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| ---- ||{{ attachment:serversat448.png }}||This is a front view - from the side, a server-sat is as thin as a piece of paper. <<BR>><<BR>> [[ attachment:serversat1800.png | full size drawing | target="_blank" ]] || Server satellites are thinned solar cells combined with thinned integrated circuit chips, some radios with antennas, and three disks of liquid crystal display (LCD) material that act as electrically controllable mirrors. The solar cell disk is thinned to reduce weight, and the chips and LCDs are thinned to match. Reduced weight reduces launch cost, and results in a more effective solar sail. The current thickness target is 100 microns, though production silicon is often thinned to as little as 20 microns for some applications. 100 micron thick silicon is very flexible, and can be rolled to diameters less than a centimeter without breaking. It is not unreasonable to assume that future server-sats can be as thin as 5 or 10 microns, weighing less than 2 grams. Everything is coplanar - the chips are arranged around the edge, and power is fed outwards (in separated zones) from the solar cell. If a portion of the cell shorts out or is otherwise damaged, the remaining circuitry should still work. |
||{{ attachment:serversat448.png }}||Server satellites are thinned solar cells combined with thinned integrated circuit chips, some radios with antennas, and three disks of liquid crystal display (LCD) material that act as electrically controllable mirrors. The solar cell disk is thinned to reduce weight, and the chips and LCDs are thinned to match. Reduced weight reduces launch cost, and results in a more effective solar sail. The current thickness target is 100 microns, though production silicon is often thinned to as little as 20 microns for some applications. 100 micron thick silicon is very flexible, and can be rolled to diameters less than a centimeter without breaking. It is not unreasonable to assume that future server-sats can be as thin as 5 or 10 microns, weighing less than 2 grams. <<BR>>Everything is coplanar - the chips are arranged around the edge, and power is fed outwards (in separated zones) from the solar cell. If a portion of the cell shorts out or is otherwise damaged, the remaining circuitry should still work<<BR>><<BR>><<BR>>''This is a front view - from the side, a server-sat is as thin as a piece of paper. '' [[ attachment:serversat1800.png | full size drawing | target="_blank" ]]|| |
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| Because the server-sat is extremely thin, some common electronic components cannot be used - electrolytic capacitors, cored inductors, etc. Bypass capacitors can be made thin, but it is better to keep peak impulse currents low. Some components such as crystal resonators may be replaced by MEMs devices. However, other devices such as radios can be operated at low voltages and low impedances, and if some devices need higher voltages at trickles of current (such as LCD electrodes) they can be powered with capacitive charge pumps. At microwave frequencies, resonators can be made with striplines and other components. | Because the server-sat is extremely thin, some common electronic components cannot be used - electrolytic capacitors, cored inductors, etc. Bypass capacitors can be made thin, but it is better to keep peak impulse currents low. Some components such as crystal frequency standards may be replaced by surface acoustic wave (SAW) devices. However, other devices such as radios can be operated at low voltages and low impedances, and if some devices need higher voltages at trickles of current (such as LCD electrodes) they can be powered with capacitive charge pumps. At microwave frequencies, resonators can also be made with striplines and other components. |
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| The server-sat will use a small array of radios (I'm assuming 6) to communicate with neighbors in the array, with other arrays, and with the ground. Besides communication, server-sats will measure radio propagation time to neighbors to accurately compute spacing and orientation, with additional location information provided by other arrays, ground stations, and possibly GPS. Multiple bands will be used, with frequencies that can penetrate atmosphere and clouds used for the downlinks, and other atmosphere-opaque bands used for server-sat to server-sat communication. The server-sats will ''not'' have dishes, but will act together as a phased array antenna. Given the wide array spacing, there will be many spurious lobes, but it will still be possible to compute solutions allowing separate beams to many ground stations, far more than a traditional dish-and-transponder comsat. While each server-sat may only dedicate one or two watts to the ground transmitters, the sum of thousands of transmitters will allow quite a lot of power for each beam. With the server-sat in a 4 hour orbit, it will be 7 times closer than a geosynchronous comsat, so there will be a 50x advantage in beam power and ground spot area. Round trip ping time will be 35 milliseconds, less than U.S. transcontinental ping time through optical fiber. | The server-sat will use a small array of radios (I'm assuming 6) to communicate with neighbors in the array, with other arrays, and with the ground. Besides communication, server-sats will measure radio propagation time to neighbors to accurately compute spacing and orientation, with additional location information provided by other arrays, ground stations, and possibly GPS. Multiple bands will be used, with frequencies that can penetrate atmosphere and clouds used for the downlinks, and other atmosphere-opaque bands used for server-sat to server-sat communication. The server-sats will ''not'' have dishes, but will act together as a phased array antenna. Given the wide array spacing, there will be many spurious lobes, but it will still be possible to compute solutions allowing separate beams to many ground stations, far more than a traditional dish-and-transponder comsat. While each server-sat may only dedicate one or two watts to the ground transmitters, the sum of thousands of transmitters will allow quite a lot of power for each beam. With the server-sat in a 4 hour orbit, it will be 7 times closer than a geosynchronous comsat, so there will be a 50x advantage in beam power and ground spot area. Round trip ping time will be 70 milliseconds, less than U.S. transcontinental ping time through optical fiber. |
