantage to add more delta/V to the stack...
As take off thrust from those 31 Raptors increases... add more prop to the BFS...
Someday... 250 tonnes RTLS may be possible... (wild guess not confirmed)

The other concept that I think helps understand it, is this example...
Lets say the Payload is a 250 tonnes water tank...
You drain 15 tonnes of water out prelaunch and put 15 tons more prop in the BFS...
You get to LEO with the 235 tonnes of payload... 9200+m/s and SECO it...
But now you have 15 tonnes of prop left in the BFS tanks...
You can eject the payload and recover the BFS (just over 1000m/s delta/v avalible + aerobraking)

So ironically... You have to expend stage 1... but can save stage 2...LMAO...  ;D

Now then... Continuing down the payload to prop weight redistribution path...
In between the 235 and 150 tonnes payload range...
Still adding more and more Prop to BFS to keep GLOW at 4400 tons spec...
You end up with enough delta/v margin at SECO, to add a MECO and start doing ASDS style BFR recoveries...
In other words... LOAN spare delta/v from the BFS back to the BFR...
At first the landings are "HOT" and very ballistic...
But with less and less payload... they get cooler and easier on the hardware with bigger delta/V loans...

At 150 tonnes payload...
You have enough delta/v to add a flip/boost back burn and start landing back on the launchpad..
And ironically... the BFS still only has 950 tonnes in the tanks at launch and still could take another 150 if the Raptor 300 bar upgrade pans out...

From 150 tonnes down to 0 tonnes payload..
If needed be... Adding more prop to BFS = more delta/v to do plane or orbit changes after reaching LEO...
You are gaining delta/v, because GLOW is now falling below 4400 tonnes...

Now to the tanker...
I'm thinking... but I'm not sure on this part...
IF the payload is reduced to zero...
And then you go a bit further on a tanker BFS by cutting weight to a bare minimum on the airframe...
I mean after all, in the big picture sense...
There is no need to have a load path thru the tank to hold a payload...
A flying gas tank could be well under 45 tonnes empty weight...
Every tonnes saved in EW helps in what is needed on the landing burn also...

It's implied by the 2017 slides and presentation...
...that something like 217 tonnes of prop can be hauled up on each load...
217x5 loads would be 1085 tonnes of prop...
...plus the 15 tonnes assumed on board that could have got it home...
Equals full tanks... 1100 tons...

But it's still not clear to me how they got from 150 tons of spare prop to a 217 or so number...
There is a 67 tonnes gap there that needs explained somehow...  :P

Anyway... My thoughts on BFS/BFR and how as a system it may work... :)

(On edit... now back to the ongoing FOD discussion)
« Last Edit: 10/07/2017 03:26 PM by John Alan »

Online edkyle99

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #56 on: 10/07/2017 02:19 PM »
Merlin was designed to ingest a nut.
Merlins have proven to be excellent engines, but we have to remember that Raptor will operate at higher pressures and will have full-flow preburners, different propellants, etc.  No guarantee that Raptor will end up as reliable as Merlin.

 - Ed Kyle

Offline RobLynn

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #57 on: 10/07/2017 02:32 PM »
That gives a SL Raptor T/W ratio of about 174:1.
That sounds aggressive, until you discover the RD270 (only other complete FFSC actually built) had a T/W of 189:1.
[EDIT. However that was from a Russian site. Astronautix give T/W as 153.25, but running the numbers gives more like 143:1, although I think the Russians sometimes leave off the TVC mass, maximum +/-12deg gimbal) ]
similar sized 50 year old NK33/AJ26 staged combustion engine with ~1500kN and ~15MPa chamber pressure has T/W ~140.  have a look at the size of its huge turbopump https://en.wikipedia.org/wiki/NK-33#/media/File:Aerojet_AJ26_in_the_Stennis_E-1_Test_Stand_-_cropped.jpg and it is pretty clear that the Raptor (if the released CAD is to be believed) is going to be much much higher T/W. Raptor T/W of 200-250 is likely.
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Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #58 on: 10/07/2017 02:52 PM »


Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.

This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane.



I'm unable to find the quote but I'm sure I remember reading about an engine where part of the design brief was to be able to ingest a loose nut into the turbopump without failure. Anyone else recall this?
(Sorry for slight thread drift...)

Ok found it- it was the Merlin!
https://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?no-ist=&page=2

Although IMHO I'm wary of the accuracy of this account and would prefer a better source. It just sounds very unlikely to me that part of qualification testing involves chucking nuts and bolts into the fuel tanks...
The reason is that inside the LOX tank were COPV Helium tanks fastened by Nuts and Bolts. If one of those came loose(unlikely because such as shown before results in a rather spectatular tank failure) or a stray got left in the tank during build (likely).

For Raptor a more likely scenario is frozen LOX or Methane in the super cooled prop.
« Last Edit: 10/07/2017 02:57 PM by oldAtlas_Eguy »

Offline rakaydos

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #59 on: 10/07/2017 02:57 PM »
Merlin was designed to ingest a nut.
Merlins have proven to be excellent engines, but we have to remember that Raptor will operate at higher pressures and will have full-flow preburners, different propellants, etc.  No guarantee that Raptor will end up as reliable as Merlin.

 - Ed Kyle
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

Offline guckyfan

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #60 on: 10/07/2017 03:05 PM »
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

I think it is so even worst case engine out does not become a RUD for the whole vehicle. With the nut the engine is no longer operational but won't destroy neighbouring engines or the tank.

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #61 on: 10/07/2017 03:48 PM »
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

I think it is so even worst case engine out does not become a RUD for the whole vehicle. With the nut the engine is no longer operational but won't destroy neighbouring engines or the tank.
These scenarios in qualification testing speak to the culture of the engine design team wanting to be able to test to destruction to find out if the engine controller software and the sensors that is being used can act fast enough to forestall a RUD on a failing engine. Starve the engine quickly enough of prop and a potential RUD will fail to occur but the engine is definitely out but when you have 31 of them or even just 4 or 2 and only need n-1 to fulfill mission with the Raptor VACs probably can do the mission with as few as 2. And for the booster probably can loose as many as 6 out of the 31.

Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #62 on: 10/07/2017 03:55 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle
A big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.

BFR doesn't use hydrogen, so it should get lower aero losses and higher averaged thrust to weight ratio (since your tanks empty sooner). Therefore the benchmark 9.2km/s need not apply, and I am nearly certain I've seen a legitimate estimate for BFR that shows a trajectory of 8.9something km/s to orbit.

If you use the exact same mass fraction as last year's ITS booster (slightly better than 96%) and your numbers for Isp and stage mass, then you get 250 tons expendable to LEO at about 8.99km/s. That fits perfectly with all the rest of the info we have.

No mystery. If you try to mess with SpaceX's numbers to fit more conservative assumptions, then of course you'll not be able to recreate their figures. That's not a mystery, either.
« Last Edit: 10/07/2017 03:58 PM by Robotbeat »
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Offline DAZ

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #63 on: 10/07/2017 05:34 PM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.  :o
How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?

There should be SX Starlink satellites around if the BFS is there at the Moon or Mars.

Distress beacons and reflective panels  from the survivors's means of escape from a doomed BFS to tracked the trajectory to the landing site from orbital and static observation asserts.

Since both the Moon and Mars are extensively survey by orbital satellites. The BFS doing the rescue can use a terrain matching navigation system to go to the survivors once they are located by orbital asserts.

SAR requires an extensive amount of infrastructure.  This infrastructure is available almost everywhere to some extent or another on earth.  It would definitely take a significant amount of infrastructure to pull off the multiple SAR missions to recover the possible 1 – 2 dozen rescue pods scattered across hundreds of square miles.  It is definitely not as simple as sending out a taxi to pick up your overly indulgent girlfriend/boyfriend.  If the BFR is successful as Musk envisions than something like the Starlink constellation will probably happen on the Moon/Mars.  This would only be part of the required infrastructure that would be needed for a successful SAR infrastructure.  The basic premise though is the same.  This extensive infrastructure would not be available until very much later when most likely the need for an emergency bailout system on the BFR most likely will no longer be needed.

The use of individual escape pods has many advantages over a single escape lifeboat type system.  They directly scale to the number of crewmen that you're trying to rescue.  They require relatively little modification to the overall BFR system.  They can be placed on the BFR system at early implementation and removed later when no longer needed with little impact to the overall system.  They could greatly reduce the need for the number of launch/reentry pressure suits.  This last alone could almost make it worthwhile to use escape pods even if to only protect against depressurization events.  The use of ejection seats over the decades has shown us two very important things.  One that they generally save more lives than they cost and 2 they don't always work.  If you have a system where everybody is on the same system it becomes an all or nothing proposition.  Using individual escape pods it is very conceivable that you could have a situation where you could eject 20 of them but only 10 of them survive.  This obviously would not be the best situation but it would be a better situation than losing all 20.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #64 on: 10/07/2017 06:06 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle
A big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.

BFR doesn't use hydrogen, so it should get lower aero losses and higher averaged thrust to weight ratio (since your tanks empty sooner). Therefore the benchmark 9.2km/s need not apply, and I am nearly certain I've seen a legitimate estimate for BFR that shows a trajectory of 8.9something km/s to orbit.

If you use the exact same mass fraction as last year's ITS booster (slightly better than 96%) and your numbers for Isp and stage mass, then you get 250 tons expendable to LEO at about 8.99km/s. That fits perfectly with all the rest of the info we have.

No mystery. If you try to mess with SpaceX's numbers to fit more conservative assumptions, then of course you'll not be able to recreate their figures. That's not a mystery, either.
Perhaps your stage 2 is not heavy enough?  I find that stage 2 is 1335 tonnes, 1100 propellant, 40 return propellant, 150 cargo, 85 structure.  This has better ISP than the first stage, hence better performance overall.  I don't get more than 9 km/s either though.  Not as good as last years vehicle though, at 9600 m/s

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #65 on: 10/07/2017 06:20 PM »
I'm calculating expendable. 250 tons expendable is what was listed for BFR in the IAC 2017 presentation.

Reusable payload has too many variables to easily replicate the calculation.
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #66 on: 10/07/2017 06:22 PM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.  :o
How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?

There should be SX Starlink satellites around if the BFS is there at the Moon or Mars.

Distress beacons and reflective panels  from the survivors's means of escape from a doomed BFS to tracked the trajectory to the landing site from orbital and static observation asserts.

Since both the Moon and Mars are extensively survey by orbital satellites. The BFS doing the rescue can use a terrain matching navigation system to go to the survivors once they are located by orbital asserts.

SAR requires an extensive amount of infrastructure.  This infrastructure is available almost everywhere to some extent or another on earth.  It would definitely take a significant amount of infrastructure to pull off the multiple SAR missions to recover the possible 1 – 2 dozen rescue pods scattered across hundreds of square miles.  It is definitely not as simple as sending out a taxi to pick up your overly indulgent girlfriend/boyfriend.  If the BFR is successful as Musk envisions than something like the Starlink constellation will probably happen on the Moon/Mars.  This would only be part of the required infrastructure that would be needed for a successful SAR infrastructure.  The basic premise though is the same.  This extensive infrastructure would not be available until very much later when most likely the need for an emergency bailout system on the BFR most likely will no longer be needed.

The use of individual escape pods has many advantages over a single escape lifeboat type system.  They directly scale to the number of crewmen that you're trying to rescue.  They require relatively little modification to the overall BFR system.  They can be placed on the BFR system at early implementation and removed later when no longer needed with little impact to the overall system.  They could greatly reduce the need for the number of launch/reentry pressure suits.  This last alone could almost make it worthwhile to use escape pods even if to only protect against depressurization events.  The use of ejection seats over the decades has shown us two very important things.  One that they generally save more lives than they cost and 2 they don't always work.  If you have a system where everybody is on the same system it becomes an all or nothing proposition.  Using individual escape pods it is very conceivable that you could have a situation where you could eject 20 of them but only 10 of them survive.  This obviously would not be the best situation but it would be a better situation than losing all 20.

It's not really analogous to an Earth-based SAR effort since the vehicle would be on a known ballistic flight path and there isn't much to hide in (ocean, forest)
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #67 on: 10/07/2017 06:35 PM »
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

I think it is so even worst case engine out does not become a RUD for the whole vehicle. With the nut the engine is no longer operational but won't destroy neighbouring engines or the tank.
These scenarios in qualification testing speak to the culture of the engine design team wanting to be able to test to destruction to find out if the engine controller software and the sensors that is being used can act fast enough to forestall a RUD on a failing engine. Starve the engine quickly enough of prop and a potential RUD will fail to occur but the engine is definitely out but when you have 31 of them or even just 4 or 2 and only need n-1 to fulfill mission with the Raptor VACs probably can do the mission with as few as 2. And for the booster probably can loose as many as 6 out of the 31.
Absolutely.

Also, FOD is a common problem with aerospace propulsion of any kind. Tolerance for incidental FOD implies reduced LOM/LOC from the start.

Many Soyuz launches had LOM due to FOD.

Now, what does Merlin's heritage have to do with Raptor's? To me, it's more about the design decision to address "face start", at the injector. (Doable in part because it was a gas generator, which meant the engine team had an intermediate "step up" before taking on FFSC.) They learned the ways the design would "go wrong". So they "paid the price of admission" to the new game early.

Also, by the design choice of going for highest chamber pressure/FFSC and avoiding ORSC, they chose a design path that would require compartmentalized design that, like with "face start", worked as a constant flow w/o sequencing restrictions, so it all depends on complexity reduction/scaling as an ensemble to work from the beginning.

Back to FOD - the weakness of fractures in moving parts magnified/multiplied the risks for Raptor over known issues with M1D - because of the FR/OR paths and pressure - note that for Block 5's engines appears will deal with that. Which makes scaled Raptor both reliable, and, more importantly, controllable with the engine controller as you have aptly described.

Raptor is on the path of needing to exceed LM ASC engine reliability. That's a tall order to fill. (If those point to point transport on Earth graphics are "real", likely engine reliability would have to approach commercial transport turbofan reliability, which is three orders plus of magnitude higher.) To prove this would require extreme testing/use/reuse.

