World Library  
Flag as Inappropriate
Email this Article

Solar thermal rocket

Article Id: WHEBN0000436208
Reproduction Date:

Title: Solar thermal rocket  
Author: World Heritage Encyclopedia
Language: English
Subject: Spacecraft propulsion, Monopropellant rocket, Interplanetary spaceflight, Rocket propellant, Rocket engine
Collection: Spacecraft Propulsion
Publisher: World Heritage Encyclopedia

Solar thermal rocket

A solar thermal rocket is a theoretical spacecraft propulsion system that would make use of solar power to directly heat reaction mass, and therefore would not require an electrical generator like most other forms of solar-powered propulsion do. The rocket would only have to carry the means of capturing solar energy, such as concentrators and mirrors. The heated propellant would be fed through a conventional rocket nozzle to produce thrust. Its engine thrust would be directly related to the surface area of the solar collector and to the local intensity of the solar radiation.

In the shorter term, solar thermal propulsion has been proposed both for longer-life, lower-cost and more-flexible cryogenic upper stage launch vehicles and for on-orbit propellant depots. Solar thermal propulsion is also a good candidate for use in reusable inter-orbital tugs, as it is a high-efficiency low-thrust system that can be refueled with relative ease.


  • Solar-thermal design concepts 1
  • Propellants 2
  • Solar-thermal for ground launch 3
  • Proposed solar-thermal space systems 4
  • References 5
  • External links 6

Solar-thermal design concepts

There are two basic solar thermal propulsion concepts, differing primarily in the method by which they use solar power to heat the propellant:

  • 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 carbides 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 K), the indirect absorption designs cannot achieve specific impulses beyond 900 seconds (9 kN·s/kg = 9 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 rockets, however, an increase that can provide substantial payload gains (45 percent for a LEO-to-GEO mission) at the expense of increased trip time (14 days compared to 10 hours).

Small-scale hardware has been designed and fabricated for the Air Force Rocket Propulsion Laboratory (AFRPL) for ground test evaluation.[1] Systems with 10 to 100 N of thrust have been investigated by SART.[2]


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 kN·s/kg) using heat exchangers made of rhenium.[3]

Conventional thought has been that hydrogen—although it gives excellent specific impulse—is not space storable. Recent design work 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.[4]

Other substances could also be used. Water gives quite poor performance of 190 seconds (1.9 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, using in-situ resources.

Ammonia has been proposed as a propellant. It offers higher specific impulse than water, but is easily storable, with a boiling point of −77 degrees Celsius. The exhaust dissociates into hydrogen and nitrogen, leading to a lower average molecular weight, and thus a higher Isp (65% of hydrogen).

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 with the asteroid belt and further from the Sun are expected to be rich in water ice.[5] [6]

Solar-thermal for ground launch

Solar thermal rockets have been proposed [7] as a system for launching a small personal spacecraft into orbit. The design is based on a high altitude airship which uses its envelope to focus sunlight onto a tube. The propellant, which would likely be ammonia, is then fed through to produce thrust. Possible design flaws include whether the engine could produce enough thrust to overcome drag, and whether the skin of the airship wouldn't fail at hypersonic velocities. This has many similarities to the orbital airship proposed by JP Aerospace.

Proposed solar-thermal space systems

As of 2010, two proposals for utilizing solar-thermal propulsion on in-space post-launch spacecraft systems have been made.

A concept to provide low Earth orbit (LEO) propellant depots that could be used as way-stations for other spacecraft to stop and refuel on the way to beyond-LEO missions has proposed that waste gaseous hydrogen—an inevitable byproduct of long-term liquid hydrogen storage in the radiative heat environment of space—would be usable as a monopropellant in a solar-thermal propulsion system. The waste hydrogen would be productively utilized for both orbital stationkeeping and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous with other spacecraft that would be inbound to receive fuel from the depot.[8]

Solar-thermal monoprop hydrogen thrusters are also integral to the design of the next-generation cryogenic upper stage rocket proposed by U.S. company United Launch Alliance (ULA). The Advanced Common Evolved Stage (ACES) is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA Centaur and ULA Delta Cryogenic Second Stage (DCSS) upper stage vehicles. The ACES Integrated Vehicle Fluids option eliminates all hydrazine monopropellant and all helium pressurant from the space vehicle—normally used for attitude control and station keeping—and depends instead on solar-thermal monoprop thrusters using waste hydrogen.[4]

The viability of various trips using Solar Thermal propulsion was investigated by Gordon Woodcock and Dave Byers in 2003[9]


  1. ^ Solar Thermal Propulsion for Small Spacecraft - Engineering System Development and Evaluation PSI-SR-1228 publisher AIAA July 2005
  2. ^ Webpage DLR Solar Thermal Propulsion of the Institut für Raumfahrtantriebe Abteilung Systemanalyse Raumtransport (SART) date = November 2006
  3. ^ Ultramet. "Advanced Propulsion Concepts - Solar Thermal Propulsion". Ultramet. Retrieved June 20, 2012. 
  4. ^ a b Zegler and Kutter, 2010, p. 3,4,7.
  5. ^ Zuppero, Anthony (2005). "Propulsion to Moons of Jupiter Using Heat and Water Without Electrolysis Or Cryogenics" (PDF). Space Exploration 2005. SESI Conference Series 001. Retrieved June 20, 2012. 
  6. ^ Zuppero, Anthony. "new fuel: Near Earth Object fuel (Neofuel, using abundant off-earth resources for interplanetary transport)". Retrieved June 20, 2012. 
  7. ^ NewMars, Solar Thermal Tech for Ground Launch?
  8. ^ Zegler, Frank; Bernard Kutter (2010-09-02). "Evolving to a Depot-Based Space Transportation Architecture" (PDF). AIAA SPACE 2010 Conference & Exposition. AIAA. p. 3. Retrieved 2011-01-25. the waste hydrogen that has boiled off happens to be the best known propellant (as a monopropellant in a basic solar-thermal propulsion system) for this task. A practical depot must evolve hydrogen at a minimum rate that matches the station keeping demands. 
  9. ^ Byers, Woodcock (2003). "Results of Evaluation of Solar Thermal Propulsion, AIAA 2003-5029". AIAA. 

External links

  • Solar Thermal Propulsion for Small Spacecraft - Engineering System Development and Evaluation (2005)
  • Pratt & Whitney Rocketdyne Wins $2.2 Million Contract Option for Solar Thermal Propulsion Rocket Engine (Press release, June 25, 2008, Pratt & Whitney Rocketdyne)
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.

Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.