Author Topic: BFR with expendable upper stage  (Read 3738 times)

Offline AC in NC

  • Full Member
  • **
  • Posts: 263
  • Raleigh NC
  • Liked: 214
  • Likes Given: 221
Re: BFR with expendable upper stage
« Reply #40 on: 02/13/2018 10:48 PM »
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.
I can grant that and will.  It's beyond my direct knowledge.  But I don't think that means anything close to what the OP is wedded to.

Online Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 9467
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 6230
  • Likes Given: 4168
Re: BFR with expendable upper stage
« Reply #41 on: 02/13/2018 11:21 PM »
This is one of those "I figured out something SpaceX didn't" threads in which OP takes on all comers and keeps mutating the idea, or the assumptions, to try to keep it alive.

Ultimately, boring.

Locked. Use the report to mod function. to appeal the decision, if you must.
"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 Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 9467
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 6230
  • Likes Given: 4168
Re: BFR with expendable upper stage
« Reply #42 on: 02/14/2018 02:43 PM »
Based on a request from a respected elder member of the forums, reopening, let's see how it goes.
"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 Robotbeat

  • Senior Member
  • *****
  • Posts: 27332
  • Minnesota
  • Liked: 7273
  • Likes Given: 4990
Re: BFR with expendable upper stage
« Reply #43 on: 02/14/2018 02:49 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.
I think I agree with this. Even with launch costs of like, say, $3-5m for BFR, for very high energy trajectories that need a lot of refueling, a $10m solid kick stage could be cheaper. Also, some trajectories would be super hard to recover the BFS from, and you don't want to expend a BFS or send it on a 5 year mission just for a small outer planets orbiter or something.
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

Online John Alan

  • Full Member
  • ****
  • Posts: 623
  • Central IL - USA - Earth
  • Liked: 307
  • Likes Given: 1477
Re: BFR with expendable upper stage
« Reply #44 on: 02/14/2018 03:51 PM »
My opinion...Once BFS comes on line...
The need for (like said above) a solid or IMHO Hypergolic (or hybrid with both working together) kick booster will better serve the needs of high energy missions...  ;)

Online Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 9467
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 6230
  • Likes Given: 4168
Re: BFR with expendable upper stage
« Reply #45 on: 02/14/2018 04:46 PM »
My view: The base BFS system can do a lot. It should (and will) be built first. As experience is gained, optimizations for certain types of missions will be found. These may be in the form of third stages carried in the cargo bay, or they may be in the form of on orbit vehicles like tugs, or in the form of specialty BFS (the tanker mk II is likely to be something weird looking, not sharing the OML of the rest of the BFS variants) . All of this specialization starts from the base BFS, not the other way round.

(the basic Model T was done first, and then Ford and others (aftermarket) elaborated it into trucks, buses, racing vehicles, the model TT heavy truck/bus chassis,  even tractors.. (that last was a rather poor idea but it WAS tried) )

Eventually we move from "missions" to "cargo runs" at least in the inner system, and missions don't always (or even usually) originate on earth, they start with cargo taken somewhere. Lewis and Clark didn't start from NYC.
« Last Edit: 02/14/2018 04:47 PM by Lar »
"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 AC in NC

  • Full Member
  • **
  • Posts: 263
  • Raleigh NC
  • Liked: 214
  • Likes Given: 221
Re: BFR with expendable upper stage
« Reply #46 on: 02/14/2018 05:40 PM »
It seems to me that many alternatives and requirements are being conflated here.  I would very much appreciate if we could differentiate what we are talking about.

BFR with ExUS to me says the part bolted on BFR.  I just don't ever see this being expendable.  I can't imagine the cost/benefit ever working out vs. working within the limitations of what you've already got with some other approach (refueling, on-orbit assembly, etc..).  There are just too many options with planned marginal launch cost.

BFS-Cargo taking up some payload with an expendable stage is a different matter.

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #47 on: 02/14/2018 05:59 PM »
What about for a BFS that has come to the end of its service life. Say itís done 4 round trips to Mars - the TPS has worn out. So itís stripped down and launched as an expendable...?

