Editing Solar thermal rocket (section)

==Solar-thermal design concepts==

There are two solar thermal propulsion concepts, differing primarily in the method by which they use solar power to heat up the propellant:{{Citation needed|date=January 2011}}
*''Indirect solar heating'' involves pumping the propellant through passages in a [[heat exchanger]] that is heated by solar radiation. The windowless heat exchanger cavity concept is a design taking this radiation absorption approach.
*''Direct solar heating'' involves exposing the propellant directly to solar radiation. The rotating bed concept is one of the preferred concepts for direct solar radiation absorption; it offers higher [[specific impulse]] than other direct heating designs by using a retained seed ([[tantalum carbide]] or [[hafnium carbide]]) approach. The propellant flows through the porous walls of a rotating cylinder, picking up heat from the seeds, which are retained on the walls by the rotation. The [[carbide]]s are stable at high temperatures and have excellent heat transfer properties.

Due to limitations in the temperature that heat exchanger materials can withstand (approximately 2800 [[Kelvin|K]]), the indirect absorption designs cannot achieve specific impulses beyond 900 seconds (900&nbsp;s·[[Standard gravity|{{math|''ɡ''<sub>0</sub>}}]] = 8.8&nbsp;km/s) (or up to 1000 seconds, see below). The direct absorption designs allow higher propellant temperatures and therefore higher specific impulses, approaching 1200 seconds. Even the lower specific impulse represents a significant increase over that of conventional [[chemical rocket]]s, however, an increase that can provide substantial payload gains (45 percent for a [[Low Earth orbit|LEO]]-to-[[Geosynchronous orbit|GEO]] mission) at the expense of increased trip time (14 days compared to 10 hours).{{Citation needed|date=January 2011}}

Small-scale hardware has been designed and fabricated for the [[Air Force Rocket Propulsion Laboratory]] (AFRPL) for ground test evaluation.<ref>[http://www.psicorp.com/pdf/library/sr-1228.pdf Solar Thermal Propulsion for Small Spacecraft - Engineering System Development and Evaluation PSI-SR-1228 publisher AIAA July 2005]</ref>  Systems with 10 to 100 N of thrust have been investigated by SART.<ref>[http://www.la.dlr.de/ra/sart/projects/sto/sto.php.en Webpage DLR Solar Thermal Propulsion of the Institut für Raumfahrtantriebe Abteilung Systemanalyse Raumtransport (SART) date = November 2006] {{webarchive|url=https://web.archive.org/web/20070706053556/http://www.la.dlr.de/ra/sart/projects/sto/sto.php.en |date=2007-07-06 }}</ref>

Reusable Orbital Transfer Vehicles (OTV), sometimes called (inter-orbital) space tugs, propelled by solar thermal rockets have been proposed. The concentrators on solar thermal tugs are less susceptible to radiation in the Van Allen belts than the solar arrays of solar electric OTV.<ref name="OTV_options">{{cite web |last1=John H. Schilling|first1=Frank S. Gulczinski III |title=Comparison of Orbit Transfer Vehicle Concepts Utilizing Mid-Term Power and Propulsion Options |url=http://erps.spacegrant.org/uploads/images/images/iepc_articledownload_1988-2007/2003index/0022-0303iepc-full.pdf |accessdate=May 23, 2018 }}</ref>

An initial proof of concept was demonstrated in 2020 with helium at the Johns Hopkins University Applied Physics Laboratory solar simulator.<ref>{{cite magazine |last1=Oberhaus |first1=Daniel |title=A Solar-Powered Rocket Might Be Our Ticket to Interstellar Space |url=https://www.wired.com/story/a-solar-powered-rocket-might-be-our-ticket-to-interstellar-space/ |magazine=[[Wired (magazine)|Wired]] |date=20 November 2020}}</ref>