Why use Kerosene?

Question

According to SpaceX's website, the Falcon 9 uses kerosene as fuel. According to Wikipedia, diesel fuel beats kerosene in energy density in terms mass by 2.8 MJ/Kg and in volume by 5.6MJ/L, so why would they use kerosene instead? In addition, liquid hydrogen beats kerosene by over three times in terms of energy density by mass, although it is about 1/4 the energy density in terms of volume, but couldn't the rocket just be built taller to accommodate the extra volume, and still retain a lower total mass since the fuel makes up most of the rocket's weight?

Answer 1

The hydrogen side of this question has been addressed here: Combination of liquid hydrogen and liquid oxygen -- short answer is that a hydrogen first stage would be lighter, loaded, but much more expensive.

Modern "kerosene"-burning rockets like the Falcon 9 use RP-1 (or RG-1 in Russian engines); this is a refined form of kerosene containing a relatively narrow mix of hydrocarbons rather than the wider spectrum found in common kerosene. RP-1 is often used as a coolant as well as a propellant, piped along the walls of the combustion chamber before entering it. The different boiling points and polymerization characteristics of different hydrocarbons causes problems:

Many early rockets had burned kerosene, but as burn times, combustion efficiencies, and combustion-chamber pressures grew, and as engine masses shrank, the engine temperatures became unmanageable. Raw kerosene used as coolant would dissociate and polymerize. Lightweight products in the form of gas bubbles, and heavy ones in the form of engine deposits, then blocked the narrow cooling passages. The coolant starvation raised temperatures further, accelerating breakdown. This cycle would escalate rapidly (i.e., thermal runaway would occur) until an engine wall ruptured.

Common diesel fuel is even more prone to polymerization and thermal breakdown, so the same pressures that drove refinement of kerosene into RP-1 apply. For SpaceX in particular, who intends to reuse their rocket engines, minimizing residue in the plumbing is a significant factor, and this is one of the reasons they are moving to methane as the fuel in their next generation of engines.

Answer 2

According to Wikipedia, diesel fuel beats kerosene in energy density in terms mass by 2.8 MJ/Kg and in volume by 5.6MJ/L, so why would they use kerosene instead?

First off, that's wikipedia for you, using unreferenced numbers that are in fact incorrect. The conflicting wikipedia page on heat of combustion states lower heating values of 43.0 and 43.4 MJ/kg for kerosene and diesel, respectively. The conflicting wikipedia pages on kerosene and diesel fuel state lower (or net) heating values of 43.1 MJ/kg for both kerosene and diesel.

Aside: The lower and higher heating values of a substance reflect whether the water produced by the combustion exits the system in gaseous or liquid form. As water exits a rocket in a gaseous state with chemical rocket engines, it's the lower (or net) heating values that are relevant to rocket engines.

Factors that argue against using diesel fuel:

• Those heating values do not account for the mass of the oxygen.
  This is fine for combustion on the surface of the Earth, but is completely incorrect with regard to rockets because rockets have to carry both the fuel and the oxidizer. Long hydrocarbon chains require more oxygen per unit mass than do shorter saturated hydrocarbon chains.

• Looking at heating values alone does not account for how rocket engines work.
  The thrust produced by a rocket is proportional to $\sqrt{Q/M}$, where $Q$ is the net heating value and $M$ is the mean molecular mass of the exhaust. Water vapor in rocket is highly preferable compared to carbon dioxide because water vapor has a molecular mass of about 18 while carbon dioxide has a molecular mass of about 44. This is the key reason why LH2-LOX rocket engines look so good on paper. This also argues against long chain hydrocarbons.

• Carbon deposits present a danger to rocket engines.
  The longer the hydrocarbon chain, the greater the risk for carbon deposits.

The slight advantage (if any) of diesel fuel over kerosene based on heating value becomes a disadvantage after accounting for those other factors.