High-Temperature, Continuous-Flow Heater Technology

The designer of future hypersonic aircraft, missiles, and space launch will be challenged by the lack of similitude and/or productivity of hypersonic ground test facilities available today. Similitude issues include test article size, altitude, enthalpy, stream uniformity, and test media effects. The use of computational modeling and empiricism based on flight results can offset to some degree similitude deficiency. Boundary-layer transition effects are less of an issue than test-media effects are on propulsion and perhaps are more amenable to empiricism. However, they are very important to the prediction of range. As Mach number increases, non-similitude issues become disproportionately larger, especially pertaining to test media effects (not having true-air flight conditions) associated with propulsion performance prediction. Heating the airflow to a condition that is hot enough to avoid liquefaction is adequate for much of the testing required. Where achieving true-temperature conditions is important, the ability to heat the test media for the length of time required is vital. Issues associated with the process of heating the airflow become increasingly more significant as Mach number increases. Work is in progress to develop a heater capability for generating true-temperature, continuous-flow conditions up to Mach number 7 without products of combustion in the test media. This technology is contrasted with in-stream combustion heating and with heating from a storage heater. To develop an operational hypersonic flight vehicle, the duty cycle required of a continuous-flow heater should match the testing cycle required for a product development test series. The durability of the heater should be sufficient to support the program testing requirements. Moreover, if testing is to be accomplished as efficiently as possible, the duty cycle should match the needs for a test program. An estimate of the amount of testing required for prospective future hypersonic vehicles is developed for classes of vehicles represented by the National Aerospace Initiative. This estimate is used to project the duty cycle and durability requirements to be met by continuous-flow heater technology.