One could "concern troll" that if AJR/BO can't test to such, then SX couldn't ever do such, omitting the fact that they seem to be able to meet reliability margins above industry norms.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #68 on: 10/07/2017 06:39 PM »
A big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.
9200 m/s is a guesstimate that includes gravity loss, drag loss, and other losses on the way up to a relatively low inclination low earth orbit from Florida specifically, as it also includes the velocity provided by the Earth's rotation at that latitude.  It is an old rule of thumb, though I would expect reality to diverge +/-100 m/s or so depending on all kinds of things, most notably thrust-to-weight ratios.

As I mentioned, I was able to find a solution if I assumed a roughly 45 tonne second stage.  Here are those numbers.  I'm not saying this is "it", but it shows a possible route to "it".  Shuttle orbiter weighed 75-ish tonnes empty.  BFR Ship is bigger, so 45 tonnes is, I would say, optimistic.

S1:  3,278 tonnes GLOW, 131 tonnes dry, 351 tonnes burnout for reuse flights, ISP avg = 347.4 sec
S2:  1,122 tonnes GLOW, 45 tonnes dry, 109.5 tonnes burnout for reuse flights, ISP = 375 sec.
PL = 250 tonnes, expendable version, delta-v = 9,499 m/s (LEO+)
PL = 150 tonnes, reusable version, delta-v = 9,355 m/s (LEO+)
PL = 20 tonnes, reusable version, delta-v = 11,700 m/s (GTO)

 - Ed Kyle
« Last Edit: 10/07/2017 06:53 PM by edkyle99 »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #69 on: 10/07/2017 08:30 PM »
A big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.
9200 m/s is a guesstimate that includes gravity loss, drag loss, and other losses on the way up to a relatively low inclination low earth orbit from Florida specifically, as it also includes the velocity provided by the Earth's rotation at that latitude.  It is an old rule of thumb, though I would expect reality to diverge +/-100 m/s or so depending on all kinds of things, most notably thrust-to-weight ratios.

It's an old rule of thumb for LOX/LH2 engines, which actually have higher gravity losses than LO2/HC engines. Gravity losses can be reduced by shifting the mixture ratio in flight. 1/2 of the 5% increase in payload of the Saturn V's was done this way (specifically between 2 values in each of the upper stages. A proportionally variable system could probably have done even better, tracking O/F ratio from high(ish) O2/low(ish) Isp at low altitude up to low(ish) O2/High Isp at MECO).

Some actual numbers from Humble (Space Propulsion Analysis & Design)


Vehicle                   Orbit specs   Launch Long   Vel to orbit m/s        Total delta V inc losses
Delta 7925            175x319           33.9                   7842                 8814
Atlas 1                   149x607           27.4                   7946                 9243
Titan IV/centaur    157x436           28.6                   7896                 9207
STS                         196x278          28.5                   7794                 9086


Which suggests that good trajectory design and selection can cut your target delta v by quite a bit. It depends what the BFS can live with.
« Last Edit: 10/07/2017 09:11 PM by john smith 19 »
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #70 on: 10/07/2017 09:18 PM »
Nowadays doing sims of vehicles to target orbits/missions is easier/better than back of the envelope, so that's where the performance numbers come from.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #71 on: 10/07/2017 09:40 PM »
Ref my earlier post for some background on what I am referring to...
http://forum.nasaspaceflight.com/index.php?topic=43920.msg1733249#msg1733249

I may be off base here...  :-\
But I only figured on 100 tonnes of prop total between the two ships to gain re usability...
15 tonnes on the BFS (assuming 46 tonnes dry mass at touchdown)*
85 tonnes on the BFR (assuming 132 tones dry mass at touchdown)
* note bringing payload back on BFS will require more prop to land is understood... plan for it...

Reason being 1x SL Raptor at 330 ISP and 170 tonnes thrust is only drinking prop at just over 1/2 metric tons a second.
And that is at full throttle... it would be modulating lower near touchdown...
And factoring in the thrust to weight ratio of the nearly empty stages... G forces are a factor here...

BFS...
Assuming 2 SL Raptors light under a 60 Tonnes wet stage at start of flip and braking/landing burn...
340 tonnes thrust on a 60 tonnes object... 5.5+ G's... serious braking power...
And it can keep that up for 15 seconds at max throttle...
I seriously think there is enough delta/v here considering the stage will be getting lighter as the burn progresses.
Aero-brake incoming offshore to under Mach 1... FL250 at start of flip as target...
Light both engines @ min throttle and pull up vertical and then tail first into the direction of travel as it quits "flying"
Modulate the engines to stop forward travel right over the landing pad... "grasshopper" it onto the pad quickly...

BFR...
85 tonnes prop is 170 engine seconds of full thrust available...
Consider engine run times on an F9 RTLS for reference...
Flip after staging and light the 7 center SL engines... to arrest forward velocity and toss it back toward land...
(note the 1-6-12-12 pattern of 31 engines is assumed)
1190 tonnes of force pushing on a 216 tonnes wet BFR... 5.5+ G's
Engines off and wait for it to fall to reentry start altitude...
Light all 7 again and brake hard up to stage structures ability or until speed arrested... off again...
Aero braking then comes into play as it sets up for the landing burn...
Now light only 4 (center and every other in next ring) and modulate thrust to land it...
At 34 tonnes minimum thrust X 4 engines.... 136 Tonnes minimum thrust
You could just hover it if needed... plus easily handle an engine out situation...

In summary... just my opinions with back of the napkin rough figuring...
Minimizing Gravity losses with quick power bursts helps...
The amount of real Delta/V it takes to land a stage with aero-braking in play... is lower then some may think...
100 tonnes prop MAY be enough... in my opinion... for BFS/BFR
« Last Edit: 10/08/2017 02:12 AM by John Alan »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #72 on: 10/07/2017 09:42 PM »
....
The one thing that is not possible or practical is an escape system at either Mars or the moon.  The reason for this is quite obvious.  All escape systems depend on being rescued after the fact.  This is possible just about any place on earth.  Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days.  There is nobody on the moon or Mars to rescue anybody.  Even if you successfully escaped the BFR you would still inevitably die.  That’s the way it would be for the foreseeable future.  Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.

That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.

Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.  :o
How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?

There should be SX Starlink satellites around if the BFS is there at the Moon or Mars.

Distress beacons and reflective panels  from the survivors's means of escape from a doomed BFS to tracked the trajectory to the landing site from orbital and static observation asserts.

Since both the Moon and Mars are extensively survey by orbital satellites. The BFS doing the rescue can use a terrain matching navigation system to go to the survivors once they are located by orbital asserts.

SAR requires an extensive amount of infrastructure.  This infrastructure is available almost everywhere to some extent or another on earth.  It would definitely take a significant amount of infrastructure to pull off the multiple SAR missions to recover the possible 1 – 2 dozen rescue pods scattered across hundreds of square miles.  It is definitely not as simple as sending out a taxi to pick up your overly indulgent girlfriend/boyfriend.  If the BFR is successful as Musk envisions than something like the Starlink constellation will probably happen on the Moon/Mars.  This would only be part of the required infrastructure that would be needed for a successful SAR infrastructure.  The basic premise though is the same.  This extensive infrastructure would not be available until very much later when most likely the need for an emergency bailout system on the BFR most likely will no longer be needed.

The use of individual escape pods has many advantages over a single escape lifeboat type system.  They directly scale to the number of crewmen that you're trying to rescue.  They require relatively little modification to the overall BFR system.  They can be placed on the BFR system at early implementation and removed later when no longer needed with little impact to the overall system.  They could greatly reduce the need for the number of launch/reentry pressure suits.  This last alone could almost make it worthwhile to use escape pods even if to only protect against depressurization events.  The use of ejection seats over the decades has shown us two very important things.  One that they generally save more lives than they cost and 2 they don't always work.  If you have a system where everybody is on the same system it becomes an all or nothing proposition.  Using individual escape pods it is very conceivable that you could have a situation where you could eject 20 of them but only 10 of them survive.  This obviously would not be the best situation but it would be a better situation than losing all 20.

It's not really analogous to an Earth-based SAR effort since the vehicle would be on a known ballistic flight path and there isn't much to hide in (ocean, forest)

My statement was assuming that they knew the locations of the escape pods with some reasonable fidelity.  When I was saying hundreds of square miles I was not referring to they would have to search in that area just that they would be scattered over that distance.  If you did not know where the escape pods were located with some fidelity you would go from an extensive amount of infrastructure to an extraordinary amount of infrastructure that would be required to do the search alone.  Just the time it would take to do the search would probably mean that there were no survivors to find and it would only be a body recovery.  The basic point is it would require more infrastructure than would be available in year one and probably not even available in year 10.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #73 on: 10/08/2017 03:05 PM »
There seem to be two cutaway view renderings to the spaceship one with the oxygen fuel tank just below a single cargo deck and one with two cargo decks.
Any idea as to which is the "correct" one?
I think they are both correct.

One shows the oxygen dome with a cargo deck above it.

The other shows the equipment packed around the dome rather than the dome itself... that's not a cargo deck (although depending on how much equipment there is a need to pack in there, maybe you have some under floor stowage space?)
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #74 on: 10/08/2017 03:06 PM »
There seem to be two cutaway view renderings to the spaceship one with the oxygen fuel tank just below a single cargo deck and one with two cargo decks.
Any idea as to which is the "correct" one?

If what you are referencing is the payload section that abuts the propellant tanks, I think it's the same spacecraft in both views, but what you are seeing is that there is equipment that packs around the top of the dome for the LOX tank, and that it is not a separate level. Kind of donut shaped.
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #75 on: 10/08/2017 03:12 PM »
My take on the above...
...is the top slide "ghosted" away the equipment bay "ring" around the top of the tank...
...highlighting the tank volume hidden in the dome shaped end there...
The other is more a"transparent" outer mold line view... showing the equipment there...

My 2 cents...  ;)

(On edit... Ron ninja'd me with a better answer)...  :P  ;D
« Last Edit: 10/08/2017 03:12 PM by John Alan »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #76 on: 10/08/2017 03:12 PM »
My statement was assuming that they knew the locations of the escape pods with some reasonable fidelity.  When I was saying hundreds of square miles I was not referring to they would have to search in that area just that they would be scattered over that distance.  If you did not know where the escape pods were located with some fidelity you would go from an extensive amount of infrastructure to an extraordinary amount of infrastructure that would be required to do the search alone.  Just the time it would take to do the search would probably mean that there were no survivors to find and it would only be a body recovery.  The basic point is it would require more infrastructure than would be available in year one and probably not even available in year 10.

If we assume a Starlink derived orbital infrastructure is in place at the time of the incident[1], there is a very high probabiliity that any survivable incident will be tracked all the way down with multiple different techs (visual, radio, GPS transciever reception). 

So I think your scenario is invalid. We will know exactly where any putative escape pod lands.

1 - I am taking this as a given. Musk always thinks ahead and it is obvious to me they will put comms/GPS/visual infrastructure in orbit. probably on the first mission.
« Last Edit: 10/08/2017 03:15 PM by Lar »
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #77 on: 10/08/2017 03:41 PM »
Ref my earlier post for some background on what I am referring to...
http://forum.nasaspaceflight.com/index.php?topic=43920.msg1733249#msg1733249

I may be off base here...  :-\
But I only figured on 100 tonnes of prop total between the two ships to gain re usability...
15 tonnes on the BFS (assuming 46 tonnes dry mass at touchdown)*
85 tonnes on the BFR (assuming 132 tones dry mass at touchdown)
* note bringing payload back on BFS will require more prop to land is understood... plan for it...
Elon Musk's 2016 presentation said 7% of total ITS first stage propellant would be reserved for first stage return to launch site burns.  I'm assuming the same for BFR as a starting point, and guessing close to the same for the "Ship", though I have no firm basis for that guess.
http://www.spacex.com/sites/spacex/files/mars_presentation.pdf

 - Ed Kyle
« Last Edit: 10/08/2017 05:18 PM by edkyle99 »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #78 on: 10/08/2017 04:07 PM »
There seem to be two cutaway view renderings to the spaceship one with the oxygen fuel tank just below a single cargo deck and one with two cargo decks.
Any idea as to which is the "correct" one?
I think they are both correct.

One shows the oxygen dome with a cargo deck above it.

The other shows the equipment packed around the dome rather than the dome itself... that's not a cargo deck (although depending on how much equipment there is a need to pack in there, maybe you have some under floor stowage space?)
How much and how big would all the ECLSS equipment be for the support of 100 people for a couple of years?

Batteries, water, scrubbers, O2 regeneration from CO2(?), moister removal from air, computers, liquid nitrogen tanks for a nitrogen oxygen "natural" atmosphere.

Also possibly some LOX regeneration equipment here as well with the LCH4 regeneration in the engine compartment.

If the water tanks are large enough they will act as a heat sink for the crew cabin and electronics until the solar array and heat radiators are deployed.

Between the LOX tank and all this equipment will be multiple layers of possibly Mylar with some sort of insinuative foam between the layers of Mylar to create a heat barrier between this equipment and the LOX tank. Similar would exist between the engine compartment equipment and the LCH4 tank.Every little bit of very lightweight insulation not in the airflow of the external surfaces will reduce power requirements for the prop regeneration. If there is enough, prop regeneration may be a small system used only during long duration coast.

There is also likely to be some sort of hard shell insulation on the outer surface of the tanks as well. Could even be various forms of PicaX which would have a secondary usage beside for reentry as an insulator for the tanks to reduce boiloff during coast.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #79 on: 10/08/2017 05:00 PM »
Merlin was designed to ingest a nut.
Merlins have proven to be excellent engines, but we have to remember that Raptor will operate at higher pressures and will have full-flow preburners, different propellants, etc.  No guarantee that Raptor will end up as reliable as Merlin.

 - Ed Kyle
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

More so reliability as FOD is a common failure mode in LVs.
The RL-10 also was designed to handle ingestion of FOD as it was expected the inner tank insulation could come loose so they tested for it by throwing chunks of the foam into the tank.

« Last Edit: 10/08/2017 05:02 PM by Patchouli »

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #80 on: 10/08/2017 05:37 PM »
Merlin was designed to ingest a nut.
Merlins have proven to be excellent engines, but we have to remember that Raptor will operate at higher pressures and will have full-flow preburners, different propellants, etc.  No guarantee that Raptor will end up as reliable as Merlin.

