[175] This is a conference to discuss aerospace food technology, and at first thought such discussion seems far removed from my experience in Raytheon's Industrial Microwave Processing Department. Our concern has been the speedy processing of large quantities of food but this is not applicable in space vehicles. If NASA were considering conveyances with many passengers, we could discuss our conveyorized microwave system for heating meals quickly; however, I have been led to believe that it may be some time before this will be necessary. It appears that we must think small and light for NASA.
Along the foregoing lines the use of highly efficient microwave heating should be considered if hot meals are desired. This leads to the question: Why should microwaves be used in a space application instead of a forced-air convection oven or some other more conventional form of electric heater ? The answer is the claim to fame of microwave heating; energy is converted to heat inside the food and is not used to heat the walls and air inside the oven.
I know of no unit currently available that would satisfy the needs of a space vehicle. There is, however, a 91-lb microwave oven being mass produced by our Amana division. This appliance was designed for the consumer market wherein heavy emphasis is placed on chrome trim, portability, and price.
A redirection of emphasis, with reliability, weight, and size as the prime requirements, could produce a usable piece of equipment for space vehicles. Factors to be considered in such a design are power supply, energy source, applicator (or, as it is commonly called, oven), construction materials, controls, food, and radiation.
Reliable, light-weight power supplies now exist for space applications. There is no doubt that a design specifically for this application is not too far away.
There are many factors to be considered in the selection of an energy source, which is the heart of a microwave heating system. Of prime importance is the power output. This should be the minimum amount necessary to heat the food, since this amount is directly related to the amount of power drain from the vehicle's electric system. Frequencies other than the commonly used 2450 mHz could be investigated. The FCC has allocated three other frequencies for this usage, namely, 915, 5850, and 22 125 mHz. Very little has been done at the two higher frequencies where parameters are generally smaller
[176] Our experience in producing high-power microwave generators which have efficiencies over 80 percent could be extrapolated to a lower power generator. To replace the present fairly heavy electromagnet, a permanent magnet utilizing Raytheon's new light-weight samarium cobalt magnet material might be considered.
Since the applicator (oven) is the part of the equipment which contains the food, its design should be conditioned by the size and shape of the food package to be heated. This requires an interface between microwave-oven and space-food designers. Most present-day microwave ovens are designed to be universal in use so that any process from heating a cup of soup to completely cooking a large toast is possible. An exception is the small unit in coin-operated vending machines used to heat sandwiches which are stored in a nearby refrigerator.
The use of strong light-weight materials in the frame and power supply is one area which warrants investigation. It may be possible to save considerable weight in the cavity by painting, plating, or laminating a microwave conductive coating over a light-weight plastic form to replace the usual, heavy stainless-steel cavity.
A wide variety of controls exist which function very well and extremely reliably. It appears that not much more than a weight-reduction program need be launched.
From a microwave engineer's point of view, the food should always have the same dielectric parameters, be of uniform density, and completely load the cavity electrically. As previously mentioned, microwave engineers and food technologists should jointly discuss these problems.
The intent of the design should be to cook food - the heat should not be wasted on equipment or people. Many excellent radiation containment techniques that may be made lighter in weight for use in a space vehicle now exist.
In summation, the design of a microwave heating unit for space vehicles is quite feasible within the current state of the art.