Orbital Composites and Virtus Solis announce space-based solar power demonstration - SpaceNews
k.a.c...@sympatico.ca
John Bucknell
--
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k.a.c...@sympatico.ca
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
Keith Henson
since they were first proposed.
I don't want to name people, but Kieran and I are not the only people
in the SBSP world who think this way. I go so far as to consider
swarm designs fraud.
Keith
John Bucknell
k.a.c...@sympatico.ca
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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Keith Lofstrom
> Virtus Solis’s approach to SBSP is to assemble near-gossamer
arrays of meter-scale monolithic “tile” satellites into highly
A very thoughtful analyis, thanks Kieran!
This vaguely resembles a bastard child of my server sky
brainfart ( http://server-sky.com ), though server sky
delivers only a few-db-above-noise-floor communication
signals, not a watts-per-square meter beam, not nearly
enough power to saturate the Low Noise Amplifiers in
all communication receivers in all bands everywhere
on Earth and near-Earth space, as sidelobes from SBSP
may carelessly do.
Server sky thinsat constellations are also orbiting,
rotating sparse arrays, though transmitting watts,
not gigawatts. As array orbits evolve, their sidelobe
patterns will also shift across the ground. Thosed
somewhat-spread-out arrays will make a "reasonably"
defined pattern on the ground, but that pattern will
stretch and un-stretch longitudinally as the array
rotates and while it orbits the Earth.
Some pretty animations of power delivery patterns
(with commented C code) here:
http://server-sky.com/RadioV01
Important note: as an array rotates and changes angle,
the main lobe is designed to hit the intended target, but
the rest of the sidelobes will drift all over the Earth's
exposed surface. Most places on Earth will see a sidelobe
drift in range, many times per orbit, multiplied by the
number of orbiting systems. Fences won't help, no place
is "safe".
The arrays that my webpage illustrates are placeholder
rectangular grids, for ease of understanding (and coding).
Production arrays will probably be geodesic ellipsoids.
Which take too much compute time, unless someone can
help me migrate the programs to CUDA or OpenCL.
The last (and most important) RadioV01 image illustrates
the effects of random antenna placement errors.
Sidelobes and spectral crap everywhere.
VERY IMPORTANTLY, femtosecond phase errors for each
individual emitter will also "fog" the emission patterns.
If the phase/timing error for each emitter is Gaussian
(and there are MANY emitters), there will be a WIDE
"Gaussian ghost" pattern surrounding the narrow main lobe.
For a watt-level communication array, that sidelobe pattern
will be no more (and no less) disturbing as interference to
other receivers "looking at" other transmitters, but again,
delivered communication signals (and their sidelobes) will
be quadrillion-times lower density than an SBSP power beam.
For power beams from MANY power satellites, with billions
of imperfect transmit elements overhead, the overlaid noise
can make terrestrial and comsat microwave communication
impossible anywhere on Earth.
Some folks dismiss these enormous side effects because
"Space Power is the only alternative to the collapse of
civilization". Sadly, also a "Final Solution" for other
uses of the microwave spectrum.
----
I read many books as a preteen. Two left a lifetime
impression; "The Rise and Fall of the Third Reich" by
William Shirer. Also a 1945 Frederick Brown short story
("The Waverlies") in Groff Conklin's 1962 "Invaders of
Earth" anthology.
The Waverlies describes the arrival of mysterious "energy
pattern" aliens, attracted to Earth by our radio emissions.
The aliens absorb the radio emissions, and grow to absorb
the energy of our power grids (and electrical appliances,
and electrical ignitions, and battery appliances, etc.).
Humanity returns to 19th century non-electrical lives.
Silly physics, but a sideways look at our dependence on
electricity and radio spectrum.
SBSP, implemented carelessly, may become a "Waverlies" for
radio communication and radar. If that happens, the first
powersats will be shut down, and (somehow) disposed of,
perhaps attacked, or eventually fragment into vast
quantities of large-cross-section space debris.
SBSP proponents might be sued (if lucky), jailed, or
lynched (worst case). A lifetime in court may feel like
the latter two options.
My early exposure to dystopian SF and dystopian history
has made me VERY skeptical of naïve utopianism and hasty
engineering. I ANALYZE THE HELL out of my designs, to
reduce the chances that they will deliver HELL to others,
especially to third parties suffering from the lazy
carelessness of my second party customers. I've had
screaming shouting matches with my marketing people
about taking money from careless or evil customers;
perhaps I am a failure as a capitalist.
But at the end of the day (or the twilight of an
engineering career), I want to look in the mirror
and see an old guy, but not an evil and ashamed guy.
Yes, we are disappointed that we haven't yet deployed
SBSP, but perhaps we are blessed that we did NOT deploy
spectrum- and orbit-destroying versions of SBSP,
and are NOT looking at ashamed old guys in the mirror.
Keep looking in the mirror. Keep maturing our juvenile
ideas. With decades of responsible adult guidance, and
willingness to scrutinize and ELIMINATE selfish careless
childish thinking, SBSP ideas can evolve and mature into
beneficial and SAFE deployed systems. Bearing as much
resemblance to our current ideas as a lungfish resembles
a human, or a twelve-year-old boy resembles an engineer
with a half-century of experience (and battle scars).
Keith L.
--
Keith Lofstrom kei...@keithl.com
Keith Lofstrom
> Long series of questions, but the first ones assume a swarm and our arrays
> are large contiguous systems and don't suffer from sidelobes at all.
dipole, or a laser diode. Radio engineering 101, or
Radio Amateur's Handbook. Not every analysis (or mind)
is accurate and realistic enough to include sidelobes.