 - Ed Kyle
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."

More so reliability as FOD is a common failure mode in LVs.
The RL-10 also was designed to handle ingestion of FOD as it was expected the inner tank insulation could come loose so they tested for it by throwing chunks of the foam into the tank.
Snice the Centaur does not use any internal insulation in it's tanks which US are you talking about that used/uses the RL-10? One of the nice properties of stainless steel is that it is a good cryo insulator, for the insulation between the LH2 and LOX tanks.

Added: Sorry I think this is taking us OT.
« Last Edit: 10/08/2017 05:39 PM by oldAtlas_Eguy »

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #81 on: 10/08/2017 06:08 PM »
On a different tack and back to the BFR.

I was playing around with rearrangement of mission planning which included using LOX derived from the ISRU on the Moon and then shipped to L2 by delivering from Earth LCH4 both to the Lunar surface and L2 to see if a reduction of number of flights total per Mars mission could be obtained by doing this. The result was that the number of total launches from Earth per Mars mission reduced to 5 from the SpaceX specification for 6 launches.

The other item is that this enables deep space outer planet missions leaving from L2 with a DV capability of >8km/s (50mt of onboard payload).

Also if 3 tankers accompany the deep space mission into a 4.2km transfer orbit that returns the tankers back to Earth, the total DV for the Deep Space Mission could be as high as 12km/s. This can be done as well from LEO but since you loose nearly 4km/s by not leaving from L2 would be like the first case L2 deep space mission.

Let us just say there are many options for creating highly capably deep space missions to do rapid transit direct transfer orbits to outer planets with a system that does refueling anywhere even after significant DV stagings from LEO. To get than the 12km/s from LEO requires that the 3 tankers all are refueled themselves by 4 tankers at the first 4km/s burn + 3 more for the mission BFR. So for a total of 65 tanker launches to LEO and 1 mission BFR launch a outer planet mission with a DV budget of 16km/s could be created.

Added: For the equvelence an L2 departing 12km/s mission is equivelent to a 16km/s departing LEO mission but with the Lunar source LOX would need just 21 launches from Earth vs the 65 for a LEO departure.

Corrected wording for better understanding.
« Last Edit: 10/08/2017 11:18 PM by oldAtlas_Eguy »

Offline DAZ

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #82 on: 10/09/2017 12:55 AM »
Just for clarification.  You are referring to ELL2 (Earth Luna Lagrange) as opposed to SEL2 (Sol Earth Lagrange).

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #83 on: 10/09/2017 05:48 AM »
Just for clarification.  You are referring to ELL2 (Earth Luna Lagrange) as opposed to SEL2 (Sol Earth Lagrange).
Yes. The advantage for Mars is only a reduction to 5/6 the cost. But for higher DV outer planet missions the savings is very significant and can be greater that 50%.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #84 on: 10/09/2017 07:10 AM »
How much and how big would all the ECLSS equipment be for the support of 100 people for a couple of years?
Turn the question on its head.

The NASA baseline for crew consumables is 5Kg/person/day, of which maybe 3.5Kg is water.

26 (31 day) months of that is 4030 Kgs. So the question is how much of that is recyclable and could you build a system to do it that did not weigh more than the consumables it recovered?

Most effort currently seems to be on exhaled breath and urine recycling with very limited work done on feces or CO2 regeneration. My instinct is a biological solution is the simplest way, but so far everyone seems to be focusing on various (complex) industrial chemistry approaches. 

So far the biological route regens the O2 but creates biomass which does not seem to have much use. Bacteria that excrete granules of plastic compatible with 3d printing maybe?

[EDIT. For anyone who's interested here is Paragon's first look at the ECLSS system for the Mars One plan.
https://www.mars-one.com/images/uploads/Mars_One_Habitat_ECLSS_Conceptual_Design_Assessment.pdf

It's a bit long and there's lots listed "TBD" but it does have various nuggets. For example 4 crew (2 standard + 2 emergency) generate 6.6Kg of sweat and exhaled water/day, requiring the removal of 4.4KwH of energy to condense it. Or that one of the reasons humidity needs to be controlled is CO2+H2O causes the formation of Carbonic acid, which could cause corrosion. Not something most people think about.

This thesis from MIT is also quite a nice introduction to the trade offs between the physiochemical (high energy, but open box) and the bioregenerative  (low energy, but black box) approaches and hybrid options

http://web.mit.edu/deweck/Public/Shaw/Shaw2014.pdf ]
« Last Edit: 10/09/2017 08:20 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Crispy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #85 on: 10/09/2017 10:08 AM »
So far the biological route regens the O2 but creates biomass which does not seem to have much use.
On Mars? It will be very useful indeed.

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #86 on: 10/09/2017 03:08 PM »
So far the biological route regens the O2 but creates biomass which does not seem to have much use.
On Mars? It will be very useful indeed.
A one step process to remove the CO2 and process some of the recovered water to produce CH4 and O2.
https://phys.org/news/2016-02-proven-one-step-co2-liquid-hydrocarbon.html

This could be used to recycle the O2 and then make a useful and easily stored byproduct CH4 which can be liquefied and stored in the large LCH4 tank. Excess O2 can be liquefied and stored in the LOX tank. But there is unlikely to be any excess O2.

So it is possible to get the consumables down to 2kg/day per person. Mostly food and some water with a little bit of O2 to replace losses in the recycling system. A water tank will be needed to store the excess water generated over time because this is where some of the losses in the regen system for O2 will end up. Also a tank to store the waste sludge to be useful in farming at a latter date.

At this rate stored consumables for 1 year for 1 person comes to 730kg. For 100 persons 73mt/yr. The weight of the person plus 400kg for personal baggage would make up the full weight capability of a BFR long duration flight and have enough consumables for their support for 1 year to handle any contingencies a 4X over supply of consumables for the expected duration of the trip. For the initial manned landings that 73mt of supplies must last 2X times the expected duration before an assured resupply event occurs. This duration is posited at being ~6.5 years or 3 synods. A 2X supply would be enough supplies for each person for 13 years. so that 73mt results in a crew size of 7.

If the assumption is only 2 synods for max time for resupply then a crew size of 11.

If the assumption is only 1 synod for max time for resupply then a crew size of 23.

Remember in all cases the available supply is 2X needed to handle unforeseen contingencies and losses.

Online cppetrie

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #87 on: 10/09/2017 03:20 PM »
So far the biological route regens the O2 but creates biomass which does not seem to have much use.
On Mars? It will be very useful indeed.
A one step process to remove the CO2 and process some of the recovered water to produce CH4 and O2.
https://phys.org/news/2016-02-proven-one-step-co2-liquid-hydrocarbon.html

This could be used to recycle the O2 and then make a useful and easily stored byproduct CH4 which can be liquefied and stored in the large LCH4 tank. Excess O2 can be liquefied and stored in the LOX tank. But there is unlikely to be any excess O2.

So it is possible to get the consumables down to 2kg/day per person. Mostly food and some water with a little bit of O2 to replace losses in the recycling system. A water tank will be needed to store the excess water generated over time because this is where some of the losses in the regen system for O2 will end up. Also a tank to store the waste sludge to be useful in farming at a latter date.

At this rate stored consumables for 1 year for 1 person comes to 730kg. For 100 persons 73mt/yr. The weight of the person plus 400kg for personal baggage would make up the full weight capability of a BFR long duration flight and have enough consumables for their support for 1 year to handle any contingencies a 4X over supply of consumables for the expected duration of the trip. For the initial manned landings that 73mt of supplies must last 2X times the expected duration before an assured resupply event occurs. This duration is posited at being ~6.5 years or 3 synods. A 2X supply would be enough supplies for each person for 13 years. so that 73mt results in a crew size of 7.

If the assumption is only 2 synods for max time for resupply then a crew size of 11.

If the assumption is only 1 synod for max time for resupply then a crew size of 23.

Remember in all cases the available supply is 2X needed to handle unforeseen contingencies and losses.
Is that all on just one ship? The current plan is send 2 crew and 2 cargo on the second wave. Could some of those supplies be on a cargo vessel? Also, would some of it be preservable enough that it could be pre-staged by a cargo ship in the first wave so you know it landed safely and is ready and waiting for you?

Offline ZachF

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #88 on: 10/09/2017 03:29 PM »
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t).  Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC.  (!)
I've been trying to rocket-equation this, with little success. 

Here are the "knowns".
GLOW 4400 t
Thrust Liftoff 5400 t, ISP = 330/356 sec
Ship dry mass 85 t
Ship Mp 1100 t
Ship Thrust 775 t (4 engines) ISP 375 sec
Ship Thrust 347 t (2 SL engines) ISP 330/356 sec

These imply a first stage mass = 4400 t - 1185 t = 3215 t
Unknown is first stage propellant mass fraction. 
When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO).  That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship".  With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent.  To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.

S1:  3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/s
S2:  1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/s
PL:  235 t, delta-v total = 9217 m/s

When I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.

S1:  3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/s
S2:  1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/s
PL:  105 t, delta-v total = 9211 m/s

Rough guesses, obviously, but I've yet to match the SpaceX charts.  When I try to model the 20 tonne GTO mass, the numbers don't converge at all.  I get no payload to GTO.

 - Ed Kyle
A big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.

BFR doesn't use hydrogen, so it should get lower aero losses and higher averaged thrust to weight ratio (since your tanks empty sooner). Therefore the benchmark 9.2km/s need not apply, and I am nearly certain I've seen a legitimate estimate for BFR that shows a trajectory of 8.9something km/s to orbit.

If you use the exact same mass fraction as last year's ITS booster (slightly better than 96%) and your numbers for Isp and stage mass, then you get 250 tons expendable to LEO at about 8.99km/s. That fits perfectly with all the rest of the info we have.

No mystery. If you try to mess with SpaceX's numbers to fit more conservative assumptions, then of course you'll not be able to recreate their figures. That's not a mystery, either.

I can get it to work for 9.3Km/s, anyway.

In fact, after further review, I think the payload numbers listed for the BFR are for the spaceship variant, not the cargo variant. I think the cargo variant will be capable of ~190 tonnes to LEO and ~55 tonnes to GTO with no refuel. Here is a Delta V chart estimate for Spaceship, Cargo, Tanker, and expendable with a list of notes.

1.) The yellow numbers are known numbers. They provide the foundation for this estimation.

2.)The original ITS had a payload of 300 tonnes, and a weight of 150 tonnes. There was no cargo variant listed, but there was a tanker variant that weighed 90tonnes. A theoretical cargo variant would have probably weighed ~100 tonnes and had a payload of >350 tonnes to LEO.

3.) The original ITS Spaceship had a landing tank of 50 tonnes. This was obtained by taking its 1,950 tonne fuel tank, subtracting 5x380 tonne tanker load required for refill, and correlating it measurements of the physical tanks from the CAD drawing.  Both seem to suggest a 50 tonne landing tank in the original ship, which would have provided a 920m/s dV for landing, this is what forms the basis for landing requirements of the upper stages. It is likely that with the addition of the delta wing, and a smaller ship having a greater sectional density, that this requirement is less in the new iteration.

4.) The original booster used 7% of it's fuel load to land, which provided ~320m/s less delta V than the booster imparted on the stage itself. This provided the basis to estimate landing fuel requirements for the various models. The weight of the booster is based on posted mass fractions of the previous ITS booster.

5.) The original ITS tanker variant had a 380 tonne to LEO payload, this was accomplished by having a larger fuel tank than the spaceship, even when accounting for the transfer. The original Tanker had a 2,400 tonne fuel tank, with 380 tonnes delivered to LEO. This means that it has 2,020 tonnes of fuel compared with 1,950 tonnes in the spaceship. The extra 70 tonnes is used to push up the higher payload. The BFR tanker will need to have a LEO payload of excess fuel around 210 tonnes in order to refill a BFS in 5 launches. I have estimated the tank, therefor, for the tanker variant to be around 1,400 tonnes. The empty weight of the tanker is based on the ratio of the previous iteration (90t vs 150t)

Im sure I'll think of more notes later
I've uploaded the XLS file if you want to play around with the numbers
« Last Edit: 10/09/2017 03:31 PM by ZachF »

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #89 on: 10/09/2017 03:47 PM »
Using the posited 2kg/person per day values:

A Lunar mission using the Lunar mission scenario encompassing a 6 month stay would be comprised of:
NOTE I do not think SpaceX actually specified how much payload for this scenario.

If 20mt payload to surface with 50% supplies for crew ->13 crew and 5.5mt of cargo to be left on surface.

If 50mt payload to surface with 50% supplies for crew ->34 crew and 13.5mt of cargo to be left on surface.

If the full 150mt payload to surface with 50% supplies for crew -> 100 crew (full complement) and 41mt of cargo to be left  on surface.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #90 on: 10/09/2017 04:35 PM »
A one step process to remove the CO2 and process some of the recovered water to produce CH4 and O2.
https://phys.org/news/2016-02-proven-one-step-co2-liquid-hydrocarbon.html

This could be used to recycle the O2 and then make a useful and easily stored byproduct CH4 which can be liquefied and stored in the large LCH4 tank. Excess O2 can be liquefied and stored in the LOX tank. But there is unlikely to be any excess O2.
How intriguing. It recalls the "SolChem" project of the US Navy in the early 80's. Essentially the NRL's effort to work out what it would take to keep the USN on the high seas if the US was cut off from external oil supplies, using solar thermal conversion and molten salt conversion heat storage built entirely from resources found in the US.

Those guys thought on a very large scale.

However this will be a very hard sell as this TRL1 or 2 level at best.  A catalyst better matched to the solar spectrum moves things along a lot.

