For those not familiar with heating by microwave, a brief explanation is in order. Food is placed in an oven cavity and heated by molecular agitation. When exposed to microwave energy the water molecules in the food try to aline themselves with the rapidly time-varying electromagnetic field. The water molecules oscillate at 2 450 000 000 cps. The oscillating molecules rub against each other and heat is generated by this intermolecular friction. Heat transfer by convection and/or conduction is a secondary process which occurs after the outer surface of the food has been exposed to the microwave excitation. For maximum speed in cooking with microwaves it is desirable to heat the product from all six sides. Materials such as glass, paper, plastics, and ceramics are used to package the food product since they allow microwave energy to pass through them with no retardation. All types of food in any state of preservation (fresh, refrigerated, or frozen) can be used provided that they are properly prepared. The microwave oven does not perform magic; the product turned out of the oven is only as good as the product put in. However, because of the speed with which the oven heats, few detrimental effects occur and the product may have better appearance, quality, and nutritional value.
In the summer of 1963 Litton Industries was approached by a major airline to undertake the development of a microwave oven for the in-flight preparation of meals. As with many new concepts, an incubation period was necessary between initial conception and a working unit. In April 1965 the first flight test of a prototype oven was conducted jointly by the airline, Litton, and the FAA.
This first Litton airborne unit was designated the T-20 model. It was a single magnetron oven, weighing 86 lb, operating at 2450 Mc, and providing approximately 1200 W of power in the oven cavity. The T-20 design utilized many of the commercial state-of-the-art concepts prevalent at the time but used 400-cycle components.
With most revolutionary concepts two design approaches may be taken; one is to make it as simple as possible and add on only when necessary, whereas the other is to provide as many systems as possible within the given space envelope and extract them after new and less complicated solutions are available. The nature of the T-20 microwave oven dictated following the later approach. Therefore, the design incorporated sensing devices to ensure complete protection  against any possible RF interference. A sophisticated mechanism was devised which raised and lowered as well as rotated the food product. Although from an operational standpoint this conservative approach was less than successful, we cannot discount the impact this design and the operational flight tests made on the state of the art of airborne microwave ovens. Both the airline and Litton take credit for pioneering a concept which will be '~e forerunner of a valuable tool for in-flight feeding in the age of the Jumbo jets and supersonic transports.
After a complete review of the in-flight tests which included both domestic and transAtlantic flights, the design underwent a series of changes which culminated in the specifications for the present Model E-30. The feeding objectives included the boarding of only frozen prepared foods to be heated to order. This plan allowed foods to be returned to inventory if not used and ideally would make it possible to feed a passenger in less than 2 min.
The E-30 airline microwave oven is a double magnetron unit weighing 110 lb, operating at 2450 Mc, and providing approximately 2400 W of power in the enlarged oven cavity. The new concept deleted a number of the original requirements and concentrated on simplicity, reliability, serviceability, and reduced weight while it retained an acceptable heating pattern for the food products to be used in night. The cavity was enlarged to permit handling of greater quantities. Advancement in the general state of the art of all types of microwave ovens and the old cliche that "necessity is the mother of invention" explain the drastic changes that took place in the evolution of the E-30 sensing devices.
A primary concern during the initial T-20 development had been the effect of RF leakage from the oven on the many navigational and communication systems of the aircraft. No interference was detected at any time during the FAA - observed flight tests or the operational flight-test program. The T-20 had both a no-load sensor and a door-seal sensor which were programmed to shut off the heating cycle in the event that there was no load in the oven or that the leakage from the door exceeded a given level. Both these devices were extremely sensitive and required intricate electronic circuitry which resulted in nuisance shutoffs during operation and increased the cost of the equipment. A light load such as a Danish roll lacked sufficient density to prevent the no-load sensor from actuating. Accumulated moisture running down the front of the oven over the seal was sufficient to actuate the door-seal sensor.
New advances in the state of the art brought inexpensive and reliable solutions to both problems. A lossy glass or ceramic shelf now serves as a medium which can absorb the microwave energy, thus negating the requirement for a no-load sensor. The glass or ceramic is not too lossy to permit bottom heating of the food product; thus no extensive detrimental effect on the cooking pattern occurs. The shelves are capable of absorbing the energy for periods of time exceeding the maximum setting of the timer.
