Солнечные тепловые ракетные двигатели: теория и перспективы

Theoretical spacecraft propulsion system
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, more efficient use of the sun 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 refuelled with relative ease.

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. 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. Ammonia has been proposed as a propellant. 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 °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.

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 proposed by JP Aerospace.

as of 2010, two proposals for utilizing solar thermal propulsion on in space post launch spacecraft systems had 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. 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) was 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. A subsequent proposal in the 2010s was the Solar Moth spacecraft that would use lightweight mirrors to focus solar radiation on a solar thermal engine.