Thinsat 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 0.3 design is 50 microns, but much thinner silicon wafers are used in current production, often loosely bonded to a thicker "handle" wafer for ease of processing. The thinsat will likely be built and tested with a thick handle wafer attached, but the handle wafer will be removed when the thinsat is attached to the deployment stack.

If the solar cell at end-of-life is 15% efficient, then the solar cell will produce 3.5 watts peak, less during maneuvering, and nothing during eclipse.

Characteristics

Electronics

Electronics: A thinsat 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 thinsats). That will be about 4 by 4 centimeters of silicon area. Computational thinsats 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 thinsats should be able to do far better.

Because the sthinsat is extremely thin, some common electronic components cannot be used: foil-wound 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 they can be powered with capacitive charge pumps. At microwave frequencies, resonators can also be made with striplines and other components. The potential of solid state integrated phased array lasers needs to be investigated for communications, small debris location and control, power transmission and active optical propulsion.

The thinsat will use a small array of radios (many more than the 14 shown) to communicate with neighbors in the array, with other arrays, and with the ground. Besides communication, thinsats 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 thinsat to thinsat communication. The thinsats 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 thinsat 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 thinsat 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.

Three optical thrusters are shown, which perform pitch and yaw control for the thinsat. Normally, the thinsat 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 thinsats on station relative to each other.

There is no direct way to control roll. However, the thinsat 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.

Thinsats 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 thinsats may stick together with vacuum welding or vander Waals forces. If a thinsat plus separators is 60 microns thick, then a stack of 20,000 3 gram thinsats will be 1.1 meters (43 inches) tall and weigh 60 kilograms (130 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 thinsat orbit is lower; a larger payload is possible. 4200 kilograms is 1,400,000 thinsats and 5.6 megawatts of electricity. Thus, a thinsat array can produce more than 400 times the power (and communication capacity) of typical comsats.

Note: This is version 0.3 of the serversat. Earlier versions were thicker and wider, and made from silicon rather than silicon-matched glass.

Thinsat geometry.