As the Mach number increases, the kinetic energy of the air increases by the square of the speed. As we saw in Figure 4.3, the kinetic energy of the air rapidly exceeds the thermal energy available to be transferred to the engine working fluid, air. The fraction of the combustion energy rejected as unavailable for conversion to useful work is also significant. In a modern turbojet engine only about 23% of the fuel combustion energy is actually converted to thrust, and 44% is discarded out of the exhaust nozzle unused except to make a hot atmosphere [Kroon, 1952]. With commercial high bypass ratio engines, about 31% is converted to thrust. It is critical then to examine what part of the energy that has been carried on board the aircraft has not converted to useful work or thrust. Any increase in the useful work conversion ratio reduces the propellant carried on board and thus the gross weight. The result of this analysis and of many efforts was the thermally integrated combined cycle propulsion system. The combined cycle engine concept's fundamental element began as a rocket ejector ramjet-scramjet [Stroup and Pontez, 1968], thermally integrated into a rocket propulsion system, and that has a long history in hypersonics. An excellent discussion of the subject, by one who was already working in supersonic combustion engines in 1958, is by E. T. Curran, [Curran, 1993]. Another early pioneer, Dr Frederick Billig, added many insights into the advantages of thermal integration [Billig, 1993]. Other nations were also working on thermally integrated concepts, and one excellent source is from TsAGI [Lashin et al., 1993]. In the class of integrated ejector ram-scramjet propulsion, the integral rocket ejectors provide both thrust and compression at lower Mach numbers. [Buhlman and Siebenhaar, 1995]. The combination of a separate ramjet and turbojet results in a poor acceleration. However, the introduction of a deeply cooled turbojet thermally integrated with an expander rocket (KLIN cycle) [Balepin and Hendrick, 1998] is
analogous to the rocket ejector ram-rocket-ramjet, with an additional benefit of excellent low-speed performance.
6. Deeply cooled turbojet-rocket (KLIN cycle). Figure 4.14 is an adaptation of Rudakov and Balepin's deeply cooled rocket ramjet into a deeply cooled turbojet-rocket. The turbojet and expander cycle rocket are thermally integrated [Balepin and Hendrick, 1998]. Unlike the ramjet, the pre-cooler on the turbojet keeps the compressor air inlet temperature low to reduce required compressor work and to increase mass flow and thrust. With the pre-cooler, the turbojet does not see the inlet temperature associated with higher Mach number flight, so it "appears" to be at lower flight speed. The pre-cooled turbojet provides a significant increase in transonic thrust. Even with the increased transonic thrust, the turbojet remains a poor transonic accelerator. So the KLIN cycle operates with the rocket as a team. Whenever the turbojet thrust is not adequate to maintain a higher value of effective specific impulse, the rocket engine operates to add additional thrust and increases the effective specific impulse, as defined below:
' airbreather sp xspe
Propellant flow Thrust — Drag Propellant flow wrocket ^ wairbreather
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