Reddit AMA — “Ask me anything about becoming a spacefaring civ” (r/spacex, 2016)

u/[deleted]: What equipment and procedures will be required for refueling operations on Mars? Will they be designed to function autonomously for the initial unmanned test flight? Also, are there any plans to introduce a third variant of the ITS with just a large shuttle-like payload bay to allow for transporting bulkier cargo?

Elon Musk: We are still far from figuring this out in detail, but the current plan is: 1. Send Dragon scouting missions, initially just to make sure we know how to land without adding a crater and then to figure out the best way to get water for the CH4/O2 Sabatier Reaction. 2. Heart of Gold spaceship flies to Mars loaded only with equipment to build the propellant plant. 3. First crewed mission with equipment to build rudimentary base and complete the propellant plant. 4. Try to double the number of flights with each Earth-Mars orbital rendezvous, which is every 26 months, until the city can grow by itself.

u/Ulysius: As a follow-up, considering the synodal reuse of the ITS spaceships, what form of permanent habitation do you foresee? Shipped modules or an (eventual) shift to in-situ resource utilization such as Martian regolith/plastic-reinforced concrete structures?

Elon Musk: Initially, glass panes with carbon fiber frames to build geodesic domes on the surface, plus a lot of miner/tunneling droids. With the latter, you can build out a huge amount of pressurized space for industrial operations and leave the glass domes for green living space.

u/MINDMOLESTER: Hi Elon, ITS question: What SpaceX technology/material still requires the most development for ITS to be a success? Thank you!

Elon Musk: It used to be developing a new metal alloy that is extremely resistant to oxidation for the hot oxygen-rich turbopump, which is operating at insane pressure to feed a 300 bar main chamber. Anything that can burn, will burn. We seem to have that under control, as the Raptor turbopump didn't show erosion in the test firings, but there is still room for optimization. Biggest question right now is sealing the carbon fiber tanks against cryo propellant with hot autogenous pressurization. The oxygen tank also has an oxidation risk problem as it is pressurized with pure, hot oxygen. Will almost certainly need to apply an inert layer of some kind. Hopefully, something that can be sprayed. If need be, will use thin sheets of invar welded together on the inside.

u/rohishimoto: In addition, what technology (if any) do you think you have nailed down/mastered at this point?

Elon Musk: Not sure that we've really mastered anything yet. Maybe starting engines...

u/Zucal: Hi Elon, and many thanks for doing this today! What level of completion is the interior habitable area layout of ITS at, and when might we expect to see renderings of it?

Elon Musk: I think we need a new name. ITS just isn't working. I'm using BFR and BFS for the rocket and spaceship, which is fine internally, but... Will aim to release details of the habitation section when we have actual live mockups. Maybe in a year or two.

u/[deleted]: [question deleted by its author]

Elon Musk: Probably just pack the pressurized space with cargo. Early missions will be heavily weighted towards cargo. First crewed mission would have about a dozen people, as the goal will be to build out and troubleshoot the propellant plant and Mars Base Alpha power system.

u/TheVehicleDestroyer: Hi Elon. I've got 3 questions on the ITS vehicle specs: Can you divulge what the Vacuum Thrust+Isp figures are for the Sea-Level Raptor variant? The ITS booster is able to hover. Will it ever use this capability to better ensure a successful landing at the expense of some small gravity losses, or is it hoverslams all the way? What is the expected maximum acceleration that the ITS booster can withstand during entry/landing? Thanks for everything.

Elon Musk: - Approx 360 sec vacuum Isp and 290 metric tons of thrust - A high acceleration landing is a lot more efficient, so there wouldn't be any hovering unless it encountered a problem or unexpected wind conditions. A rocket that lands slowly is wasting a lot of fuel. - Aiming for 20 g's

u/Tesla_X_City: If I recall correctly on one of the slides it mentioned that there it will be 4-6 G's upon reentry. It does not specify, however, whether that will be during the landing burn or aerobraking. It would be nice if that is clarified as well.

