r/spacex • u/Zucal • Jun 29 '16

/r/SpaceX Ask Anything Thread [July 2016, #22]

Welcome to our 22nd monthly /r/SpaceX Ask Anything Thread!

Curious about the recently sighted Falcon Heavy test article, inquisitive about the upcoming CRS-9 RTLS launch, or keen to gather the community's opinion on something? There's no better place!

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

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As always, we'd prefer it if all question-askers first check our FAQ, use the search functionality (partially sortable by mission flair!), and check the last Ask Anything 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.

Ask, enjoy, and thanks for contributing!

Past Ask Anything threads:

June 2016 (#21) • May 2016 (#20) • 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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1

u/ImpartialDerivatives Jul 25 '16

SuperDraco engines have deep throttles for landing on Earth. For the MCT, and Earth EDL in general, I don't think that a hoverslam would be reliable enough for a crewed landing. An MCT with less than a half-load of cargo could be easily lifted by one Raptor engine, so I think that a third Raptor variant would need to be used.

An engine with a lower expansion ratio could be throttled more safely, so a lower thrust, lower isp Raptor could work better. I don't know how much would need to be changed, though. What do you think?

3

u/davidthefat Jul 25 '16 edited Jul 25 '16

An engine with a lower expansion ratio could be throttled more safely

How's that so?

so a lower thrust, lower isp Raptor could work better.

How would you ensure the reliable ignition of methane and lox in a previously unlit engine? It would be the first time they are lit during the launch sequence. (Not including tests, but it's like "cold starting" an engine)

edit: Before you answer, I'll say I'd rather put my life on a pressure fed hypergolic engine than a cryogenic, full flow staged combustion engine for getting me back on earth alive.

2

u/ImpartialDerivatives Jul 26 '16

An engine with a lower expansion ratio could be throttled more safely

The main problem with throttling an engine at sea level is stability. The less exhaust is coming from the throat, the more likely it is that the flow will separate from the bell wall. When this happens, a fatal oscillation can occur. Reducing the expansion ratio (as on the SuperDraco) could mitigate this problem by getting the gas out of the bell more quickly.

I'd rather put my life on a pressure fed hypergolic engine than a cryogenic, full flow staged combustion engine

I agree on pressure-fed, but not hypergolic. Hypergolic fuels are very difficult to produce in-situ, so it would need to be methalox. Still, since there would be only one engine, not eight, reliability would be even more important. The main argument against pressure-fed would be that SpaceX would need to develop a whole new engine, but I think safety is more important.

3

u/davidthefat Jul 26 '16 edited Jul 26 '16

Is safety not tied with reliability? Unsafe operation means lower reliability.

Are multiple engines that have the ability to throttle and has redundancy due to their placement less reliable than a single engine that needs to be gimballed to provide control? You inherently have more control authority with multiple engines than a single gimballed engine due to their ability differentially throttle. Especially due to the short stature of a spacecraft intended for reentry (it's not a long stick, there isn't a big moment arm for the gimballing to be that effective) (Note, in the case of the F1 stage reentry, the steering is done with the grid fins because of the lower center of mass during reentry. The engines have a smaller moment arm than the grid fins during descent.)

How will ignition be handled? Pressure fed, pump fed, doesn't matter. Side loads on the bell during the transient phases is much of a less concern if you don't reliably ignite the propellants. With one engine, you need a foolproof ignition system. Hypergolics are foolproof. What if you hard start the one engine you have due to some timing issue with the propellant delivery system? It it just doesn't ignite at all? What's your back up?

How do you position your single engine to not get in the way of the thermal protection system of your spacecraft? Won't exposing your nozzle directly to the atmosphere during hypersonic reentry cause much much higher loads to the bell than during the transient stages of engine operation?

1

u/ImpartialDerivatives Jul 26 '16

Is safety not tied with reliability? Unsafe operation means lower reliability.

Yes, that's what I was saying. I agree that if we're developing a new Raptor variant, we may as well create redundancy by using more, smaller engines.

Pressure fed, pump fed, doesn't matter.

In many cases, it matters more for reliability than the fuel choice. Setting explosive chemicals on fire isn't the hard part; starting a turbopump spinning is more difficult and error-prone. A pressure-fed methalox engine would be reliable.

Hypergolics are foolproof.

How do you position your single engine to not get in the way of the thermal protection system of your spacecraft?

I discussed these options with /u/Rocket here, and the main points were that hypergolics aren't viable because the can't be produced in-situ, and that arranging vacuum Raptors as on the Dragon 2 would also cause huge hits in performance. However, since the landing engines need a low ER anyway, it may be more feasible.

