Author Topic: IAC 2017 -- BFR v0.2 - DISCUSSION THREAD 3 (Post Speech)  (Read 23137 times)

Online cppetrie

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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 canít really be expended unless they donít 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. Itíll never be used expendable so it hardly matters but just curious.

Online John Alan

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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 »

Offline DAZ

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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 »

Offline Robotbeat

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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 »

Offline Robotbeat

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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.  Thatís 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.  Thatís 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.  Thatís 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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Offline Kaputnik

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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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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 »

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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.
I'm a "glass is twice as big as it needs to be" kinda guy

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."

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