Trevor Brown proposes spherical solar power satellites.
This isn’t a new idea. I wrote a paper on it back in the early nineties for an SPS conference, and I think that Geoff Landis has done some work on it as well (for instance, here’s a report of a talk that he gave on it at the 1996 ISDC, which was the last one that I attended prior to by going to Dallas two years ago — ctrl-F for “spherical”). It does vastly simplify the design issues, because it is no longer necessary to point the panels at the sun. One of the comments there needs some elaboration:
While the surface area of the sphere facing the Sun matches your calculations, the whole side would not be available for power generation. The so-called Beta angle, or the Sun angle, affects the total amount of power converted. Also, while a sphere would not need rigid station-keeping and attitude control to collect solar energy, the transmitter back to Earth certainly will. Also, a large spherical structure would be more taxing on a station-keeping/attitude control system than a more planar design. These caveats in mind, this is a creative alternative.
With regard to the needed area, the beta-angle effect means that at any orientation, you’re only getting the effective solar panel area of the cross-section of the sphere. That is, while the hemisphere has twice the area of the circular cross section, the non-zero beta angle of all points except that at the center of the illuminated area means that you need twice the solar panel that would be necessary if it were a flat circle. Add to this the fact that you have just as much area on the side in shadow, and it means that you need four times the total solar panel area to get the equivalent collection capacity of a pointed flat plate. So you have to postulate very cheap panels for this to make economic sense. But if you can get them, the simplification of the design is worth a lot.
As for pointing the transmitter, that’s actually not so tough a job. You hang it down below the sphere, and it will remain vertical, due to gravity gradient restoring torques. You could point it with control cables all around its circumference, attached to the sphere. In addition to inflating it, I also considered putting a charge on its surface to keep it spherical, but it would take a lot of ions, particularly for a big one, and inflating is probably a better solution, though subject to leaks, and the need for gas resupply.
One other point. I actually considered a fleet of them in MEO, continuously switching from one rectenna to the next as they orbit, to reduce the size of the transmitter antenna, which gets kind of humungous out at GEO.
Could you have an internal, unfolding frame that “snaps into place” once the sphere has been inflated? Sort of like a paper mache pinate that keeps its shape after the balloon inside has been popped? That way you wouldn’t have to worry about gas loss, though I suppose the frame would obviously require more mass than one that relies on gas alone.
Rand,
Here’s a slightly related idea that may be synergistic with SPS, but might be entirely without merit. I haven’t got much feedback from NSF.com, and I was wondering about your opinion. As I said, it may be entirely without merit, please be gentle yet honest. 😉
As I am a big fan of incrementalism, I like precursor systems. Anything that has large solar panels can be a precursor to at least three things:
– a solar power satellite
– a solar electric tug
– a laser/microwave thermal propulsion base station
All three are currently considered hard, for various reasons. Size is one, solar panel degradation in the van Allens is another, and getting anything of non-neglible mass from Earth to orbit before it vanishes below the horizon requires enormous power.
But what if the power source, beam and spacecraft were all in LEO/MEO/GEO? Then the beam wouldn’t have to pass through the atmosphere, you would need less thrust and the delta-v’s involved would be a lot smaller. Also your spacecraft wouldn’t have to be aerodynamically shaped, it could be larger and it could have a larger heat exchanger. All these should make the problem easier. On the other hand, you now need infrastructure in space, which is a lot harder than infrastructure on the ground.
There may however be synergies with other programs that need in-space precursors. Take solar power satellites for instance, which in a way do the opposite of laser thermal propulsion, namely beaming down energy to earth. One precursor of this would be a solar power satellite for use on the moon, as was recently proposed by ENTECH. This system would not only be a precursor of a solar power satellite for use on earth, but also a precursor for a SEP tug, because it would use ion thrusters to get from LEO to LLO. And perhaps it could also be a precursor for the sort of thing you are working on. A power beaming satellite in LLO or more usefully at L1 or in GEO could support laser thermal or microwave thermal cargo transporters as part of a beyond-LEO supply chain.
The difficulty would be to get enough power into space, but people have been thinking of precursors of SPSs based on the ISS solar panels, with improved PV cells. This has long been a dream application for HLV proponents. Should NASA to decide to build J-130 they would initially have a worse than useless oversized LEO launcher without useful payloads and a small solar power satellite might provide a credible payload. And if they are not going to make it to the moon by 2020 as seems likely, they would need a credible destination beyond LEO but short of the moon. L1 sounds perfect for that.
Great idea, as long as it has a laser as the power downlink so the vehicle can have a dual mission.
http://en.wikipedia.org/wiki/Death_Star
A bit off-topic, but I watched a show on Nat Geo last night about the solar-powered rovers currently wandering about Mars. I was rather fascinated by the efforts of the JPL engineers to “aim” the flat solar panels at the sun by rolling the robots up and down the sides of hills and other available contours in the terrain. The name of the program is, “Five Years on Mars.”
It’s starting to sound more like Spaceballs than space powerballs. 🙂
Wouldn’t a rotation on the powerball spin stabilize things enough to help maintain positioning for the transmitter?
“Wouldn’t a rotation on the powerball spin stabilize things enough to help maintain positioning for the transmitter?”
Positioning becomes even harder. You then need some way to keep the transmitter from rotating with the powerball.
I really like Rand’s gravity gradient idea to give a stable base for keeping the transmitter pointed properly. It really appeals to the astrophysicist in me. But wouldn’t all the rectenna’s spread out around the Earth require the purchase or lease of a lot of real estate?
But wouldn’t all the rectenna’s spread out around the Earth require the purchase or lease of a lot of real estate?
Not as much as solar power, and it could be dual use (e.g., pasture) since it would be transparent to visible light, and the radiation would be absorbed by the rectenna above.
Yeah, Rand, I see what you mean. And there was something else I thought about right after I made the post that relates to the fact that the balls would be supplying power to an area around each rectenna, rather than only one locality benefiting (as would be the case with a GEO station). Since the locals would be benefiting, it might not be that hard to get them to give up a large piece of property. So even though we’re talking about a lot of land globally, it’s the locally affected people that count. But of course, your dual use argument makes it an even more attractive deal, because they would not have to give up the use of that area for other things after all!
Rand, any comment on the laser/microwave thermal propulsion idea?