It certainly suggests other options are possible. That said a fully closed cycle ECLSS system would carry everything it brought from Earth to Mars. Long term it would be better if that had been converted into something that was usable on the surface.
« Last Edit: 10/09/2017 04:58 PM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #91 on: 10/09/2017 04:59 PM »
A one step process to remove the CO2 and process some of the recovered water to produce CH4 and O2.
https://phys.org/news/2016-02-proven-one-step-co2-liquid-hydrocarbon.html

This could be used to recycle the O2 and then make a useful and easily stored byproduct CH4 which can be liquefied and stored in the large LCH4 tank. Excess O2 can be liquefied and stored in the LOX tank. But there is unlikely to be any excess O2.
How intriguing. It recalls the "SolChem" project of the US Navy in the early 80's. Essentially the NRL's effort to work out what it would take to keep the USN on the high seas if the US was cut off from external oil supplies, using solar thermal conversion and molten salt conversion heat storage built entirely from resources found in the US.

Those guys thought on a very large scale.
Which is what should be thought of when trying to design a system to regen for a crew size of 100 or several 1,000s in a base or colony. Tiny systems designed for crew sizes of 6 is not what is needed.

This also brings up thoughts on regeneration of packaging. Plastic packaging including aluminum coated plastic can be ground into a fine powder melted to remove the plastic then the remaining aluminum melted to produce ingots of 3D printer blanks for a plastic and aluminum 3D printers. But this would not be done on the BFR but at an established base or colony.

Also the waste sludge would also be processed at a base or colony and not on the BFR. The BFR would just store it and offload at destination where it becomes a useful commodity able to be processed by a large industrial size system.

Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #92 on: 10/09/2017 05:08 PM »
A packaging recycling machine that spits out 3D printer filament is being flown to Station soon.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #93 on: 10/09/2017 05:31 PM »
A packaging recycling machine that spits out 3D printer filament is being flown to Station soon.
That's a good start too.

It's pretty clear that any flights will continue to use a lot of packaging and being able to convert it into something useful will pay major dividends into the future.

The challenge of course is to see what (and how much) of the ships components can be resigned to use the parts that can be made.  Ideally you'd want them to be made on a bigger version of the same machine on Earth of the same material, but I don't see that happening much without a lot of effort.  :(
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Steve D

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #94 on: 10/09/2017 07:45 PM »
Organic waste from 100 people on board the BFR should produce a fair amount of methane on its own. Any thoughts on how much could be produced during a 4 to 6 month voyage and if it would be worth collecting?

Offline Johnnyhinbos

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #95 on: 10/09/2017 07:50 PM »
This is an interesting conversation. Perhaps it should be continued on the "BFS - the Human Factors - Updates and Discussion" thread. Regardless, I'll cross link over there...

https://forum.nasaspaceflight.com/index.php?topic=43884.0
John Hanzl. Author, action / adventure www.johnhanzl.com

Offline dror

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #96 on: 10/09/2017 08:13 PM »
...
Also if 3 tankers accompany the deep space mission into a 4.2km transfer orbit that returns the tankers back to Earth, the total DV for the Deep Space Mission could be as high as 12km/s. This can be done as well from LEO but since you loose nearly 4km/s by not leaving from L2 would be like the first case L2 deep space mission...

Is it possible to design a mission in  a similar manner, which could land on Mars with enough fuel for take off without ISRU?
Maybe another fueled tanker could wait at LMO.
How many flights will it require in total?
what payload could it take and bring back?

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Offline maitri982

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #97 on: 10/09/2017 10:54 PM »
Since I could not reply to Ed's post on the prior thread and quote it here, i want to note that making extrapolations on Musk projects by looking back at what he said in 2005 is ridiculous really.

He was very new to running a rocket company and had a helluva lot to learn.  Couple that with his typical over enthusiasm for forecasting shorter than realistic time frames and you got things that turned out to appear to be wildly optimistic.

BUT...he is a veteran now of creating brand new rocket engines, air frames, spaceships, software, electronics, assembly lines, friction stirred tanks and on and on and the result is he is MUCH smarter nowadays.  I run a software company and i can say that my own ability to forecast project time frames is much better than it was in 2005 when i started the company since i have a bunch of experience to go on now.

We still have to take account for his aggressive nature for time frame predictions, but i would guess his margin for error is quite lower now than it was 12 years ago.  So before it was probably like add ~5 years onto estimates and now I would guess its add <~2 years.


Offline intrepidpursuit

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #98 on: 10/09/2017 11:42 PM »
My biggest concern with BFS is the heat shield. It covers a massive area. Nothing will fall on it and it shouldn't be in bird range while going fast enough to lose a battle, but MMOD damage seems like a real risk. It may be that because of its size and corresponding lighter heat load that it could take the inch or two damage that is likely from an MMOD strike and survive reentry. It certainly seems like a potential problem at really high flight rates.

It is also inevitable that the heat shield will have to be replaced fairly regularly. (It is PICA-X so it is ablative right? why was it rendered silver...) Whether it is 10 flights or 100 flights it will likely be a pacing factor in refurbishing the ship. Tiles seem like the most likely method considering they are being used for dragon. It would be hard to make a shield as difficult to maintain as shuttle's, but it will still require a huge number unique tiles to be removed and reapplied and in this case they have to be replaced with new tiles or remanufactured.

Are there any other technologies that may be helping them avoid these issues do they just consider them manageable?

Offline CapitalistOppressor

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #99 on: 10/10/2017 04:01 AM »
My biggest concern with BFS is the heat shield. It covers a massive area. Nothing will fall on it and it shouldn't be in bird range while going fast enough to lose a battle, but MMOD damage seems like a real risk. It may be that because of its size and corresponding lighter heat load that it could take the inch or two damage that is likely from an MMOD strike and survive reentry. It certainly seems like a potential problem at really high flight rates.

It is also inevitable that the heat shield will have to be replaced fairly regularly. (It is PICA-X so it is ablative right? why was it rendered silver...) Whether it is 10 flights or 100 flights it will likely be a pacing factor in refurbishing the ship. Tiles seem like the most likely method considering they are being used for dragon. It would be hard to make a shield as difficult to maintain as shuttle's, but it will still require a huge number unique tiles to be removed and reapplied and in this case they have to be replaced with new tiles or remanufactured.

Are there any other technologies that may be helping them avoid these issues do they just consider them manageable?

IIRC in his most recent presentation Elon made a fairly strong claim that the heat shield would only experience noticible ablation during a Mars Entry, and would not do so when entering Earths atmosphere.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #100 on: 10/10/2017 04:14 AM »
(It is PICA-X so it is ablative right? why was it rendered silver...)
IIRC the silver lining is a layer put on to prevent moisture intrusion prelaunch. It wasn't used on the first few launches, again IIRC.

Offline intrepidpursuit

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #101 on: 10/10/2017 04:18 AM »
My biggest concern with BFS is the heat shield. It covers a massive area. Nothing will fall on it and it shouldn't be in bird range while going fast enough to lose a battle, but MMOD damage seems like a real risk. It may be that because of its size and corresponding lighter heat load that it could take the inch or two damage that is likely from an MMOD strike and survive reentry. It certainly seems like a potential problem at really high flight rates.

It is also inevitable that the heat shield will have to be replaced fairly regularly. (It is PICA-X so it is ablative right? why was it rendered silver...) Whether it is 10 flights or 100 flights it will likely be a pacing factor in refurbishing the ship. Tiles seem like the most likely method considering they are being used for dragon. It would be hard to make a shield as difficult to maintain as shuttle's, but it will still require a huge number unique tiles to be removed and reapplied and in this case they have to be replaced with new tiles or remanufactured.

Are there any other technologies that may be helping them avoid these issues do they just consider them manageable?

IIRC in his most recent presentation Elon made a fairly strong claim that the heat shield would only experience noticible ablation during a Mars Entry, and would not do so when entering Earths atmosphere.

I heard him say, "It is a multi-use heat shield, but unlike for Earth operations, it's coming in hot enough that you really will see some wear of the heat shield." But that passing reference isn't enough for me to believe that a heat shield would get more use in mars' thin atmosphere than earth's atmosphere. Bleeding off the speed propulsively makes reusability sense, but would use an incredible amount of fuel. Hard to believe that is the plan.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #102 on: 10/10/2017 04:21 AM »
(It is PICA-X so it is ablative right? why was it rendered silver...)
IIRC the silver lining is a layer put on to prevent moisture intrusion prelaunch. It wasn't used on the first few launches, again IIRC.

If they are planning to put that on BFS that is a huge project every launch by itself. It is shown on the booster too. That seems very unlikely.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #103 on: 10/10/2017 05:56 AM »
Hi, any thoughts what goes into the top two windowless levels of the BFS?

Link to image from presentation here:

...

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #104 on: 10/10/2017 06:37 AM »
How much and how big would all the ECLSS equipment be for the support of 100 people for a couple of years?
Turn the question on its head.

The NASA baseline for crew consumables is 5Kg/person/day, of which maybe 3.5Kg is water.

So is that part of the ECLSS solution for the trip? If 150 mt ascent and 50 mt descent cargo mass is the generic load (but is it?) then the BFS "has to" get rid of 100 mt during travel. For Mars with 6 months travel that is about 1 mt/person, or 100 mt for the fully crewed ship.

Venting used consumables would remove some heat as well (~ 0.5 kWh/day if superheated steam, I think) but I guess not enough to take care of the cooling needs.

EDIT: Clarified wording.
« Last Edit: 10/10/2017 06:41 AM by Torbjorn Larsson, OM »

Offline Cheapchips

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #105 on: 10/10/2017 07:16 AM »
Organic waste from 100 people on board the BFR should produce a fair amount of methane on its own. Any thoughts on how much could be produced during a 4 to 6 month voyage and if it would be worth collecting?

I did a cursory look into this a while ago. Human digestion produces very little methane.  In terms of sold waste, we produce around 127kg a year.  I'm sure it'll be useful for something on Mars, but it's not going to contribute to ISRU.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #106 on: 10/10/2017 07:29 AM »
(It is PICA-X so it is ablative right? why was it rendered silver...)
IIRC the silver lining is a layer put on to prevent moisture intrusion prelaunch. It wasn't used on the first few launches, again IIRC.

Does the Big frakking Sponge ohhh, I mean spacecraft need to be painted after every reentry? BFS really need real reusable thermal protection system; Toughened Uni-piece Fibrous Reinforced Oxidation-Resistant Composite or other similar solution.
« Last Edit: 10/10/2017 07:30 AM by HVM »

Offline hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #107 on: 10/10/2017 07:57 AM »
How much and how big would all the ECLSS equipment be for the support of 100 people for a couple of years?
Turn the question on its head.

The NASA baseline for crew consumables is 5Kg/person/day, of which maybe 3.5Kg is water.

So is that part of the ECLSS solution for the trip? If 150 mt ascent and 50 mt descent cargo mass is the generic load (but is it?) then the BFS "has to" get rid of 100 mt during travel. For Mars with 6 months travel that is about 1 mt/person, or 100 mt for the fully crewed ship.

Venting used consumables would remove some heat as well (~ 0.5 kWh/day if superheated steam, I think) but I guess not enough to take care of the cooling needs.

EDIT: Clarified wording.

50 tonnes is the descent mass to EARTH. The mars descent mass is much higher.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #108 on: 10/10/2017 01:23 PM »
...
...
I heard him say, "It is a multi-use heat shield, but unlike for Earth operations, it's coming in hot enough that you really will see some wear of the heat shield." But that passing reference isn't enough for me to believe that a heat shield would get more use in mars' thin atmosphere than earth's atmosphere. Bleeding off the speed propulsively makes reusability sense, but would use an incredible amount of fuel. Hard to believe that is the plan.
Mars lacks the thick atmosphere of Earth, but it carries a heavy dust burden of very fine particles, predominately iron oxide, that remain airborne indefinitely. Reentry through this atmosphere will be like sandblasting the heatshield.
* Mars: a convenient service station for an asteroid-sized spaceship en-route to Ceres. *

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #109 on: 10/10/2017 01:44 PM »
Not heavy at all.
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #110 on: 10/10/2017 02:44 PM »
In terms of sold waste, we produce around 127kg a year.  I'm sure it'll be useful for something on Mars
Growing potatoes?  :)

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #111 on: 10/10/2017 03:02 PM »
...
...
I heard him say, "It is a multi-use heat shield, but unlike for Earth operations, it's coming in hot enough that you really will see some wear of the heat shield." But that passing reference isn't enough for me to believe that a heat shield would get more use in mars' thin atmosphere than earth's atmosphere. Bleeding off the speed propulsively makes reusability sense, but would use an incredible amount of fuel. Hard to believe that is the plan.
Mars lacks the thick atmosphere of Earth, but it carries a heavy dust burden of very fine particles, predominately iron oxide, that remain airborne indefinitely. Reentry through this atmosphere will be like sandblasting the heatshield.

There are also some not fully described radiative heat flux effects at high entry velocities in a predominantly carbon dioxide atmosphere like Mars in which the vibrational energy of diatomic molecules (CO, CN, etc.) formed in the shock wave produce even more radiative heat impinging on the heat shield.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #112 on: 10/10/2017 04:24 PM »
If they are planning to put that on BFS that is a huge project every launch by itself. It is shown on the booster too. That seems very unlikely.
Well supposedly the PICAX will be be so robust it it will give negligible wear in most situations, which also implies it will be a poor enough thermal conductor so that what heat is absorbed won't get to the cabin before it can be re-radiated or convected outward.

Note that the BFS is somewhat more symmetrical than (basically a cylinder, not a cylinder + slab sectionsetc) Shuttle and I suspect every tile (which should be bigger than 6"x6") will not be unique.

However the wing area remains a tricky area. OTOH I'll bet that SX will be very much more automated in their testing and replacement than NASA ever managed. I'll also note that BFS eliminates hypergols, which were very dangerous to Shuttle TPS (not to mention the people working on the area).

Orbital grade, reusable TPS and large composite aerostructures remain areas high risk areas for this design.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #113 on: 10/10/2017 04:37 PM »

So is that part of the ECLSS solution for the trip?
I don't think anyone knows at this stage.

That said the open loop system takes about 4 tonnes of consumables to support a person for 26 Months from Earth takeooff to Earth landing. I strongly expect SX will study the various ECLSS systems developed on ISS, and will do some kind of partial closed loop system to reduce it by quite a lot.

One of the reports I linked to was the one one Paragon (who do ECLSS for Dragon 2) did for the Mars One project

The figure I quoted is what NASA reckon one person needs in terms of O2/Water/Food (and it's associated packaging) for 1 day. Water is the big item. For drinking and food rehydration (I'm not sure any of that is for washing, which remains a deeply primitive business on the ISS  :(  ).