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| '''Solar sailing:''' The disk has a sail area of about 0.1 square meters, and assuming low albedo, produces an optical thrust of approximately 4 nano-Newtons in 1300W/m^2^ sunlight. Additional thrust is produced by heat dissipation, but that is isotropic and sums to zero. That produces an acceleration of 5cm/sec/hour for a 30 gram server-sat, which does not seem like much. However, in a year, that is 400 meters per second, enough for small plane changes. Thinner disks will be more responsive. Actually, only a fraction of the thrust is available, as the disk must be turned one way or the other on opposite sides of the orbit for there to be a net effect. | ||'''LCD thrusters''' are two thin (30 micron) layers of commercial glass coated with of transparent Indium Tin Oxide (ITO) conductor on inner surfaces. The bottom layer glass is coated in separately controlled strips to permit partial functionality in spite of top to bottom shorts. In typical applications, a 1 micron gap is filled with nematic LCD material and flat 1 micron diameter glass beads, though a different spacing may be chosen if that improves performance or survivability.||{{attachment:thruster.png|}}|| |
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| Three LCDs are shown, which perform pitch and yaw control for the server-sat. Normally, the server-sat is pointed straight at the sun, and the LCDs are electrically stimulated to into transparency. If one or two of the disks are unstimulated, they turn reflective, and produce about half a nanoNewton of thrust. This is enough to slowly rotate the disk. If all the LCDs are stimulated, this increases centerline thrust. Again, the thrust difference is not much, but it is enough to keep an array of server-sats on station relative to each other. Server-sats will feel some very small tidal forces relative to the center of the array, and some offset thrust will be needed to keep them all aligned properly. | Unlike regular LCD displays, the thrusters need not operate quickly and small optical defects are not important. Bias fields in most liquid crystals are alternated at a 120Hz (or faster) rate to prevent plateout on one electrode. This is much faster than necessary for thruster purposes. However, this provides an intriguing alternate communication channel between server-sats; if one is close enough for its thrusters to shade the edges of one behind it, then it can modulate the light into a portion of the behind solar cell, perhaps generating measurable voltage fluctuations. This could be used for half-duplex communication between server-sats, at a 60 bps rate. That provides a secure and radio-silent way to propagate short messages such as encryption keys, for example. |
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| '''QUESTION(TBD): What about roll control? Something may need to stick out normal to the plane of the disk!''' | Three LCDs are shown, which perform pitch and yaw control for the server-sat. Normally, the server-sat is pointed straight at the sun, and the LCDs are electrically stimulated to into transparency. If one or two of the disks are unstimulated, they turn black, and produce about half a nanoNewton of thrust. This is enough to slowly rotate the disk. If all the LCDs are stimulated, this increases centerline thrust. Again, the thrust difference is not much, but it is enough to keep an array of server-sats on station relative to each other. There is no direct way to control roll. However, it should be possible to roll the system by [[ RollControlV01 | combinations of pitch and yaw ]]. |
Server Sat Details
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Server satellites are thinned solar cells combined with thinned integrated circuit chips, some radios with antennas, and three disks of liquid crystal display (LCD) material that act as electrically controllable mirrors. The solar cell disk is thinned to reduce weight, and the chips and LCDs are thinned to match. Reduced weight reduces launch cost, and results in a more effective solar sail. The current thickness target is 100 microns, though production silicon is often thinned to as little as 20 microns for some applications. 100 micron thick silicon is very flexible, and can be rolled to diameters less than a centimeter without breaking. It is not unreasonable to assume that future server-sats can be as thin as 5 or 10 microns, weighing less than 2 grams. |
![[ATTACH]](/ThinsatV3?action=AttachFile&do=upload_form&ticket=006724292b.733e0d1b66a681f356361b3f83c3495a33226fd7&target=serversat448.png "[ATTACH]"){kind=small}
Electronics: A server-sat used for database and web service may need as much as a terabit or more of flash memory (note - databases will be distributed over many server-sats). That will be about 4 by 4 centimeters of silicon area. Computational server-sats will probably need less. While some high-performance processors and chipsets use hundreds of watts, Giga-instruction-per-second level machines can get by with far less. Here is a complete 4 watt system (including IO and power conversion losses) with 990 bogomips performance. Optimized server-sats should be able to do far better.
