Hypergolic rocket engines typically use NTO and hydrazine based propellants.

Wikipedia page on Liquid Rocket Propellants shows N2O4 and N2H4 at 1.36 ratio to have 286s Isp at sea level.

My question is that can ratio of N2O4 be decreased so that it is used only for making sure the propellants burn and LOX be used as oxidizer instead? Will such an engine increase efficiency and will it be practical for launch vehicles which use hypergolic propellants like Proton and GSLV?

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    $\begingroup$ such a propellant combination could only be used in a tripropellant configuration as the NTO freezes at LOX temp. $\endgroup$
    – R. Hall
    May 27, 2021 at 3:41
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    $\begingroup$ The logical extension of this would be to just use the hypergolic propellants to ignite the engine, and switch entirely to non-hypergolic propellants afterward. This is in fact how the Merlin engines ignite, though they use a special ignition fuel (TEA-TEB) that is hypergolic with LOX rather than an oxidizer that's hypergolic with the fuel. $\endgroup$ May 27, 2021 at 4:04

1 Answer 1


I think you've mistaken why NTO and N2H4 are used. That is because they are stable at room temperature and easy to store.

While you could plausibly improve the energetics of the system with LOX, you have the problem that it's going to boil off over time. If you're in a position to handle cryogenics, you wouldn't use NTO etc anyway.

If you need storable, then if you end up messing around with cryogenics when you don't want to.

  • $\begingroup$ Yes, i know hypergolics are easy to store hence they are used. That is why my question specifically mentions usage of LOX in launch systems and not spacecraft. $\endgroup$
    – Ashvin
    May 28, 2021 at 6:20
  • $\begingroup$ In which case, my answer remains. Yes you improve the energetics of the system. You just wouldn't start from there? $\endgroup$ May 28, 2021 at 12:37

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