When combustion takes place in a chemical rocket (LOX/LH2), do the gases ionize due to high temperatures? If they do, how do they get around the problem?

Ionization is a waste of energy I believe, which could be used to increase thrust.


1 Answer 1


Yes, a proportion of the propellant products attain excited molecular states, ionize or dissociate and the fraction that doesn't recombine before leaving the nozzle, termed "frozen flow", results in a performance loss.

That's as far as I'll go as the topic of how to manage the problem is out of my field. You could try Rocket Propulsion Elements by Sutton for background and perhaps something like this if you want to get into it further.

  • 6
    $\begingroup$ Dissociation (H2 -> H +H) is not quite the same thing as ionization (H2-> H+ + H-) Actual ionization doesn't occur to any great extent in the absence of electric fields. Some dissociation is inevitable at these temperatures, but to first approximation can be neglected. Rocket engines aren't my specialty (I'm a chemical engineer) but I suspect there's very little that can actually be done about it other than controlling mixture ratio (and possibly the way the reactants mix.) High temps and pressures are used for good efficiency in the first place so reducing combustion temp is counterproductive $\endgroup$ Jan 3, 2016 at 19:22
  • $\begingroup$ I don't know what the proportion of excited states/ionisation/dissociation are in frozen flow. My understanding is that the end result is the same to the limited extent of reducing the kinetic beam power. $\endgroup$
    – Puffin
    Jan 4, 2016 at 13:34
  • $\begingroup$ Interestingly, regarding combustion and the beam temperature, other than delving into nozzle design one clear limitation does apply to augmented thermal rocket motors, such as resistojets and hydrazine arc-jets. The designer could in these cases add an arbitrarily large degree of electrical power but will find diminishing returns due to frozen flow considerations. $\endgroup$
    – Puffin
    Jan 4, 2016 at 13:37
  • $\begingroup$ @LevelRiverSt: Exhaust can be ionised without an external electric field if it's hot enough. That's what plasma is. $\endgroup$
    – Vikki
    May 3, 2018 at 22:31

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