4 ms·

Cryogenic temperatures. They introduced a large number of additional costs. From boil off management systems to more expensive cryogenic pumps, to insulation ma

Cryogenic temperatures. They introduced a large number of additional costs. From boil off management systems to more expensive cryogenic pumps, to insulation materials and extra fuel to cover the fuel that boils off. Cryogenic fuel and oxidiser combinations are generally more powerful, but Litre for Litre, a cryogenic liquid costs more to get and keep around than a room temperature one, even one that needs to be kept at moderate pressure, cryogenic related costs didn’t get exceeded till you look at less common chemicals with higher bulk manufacturing costs. One big factor in favour of cryogenic fuel and oxidiser is they are much better engine and nozzle coolants in regeneratively cooled rocket engines. This is why they are so common. My analysis was based on Sea Dragon sized rockets with similarly simplistic engines not designed for reuse. The rapid reuse goals of SpaceX Starship pretty much demands regeneratively cooled rocket engines which are easiest with cryogenic at least a cryogenic oxidiser and once your working with one cryogenic liquid you may as well just go fully cryogenic with a cryogenic fuel to get even more cooling capacity.

panick21_ 5y ago
Makes sense. Nice analysis. SpaceX goes even further and does deep cryo going beyond what anybody else does. This additionally makes the fuel even denser so you can carry more fuel in the same rocket. They are the only company that has ever done that. Its huge win, for Ox its 10%, for RP-1 its about 2%, for Methane 7%. Quite amazing. Re-usability changes everything and going to Mars as a goal also changes the design.
- techdragon 5y ago
  Thanks! Your totally right about deep cryogenic subcooling/densification, it’s a technique no one else has really worked on in production systems meanwhile SpaceX has made it normal. Not only does it get you even more performance for a given unit of volume (because not only do you care about performance per unit weight you care about the density of the fuel as well) but it also gives you a better engine coolant because you can let the fuel and/or oxidiser soak more heat without running the risk of boiling your fuel in the tiny coolant pipes/channel wrapped around the engine and nozzle. This is a big win for engine life as a cool engine is an engine not trying to melt itself to bits from the inside. Your also right about their Mars goals too, when you factor in their Mars goals it’s the obvious fuel and oxidiser combination. While it’s a process that requires lots of energy, there are extremely well established chemical processes to go from CO2 and H2O to both oxygen and methane. It makes their choice of a methalox engine basically a no-brainer.
  - panick21_ 5y ago
    Lets hope it happens. I still think they will need nuclear reactors to produce the energy. Using solar seems kind of insane.
    - techdragon 5y ago
      As a general rule of thumb I think we need more adoption of nuclear technology to normalise the safe use of the technology, and nuclear would be a great fit for bases on the moon and on mars… But the problem for Elon is regulatory. Doing anything nuclear requires navigating significant regulation from start to finish. I wouldn’t even be surprised if the regulations would require SpaceX to continue filing paperwork on Earth for any humans near a reactor on Mars being exposed to more radiation from space than the reactor is emitting. This is the antithesis of how Elon likes to work. If he’s going to get people onto Mars they can’t go and die from air and the rockets can’t be trapped there and he’s going to want to be sure about that before he launches them. In order to do that he’s going to want to develop multiple iterations of the chemical plant equipment on earth, testing it’s robustness reliability and effectiveness. They know the solar panels will work fine, if they don’t user super expensive ones they’ll be fine using lower efficiency and just adding more square meters of solar panels… but they can’t do that with the oxygen and methane production equipment. They need to know it will work dusty, with perchlorate contamination, with variable electrical supply, and probably even survive a military style airdrop in terms of rough landing with zero impact on its ability to generate and liquefy CH4 and O2. He could split his resources and add the development of a similarly complex reactor (the majority of the complexity is likely to be regulatory since shipment to and operating on Mars rather constrains the design space for a practical reactor) or he can use heaps of reliable panels, trust he can get enough mass to the surface of mars and focus entirely on the development of the critical chemical process equipment he needs on Mars.

panick21_ 5y ago

Makes sense. Nice analysis. SpaceX goes even further and does deep cryo going beyond what anybody else does. This additionally makes the fuel even denser so you can carry more fuel in the same rocket. They are the only company that has ever done that. Its huge win, for Ox its 10%, for RP-1 its about 2%, for Methane 7%. Quite amazing. Re-usability changes everything and going to Mars as a goal also changes the design.

techdragon 5y ago
Thanks! Your totally right about deep cryogenic subcooling/densification, it’s a technique no one else has really worked on in production systems meanwhile SpaceX has made it normal. Not only does it get you even more performance for a given unit of volume (because not only do you care about performance per unit weight you care about the density of the fuel as well) but it also gives you a better engine coolant because you can let the fuel and/or oxidiser soak more heat without running the risk of boiling your fuel in the tiny coolant pipes/channel wrapped around the engine and nozzle. This is a big win for engine life as a cool engine is an engine not trying to melt itself to bits from the inside. Your also right about their Mars goals too, when you factor in their Mars goals it’s the obvious fuel and oxidiser combination. While it’s a process that requires lots of energy, there are extremely well established chemical processes to go from CO2 and H2O to both oxygen and methane. It makes their choice of a methalox engine basically a no-brainer.
- panick21_ 5y ago
  Lets hope it happens. I still think they will need nuclear reactors to produce the energy. Using solar seems kind of insane.
  - techdragon 5y ago
    As a general rule of thumb I think we need more adoption of nuclear technology to normalise the safe use of the technology, and nuclear would be a great fit for bases on the moon and on mars… But the problem for Elon is regulatory. Doing anything nuclear requires navigating significant regulation from start to finish. I wouldn’t even be surprised if the regulations would require SpaceX to continue filing paperwork on Earth for any humans near a reactor on Mars being exposed to more radiation from space than the reactor is emitting. This is the antithesis of how Elon likes to work. If he’s going to get people onto Mars they can’t go and die from air and the rockets can’t be trapped there and he’s going to want to be sure about that before he launches them. In order to do that he’s going to want to develop multiple iterations of the chemical plant equipment on earth, testing it’s robustness reliability and effectiveness. They know the solar panels will work fine, if they don’t user super expensive ones they’ll be fine using lower efficiency and just adding more square meters of solar panels… but they can’t do that with the oxygen and methane production equipment. They need to know it will work dusty, with perchlorate contamination, with variable electrical supply, and probably even survive a military style airdrop in terms of rough landing with zero impact on its ability to generate and liquefy CH4 and O2. He could split his resources and add the development of a similarly complex reactor (the majority of the complexity is likely to be regulatory since shipment to and operating on Mars rather constrains the design space for a practical reactor) or he can use heaps of reliable panels, trust he can get enough mass to the surface of mars and focus entirely on the development of the critical chemical process equipment he needs on Mars.