Microrocket for smart dust

Abstract This paper details design and fabrication aspects 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 silicon wafers; however, thermal losses during combustion through a 1.5 mm² cross section of fuel were too high to reliably maintain a burn. Therefore, cylindrical alumina ceramic combustion chambers with thermal conductivities five times lower than silicon are being used, in which combustion through cross sections from 1 to 8 mm 2 has been demonstrated. Thrusts of 10 mN to 15 mN have been measured for ceramic rockets weighing under 1 g, with specific impulses up to 15 s. 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 less than one watt of power to reach the ignition temperature of HTPB/AP propellant, approximately 600 o C. 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, 0.5 µW of power could feasibly be produced during the microrocket's flight and used to augment the Smart Dust power supply.