Editing Solar thermal rocket (section)

==Propellants==

Most proposed designs for solar thermal rockets use [[hydrogen]] as their propellant due to its low [[molecular weight]] which gives excellent [[specific impulse]] of up to 1000 seconds (10&nbsp;kN·s/kg) using heat exchangers made of rhenium.<ref name=ultramet_rhenium>{{cite web|last=Ultramet|title=Advanced Propulsion Concepts - Solar Thermal Propulsion|url=http://www.ultramet.com/propulsionsystem_components_advanced.html|publisher=Ultramet|accessdate=June 20, 2012}}</ref>

Conventional thought has been that hydrogen—although it gives excellent specific impulse—is not space storable.  Design work in the early 2010s has developed an approach to substantially reduce hydrogen boiloff, and to economically utilize the small remaining boiloff product for requisite in-space tasks, essentially achieving zero boil off (ZBO) from a practical point of view.<ref name=aiaa20100902/>{{rp |p. 3,4,7}}

Other substances could also be used. Water gives quite poor performance of 190 seconds (1.9&nbsp;kN·s/kg), but requires only simple equipment to purify and handle, and is space storable and this has very seriously been proposed for interplanetary use, [[In-situ resource utilization|using in-situ resources]]. <ref name = NASA>{{cite web|last=NASA|title=Robotic Asteroid Prospector NIAC Phase 1 Final Report|url=https://www.nasa.gov/sites/default/files/files/Cohen_2012_PhI_RAP.pdf|publisher=NASA|accessdate=March 11, 2021}}</ref>

[[Ammonia]] has been proposed as a propellant.<ref name=physical_sciences_inc>{{cite web|last=PSI|title=Solar Thermal Propulsion for Small Spacecraft_Engineering System Development and Evaluation|url=http://www.psicorp.com/pdf/library/sr-1228.pdf|publisher=PSI|accessdate=August 12, 2017}}</ref> It offers higher specific impulse than water, but is easily storable, with a freezing point of −77 degrees Celsius and a boiling point of −33.34&nbsp;°C.

A solar-thermal propulsion architecture outperforms architectures involving electrolysis and liquification of hydrogen from water by more than an order of magnitude, since electrolysis requires heavy power generators, whereas distillation only requires a simple and compact heat source (either nuclear or solar); so the propellant production rate is correspondingly far higher for any given initial mass of equipment. However its use does rely on having clear ideas of the location of water ice in the [[Solar System]], particularly on lunar and asteroidal bodies, and such information is not known, other than that the bodies within the [[asteroid belt]] and further from the Sun are expected to be rich in water ice.<ref name=zupper_moons>{{cite journal|last=Zuppero|first=Anthony|title=Propulsion to Moons of Jupiter Using Heat and Water Without Electrolysis Or Cryogenics|journal=Space Exploration 2005|year=2005|volume=001|series=SESI Conference Series|url=http://www.neofuel.com/2005.04.06-sesi-space2005/Zuppero%20SESI%20Space%202005%20per%20template.pdf|accessdate=June 20, 2012}}</ref><ref>{{cite web|last=Zuppero|first=Anthony|title=new fuel: Near Earth Object fuel (Neofuel, using abundant off-earth resources for interplanetary transport)|url=http://www.neofuel.com|accessdate=June 20, 2012}}</ref>