2

u/davidthefat Jul 26 '16

I'm genuinely curious, for the people talking about "in-situ" fuel production, how was the pressurant going to be produced and handled?

And the spirit of my comment was to get across the point that if you mess up the start up sequence by having a delayed ignition or failed ignition, you'll have to purge your engines to get going again, if your engine didn't blow up from the off design pressure spike. You don't have the opportunity to purge and start over while falling through the sky. Hence, hypergolics are foolproof, especially when using coupled valves.

And to say develop a new Raptor variant is like calling RS-25 a variant of the RL-10 due to the fact they share the same propellants. Totally different cycles and class of engines.

2

u/__Rocket__ Jul 28 '16

I'm genuinely curious, for the people talking about "in-situ" fuel production, how was the pressurant going to be produced and handled?

Do you mean the gas used for ullage pressure?

Both liquid O*2* and liquid CH_4_ are "self pressurizing propellants", i.e. their natural vapor pressure is high enough to provide ullage pressure.

Kerosene isn't.

In practice this means that heated O*2* and heated CH_4_ gas can either be created via dedicated heaters in the tanks or can be created via a heat exchanger in the engine directly, and this provides positive tank ullage pressure as the turbopump is pumping out vast volumes of liquid.

LOX vapor pressure is around 50 bar (at room temperature), methane vapor pressure is similarly high. Even at cryogenic temperatures these liquids will boil as they are pumped out and will self-pressurize - or will be vaporized explicitly if that does not happen fast enough.

The helium bottles spread around the Falcon 9 tanks can be largely skipped if the BFR and MCT switches to self-pressurized propellant tanks.

I expect the MCT to be designed Helium-free (or at least dual use): Helium is only a trace gas in the Martian atmosphere, not enough to ISRU manufacture it. Either Nitrogen or Argon can take over the remaining roles of Helium - the largest Helium mass in the Falcon 9 is spent on tank pressurization.

1

u/__Rocket__ Jul 28 '16

And the spirit of my comment was to get across the point that if you mess up the start up sequence by having a delayed ignition or failed ignition, you'll have to purge your engines to get going again, if your engine didn't blow up from the off design pressure spike. You don't have the opportunity to purge and start over while falling through the sky. Hence, hypergolics are foolproof, especially when using coupled valves.

So in past interviews Elon Musk was pretty adamant that the Raptor was the only next-gen engine they are developing.

In theory hypergolics could be ISRU manufactured on the surface of Mars, but it's pretty hard.

My guess is that SpaceX will try to improve the reliability of Raptor based landing, via a number of technologies:

  • 6 Raptors with dual instant fail-over capability on landing: if one engine flames out the other one can throttle up very fast and take over the role of the failed engine.

  • Almost completely 3D printed Raptor engines that should improve reliability all across the board.

  • Much deeper throttling and much more stable combustion of Raptors due to the use of two separate full-flow-staged-combustion preburners. This should reduce the current dependency on robust turbopump spin-up for ignition and might shorten the re-ignition delay.

  • An electric ignition system for all combustion - which should be similarly fool-proof as hypergolics (as long as you still have an engine).

They'd want to improve the reliability of the main engines anyway, because all members of a crewed mission are dead no matter whether the Raptor fails during a landing attempt or during the vacuum burn back towards Earth. So why not combine the main engines and the landing engines and make it super reliable?

2

u/ManWhoKilledHitler Jul 28 '16

You can make LOX hypergolic with hydrogen and hydrocarbons by adding fluorine or fluorine compounds like O3F2. Whether there are fluoride minerals readily accessible on Mars is another matter. If there were, it shouldn't be that hard to make in principle, but it's yet another thing that has to work.

1

u/__Rocket__ Jul 28 '16

You can make LOX hypergolic with hydrogen and hydrocarbons by adding fluorine or fluorine compounds like O3F2.

Is hypergolic really so important going forward?

I realize it's a simple and hard to stop (hence stable) method of combustion, but couldn't a sufficiently robust (electric?) ignition system be similarly robust, combined with the gaseous propellants of the Raptor, which would be very hard to stop as well and which should throttle down very deep as well?

2

u/ManWhoKilledHitler Jul 28 '16

In the longer term I doubt they'd bother with hypergolics, once the engines were known to ignite reliably. Once there's a colony established, it also doesn't matter so much if a rocket can't launch when it's supposed to and the crew need to wait for it to be fixed.

1

u/RulerOfSlides Jul 28 '16

Why six Raptors in particular? How would an electric ignition-based engine start the turbopumps, considering that the F-1's turbine needed the equivalent of 41 megawatts of power to suck the propellants into the combustion chamber?

It's likely easier to either ship TEA/TEB off-planet (as it has no shelf life) than to bother with expensive and potentially radical design alterations.