Editing Hypergolic propellant (section)

=== Proposed, remain unflown ===

* [[Chlorine trifluoride]] (ClF<sub>3</sub>) + all known fuels – Briefly considered as an oxidizer given its high hypergolicity with all standard fuels, it was ultimately abandoned in the 1970s due to the difficulty of handling the substance safely. ClF<sub>3</sub> is known to burn concrete and gravel.<ref name="Ignition">{{Cite book | last = Clark | first = John D. | author-link = John Drury Clark | title = Ignition! An Informal History of Liquid Rocket Propellants | publisher = Rutgers University Press | year = 1972 | isbn = 978-0-8135-0725-5 |url = https://library.sciencemadness.org/library/books/ignition.pdf | url-status=live | archive-url=https://web.archive.org/web/20220710061023/https://library.sciencemadness.org/library/books/ignition.pdf |archive-date=10 July 2022}}</ref>{{rp|74}} [[Chlorine pentafluoride]] (ClF<sub>5</sub>) presents the same hazards, but offers higher [[specific impulse]] than ClF<sub>3</sub>.
* [[Pentaborane(9)]] and diborane + nitrogen tetroxide – Pentaborane(9), a so-called [[zip fuel]], was studied by Soviet rocket scientist [[Valentin_Glushko|V. P. Glushko]] for use in combination with nitrogen tetroxide in the [[RD-270|RD-270M]] rocket engine. This propellant combination would have yielded a significant increase in performance, but was ultimately given up due to toxicity concerns.<ref name=L1>[http://www.astronautix.com/engines/rd270.htm Astronautix: '''RD-270'''] {{webarchive|url=https://web.archive.org/web/20090430234903/http://astronautix.com/engines/rd270.htm |date=2009-04-30 }}.</ref>
* [[Tetramethylethylenediamine]] +red fuming nitric acid is a sightly less-toxic alternative to hydrazine and its derivatives.