Server Sat Details

Silicon circuits, solar cells, interconnect, are essentially two-dimensional systems. The horizontal dimensions of an integrated circuit die may be measured in millimeters, but all the important action occurs within a few microns of the top surface. Indeed, modern IC die are thinned to increase thermal conductivity and reduce package height. The target thickness for this version 0.1 design is 100 microns, but much thinner silicon wafers are used in current production, often loosely bonded to a thicker "handle" wafer for ease of processing. The server-sat will likely be built and tested with a thick handle wafer attached, but the handle wafer will be removed when the server-sat is attached to the deployment stack.

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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 a gram.
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


This is a front view - from the side, a server-sat is as thin as a piece of paper. full size drawing

If the solar cell at end-of-life is 13% efficient, then the solar cell will produce 6 watts.

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 (many more than the 6 shown) 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.

Optical 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 electro-optic shutter material and 1 micron diameter glass beads. A different spacing may be chosen if that improves performance or survivability.

Liquid crystals (as shown) are well known, but they respond very slowly at low temperatures and do not have the best contrast ratio. A superior alternative may be electrochromic materials

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Unlike regular LCD displays, the optical 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 optical thrusters are shown, which perform pitch and yaw control for the server-sat. Normally, the server-sat is pointed straight at the sun, and the thrusters are electrically stimulated 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 thrusters 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, the server-sat can be rolled 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. Some kind of very thin separator will be needed, or the server-sats may stick together with vacuum welding or vander Waals forces. If a server-sat plus separators is 110 microns thick, then a stack of 10,000 server-sats will be 1.1 meters (43 inches) tall and weigh 70 kilograms (150 pounds).

A typical modern geosynchronous communication satellite such as HotBird 9 weighs 4880 kilograms (10,800 pounds) and has a 14kW array, launched by an Ariane 5 ECA, which can put 10,500 kg of satellite and apogee kick motor into a geosynchronous transfer orbit. The planned m288 server-sat orbit is lower; a larger payload is possible. 4200 kilograms is 600,000 server-sats and 3.6 megawatts of electricity. Thus, a server-sat array can produce more than 250 times the power (and communication capacity) of typical comsats.