Solar-powered space flight
5a. Creating ultra-lightweight solar
power concentrators: Design requirements
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5a. Design
requirements
5.1 The above
analysis suggests that to make a significant contribution to any pre-orbital
trajectory the vehicle would need to collect at least, say, circa 20 – 100
kW/kg of solar energy. How practical is this?
5.2 The solar
power incident on each 1 m2 perpendicular to the sun’s rays in the
vicinity of the earth (i.e. the solar constant) is roughly 1.37 kW. So if the
collector were wholly perpendicular to the sun’s rays and conversion of solar
energy to thrust were 100% efficient then a power requirement of 20 – 100 kW/kg
would equate to 15 – 73 m2 per unit lifted mass.
5.3 Over the
last few years there have been substantial advances in potential fabrication of
giant ultra-low mass mirrors, principally driven by a renewed interest in solar
sail technology. According to NASA (2005),
conventional light solar sail film has comprised of 5-micron thin aluminised
Mylar with a thin film aluminium layer (approximately 100 nm thick) deposited on
one side to form a mirror with 90% reflectivity, weighing circa 7 g m-2.
The solar sail that Team
Encounter (2005) was planning to launch was reported to have a mass of less
than 4 g m-2 (including payload). 15 – 73m2 of a 100%
reflecting 5 g m-2 mirror would weigh only 0.08 – 0.37 kg, implying
that, in theory, it ought to be possible for such a mirror to lift itself into
orbit with some payload to spare, following use of chemical rocketry to lift
the vehicle to a height at which atmospheric drag becomes small.
5.4 We next
identify a way of concentrating sunlight by using mirrors of the sort now being
developed for solar sails. Ideally the mirrors should:
(a) Concentrate the
sunlight by as much as possible. This should reduce the mass of any means of
converting the sunlight into usable thrust and increase conversion efficiency;
(b) Have a surface area as
little as possible more than the collector area perpendicular to the sun’s
rays; and
(c) Be dragged
‘behind’ the vehicle (but without fouling propellant ejection), to enhance the
stability of the vehicle in flight.
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