Solar thermal rocket
Encyclopedia
Solar thermal propulsion is a form of spacecraft propulsion
Spacecraft propulsion
Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. There are many different methods. Each method has drawbacks and advantages, and spacecraft propulsion is an active area of research. However, most spacecraft today are propelled by forcing a gas from the...

 that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar energy, such as concentrators and mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is 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 depot
Propellant depot
An orbital propellant depot is a cache of propellant that is placed on an orbit about the Earth or another body to allow spacecraft to be fuelled in space. Launching a spacecraft separately from some of its propellant enables missions with more massive payloads...

s. 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

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
    Heat exchanger
    A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

     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
    Specific impulse
    Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

     than other direct heating designs by using a retained seed (tantalum carbide
    Tantalum carbide
    Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic electrical conductivity. They appear as brown-gray powders...

     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
    Carbide
    In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: salt-like, covalent compounds, interstitial compounds, and "intermediate" transition metal carbides...

    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 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

), the indirect absorption designs cannot achieve specific impulses beyond 900 seconds (9 kN·s/kg). 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
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

-to-GEO
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...

 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. Systems with 10 to 100 N of thrust have been investigated by SART.

Propellants

Most proposed designs for solar thermal rockets use hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 as their propellant due to its low molecular weight which gives excellent specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...

 of 900 seconds (9 kN·s/kg).

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.

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 storeable 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
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

 and further from the Sun are expected to be rich in water ice.

Solar-thermal for ground launch

Solar thermal rockets have been proposed 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
Orbital airship
The orbital airship, also called the space blimp, is a proposed space transportation system that carries payloads to and from low Earth orbit...

 proposed by JP Aerospace
JP Aerospace
JP Aerospace is a volunteer-based organization dedicated to achieving affordable access to space. They have been hired by the U.S. Air Force to provide concepts to allow rapid launch of battlefield communication and monitoring systems....

.

Proposed solar-thermal space systems

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

A concept to provide low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...

 (LEO) propellant depot
Propellant depot
An orbital propellant depot is a cache of propellant that is placed on an orbit about the Earth or another body to allow spacecraft to be fuelled in space. Launching a spacecraft separately from some of its propellant enables missions with more massive payloads...

s 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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

—an inevitable byproduct of long-term liquid hydrogen
Liquid hydrogen
Liquid hydrogen is the liquid state of the element hydrogen. Hydrogen is found naturally in the molecular H2 form.To exist as a liquid, H2 must be pressurized above and cooled below hydrogen's Critical point. However, for hydrogen to be in a full liquid state without boiling off, it needs to be...

 storage in the radiative heat environment of space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....

—would be usable as a monopropellant
Monopropellant rocket
A monopropellant rocket is a rocket that uses a single chemical as its propellant.-Chemical-reaction monopropellant rockets:...

 in a solar-thermal propulsion system. The waste hydrogen would be productively utilized for both orbital stationkeeping
Orbital stationkeeping
In astrodynamics orbital station-keeping is a term used to describe the orbital maneuvers made by thruster burns that are needed to keep a spacecraft in a particular assigned orbit.For many Earth satellites the effects of the non-Keplerian forces, i.e...

 and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...

 with other spacecraft that would be inbound to receive fuel from the depot.

Solar-thermal monoprop hydrogen thrusters are also integral to the design of the next-generation cryogenic upper stage rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

 proposed by U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 company United Launch Alliance
United Launch Alliance
United Launch Alliance is a joint venture of Lockheed Martin and Boeing. ULA was formed in December 2006 by combining the teams at these companies which provide spacecraft launch services to the government of the United States. U.S...

 (ULA). The Advanced Common Evolved Stage
Advanced Common Evolved Stage
The Advanced Common Evolved Stage, or ACES, is a proposed upper stage rocket for use on space launch vehicles. The design concept is from the U.S. company United Launch Alliance . ACES is intended to boost satellite payloads to geosynchronous orbit or, in the case of an interplanetary space...

 (ACES) is intended as a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA Centaur
Centaur (rocket stage)
Centaur is a rocket stage designed for use as the upper stage of space launch vehicles. Centaur boosts its satellite payload to geosynchronous orbit or, in the case of an interplanetary space probe, to or near to escape velocity...

 and ULA Delta Cryogenic Second Stage (DCSS) upper stage vehicles. The ACES Integrated Vehicle Fluids option eliminates all hydrazine
Hydrazine
Hydrazine is an inorganic compound with the formula N2H4. It is a colourless flammable liquid with an ammonia-like odor. Hydrazine is highly toxic and dangerously unstable unless handled in solution. Approximately 260,000 tons are manufactured annually...

 monopropellant and all helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 pressurant from the space vehicle—normally used for attitude control and station keeping—and depends instead on solar-thermal monoprop thrusters using waste hydrogen.

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

External links

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