SpaceX MCT – Eliminating Helium from the Pressurization Purge Start Restart Sequence with Hydrogen and Methaneby Tommy on 24/10/2012
There has been a lot of discussion in the blogs and forums about the SpaceX second generation propulsion trades (MCT), but lost in the discussion is a comment from SpaceX management about the dire need to replace or eliminate helium from the purge and cooling cycles in the engine start restart sequences – which was clearly stated from the start.
Ideally any engine would want to utilize existing engine power and fluids wherever possible. An example would be the use of RP-1 in the hydraulic thrust vector control (TVC) system, or using hydrogen and oxygen propellants in the tank pressurization schemes (SSME). Additionally, any residual fuel would need to be retained for future scavenging or restarts rather than massive fuel dumps to purge and safe the system after shutdown.
The vaporization and mixing properties of both hydrogen and methane amply satisfy the criteria just set forth when applied to either methane and propane and their associated mixing agents – ethane and butane. I propose that thrust vector control systems will be electrically operated, with a subset of the engines in the engine cluster being fixed or piezoelectrically tweaked.
Remember, this is the vacuum of space we’re talking about here. He’s already got great RP-1 boosters, all he needs now is something that will reliably start and shut down in space, using and saving the fuel or fluids you already have and use – oxygen, hydrogen and methane.
I can’t see him replacing RP-1 for atmospheric boost if he wishes to attain the simplicity of airline like operations, but for deep space methane and hydrogen together can be very useful especially if you can start and restart a methane engine simply and reliably without wasting precious bodily fluids. And to make hydrogen you need a methane steam reformer anyways.
If you want to learn how to manufacture, store and utilize methane and hydrogen in space – Earth is where you start.
So this in turn brings up sunshades, insulation and boil off recycling issues, the behavior of volatile cryogenic liquids and gases in zero gravity and fuel settling thrusters – all non-trivial endeavors in rocket science. We certainly have the engines right now to get the job done (Falcon 9 Merlin 1Ds, RL-10s, J2s and SSMEs), the problem rather is how to get these things together so that it will work and perform something dramatic enough to get people interested.
Methane engines are a good ways off for now, but the SSMEs are right here and now. They are proposing to toss them away anyways, so I propose launching the SLS with as many Falcon 9 boosters as necessary (which means extra attachment points and a complete redesign of the launch vehicle) and fitting the core stage out with landing engines for direct lunar polar landing abilities, and forgetting about the upper stages for these things entirely, rather than waiting for the methane and hydrogen holy grail to appear. If they intend to throw all those engines and that great tankage away anyways, I would rather store them indefinitely in an area where they will stand as a monument to human ingenuity and do useful work as well. Towering above a deep dark cold crater in full view of the Earth and sun on the poles of the moon is the appropriate place to deposit this kind of hardware. And once those SSMEs are used up then the methane and hydrogen engines will soon follow.