Author Topic: MCT Speculation and Discussion Thread 4  (Read 569620 times)

Offline cscott

  • Senior Member
  • *****
  • Posts: 2848
  • Liked: 1970
  • Likes Given: 664
Re: MCT Speculation and Discussion Thread 4
« Reply #1180 on: 12/11/2015 03:34 PM »
"Big Falcon Spaceship"

Offline philw1776

  • Full Member
  • ****
  • Posts: 957
  • Seacoast NH
  • Liked: 559
  • Likes Given: 267
Re: MCT Speculation and Discussion Thread 4
« Reply #1181 on: 12/11/2015 05:29 PM »
A BFR can be conceived with lots less than 30 1st stage engines. 
As a point of reference, I’ve taken these Musk’s statements as a given even though I bet they’ll be modified at his supposed late 2015 (likely spring 2016) briefing.
100mT land on Mars cargo
Land the whole thing and re-use it; i.e. return it to Earth
“there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster”
Raptor thrust over 230mT; use a lot of them.
380 seconds ISP vacuum

My BFR is 12.5m diameter, making a relatively squat first stage under 30m tall.   Propellant tanks alone 23.5m.  Delivers ~2.8Km/sec after gravity losses.  The wider diameter leaves room for growth for later larger versions, allowing for lengthening the tanks in both stages, adding engines and lengthening the cargo hold.  Assumed a heavy dry weight of 90mT for stage 2 MCT.

No worries about pad towers for 100m high rockets.  BFR with MCT 2nd stage is shorter than the 70m F9 but 10 times more massive.  BFR’s multi Raptor engine driven 12.5m diameter is a nice size to fit living quarters and various colonial heavy equipment.

Don’t need 15 million LBS thrust, but with mass ratios ~4.3%  need a bit over 12 million LBS (54 million Newtons) thrust, 24 engines arranged in rings of 16 and 8 with room for a future center engine or 2.

Given the huge delta V requirements for both Mars departure from refueling at LEO AND later functioning as a SSTO taking off from Mars’ surface and return to Earth, I put the Km/sec budget into the 2nd stage.  Stage one goes low & slow, under 3 Km/sec, boosting the heavy 2nd stage before return to launch site, RTLS.  The dry weight plus fuel of the returning 1st stage exceeds Raptor thrust so any 2 of the 8 engine inner ring engines throttled down provide landing thrust.

Total BFR mass 4450mT or 9.8 million LBS.  LEO mass fraction 4.3%.

Stage One:
12.5m diameter with 23.5m length propellant tanks
3280mT   7.2 million LBS  1st stage fueled mass
230mT thrust engines  506K LBS
24 engines 56 million Newtons  12.2 million LBS Thrust; T/W 1.24
Rings of 16 engines, and 8 engines
Avg ISP from sea level to vacuum 325
After 1 Km/sec additional Delta V reserved for RTLS Rocket Equation gives 3.4 Km/sec but nets under 3 Km/sec Delta V after gravity losses

Stage Two The MCT:
Dry Mass 90mT; 100mT is cargo
12.5m diameter with 7.5 m length for propellant tanks. 
Cargo 8.5m length ; 1040 m3 volume
(Plenty of space for expanded fuel tanks in a simple modified tanker version.)
1165mT fueled mass (2.6 million LBS)
380 seconds ISP vac; Rvac engines assumed 14% higher thrust as with F9 FT
6 Rvac engines   3.5 million LBS Thrust
6.75Km/sec Delta V capability, via Rocket eq.
8.4Km/sec Mars liftoff with only 25mT return cargo

I also modeled another small version to see just how small a BFR could meet Elon’s goals. I optimistically lowered the MCT 2nd stage dry weight from 90mT to 75mT.  This minimal, 12m diameter optimistic version needs only 19 engines with rings of 12, 6 and two center engines.  The MCT stage 2 has only 5 engines, reducing the cost of building this BFR/MCT from 30 Raptors to 24.  Engines are the big cost driver for mass producing BFRs.
« Last Edit: 12/11/2015 05:31 PM by philw1776 »
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Lars-J

  • Senior Member
  • *****
  • Posts: 3845
  • California
  • Liked: 3095
  • Likes Given: 1965
Re: MCT Speculation and Discussion Thread 4
« Reply #1182 on: 12/11/2015 05:55 PM »
From another thread, here is a recent quote by Musk in an interview:

