Solar-powered space flight
6a. Efficiently converting sunlight to
thrust: Possible thrust mechanisms
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6.1 The highest
specific impulses currently available (other than via solar sails) involve
gridded ion engines and propellant ejection speeds of circa 40,000 ms-1,
see US
National Research Council Committee on Thermionic Research and Technology
(2001), henceforth ‘NRCTRT’. Limiting the ejection speed to 40,000 ms-1
increases the required power per unit final mass (versus the no limit case)
only very modestly. However, the conversion efficiency of sunlight into
electricity is unlikely to be much above, say, 25 – 30%, and the conversion
efficiency of electricity into thrust is unlikely to be much above 60%, see
NRCTRT. There would probably also be other components of the energy conversion
process that might add significantly to the launcher mass (e.g. the power
converter).
6.2 More
promising is solar thermal propulsion, see NRCTRT. Conceptually, all one needs
is a slab with a high melting point and good thermal conductivity placed at the
focal point of the layout and heated by the concentrated sunlight. Into the
slab would be injected at high pressure some suitable liquid or gas (probably
liquid hydrogen, given its low molecular mass and therefore high potential
specific impulse). The H2 would be vaporised by the intense heat of
the slab, generating thrust probably by being expelled through narrow outlets
(akin to the throats of a chemical rocket) and then expanding through suitable
rocket shaped nozzles. Reducing the flow of propellant through the engine
increases the engine temperature and hence the propellant ejection speed
essentially continuously (up to some upper limit defined by the propellant and
by the engine’s maximum operating temperature). Thus it should be relatively
simple to achieve the continuously varying optimal propellant ejection speed so
helpful in keeping down the required collector area per unit mass lifted into
orbit.
6.3 Solar
thermal propulsion has been proposed for the Solar Orbit Transfer Vehicle
(SOTV), see NRCTRT. The SOTV aimed to use solar thermal propulsion to move a
payload from lower earth orbit to geostationary orbit over circa 15 – 30 days,
delivering a low thrust but with a high specific impulse (750 – 800 seconds,
versus the circa 450 delivered by a liquid hydrogen/liquid oxygen chemical
rocket as used by, say, the NASA Space Shuttle). The SOTV envisaged
concentrating sunlight using parabolic mirrors into a cavity in which hydrogen
gas was heated to circa 2300 K. The resulting expansion of the hydrogen gas
would provide the thrust, the parabolic mirrors being reused in conjunction
with a thermionic power converter to provide power to the payload after
reaching geostationary orbit.
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