Highlights of Elon Musk’s Reddit Ask Me Anything Session

Elon Musk (Credit: SpaceX)
Elon Musk (Credit: SpaceX)

SpaceX CEO Elon Musk did an interactive Ask Me Anything Q&A last night on Reddit. Here are some excerpts from that session.

Q. Previously, you’ve stated that you estimate a 50% probability of success with the attempted landing on the automated spaceport drone ship tomorrow. Can you discuss the factors that were considered to make that estimation?

In addition, can you talk more about the grid fins that will be flying tomorrow? How do they compare to maneuvering with cold-gas thrusters?

Elon Musk: I pretty much made that up. I have no idea 🙂

The grid fins are super important for landing with precision. The aerodynamic forces are way too strong for the nitrogen thrusters. In particular, achieving pitch trim is hopeless. Our atmosphere is like molasses at Mach 4!

Q. In your recent MIT talk, you mentioned that you didn’t think 2nd stage recovery was possible for the Falcon 9. This is due to low fuel efficiency of kerosene fuel, and the high velocities needed for many payloads (high orbits like Geostationary orbit). However, you also said that full reusability would be possible for the Mars Colonial Transporter launch vehicle.

What have you learned from flights of Falcon 9 that taught you

a) that reuse of its second stage won’t be possible and

b) what you’ll need to do differently with MCT to reuse its second stage.

Elon Musk: Actually, we could make the 2nd stage of Falcon reusable and still have significant payload on Falcon Heavy, but I think our engineering resources are better spent moving on to the Mars system.

MCT will have meaningfully higher specific impulse engines: 380 vs 345 vac Isp. For those unfamiliar, in the rocket world, that is a super gigantic difference for stages of roughly equivalent mass ratio (mass full to mass empty).

Q. What kind of mass ratio do your upper stages have?

Elon Musk: With sub-cooled propellant, I think we can get the Falcon 9 upper stage mass ratio (excluding payload) to somewhere between 25 and 30. Another way of saying that is the upper stage would be close to 97% propellant by mass.

Q. Follow-up question: How much do you sleep per night, on average?

Elon Musk: I actually measured this with my phone! Almost exactly 6 hours on average.

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

Elon Musk: 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 🙂

Q. Hi Elon! I’m asking three questions on behalf of the nearly 20,000-strong fan community /r/SpaceX. We consider these the best questions we’d like you to answer for us (trust me, there were hundreds more), so a response to each would be much appreciated!

Falcon Heavy. Some have speculated that at stage separation the Falcon Heavy center core is too far downrange and travelling too fast to be feasibly returned to the launch site. Could you go into some detail on whether you plan to use barge landings permanently for this core, expend it depending on the mission, or take the payload loss and boost back to the launch site?

Mars. Could you please clarify what the Mars Colonial Transporter actually is? Is it a crew module like Dragon, a launch vehicle like Falcon, or a mix of both? Does it have inflatable components? Is MCT just a codename?

Spacesuits. How does SpaceX plan to address the limitations and contribute to the advancement of current spacesuit technology to best serve humans enroute and on the surface of Mars? You mentioned in 2013 that there’d be an update to SpaceX’s “spacesuit project” soon – how is it coming along?

Elon Musk: Yes, the Falcon Heavy center core is seriously hauling a** at stage separation. We can bring it back to the launch site, but the boost back penalty is significant. If we also have to the plane change for geo missions from Cape inclination (28.5 deg) to equatorial, then a downrange platform landing is needed.

The Mars transport system will be a completely new architecture. Am hoping to present that towards the end of this year. Good thing we didn’t do it sooner, as we have learned a huge amount from Falcon and Dragon.

Our spacesuit design is finally coming together and will also be unveiled later this year. We are putting a lot of effort into design esthetics, not just utility. It needs to both look like a 21st century spacesuit and work well. Really difficult to achieve both.

Q. Hello Elon, HUGE HUGE fan here!! Question about the Mars Colonial Transporter:

There has been a lot of speculation over comments about exactly how much mass you are hoping to send to the Martian surface with the MCT. Can you tell us how much cargo you would like to be able to land on Mars with MCT, not including the mass of the MCT itself?

Elon Musk: Goal is 100 metric tons of useful payload to the surface of Mars. This obviously requires a very big spaceship and booster system.

Q. Emily Shanklin indicated in late 2013 that the Raptor would be the first of a “family of engines” designed for the exploration and colonization of Mars. Could you elaborate on her wording, i.e. was she simply referring to a vacuum version and standard version, or do you plan on building multiple methane-based engines with significantly different thrust and size specifications?

Elon Musk: Default plan is to have a sea level and vacuum version of Raptor, much like Merlin. Since the booster and spaceship will both have multiple engines, we don’t have to have fundamentally different designs.

This plan might change.

Q. Design life of Merlin 1D has been mentioned to be 40 “cycles”. Could you expand on what a “cycle” is? Is it just a start of the engine?

Elon Musk: There is no meaningful limit. We would have to replace a few parts that experience thermal stress after 40 cycles, but the rest of the engine would be fine.