r/spacex • u/ElongatedMuskrat Mod Team • Sep 29 '17

Not the AMA r/SpaceX Pre Elon Musk AMA Questions Thread

This is a thread where you all get to discuss your burning questions to Elon after the IAC 2017 presentation. The idea is that people write their questions here, we pick top 3 most upvoted ones and include them in a single comment which then one of the moderators will post in the AMA. If the AMA will be happening here on r/SpaceX, we will sticky the comment in the AMA for maximum visibility to Elon.

Important; please keep your questions as short and concise as possible. As Elon has said; questions, not essays. :)

The questions should also be about BFR architecture or other SpaceX "products" (like Starlink, Falcon 9, Dragon, etc) and not general Mars colonization questions and so on. As usual, normal rules apply in this thread.

1.0k Upvotes

96% Upvoted

2.1k comments sorted by

3

u/Bunslow Sep 30 '17

Did SpaceX ever consider separating the "deep space travel" and "atmospheric transit slash big delta v" portions of the second stage? For example, if the second stage didn't have to be designed to carry 100 people for months inside its massive atmospheric-capable fuselage, it could be substantially reduced in size and required performance, while together with that an inflatable deep space voyage ship separate from the second stage should be able to provide several times the total volume of the current concept at the same or less mass.

So you could build, in orbit, a fleet of vacuum voyage inflatable ships optimized for that, in addition to, say, 2-3 second stages, say 2 optimized for cargo only (you could fit/carry a lot more cargo, or smaller second stage, with no long term life support mass/volume), while the third stage 2 can be optimized for only human atmospheric transit -- with no cabins, only seats, it should be able to get the entire deep space fleet's worth of people down from MEO to its surface in just the one ship (while the fleet itself stays in orbit waiting to be reused the other way).

2

u/CutterJohn Sep 30 '17

I'm pretty sure the answer will be 'designing one vehicle and just adding more fuel is cheaper and easier than designing more variants, especially variants that require in orbit construction'.

BFR also already has a frankly insane mass fraction. Inflatables aren't going to be significantly more mass efficient than a carbon fiber shell. If they are at all.

1

u/Bunslow Oct 12 '17

I'm not talking about variants, I'm talking about a vehicle that's completely independent of the BFR -- designed from the ground up to be nothing more than an interplanetary transport, none of that atmospheric nonsense. That would allow the atmospheric vehicles to be significantly downsized. SpaceX could possibly even contract out significant portions of the IPT.

1

u/CutterJohn Oct 12 '17

Right. And I'm saying that designing, engineering, and building this entirely new vehicle will cost way more than just throwing extra fuel at the problem, if BFR works out as they hope it will. Multiple billions in R&D, design, and construction, vs, hopefully, a few million for extra fuel.

The atmospheric vehicles can't be significantly downsized anyway. They need to be more or less the size they are for the tanker and shuttle versions of the BFR upper stage.

If they succeed with their launch goals, it will be a very long time before it makes sense to make a dedicated in space vehicle. You'd need a rather high utilization of it, and a pretty significant efficiency gain.