Patent Number: 
Section: description

Referring now to the drawings, and more particularly to FIGS. 2-4, an atomic-based combined cycle propulsion system is illustrated schematically in each of its ejector (FIG. 2), ramjet (FIG. 3) and rocket modes (FIG. 4) in accordance with the present invention. Each of the views will use the same reference numerals for the elements that are common therebetween. The system of the present invention is incorporated into an aerodynamic vehicle body 100 suitable for earth-to-space travel. The particular design of vehicle body 100 is not a limitation of the present invention. Housed within vehicle body 100 is a propellant tank or store 102 containing a fuel of choice. In most instances, the fuel will be liquid hydrogen owing to its low molecular weight, cost, availability, etc. However, methane and other hydrocarbon fuels are possible candidates due to their even lower cost, broad availability and ease with which they can be handled. Coupled to propellant store 102 via fuel lines 104 is a nuclear-based thermal rocket 106, the particular design of which can vary without departing from the scope of the present invention. A variety of nuclear-based thermal rockets are disclosed in the prior art. For example, see R. W. Bussard et al., xe2x80x9cFundamentals of Nuclear Flight,xe2x80x9d McGraw Hill, 1965; S. K. Borowski et al., xe2x80x9cNuclear Thermal Rockets: Key to Moon-Mars Exploration,xe2x80x9d Aerospace America, Vol. 30, No. 7, July 1992; and C. W. Watson, xe2x80x9cNuclear Rockets: High-Performance Propulsion for Mars,xe2x80x9d Los Alamos National Laboratory, Publication LA-12784-MS, May 1994. Regardless of the design of rocket 106, exhaust gases 108 are expelled through an exhaust nozzle 106A. An air induction system 110 is provided to selectively introduce surrounding ambient air 112 into exhaust gases 108. Such selective introduction would be controlled by adjusting, for example, a plurality of intakes 110A distributed about vehicle body 100. Operation of intakes 110A would typically be controlled in a pre-programmed fashion by an on-board processor 110B. Control of intakes 110A could also come from a remotely-located, pre-programmed processor, or by an on-board (or remotely-located) adaptive control system. The choice of control for intakes 110A can be tailored to meet specific mission requirements. In general, at vehicle speeds of less than approximately Mach 6 and vehicle altitudes less than approximately 40 kilometers, air 112 is introduced into exhaust gases 108 via intakes 110A where exhaust gases 108 and air 112 mix together to form mixture 114. The mixing action transfers energy to air 112 and increases momentum flux exiting nozzle 116. In other words, the introduction of air 112 augments the thrust force F applied to vehicle body 100. More specifically, from the static at-launch condition of zero velcity up to a vehicle speed of approximately Mach 2.5, the system of the present invention will typically operate as an ejector as shown in FIG. 2. That is, thrust augmentation (due to the introduction of air 112) is derived primarily from the physical mixing of exhaust gases 108 with air 112. (Note that even at these lower velocities additional energy may be obtained from some combustion of exhaust gases 108 and air 112). The term xe2x80x9cejectorxe2x80x9d refers to the pumping action that a low-pressure fluid stream (i.e., exhaust gases 108) exerts on a higher pressure fluid (i.e., air 112 introduced into system 110). Thus, when intakes 110A are opened, air 112 is sucked into the lower pressure exhaust gases 108 thereby causing the two to mix as indicated in FIG. 2 at reference numeral 114. This mixing process transfers momentum and energy from exhaust gases 108 to air 112 in mixture 114. Although the total energy of both exhaust gases 108 and air 112 remains constant, the total momentum increases and reaches a theoretical maximum when the exit velocity of mixture 114 exiting nozzle 116 is uniform, i.e., the velocity of exhaust gases 108 and air 112 in mixture 114 is the same. From vehicle speeds of approximately Mach 2.5 to Mach 6, the benefits from ejector-mode pumping will diminish, and the primary source of thrust augmentation will be due to combustion. That is, most of the mixture of exhaust gases 108 and air 112 will combust as indicated by burning mixture 115 in FIG. 3. This mode is known as the ramjet mode of operation. Assuming the fuel being burned in rocket 106 is hydrogen-based, nuclear-based thermal rocket 106 produces hydrogen exhaust gases 108 having an extremely high temperature that can reach 1500xc2x0 Kelvin. Since exhaust gases 108 are generated at such high temperatures, and since the pressure of air 112 increases at higher vehicle velocities, burning mixture 115 is generated without requiring any flame stabilization or auxiliary ignition devices. Note that as the speed of vehicle 100 increases, the amount (e.g., mass) of air 112 introduced into exhaust gases 108 is adjusted by controlling intakes 110A. Typically, the mass of air 112 introduced into exhaust gases 108 is reduced with increasing vehicle speed and altitude. In the present invention, aerodynamic heating is kept under control by limiting the above-described ramjet operation to vehicle speeds of approximately Mach 6 or less. Further, at altitudes above 40 kilometers, the density of air 112 is too low to derive any benefit from its introduction into exhaust gases 108. Accordingly, at the combination of a vehicle speed of approximately Mach 6 and vehicle altitude of 40 kilometers, intakes 110A are closed as illustrated in FIG. 4. This mode is known as the rocket mode of operation. That is, no more mixing of exhaust gases 108 and air takes place and only exhaust gases 108 exit nozzle 114. The advantages of the present invention are numerous. The higher exhaust gas temperatures generated by a nuclear-based thermal rocket are used advantageously in an air-breathing mode propulsion system. Thrust augmentation is achieved at lower speeds primarily through the mixing action of (inducted) ambient air with the exhaust gas and, secondarily, through combustion of some of the exhaust stream with oxygen in the inducted air. At medium speeds up to Mach 6, thrust augmentation is achieved primarily through combustion when inducted air mixes with the rocket""s exhaust stream. Such combustion is achieved without any flame stabilization and/or auxiliary ignition devices. Further, because of the higher specific impulse produced by the nuclear-based thermal rocket, the transition from combined nuclear rocket/air-breathing mode to pure nuclear rocket mode can occur at slower vehicle speeds thereby reducing aerodynamic heating effects. Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.