Because the server-sat is extremely thin, some common electronic components cannot be used - electrolytic capacitors, cored inductors, etc. Bypass capacitors can be made thin, but it is better to keep peak impulse currents low. Some components such as crystal frequency standards may be replaced by surface acoustic wave (SAW) devices. However, other devices such as radios can be operated at low voltages and low impedances, and if some devices need higher voltages at trickles of current (such as LCD electrodes) they can be powered with capacitive charge pumps. At microwave frequencies, resonators can also be made with striplines and other components.
The server-sat will use a small array of radios (I'm assuming 6) to communicate with neighbors in the array, with other arrays, and with the ground. Besides communication, server-sats will measure radio propagation time to neighbors to accurately compute spacing and orientation, with additional location information provided by other arrays, ground stations, and possibly GPS. Multiple bands will be used, with frequencies that can penetrate atmosphere and clouds used for the downlinks, and other atmosphere-opaque bands used for server-sat to server-sat communication. The server-sats will not have dishes, but will act together as a phased array antenna. Given the wide array spacing, there will be many spurious lobes, but it will still be possible to compute solutions allowing separate beams to many ground stations, far more than a traditional dish-and-transponder comsat. While each server-sat may only dedicate one or two watts to the ground transmitters, the sum of thousands of transmitters will allow quite a lot of power for each beam. With the server-sat in a 4 hour orbit, it will be 7 times closer than a geosynchronous comsat, so there will be a 50x advantage in beam power and ground spot area. Round trip ping time will be 70 milliseconds, less than U.S. transcontinental ping time through optical fiber.
LCD thrusters are two thin (30 micron) layers of commercial glass coated with of transparent Indium Tin Oxide (ITO) conductor on inner surfaces. The bottom layer glass is coated in separately controlled strips to permit partial functionality in spite of top to bottom shorts. In typical applications, a 1 micron gap is filled with nematic LCD material and flat 1 micron diameter glass beads, though a different spacing may be chosen if that improves performance or survivability. |
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![[ATTACH]](/ThinsatV3?action=AttachFile&do=upload_form&ticket=006724292b.733e0d1b66a681f356361b3f83c3495a33226fd7&target=thruster.png "[ATTACH]"){kind=small}
Unlike regular LCD displays, the thrusters need not operate quickly and small optical defects are not important. Bias fields in most liquid crystals are alternated at a 120Hz (or faster) rate to prevent plateout on one electrode. This is much faster than necessary for thruster purposes. However, this provides an intriguing alternate communication channel between server-sats; if one is close enough for its thrusters to shade the edges of one behind it, then it can modulate the light into a portion of the behind solar cell, perhaps generating measurable voltage fluctuations. This could be used for half-duplex communication between server-sats, at a 60 bps rate. That provides a secure and radio-silent way to propagate short messages such as encryption keys, for example.
Three LCDs are shown, which perform pitch and yaw control for the server-sat. Normally, the server-sat is pointed straight at the sun, and the LCDs are electrically stimulated to into transparency. If one or two of the disks are unstimulated, they turn black, and produce about half a nanoNewton of thrust. This is enough to slowly rotate the disk. If all the LCDs are stimulated, this increases centerline thrust. Again, the thrust difference is not much, but it is enough to keep an array of server-sats on station relative to each other.
There is no direct way to control roll. However, it should be possible to roll the system by combinations of pitch and yaw.
In orbit, the stresses are very small; the disk is relatively rigid by comparison. Maneuvers such as rotation and acceleration will take hours to days; microgee forces are involved. The main stresses on the disk will be from thermal contraction. When exposed to sunlight, the disk will be at 300K; as it passes into earth shadow, it will quickly cool to 150K or less, heated only by the infrared radiation from the night side of the earth. The system is mostly silicon, with some glass and metal. Interconnect metal layers should be designed with strain relief, and the whole metal and insulator stack should have about the same average thermal coefficient as the silicon.
Server-sats are designed to have uniform thickness, so they can be stacked in cylinders for launch. If a server-sat plus metalization plus separators are 110 microns thick, then a stack of 10000 server-sats will be about one meter tall and weigh 300 kilograms. Some kind of very thin separator will be needed, or the server-sats may stick together with vacuum welding or vander Waals forces.