What is the reason behind America's switch to LH2/LOX, and (USSR's) Russia's decision to stay with it and develop it to spectacular technical sophistication such as the Oxygen-rich staged combustion technology used on RD-170 series? If LH2/LOX offers so clear an advantage in terms of $I_\mathrm{sp}$, then why did only Americans make the switch? The advantage of LH2/LOX seems to be partially offset by the complication of storing the deeply cryogenic LH2 and much heavier turbopump, which adds back some dead weight.
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3$\begingroup$ To be clear, the US never stopped working with kerosene--it just wasn't used on the main launch platform: the Space Shuttle. And since then it has reentered mainstream use with the Falcon rockets. $\endgroup$– called2voyage ♦May 10, 2019 at 20:40
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1$\begingroup$ Many rockets used kerosene in the first stages, particularly the Delta and Atlas families. NASA thought at first that the Space Shuttle would be taking everything into space, but after the Challenger disaster the expendable rockets made a comeback. So I think we can say that kerosene was always mainstream except for a brief hiatus in the Shuttle era. LH2/LOX has been long been popular in upper stages. $\endgroup$– GregMay 10, 2019 at 21:31
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1$\begingroup$ For the record @called2voyage, it has been in near continuous use in the US. Atlas has always used Kerosene. $\endgroup$– PearsonArtPhoto ♦May 10, 2019 at 21:45
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$\begingroup$ @PearsonArtPhoto Yeah, that's what I was referring to--thanks for clarifying. $\endgroup$– called2voyage ♦May 10, 2019 at 21:48
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$\begingroup$ @Greg Yeah, mainstream probably wasn't quite the right word perhaps "flagship" would have been a little more accurate. $\endgroup$– called2voyage ♦May 10, 2019 at 21:59
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1 Answer
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Each technology has their own strengths and difficulties. It's really difficult to explain this without going in to a great detail about how rocket engines work, but let me try and give a brief overview.
There are a number of different ways you can power a rocket engine. In general, for the same fuel and engine size, the higher the chamber pressure, the more thrust you get. To get the highest thrust you need to pump it. This is usually done with some degree of rocket fuel. One of the biggest problems facing high efficiency rocket engines is what do you do with the exhaust from this system.
The most efficient rocket engines will use the exhaust from the rocket as exhaust itself. Many less efficient ones will simply throw the exhaust away. The basic idea to using the fuel itself somehow is in only partially burning the fuel, either a fuel rich or an oxygen rich method.
The fuel rich method for kerosene leads to a lot of carbon, which can clog up the engines, and thus isn't desired. The way the US and Russia overcame this problem diverged down two very different paths.
The US path was to switch to hydrogen, which is more efficient in and of itself, and won't deposit any carbon. The Soviets figured out a way to go oxygen rich by inventing new alloys that could handle the very hot very oxygen rich combustion. Both of them have a lot of complexities, it really just came down to the path each decided to go down.
Lastly, which of them is fundamentally better? Theoretically kerosene is better for a lower stage, because it offers more thrust, and hydrogen/oxygen for the upper stage, because ISP matters more once you leave the gravity of the Earth.
BTW, the Everyday Astronaut has a video coming out about the strengths of the 3 main rocket fuels coming out soon, keep an eye out for that! The video will be more focused on Raptor, which uses Methane, but...
answered May 10, 2019 at 21:55
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2$\begingroup$ @PearsonArtPhoto I understand what you mean. $T=\dot{m}\cdot\Delta v$, LH2 is very good on the second term but only $7\%$ as dense as water, while kerosene is $80\%$ as dense as water. So a kerosene rocket will be less bulky but heavier than an LH2 first stage given that they lift the same upper stage flying the same mission. $\endgroup$ May 11, 2019 at 1:39