"Open loop" life support may be acceptable for a flags and footprints exploration mission but it's not sustainable for any sizeable settlement. This problem is well overdue for serious attention.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #114 on: 10/10/2017 04:45 PM »
There are also some not fully described radiative heat flux effects at high entry velocities in a predominantly carbon dioxide atmosphere like Mars in which the vibrational energy of diatomic molecules (CO, CN, etc.) formed in the shock wave produce even more radiative heat impinging on the heat shield.
Good point.

Mars entry, and Earth re-entry from Mars, are at speeds high enough not just to split diatomic molecules into free radicals (IE N2 --> 2N: ) as a re-entry from LEO would be, but actually ionize the molecules and get raw electron clouds as well.

Modelling the effects of real gases (especially the difference between the temperatures of different species, which is a proxy for their velocity) is a key part of explaining the differences between calculated and measured entry temperatures.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Archibald

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #115 on: 10/10/2017 04:55 PM »
The B-58, B-70, F-111 and B-1A escape pods all killed pilots.
Carl Cross was killed in the B-70 escape pod.
 F-111B pilots also died in February 1968
And finally, the 1984 B-1A crash.
Plus they are extremely heavy.
They were considered for both Shuttle and Hermès, and rejected.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #116 on: 10/10/2017 04:58 PM »
What might be the hatches used for the BFR?

It seems unlikely to me that existing mating mechanisms will be large enough, and berthing using future Canadarm like systems also seems unlikely.

In the videos the same hatch seems capable of having passengers walk in vertically and connect to the ISS, etc.  Sounds too good to be true.  and there is the question of male vs female systems.  how do you connect two BFR together?

« Last Edit: 10/10/2017 05:00 PM by lamontagne »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #117 on: 10/10/2017 05:04 PM »
Did previous Mars probes do any extensive inspection of their heat shields ? I think that Curiosity may have had a quick look, but it seems that it might be valuable to have a close look at the pitting and damage if we ever want to design reusable heat shields for Mars.

I'm also a bit worried about claims that BFR will be able to reuse its heatshield hundreds of times without any maintenance. So far, SpaceX has always refitted new Pica-X heatshields on their reflown Dragons, so there really isn't much basis into knowing how reusable it really is or how much maintenance is required.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #118 on: 10/10/2017 05:09 PM »
What might be the hatches used for the BFR?

It seems unlikely to me that existing mating mechanisms will be large enough, and berthing using future Canadarm like systems also seems unlikely.

In the videos the same hatch seems capable of having passengers walk in vertically and connect to the ISS, etc.  Sounds too good to be true.  and there is the question of male vs female systems.  how do you connect two BFR together?

I would imagine they would go with some flavor if IDS. I don't think the same hatch would be used for be used for docking and boarding. Passenger flights and ISS-servicing missions are different enough to require different vehicles, and docking doesn't seem to be a requirement for most of BFS' missions.

Like most of the pictures shown during the presentation, the "docked to ISS" picture was an artists impression that only illustrated the concept. There are so many things wrong with the idea that you shouldn't really be taking anything in those illustrations at face value.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #119 on: 10/10/2017 05:23 PM »
I'm also a bit worried about claims that BFR will be able to reuse its heatshield hundreds of times without any maintenance. So far, SpaceX has always refitted new Pica-X heatshields on their reflown Dragons, so there really isn't much basis into knowing how reusable it really is or how much maintenance is required.
TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #120 on: 10/10/2017 05:46 PM »
I'm also a bit worried about claims that BFR will be able to reuse its heatshield hundreds of times without any maintenance. So far, SpaceX has always refitted new Pica-X heatshields on their reflown Dragons, so there really isn't much basis into knowing how reusable it really is or how much maintenance is required.
TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.

You'd get significant g-loads doing this, maybe not an issue for a returning unmanned craft, but very much a problem for a crew nearing the end of a long transit.
Waiting for joy and raptor

Offline zhangmdev

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #121 on: 10/10/2017 06:47 PM »
Carl Cross was killed in the B-70 escape pod.


IIRC Carl Cross didn't eject from XB-70. G-load from the spin prevented his seat from retracting into escape pod.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #122 on: 10/10/2017 08:08 PM »
You'd get significant g-loads doing this, maybe not an issue for a returning unmanned craft, but very much a problem for a crew nearing the end of a long transit.
Well the 2016 slidshow deck talked about Mars entry pulling 6-8g and returning to Earth 2-3g's

Neither of which is good after 90+ days in zero g (so compression suits 24/7, longish hours in the gym or spinning pairs of ships for some sort of AG look like they are on the cards)

However that was last years plan. 

The addition of wings (and hence increased lifting surface) should reduce that a bit, but I'm not sure it's halve the loads or what.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline CapitalistOppressor

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #123 on: 10/10/2017 08:19 PM »
My biggest concern with BFS is the heat shield. It covers a massive area. Nothing will fall on it and it shouldn't be in bird range while going fast enough to lose a battle, but MMOD damage seems like a real risk. It may be that because of its size and corresponding lighter heat load that it could take the inch or two damage that is likely from an MMOD strike and survive reentry. It certainly seems like a potential problem at really high flight rates.

It is also inevitable that the heat shield will have to be replaced fairly regularly. (It is PICA-X so it is ablative right? why was it rendered silver...) Whether it is 10 flights or 100 flights it will likely be a pacing factor in refurbishing the ship. Tiles seem like the most likely method considering they are being used for dragon. It would be hard to make a shield as difficult to maintain as shuttle's, but it will still require a huge number unique tiles to be removed and reapplied and in this case they have to be replaced with new tiles or remanufactured.

Are there any other technologies that may be helping them avoid these issues do they just consider them manageable?

IIRC in his most recent presentation Elon made a fairly strong claim that the heat shield would only experience noticible ablation during a Mars Entry, and would not do so when entering Earths atmosphere.

I heard him say, "It is a multi-use heat shield, but unlike for Earth operations, it's coming in hot enough that you really will see some wear of the heat shield." But that passing reference isn't enough for me to believe that a heat shield would get more use in mars' thin atmosphere than earth's atmosphere. Bleeding off the speed propulsively makes reusability sense, but would use an incredible amount of fuel. Hard to believe that is the plan.

You've got to make your roll to disbelieve first.

It was a pretty clear statement during a presentation that was planned for months about a system that is supposed to be highly reusable during standard operations around Earth.  Ships going to Mars become the exception to the rule on reusability and thus merited this statement.

If there is room for argument it would probably be based on the wide variety of Earth reentry profiles the system anticipates and the possibility that only some reentry modes are not asssociated with significant ablation.

The ship having a significant wing is also a new variable that might be impacting this.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #124 on: 10/10/2017 10:29 PM »
Carl Cross was killed in the B-70 escape pod.


IIRC Carl Cross didn't eject from XB-70. G-load from the spin prevented his seat from retracting into escape pod.
There is a thread for discussion of escape systems. Please use it, or the one about should there be such, rather than here.

Should: https://forum.nasaspaceflight.com/index.php?topic=43438
How: http://forum.nasaspaceflight.com/index.php?topic=43923

Thanks.
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #125 on: 10/10/2017 10:45 PM »
The "split body flaps" that we've seen mentioned for the BFS - would they be like the ones on the IXV tested by ESA in 2015? See:

https://en.wikipedia.org/wiki/Intermediate_eXperimental_Vehicle

... and that design in turn may have used some of the experimental work done for the unbuilt HL-20:

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19990117251.pdf

... for example skewing the hinge lines to eliminate adverse yaw.

Do we know any more about these flaps?
« Last Edit: 10/10/2017 10:50 PM by SLC »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #126 on: 10/11/2017 10:11 AM »
I believe the ones we were shown are also "conceptual" artist impressions.

The flaps on the IXV trail at the back of the vehicle, therefore they can move up and down. The ones that were shown in the BFS illustrations are more like the HL20 integrated into the wing surface. They don't seem to have enough clearance against the Raptor nozzle to move significantly upwards, which will limit pitch adjustment. The HL20 design had upper body flaps to compensate.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #127 on: 10/11/2017 11:10 AM »

I think that this is the mechanism for the flaps as seen in this slide from the IAC presentation.

Also of interest is the landing leg positions. I don't see any in this frame.

In this one though, there is what appears to be bits of angle iron welded onto the outside of the hull.

At first glance, it doesn't appear to be very mechanical for retraction.

Offline octavo

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #128 on: 10/11/2017 11:51 AM »


. They don't seem to have enough clearance against the Raptor nozzle to move significantly upwards, which will limit pitch adjustment.

This. It looks to me like those flaps can't pitch the nose down at all.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #129 on: 10/11/2017 12:04 PM »


. They don't seem to have enough clearance against the Raptor nozzle to move significantly upwards, which will limit pitch adjustment.

This. It looks to me like those flaps can't pitch the nose down at all.

flaps down == nose down. can do this.
flaps up == nose up. cannot do this.

But maybe the skirt around the engines will be made differently shaped so that in the actual version the flaps can also turn upwards.

« Last Edit: 10/11/2017 12:06 PM by hkultala »

Offline octavo

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #130 on: 10/11/2017 12:21 PM »




. They don't seem to have enough clearance against the Raptor nozzle to move significantly upwards, which will limit pitch adjustment.

This. It looks to me like those flaps can't pitch the nose down at all.

flaps down == nose down. can do this.
flaps up == nose up. cannot do this.

But maybe the skirt around the engines will be made differently shaped so that in the actual version the flaps can also turn upwards.

Thanks! I even spent a minute thinking about which was which and still got it wrong!

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #131 on: 10/11/2017 12:47 PM »
In which flight regime do the flaps actually work? I cant think of them having much effect during the flaming part of the reentry. Is my intuition wrong here? I would expect to still have extensive steering thruster usage to get the nose onto the correct angle. Since the thrusters are hard to point downwards due to the heat shield at the bottom, how DO they pitch the nose up? With thrusters firing upwards at the top of the engine section, behind the center of mass?

Offline AncientU

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #132 on: 10/11/2017 01:26 PM »
In which flight regime do the flaps actually work? I cant think of them having much effect during the flaming part of the reentry. Is my intuition wrong here? I would expect to still have extensive steering thruster usage to get the nose onto the correct angle. Since the thrusters are hard to point downwards due to the heat shield at the bottom, how DO they pitch the nose up? With thrusters firing upwards at the top of the engine section, behind the center of mass?

Could be that the airframe design has a natural nose up moment. 
Flaps counter this with small extension, and pitch nose down with larger extension.
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #133 on: 10/11/2017 01:35 PM »
In which flight regime do the flaps actually work? I cant think of them having much effect during the flaming part of the reentry. Is my intuition wrong here? I would expect to still have extensive steering thruster usage to get the nose onto the correct angle. Since the thrusters are hard to point downwards due to the heat shield at the bottom, how DO they pitch the nose up? With thrusters firing upwards at the top of the engine section, behind the center of mass?

Almost the entire entry is in this very high AOA orientation relative to the flow field, so they will probably try to make this as naturally stable as possible. This orientation has both a lot of drag, and a lot of lift, so it's optimal for entry. In this orientation, then can actually get some pitch-down torque by retracting the flaps above the plane of the lower wing surface.

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.

I think they will only need RCS for 1) vacuum control before the aerosurfaces get enough flow for control; 2) for yaw control; and 3) for the final backflip before the Raptors start.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #134 on: 10/11/2017 01:40 PM »
As for the flight regime where the body flaps are effective, the experimental HL-20 study I cited a few posts back was done at Mach 10, and there the body flaps gave "effective roll control".

For pitch control, I'm guessing (like AncientU) that the fore-and-aft mass distribution of the BFS would be trimmed to give a default pitch up, and both flaps would normally be half-extended to counteract that.  They could then move around that default position to control both roll and pitch.

The first page of this paper on the IXV has an image of heat distribution during re-entry:

https://www.esa.int/esapub/bulletin/bulletin128/bul128h_tumino.pdf

If that's any guide, the body-flaps will get very hot.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #135 on: 10/11/2017 05:21 PM »

Could be that the airframe design has a natural nose up moment. 
Flaps counter this with small extension, and pitch nose down with larger extension.

I am of two minds about this, as I beleive a non neutral airframe will have increased drag since the flaps will have to be half extended. On the way down that seems like a good thing, drag is goodness. But on the way up, it puts an additional load, however slight, on the ascent engines, compared to a similar airframe that's neutral.... And worse, it might introduce some asymmetric forces on the coupling between the booster and the ship. That may or may not be significant, IANARS
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #136 on: 10/11/2017 05:35 PM »

Could be that the airframe design has a natural nose up moment. 
Flaps counter this with small extension, and pitch nose down with larger extension.

I am of two minds about this, as I beleive a non neutral airframe will have increased drag since the flaps will have to be half extended. On the way down that seems like a good thing, drag is goodness. But on the way up, it puts an additional load, however slight, on the ascent engines, compared to a similar airframe that's neutral.... And worse, it might introduce some asymmetric forces on the coupling between the booster and the ship. That may or may not be significant, IANARS
But on the way up, the flaps would be fully retracted, flush with the skin, wouldn't they?  No need for fore-and-aft trim if you're accelerating vertically upwards, no matter how the fore-and-aft mass distribution changes as the propellants are consumed.

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #137 on: 10/11/2017 07:08 PM »

Could be that the airframe design has a natural nose up moment. 
Flaps counter this with small extension, and pitch nose down with larger extension.

I am of two minds about this, as I beleive a non neutral airframe will have increased drag since the flaps will have to be half extended. On the way down that seems like a good thing, drag is goodness. But on the way up, it puts an additional load, however slight, on the ascent engines, compared to a similar airframe that's neutral.... And worse, it might introduce some asymmetric forces on the coupling between the booster and the ship. That may or may not be significant, IANARS
But on the way up, the flaps would be fully retracted, flush with the skin, wouldn't they?  No need for fore-and-aft trim if you're accelerating vertically upwards, no matter how the fore-and-aft mass distribution changes as the propellants are consumed.

