Effects of thermal compression on the performance estimates of hypersonic ramjets.

A relatively simple gasdynamic model is introduced to determine the effects of thermal compression on the performance of hypersonic ramjets using supersonic combustion. Real-gas thermodynamic cycle calculations based on hydrogen-air combustion are summarized. Although a discussion of the practical problems concerned in the development of a workable engine is not presented, the results show that improvements in specific impulse can be realized with thermal compression. Maximum performance is shown to occur when both the combustion process in each stream and the mass fraction between the streams are tailored. In general, the maximum twostream mass averaged specific impulse is obtained with nonoptimized processes in the individual streams. For a given minimum engine contraction ratio, the mass fraction of secondary air was found to be independent of flight Mach number. The gains due to thermal compression are greatest at low Mach numbers and small contraction ratios. At Mach 5 with a contraction ratio of 4, thermal compression increases specific impulse by 61%, whereas, at Mach 12 with a contraction ratio of 10, the gain is only 3.5%. Calculations also show that there is little gain in going from a two-stream to a three-stream model.