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For a smoothly burning liquid-vapor mixture, I understand that burning hydrogen and oxygen is the best possible solution, not just for now but even theoretically. (?)

But I read about ideas, that apparently go back decades, about detonating fission or even fusion nuclear bombs to generate thrust.

Why not start with nitrogen-based high-explosives, like HNIW or octonitrocubane?

Also, is it perhaps possible to create a volatile liquid out of HNIW or CL-20? Would it then, maybe, create more thrust per unit mass (or atom-to-atom) as a liquid-vapor fuel like in a 'regular' engine? (Despite what I assumed in the first paragraph...)

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The thing that matters for rockets is not how explosive something is, but how efficient it is in converting fuel to thrust. I don't know about all the values for the explosives you listed, but hydrogen and oxygen are very efficient, way more than alternatives.

Actually not THE most efficient, from wikipedia I found that a combination of hydrogen, lithium and fluorine is even better, just very hard to handle. What matters for efficiency is how much energy is released in the reaction (its caloric value), and how that energy results in thrust, where smaller/lighter reaction products are more effective in generating thrust.

On that score, we see hydrogen/oxygen scores high on caloric value (13 MJ/kg when you count hydrogen and oxygen), and it has a simple reaction product (water). Your example Octanitrocubane also has nice simple reaction products, but is no match in caloric value (just 8.5 MJ/kg)

This is, as high explosives are generally selected for having a lot of energy in a small volume (in contrast to mass), and by well, exploding well (its combustion front should move as fast as possible). While for rockets a smaller volume is nice, but energy by mass is far more important. And how fast something burns is irrelevant (as long as it burns fast enough so that it completely reacts before leaving the nozzle).

You're right that nuclear weapons were once considered as a means of propulsion (this was called "project Orion"), but nuclear weapons have one very big advantage: they release lots of energy for a small mass. Even when their energy-to-thrust conversion was not the best, the energy output was so huge, they were more efficient than anything else.

Basically, you can already see that in how nuclear yield is epxressed: in tons of TNT equivalent. While TNT may not be the most energetic explosive, most are in the same ballpark. Except for nuclear weapons. A normal bomb weighing 60,000 pounds might, when really pushing it, maybe create an explosion of 50 ton TNT-equivalent. Except the Tsar Bomba weighed the same, yet had a yield of 50 million tons of TNT-equivalent.

So once you find any epxlosive that creates more bang for the same weight than hydrogen/oxygen combination, we can start talking. But even then, it also needs to be practical. Most explosives are solids, so it will be hard to just explode parts of it at a time instead of in one big bang. Possible (for example in using pellets in sequence), but hard. And even the Orion project just remained a concept after all.

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Don't you think a rocket can prevent itself from being destroyed by a Super Powerful Thermonuclear Device (SPTD) or even explosives?

As you can read above there exists a chemical combination more powerful than the (separated mix of hydrogen and oxygen.

The truly MOST powerful way is making use of pure energy, i.e. photons. But that's something that can be developed far far away in the future.

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