There's still aero loads, though.
If the flaps need to be slightly extended to fly straight, that means the vehicle has asymmetrical aerodynamic properties. Like shuttle, you'd need vectored thrust to maintain straight flight.
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Offline ncb1397

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #138 on: 10/11/2017 07:20 PM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

That's impossible to judge unless you know how much insulation the cabin has, and how much insolation it receives (which depends on vehicle attitude).
In space the only way you lose heat is radiation. I didn't see any radiators, but they could have been hidden on the other side of the solar panels. Also, the heatshield on the bottom should be dark which will serve as a radiator. Waste heat from electronics, the life support system, chemical reactions powering human bodies, and whatever else needs energy on the ship should keep things plenty warm, they just have to keep the radiators sized and positioned correctly to balance the energy needs. If they need more heating, they just need to point the black side of the ship a bit more towards the sun.

The point is, you couldn't run 20 hair dryers on 35 KW. Shrinking the design, power, fuel and everything else but not shrinking the passenger count tends to lead to less technical credibility. As far as tilting the ship, I've never seen any indication that the solar fins pivot. They are always rendered in the same position. So, tilting the ship just exacerbates the power situation. For comparison, the Shuttle supported a crew of 8 and the 3 fuel cells could produce 21 KWs of continuous power.
« Last Edit: 10/11/2017 07:26 PM by ncb1397 »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #139 on: 10/11/2017 07:45 PM »

Could be that the airframe design has a natural nose up moment. 
Flaps counter this with small extension, and pitch nose down with larger extension.

I am of two minds about this, as I beleive a non neutral airframe will have increased drag since the flaps will have to be half extended. On the way down that seems like a good thing, drag is goodness. But on the way up, it puts an additional load, however slight, on the ascent engines, compared to a similar airframe that's neutral.... And worse, it might introduce some asymmetric forces on the coupling between the booster and the ship. That may or may not be significant, IANARS

Total drag during first stage ascent will be on the order of 30 m/s, the slight addition from partially extended flaps would be on the order of single-digit m/s losses. Drag losses during 2nd stage ascent are entirely trivial on Earth and extremely small on Mars.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #140 on: 10/12/2017 03:17 PM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.
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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #141 on: 10/12/2017 03:25 PM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.

More like 30 Mach numbers, from interplanetary entry to subsonic on Earth. Will be interesting to see whether they can make it stable (or nearly so) through hypersonic, supersonic, transonic, and subsonic regimes. At least it's only hypersonic and high supersonic at Mars.

Not sure that they don't want it to fly like HOTOL; the only time lift is a significant factor is during entry and descent, where they want as much drag as possible and a very high AOA. They don't have to orient for lift during terminal landing or at all during ascent.

Offline spacenut

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #142 on: 10/12/2017 03:29 PM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Also when returning to earth through the thick atmosphere, are the wings or winglets used to keep the lower mass level with the upper mass to keep from coming in bottom first? 

I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned. 

Offline octavo

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #143 on: 10/12/2017 03:51 PM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Also when returning to earth through the thick atmosphere, are the wings or winglets used to keep the lower mass level with the upper mass to keep from coming in bottom first? 

I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned.
I think you're confusing the booster and spaceship. Only 6 engines on the the ship.

Offline Oersted

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #144 on: 10/12/2017 07:50 PM »
The BFR will be built in the Los Angeles port area.

Gwynne Shotwell Q&A. The quote below is not verbatim but from notes by Reddit-user "Sticklefront": https://www.reddit.com/r/spacex/comments/75ufq9/interesting_items_from_gwynne_shotwells_talk_at/

"Where will the BFR be built?

We're looking at building a facility by the water in LA. We thought we'd build it in our factory in Hawthorne, but we priced transport to the harbor, and it came out to $2.5m per trip. It would require taking down stoplights, and just wouldn't be worth it. So we will build a new facility by the water. We will eventually also have a number of production sites by out launch sites."

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #145 on: 10/12/2017 09:06 PM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.  There would be no windows in the lower rooms (on the heat protected side), but this was easier to model  :-)

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

« Last Edit: 10/12/2017 09:07 PM by lamontagne »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #146 on: 10/12/2017 09:07 PM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

BFS was to be used as a an early surface hab per the 2016 presentation. That doesn't seem to have changed.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #147 on: 10/12/2017 09:29 PM »
40 rooms cannot evenly be distributed into 3 floors.  So do we have 3 floors with 42 spaces, two that are not rooms but some kind of common spaces?  The video shows 2 sizes of rooms, perhaps there are 4 larger rooms per floor and 10 smaller ones? And are the narrow spaces utility spaces and bathrooms?
Or did Elon just round up?
« Last Edit: 10/12/2017 09:30 PM by lamontagne »

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #148 on: 10/12/2017 09:30 PM »
Yes,  I was confusing the booster from the ship.  I'm not concerned about the booster, it will be like F9.  The ship yes will have 6 engines, 4 vacuum and 2 sea level.  Ok, that is 6 tons or so, seems like with a 50 ton down mass, the ship will be top heavy when it lands almost empty of fuel/lox.  Same with landing on the moon or Mars, with 150 tons, seems too top heavy.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #149 on: 10/13/2017 12:25 AM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.  There would be no windows in the lower rooms (on the heat protected side), but this was easier to model  :-)

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

Keep in mind that the Mars re-entry profile will need to be taken into account. Gravity will be shifting, from zero, to sideways (when entering the atmosphere at high angle of attack), and finally down during the landing burn and stay on Mars.

And each bed might need to double as a launch/landing couch too. So it is very likely that all couches/beds need to face in the same direction to accommodate there shifts. This is what I had in mind when trying to create some sort of interior layout for the 2016 ITS (see image)

Of course if the ship is less full and there are separate launch/entry seats, then you have much more freedom to lay things out.


Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #150 on: 10/13/2017 02:50 AM »
Another thing that doesn't seem to quite add up is power. Design appears to use maximum - 12 m radius PV fins, which yields something around 90 KW @ earth and 35 KW @ Mars aphelion. Doesn't seem like enough to heat the 853 cubic meters of pressurized volume. Going to need some of that natural gas for heat in a lot of scenarios.

That's impossible to judge unless you know how much insulation the cabin has, and how much insolation it receives (which depends on vehicle attitude).
In space the only way you lose heat is radiation. I didn't see any radiators, but they could have been hidden on the other side of the solar panels. Also, the heatshield on the bottom should be dark which will serve as a radiator. Waste heat from electronics, the life support system, chemical reactions powering human bodies, and whatever else needs energy on the ship should keep things plenty warm, they just have to keep the radiators sized and positioned correctly to balance the energy needs. If they need more heating, they just need to point the black side of the ship a bit more towards the sun.

The point is, you couldn't run 20 hair dryers on 35 KW. Shrinking the design, power, fuel and everything else but not shrinking the passenger count tends to lead to less technical credibility. As far as tilting the ship, I've never seen any indication that the solar fins pivot. They are always rendered in the same position. So, tilting the ship just exacerbates the power situation. For comparison, the Shuttle supported a crew of 8 and the 3 fuel cells could produce 21 KWs of continuous power.
I feel to see why everyone would need to be running a hairdryer full blast. That's more energy than even a closed loop ECLSS.

Hair dryers use a LOT of power. Equivalent to like 200 bright LED light bulbs.
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Offline livingjw

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #151 on: 10/13/2017 05:19 AM »

To make this stable, both the CG and COP need to be roughly near the center of the vehicle fore/aft, which is exactly where the intersection of the blue and green axes is in the animation. The COP need to be towards the dorsal side of the vehicle, and the CG towards the ventral side.
And to remain there throughout the deceleration through about 20 Mach numbers?

Maybe all that mass above the propellant tanks will do the trick and the BFS will fly like Skylon, not like HOTOL.

- The CP needs to be near the CG to be controllable, so they added the wing to drag the CP towards the tail. The wing also gives them some more volume in the back for subsystems. Design is all about lining up you CG and CP and providing adequate control authority.

- The CP will be nearer the bottom (heatshield side) than the CG, hence BFS will not be stable, nor should it be. It has to fly AoAs from -10 to 190 degrees.

- BFS will be actively controlled using the flaps for roll and pitch and RCS thrusters for yaw.

- But it does need adequate aerodynamic control authority in pith and roll. Flap pitch deflections must be able to move the CP from in front of the CG to behind it as commanded by the autopilot.

- Mars entry, down to about 600 m/s, will be flown somewhere between 45 and 65 degrees AoA, maybe even a little higher. The vehicle maintaining high AoA to maximize drag. AoA can be set within these limits to extend or shorten downrange landing position.

-  Roll angle is the principle control. Lift is controlled by rolling alternately left and right. Roll angles vary as much as 360 degrees to control g's, dynamic pressure, altitude and position (similar to the shuttle). The easiest way to think about it is to picture and arrow sticking out of the vehicle in the direction of flight. It will be sticking through the CG and out of the forward bottom of the BFS at an angle of about negative 45-65 degrees and yaw = 0. You will always be flying in the direction of this arrow. If you want to get to higher dynamic pressure, you kill off lift by rolling alternately left and right. If you need down force, you roll nose down and alternate left and right. You can see the BFS nose down early in its Mars entry, keeping it from skipping out of the atmosphere.

- One nice thing about hypersonics is that much above Mach 4 flow becomes Newtonian and aerodynamics don't change much, they just get hotter. Stability derivatives tend to not change much.

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #152 on: 10/13/2017 05:36 AM »
The BFR will be built in the Los Angeles port area.

Gwynne Shotwell Q&A. The quote below is not verbatim but from notes by Reddit-user "Sticklefront": https://www.reddit.com/r/spacex/comments/75ufq9/interesting_items_from_gwynne_shotwells_talk_at/

"Where will the BFR be built?

We're looking at building a facility by the water in LA. We thought we'd build it in our factory in Hawthorne, but we priced transport to the harbor, and it came out to $2.5m per trip. It would require taking down stoplights, and just wouldn't be worth it. So we will build a new facility by the water. We will eventually also have a number of production sites by out launch sites."

This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory
b) F9 production will have to end to allow BFR to be built

I don't know how flexible the factory floor is. Can they use the existing facility to produce the first test articles and prototypes for BFR, or would that require removing all the F9 equipment?
Waiting for joy and raptor

Offline Patchouli

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #153 on: 10/13/2017 06:09 AM »

TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.

I think BFS can partly use propulsive capture into a low energy orbit on Earth return as it would leave Mars with full propellant load and much less cargo.
If Spacex can meet the mass fractions claimed it probably can almost get into LEO by propulsion alone.
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.


This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory
b) F9 production will have to end to allow BFR to be built

I don't know how flexible the factory floor is. Can they use the existing facility to produce the first test articles and prototypes for BFR, or would that require removing all the F9 equipment?

They'll have to keep F9 upper stage production going as this is not reusable.
This also opens up an option for an interim vehicle if there are delays with BFR/BFS in that it might be possible to fly a F9 upper stage on a half length BFR stage.
« Last Edit: 10/13/2017 06:18 AM by Patchouli »

Online Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #154 on: 10/13/2017 06:19 AM »
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.

Huh? It will be far cheaper to transport ANY commodity from from Earth to the Moon directly, rather than from Mars.

Offline Patchouli

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #155 on: 10/13/2017 06:56 AM »
Depends on how things work out but energy wise it's a lot easier to take something from Mars to the Moon than from Earth.

Offline hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #156 on: 10/13/2017 07:11 AM »

TBF the Dragon reentry is "fast and furious" due to it being quite dense.

The BFS and the other variants have large surface areas and (in principal) much lower ballistic coefficients (or "wing loadings"  if they were aircraft) so in principle should have lower wear.

I think SX are aiming at a part of the size/TPS solution space that has not really been looked at Historically "reusable" has actually meant, winged/longish entry/heat re-radiated. But what if you built a vehicle with the "fluffiness" of a winged, internally tanked vehicle (Like a Shuttle but with the ET internal) but brought it down with a profile more like a capsule? Coming down fast(ish) sacrifices cross range (but how much of that do you really need?) but reduced heat soak to your TPS (and reduced ablation to negligible levels?)

This maybe another area where the lore of the subject (driven by assumptions made by engineers in a tearing hurry to make something work?) is about to be challenged.

I think BFS can partly use propulsive capture into a low energy orbit on Earth return as it would leave Mars with full propellant load and much less cargo.

Please, use numbers and calculations when you think. It might give you better thoughts.

Delta-V from LEO to mars tranfer orbit is only 3.8 km/s. And braking / landing to surface from the transfer orbit to mars is 99% aerobraking.
Delta-V from Mars surface to earth transfer orbit is 6.4 km/s.

This is the reason the return payload is much smaller (50 tonnes) that outgoing payload

Quote

If Spacex can meet the mass fractions claimed it probably can almost get into LEO by propulsion alone.

Delta-V from Mars surface to LEO is 10.2 km/s.

So no, far from that. Cannot do it. And try to do it with any cargo.

Quote
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.

this "some" would probably be a negative amount.
« Last Edit: 10/13/2017 07:13 AM by hkultala »

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #157 on: 10/13/2017 08:41 AM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Good question. In theory as long as all the mass stays inside the base legs things should be OK, but then
there are external wind forces. [EDIT. Oops. The big joker here is how flat the landing area is (and how flat will it be after the Raptors have hosed it for a bit). That will be a recurring issue, so landing the first vehicle light doesn't help much. A permanent landing area needs to be checked for soft spots (partly landing on a sink hole would be a Very Bad Day) and flattened for future landings probably just after the crews have "dug in," which I assume is what TBC is for) ]

On Earth the stage should lock into its landing cradle (now I think of it very EE Smith, but without an inertialess drive ) on Mars wind forces (even at high speeds) should be much lower due to the pressure being 1/160 that of Earth SL (  "The Martian" is quite realistic in many ways, but not this. OTOH visibility and dust damage is likely to be a real problem. an Earth hurricane can hit an area for a few hours before moving on or dissipating. A Mars sandstorm can last for months. )
Quote from: spacenut
I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned.
Doesn't 31 engines only apply to the booster? Otherwise 1 tonne implies no better than a T/W of 174:1, although the joker is the complexity of the feed piping.
« Last Edit: 10/13/2017 09:01 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #158 on: 10/13/2017 09:01 AM »
More like 30 Mach numbers, from interplanetary entry to subsonic on Earth. Will be interesting to see whether they can make it stable (or nearly so) through hypersonic, supersonic, transonic, and subsonic regimes. At least it's only hypersonic and high supersonic at Mars.
Noted. I was still thinking in terms of Earth re-entry.  30 Machs
Quote from: envy887
Not sure that they don't want it to fly like HOTOL; the only time lift is a significant factor is during entry and descent, where they want as much drag as possible and a very high AOA. They don't have to orient for lift during terminal landing or at all during ascent.
The point about HOTOL and Skylon was the HOTOl design didn't really work precisely because of the huge shift in CoG and CoM due to the massive shift in mass properties (through propellant burn off) and lifting forces, needing an enormous set of servos on the nose canards to keep it horizontal.

