Brian Wang reports on another small step towards a flying SKYLON SSTO…
…massing 275 tons fuelled and orbitting ~12 tons of payload, capable of ~ 200 flights, it would drop the LEO access cost dramatically – if it had sufficient business. One SKYLON can orbit 2400 tons over its life-span – what could we make with 30 SKYLONs? Some 72,000 tons to LEO…
There’s been lots of paper-studies of Solar Power Satellites – in the 1970s the assumed photovoltaic efficiency was 10-15% and they estimated SPS masses at about 10,000 tons per gigawatt. Concentrator systems, using space-capable inflatables, could get that down to ~1500-3000 tons per gigawatt. The latest SPS designs get rid of the big flat arrays and now use mirrors to reflect light onto an integrated PV converter/microwave rectenna system, to avoid mass penalties from wiring up the array and heavy slip-rings for power-flow from arrays to rectenna. The rectenna can also produce a tighter beam that can be transmitted over a range of angles thanks to improved “optics” that modern microwave arrays now have. Metamaterials could be used to overcome diffraction issues, thus allowing ground-receivers to be flexible in size.
I went to check the makers of SKYLON, Reaction Engines Limited, website and they’ve just completed a study on the economics of SPS using SKYLON for space-launch…
…thus they’re thinking the same thing. Perhaps the times are pregnant with opportunity for ideas like SPS?
GEO SPS could be eventually superceded by Criswell arrays on the Moon. And a SKYLON system can support Moon operations easily too. The ESA’s predecessor, ELDO, designed a two-stage Moon-capable OTV system that each component massed under 12.5 tons and could deliver 7 tons to the Moon. This could be used to build up equipment needed for cooking arrays straight out of the regolith, eventually wrapping the Moon in an equatorial power network.
Either way, once power is feasibly seen as coming from space it might be a driver for further serious investment in space-access and eventual commercial flights to the rest of the Solar System. With large space-power available we can use beamed power to fly a lot faster than via rockets. Robert Winglee & crew have proposed the Mag-Beam for an Earth-Mars transport system which is eminently feasible and able to get round-trip times down to ~3 months, for a beam-power of 300 MW. Not quite quick enough for a short holiday, but looking good.
Eventually robotics will enable self-replicating SPS factories to be constructed which, 10 years ago now, Gerald Nordley pointed out would enable large-scale interstellar travel. How so? Imagine a 1000 ton starship being propelled to 0.86c – or a 200,000 ton freighter pushed to 0.1c – which would require 180,000 exajoules of energy input. If it accelerates to that speed over 5 weeks the power level is 60 petawatts – or 4,000 times present World Power usage. But it represents but a fraction of the Sun’s 385 million exawatts of power. And that power could be tapped by an SPS array – if we could build it big enough. How big? Say we put the array at 0.1 AU where it’s exposed to 100 times the solar energy flux that Earth feels. Assuming 50% conversion efficiency the array needs to be a 936 km square, or 876,000 km2, some ~0.172% of the Earth’s surface area. If we built SPS at a pace sufficient to keep pace with our growing demand for energy ~2.6% it’d take 320 years. That’s a long wait, which some have accepted.
But I’m impatient. I want interstellar travel, at least in embryo, by 2100. So what if our self-replicating SPS starts off at 1 GW and doubles once a year. How many doublings do we need? Just 26. If we began in 2064 we’d be in time for the deadline. If we waited for a few more doublings then by the year 38 we’d be able to launch 30,000 of those starships per year. If we enshrouded 95% of the Sun, then we’d be able to fling the equivalent of 34 trillion tons of shipping to 0.87 c using the same efficiency I’d assumed before. Just another 20 years of SPS self-replication…
Kind of makes the Fermi Paradox more glaring, doesn’t it?
Except… Robert Freitas discussed interstellar trade in his “Xenology” book and a piece on “Galactic Empires”, basically concluding that if cargo pods can be sent and their kinetic energy recovered during decceleration, then no problem. It can be done for a lot cheaper than we currently imagine. You just have to be patient :o) Maybe ETIs get by with a lot less energy expenditure than my exuberant imaginary interstellar humanity.
NB – I’ve corrected the single ship power level from 60 exawatts to 60 petawatts, adjusting the resulting figures accordingly and I owe a debt of inspiration to Gerald Nordley for the modern day revival of the mass-beam concept.