Direct quote from Elon implying no second stage, just booster and mars spacecraft from an article in GQ:

"Well, there's two parts of it—there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has quite a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster, because Mars's gravity is weaker and the atmosphere's thinner, so it's got enough capability to get all the way back here by itself. It needs a helping hand out of Earth's gravity well. So, technically, it would be the BFR and the BFS." As in "Big frakking Spaceship."

http://www.gq.com/story/elon-musk-mars-spacex-tesla-interview?utm_source=10370

So... I guess we are back to basics, what many of us have arguing, it seems? This thread has been devalued (and made less interesting) in the last few months by people wanting to discuss their own architectures (yes, you know who you are), so hopefully this can narrow down the discussion again.

Offline JamesH

  • Full Member
  • ****
  • Posts: 525
  • United Kingdom
  • Liked: 283
  • Likes Given: 7
Re: MCT Speculation and Discussion Thread 4
« Reply #1183 on: 12/11/2015 06:37 PM »
He also stated it was a very unusual (crazy) architecture, so  I suspect many of the ideas put forward that are similar to 'large dragon' are likely to be wrong.

Offline Occupymars

  • Full Member
  • *
  • Posts: 158
  • Liked: 39
  • Likes Given: 58
Re: MCT Speculation and Discussion Thread 4
« Reply #1184 on: 12/11/2015 08:03 PM »
So the second stage is the spaceship confirmed. The longer this speculation goes on the more people have complicated it. Second stage's third stage's ect. KISS
This is what we know about his mars architecture so far:
1. Fully reusable
2. Mars ISRU
3. Two stage's
4. 100mt to mars surface
From those points I think we can safely say that his architecture includes on orbit refueling probably with a second BFS to remain fully reusable.

Oh man can't wait to see his architecture presented just awesome.
I wonder if the BFS will be a biconical design(Mars thin atmosphere) like blue origin's spaceship or a capsule.
« Last Edit: 12/11/2015 08:06 PM by Occupymars »
"Those who would give up essential liberty to purchase a little temporary safety deserve neither liberty nor safety." ~ Benjamin Franklin

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: MCT Speculation and Discussion Thread 4
« Reply #1185 on: 12/11/2015 08:56 PM »
A BFR can be conceived with lots less than 30 1st stage engines. 
As a point of reference, I’ve taken these Musk’s statements as a given even though I bet they’ll be modified at his supposed late 2015 (likely spring 2016) briefing.
100mT land on Mars cargo
Land the whole thing and re-use it; i.e. return it to Earth
“there's a booster rocket and there's a spaceship. So the booster rocket's just to get it out of Earth's gravity because Earth has a deep gravity well and thick atmosphere, but the spaceship can go from Mars to Earth without any booster”
Raptor thrust over 230mT; use a lot of them.
380 seconds ISP vacuum

My BFR is 12.5m diameter, making a relatively squat first stage under 30m tall.   Propellant tanks alone 23.5m.  Delivers ~2.8Km/sec after gravity losses.  The wider diameter leaves room for growth for later larger versions, allowing for lengthening the tanks in both stages, adding engines and lengthening the cargo hold.  Assumed a heavy dry weight of 90mT for stage 2 MCT.

No worries about pad towers for 100m high rockets.  BFR with MCT 2nd stage is shorter than the 70m F9 but 10 times more massive.  BFR’s multi Raptor engine driven 12.5m diameter is a nice size to fit living quarters and various colonial heavy equipment.

Don’t need 15 million LBS thrust, but with mass ratios ~4.3%  need a bit over 12 million LBS (54 million Newtons) thrust, 24 engines arranged in rings of 16 and 8 with room for a future center engine or 2.

Given the huge delta V requirements for both Mars departure from refueling at LEO AND later functioning as a SSTO taking off from Mars’ surface and return to Earth, I put the Km/sec budget into the 2nd stage.  Stage one goes low & slow, under 3 Km/sec, boosting the heavy 2nd stage before return to launch site, RTLS.  The dry weight plus fuel of the returning 1st stage exceeds Raptor thrust so any 2 of the 8 engine inner ring engines throttled down provide landing thrust.

Total BFR mass 4450mT or 9.8 million LBS.  LEO mass fraction 4.3%.

