RP-1 - Comparison With Other Fuels

Comparison With Other Fuels

Chemically, a hydrocarbon propellant will be less efficient than hydrogen fuel. Hydrocarbons can absorb significant amounts of combustion energy by generating any of several oscillating modes between the atoms. This is energy that could have instead gone into exhaust velocity, and thus, thrust. The heavier oxygen atoms in the CO₂ exhaust absorb much more energy than the two hydrogens of H₂O. American designed hydrocarbon engines are also run fuel-rich, which produces some CO instead of CO₂. But this also results in incomplete combustion, producing some organics of high molecular weight and numerous vibration modes. All told, kerosene engines generate an I_sp in the range of 270 to 360 seconds, while hydrogen engines achieve 370–465 seconds.

During engine shutdown, fuel flow goes to zero rapidly, while the engine is still quite hot. Residual and trapped fuel can polymerize or even carbonize at hot spots or in hot components. Even without hot spots, heavy fuels can create a petroleum residue, as can be seen in gasoline, diesel, or jet fuel tanks that have been in service for years. Rocket engines have cycle lifetimes measured in minutes or even seconds, preventing truly heavy deposits. However, rockets are much more sensitive to a deposit, as described above. Thus, kerosene systems generally entail more teardowns and overhauls, creating operations and labor expenses. This is a problem for expendable engines as well as reusable ones, because engines must be ground-fired some number of times beforehand. Even cold-flow tests, in which the propellants are not ignited, can leave residues.

On the upside, below a chamber pressure about 1000 psi (6.9 MPa), kerosene can produce sooty deposits on the inside of the nozzle and chamber liner. This acts as a significant insulation layer, and can reduce the heat flow into the wall by roughly a factor of two.

Recent heavy-hydrocarbon engines have modified components and new operating cycles, in attempts to better manage leftover fuel, achieve a more-gradual cooldown, or both. This still leaves the problem of non-dissociated petroleum residue. Other new engines have tried to bypass the problem entirely, by switching to light hydrocarbons such as methane or propane gas. Both are volatiles, so engine residues simply evaporate. If necessary, solvents or other purgatives can be run through the engine to finish dispersion. The short-chain carbon backbone of propane (a C₃ molecule) is very difficult to break; methane, with a single carbon atom (C₁), is technically not a chain at all. The breakdown products of both molecules are also gases, with fewer problems due to phase separation, and much less likelihood of polymerization and deposition. However, methane (and to a lesser extent propane) reintroduces handling inconveniences that prompted kerosenes in the first place.

The low vapor pressure of kerosenes gives safety for ground crews. However, in flight the kerosene tank will need a separate system of pressurization, to replace fuel volume as it drains. Generally, this is a separate tank of liquid or high-pressure inert gas, such as nitrogen or helium. This creates extra cost and weight. Cryogenic or volatile propellants generally do not need a separate pressurant; instead, some propellant is expanded (often with engine heat) into low-density gas, and routed back to its tank. A few highly-volatile propellant designs do not even need the gas loop; some of the liquid automatically vaporizes to fill its own container. Some rockets use gas from a gas generator to pressurize the fuel tank; usually, this is exhaust from a turbopump. Although this saves the weight of a separate gas system, the loop now has to handle a hot, reactive gas instead of a cool, inert one.

Regardless of chemical constraints, RP-1 has supply constraints, due to the very small size of the launch-vehicle industry versus other consumers of petroleum. While the material price of such a highly-refined hydrocarbon is still less than many other rocket propellants, the number of RP-1 suppliers is limited. A few engines have attempted to use more standard, wide-distribution petroleum products such as jet fuel or even diesel. By using alternate or supplemental engine cooling methods, some can tolerate the non-optimal formulations.

Any hydrocarbon-based fuel when burned produces more air pollution than hydrogen. Hydrocarbon combustion produces carbon dioxide (CO₂, a greenhouse gas), toxic carbon monoxide (CO), hydrocarbon (HC) emissions, and oxides of nitrogen (NO_x), while hydrogen (H₂) reacts with oxygen (O₂) to produce only water (H₂O), with some unreacted H₂ also released.