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To review, liquid rockets utilize liquid fuel and liquid oxidizer stored in tanks. By either pressure feeding or by mechanically pumping the propellants from their tanks, they are forced into a mixing chamber where chemical combustion occurs. These types of systems generally provide good thrust and can be thrust-controlled (throttled). In addition, they tend to be the most efficient of high-thrust engines. However, the complexity of these systems is also high. There are stop-valves, pressure regulators, injectors, turbopump machinery and all sorts of “plumbing”. When considering that there needs to be redundancies on all of these systems in order to make a reliable motor, it easy to see that the overall cost and weight of liquid rockets will be excessive. In addition, due to the liquid nature of the propellants involved, there can also be storage problems.
Solid rockets are somewhat different in nature, but also have a specific set of advantages and drawbacks. In solid rocket motors, the fuel and oxidizer are chemically premixed to form a solid fuel grain. By simply igniting this substance, the oxidizer and fuel in the solid react and produce the high-energy combustion gases desired. A variety of designs for the central burning port of the solid fuel can be created so as to produce the desired thrust performance. Solid rockets provide good thrust and are the most simple systems available. On the down side, they also are fairly inefficient fuel burners and cannot be throttled. In some cases there may also be explosion dangers since the oxidizer and fuel are not separated.
It appears necessary to obtain some "optimal" solution to this dilemma. On the one hand, we have a high-thrust rocket engine with good performance but high complexity and cost, while on the other hand to get low complexity we must accept lower performance as well. It is at this point where hybrid rockets become an attractive alternative. Hybrid rockets combine elements from both types of rockets. In a hybrid rocket, a gaseous or liquid oxidizer is stored in a tank separate from a solid fuel grain. The fuel grain is placed inside a pressure chamber which lies between an oxidizer injector and the exit nozzle. The solid grain is hollowed out in the same fashion to produce a combustion port, very similar to that of a solid rocket motor type system. Unless the fuel is hypergolic (spontaneously combustible in the presence of an oxidizer), the fuel must be initially ignited in order to vaporize some of the fuel into a region just above the solid surface. Then, by injecting the oxidizer at a high mass flow rate and pressure into the pressure chamber / combustion port area, the oxidizer and fuel are free to react in a thin boundary layer just above the surface of the fuel grain. The high energy released and the high temperature attained both increase the energy in the flow and sustain the solid fuel vaporization. The combustion gases pass down the remainder of the combustion port and are expanded via nozzle. By changing the flow rate of the oxidizer, the total production of combustion gases and the energy going into them will be changed in a like fashion (increasing or decreasing). This fact demonstrates that hybrid rockets can be throttled. Given a simple ignition system that would efficiently initiate fuel burning prior to injecting the oxidizer, it also shows that hybrid rockets have start-stop-restart capabilities.
On the down side, the nature of the system renders itself to marginally higher combustion inefficiencies and also to variations in specific impulse. this has much to do with the (in)completeness of the mixing in the active combustion zone above the fuel surface. These effects are not so bad however; as far as specific impulse is concerned, hybrid rockets actually sit on the median between liquid and solid systems. Typical performance numbers are not difficult to find : for liquid systems, impulse can range from 300 sec up to 400 sec for the SSME; most solid systems operate at a specific impulse of 200 to 270 sec. Performances generated thus far for experimental hybrid test engines lie in the range of 275 to 350 sec. Moreover, it is not beyond possibility that further dedicated research and development of hybrid rocket motors will relieve some of the inefficiency problems and therefore boost the performance figures even higher. Another disadvantage to hybrids is that there will usually be unburned fuel slivers remaining after burning; however, this effect also plagues solid rockets. Clearly in many of these respects, the disadvantages of hybrid rockets are non-critical, and many are clearly not disadvantages with respect to solid systems.
Several advantages of hybrid systems are fairly simple to point out. The major benefit of solid rockets over hybrid rockets (and liquid systems, too) is their simplicity. In hybrid systems, then, it seems that higher complexity is the price paid for better performance. However, note that the performance for these rockets is rival to that of liquid systems. Furthermore, note that hybrid rocket systems require support for only one fluid systems, including tanks, valves, regulators, etc. In other words, although hybrid rockets are more complex than solid systems, they compare in performance to liquid systems while requiring only half of the “plumbing”. This vastly reduces the overall systems weight and cost, while increasing its reliability (there will be fewer parts to fail). Hybrid rocket systems are also safer to produce and store, can be more ecologically safe with proper propellant choice, and the fuel grain, being inert, is stronger then manufactured solid propellant grains (for solid rockets), and is therefore more reliable. Finally the solid fuel grain of the hybrid gives it volumetric sizing advantages over the tankage required for liquid systems. Clearly, hybrid rocket motors offer numerous benefits at a small price. It is in light of this prospect for hybrid rockets, that a project group has been formed at the University of Illinois to design and test a test-scale hybrid rocket motor.