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Propyne is a light hydrocarbon which packs quite a lot of energy in a triple carbon bound. It poses no significant health or explosion hazard by itself, and some European studies have found it to be a good potential fuel.

It is an energetic, relatively dense fuel, though it burns rather hot. it also exists in equilibrium with propadylene, though I don't know what kind of impact that would have.

However, designing engines for a new fuel type can cause significant challenges, not all of them obvious. So let's assume that some company decides to follow the steps of Vector, but go for propyne instead of propylene. Leaving aside the production of the fuel, what would be the difficulties in designing such a new rocket?

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In general, all other things being the same, saturated hydrocarbons (i.e., alkanes) are probably going to do better than their equivalent double- or triple-bonded relatives.

Though the double and triple bonds do pack a lot of specific energy, there's more to rocket thrust than just energy input. Ultimately, you are trying to accelerate an exhaust gas as fast as you possibly can. By using thermodynamic processes, you are ultimately limited in how fast the exhaust products can actually expand. If you hold inlet temperature and pressure conditions constant, the exit velocity is inversely proportional to the square root of the molecular weight of the gas.

It's likely that, regardless of your propellant choice, your inlet temperature and pressure conditions are going to be constrained by other engineering considerations (e.g., not melting or rupturing your combustion chamber). In that case, you get the most bang for your buck by ensuring your exhaust products are as light as possible. Hydrocarbons burned with oxygen will give off, in order of increasing molecular weight, water, carbon monoxide, and carbon dioxide as primary exhaust species. Saturated hydrocarbons, having more hydrogen per unit mass, will give off more water than unsaturated hydrocarbons, making the exhaust gas comparably lighter and faster.

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  • $\begingroup$ How big a constraint is the inlet temperature in practice? This had crossed my mind, and it's clearly relevant for non-chemical thermal rockets (NERVA etc) which all use hydrogen as reaction mass for that reason, but it's not something people mention much in connection with choice of chemical propellants. $\endgroup$ – Steve Linton Oct 4 '18 at 15:40
  • $\begingroup$ @SteveLinton: Big. The V2 rocket had its fuel (ethanol) watered down to prevent melting the engine. Material engineering has progressed a lot since then, but most engines lean against that limit, and many run in non-stoichiometric ratio to reduce the temperature and pressure of combustion. And while there are solutions that allow running hotter, they all weight a bit and add complexity; extra costs that are rarely offset by the performance gain. People are no longer chasing squeezing extra seconds of specific impulse out of chemical engines - other priorities have taken precedence. $\endgroup$ – SF. Oct 5 '18 at 12:01
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In addition to Tristan's answer, there are a few other considerations:

  1. Propyne is not a natural material; it must be synthesized. In contrast, kerosene occurs naturally in crude oil, and hydrogen has many industrial processes to produce it cheaply. Therefore, propyne will be more difficult and expensive to produce than more commonly-used fuels.

  2. Propyne's boiling point is $-$23°C. It can be held as a liquid at 20°C under 5.2 atm pressure. This means it is a cryogenic fuel, although a lot easier to use than most cryogenic fuels.

  3. Carbon double-bonds and triple-bonds have a habit of interacting with other organic molecules. In fact, propyne is used to synthesize larger molecules. This means some of your fuel could self-polymerize while it is stored under pressure. (In fact, Teflon was discovered when a cylinder of compressed tetrafluoroethylene appeared to lose its pressure without losing weight. When they opened up the cylinder, it was filled with a white powder, which was the polymerized TFE.)

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