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As I stated in the answer to another question, LH2 suffers some serious drawbacks compared to other fuels.

Off the top of my head, they are

  • Extremely low density, resulting in:

    • Lower mass-fraction because of high tank mass
    • High aerodynamic drag due to tank volume
    • Larger and heavier vehicle structure due to tank volume

  • Boiloff and tank seepage, resulting in:

    • Even higher tank mass because of the need for insulation
    • More complicated tank arrangements because of the inability to butt LH2 tanks against relatively warm cryopropellants like LOX.
    • Long-term storage issues from propellant boiloff

The only possible advantages I can think of are

  • Marginally higher specific impulse
  • Producing only water as a byproduct, keeping environmentalists happy.

In light of all this, what rationale is there for a LH2/LOX Rocket?

UIDAlexD
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  • LH2 is extremely cold, a lot of materials become very brittle at those temperatures. Construction of valves and hoses are difficult.

    LH2 can't build deposits in the cooling channels of the rocket engine, it can't polymerize or even carbonize like kerosene.

     – Uwe Oct 21 '16 at 11:35
  • Con: GH2 can embrittle metals causing them to lose strength over time. Pro: H2/O2 combustion is a very quick and simple chemical process (very much unlike hydrocarbon combustion). It is usually very stable. Pro: The specific impulse is not marginally higher, it's a lot higher, and in the end that's what counts. – Rikki-Tikki-Tavi Jan 20 '17 at 12:49

2 Answers2

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It's not a "marginally higher" specific impulse. High performance hydrogen engines typically have a vacuum Isp of around 420-450 seconds, compared to 310-350 for hypergolics or kerosene. That's about 30% delta v advantage, ton for ton, which more than offsets the structural volume penalty.

The drag penalty is mostly irrelevant for upper stages as long as their diameter is no larger than lower stages.

Another drawback to hydrogen is that low propellant density yields lower thrust from a similar chamber size; again not a big problem for upper stages.

The clean exhaust is nice, but not a huge advantage over kerosene. It is much more attractive than toxic hypergolics, of course.

Consider two 22-ton upper stages each pushing a 5 ton payload.

  • Stage H is 20 tons hydrogen-LOX, 2 tons dry mass, 450s Isp.
  • Stage K is 21 tons kerosene-LOX, 1 ton dry mass, 350s Isp.

H delivers 5956 m/s of delta-v versus K's 5162 m/s -- a 15% improvement despite hauling twice the dry structural mass.