BFS should be easier to manage on Mars as fully loaded I doubt it will be bottom heavy the way the BFR is but Earth re-entry may be more doubtful, assuming it's left all it's payload on Mars.
[EDIT. It depends on how much all that nose section masses. If it's too light those tail flap actuators are going to be fighting a tendency to flip all the way down. Too heavy and the won't be able to go up far enough to keep it nose front. I can't remember if those wings are expected to have trailing edge control surfaces. If they aren't, I'm betting by the time this flies they will.  ]
« Last Edit: 10/13/2017 09:10 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline hkultala

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #159 on: 10/13/2017 12:15 PM »

This does slightly mess up the notions that
a) 9m BFR was chosen to fit existing factory

EM never talked about existing FACTORY. He talked about exsiting FACILITIES.

Launch pads, testing pads, integration facilities etc are also facilities.

Offline Xentry

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #160 on: 10/13/2017 02:50 PM »
Quote
It would be tempting though to capture in a high orbit around Luna and have a dedicated Lunar lander transfer some methane and water from Mars off BFS for use on a moon base.

this "some" would probably be a negative amount.
Not necessarily. Taking methane/water from Mars surface to lunar surface would be ~9km/s. Taking methane/water from Earth surface to the Moon costs ~15km/s. Comparatively speaking, it's a good deal.

Offline philw1776

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #161 on: 10/13/2017 02:58 PM »
Here are possible arrangements for one of the room floors shown in the presentation.

The horizontal arrangement holds 30 people, the vertical one at least 24, with 4 rooms for couple.  The vertical arrangement is identical with the presentation, while the horizontal has 2 less rooms but 2 more bathrooms.  There would be no windows in the lower rooms (on the heat protected side), but this was easier to model  :-)

If the BFS is used as a surface habitat I guess the horizontal arrangement is more likely.  Was that clearly part of the plan?

It's been stated that BFS will initially be used as a surface habitat.  I'd design bunks to be able to pivot so as to function as acceleration couches during atmospheric entry.  Separate acceleration couches mean excess mass. 
You nan never have too many bathrooms especially with quantities of people waking up at the same time in the morning.  Plus redundancy for that most likely to fail onboard item.
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Offline spacenut

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #162 on: 10/13/2017 03:52 PM »
I think people will wake and sleep in shifts.  That is the reasoning behind sending 3 astronauts to the moon.  One person slept every 8 hours, while two stayed awake.  Say they send 90 to Mars.  I predict that 30 will be sleeping at any one time during transit and while on Mars.  This is the way 24 hour running factories do it on earth, also hospitals and first responders, such as police, fire, utility workers.  Someone is always awake to watch and monitor things. 

In transit, everyone will have a seat/bed during take-off, landings to be strapped in, and probably be one in the same.  Probably also during engine firings for acceleration/deceleration.  They will know the times and be given ample time to strap in. 

Offline groundbound

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #163 on: 10/13/2017 07:42 PM »

- Mars entry, down to about 600 m/s, will be flown somewhere between 45 and 65 degrees AoA, maybe even a little higher. The vehicle maintaining high AoA to maximize drag. AoA can be set within these limits to extend or shorten downrange landing position.


A really ignorant question popped into my head when I read this. Do we know for certain that Mars entry will never need negative lift?

I'm not suggesting anything, just asking out of ignorance and the tiny amount of knowledge that skipping out would be bad.

Offline RocketmanUS

I think people will wake and sleep in shifts.  That is the reasoning behind sending 3 astronauts to the moon.  One person slept every 8 hours, while two stayed awake.  Say they send 90 to Mars.  I predict that 30 will be sleeping at any one time during transit and while on Mars.  This is the way 24 hour running factories do it on earth, also hospitals and first responders, such as police, fire, utility workers.  Someone is always awake to watch and monitor things. 

In transit, everyone will have a seat/bed during take-off, landings to be strapped in, and probably be one in the same.  Probably also during engine firings for acceleration/deceleration.  They will know the times and be given ample time to strap in.
Good idea, but 30 waking up, that is a line at the bathroom. Food prep for 30 at once would be a lot also.

Each hour some of the passengers wake up and others go to sleep. This helps with ship resources.  Not everyone doing the same thing at once, such as eating, restroom, exercise, checking email that has come in,looking out the window, ect.
Mars and beyond, human exploration
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Online Lars-J

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IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #165 on: 10/13/2017 07:58 PM »

- Mars entry, down to about 600 m/s, will be flown somewhere between 45 and 65 degrees AoA, maybe even a little higher. The vehicle maintaining high AoA to maximize drag. AoA can be set within these limits to extend or shorten downrange landing position.


A really ignorant question popped into my head when I read this. Do we know for certain that Mars entry will never need negative lift?

I'm not suggesting anything, just asking out of ignorance and the tiny amount of knowledge that skipping out would be bad.

The mars entry/landing “simulation” that was shown used negative lift. (BFS in nose down attitude) AoA refers to angle in relation to the direction of travel, not “up” from the surface.

Negative lift shown here at 2:18...
« Last Edit: 10/13/2017 08:03 PM by Lars-J »

Offline spacenut

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #166 on: 10/13/2017 08:52 PM »
Waking up every hour, sound good, say you had 96 passengers, 24 hours into 96 people would be only 4 per hour.  Much less strain on bathrooms and eating. 

Offline groundbound

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #167 on: 10/13/2017 08:54 PM »
Wow, thanks. The video shows an interesting maneuver to get from negative lift inverted body position to the more normal orientation.

Offline philw1776

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #168 on: 10/13/2017 09:46 PM »
Nobody has ever run a ship or anything with  1/24th of the crew waking up every hour.  Not amenable to human social interaction.  3 shifts was assumed by me.  Build more bathrooms.  You need redundancy there.  Spare parts.  For once magic unicorn 3D printers for piping & valves makes sense for repair.
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Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #169 on: 10/13/2017 10:02 PM »
The mars entry/landing “simulation” that was shown used negative lift. (BFS in nose down attitude) AoA refers to angle in relation to the direction of travel, not “up” from the surface.
Musk said at the start this is a  "true physics" simulation.

That implies that while the GUI is a bit rough it's not an "open" flight path. It's being driven by a need to land at a certain point on Mars and to keep at least one (and probably more) parameters (probably structural stress and skin temperature) within acceptable ranges while traveling through a chemically accurate Mars atmosphere (although this is something that can vary a lot)

Quote from: Lars-J
Negative lift shown here at 2:18...

Seeing it again I noted the timeline of about 470 seconds from start to finish, a little under 8 minutes.

That really does suggest they are looking at a different way of doing winged lifting entry compared to Shuttle or X37b. IIRC Shuttle entries were in the 60-90 minute range.

Those shifts in attitude strongly suggest you're going to need to be strapped into a chair of some kind.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #170 on: 10/13/2017 11:05 PM »
Shuttle and X37b came in from LEO. This is coming in on a hyperbolic trajectory. And because of vertical landing and the thin atmosphere, they're not going to mess around long deep in the atmosphere.
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Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #171 on: 10/14/2017 09:49 AM »
Shuttle and X37b came in from LEO. This is coming in on a hyperbolic trajectory. And because of vertical landing and the thin atmosphere, they're not going to mess around long deep in the atmosphere.
SX are very keen on leveraging their existing experience, which should not be surprising given how hard they fought to earn it.

It seems very odd to me that they wouldn't do the same for a LEO re-entry, which is a very different profile than that of all known winged vehicles, although they also have a very different TPS than all known winged vehicles, that have done re-entry from orbit.

A Mars entry on a vehicle of this size and shape already has enough "unknown unknowns" that you'd want to operate as much of it as possible on as firm a basis as possible.

That's simple logic and it plays to the strengths of known ablators.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline M.E.T.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #172 on: 10/14/2017 10:08 AM »
Ok, so I am a bit confused now.

After Shotwell's latest revelations that a new factory will be built for BFR on the LA waterfront, and that BFR will be launching from a new pad at Boca Chica, can someone explain to me again what the remaining benefit is of shrinking the BFR from 12m to 9m?

The 9m factory limit of the existing Hawthorne facilities will no longer be a limiting factor, and the 12 million pound thrust limit of the existing launch pads will no longer be a limit either, since a new pad is being built at Boca Chica in any case.

So where are the remaining cost savings associated with the smaller 9m BFR to justify the significantly reduced efficiencies compared to the 12m version?

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #173 on: 10/14/2017 10:45 AM »
Ok, so I am a bit confused now.

After Shotwell's latest revelations that a new factory will be built for BFR on the LA waterfront, and that BFR will be launching from a new pad at Boca Chica, can someone explain to me again what the remaining benefit is of shrinking the BFR from 12m to 9m?

So where are the remaining cost savings associated with the smaller 9m BFR to justify the significantly reduced efficiencies compared to the 12m version?
Well a smaller rocket --> smaller raw materials bill and smaller test & mfg facilities
BFR is not just a bigger F9, it's a step change in the materials used.
You don't FSW CF composite and you don't filament wind LiAl alloy (although, hmmm....)

It could be buying a 12m filament winder (or the quotes from suppliers to have it sub contracted out) were just too much. 

The Beal BA-2 was filament wound at 6.2m in diameter. At the time (1999/2000) it's filament winder (stages were built in house and tested at McGregor TX) was said to be one of the biggest in the world.

So a sub contractor quote would probably include the bill for them to go out and have a new biggest-one-of-its-kind-in-the-world filament winder made (and really worst case the company building it has to get a new factory to make it in. This hardware is big  :o )

This stuff is made to order, has lots of finicky little details, and built on the machine builders schedule.

A re-scoping may also have something to do with the engines.

It may be that scaling up has identified severe issues that are too tough to solve if they want to keep to  their planned (or anything like planned) schedule.  As the thrust chamber and preburners get bigger the number of possible instability modes grows as well (Merlin uses the Pintle injector design which TRW claimed has never had an instability issue, but I'm not sure you can use this with any kind of SC cycle, especially feeding the main chamber).

So between a possible step change in schedule and mfg costs (of the mfg hardware) and scaling issues with the engine building a new factory (without all the compromises of fitting it into the existing layout while keeping the existing F9 line running, a non trivial exercise), expensive as it seems, may still be the cheapest option.

This is engineering and manufacturing in the big league.
« Last Edit: 10/14/2017 10:59 AM by john smith 19 »
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #174 on: 10/14/2017 03:57 PM »
They've also already ordered tooling.
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Offline Xentry

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #175 on: 10/14/2017 04:14 PM »
Musk said at the start this is a  "true physics" simulation.
That implies that while the GUI is a bit rough it's not an "open" flight path. It's being driven by a need to land at a certain point on Mars and to keep at least one (and probably more) parameters (probably structural stress and skin temperature) within acceptable ranges while traveling through a chemically accurate Mars atmosphere (although this is something that can vary a lot)
Interesting. A lift-down reentry attitude combined with such a low ballistic coefficient (near-empty BFS upon arrival at Mars + high AoAs) should mean both a faster descent and a higher deceleration at higher altitudes -> lower heat fluxes + definitely much lower integrated loads (ablation) as a whole. Since Mars is known to have a really thin atmosphere, this could make it harder to decelerate prior to hitting the ground, but that's why you have supersonic retro-propulsion...

Seeing it again I noted the timeline of about 470 seconds from start to finish, a little under 8 minutes.

That really does suggest they are looking at a different way of doing winged lifting entry compared to Shuttle or X37b. IIRC Shuttle entries were in the 60-90 minute range.

Those shifts in attitude strongly suggest you're going to need to be strapped into a chair of some kind.
8 minutes is longer than most prior Mars atmospheric EDL (they're usually ~6-7min at most - again, it's a thin atmosphere so the flight-path angles are much steeper than on Earth - ensuring a shorter flight -, in order to have the atmosphere slow the spacecraft to low supersonic speeds before it hits the ground).
The reason for a longer flight than usual at Mars is no doubt the prescribed upper g-load which is imposed on the trajectory, as compared to robotic EDLs (peak accelerations of 7 to 15g, depending on the mission). Still, 470s to decelerate from 7km/s to zero is ~1.5g on average...

Offline Pipcard

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #176 on: 10/14/2017 04:36 PM »
Are the delta wings more advantageous for c.g. control than having the payload be in the middle of the vehicle?

https://forum.nasaspaceflight.com/index.php?topic=43035.msg1684500#msg1684500
Quote from: Nathan2go
Umm, remember the McDonnell Douglas Delta Clipper SSTO proposal?  Like the ITS ship, it was supposed to do a nose-first (or broad-side) entry, followed by rotation and tail-first landing.  As a result, it needed the payload sandwiched between the two propellant tanks for cg control.   Similarly, the ITS ship shows "densely packed cargo" in between the tanks and the loosely filled passenger area.  It has to do the rotation maneuver with full payload at Mars, and nearly empty back at Earth, so cg control is very crucial.


Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #177 on: 10/14/2017 04:37 PM »
Seeing it again I noted the timeline of about 470 seconds from start to finish, a little under 8 minutes.

That really does suggest they are looking at a different way of doing winged lifting entry compared to Shuttle or X37b. IIRC Shuttle entries were in the 60-90 minute range.