Stage One:
12.5m diameter with 23.5m length propellant tanks
3280mT   7.2 million LBS  1st stage fueled mass
230mT thrust engines  506K LBS
24 engines 56 million Newtons  12.2 million LBS Thrust; T/W 1.24
Rings of 16 engines, and 8 engines
Avg ISP from sea level to vacuum 325
After 1 Km/sec additional Delta V reserved for RTLS Rocket Equation gives 3.4 Km/sec but nets under 3 Km/sec Delta V after gravity losses

Stage Two The MCT:
Dry Mass 90mT; 100mT is cargo
12.5m diameter with 7.5 m length for propellant tanks. 
Cargo 8.5m length ; 1040 m3 volume
(Plenty of space for expanded fuel tanks in a simple modified tanker version.)
1165mT fueled mass (2.6 million LBS)
380 seconds ISP vac; Rvac engines assumed 14% higher thrust as with F9 FT
6 Rvac engines   3.5 million LBS Thrust
6.75Km/sec Delta V capability, via Rocket eq.
8.4Km/sec Mars liftoff with only 25mT return cargo

I also modeled another small version to see just how small a BFR could meet Elon’s goals. I optimistically lowered the MCT 2nd stage dry weight from 90mT to 75mT.  This minimal, 12m diameter optimistic version needs only 19 engines with rings of 12, 6 and two center engines.  The MCT stage 2 has only 5 engines, reducing the cost of building this BFR/MCT from 30 Raptors to 24.  Engines are the big cost driver for mass producing BFRs.

Just to confirm: you have the BFR contributing only 1km/s delta V before separation?

Offline GregA

  • Full Member
  • ****
  • Posts: 489
  • Liked: 254
  • Likes Given: 58
Re: MCT Speculation and Discussion Thread 4
« Reply #1186 on: 12/11/2015 09:05 PM »
From another thread, here is a recent quote by Musk in an interview:

"So, technically, it would be the BFR and the BFS." As in "Big frakking Spaceship.""
I'd just read that article and came here.

So new naming?
Mars Colonial Transport Architecture with the BFR and BFS? :-)

Offline philw1776

  • Full Member
  • ****
  • Posts: 957
  • Seacoast NH
  • Liked: 559
  • Likes Given: 267
Re: MCT Speculation and Discussion Thread 4
« Reply #1187 on: 12/11/2015 11:23 PM »


Just to confirm: you have the BFR contributing only 1km/s delta V before separation?

Sorry about any confusion, "nets under 3 Km/sec Delta V after gravity losses"  So BFR goes low & slow (under 3 Km/sec)
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 363
  • Likes Given: 0
Re: MCT Speculation and Discussion Thread 4
« Reply #1188 on: 12/12/2015 03:38 AM »
philw1776:  Your engine count is off because you forgot to account for lower thrust at Sea-level, 230 mt is the VAC performance goal for Raptor.  I'm estimating that Sea-level thrust is ~77% of Vac which raises the engine count to ~30 to get very nearly identical total thrust as your estimate.

Offline Zed_Noir

  • Senior Member
  • *****
  • Posts: 2360
  • Canada
  • Liked: 320
  • Likes Given: 530
Re: MCT Speculation and Discussion Thread 4
« Reply #1189 on: 12/12/2015 07:58 AM »
If there is going to be a BFR boosting a BFS.

Then someone can modified the BFS as part of a cislunar base station for small reusable Lunar landers. Along with a few BFS tanker variants docked to a frame (Space Dock :D) with solar power & station keeping capabilities. The BFS mods is mostly for lander handling and maintenance with modules in the cargo bay.

Should be available in about early mid to late mid  2020s IMO if SpaceX manages to be on schedule.


Offline Dante80

  • Full Member
  • ****
  • Posts: 883
  • Athens : Greece
  • Liked: 801
  • Likes Given: 500
Re: MCT Speculation and Discussion Thread 4
« Reply #1190 on: 12/12/2015 10:44 AM »
philw1776:  Your engine count is off because you forgot to account for lower thrust at Sea-level, 230 mt is the VAC performance goal for Raptor.  I'm estimating that Sea-level thrust is ~77% of Vac which raises the engine count to ~30 to get very nearly identical total thrust as your estimate.

A question. Do we know that the 230mt quoted (I assume in the reddit Q&A Elon did) was for Vac? Or has there been any more information on this elsewhere? I'm asking because Elon was answering a question that was about SL thrust.

The quote I remember.

Quote
Q: Has the Raptor engine changed in its target thrust since the last number we have officially heard of 1.55Mlbf SL thrust?

