Chemically, a hydrocarbon propellant is less mass-efficient than hydrogen, although typically achieving a higher density and simpler handling than hydrogen. For rocket engines,
specific impulse () differs from other engines' (
turbines' or
pistons') efficiencies: due to the
rocket equation, efficiency is derived from exhaust velocity, not from total energy. As such, it can be beneficial to use
less energy overall in exchange for lower-
molecular-mass exhaust, meaning that chemical rocket engines achieve their peak efficiency at
non-stoichiometric ratios. In particular, since the oxygen is heavier than the carbon or hydrogen, essentially all combustion rocket engines run fuel-rich to reduce the exhaust molecular mass, increasing exhaust velocity and thus specific impulse (and as a side benefit, temperature and cooling are reduced too). This effect favors lighter elements like pure hydrogen. However, total thrust also matters, especially
deep inside a gravity well, and the density of kerosene enables considerably higher power and thrust than hydrogen (relative to engine mass). All told, due to higher energy-per-mass and lower molecular mass, hydrogen engines achieve , while kerosene engines generate an in the range of ; conversely, kerosene has the better handling, density, and thrust-to-weight properties. One common solution is to use a multistage rocket, where the first stage uses kerosene where thrust matters most, and the upper stages use hydrogen where specific impulse matters more. Examples of this dual-fuel architecture include the
Saturn V moon rocket and the
Atlas V workhorse.
Methane serves as a middle-ground between hydrogen and kerosene, offering middling molecular mass and efficiency, middling handling, middling coking/buildup properties, and density only slightly worse than kerosene. Since methane's handling difficulties, while worse than kerosene, are about the same as liquid oxygen, that means a methlox rocket is nearly as easy to handle as a kerolox rocket, but with the improved efficiency and cleanliness (which remain worse than hydrogen). Furthermore, these balances in efficiency-vs-power makes methane more suitable for a single-fuel rocket, which have proven more economical than dual-fuel rockets (due to less complexity). As such, methalox has made a resurgence in popularity in 21st century rockets, at the expense of hydrolox (better efficiency) and kerolox (better handling). Examples include
Starship,
New Glenn, the first stage of
Vulcan, and . 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 before launch. Even cold-flow tests, in which the propellants are not ignited, can leave residues. On the upside, below a chamber pressure of about , 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. Most modern hydrocarbon engines, however, run above this pressure, therefore this is not a significant effect for most engines. 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 needs a separate pressurization system 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 adds 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, widely distributed petroleum products such as jet fuel or even diesel (for example, ABL Space Systems' E2 engine can run on either RP-1 or
Jet-A). By using alternate or supplemental engine cooling methods, some engines can tolerate the non-optimal formulations. Any hydrocarbon-based fuel produces more air pollution when burned than hydrogen alone. Hydrocarbon combustion produces carbon dioxide (CO2), carbon monoxide (CO), and hydrocarbon (HC) emissions, while hydrogen (H2) reacts with oxygen (O2) to produce only water (H2O), with some unreacted H2 also released. Both hydrocarbon-based fuels and hydrogen fuel will create oxides of nitrogen (NO
x) pollutants, because
rocket exhaust temperatures above will
thermally combine some of the nitrogen (N2) and oxygen (O2) already present in the atmosphere, to create oxides of nitrogen. == RP-1-like fuels ==