Solar-powered space flight
1. Introduction and Conclusions
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1.1 Solar power is routinely used to provide
on-board power for space vehicles. However, it is rarely used for actual
propulsion purposes. Instead, nearly all space vehicles currently use chemical
rocketry. This is despite solar power being plentiful in space (at least in the
vicinity of the earth).
1.2 The aim of this paper is to analyse the
practicality or otherwise of solar-powered space vehicles that deliberately aim
to use solar power for propulsion purposes for as much as possible of a
vehicle’s trajectory. Almost certainly, such a vehicle would need to employ
chemical rocketry in its early pre-orbital trajectory, to lift itself high
enough to limit atmospheric drag. So, in practice our aim is to analyse the use
of solar-powered propulsion in a vehicle’s late pre-orbital trajectory phase,
for orbital transfer and for any subsequent post-orbital trajectory. Ideally,
we would use the same solar power collector arrangement in each phase, to
minimise the mass of the propulsion system and to make the vehicle as reusable
as possible.
1.3 We do this by describing a ‘concept’ vehicle
that uses a two-mirror collector arrangement with optical characteristics that
ought to be particularly attractive for this purpose. Such a vehicle would
undoubtedly be quite flimsy, and some of the practical engineering challenges
that this would introduce are ignored, with focus instead being on some of the
more fundamental challenges that such vehicle would face irrespective of how
well it was manufactured.
1.4 Hopefully this paper will stimulate others to
consider further the potential for solar-powered space flight. Its overall
conclusions are:
(a) There
appears to be a big disparity between currently available component performance
for solar-powered thrust in space and what ought theoretically to be
achievable. Without some bridging of this gap, usage of any form of
solar-powered thrust in space may remain limited.
(b) Longer-term, if
component performance can reach closer to what ought theoretically to be
achievable then solar-powered propulsion may have a bright future. With big
enough component improvements, solar-powered space propulsion should become
practical for late pre-orbital, orbital transfer and post-orbital flight, and
particularly when a flight involves all three, since the same collector
arrangement can be used in all three of these stages.
(c) The ideal
size for ultra-low mass optical concentrators for use in space is probably of
the order of at least 
where 
is a tension
sufficient to keep the concentrator taut but not so great as to cause inelastic
deformation, 
is the
effective acceleration being generated by thrust and 
is the
average density of the thin film. Vehicles aiming to use solar power
for late pre-orbital flight seem likely to be broadly consistent with this
design characteristic. Current astronautical optical concentrators, being quite
small and typically designed to operate in essentially zero 
environments, are
typically too small to satisfy this design criterion.
1.5 A major technical challenge faced by such a
vehicle would be the need to keep a large thin-film mirror accurately
positioned during flight. Based purely on experience to date with (relatively
modest sized) optical concentrators in a zero environment,
it might be considered impractical to construct a large ultra-low mass
concentrator that achieves the desired position accuracy when undergoing the
appreciable accelerations applicable to any pre-orbital use of solar power. But
this is not necessarily sound logic. This paper argues that it ought to be easier
to achieve the desired position accuracy with the proposed vehicle design in
its pre-orbital trajectory (when very approximately the vehicle might be in a
roughly 
environment) than it
would be for a smaller optical concentrator in an essentially zero environment.
1.6 Another major technical challenge is posed by
atmospheric drag. Once the concept vehicle unfurled its solar power collector,
atmospheric drag would be proportional to the cross-sectional area of the solar
power collector (which would by necessity be large). Therefore atmospheric drag
cannot be ignored except in deep space. However, careful optimisation of the
flight speed and trajectory, following a conventional rocket launch to lift the
vehicle above the lower atmosphere, may be able to mitigate this problem rather
more than might be expected, because atmospheric drag is also proportional to
the square of the vehicle’s velocity.
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