The success of seal-plate and choke-type doors in over 25 000 commercial applications has virtually eliminated the necessity for a door-seal sensor. Simplicity in design has provided adequate protection and reliability at reduced initial and sustaining cost. The E-30 oven which has a seal-plate-type door was tested before and after 30 days of flight testing and no leakage even close to allowables established by reference 1 was measured.
One of the major problems facing microwave oven designers is directing the waves uniformly to the food product. Since the heating is by direct interception of the RF waves and not by conduction or convection, hot and cold spots can occur. Some early designers utilized the rotatingshelf concept to balance the exposed food product to the energy. Other concepts broke up the direction of the waveforms by putting stirrers in the feedbox or waveguide. The T-20 utilized both approaches. The result was a near-perfect heating pattern for nearly all types of food products regardless of their geometric configuration or their density. However, the complexity and reliability of the mechanical system required to provide this optimum heating pattern were not compatible with the aircraft environment. Therefore, only the stirrer concept is used in the present E-30. The approach has been quite successful and it is possible to obtain a uniform heating pattern over a 1 1/2 -sq-ft area.
The primary and most dramatic improvement in the E-30 design is in weight reduction. New components and new electric concepts permitted a 30-percent reduction in weight. The new components also improved reliability and permitted use of modular construction techniques which improve serviceability.
The magnetron used in the T-20 was of an electromagnet type and weighed 13 lb. It was prone to filament failures, especially in the shock and vibration environment of an aircraft. The new L-5181 tubes have permanent magnets and weigh 6. 5 lb. New construction techniques make it possible to fabricate a magnetron which can withstand shock and vibration and provide 4000 to 5000 hr of operation.
A plate transformer is required to raise the input voltage from 200 to 3500 V and to protect the magnetrons from transient voltages present in the aircraft electric system. New design techniques and new high-temperature epoxies have been instrumental in reducing the weight by 50 percent. The transformer in the Model E-30 weighs 13 lb. The original T-20 transformer weighed 26 lb.
The E-30 concept was reduced to practice in early 1968 and underwent an in-flight evaluation during the summer of 1968. Results of these flights were very encouraging. No serious operation or technical problems were encountered. The feeding objectives were achieved, and several airlines are currently designing in-flight feeding systems which include a Litton E-30 microwave oven as an integral part. Litton is engaged in a full-scale marketing effort for both new aircraft installations and older aircraft retrofits.
Two other programs influenced to a large extent the state of the art of airborne microwave ovens. The first was the development of the E-31 oven. The E-31 is a single-magnetron oven  operating at 2450 Mc providing approximately 1100 W to the oven cavity. It weighs 82 lb and has a volume of 4.3 cu ft. The E-31 was developed specifically for the presidential fleet and is a 400 cycle version of a standard commercial model. These ovens have been in operation for 14 months The success of this equipment has demonstrated the ability of the electric components to withstand the aircraft shock and vibration environment.
The Speed oven is a large-cavity, 4-magnetron, 400-cycle oven developed for an advanced concept of a U. S. Army field kitchen. It is designed to feed 200 men or to supply 5000 men with bakery products. The ovens used in the Speed kitchens have demonstrated good reliability under actual field operations. At last report the ovens had over 800 hr of operation without magnetron failure.
The state of the art of airborne microwave ovens has improved significantly during the last 6 years. We are now able to provide 22 W/lb of oven, whereas originally we could provide only 14 W/lb. The use of solid-state power supplies could conceivably increase this ratio even more.
The use of microwave ovens for any mode of transportation depends to a large degree on the total feeding system. The type of food, food packaging, food storage, and oven must be compatible. The microwave oven system is superior to more conventional methods for small amounts of food and individual meals. Full power is attained in seconds; no warmup of the oven is required Therefore, the actual high electric power drawn from the vehicle power system is required only during the actual cooking time.
Although cooking by microwave in aircraft is a relatively new development, it has made significant advances in the last few years. The speed, cleanliness, and reliability of the concept make it an ideal system for all forms of transportation.