Measurement and control systems based on microwave radiometers would be used to prevent unstarts in the next generation of high-speed (up to mach 2.4) civil-transport airplanes, according to a proposal. "Unstart" denotes a compressor stall in a supersonic aircraft engine, and is caused by a change in the temperature of the incoming air, as explained in more detail below.
Unstart could be prevented by adjusting engine bypass baffles according to changes in temperature, but sensing the temperature of air as it enters an engine does not give enough lead time for performing the adjustment. Accordingly, the microwave radiometer in a system of the proposed type would measure air temperatures far enough ahead of the airplane to enable baffle adjustments in anticipation of changes in the temperature of the entering air.
The unstart problem arises in a supersonic-aircraft engine because the operation of the most efficient type of engine involves the formation of a shock front inside the engine at a location for which the dimensional tolerance is very small. Changes in the temperature of the incoming air move the shock front, and if these changes cannot be anticipated in time to adjust the baffles to maintain the shock front at the desired position, compression is lost, and thus engine thrust is lost. The engine is then restarted automatically. However, from the perspective of safety and the comfort of passengers, restarting is not acceptable as a routine procedure; what is needed is to prevent unstart in the first place.
Airborne microwave radiometers have been developed for measuring temperature-vs.-altitude profiles (see figure) and to warn against clear-air turbulence ahead of airplanes; similar microwave radiometers could be used in unstart-prevention systems. For example, a microwave radiometer operating at a frequency of 58.3 GHz, while aimed ahead of an airplane flying at mach 2.4 at an altitude of 60,000 ft (18 km), would measure the temperature of the air about 1 km ahead, giving an indication with about 1.4 seconds of lead time before the airplane encountered the air. This amount of lead time would be sufficient to enable the changes in baffle settings in time to prevent unstart.
If the microwave radiometer were to operate at several nearby frequencies in the range of 53 to 59 GHz, then it could generate more detailed information about upcoming changes in air temperature. The readings from the different radiometer channels could be processed into crude profiles of air temperature versus distance ahead.
One potential source of complication would be the shock front, which would produce a 4-in. (10-cm) layer of air with a temperature of 500 K over the front surface of the airplane. The heating would make it necessary to protect the forward-looking antenna of the microwave radiometer antenna with a heat-resistant, microwave-transparent fairing. For a microwave radiometer with a uniform absorption coefficient versus frequency throughout its frequency band, the heating in the shock front would contaminate the temperature readings by about 0.03 K - an amount small enough that it could be ignored.
This work was done by Bruce L. Gary of Caltech for NASA's Jet Propulsion Laboratory.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
NASA Management Office-JPL
Refer to NPO-19742.