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Never mind that this is the 18th question to have the tag.

To the question "Would the Dragon escape pod have survived this event?" asked about the AMOS-6 mission, Elon Musk tweet-responded:

yes. This seems instant from a human perspective, but it really a fast fire, not an explosion. Dragon would have been fine.

The speed of combustion was limited by the rate at which fresh oxygen and kerosene could mix and be heated. Compare that to what happens when high explosives detonate where the reaction travels through the material at the speed of sound, and no mixing is required.

So for the purposes of this question let's say that LOX/RP-1 rockets don't explode.

Question: Have there been tests of rockets that had the potential to detonate proper? Just for an illustrative example: oxidizer and fuel thoroughly mixed but kept so cold that nothing happens until some is warmed up in a combustion chamber?

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  • $\begingroup$ I think you need to disambiguate your terms a little bit more. Making a rocket explode is easy: just poke a hole in a tank. The line that I think you are drawing is between a deflagration and a detonation. And explosion can be a deflagration or a detonation, and it can even be neither of those things. $\endgroup$ – Jörg W Mittag Feb 12 at 12:23
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    $\begingroup$ In short: explosion = rapid increase in volume, deflagration = subsonic combustion, detonation = supersonic combustion (relative to the local speed of sound in the flame front). $\endgroup$ – Jörg W Mittag Feb 12 at 12:26
  • $\begingroup$ Thermobaric explosions are detonations. $\endgroup$ – Heopps Feb 12 at 12:43
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    $\begingroup$ @JörgWMittag That's why "detonate" already appears in the title and twice in the body of the question. and I've already addressed the speed of sound as well. Can you give it anothre read through and let me know if there are really any ambiguities there? And if not, consider deleting the comment? Thanks! $\endgroup$ – uhoh Feb 12 at 13:40
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    $\begingroup$ Will you count only successful system? John D. Clark's "Penelope" fuel was unsuccessful because it was such a good high explosive it couldn't be forced to burn normally. $\endgroup$ – SF. Feb 13 at 18:34
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As a rule of thumb, no rocket fuel (or oxidizer) will ever be able to detonate on its own. For the simple reason that the maximum flame front propagation through it needs to be slower than the fluid speed through the injectors, otherwise the flame would propagate back into the tanks. NO, no-one has ever managed to pump flammable fluids faster than their speed of sound.

As for the mixture? Absolutely. Even a simple beast such as the Proton rocket's mixture of unsymmetrical dimethyl hydrazine and nitrogen tetroxide will detonate, if you somehow achieve complete mixing before igniting it. This may be difficult though, as this combination is exceedingly hypergolic, and even frozen solid the materials will ignite each other.

Again, as a general rule, no. Rockets thrive on very rapid combustion, not supersonic detonations. The fuels are selected such that this is true.

P.S. Even liquid Kerosene and LOX will detonate. Quite violently, too! You just need to thoroughly mix them and emulsify the mix so the particle size is approaching zero. This just does not happen in any realistic accident or failure mode. It helps for safety that the kerosene freezes hard at a much higher temperature than the LOX boils, and liquid or solid kerosene and gaseous oxygen is very hard to mix.

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  • $\begingroup$ Thanks for your informative answer! As far as mixtures are concerned, I'm looking for a rocket where they would exist in a premixed state, perhaps at low temperature, (as an example), rather than a hypothetical situation where they are mixed. There is no likely candidate today, but in the early days of experimentation there may have been a search for combinations that could be premixed; it would be tempting as it would allow for some simplification of design. $\endgroup$ – uhoh Feb 13 at 1:05
  • $\begingroup$ @uhoh a premix, capable of detonation, would require that the motor somehow prevent the flamefront from running up the injectors throught he fuel pumps into the fuel tank. This would require pumping the fuel faster than the flame font can move. but being a detonation,t he flame front is supersonic. That is supersonic in a liquid, which means a couple km/s! no pump can manage that. $\endgroup$ – PcMan Feb 13 at 8:41
  • $\begingroup$ Yes it seems like a bad idea but that's not the question that I've asked here. It doesn't preclude the possibility that in the early days of experimentation there may have been some tests or experimentation. One can't say that nobody would ever try something 80 years ago based on more recent understanding. $\endgroup$ – uhoh Feb 13 at 9:42
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    $\begingroup$ @uhoh anyone that tried it, would have created a crater. They almost certainly did try it (although i do not recall anything from Ignition! discussing that). But with available technology, especially in early rocketry, any experiment using detonate-able premix fuel....would have detonated. In the fueltank. $\endgroup$ – PcMan Feb 13 at 9:48
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    $\begingroup$ Yep. but pumping pellets is a bit awkward. Still, it has merits. Taken to a (huge) extreme, you end up with an Orion drive. $\endgroup$ – PcMan Feb 13 at 14:00
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Some rockets (especially hybrids) use nitrous oxide oxidizer, which exothermically decomposes into nitrogen and oxygen and is capable of detonation. This is not just a theoretical hazard, Virgin Galactic experienced an explosion during a cold-flow injector test in 2007 that killed 3 people and injured 3 more.

Acetylene is typically handled as a solution in acetone due to it being prone to explosive decomposition in liquid form or as a high pressure gas, which can happen supersonically in a detonation. However, the DARPA ALASA program attempted to use a monopropellant consisting of a mixture of acetylene and nitrous oxide. The program was terminated in 2015 due to the propellant exploding in testing. The explosion of such a mixture seems likely to progress to detonation.

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I think the answer is possibly yes, but the place to look is probably solid-fueled rockets not liquid-fueled ones. I am not a chemist but I believe it is the case that ammonium nitrate is used as an oxidiser in some such propellants: Wikipedia also seems to think so and says such mixtures are often known as 'ANCP'.

Well, if you do the right thing to ammonium nitrate you can make it detonate, on its own (so it's a high explosive, not a low explosive in this case). I believe that 'the right thing' is rather hard to arrange, but you definitely do not want to be trying to break up large masses of it with dynamite however.

The right thing is certainly sufficiently hard to manage that I've seen pretty substantial barns full of sacks of ammonium nitrate fertiliser: no-one worried about it exploding (people nicking it from farms and using it as an oxidiser for IEDs is another matter: I think that still goes on, unfortunately).

So I think that it's at least reasonably possible that solid-fueled rockets which use ammonium nitrate as an oxidiser are potentially using high-explosive fuels, but ones which are so hard to detonate as not to be a risk in practice. However I think that whether the thing is a high explosive depends on the concentration of ammonium nitrate, and I'm not sure what that is in rocket fuels, but it may be below the critical threshold (misjudging what the threshold was seems to have been one of the mistakes made at Oppau).

(Incidentally the speed of a detonation isn't the speed of sound in the material: it's much higher than that.)

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