Editing Liquid rocket propellant (section)

==Hydrogen==
Many early rocket theorists believed that [[Hydrogen gas|hydrogen]] would be a marvelous propellant, since it gives the highest [[specific impulse]]. It is also considered the cleanest when oxidized with [[oxygen]] because the only by-product is water. Steam reforming of [[natural gas]] is the most common method of producing commercial bulk hydrogen at about 95% of the world production<ref name="Ogden 1999">{{cite journal |last=Ogden |first=J.M. |year=1999 |title=Prospects for building a hydrogen energy infrastructure |journal=[[Annual Review of Energy and the Environment]] |volume=24 |pages=227–279 |doi=10.1146/annurev.energy.24.1.227 |doi-access=}}</ref><ref>{{cite report |title=Hydrogen production: Natural gas reforming |publisher=U.S. [[Department of Energy]] |url=https://energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming |access-date=6 April 2017}}</ref> of {{nobr|500 billion m<sup>3</sup>}} in 1998.<ref>{{cite report |last1=Rostrup-Nielsen |first1=Jens R. |last2=Rostrup-Nielsen |first2=Thomas |date=2007-03-23 |df=dmy-all |title=Large-scale Hydrogen Production |page=3 |publisher=[[Haldor Topsøe (company)|Haldor Topsøe]] |url=http://www.topsoe.com/sites/default/files/topsoe_large_scale_hydrogen_produc.pdf |url-status=dead |access-date=2023-07-16 |archive-url=https://web.archive.org/web/20160208011417/http://www.topsoe.com/sites/default/files/topsoe_large_scale_hydrogen_produc.pdf |archive-date=2016-02-08 |quote=The total hydrogen market in 1998 was 390×{{10^|9}}&nbsp;Nm³/y + 110×{{10^|9}}&nbsp;Nm³/y co-production.}}</ref> At high temperatures (700–1100&nbsp;°C) and in the presence of a [[metal]]-based [[catalyst]] ([[nickel]]), steam reacts with methane to yield [[carbon monoxide]] and hydrogen.

Hydrogen is very bulky compared to other fuels; it is typically stored as a cryogenic liquid, a technique mastered in the early 1950s as part of the [[Thermonuclear weapon#American developments|hydrogen bomb development program]] at [[Los Alamos National Laboratory|Los Alamos]]. [[Liquid hydrogen]] can be stored and transported without boil-off, by using [[helium]] as a cooling refrigerant, since helium has an even lower boiling point than hydrogen. Hydrogen is lost via venting to the atmosphere only after it is loaded onto a launch vehicle, where there is no refrigeration.<ref>{{cite book |first=Richard |last=Rhodes |author-link=Richard Rhodes |year=1995 |title=Dark Sun: The making of the hydrogen bomb |pages=483–504 |publisher=[[Simon & Schuster]] |place=New York, NY |isbn=978-0-684-82414-7 }}</ref>

In the late 1950s and early 1960s it was adopted for hydrogen-fuelled stages such as [[Centaur (rocket stage)|Centaur]] and [[Saturn I|Saturn]] upper stages.{{citation needed|date=March 2017}} Hydrogen has low density even as a liquid, requiring large tanks and pumps; maintaining the necessary extreme cold requires tank insulation. This extra weight reduces the mass fraction of the stage or requires extraordinary measures such as pressure stabilization of the tanks to reduce weight. (Pressure stabilized tanks support most of the loads with internal pressure rather than with solid structures, employing primarily the [[tensile strength]] of the tank material.{{citation needed|date=March 2017}})

The Soviet rocket programme, in part due to a lack of technical capability, did not use liquid hydrogen as a propellant until the [[Energia (rocket)|Energia]] core stage in the 1980s.{{citation needed|date=March 2017}}

===Upper stage use===
The liquid-rocket engine bipropellant [[liquid oxygen]] and hydrogen offers the highest specific impulse for conventional rockets. This extra performance largely offsets the disadvantage of low density, which requires larger fuel tanks. However, a small increase in specific impulse in an upper stage application can give a significant increase in payload-to-orbit mass.<ref name="Sutton 2010">{{cite book |last1=Sutton |first1=E.P. |last2=Biblarz |first2=O. |year=2010 |title=Rocket Propulsion Elements |edition=8th |publisher=Wiley |location=New York |isbn=9780470080245 |url=https://archive.org/details/Rocket_Propulsion_Elements_8th_Edition_by_Oscar_Biblarz_George_P._Sutton |via=Internet Archive}}</ref>

===Comparison to kerosene===
{{unreferenced section|date=March 2017}}
Launch pad fires due to spilled kerosene are more damaging than hydrogen fires, for two main reasons:
*Kerosene burns about 20% hotter in absolute temperature than hydrogen.
*Hydrogen's buoyancy. Since hydrogen is a deep cryogen it boils quickly and rises, due to its very low density as a gas. Even when hydrogen burns, the [[Steam|gaseous {{chem|H|2|O}}]] that is formed has a molecular weight of only 18&nbsp;[[Atomic mass unit|{{sc|amu}}]] compared to 29.9&nbsp;[[Atomic mass unit|{{sc|amu}}]] for air, so it also rises quickly. Spilled kerosene fuel, on the other hand, falls to the ground and if ignited can burn for hours when spilled in large quantities.
Kerosene fires unavoidably cause extensive heat damage that requires time-consuming repairs and rebuilding. This is most frequently experienced by test stand crews involved with firings of large, unproven rocket engines.

Hydrogen-fuelled engines require special design, such as running propellant lines horizontally, so that no "traps" form in the lines, which would cause pipe ruptures due to boiling in confined spaces. (The same caution applies to other cryogens such as liquid oxygen and [[liquid natural gas]] (LNG).) Liquid hydrogen fuel has an excellent safety record and performance that is well above all other practical chemical rocket propellants.