Liquid hydrogen has about the cleanest exhaust imaginable.
I'm not sure I'd call it "rocket fuel" in the traditional sense, hydrazine is used (these days) as a thruster monopropellant when already out in space. It's pretty fascinating actually - a monopropellant doesn't combust with another fuel like you'd think, it gets passed through a solid lattice that creates a reaction within the monopropellant alone and expands, causing thrust. Since it's only used in space it dissipates and doesn't get in contact with anything it can be toxic to. However, this is usually why they tell you to stay away from crashed satellite bits if you happen to find them randomly, there's a chance they're contaminated with hydrazine.
It's hard to do small precise burns with H2 and O2 with a small thruster.
Hydrazine is one of the big reasons why when NASA or any other space agency has an accident they make a big deal about telling people not to touch any debris they find. Hydrazine is poisonous, mutagenic, tetragenic, carcinogenic, inflammable, and about the only thing it isn't is radioactive.
Thanks guys, I learned something. "less toxic" can also mean "cheaper to handle"
Straight hydrogen peroxide has been used as a monopropellant in atmosphere quite a bit - most famously in the Thiokol jump belt. The catalytic grid is silver.
The holy grail of propellants would run at ambient temperature (no heat signature), contain no metals (no radar signature), burn invisibly (no light signature at night or smoke trail in the day) and inaudibly (good luck with that one), and have no detectable odor or health effects. When I was in the rocket science racket we acknowledged all these academic points and moved on to maximizing specific impulse.
True, but only if you don't permit anyone to externalize the costs of poisoning the working class.
When a government spends tax dollars to protect large corporations from the consequences of their own actions, common sense just flies right out the window, and we all suffer.
This makes me think of John D Clark's book, "Ignition!" - a history of liquid fuel rocket propellants. It's been out of print for decades but is an entertaining read, and Mr. Clark even made the chemistry accessible for lay people like me.
Things the book pointed out were that multiple organizations tended to have the same ideas nearly simultaneously, that almost every possible branch of possible liquid rocket fuels have been explored, and that you generally have to give something up to get something else. I wish them luck, but I won't hold my breath.
The UK's space programme was built around hydrogen peroxide. Initially it was used to assist jet aircraft off of the ground by direct decomposition with a silver catalyst to produce high pressure steam and oxygen. When rockets were required, they used kerosene as a fuel. The hydrogen peroxide was decomposed with a catalyst and the exhaust was used to spin the pumps. The hot, oxygen rich exhaust was then combined with kerosene to drive the main engine.
If you look at the British rockets they were TINY and their exhaust is near invisible.
Sadly the programme was cancelled before the first satellite launch. The UK did launch one satellite (Prospero - still up there and still transmitting), but then everything was broken up and the expertise lost.
Liquid hydrogen has a huge drawback of being impossible to store, so it isn't much use for in-flight boosts. There are also huge engineering challenges associated with plumbing and pumping liquid hydrogen - it makes for very expensive rockets.
...if you completely discount the logistics train leading to the rocket engine.
And then, within the engine, pumping losses are huge. The drag penalty of the huge hydrogen tank and the mass penalty of its mondo insulation together nearly eliminate any advantage within the atmosphere; every attempt at LH2 propulsion within the atmosphere has, at best, lost market share. At worst, the boondoggle burnt up many millions in capital.
The logistics are why I said that it had the cleanest EXHAUST.
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