by Eric Drexler on 2009/07/20

We are still in the prehistory of convenient spaceflight

We are still in the prehistory of effective space technology.
The problem is that we aren’t (yet) very good at making things.

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{ 3 comments… read them below or add one }

Chris Phoenix July 20, 2009 at 12:53 am UTC

There’s a bit of good news: Falcon 1 recently orbited a satellite successfully. It’s still crude, by future standards: the second stage is machined from plate aluminum.

The back of my envelope says that a molecular manufactured package luggable by a single human – including fuel (it would gather oxidizer on the way up) – should be able to put a kg into orbit. With negligible avionics weight, and small airframe weight, it should be able to use several airplane stages and one or two rocket stages for quite good efficiency. Of course, this assumes excellent material properties – I doubt you could do it with biopolymers.


Bruce Cohen (SpeakerTo July 21, 2009 at 3:32 pm UTC

I’d argue that the reason is that we haven’t found efficient mechanisms for converting source energy (chemical bond energy, nuclear bond energy, what have you) into kinetic energy. Rockets are horribly inefficient; the worse the specific impulse, the more horrible, and we’ve got only one technology for getting off Earth right now: chemical rockets, about the worst for specific impulse. Nuclear rockets get at most an order of magnitude better impulse, and that requires designs like the gas phase reactor, which is as yet purely theoretical. Building better rockets is largely an exercise in choosing materials because specific impulse is dependent on the temperature of the exhaust, the higher the better.

Ion rockets give high specific impulse, but aren’t useful for getting off earth because we can’t build one with enough thrust; power requirements for a given thrust increase as the square of the impulse.

Any system that leaves the power source on Earth beats a system that has to carry its power, so catapults, launch loops, skyhooks, and space elevators are better candidates for industrial scale operations into LEO than rockets. Clearly nanotech could be useful in creating such systems, since they involve either direct contact or magnetic levitation and transfer of momentum between structure and payload. The large power density and cooling capability of nanomachines could make launch systems very efficient and (eventually) very cheap to build.

Eric Drexler July 28, 2009 at 12:17 pm UTC

Actually, calculations show that a LOX/LH2 fueled vehicle with about the mass of an SUV (that’s with fuel: much less when empty) could transport several people to orbit, with some luggage. (Several years ago, I dusted off my MIT aerospace engineering training for this calculation, was surprised by the result, and cross-checked it to be sure.) This says that conventional rocket technologies can be quite effective, provided that tanks and other structures are sufficiently lightweight.

The example assumes structures made of carbon-based materials with strength-to-density ratios in the graphene/diamond range (comparable to carbon nanotubes), but these can’t yet be manufactured with the necessary microstructures and at the necessary scale.

Once in orbit, ion engines work, and scaling laws permit remarkable performance for systems powered by photovoltaic devices with micron-scale cells and 100-micron-scale concentrators and radiators. The key physical facts are (1) photovoltaics can operate, and actually gain some efficiency, under sunlight concentrated by factors of 100 or more, provided they are kept cool, and (2) small cells and short heat-flow paths enable thin, low-mass thermal radiators to provide effective cooling: thermal loads are small (because amount of intercepted light per unit is small), small temperature differences (~ 1 K) produce large thermal gradients (~ 104 K/m), and as a consequence, thin-film structures can serve both as reflectors and as radiators, given some tricks that can raise the emissivity of the back surface with a low-density structure of metal filaments. The resulting improvements in power-to-mass ratio, the key figure of merit for electric propulsion, are orders of magnitude beyond current practice.

Note that this involves no new device physics whatsoever — the results are straightforward consequences of scale and geometry in intricate structures that (again) can’t yet be manufactured.

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