Offline Michael Bloxham

  • Regular
  • Full Member
  • ****
  • Posts: 632
  • Auckland, New Zealand
  • Liked: 2
  • Likes Given: 6
Re: BFR with expendable upper stage
« Reply #48 on: 02/14/2018 06:05 PM »
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.

Exactly! I could also imagine SpaceX wanting to retire their Block 1ís & 2ís this way, once they start flying Block 3ís & 4ís. Even 747ís retire eventually ;-)

Offline 1

  • Full Member
  • *
  • Posts: 120
  • El Segundo, CA
  • Liked: 243
  • Likes Given: 4
Re: BFR with expendable upper stage
« Reply #49 on: 02/14/2018 06:39 PM »
What about for a BFS that has come to the end of its service life. Say itís done 4 round trips to Mars - the TPS has worn out. So itís stripped down and launched as an expendable...?

You're probably better off landing it one last time and using it as a permanent habitat module on Mars; or even just for spare parts. I think one of Robobeat's previous points is worth exploring in greater detail.

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.

Let's look at a breakdown of a couple recent missions. From wiki:

Mars Science Laboratory:
 -Cruise Stage: 539kg
 -Curiosity Rover: 889 kg
 -Entry-Decent-Landing system: 2401 kg
 
 -Total mass: 3893 kg

Mars Exploration Rovers:
 -Instruments: 5 kg
 -Propellant: 50 kg
 -Heat Shield: 78 kg
 -Rover: 185 kg
 -Cruise Stage: 193 kg
 -Backshell / Parachute: 209 kg
 -Lander: 348 kg
 
 -Total Mass: 1,063 kg

Schiaparelli:
 -rear heat shield: 20 kg
 -hydrazine: 45 kg
 -main heat shield: 80 kg
 -lander: 280 kg
 
 -Total Mass: 577 kg


Even for a relatively "simple" missions, it's clear that incorporating EDL gear comes at great cost to your mission payload, for lack of a better phrase. At best, if you abandon BFS's built-in EDL capabilityes, one half if your total payload will be "useful". At worst, perhaps one fifth. Expending BFS would need to result in at least twice as much payload as a refeuled BFS. Likely better to use an end-of-life BFS as payload.
   

Online Norm38

  • Full Member
  • ****
  • Posts: 1073
  • Liked: 451
  • Likes Given: 617
Re: BFR with expendable upper stage
« Reply #50 on: 02/19/2018 02:39 AM »
One of the main concerns made by the OP on the first page is large scale fuel production on Mars.  But there's no avoiding that. ISRU is needed to get crew back. We certainly won't be landing return fuel with expendables.
So there's little point coming up with Plan Bs for that. There is no plan B to large scale ISRU.

Offline AC in NC

  • Full Member
  • **
  • Posts: 263
  • Raleigh NC
  • Liked: 214
  • Likes Given: 221
Re: BFR with expendable upper stage
« Reply #51 on: 02/19/2018 02:46 AM »
One of the main concerns made by the OP on the first page is large scale fuel production on Mars.  But there's no avoiding that. ISRU is needed to get crew back. We certainly won't be landing return fuel with expendables.
So there's little point coming up with Plan Bs for that. There is no plan B to large scale ISRU.

Pretty sure the OP wasn't interested in returning crew with that wide diameter low density lander.

Online Norm38

  • Full Member
  • ****
  • Posts: 1073
  • Liked: 451
  • Likes Given: 617
Re: BFR with expendable upper stage
« Reply #52 on: 02/19/2018 12:22 PM »
Is there a payload that needs that new lander?  Without crew, all we need for science is some orbiters and Curiosity sized rovers.

Also, I'm not worried about the first few BFS not coming back. It's not a waste, they're prototypes and SpaceX likes to iterate. The first few BFS build up the base. They're the ISRU plant, tanks, supplies, shelter. That's 4 ships right there that don't need return fuel, but do need to be the full configuration to test everything out. Using a new expendable lander configuration doesn't get us anywhere.
« Last Edit: 02/19/2018 02:21 PM by Norm38 »

Tags: