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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. $\endgroup$ 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
  • $\begingroup$ @Tristan, your answer is actually trying to explain that propyne is worst than RP-1 in terms of performance which is incorrect. Propyne is at high end between hydrocarbons when it comes to performance. So actually is incorrect. $\endgroup$
    – WOW 6EQUJ5
    Nov 11 '20 at 9:50
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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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In contrast to Tristan's answer, which claims that propyne performance is “probably” worse than saturated hydrocarbons currently used due to higher average mass of combustion gasses, propyne (methylacetylene) is actually superior in terms of Isp and according to this source is 4th performing fuel among all hydrocarbons, while propadiene (allene) being 2nd. There are many other factors such as temperature, specific heat capacity of exhaust gasses, O/F ratio etc. which influence specific impulse.

Propyne (methyacethylene) exists in equilibrium with propadiene (allene). Mixture is often abbreviated as MA-PD.

H3CC≡CH ⇌ H2C=C=CH2

Higher temperatures favor propadiene and its ratio increases in mixture.

All this being said, what are then challenges to it’s use in rocketry?

  1. High combustion temperature – Mixture is traditionally used in welding/cutting industry as a major constituent of MAPP gas (MethylAcetylene-Propadiene-Propane) because of high flame temperatures. In rocket engines temperature rises further due to high pressure and it would be more challenging to efficiently cool combustion chamber. Usual approach to reduce combustion temperature in chamber with fuel rich mixtures in case of MA-PD would create more soot than RP-1. Presence of solid particles in exhaust would ruin performance gains.

  2. Explosion hazard – Both propyne and propadiene have high enthalpies of formation, 185,67 KJ/mol and 190.06 KJ/mol respectively. They are capable of detonation or explosive decomposition or explosive reaction but require a strong initiating source or must be heated under confinement before initiation. To mitigate explosion hazard in large-scale industry MA-PD mixture is diluted with propane (which has high negative enthalpy of formation) and this mixture is considered stabilized. Similar approach in rocketry would waste performance gains. Besides, each explosive matter has parameter called “critical diameter” (minimal diameter of confinement at which detonation can occur). For MA-PD mixtures this parameter is not well defined and is possible that detonation can occur in rocket or storage tanks although it will not appear in storage tanks used in welding industry because of much smaller diameter.

  3. Polymerization – MA-PD is prone to polymerize at elevated temperatures and pressures because of high reactivity of triple bond between carbon atoms. Regenerative cooling with MA-PD would be risky both from polymerization an explosion reasons, leaving oxygen as only cooling medium and this is also avoided in practice (although not without successful examples). In welding industry substituted amines are uses as polymerization inhibitors but would this work in rocketry is a question.

  4. Water hammer pressure surges – Severe pressure peaks can occur in rocket engines when liquids are accelerated and then suddenly stopped. This events can occur during priming, starting and throttling the engine. When fuel at the same time can be used as mono-propellant (propyne was investigated as mono-propellant) there is a risk of adiabatic compression detonation which can lead to structural failure of engine. ISPM (International Solar Polar Mission) satellite propulsion system (which used hydrazine as mono-propellant) was damaged during testing when this effect appeared. This presents an additional challenge in designing rocket engine feed-line subsystems.

  5. Toxicity – Propyne (along with MA-PD) is toxic when inhaled in high concentrations. IDLH (immediately dangerous to life of health) level is 3400ppm, or 0,34%. This limit can be easily reached when handling large amounts of gas, which would be the case in rocketry.

  6. Cost – Demand in large-scale industry decreased because oxygen/acetylene is more economic in welding applications and propane/air is more economic in heating applications . For comparison MAPP (which at best has 71% of MA-PD) is up to four times more expensive than propane.

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