A: Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them :)

Offline philw1776

  • Full Member
  • ****
  • Posts: 957
  • Seacoast NH
  • Liked: 559
  • Likes Given: 267
Re: MCT Speculation and Discussion Thread 4
« Reply #1191 on: 12/12/2015 04:49 PM »
philw1776:  Your engine count is off because you forgot to account for lower thrust at Sea-level, 230 mt is the VAC performance goal for Raptor.  I'm estimating that Sea-level thrust is ~77% of Vac which raises the engine count to ~30 to get very nearly identical total thrust as your estimate.

A question. Do we know that the 230mt quoted (I assume in the reddit Q&A Elon did) was for Vac? Or has there been any more information on this elsewhere? I'm asking because Elon was answering a question that was about SL thrust.

The quote I remember.

Quote
Q: Has the Raptor engine changed in its target thrust since the last number we have officially heard of 1.55Mlbf SL thrust?

A: Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them :)

I read that same quote to refer to sea level thrust of Raptor over 230mT or over 507 KLBs.  I uprated the Rvac thrust by the same % as the Merlin Vac/SL.

Regardless, I'm in the camp that says the now Q1 2016 big reveal will have a higher thrust for Raptor than 230mT
“When it looks more like an alien dreadnought, that’s when you know you’ve won.”

Offline MichaelBlackbourn

  • Member
  • Posts: 41
  • Liked: 63
  • Likes Given: 0
Re: MCT Speculation and Discussion Thread 4
« Reply #1192 on: 12/12/2015 05:52 PM »
It would be amazing to one day see hardware with a giant 'SpaceX BFR' Decal on the side.   I wonder if the 'name' will survive that long... Of it gets renamed something else... 'Tiny' maybe.

Offline Jcc

  • Full Member
  • ****
  • Posts: 855
  • Liked: 197
  • Likes Given: 89
Re: MCT Speculation and Discussion Thread 4
« Reply #1193 on: 12/12/2015 10:51 PM »
It would be amazing to one day see hardware with a giant 'SpaceX BFR' Decal on the side.   I wonder if the 'name' will survive that long... Of it gets renamed something else... 'Tiny' maybe.

Eagle!

Online guckyfan

  • Senior Member
  • *****
  • Posts: 6568
  • Germany
  • Liked: 1686
  • Likes Given: 1597
Re: MCT Speculation and Discussion Thread 4
« Reply #1194 on: 12/13/2015 03:50 PM »
I understand that usually it is assumed that the first stage will give the upper stage/MCT a horizontal speed of ~3km/s, similar to what the Falcon first stage does. For RTLS of the first stage that needs to be reversed, cutting into payload.

I do wonder if another approach could be effective. MCT will have a very large delta-v budget. It needs it to perform its functions towards Mars. Could the first stage go up almost straight similar to the BO New Shepard, but maybe up to 150km peak altitude, eating all the gravity and air resistance losses and use the second stage for the task of building up orbital speed? On the way down with its large diameter it may not need a reentry burn or only a very small one. Reuse fuel would be mainly only the small amount of landing fuel.

Did anyone of those who did thorough analysis ever consider such a scenario or am I way off?
« Last Edit: 12/13/2015 03:51 PM by guckyfan »

Offline Dante80

  • Full Member
  • ****
  • Posts: 883
  • Athens : Greece
  • Liked: 801
  • Likes Given: 500
Re: MCT Speculation and Discussion Thread 4
« Reply #1195 on: 12/13/2015 04:13 PM »
I understand that usually it is assumed that the first stage will give the upper stage/MCT a horizontal speed of ~3km/s, similar to what the Falcon first stage does. For RTLS of the first stage that needs to be reversed, cutting into payload.

I do wonder if another approach could be effective. MCT will have a very large delta-v budget. It needs it to perform its functions towards Mars. Could the first stage go up almost straight similar to the BO New Shepard, but maybe up to 150km peak altitude, eating all the gravity and air resistance losses and use the second stage for the task of building up orbital speed? On the way down with its large diameter it may not need a reentry burn or only a very small one. Reuse fuel would be mainly only the small amount of landing fuel.

Did anyone of those who did thorough analysis ever consider such a scenario or am I way off?

The vast majority of the energy in a rocket launch is expended reaching orbital velocity, not orbital height. Getting to orbital velocity requires a large amount of horizontal velocity. Getting that horizontal velocity takes about the same amount of dV, no matter what altitude you are at. If you started at LEO altitude, and started accelerating horizontally, you'd still need to obtain 17,100 mph of horizontal velocity. Without the horizontal velocity, you're suborbital.

