This paper details the design, fabrication and testing of millimeter scale solid propellant rockets for use as one-time deployment platforms carrying communication-equipped MEMS sensor systems, known as Smart Dust. Each rocket 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.
Combustion chambers were fabricated in various materials, including silicon wafers; however, thermal losses were too high to reliably maintain a burn. Therefore, cylindrical alumina ceramic combustion chambers with thermal conductivities five times lower than silicon are used. Thrusts of up to15 mN have been measured for ceramic rockets weighing under 1 g, with specific impulses in the 10 to 20 s range. By reducing the propellant mass fraction and optimizing the nozzle geometry, calculations show that flight producing thrusts can be generated.
Silicon nozzles integrated with polysilicon igniters and thermopiles for thermal power conversion have been microfabricated in a single process. Fuel ignition by polysilicon heaters suspended on a low-stress nitride (LSN) membrane has been demonstrated. Igniters require as little as 0.2 W to ignite composite propellant, primarily composed of hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP). The igniter is suspended for thermal isolation through bulk post-processing by a backside deep reactive ion etch (DRIE). The etched hole beneath the igniter 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. 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 power Smart Dust circuitry or potentially rocket control surfaces.