Editing Rocket engine (section)

===Combustion chamber===
For chemical rockets the combustion chamber is typically cylindrical, and [[flame holder]]s, used to hold a part of the combustion in a slower-flowing portion of the combustion chamber, are not needed. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different [[rocket propellant]]s require different combustion chamber sizes for this to occur.

This leads to a number called <math>L^*</math>, the [[characteristic length]]:
:<math>L^* = \frac {V_c} {A_t}</math>
where:
*<math>V_c</math> is the volume of the chamber
*<math>A_t</math> is the area of the throat of the nozzle.
L* is typically in the range of {{convert|64|-|152|cm|in}}.

The temperatures and pressures typically reached in a rocket combustion chamber in order to achieve practical [[thermal efficiency]] are extreme compared to a [[afterburner|non-afterburning]] [[airbreathing jet engine]]. No atmospheric nitrogen is present to dilute and cool the combustion, so the propellant mixture can reach true [[stoichiometric]] ratios. This, in combination with the high pressures, means that the rate of heat conduction through the walls is very high.

In order for fuel and oxidiser to flow into the chamber, the pressure of the propellants entering the combustion chamber must exceed the pressure inside the combustion chamber itself.  This may be accomplished by a variety of design approaches including [[turbopump]]s or, in simpler engines, via [[Pressure-fed cycle (rocket)|sufficient tank pressure]] to advance fluid flow. Tank pressure may be maintained by several means, including a high-pressure [[helium]] pressurization system common to many large rocket engines or, in some newer rocket systems, by a bleed-off of high-pressure gas from the engine cycle to [[autogenous pressurization|autogenously pressurize]] the propellant tanks<ref name=nsf20160927>
{{cite news |last=Bergin|first=Chris |url=https://www.nasaspaceflight.com/2016/09/spacex-reveals-mars-game-changer-colonization-plan/ |title=SpaceX reveals ITS Mars game changer via colonization plan |work=[[NASASpaceFlight.com]] |date=2016-09-27 |access-date=2016-09-27 }}</ref><ref name=sfi20160927/>  For example, the self-pressurization gas system of the [[SpaceX Starship]] is a critical part of SpaceX strategy to reduce launch vehicle fluids from five in their legacy Falcon 9 vehicle family to just two in Starship, eliminating not only the helium tank pressurant but all [[hypergolic propellant]]s as well as [[nitrogen]] for cold-gas [[reaction control system|reaction-control thrusters]].<ref name=nsf20161003/>