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Liquid rockets usually use a propellant pair of fuel and oxidizer. I've seen some rockets from 1960's that used LOX/UDMH as propellant pair but why they are not used any more although it is mentioned that LOX/UDMH has higher performance than N2O4/UDMH but N2O4/ UDMH is used abundantly.

If there are any rockets with LOX/UDMH kindly mention them.

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  • $\begingroup$ Was LOX/UDMH ever used in a staged combustion engine? $\endgroup$ – Organic Marble Aug 2 at 1:52
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N2O4/UDMH is the choice if you need a non cryogenic storable hypergolic fuel. But UDMH is toxic and carcinogenic and should be avoided if possible. UDMH is also corrosive. LOX/UDMH is not storable (cryogenic LOX) and not hypergolic.

There are a lot other fuels to be combined with LOX and they are not toxic or carcinogenic and thus easier to handle. There are combinations of LOX with higher performance than LOX/UDMH.

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    $\begingroup$ LOX/UDMH combines the worst of both worlds: the cooling infrastructure of cryogenic fuels, and the hazards of the storable hypergolics. $\endgroup$ – Dohn Joe Aug 1 at 8:48
  • $\begingroup$ @DohnJoe well said, the worst of both worlds! But there are a lot of combinations of cryogenic LOX with non cryogenic storable fuels. $\endgroup$ – Uwe Aug 1 at 8:53
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    $\begingroup$ LOX is stored below the freezing point of UDMH, which complicates your design. reddit.com/r/askscience/comments/4acvz2/… $\endgroup$ – Hobbes Aug 1 at 9:05
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    $\begingroup$ The bottom line is that there's always a Very Good Reason why They stopped using this or that high-performance fuel/oxidizer combo. Always. $\endgroup$ – RonJohn Aug 1 at 20:25
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The reason you go with UDMH/NTO F/O pair is storability, i.e. the ability of your rocket or missile to be stored, fully fueled straight from the factory, in a silo or stand on a launch rack for an extended amount of time, possibly years or even decades, and ready to use at a moment's notice, and spontaneous combustion upon contact with no ignition, meaning no fireplugs or glow wires or squib or matches needed, or deep space mission, e.g. main propulsion engine on the Cassini probe, where any cryogenic propellent eventually boils away. There's simply no other advantage to hypergolics if these two scenarios are not your concern. Going half-measure, either UDMH/LOX or kerosene/NTO defeats the only reason for choosing hypergolics. The weird combo you mentioned is mostly a product of its time when rocket propulsion engineers are making the switch from more primitive stuff like RFNA or H2O2 and trying out new combos. As soon as the UDMH/NTO pair was found and demonstrated reliability all other combos were simply dropped.

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  • $\begingroup$ RFNA-based oxidizers were not dropped. They were used in the military rockets because of the lower freezing point and in the derived launch systems $\endgroup$ – OON Sep 27 at 21:15

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