Author Topic: BFR with expendable upper stage  (Read 13476 times)

Online Robotbeat

  • Senior Member
  • *****
  • Posts: 28186
  • Minnesota
  • Liked: 7981
  • Likes Given: 5329
Re: BFR with expendable upper stage
« Reply #20 on: 02/13/2018 04:13 AM »
If you have to do 20+ flights of BFR/BFS to LEO to set up the infrastructure to return one BFS from Mars surface...
No, you just need one booster, one BFS, and maybe another tanker version (which can be used for other tasks) to setup the required infrastructure. The launches themselves are cheap.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #21 on: 02/13/2018 04:17 AM »
Without upper stage reuse, launch costs will never approach the low levels needed for mass settlement. With partial reuse we can get down to $1000/kg. We might need to get nearly $10/kg (consistent with spaceX’s numbers for ITS/BFR) to really make mass settlement feasible. Just not possible without upper stage reuse.

You're implying you're not interested in anything less than the economic settlement of Mars being opened up by BFS. I don't see things that way... It could be that BFS will just not be able to make things as easy and as cheaply as Musk envisions. This stuff is hard. So what I'm asking (the performance of a BFR with expendable BFS derived upper stage) is maybe to look at plan B.

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #22 on: 02/13/2018 04:28 AM »
With Plan B you could concentrate on an exploration phase of Mars, before committing to settlement. You could also look at cooperating more with NASA, and other potential stakeholders. The empty shells of the big dumb landers could also become very useful when you do commit to the base building phase. But anyway... All I want to know is the performance of BFR with an expendable upper stage! :-P

Online Robotbeat

  • Senior Member
  • *****
  • Posts: 28186
  • Minnesota
  • Liked: 7981
  • Likes Given: 5329
Re: BFR with expendable upper stage
« Reply #23 on: 02/13/2018 04:32 AM »
You still don’t seem to have a good grasp of the situation, though. Performance to Mars isn’t actually lower because of reuse. No significant mass is wasted on the BFS. Everything you might need for reuse is also what you need to land on Mars.

A large expendable Mars lander still needs a huge heatshield. It still needs supersonic retropropulsion. It still needs landing thrusters. It still needs landing legs. You even still need large tanks on your trans Mars insertion stage.

You might save a little bit of IMLEO by staging off pieces (like tanks, heatshield, etc) as they are used, but that also increases complication and costs in its own way. How much did the EDL system for Curiosity cost? I bet it’s still about the same as a BFS, even though it’s purely expendable and can only land a hundredth of the mass.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #24 on: 02/13/2018 04:41 AM »
Okay, I get it. But there still may be road-stoppers ahead. I'm curious about a Plan B. Okay? :-P

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #25 on: 02/13/2018 04:46 AM »
One of the potential problems cited with BFS was the high g-loads during Mars EDL. That's because it's still a high-density design even with mostly empty tanks. That could be a problem considering your human cargo - and there's no provision for artificial gravity with BFS. So I was thinking about very low-density high-volume landers to keep g-loads as low as possible. But that question warrants a different thread, and after I'd gotten the answer to this one!

Online Robotbeat

  • Senior Member
  • *****
  • Posts: 28186
  • Minnesota
  • Liked: 7981
  • Likes Given: 5329
Re: BFR with expendable upper stage
« Reply #26 on: 02/13/2018 04:50 AM »
High density design? citation badly needed.

The gee loads are pretty low since the body generates significant lift. The only reason they might look fairly high is the fast transit, which also minimizes microgravity exposure. If you wanted, you could always choose a slower trajectory which would have lower fee loads.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #27 on: 02/13/2018 05:17 AM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Offline Valerij

  • Member
  • Posts: 26
  • Russia, SPb
  • Liked: 4
  • Likes Given: 30
Re: BFR with expendable upper stage
« Reply #28 on: 02/13/2018 09:21 AM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
Very small. The point is that the efficiency of a two-stage rocket starting from Earth, with the deltaV necessary for a flight to Mars, is very small.
   
For some specific missions, it may be advantageous to use disposable, consumable upper stages on the BFR. First of all, to create a depot of fuel in orbit. It is also possible to deliver the first elements of the fuel complex to Mars. But without refueling this stage on LEO you will not be able to deliver large modules not to the Moon, nor to Mars.
   
