Editing Hypergolic propellant (section)

=== Advantages ===
Hypergolically fueled rocket engines are usually simple and reliable because they need no ignition system. Although larger hypergolic engines in some launch vehicles use [[turbopump]]s, most hypergolic engines are pressure fed. A gas, usually [[helium]], is fed to the propellant tanks under pressure through a series of [[check valve|check]] and [[safety valve]]s. The propellants, in turn, flow through control valves into the combustion chamber; there, their instant contact ignition prevents a mixture of unreacted propellants from accumulating and then igniting in a potentially catastrophic [[hard start]].

As hypergolic rockets do not need an ignition system, they can fire any number of times by simply opening and closing the propellant valves until the propellants are exhausted, so are uniquely suited for spacecraft maneuvering and well-suited, though not uniquely so, as upper stages of such space launchers as the [[Delta II]] and [[Ariane 5]], which must perform more than one burn. Restartable nonhypergolic rocket engines nevertheless exist, notably the cryogenic (oxygen/hydrogen) [[RL-10]] on the [[Centaur (rocket stage)|Centaur]] and the [[J-2 (rocket engine)|J-2]] on the [[Saturn V]]. The [[RP-1]]/LOX [[Merlin (rocket engine family)|Merlin]] on the [[Falcon 9]] can also be restarted.<ref>{{Cite web|title=SpaceX|url=http://www.spacex.com/|access-date=2021-12-29|website=SpaceX|language=en}}</ref>

The most common hypergolic fuels, [[hydrazine]], [[monomethylhydrazine]], and [[unsymmetrical dimethylhydrazine]] (UDMH), and oxidizer, [[nitrogen tetroxide]], are all liquid at ordinary temperatures and pressures.  They are therefore sometimes called "storable liquid propellants".  They are suitable for use in spacecraft missions lasting many years. The [[cryogenics|cryogenity]] of [[liquid hydrogen]] and [[liquid oxygen]] has so far limited their practical use to space launch vehicles where they need to be stored only briefly.<ref>{{cite web
|url=https://www.permanent.com/space-transportation-propellants.html
|title=Fuel Propellants - Storable, and Hypergolic vs. Ignitable by Mike Schooley
|url-status=live
|archive-url=https://web.archive.org/web/20210724084512/https://www.permanent.com/space-transportation-propellants.html
|archive-date=24 July 2021}}</ref><!-- could be, historically; but would need a source.  For more modern rocket designs, after c. 2016, there are certainly cryogenic-propellant rocket stages that are planning on months-long duration for use of cryopropellants on interplanetary flights  --> As the largest issue with the usage of cryogenic propellants in interplanetary space is boil-off, which is largely dependent on [[Square–cube_law|the scale]] of spacecraft, for larger craft such as [[SpaceX_Starship#Starship_spacecraft|Starship]] this is less of an issue.

Another advantage of hypergolic propellants is their high density compared to cryogenic propellants. [[Liquid oxygen|LO<sub>2</sub>]] has a density of 1.14 g/ml, while  hypergolic oxidizers such as [[nitric acid]] or [[nitrogen tetroxide]] have a density of 1.55 g/ml and 1.45 g/ml, respectively. [[Liquid_hydrogen|LH<sub>2</sub>]] fuel offers extremely high performance, yet its density only warrants its use in the largest of rocket stages, while mixtures of hydrazine and UDMH have a density at least 10 times greater.<ref>{{cite web
|url=http://www.braeunig.us/space/propel.htm#tables
|title=PROPERTIES OF ROCKET PROPELLANTS
|website=braeunig.us
|url-status=live
|archive-url=https://web.archive.org/web/20220526100431/http://www.braeunig.us/space/propel.htm#tables
|archive-date=26 May 2022}}</ref> This is of great importance in [[Space probe|space probes]], as the higher propellant density allows the size of their propellant tanks to be reduced significantly, which in turn allows the probes to fit within a smaller [[payload fairing]].