Russell Borogove
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  • It's worth pointing out that one of the drivers in fuel selection for optimal Isp is to have the exhaust gases be as light as possible, as the maximum exhaust velocity is inversely proportional to the square root of molecular weight. A hydrogen-based engine will have almost exclusively water as its exhaust gas. Long chain hydrocarbons like kerosene will have (in addition to the water) carbon dioxide, which is much heavier, as well as other constituents due to incomplete combustion, like carbon monoxide (which is actually better than CO2) and residual unburned hydrocarbons (worse). – Tristan Jul 29 '16 at 15:24
  • Yeah, that's a difference. I can see how it makes sense for upper stages now. What about orbital operations? I've been lead to believe the boiloff is considerable, but is it really that bad? – UIDAlexD Jul 29 '16 at 15:43
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    It depends on the timeframe under consideration. Hydrogen boil off rate for a stage like Centaur is about 4% per day, so it's not a major issue for typical LEO/GEO insertion operations, but a definite concern for lunar and a showstopper for interplanetary operations, which is why you see hypergolics used for orbital insertion on those kinds of missions. LOX boil off is about 2% per day, so kerolox has similar limitations. – Russell Borogove Jul 29 '16 at 15:53
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    Saturn/Apollo is a terrific example of the tradeoffs in fuel selection. Kerosene first stage to meet the colossal liftoff thrust requirement and keep the stage diameter somewhat sane; hydrogen second and third stages to make the delta-v needed for orbital and translunar injection; hypergolics on the CSM and LM to store fuel for a two-week mission and start reliably, repeatedly, over many short burns. – Russell Borogove Jul 29 '16 at 16:01
  • There any way to avoid or prolong boiloff? Nuclear Thermal Rockets look great for deep space propulsion but they only function with Hydrogen. – UIDAlexD Jul 29 '16 at 16:31
  • It's a tough problem. Shuttle-Centaur was going to have a thermodynamic vent system to mitigate the problem. Don't know if operational Centaurs ever used such a system. http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880006561.pdf – Organic Marble Jul 29 '16 at 16:35
  • ULA has a white paper on insulation based strategies, but I think you need active cooling to really make it work, which means a lot of power and a bulky/massive coolant loop. – Russell Borogove Jul 29 '16 at 16:35
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    Does hydrolox = LOX/LH2? – uhoh Aug 03 '16 at 12:02
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    Yes. Kerolox = kerosene/LOX, methalox = methane/LOX. In casual rocket-engine-discussion use, these are synonymous with the name of the fuel by itself (because other oxidizers are rarely used with those fuels). – Russell Borogove Aug 03 '16 at 14:41
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    @UIDAlexD: Thermal nuclear rockets may work on pretty much damn everything as long as its boiling temperature is less than the chamber melting temperature. They are way more efficient on hydrogen than anything else, but they won't suffer too badly if other, not too heavy elements are introduced into the mix. – SF. Jan 19 '17 at 12:54
  • @RussellBorogove: IIRC: "if you isolate a house with aerogel and keep a lit candle in one of the rooms, in a few days living inside will be impossible due to the heat from the candle". It has truly miraculous thermal properties, but the manufacture process currently is too expensive for stuff like isolation on a non-reusable 10-ton tank of LH2. Although it seems such tank, or at least its isolation wouldn't have much problems landing in one piece on Earth. It's ludicruously light so it should brake to sane speeds long before entering dense atmosphere. – SF. Jan 19 '17 at 12:58
  • Water is not a good choice for thermal nuclear rockets. If very hot metals get in contact with water vapour they react with the oxygen of the water and leave hydrogen. – Uwe Jan 19 '17 at 14:39
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Another advantage (relating to efficiency) is that it's easier to build a staged combustion rocket engine using LOX/LH2. Specifically, you can use LH2 for a fuel-rich staged combustion engine, like the Space Shuttle main engines (and the Energia main engines). Most other common rocket fuels don't work for fuel-rich combustion, I believe due to the risk of coking. Oxidizer-rich cycles don't have that problem, but require very advanced metallurgy to safely handle the (extremely corrosive) hot oxidizer. Russia / the USSR figured out how to do that in the 60s, but the USA never built a working oxygen-rich (or full-flow) staged combustion engine until the 2000s.

In the end, efficiency is efficiency. Even if, hypothetically, some new propellants could get 600s ISP with a good TWR but could only be used in a gas generator cycle (or something similarly wasteful), it would still be more efficient than the best staged combustion engines available today. With the currently available propellants, though, the fact that LH2 makes it easier to build higher-efficiency rockets (not just because of the inherent suitability of the propellants for high ISP) is - or at least was - significant. (It still is for rockets that need maximum efficiency, but some focus on other desiderata now, like storage, transportation, cost, in-situ generation off earth, etc.)

CBHacking
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    fuel-rich combustion with methane seems like it has some advantages over the alternatives. –  Jun 17 '18 at 18:13
  • What propellants are "new" and could get more than 500 s of ISP? The only thing that I'm familiar with getting anywhere near that high are ultra-dangerous (often fluorine) propellants that were explored in the 60s but never actually used. – ikrase Jun 04 '22 at 07:58
  • @ikrase I'm not aware of any; that was a hypothetical example of why overall efficiency is all that matters and efficiency from any particular source only matters to the extent that it modifies the final number. Editing to make that clearer... – CBHacking Jun 04 '22 at 08:16