You will still need reusable upper stages as tankers for refueling the stage sent to Mars.

Offline DJPledger

  • Full Member
  • ****
  • Posts: 452
  • Liked: 127
  • Likes Given: 2939
Re: BFR with expendable upper stage
« Reply #29 on: 02/13/2018 09:33 AM »
Expendable US for BFR defeats the objective as BFR is being designed for fully reusability. Expending any BFR hardware will dramatically increase it's cost per launch. Full reusability is what will give BFR extremely low launch costs.

Offline uhuznaa

  • Full Member
  • *
  • Posts: 129
  • Liked: 69
  • Likes Given: 9
Re: BFR with expendable upper stage
« Reply #30 on: 02/13/2018 11:55 AM »
Expendable US for BFR defeats the objective as BFR is being designed for fully reusability. Expending any BFR hardware will dramatically increase it's cost per launch. Full reusability is what will give BFR extremely low launch costs.

"Full reusability" does not necessarily mean to only and always fly reusable second stages on BFR. If what you want to launch is something like a big space station module or some large module to land on the Moon, building a one-off second stage from parts or from a BFS near its end of life would be totally reasonable.

For one-way missions dragging a reusable stage with heat shield, landing engines, legs etc. all the way to the destination and back would be pointless.

Online Robotbeat

  • Senior Member
  • *****
  • Posts: 28186
  • Minnesota
  • Liked: 7981
  • Likes Given: 5329
Re: BFR with expendable upper stage
« Reply #31 on: 02/13/2018 12:44 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldn’t want to do that from LEO. Instead, you’d want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start it’s burn while already very close to escape velocity at perigee.

You’d want basically the biggest Star motor in OrbitalATK’s catalogue. It, that’d be one of the cheapest options.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline acsawdey

  • Full Member
  • ***
  • Posts: 377
  • Minnesota
  • Liked: 220
  • Likes Given: 405
Re: BFR with expendable upper stage
« Reply #32 on: 02/13/2018 04:12 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldn’t want to do that from LEO. Instead, you’d want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start it’s burn while already very close to escape velocity at perigee.

You’d want basically the biggest Star motor in OrbitalATK’s catalogue. It, that’d be one of the cheapest options.

The step beyond that is a departure stage assembled in orbit (or brought up in a BFS if it can fit). Consists of just a few elements:

* tanks
* BFS-compatible refueling connection
* one Raptor-vac, perhaps with oversize radiatively cooled nozzle extension

Fuel it up (perhaps in a highly elliptical orbit), stick an outer solar system probe on it, and send it on it's way.

Since high chamber pressure isn't all that necessary for vacuum engines, one could also imagine using a low pressure engine that is pressure fed at something like 50 psi so you don't need special tanks. Perhaps Raptor would be affordable enough that it wouldn't be worth bothering.

Offline groundbound

  • Full Member
  • *
  • Posts: 199
  • Liked: 158
  • Likes Given: 8
Re: BFR with expendable upper stage
« Reply #33 on: 02/13/2018 05:17 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?
An expendable kick stage for one-off high energy missions may make sense! But you wouldn’t want to do that from LEO. Instead, you’d want to send the BFS (with payload) to a high, elliptic orbit, then the kick stage would start it’s burn while already very close to escape velocity at perigee.

You’d want basically the biggest Star motor in OrbitalATK’s catalogue. It, that’d be one of the cheapest options.

The step beyond that is a departure stage assembled in orbit (or brought up in a BFS if it can fit). Consists of just a few elements:

* tanks
* BFS-compatible refueling connection
* one Raptor-vac, perhaps with oversize radiatively cooled nozzle extension

Fuel it up (perhaps in a highly elliptical orbit), stick an outer solar system probe on it, and send it on it's way.

Since high chamber pressure isn't all that necessary for vacuum engines, one could also imagine using a low pressure engine that is pressure fed at something like 50 psi so you don't need special tanks. Perhaps Raptor would be affordable enough that it wouldn't be worth bothering.

