Shirtbutton-sized Gas Turbines: the Engineering Challenges of Micro High Speed Rotating Machinery

MIT is developing micro-electro-mechanical systems (MEMS)-based gas turbine engines, turbogenerators, and rocket engines. Fabricated in large numbers in parallel using semiconductor manufacturing techniques, these engines-on-a-chip are based on micro-high speed rotating machinery with power densities approaching those of their more familiar, full-sized brethren. The micro-gas turbine is a 2 cm diameter by 3 mm thick Si or SiC heat engine designed to produce about 10 W of electric power or 0.1 N of thrust while consuming about 15 grams/hr of H2. Later versions may produce up to 100 W using hydrocarbon fuels. This paper gives an overview of the project and discusses the challenges faced in the design and manufacture of high speed microrotating machinery. Fluid, structural, bearing, and rotor dynamics design issues are reviewed. INTRODUCTION High speed rotating machinery comes in many sizes. In recent years much emphasis has been placed on the large end of the business – 10 m diameter hydroelectric turbines, 300 ton ground-based gas turbine generators, 3 m diameter aircraft engines. These machines are engineered to produce hundreds of megawatts of power. The focus of this paper is the opposite end of the rotating machine size scale, devices a few millimeters in diameter and weighing a gram or two. These machines are about one thousandth the linear scale of their largest brethren and thus, since power level scales with fluid mass flow rate and flow rate scales with intake area, they should produce about one millionth the power level, a few tens of watts. The interest in rotating machinery of this size range is fueled by both a technology push and a user pull. The technology push is the development of micromachining capability based on semiconductor manufacturing techniques. This enables the fabrication of complex small parts and assemblies – devices with dimensions in the 1-10,000 micron size range with micron and even submicron precision. Such parts are produced using photolithography defined features and many can be made simultaneously, holding out the promise of low production cost. Such assemblies are known as micro-electrical-mechanical systems (MEMS) and have been the subject of thousands of publications over the last decade. Early work in MEMS focused on sensors and many devices based on this technology are in large scale production (such as pressure sensors and airbag accelerometers for automobiles). More recently, work has been done on actuators of various sorts. Fluid handling is receiving attention as well, for example MEMS valves are commercially available. The user pull is predominately one of electric power. There is proliferation of small, portable electronics – computers, digital assistants, cell phones, GPS receivers, etc. – which require compact energy supplies. The demand is for energy supplies whose energy and power density exceed that of the best batteries available today. Also, the continuing advance in microelectronics permits the shrinking of electronic subsystems of mobile devices such as robots and air vehicles. These small, and in some cases very small, systems require increasing compact power and propulsion. For compact power production, hydrocarbon fuels burned in air have 20-30 times the energy density of the best current lithium chemistry-based batteries. Thermal cycles and high speed rotating machinery offer high power density compared to other power production schemes and MEMS technology is advancing rapidly. Recognizing these trends, a group at MIT began research in the mid 1990’s on a MEMS-based “micro-gas turbine generator” capable of producing tens of watts of electrical power