[AR] Re: Freeman Dyson, RIP

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: Arocket List <arocket@xxxxxxxxxxxxx>
  • Date: Wed, 4 Mar 2020 18:08:09 -0500 (EST)

On Tue, 3 Mar 2020, I wrote:

...So it is fairly obvious that you could make a bunch of "fixed" solar sail like structures in a sphere around the sun...

If you can get total mass down to about 1.5g/m^2, which is what sunlight will support at 1AU. That's challenging -- the Team Encounter sail design, circa 20 years ago, was aiming for 3.63g/m^2 (including structure and a minimal payload), and that was a very aggressive design... The fundamental limits are much lower, but...
At the cost of having only half the thrust, your sails can be black instead of reflective... Good solar cells are actually pretty close to black, although it's not clear that you can make them thin enough...

Fast answer:  barring breakthroughs, I suspect not.

Looking at some (admittedly rather old) references on thin-film solar cells, the thinnest of the technologies can give you working cells where the active layer plus the contact layers on top and bottom total circa 2.5um thick. Given what materials they're made of, a first guess is maybe 10g/m^2 for that. Such cells are normally made on *much* thicker substrates, but if you can make them on L'Garde's 0.9um aluminized Mylar, or make them on something thicker and then transfer them to that, you just might be able to make working cells at 12g/m^2. Budget a bit for things like seams and first-level wiring, and 15g/m^2 "membrane" mass would be plausible; add the ultra-light structure L'Garde was proposing for the Team Encounter design, and then a modest payload fraction, and you might optimistically be at 20g/m^2.

Taking that down by a factor of ~25 will be tough. In particular, the active layer, which is circa 5g/m^2 all by itself, is already about as thin as it can be made and still be opaque! You'd have to start by finding a much better cell material. I can't say for sure it's impossible but I have my doubts.

Actually, what you want on the sunward side is something that absorbs most of the visible but doesn't emit much thermal IR (with an IR-emitter surface on the starward side)...

Thinking about this led me to notice another issue: the momentum of a beam of photons carrying power P is just P/c, and this is independent of wavelength. If you are absorbing all the sunlight in solar cells, then re-radiating that energy outward as heat (some of it directly as waste heat from the cells themselves, some of it from cooling radiators for the electrical equipment the cells are powering), the photon thrust from the waste heat has to be subtracted from the absorption thrust on the cells!

(Yes, radiators do produce thrust. The Topex/Poseidon spacecraft, with a couple of radar altimeters using a lot of power and producing a lot of heat, exploited its radiator thrust to help with orbit maintenance...)

Since the sunlight is arriving as a unidirectional (well, more or less) beam, while the waste heat is radiated diffusely, reducing its thrust due to divergence loss, I think you still end up with net thrust, but it's even less than the simplistic calculation would indicate.

Henry

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