I've actually wondered if someone might come up with a kick stage / tug hybrid that used methalox boost and solar electric return. Cycle time might be a year or more until it could get back to some earth orbit where BFS could refuel it, but that roughly matches the target market -- outer planet exploration missions.


Offline Ludus

  • Full Member
  • ****
  • Posts: 1071
  • Liked: 459
  • Likes Given: 228
Re: BFR with expendable upper stage
« Reply #34 on: 02/13/2018 06:40 PM »
But then you end up with this huge expensive BFS on Mars surface, which you need to refuel to send back. The whole idea of reusing the BFS for Mars relies on huge refuelling infrastructure on Mars. If this ends up being uneconomic, then the benefit of BFS for Mars goes away (still useful for LEO tho). Also, even with successful reuse, you’re still looking at the expense of setting up the fuel manufacturing on Mars in the first place. You’re also taking the productive asset that is BFS and tying it up with incredibly infrequent Mars missions (one launch every 2.5 years). It might be better to use that asset for LEO work, where it could be flown hundreds of times per year. There’s always a trade off. I’m suggesting it might make sense to go the expendable route for missions beyond LEO.

If you like the frugality of using the BFS hundreds of times near earth, do that first. Once you’ve amortized and depreciated your BFS so it has little book value, send it to Mars and treat it as expendable.

Online TrevorMonty

Re: BFR with expendable upper stage
« Reply #35 on: 02/13/2018 07:26 PM »
An expendable 3rd stage would be better option for BFR. This could be fully fuelled 100t 1xRaptor stage + payload, delivered to LEO inside BFR 2nd stage. Shouldn't cost much more than F9 2nd stage, ideally have life of few days so it can deliver payloads direct to anywhere in cis lunar space.

Why initially expendable could turn into reuseable OTV that returns to LEO for BFR delivered payload and fuel. Much like what ULA plan to do with ACES.

The long life and refuelling capabilities this OTV needs are also required by BFR, so good test bed.

In expendable mode can deliver about 40t to  LLO or 30t when reusable. 30t to LLO in single launch using fully reuseable vehicles    would make for cheap lunar missions.
« Last Edit: 02/13/2018 07:37 PM by TrevorMonty »

Offline spacenut

  • Senior Member
  • *****
  • Posts: 2378
  • East Alabama
  • Liked: 397
  • Likes Given: 231
Re: BFR with expendable upper stage
« Reply #36 on: 02/13/2018 09:02 PM »
Musk mentioned it would take 6 tanker launches to completely fill the BFS or second stage for transport to Mars.  He is also sending two cargo ships to Mars 2 years prior to the human trip.  These would have the equipment to make Methane and LOX for refueling the human lander for return to earth.  He also said for a base, it would probably take 10 cargo flights to Mars with all the equipment and food for one human ship.  He also said the trip will be during the time when Mars is closest to earth and only require about a 2 month trip, so no real worries about zero g for humans for two months.  The stay on Mars would be about 2 years for the crew, to then return or continue to stay longer, eventually setting up the colony.  SEP tugs could also bring cargo via slow route on a continuous basis. 

Offline AC in NC

  • Full Member
  • ***
  • Posts: 361
  • Raleigh NC
  • Liked: 283
  • Likes Given: 335
Re: BFR with expendable upper stage
« Reply #37 on: 02/13/2018 09:44 PM »
This is Plan B:

The Hunt for Red October (1990)
Quotes
Captain Ramius: When he reached the New World, Cortez burned his ships. As a result his men were well motivated.

Offline AC in NC

  • Full Member
  • ***
  • Posts: 361
  • Raleigh NC
  • Liked: 283
  • Likes Given: 335
Re: BFR with expendable upper stage
« Reply #38 on: 02/13/2018 09:51 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Why?

Offline groundbound

  • Full Member
  • *
  • Posts: 199
  • Liked: 158
  • Likes Given: 8
Re: BFR with expendable upper stage
« Reply #39 on: 02/13/2018 10:32 PM »
Ah okay. Well how about for a one-off exploration probe to say Titan, for example? How much extra performance could you get out of an expendable upper stage?

Why?

Because BFS has a dry mass that cripples it for launching things to extreme energy trajectories. I buy that it is useful to Mars and maybe a little beyond. More than that and the final boost stage needs to go on a diet.

Tags: