Solar- & laser thermal systems

Solar-thermal rockets use concentrated solar energy to generate a high temperature gas. Laser-thermal rockets differ from solar thermal rockets in the energy source used. The advantage of using laser energy (or microwave energy) is that laser propelled systems do not have a power limitation as for solar thermal rockets.

 A schematic of a laser (or solar)-thermal rocket is given below.

Figure: Schematic of laser-thermal rocket system (courtesy NASA)

A collector or concentrator is used to collect power over a large area, which is than focussed into a smaller area situated in the engine. To concentrate the energy into a smaller area, the collector is shaped into a lens-like form or a parabolic mirror. The size (diameter) of the collector depends on the power needed and the concentration ratio. Typical sizes found for in literature for solar-thermal systems are up to 100 m diameter. Because of this large size and to minimise the mass of the collector the collecting surface is made of lightweight, metal covered, plastic film with a spherical or parabolic  curvature. Inflatable structures  are the preferred structure. In the engine, the propellant is heated to a high temperature. For a solar-thermal engine, essentially two ways of heating the propellant exists:

  1. solar energy is used to heat some heat exchanger, which is then used to heat up the working fluid, and
  2. by  molecular resonance coupling, wherein the vibration-rotation energy status of the propellant or some seedant material is excited.

The second method allows for highest temperature to be reached. Because of the high temperature, the engine typically is constructed of a high-temperature material, like tungsten or rhenium. With regard to the propellants there are no important limitations. To achieve high exhaust velocity hydrogen is preferred, because of its low molar mass.  The engine essentially is a long cylinder (the receiver or absorber cavity) with a 'window' on one end and a hemispherical cap at the other. The window may be either open or consist of a lens that acts as a secondary concentrator. The long cylindrical construction of the cavity allows most of the radiation that enters the engine to be used for heating the working fluid, i.e. the propellant. The latter enters the engine, flows through a foam-filled annulus around the receiver cavity, is heated to a high temperature, and expands through the nozzle, increasing the specific impulse over that of the cold fluid.Like for all thermal thrusters the materials’ properties limit the achievable temperature of the working fluid. Typical construction materials for the engine include refractory metals like molybdenum, rhenium or tungsten. Materials for the construction of the secondary concentrator include single-crystal zirconia, sapphire, or yttrium-aluminium-garnet (YAG). In addition to the collector and the engine, there is also a propellant storage and feed system,  a structure to tie the collector and the engine together and to the vehicle structure, a pointing and tracking system that allows for tracking the Sun and a management system that provides for command and data handling.

Development currently focuses on solar-thermal systems mainly by the USA and Japan. Efforts in the USA currently focus on the Shooting Star Solar Engine by NASA. This is a technology program meant to quantify the effectiveness of the heat exchanger and rocket, verify the structural integrity of the engine module assembly in a launch configuration and demonstrate the mission worthiness of the design configuration, materials, structure and instrumentation. In 1998, USA Air Force signed Boeing to build $48 Million solar rocket, to fly in 2001 (Space News, Feb. 1998). A recent design of a 1 kN thrust system indicates a mass of 720 kg for the collector system on a total rocket system mass of 890 kg. The next figure shows an example mirror (reflector) used for collecting and concentration solar energy. This reflector together with the receiver unit is reported to cost about 45,000 euros (FY 2003) for commercial manufacture.

Figure: Space reflector for solar energy collection and concentration (including receiver unit in focal point of mirror).

Solar-thermal systems have been reported to be evaluated for application to a wide variety of interplanetary and science missions including:

  • LEO-to-GEO payload delivery
  • Outer planet (Jupiter, Saturn, Uranus and Pluto) flyby and orbiter class missions
  • Science missions to the Sun and deep space (1000 AU)
  • Lunar/Mars cargo transfer
  • Observatory class missions such as the Next Generation Space Telescope
Name author: SSE
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