Editing Rocket engine (section)

==Chemistry==
[[Rocket propellant]]s require a high energy per unit mass ([[specific energy]]), which must be balanced against the tendency of highly energetic propellants to spontaneously explode. Assuming that the chemical potential energy of the propellants can be safely stored, the combustion process results in a great deal of heat being released. A significant fraction of this heat is transferred to kinetic energy in the engine nozzle, propelling the rocket forward in combination with the mass of combustion products released.

Ideally all the reaction energy appears as kinetic energy of the exhaust gases, as exhaust velocity is the single most important performance parameter of an engine. However, real exhaust species are [[molecule]]s, which typically have translation, vibrational, and [[rotational modes]] with which to dissipate energy. Of these, only translation can do useful work to the vehicle, and while energy does transfer between modes this process occurs on a timescale far in excess of the time required for the exhaust to leave the nozzle.

The more [[chemical bond]]s an exhaust molecule has, the more rotational and vibrational modes it will have. Consequently, it is generally desirable for the exhaust species to be as simple as possible, with a diatomic molecule composed of light, abundant atoms such as H<sub>2</sub> being ideal in practical terms. However, in the case of a chemical rocket, hydrogen is a reactant and [[reducing agent]], not a product. An [[oxidizing agent]], most typically oxygen or an oxygen-rich species, must be introduced into the combustion process, adding mass and chemical bonds to the exhaust species.

An additional advantage of light molecules is that they may be accelerated to high velocity at temperatures that can be contained by currently available materials - the high gas temperatures in rocket engines pose serious problems for the engineering of survivable motors.

Liquid [[hydrogen]] (LH2) and [[oxygen]] (LOX, or LO2), are the most effective propellants in terms of exhaust velocity that have been widely used to date, though a few exotic combinations involving boron or liquid ozone are potentially somewhat better in theory if various practical problems could be solved.<ref>[http://yarchive.net/space/rocket/fuels/fuel_ratio.html Newsgroup correspondence], 1998–99</ref>

When computing the specific reaction energy of a given propellant combination, the entire mass of the propellants (both fuel and oxidiser) must be included. The exception is in the case of air-breathing engines, which use atmospheric oxygen and consequently have to carry less mass for a given energy output. Fuels for car or [[turbojet engine]]s have a much better effective energy output per unit mass of propellant that must be carried, but are similar per unit mass of fuel.

Computer programs that predict the performance of propellants in rocket engines are available.<ref>[http://rocketworkbench.sourceforge.net/equil.phtml Complex chemical equilibrium and rocket performance calculations], Cpropep-Web</ref><ref>[http://propulsion-analysis.com/ Tool for Rocket Propulsion Analysis], RPA</ref><ref>[https://web.archive.org/web/20000901045039/http://www.grc.nasa.gov/WWW/CEAWeb/ NASA Computer program Chemical Equilibrium with Applications], CEA</ref>