Elon Musk: The spaceship would be limited to around 5 g's nominal, but able to take peak loads 2 to 3 times higher without breaking up. Booster would be nominal of 20 and maybe 30 to 40 without breaking up.

u/termderd (Tim Dodd, Everyday Astronaut): We got a pretty good idea of what a Mars EDL looks like, but can you explain how the ITS and the Tanker plan to do an Earth EDL? Having talked with you at IAC about the Mars entry, we learned that there's very powerful thrusters that can handle attitude control. These work great for the Martian atmosphere, but what about on Earth? There doesn't appear to be grid fins and the thrusters obviously have less authority here on Earth, so what's the trick?

Elon Musk: Good question -- that wasn't shown at IAC. The spaceship and tanker would have split body flaps for pitch and roll. Probably just use the attitude control thrusters for yaw.

u/nalyd8991: I think one of the most shocking things in your ITS presentation for many people was the full size carbon fiber tank SpaceX built. Can you tell us a little bit more about the design, construction, and role of that particular test article?

Elon Musk: Yeah, for those that know their stuff, that was really the big news :) The flight tank will actually be slightly longer than the development tank shown, but the same diameter. That was built with latest and greatest carbon fiber prepreg. In theory, it should hold cryogenic propellant without leaking and without a sealing linker. Early tests are promising. Will take it up to 2/3 of burst pressure on an ocean barge in the coming weeks.

u/FoxhoundBat: Overall is the landing architecture of the ITS booster and the distances to be covered the same as Falcon 9's? Boostback, re-entry burn, landing burn? Could you give us nuggets on what changes the “final” Falcon 9 version (“v1.3”) you mentioned will have? Gwynne mentioned two weeks ago that F9 v1.2 will be reused only once or twice while “v1.3” should be reused up to 10 times. Can you talk about what the limiting factors for Falcon 9 reuse are?

Elon Musk: The big booster will have an easier time of things than Falcon, as the mass ratio of the stages is lower and it will have lower density. Net result is that it won't come in quite as hot and fast as Falcon, so Falcon should be a bounding case on the big booster. Final Falcon 9 has a lot of minor refinements that collectively are important, but uprated thrust and improved legs are the most significant. Actually, I think the F9 boosters could be used almost indefinitely, so long as there is scheduled maintenance and careful inspections. Falcon 9 Block 5 -- the final version in the series -- is the one that has the most performance and is designed for easy reuse, so it just makes sense to focus on that long term and retire the earlier versions. Block 5 starts production in about 3 months and initial flight is in 6 to 8 months, so there isn't much point in ground testing Block 3 or 4 much beyond a few reflights.

u/__Rocket__: ITS Spaceship design question: the ITS Spaceship has two mystical spherical tanks. The whole tank design looks very exciting, and there's rampant speculation on this sub about the purpose of those spherical tanks: are they for landing fuel? Or are they storing “hot” gaseous propellants as part of the autogenous propellant pressurization system? Or are they used for on-orbit propellant densification to store vapor before it's liquefied again? All of the above perhaps?

Elon Musk: Those are the header tanks that contain the landing propellant. They are separate in order to have greater insulation and minimize boil-off, avoid sloshing on entry and not have to press up the whole main tank.

u/_rocketboy: Also, why does the booster only have one header tank?

Elon Musk: The liquid oxygen transfer tube serves as the header tank for ox

u/__Rocket__: ITS Booster engine placement design question: the tight cluster of 42 engines of the ITS Booster (cool number!) has created speculation on this sub that maybe they are packed so tightly because that way there's a “virtual nozzle” or “virtual aerospike” effect they can take advantage of: they can have shorter nozzles while most of the exhaust momentum of the inner engines is still axial. Is there any truth to this speculation, or is the tight packing done purely to scale up liftoff thrust-to-weight?

Elon Musk: It had to be 42 for important scientific and fictional reasons! The dense packing is just to max out thrust to weight, but it would be cool if there was a virtual nozzle side effect.

u/__Rocket__: ITS Spaceship capabilities question: according to your IAC presentation the ITS Spaceship has a Δv budget of 7.5 km/s when returning from Mars to Earth, with 150 tons of payload. With a much smaller payload it has a Δv budget in excess of 9.0 km/s — which is amazing! Could this unprecedented amount of Δv be used to fly between Mars and Earth even outside the launch windows enforced by the synodic period, when payload mass is not a primary factor? It could be used for emergency purposes such as medical supplies/instruments and experts, or for other high priority but low mass cargo like critical replacements.

Elon Musk: yes