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John;
Thanks for sharing those links.
I have a couple of technical questions (which I’m sure others in this list also have), if you’re willing to discuss such things. You wrote:
Virtus Solis’s approach to SBSP is to assemble near-gossamer arrays of meter-scale monolithic “tile” satellites into highly elliptical ground synchronous Molniya orbits which feature long dwell times over a given site on the Earth’s surface. The thin “tile” architecture includes PV on one face, power electronics, communications and control in a center layer and WPT phased array antennas (PAAs) on the opposite face. A co-orbiting modular gossamer mirror adjacent to each array ensures that the satellite’s solar collectors receive sunlight continuously by rotating about the array’s normal axis once per year. Wireless power transfer is accomplished via integrating the satellite tiles into kilometer scale PAAs operating at 10GHz.
- It sounds like each of the meter-scale tile satellites is an independent satellite, each of which is orbiting the Earth independently, but with the orbit of each chosen in order to maintain a desired position with respect to its neighbors in a swarm formation; is that right? With the swarm geometry designed to form a large (km-scale) phased-array antenna, comprised out of the many meter-scale PAA’s all being in the right place with respect to each other, at all times (or at least, at all times that they are intended to transmit power, up near apogee). Is that right?
- If so, then there will be gaps between each of the meter-scale tile satellites, presumably larger than a quarter of a 10 GHz wavelength. As a result, the km-scale PAA will produce much larger sidelobes, than would a monolithic km-scale PAA in which all the radiating elements are equally spaced at a distance of rather less than 1 wavelength. Which is to say, this “swarm” PAA will “suffer from the thinned-array curse”: https://en.wikipedia.org/wiki/Thinned-array_curse. Do you agree?
- Assuming that you agree with that point, how does it make sense to compare the Virtus Solis architecture head-to-head with other architectures that use filled aperture arrays for power transmission, rather than spare arrays (i.e., the other designs in your paper)? The amount of power delivered to the target area on the ground will be smaller for the sparse array case, with the remaining power showing up in sidelobes some distance away from the main-lobe’s footprint on the ground.
- Also in that case, what’s the concept for dealing with the greatly-increased power in the sidelobes? It will greatly increase the RF interference (both at the main transmit frequency, and at other frequencies due to passive intermodulation) experienced by radio receivers far from the main lone of the power beam, reducing the likelihood that such an SPS would be deemed acceptable by other users of the RF spectrum (i.e., everyone). If the sidelobes are powerful enough, they could end up exceeding the deemed-safe level of power flux density, and hence require large other areas on the ground to be fenced off for safety reasons (not just the area around the main lobe).
- (Note: I and others have the same questions for the CalTech swarm architecture, haven’t heard any answers from them yet.)
- The gossamer mirror that is co-orbiting with the arrays will have a different mass/area ratio than those arrays, and be much more reflective, and as a result will experience a larger solar radiation pressure force, and resultant acceleration, than will the arrays. Because of this, it seems to me that the orbit of the mirror will quickly evolve away from the orbits of the meter-scale tile satellites, unless the mirror is equipped with a propulsion system to counter the SRP thrust. Have you taken that into account in your concept design and mass budgeting?
- More generally, have you yet done orbital dynamics simulations for the tile satellites and the mirror, that take solar radiation pressure effects into account, using a realistic solar radiation pressure model for each? (A reasonably realistic model can be found in this paper that I co-authored: https://www.researchgate.net/publication/313851304_Real_Solar_Sails_are_Not_Ideal_and_Yes_It_Matters.) If so, I’d be fascinated to hear what results came from that. Also, what assumptions you made regarding the absorptivity and reflectivity of the faces of the tiles that are covered with photovoltaic cells --- I’ve had a heck of a time finding decent estimates of these parameters for solar cells, to the level needed in order to properly estimate SRP effects. (Note that those properties vary with the amount of power being pulled out of each solar cell, because that changes its absorptivity.)
- (I have the same question about CalTech’s swarm satellites --- the most detailed paper I’ve found amongst their publications says that SRP effects were explicitly neglected.)
Note: I’m somewhat spun up on the SRP effect on SPS, having recently completed a study for ESA on the topic (https://activities.esa.int/4000137232). ESA hasn’t yet published our final report; the main result is that SRP has a significant effect on the orbits of satellites in GEO (as is well known by anyone who owns and operates a GEO commsat, as this has a noticeable effect on station-keeping), and the effect goes up linearly with area-to-mass ratio. Hence it’s quite a bit larger for any reasonable SPS design, than it is for typical GEO commsats; and it’s hugely larger for a “gossamer mirror” (AKA a solar sail). Which is to say, any SPS design *must* take SRP effects on its orbit into account, and swarm designs even more so, and swarms that include co-orbiting mirrors more so yet.
- Kieran
John;
Glad to hear your architecture is not a swarm! That’s unclear from the write-up in your paper; something to keep in mind when you’re describing it to others.
Re: the stationkeeping/SRP thing, looking forward to the day when you can tell us more!
- Kieran
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