Also, take a look at this.
https://gravityloss.wordpress.com/2008/01/10/drag-loss-in-ascent-gain-in-descent-and-what-it-means-for-scalability/

Online guckyfan

  • Senior Member
  • *****
  • Posts: 6568
  • Germany
  • Liked: 1686
  • Likes Given: 1597
Re: MCT Speculation and Discussion Thread 4
« Reply #1196 on: 12/13/2015 04:30 PM »
The vast majority of the energy in a rocket launch is expended reaching orbital velocity, not orbital height. Getting to orbital velocity requires a large amount of horizontal velocity. Getting that horizontal velocity takes about the same amount of dV, no matter what altitude you are at. If you started at LEO altitude, and started accelerating horizontally, you'd still need to obtain 17,100 mph of horizontal velocity. Without the horizontal velocity, you're suborbital.

I know all of this. I even explicitly stated it in my post. So what's your point?

Usually, I understand, a first stage does a lot of delta-v towards orbital speed. The Falcon 9 does much less, to facilitate RTLS and leaves more of the buildup of orbital speed to the second stage. That is not the optimum approach for expendable vehicles. It is a better approach for reusable vehicles.

My suggestion was to carry the idea to the extreme. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

Online MikeAtkinson

  • Full Member
  • ****
  • Posts: 1714
  • Bracknell, England
  • Liked: 482
  • Likes Given: 61
Re: MCT Speculation and Discussion Thread 4
« Reply #1197 on: 12/13/2015 04:52 PM »
The vast majority of the energy in a rocket launch is expended reaching orbital velocity, not orbital height. Getting to orbital velocity requires a large amount of horizontal velocity. Getting that horizontal velocity takes about the same amount of dV, no matter what altitude you are at. If you started at LEO altitude, and started accelerating horizontally, you'd still need to obtain 17,100 mph of horizontal velocity. Without the horizontal velocity, you're suborbital.

I know all of this. I even explicitly stated it in my post. So what's your point?

Usually, I understand, a first stage does a lot of delta-v towards orbital speed. The Falcon 9 does much less, to facilitate RTLS and leaves more of the buildup of orbital speed to the second stage. That is not the optimum approach for expendable vehicles. It is a better approach for reusable vehicles.

My suggestion was to carry the idea to the extreme. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

That is not a bad approach, it was used in The Rocket Company book (fiction, but at times read more like a design manual), but I don't think that is what SpaceX are doing.

Offline docmordrid

  • Senior Member
  • *****
  • Posts: 4624
  • Michigan
  • Liked: 1745
  • Likes Given: 1
Re: MCT Speculation and Discussion Thread 4
« Reply #1198 on: 12/13/2015 04:55 PM »
"Big Falcon Spaceship"

'Falcon' is not what he was quoted as saying 'F' stood for throughout the interview. Quite the contrary.
DM

Offline Dante80

  • Full Member
  • ****
  • Posts: 883
  • Athens : Greece
  • Liked: 801
  • Likes Given: 500
Re: MCT Speculation and Discussion Thread 4
« Reply #1199 on: 12/13/2015 05:02 PM »
I know all of this. I even explicitly stated it in my post. So what's your point?

Usually, I understand, a first stage does a lot of delta-v towards orbital speed. The Falcon 9 does much less, to facilitate RTLS and leaves more of the buildup of orbital speed to the second stage. That is not the optimum approach for expendable vehicles. It is a better approach for reusable vehicles.

My suggestion was to carry the idea to the extreme. Let the first stage eat all the gravity loss and drag loss and leave all or almost all of the buildup of orbital speed to the second stage. MCT needs the big delta-v budget anyway to get to Mars from LEO and land.

The point I tried to make is that this is a lot of delta-v needed. MCT might be able to do it, if its designed to stage from LEO, but in that case it might be almost empty when it reaches orbital speed. You will then need to transfer more fuel to it so that it can start the mission to Mars.

At the same time, the "slingshot to orbital height S1" might make re-usability easier, but you would still have to design it so that it could carry other payloads than MCT (for example, a tanker, or a hub, or mars infrastructure, or commercial payloads etc). If S1 is designed to launch more things than strictly MCT, then it would not be easy (I think) to merge both capabilities in the same structure.
« Last Edit: 12/13/2015 05:03 PM by Dante80 »

Tags: