Volatility is the ability for something to give off a vapor. Water is usually the relative zero point. Ethanol, the alcohol in liquor, is more volatile than water meaning if gives off a lot of vapor compared to water under identical conditions. Something like honey is very nonvolatile, it’s mostly gluconic acid and is quite strongly attracted to itself so it prevents much from coming off as a vapor. An open bottle of ever clear will make a room smell like a bar in a short time but honey doesn’t fill a room.
Energy density is the amount of energy we get from a unit mass of fuel. Usually given as joules(or mega joules) per kg or sometimes calories per kg but it’s all the same. I won’t do any combustion analysis or calorimeter so take it on good faith that these ideas are disconnected from one another.
Kerosene has a little bit more energy density than gas, but this is only one of the many reasons it’s used. First off if you Google “energy density of fuels” you’ll see than Natural gas, primary methane (CH4) and Propane (C3H8) are towards the top with methane being higher than propane and hydrogen, H2 being the highest. This isn’t because of the volatility of each, you have cause and effect backwards.
Each of those with high energy density have bonds that take little energy to break and produce very stable bonds that release a lot of energy when they react with oxygen. Hydrogen- oxygen is the best. Elemental hydrogen is just one proton and one electron, this is the simplest element possible. If you have this hydrogen reach with hydrogen you get something with a mass of 18, where only 2 of those amu on the combustion by product came from the fuel almost all the mass came from oxygen. Whereas if we burn methane we get CO2 and H2O and co2 has a mass of 48 where 12 of those amu came from the fuel and the water that came off the fuel. So with H2 we had a 2:18 or 1:9 fuel to oxygen mass ratio and with methane we have a 12:48 or 1:4. We have the water present in both but a larger and larger portion of the mass of byproducts come from the carbon in heavier and heavier hydrocarbons rather than the hydrogens. This decreases energy density as this ratio grows closer and closer.
The key is that the hydrogen releases almost as much energy as the carbon when it oxidizes as the carbon, but because of how much lighter that hydrogen it plays a bigger role in effecting the energy density. As such, the lightest compounds tend to have a higher energy density.
Ok so with that background I can answer the question. Gas has a lot of stuff that’s lighter but releases less energy than the things in kerosene. Mostly Benzene but many other aromatics. These aromatics are heavy and very stable, meaning they require a lot of energy to break bons in but release a comparable amount of energy and so the net energy release is actually lower. These aromatics have especially stable bonds due to resonance meaning that electrons can flow more freely though their structure. They can be quite light but be very bad as far as energy density goes.
Tldr: volatility and energy density aren’t the same thing. Gas is more volatile but less energy dense than kerosene because it contains junk that is light but doesn’t release that much energy
P.s. also kerosene has a higher enthalpy of vaporization which means it absorbs more energy when it makes the phase change from liquid to gas this actually helps keep the engine within a safe temperature range. The sr-71 actually used the fuel as a coolant around the body of the aircraft.
Latest Answers