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Russia has two types of hypergolic liquid fuel ICBMs deployed, UR-100N and R-36M2 Vojewoda and they are working on the big Sarmat. There's in this category also the Chinese DF-A5. (A list of ICBMs here)

What are the main reasons for the US going for solid fuel ICBMs while Russia and China develop hypergolic liquid fuel ones? Why is hypergolic fuel prefered for ICBMs but not used for any new orbital launcher (except for India's GSLV)? Are there economic motives in the form of significant synergies between for example the UDMH/NTO Sarmat and the H+LOX Angara?

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It's not hypergolics per se that are super-desirable for ICBMs, but room-temperature-storable fuels.

ICBMs have to stay ready for long periods of time and be launched on short notice, so that means they have to stay fueled up more or less constantly. In practice, that means solid fuels or something in the UDMH/NTO family. A cryogenic fuel ICBM would need hours of lead time before a launch.

Solid fuels are less dangerous to store (and possibly less toxic to launch?) than hypergolic liquids; this may be why the US prefers solids, even though liquid fuels yield higher specific impulse.

I think ICBM vs orbital launcher development is generally going to tend to be divergent. ICBMs have an upper limit of practical required payload (like a handful of warheads under a ton each) and their payloads don't need to reach orbital velocity, so the design constraints are very different.

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In addition to what the previous answers are already saying, I would like to point out that recent developments have almost all been solid, even on the Russian and Chinese side. If you look at the DF-31, the Topol-M or Yars, you will see why military leaders like them: They are doomsday devices in the truest sense of the word. They can be deployed anywhere, even through rough terrain, kept there for years, and then fired within 10 minutes at the push of a button.

Not only is the propellant no immediate danger to the surroundings, they also don't need elaborate tubing to route the propellants, that eliminates problems that are known for hydrazine-based solutions, like the tendency of gasket materials to soak up hydrazine (or derivatives) and thereby gain volume and lose tensile strength. This could cause a rocket to suddently start leaking deadly propellant after quietly sitting somewhere for years.

As for synergy, I would say you're right on the money. France has long fought to replace the main engine of the Ariane 6 Launch vehicle with a solid rocket motor. This was in hopes that they could cut one of the two engine development programs that they finance. You see, for much the same reasons as the ICBMs discussed above France needs the solid fuels for it's SLBM arsenal.

For first stages this kind of makes sense. The specific impulse problem that jxexk talked about mostly rears its head in the final stage of a rocket. In the lower stages generating a lot of thrust is key, and solids are good at that.

However, they are expensive to make, and satellite providers are nervous about the additional vibrations that they introduce harming their precious devices.

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    $\begingroup$ You confirm my impression that solids are most practical for military purposes. But why then are the Russians developing the hypergolic Sarmat to be deployed around 2020? Because that's better fuel for an extra big one? Or building upon domestic legacy technology? The Russians have expertise about liquid rocket engines, but maybe they are bad at solid ones? $\endgroup$ – LocalFluff Jun 7 '15 at 18:41
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    $\begingroup$ @LocalFluff Sorry, I know nothing specific about Sarmat. Perhaps they anticipate that with a possible rocket shield they need a very large payload to carry enough individually targetable warheads to penetrate it. May also just be a political decision, to give the people working with storables something to do, in case they are needed in the future. I think the Russians have plenty of expertise with solids also. After all they make solid ICBMs. $\endgroup$ – Rikki-Tikki-Tavi Jun 8 '15 at 10:08
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    $\begingroup$ @LocalFluff Maybe they need additional delta-v to fly on a depressed trajectory. $\endgroup$ – Rikki-Tikki-Tavi Jun 8 '15 at 10:10
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LOX/LH2 motors can achieve a higher specific impulse than hypergolic motors. According to the liquid rocket propellant page on wikipedia, theoretically LOX/LH2 has 111 second lead over UDMH/NTO. This might not seem like much, but changes in the specific impulse affect the rocket's delta-v significantly according to the rocket equation, so any gain is a big deal.

However, LOX/LH2 is not used in ICBMs because cryogenic fuels are difficult to store and must be loaded directly before launch. Since ICBMs do not need to achieve orbital velocity and carry a fixed payload, efficiency can be sacrificed for robustness and storability.

For heavy orbital launchers, efficiency matters a lot. So designers will tend towards more efficient propulsion systems. This means that orbital launcher designs will diverge from ICBMs.

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  • $\begingroup$ Why couldn't you load the ICBM ahead of time and keep the contents liquid pre-launch with some sort of refrigeration mechanism? $\endgroup$ – Sean May 24 at 3:36

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