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Comment by jabl | Hacker News Reader

This is a classic. Although it's a subject with absolutely no relevance to the vast majority of readers, it's written in a funny dry style (for a good example, see e.g. the quote posted in another comment).

It's also interesting to reflect on rocket fuels since the publication of the book.

- Military rockets have mostly shifted from hypergolics to solid fuels.

- Orbital rockets mostly use LOX + suitable fuel. For the fuel, seems to be good arguments for and against everything from hydrogen to heavier hydrocarbons like kerosene.

- Exotics (flourine, boron, etc.) have disappeared.

- For hypergolics, the remaining use is mostly RCS's, deep space stuff, but not "main" propulsion. And the oxidizer is mostly NTO rather than acid that seemed to be Clark's favorite.

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India and China still make use of hypergolics for their main launch vehicles, though both are moving away from that.

Regarding hypergols, I love the fact that you are absolutely correct that they’re mostly relegated to places where you _need_ autoignition these days, but then you look at the Proton rocket family...

https://en.wikipedia.org/wiki/Proton_(rocket_family)

I should have made it clearer that I was writing about trends, not absolutes.

So yes, hypergolic powered rockets are still around, but mostly they are old designs on the way out. The Proton family you mention was on the drawing board before the book was published, and is to be replaced by the kerolox powered Angara family.

Similarly, the Chinese hypergolic Long March 2/3 families are old designs in the process of being replaced by the kerolox powered Long March 5/6/7 families.

Ha, TIL!

I’m not too familiar with the Long March family but that does make sense.

More random engine nerdery:

- It’s really exciting to see all of the development going into full-flow combustion metholox engines these days; BE-4 and Raptor have been incredible projects to follow.

- The Rocketdyne F-1 is still the most impressive rocket engine to me, although the RS-25 is a very close second. Watching videos of the Apollo launches and seeing those things in action is absolutely awe-inspiring.

Yes, the staged combustion methalox engines under development are interesting.

Though there's value in keeping it simple too. Gas generator engines got us to the moon and back.

And if the goal is routine travel to Mars, I think we really need nuclear propulsion.

And yeah, F1 is really impressive. Heck, the entire Apollo program was an astounding achievement in so many ways.

Though the geek in me is disappointed that kerolox engines produce so much soot that you can't see the Mach diamonds..

If it ain't broke, don't fix it. Titan IIs are no longer around, but they too were used into the 21st century.

Not broken, but better choices are available.

Many early space rockets were outgrowths of military ballistic missile programs, where hypergolics were favoured due to storability, as being able to launch on a moment's notice was critical. But for launching satellites or humans into orbit, that's not a particularly relevant consideration.

On the downside, hypergolics are expensive, and their toxicity makes handling them more expensive still. Meanwhile, cryogenic technology has developed and is today affordable and reliable. Heck, even amateur efforts like Copenhagen Suborbitals are using LOX.

Unsymmetrical dimethylhydrazine is still used for small thrusters.

I think that goes under the heading of RCS that I mentioned (https://en.wikipedia.org/wiki/Reaction_control_system ).

Though, to be fair, I didn't mention monopropellant thrusters, which often use UDMH, like you said.

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