Sadly, all the "good" (storable, hypergolic, and having good performance) rocket fuels and oxidizers -- Hydrazine, methylated hydrazine, nitrogen tetroxide, and fuming nitric acid -- are highly toxic.

The theoretical combustion products of these -- N2, H2, and H2O -- are pretty harmless. But incomplete combustion and the potential for leaks means that when spacecraft land, their crews (or recovery crews) often are limited in the ability to approach due to the risk of propellant fumes (as prominently seen when the first manned Dragon landed). And having hypergolic-fueled rockets fly on earth near personnel is right out.

Is there potentially a way to make combustion complete enough that this risk will be vastly mitigated?

  • $\begingroup$ There was a bit on this problem in Ignition!... Something along the lines that if you burn hydrogen and oxygen in stoichiometric ratio, you'd think your product will be H2O. In reality you'll be getting H2, O2, H+, O(2-), OH- H2O2, O3, a whole zoo of possible unstable particles and ions in the exhaust. $\endgroup$
    – SF.
    Dec 12, 2020 at 2:54
  • $\begingroup$ That's definitely true of the combustion at high temperature and pressure, but what about out of the nozzle when the exhaust has cooled down a bit? $\endgroup$
    – ikrase
    Dec 12, 2020 at 3:56
  • $\begingroup$ as water becomes the dominant species it's increasingly hard for the radicals to find something to bind to, Especially in vacuum. $\endgroup$
    – SF.
    Dec 12, 2020 at 9:08

1 Answer 1


The combustion needs some time and enough heat to be complete, at least 99 or 99.9 %. The high temperature should be given before the combustion of N2O4 and hydrazin is started and should be hold some time after the end of combusting the hyergolics. A turbulent flow and a high pressure may enhance a complete combustion.

So you need at hot hydrogen and oxygen flame with a long and hot exhaust pipe. When the combustion chamber and exhaust pipe is well preheated, injection of the hypergolics is started. After all the hydrazine is burnt, let the hydrogen and oxygen alone burn for some additional minutes.

But you need a water cooled chamber and exhaust pipe to resist the high temperatures. Too much water cooling is bad, the hot gases should stay hot for enough time.

So this method is applicable for a waste combustion plant on ground but not for a space capsule. But making combustion complete enough for a rocket engine is hard as shown above. Using a longer nozzle does not help at engine start and stop.

  • $\begingroup$ I think the question is asking about designing a Hydrazine/N2O4 rocket so that its engines burn the fuel all the way to nitrogen and water. Not disposing of waste fuel on the ground. $\endgroup$ Dec 10, 2020 at 11:32
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    $\begingroup$ @SteveLinton I think the answerer is explaining what is done on the ground for complete combustion, which illustrates why this would be difficult for a rocket. "A long and hot exhaust pipe" would be heavy and likely would seriously degrade performance. $\endgroup$ Dec 10, 2020 at 16:22

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