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Multiple rockets, such as the Atlas V or Saturn V, have used the combination of a kerosene/liquid oxygen (kerolox) lower stage for higher density and thrust within the atmosphere and a liquid hydrogen/oxygen (hydrolox) upper stage for better efficiency or specific impulse in a vacuum.

SpaceX has decided to avoid the use of hydrogen on their Falcon 9 rocket, which is said to be an "optimization for cost," as liquid hydrogen is very cold and therefore more expensive to handle. The Falcon 9 uses the same kerolox fuel and mostly the same engine on both stages to take advantage of commonality and mass production. The lower specific impulse however gives it a disadvantage if you are sending a probe on an interplanetary trajectory, but the reduction in systems, manufacturing, and operating costs is said to be worth it for most launches.

(Why does the Falcon 9 use RP-1/LOx and not LH2/LOx?)

SpaceX's Starship rocket currently in development plans to use methane on all stages as it can be stored for long periods of time (liquid hydrogen can boils off and leak out), can be made on Mars more easily, and is good for reusable engines as it produces less carbon deposits (coking) compared to kerolox and less hydrogen embrittlement compared to hydrolox.

However, most other launch service providers that have already developed hydrolox engines are still planning to use them, such as Blue Origin. If another company or space agency has already developed hydrolox engines, would a switch from hydrolox to methalox be a step backward since a new engine (which has less specific impulse compared to hydrolox) needs to be developed, or a step forward in terms of reducing overall cost in the long-term?

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  • $\begingroup$ The first stage of Blue Origin's New Glenn burns methane, only the second stage is hydrolox. This actually meant designing two new engines, since the BE-3U is actually a different engine from the BE-3...it doesn't even use the same combustion cycle. And what existing hydrolox engines do you have in mind that have a prayer of being competitive in the long run? Especially considering that Starship is a fully reusable system. $\endgroup$ – Christopher James Huff Feb 13 at 4:13
  • $\begingroup$ I already know that New Glenn has a methalox first stage. "And what existing hydrolox engines do you have in mind that have a prayer of being competitive in the long run?" That is basically in the same vein to the question I was asking. Should hydrolox be abandoned if you've already invested into it, or is the capability worth keeping? $\endgroup$ – user39270 Feb 13 at 8:37
  • $\begingroup$ Very interesting that hydrogen has a much higher flammability range than methane, possibly making it more suitable for reliable re-lights. But LNG is very inexpensive and becoming widely available. Gaseous (or liquid) H2 injection may be considered (for re-lights), but i would stay with "methalox". $\endgroup$ – Robert DiGiovanni Feb 13 at 13:49
  • $\begingroup$ @user39270 you cite Blue Origin as a company that already has hydrolox engines that they intend to use. They aren't planning to use the hydrolox engine they have (which is a sea-level engine with limited thrust), New Glenn uses all newly-developed engines. Anyone interested in cost reduction is going to need to do this, current hydrolox engines are eye-wateringly expensive. (\$100M/engine for the "cost-reduced" RS-25E, \$26M for the "low cost" RS-68, the RL-10 family of engines were known for their high cost even when nobody took launch costs seriously...) $\endgroup$ – Christopher James Huff Feb 13 at 14:29
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That is a complex question that depends on what the company's goals are among many other things. If the intention is to create a rocket with the highest specific impulse then hydrogen is the way to go. Although why that of itself would be the goal is unclear.

Hydrolox engines have a potential performance advantage in upper stages. However the benefits are not as big as might be thought. Liquid hydrogen is not only problematic due to ultra low cryogenic storage temperatures and embrittlement; it is also has ultra low density so requires a very large tank for storage which increases the mass of the tank. An issue only made worse by the need to use a fuel rich mixture (as in all rocket engines). It is also necessary to insulate the common bulk head to prevent lox from being frozen solid.

Most companies are ultimately about making money as their prime goal. But this is not true of SpaceX, their mission is to “make humanity a multi-planet species”. However fantastical this sounds it really is what SpaceX is about. For SpaceX money is just a vitally important means to an end not as an end in itself.

Making humanity a multi-planet species will require vast numbers of spacecraft carrying a vast tonnage of cargo and this will only be remotely feasible if those rockets are affordable. And the only way to do that is to make them reusable and to optimize for cost. That is why Starship uses full flow stage combustion engines burning methane.

Would it be a step backward for other companies to switch between a hydrolox engine they have already developed to a methalox engine? It depends on the company’s goals, how much they have already spent and how much more would need to be spent. But in my opinion in many cases it would not be a step backward.

The drive for maximum payload capability at all costs is no longer the only way forward and in some ways is a throwback. The advantage of hydrogen’s specific impulse and the extra thrust imparted by allowing whole rocket stages to crash into the ocean can be negated by using a larger methalox stage and orbital re-tanking. And beyond a certain point the question must be asked why do you need to put so much in orbit as one monolithic mass?

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