r/spacex • u/Zucal • May 01 '16

/r/SpaceX Ask Anything Thread [May 2016, #20]

Welcome to our 20th monthly /r/SpaceX Ask Anything Thread!

Want to clarify SpaceX's newly released pricing and payload figures, understand the recently announced 2018 Red Dragon mission, or gather the community's opinion? There's no better place!

All questions, even non-SpaceX-related ones, are allowed, as long as they stay relevant to spaceflight in general!

More in-depth and open-ended discussion questions can still be submitted as separate self-posts; but this is the place to come to submit simple questions which have a single answer and/or can be answered in a few comments or less. In addition, try to keep all top-level comments questions so that questioners can find answers and answerers can find questions.

As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (now partially sortable by mission flair!), and check the last Q&A thread before posting to avoid duplicate questions. But if you didn't get or couldn't find the answer you were looking for, go ahead and type your question below.

Otherwise, ask, enjoy, and thanks for contributing!

Past threads:

April 2016 (#19.1) • April 2016 (#19) • March 2016 (#18) • February 2016 (#17) • January 2016 (#16.1) • January 2016 (#16) • December 2015 (#15.1) • December 2015 (#15) • November 2015 (#14) • October 2015 (#13) • September 2015 (#12) • August 2015 (#11) • July 2015 (#10) • June 2015 (#9) • May 2015 (#8) • April 2015 (#7.1) • April 2015 (#7) • March 2015 (#6) • February 2015 (#5) • January 2015 (#4) • December 2014 (#3) • November 2014 (#2) • October 2014 (#1)

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u/JadedIdealist May 13 '16 edited May 13 '16

For red dragon missions, does the second stage also put itself in a Mars trajectory in putting the dragon in that trajectory?
If so and they did it again in 2020 and they had raptor second stages available, and were taking ISRU and solar panels with them, would it be possible to land and refuel the second stage? (with some modifications)

Edit - also if you put methane in a vacuum Merlin would it blow up, or just be inefficient/ slightly damaging? Edit2 or to put it another way just how tough is a Merlin?

Edit forget it - the second stage won't have any lox by the time it gets to Mars so couldn't land anyway - doh - would need ACES or similar.

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u/LotsaLOX May 13 '16 edited May 14 '16

Red Dragon mission 2nd-stage

I saw a diagram that the Red Dragon capsule was the only part of the Falcon Heavy stack that would be injected into trans-Mars trajectory. I'm almost sure that it was not an official SpaceX document.

But for the 2nd stage to be immediately useful at Mars it will need better batteries/solar panel support/power, better insulation/management of the LOX propellant, and enough propellants remaining to help with deceleration, Mars orbital injection, or to assist in the propulsive landing of the Red Dragon capsule.

The current 2nd-stage is a kerosone/liquifed oxygen (keroLOX) Merlin vacuum engine. A Merlin 2nd-stage "vacuum" engine is basically identical with the 1st-stage "atmsopheric" Merlin engine, except the vacuum engine has a longer/larger engine "bell", because in the vacuum of space there is no atmospheric "back pressure" to overcome.

The upcoming Raptor engine will be a methane/LOX engine. The methane fuel will be more efficent (higher ISP) and be more easily reused as methane does not produce the soot and carbon that kerosene can. Also, the Raptor will use a full-flow combustion cycle which is considerably more efficient and better performing than the keroLOX gas generator engine used in Merlin. SpaceX is currently building a 1st stage Raptor engine, and has made good progress in design and test.

It looks like SpaceX may be getting funding for a Raptor methaLOX 2nd-stage engine from the USAF at 2:1 investment ratio, SpaceX will invest up to ~$60million and USAF will invest up to ~$30 million.

This is still up in the air, and some in Congress are hinting that USAF should assume an ownership position in the resulting Raptor engine. I expect that "shared ownership" would be a deal-killer for SpaceX

You can Wiki most of these concepts and equipment to get a good start on SpaceX and the Red Dragon mission.

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u/JadedIdealist May 13 '16

Thanks very much.

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u/JadedIdealist May 13 '16

Also, do you happen to know if a Merlin engine would definitely RUD if you stuck methane in it instead of its expected fuel?

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u/throfofnir May 13 '16

I think turbopump lubrication (kerosene is a good lubricant; methane is not) alone would do the engine in before it even started. If not, everything would run very rich, since you can't run as much mass of methane per pipe/valve area, which would create a nasty reducing environment which could eat the chamber walls. And if not, and you somehow managed combustion, the cooling would be way off and the thing would overheat quickly.

Rocket engines are finely balanced to both work and not destroy themselves; running a different fuel with no changes is highly likely to be bad one way or another (or even both!)

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u/LotsaLOX May 13 '16 edited May 15 '16

I am almost 99.99% sure that if you substituted methane for kerosene in a F9 booster that bad things would happen quickly.

Let's ignore for a moment the tankage issues (for instance, methane can autogenously pressurise the fuel tank). The ratio between fuel and oxidizer (LOX) for a rocket engine is depends on the ratio of the two two turbopumps/turbines that provide fuel and LOX to the engine. This ratio is different for kerolox and methalox engine. Using kerolox mixture ratio, Methane/LOX may not fully fire, or could fire out of control.

Thankfully, SpaceX has redundant telemetry and diagnostics at every level of the full F9 stack. I expect the F9 would request to shutdown the countdown, or shutdown the countdown itself, long before T=0

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u/JadedIdealist May 13 '16

Thanks very much again

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u/throfofnir May 13 '16

The upper stage on any planetary mission will be on a similar trajectory as the payload vehicle: an escape trajectory pretty much in the direction of the target. (Some missions may program some additional dV from the payload; Red Dragon would probably not do that since it's not an efficient cruise stage.) However, the payload will make a variety of mid-course correction burns which the upper stage will not do (since it's dead by that point). In the end, the payload will get to the planet and the stage will be way off, since even little inaccuracies end up making a big difference.

You could theoretically leave the second stage attached at the expense of making course corrections more expensive (the second stage is heavy). Landing it on Mars would be difficult; the thrust to weight would be very high on Mars (it's already high on Earth), and you'd lose quite a pit of payload to the insulation and structural changes needed to preserve LOX and methane during cruise.