Those shifts in attitude strongly suggest you're going to need to be strapped into a chair of some kind.
8 minutes is longer than most prior Mars atmospheric EDL (they're usually ~6-7min at most - again, it's a thin atmosphere so the flight-path angles are much steeper than on Earth - ensuring a shorter flight -, in order to have the atmosphere slow the spacecraft to low supersonic speeds before it hits the ground).
The reason for a longer flight than usual at Mars is no doubt the prescribed upper g-load which is imposed on the trajectory, as compared to robotic EDLs (peak accelerations of 7 to 15g, depending on the mission). Still, 470s to decelerate from 7km/s to zero is ~1.5g on average...
I did not know this. This iwll be the first winged vehicle to make a Mars entry and all the historical data we have for winged re-entry to Earth is around 60-90 mins.  Hence my interest in such a short (by Earth standards) entry.

The slide show deck from 2016 has landing g loads at 6-8g. I speculated smaller vehicle --> lower g loads but apparently not.

An average of 1.5g does not sound bad but IRL it's not going to be averaged, and that's going to be pretty rough after at least 90 days in zero g, especially if you've just paid north of $500K for the experience.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline livingjw

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #178 on: 10/14/2017 09:35 PM »
Shuttle and X37b came in from LEO. This is coming in on a hyperbolic trajectory. And because of vertical landing and the thin atmosphere, they're not going to mess around long deep in the atmosphere.
SX are very keen on leveraging their existing experience, which should not be surprising given how hard they fought to earn it.

It seems very odd to me that they wouldn't do the same for a LEO re-entry, which is a very different profile than that of all known winged vehicles, although they also have a very different TPS than all known winged vehicles, that have done re-entry from orbit.

A Mars entry on a vehicle of this size and shape already has enough "unknown unknowns" that you'd want to operate as much of it as possible on as firm a basis as possible.

That's simple logic and it plays to the strengths of known ablators.

The Video appears to be consistent with active lifting reentry guidance logic, ala shuttle. See attached document starting at page 237 for an explanation of the banking to control lift logic. It is initially counter intuitive. Drag and L/D are functions of angle of attack, but you want high drag (AoA~45 deg) so you modulate effective L/D by rolling your lift vector left and right. Your initial entry is with ~45 deg AoA with nose rolled 180 deg (nose towards planet). BFS L/D at 45 deg should be somewhere around .5-.8 .

John

Offline oldAtlas_Eguy

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #179 on: 10/15/2017 12:08 AM »
An aside from the discussion on SLS schedules is that a likely date for the launch date of the SLS 1B whether that is  EM-2 or Europa Clipper is a value being disbelievingly discussed of NET January 2023.

That is a date of > 5 years from now. It starts to begin to be a race between SLS 1B and BFR for which will launch first. 5 years for SpaceX is a very long time. They can accomplish a lot in 5 years. From the first launch of V1.1 with the first M1Ds to the launch of the 3rd reused booster, regular recovery of boosters, and on the door step of FH that is itself an SHLV (50+mt capable vehicle). By the time SLS 1A flies (EM-1) FH will have been flying for > 2 years almost 2.5. Also SpaceX may get the manned FH/Dragon Lunar flight launched prior to EM-1 as well.

So BFR? What will it's schedule be? Suggestion is to ask these questions again once the factory is being setup to manufacture BFR. Supposedly that may occur in 6 months.

Offline Kaputnik

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #180 on: 10/15/2017 11:38 AM »
In the AMA, Elon said that the BFS tanks would be vented to vacuum for the transit to Mars.
Presumably they will need to be repressurised for entry, by boiling off some of the landing props. What sort of penalty is involved in this in terms of lost propellant?
Is there any benefit to venting the tanks, other than providing a better thermal environment for the header tanks (and if this is the case, why vent both tanks?)
Waiting for joy and raptor

Offline LucR

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #181 on: 10/15/2017 11:43 AM »
He also said they aren't too happy about the current header tank arrangement (i.e., both header tanks embedded in the bottom large tank) and it might change again...

Considering that they have already added a third sea-level Raptor, the design details still appear to be in flux.

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #182 on: 10/15/2017 11:45 AM »
I have a couple of questions on the ITS vehicle.  From what I see, the center of gravity for the vehicle would need to be low enough not to topple over when landing vertical.  When landing most if not all fuel will be expended, thus lower mass on the bottom of the vehicle.  If it lands on earth with 50 tons downmass and on Mars with 150.  How will this weight be balanced on the bottom? 

Also when returning to earth through the thick atmosphere, are the wings or winglets used to keep the lower mass level with the upper mass to keep from coming in bottom first? 

I'm assuming the engines will weight about a ton so that is 31 tons on the bottom.  I'm assuming a titanium bottom around the engines adding more weight, I suppose this totals 50 tons to offset the 50 tons in the cargo bay if 50 tons is returned.
I think you're confusing the booster and spaceship. Only 6 engines on the the ship.

Severn now, from the reddit AMA they have added another sea level raptor to increase the max possible landed mass on earth. (for the point to point travel)

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #183 on: 10/15/2017 01:07 PM »
The Video appears to be consistent with active lifting reentry guidance logic, ala shuttle.
I don't recall the Shuttle spending much time "nose down," and the Shuttle re-entry was IIRC more like an hour to an hour and a half, due to the tile re-radiating the heat they'd absorbed.
Quote from: livingjw
See attached document starting at page 237 for an explanation of the banking to control lift logic. It is initially counter intuitive. Drag and L/D are functions of angle of attack, but you want high drag (AoA~45 deg) so you modulate effective L/D by rolling your lift vector left and right. Your initial entry is with ~45 deg AoA with nose rolled 180 deg (nose towards planet). BFS L/D at 45 deg should be somewhere around .5-.8 .

John
It's only counter intuitive if you a) Don't know how Apollo GNC worked and b)Thought of the Shuttle as a conventional aircraft.  Very few aircraft spend any significant length of time at 10s of degrees to their line of flight, except fighters in real (or simulated) combat.

45deg means 45deg above the horizontal. and rolling means along the axis of the vehicle, left or right, which is what Apollo did, relying IIRC on some heavy point masses inside the capsule to shift the lift vector but not the angle of attack.

Going from High AoA to zero forward speed prior to a vertical landing is where it will get very interesting. The only thing that comes close are those "tail sitter" VTOl aircraft of the 1950's and 60's. I don't think they had a very good safety record.

In the AMA, Elon said that the BFS tanks would be vented to vacuum for the transit to Mars.
Presumably they will need to be repressurised for entry, by boiling off some of the landing props. What sort of penalty is involved in this in terms of lost propellant?
Is there any benefit to venting the tanks, other than providing a better thermal environment for the header tanks (and if this is the case, why vent both tanks?)
Good question. The Shuttle ET took a surprising amount of GO2/GH2 to orbit

Obviously it depends on pressure and average tank temperature.  The NIST database for fluid properties says that at 92K GO2 density is about 4.36 Kg/m^3, Methane is 1.68 Kg/m^3 at about 120K, both for a 1 atm absolute tank pressure.

http://webbook.nist.gov/chemistry/fluid/

On the upside given Mars very low atmospheric pressure and Mars gravity it might need only a fairly low (IE sub 1 Earth atmosphere) pressure to stiffen the structure. OTOH if that's not the case you could be looking at tonnes of gas, given the size of the tanks.
He also said they aren't too happy about the current header tank arrangement (i.e., both header tanks embedded in the bottom large tank) and it might change again...
I did not notice that.  If I had to guess I'd have thought each one at the base of its respective main propellant tank, to minimize the additional plumbing. With a reflective coating on the outside of the header tanks any solar radiation would have to go through the TPS, the tank wall and the reflective coating to raise the tank pressure. The other possibility would be through the tank connections to the rest of the vehicle. The issue of course is what happens over the course of the cruise to Mars.
Quote from: LucR
Considering that they have already added a third sea-level Raptor, the design details still appear to be in flux.
It does look that way.

Not to worry though, the renders are driven directly by the CAD system, so creating a new presentation won't be difficult   :)
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Online lamontagne

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #184 on: 10/15/2017 03:30 PM »
Banking during re entry.

I need advice on what the plasma envelope on the BFS at re entry would look like.  Would it be very wide and visible, or rather transparent?  It seems that despite using pica-x there will be little ablation, so not all that much trailing gas glow?
I've got the vehicle looking hot, but not suffering enough, I feel  :-)
« Last Edit: 10/15/2017 03:31 PM by lamontagne »

Offline LucR

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #185 on: 10/15/2017 05:19 PM »
He also said they aren't too happy about the current header tank arrangement (i.e., both header tanks embedded in the bottom large tank) and it might change again...
I did not notice that.

Quote from: ElonMusk
The aspiration by the change was to avoid/minimize plumbing hell, but we don't super love the current header tank/plumbing design. Further refinement is likely.
https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodhl3u/

If I had to guess I'd have thought each one at the base of its respective main propellant tank, to minimize the additional plumbing.
AIUI that was the IAC 2016 arrangement, but apparently they have not yet found an optimal solution.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #186 on: 10/16/2017 09:59 AM »
If I had to guess I'd have thought each one at the base of its respective main propellant tank, to minimize the additional plumbing.
AIUI that was the IAC 2016 arrangement, but apparently they have not yet found an optimal solution.
I would have not thought it was that big an issue.

However with this stuff so much is interlinked in different ways, so apparently trivial design decisions (or changes to them) have significant knock on effects on other systems. It's not just how they work during any given stage, it's how they work throughout the whole mission.

In case it's not obvious I think it's great the design is still evolving before major final hardware gets built.
This is much better sorted out on the CAD screen than when you're just about to fix the tank end cap on.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Nibb31

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #187 on: 10/16/2017 03:49 PM »
I find it hard to wrap my head around the idea that such fundamental layout issues as "where do we put the tanks?" and "how many engines do we need?" haven't been sorted 6 months before construction starts.

There is a whole lot of work to do between "figure out where major parts go" and "start bolting parts together" (things like high-level design, in-depth design, interfacing, sourcing, tooling, facility layout, prototyping, testing, etc.) and 6 months seems awfully short.

It's hard to engineer a test stand or manufacturing fixtures when you're still adding and removing engines to the basic design.

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #188 on: 10/16/2017 04:50 PM »
I find it hard to wrap my head around the idea that such fundamental layout issues as "where do we put the tanks?" and "how many engines do we need?" haven't been sorted 6 months before construction starts.
Which suggests we are more than 6 months away from construction on those parts perhaps?
Quote from: Nibb31
It's hard to engineer a test stand or manufacturing fixtures when you're still adding and removing engines to the basic design.
I'd suggest the test stands won't be a problem as long as they are still within the same outer mold line. If that changes again then there will be trouble.

Mfg fixtures for the main structure will be more of a problem (depends if engines mounted directly, or  to a sub-structure that can be modified). But again, are they less than 6 months from first build?

Only time will tell.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

Offline Robotbeat

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #189 on: 10/16/2017 11:43 PM »
Nah. They're committed to start building it within 6-9 months.

Think Grasshopper or Falcon 9 v1.0. If they need to change things, they will.

It's better to catch things in CAD if that doesn't slow you down, but better to build it sooner rather than get stuck in paralysis by analysis. Build one, even if it needs a battleship thrust structure like the v1.0 falcon 9.
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Offline TomH

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #190 on: Today at 02:10 AM »
Going from High AoA to zero forward speed prior to a vertical landing is where it will get very interesting. The only thing that comes close are those "tail sitter" VTOl aircraft of the 1950's and 60's. I don't think they had a very good safety record.

That's not a valid analogy. In the 1950s and '60s it would have been impossible for a two wheeled upright vehicle to remain upright for hours, much less so while people were pushing on it and unsuccessfully trying their best to knock it over. But that accurately describes a Segway Personal Transporter. What's the difference? While accelerometers and actuators are more advanced, the major difference is computer processor power. The YB-49 was an unstable aircraft even in calm conditions, yet a B-2 can fly through a hurricane with no problems. The ability of processors to make almost uncountable ultra-minute adjustments in nanoseconds is what makes the difference. Even five years ago, landing any rocket on its tail seemed pretty farfetched. This is nothing more than finessing the same process. Put today's electronics on those tail landing aircraft of a half century ago and they would work fine.

And BTW, find a parked Segway PT somewhere and try to push it over. It will make you doubt your understanding of physics. As well as change your POV on this subject.
« Last Edit: Today at 02:14 AM by TomH »

Online Lars-J

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #191 on: Today at 03:11 AM »
Nah. They're committed to start building it within 6-9 months.

They are committed to no such thing. It may be their current intention, but plans can change.

Offline Patchouli

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #192 on: Today at 03:39 AM »

It's only counter intuitive if you a) Don't know how Apollo GNC worked and b)Thought of the Shuttle as a conventional aircraft.  Very few aircraft spend any significant length of time at 10s of degrees to their line of flight, except fighters in real (or simulated) combat.

45deg means 45deg above the horizontal. and rolling means along the axis of the vehicle, left or right, which is what Apollo did, relying IIRC on some heavy point masses inside the capsule to shift the lift vector but not the angle of attack.

Going from High AoA to zero forward speed prior to a vertical landing is where it will get very interesting. The only thing that comes close are those "tail sitter" VTOl aircraft of the 1950's and 60's. I don't think they had a very good safety record.



During the DC-X program they had a vehicle fly such maneuver.

« Last Edit: Today at 03:40 AM by Patchouli »

Offline john smith 19

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Re: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)
« Reply #193 on: Today at 08:05 AM »
Nah. They're committed to start building it within 6-9 months.

They are committed to no such thing. It may be their current intention, but plans can change.
Indeed.  With SX change is a virtual certainty.  :)
During the DC-X program they had a vehicle fly such maneuver.

The obvious question is what was its forward speed before it started to slow down?

If it's higher than the BFS's planned transition speed then BFS is within the known SoA. If not then that's going to be another area for the R&D list.

It's pretty impressive given (IIRC) the DC-X used an F15 flight computer, so probably a 1715A processor executing at < 1 MIPS.
"Solids are a branch of fireworks, not rocketry. :-) :-) ", Henry Spencer 1/28/11  Averse to bold? You must be in marketing."It's all in the sequencing" K. Mattingly.  STS-Keeping most of the stakeholders happy most of the time.

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