Microrockets for Smart Dust

This paper details the design and fabrication of millimeter-scale solid propellant rockets for one-time deployment of wireless sensor platforms; known as Smart Dust. Each microrocket assembly is an integrated system, incorporating a combustion chamber, composite propellant grain, nozzle, igniter, and thermoelectric power converter. Solid propellant is advantageous for a millimeter-scale single-use device because of its simple implementation, unlike liquid propellants, which require a more elaborate system of pumps and valves. Therefore the total system volume and complexity are minimized. One type of combustion chamber was fabricated in silicon; however, thermal losses to the silicon sidewalls during combustion through a 1:5 mm(2) cross section of fuel were too high to reliably maintain a burn. Successful combustion was demonstrated in cylindrical alumina ceramic combustion chambers with thermal conductivities five times lower than silicon and cross sections of 1-8 mm(2). Thrusts of 10-15 mN were measured for ceramic rockets weighing under 1 g, with specific impulses up to 15 s. Silicon nozzles integrated with polysilicon microheaters and thermopiles for thermal power conversion were microfabricated in a single process. Fuel ignition by polysilicon microheaters suspended on a low-stress nitride (LSN) membrane was demonstrated. Microheaters require less than 0.5 W of power to ignite a propellant composed primarily of hydroxyl-terminated polybutadiene (HTPB) with ammonium perchlorate (AP) oxidizer. They are suspended for thermal isolation through bulk post-processing by a. backside deep reactive ion etch (DRIE). The etched hole beneath the igniter area also serves as a nozzle through which high-velocity combustion gases exit the rocket. Thermopiles, which generate voltages proportional to hot and cold junction temperature differentials, have been fabricated in the same process as igniters, and span backside DRIE thermal isolation cavities. Ten junction thermopiles produced a maximum power of 20 muW. With potential temperature differences of hundreds of degrees and a total of 120 thermocouple junctions fabricated on the silicon nozzle chip, hundreds of milliwatts of power could feasibly be produced during the microrocket's flight and used to augment the Smart Dust power supply.