Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 102

ber of ongoing micropower (milliwatt-to-watt) development projects that include
(1) gasdynamic (microturbines, micro Wankel engines, etc.) and thermoelectric
energy conversion that use hot gas from microcombustors, and (2) hydrocarbon
fuel reformers that would enable fuel cells to operate using normal liquid fuels.
Participants in the micro power generator (MPG) program include the Massachu-
setts Institute of Technology, California Institute of Technology, Princeton, Uni-
versity of California at Berkeley, Case Western Reserve University, Georgia
Institute of Technology, the University of Southern California, Pacific Northwest
National Laboratory, Honeywell, and the Aerospace Corporation.
High-efficiency MEMS-based refrigerators, packaged within microelectron-
ics chips, may reduce the scope of, or even eliminate the need for, the complex
avionics cooling systems now used on high-performance military aircraft. This
technology would also reduce the weight and cost and improve the operability of
sensor coolers on aircraft, ordnance, and spacecraft.
Launch Vehicle Propulsion
Solid Propellant Rocket Motors
Solid rockets use a pressure vessel that surrounds a combusting solid to
direct the emerging hot gas through a converging/diverging nozzle. Each pound
of mass that can be removed from the pressure vessel, or casing, allows an extra
pound of payload to be delivered. Metal casings have given way to composite
casings made of carbon or glass fibers, which have higher strength-to-weight
ratios. Since solid rockets burn for a limited time once ignited, a layer of internal
insulation is used to shield the casing from high-temperature exhaust; this allows
composites with polymer binders to be used. An obvious application of micro-
and nanoengineering to solid rockets might be the use of carbon nanotube com-
posites if significantly improved strength-to-weight ratios are achievable; this
could significantly decrease casing mass. The technical challenge is to fabricate
carbon nanotubes of suitable length (millimeters or even centimeters) in large
enough quantities at reasonable cost (see the preceding section).
The propellant in solid rockets is a mechanical mixture of oxidizer, fuel, and
binder solids. When the local temperature gets sufficiently high, an exothermic
chemical reaction takes place. The key is to create a sufficient activation barrier
between highly energetic components so that both species can coexist at typical
storage and handling temperatures. Micro- and nanoengineered coatings may
enable more energetic species with overall higher performance to coexist under
normal conditions.
High-performance solid rocket propellants usually include aluminum pow-
der as fuel. Typical large solid rocket engines contain 14 to 18 percent aluminum
powder by weight. Nanopowder aluminum provides more rapid combustion ow-
ing to its increased surface-area-to-volume ratio, which results in faster linear