Thermally Integrated Combined Cycle Propulsion

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

Figure 4.14. KLIN cycle, thermally integrated turbojet-rocket.

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:

Thrust

- rocket

' airbreather sp xspe

Propellant flow Thrust — Drag Propellant flow wrocket ± wairbreather

Because of its lower thrust, a hydrogen-fueled turbojet is about equivalent in effective specific impulse in the transonic region to a hydrogen-oxygen rocket. In afterburner operation, the rocket outperforms the turbojet. Thermally integrated together the combination is better that the sum of individual engines, as demonstrated in Figure 4.16. The thermal energy from both the rocket and turbojet is used to power the expansion turbines that drive the propellant turbopumps. If there is remaining excess energy it can be added to a heat exchanger upstream of the turbojet combustor. The pre-cooled turbojet provides operation from takeoff to Mach 5.5 with rocket thrust augmentation when required, such as in the transonic region. Above Mach 5.5 turbomachinery is shut down and the rocket operates as a conventional cryogenic rocket.

7. LACE rocket-ram-scramjet. Figure 4.15 is the engine family in Figure 4.11 integrated with a ramjet. As in Figure 4.16, the results with a LACE rocket will

LAC H based Combined Cycle Deeply Cooled Combined Cycle

Figure 4.15. Airbreathing rocket thermally integrated combined cycle.

LAC H based Combined Cycle Deeply Cooled Combined Cycle

Figure 4.15. Airbreathing rocket thermally integrated combined cycle.

be similar to the deeply cooled rocket. The airbreathing rocket operates only to Mach 6 or less, so the companion engine is a subsonic through-flow ramjet. In this cycle the thermal energy from the incoming air and hydrogen combustion is used to drive an expansion turbine that in turn drives a turbopump. A rocket motor combustion chamber heat exchanger is necessary to provide sufficient energy to drive the turbomachinery. After leaving the expansion turbine, the hydrogen is introduced into the ramjet combustion chamber. The inlet air is cooled to nearly saturation by an air-hydrogen heat exchanger, and then pressurized to a few atmospheres. It then flows into the pressurized liquefying heat exchanger. The turbopump pressurizes the liquid air to rocket operating pressures so it can be introduced into the rocket combustion chamber. After exiting the turbomachinery, the hydrogen is introduced into the ramjet combustion chamber. At Mach 6 or less, the rocket is essentially an airbreathing rocket operating in parallel with a ramjet. The ramjet can convert to a supersonic through-flow engine (scramjet) at Mach above 6, but the rocket is now a conventional cryogenic rocket, not an airbreathing rocket. Above Mach 6, the rocket is normally not used when the scramjet is operating. After scramjet shutdown the rocket operates as a conventional expander cycle cryogenic rocket.

8. Deeply cooled rocket-ram-scramjet. Figure 4.15 is the integration of the deeply cooled cycle developed by Rudakov and Balepin at CIAM and Alan Bond for HOTOL [Anon., BAC, 1991] with a subsonic through-flow ramjet. In this cycle the recovered thermal energy from the incoming air and hydrogen combustion in both the rocket and ramjet is used to drive an expansion turbine, which in turn drives a turbocompressor. The incoming inlet air is cooled to nearly saturation in an air-hydrogen heat exchanger, and then compressed to rocket operating pressures by the turbocompressor so it can be introduced into the rocket combustion chambers. A rocket motor combustion chamber heat exchanger is necessary to provide sufficient energy to drive the turbomachinery. After leaving the expansion turbine, the hydrogen is introduced into the ramjet combustion chamber. At Mach 6 or less, the rocket is essentially an airbreathing rocket operating in parallel with a ramjet. Above Mach 6, the rocket is normally not used, and the ramjet operates as a supersonic through-flow ramjet (scramjet). After scramjet shutdown the rocket operates as a conventional cryogenic rocket.

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