PhD Candidate, Stanford University
A hybrid rocket can be thought of as a combination of the best aspects of pure liquid and pure solid rockets. By placing a solid fuel grain in the combustion chamber and a gaseous oxidizer upstream in a separate tank, they offer mechanical simplicity and higher fuel density as compared to liquid rockets, and are much safer and easier to control than solid rockets. One of the major theoretical advantages of hybrid rockets is their ability to be extinguished and restarted thanks to the presence of a control valve located between the oxidizer tank and combustion chamber. In practice, however, current ignition technologies preclude researchers and engineers from obtaining multiple ignitions of hybrid rocket motors. A typical example of an existing ignition system for hybrid motors is a pyrotechnic ignitor, which can be thought of as a very small solid rocket motor that gets mounted on the outside of the combustion chamber and is lit by passing a high voltage through the pyrotechnic device. These types of devices must be replaced after each use and are therefore not well suited to restarting hybrid motors. As an alternative ignition technology, I am currently researching a laser-based ignition device. Semiconductor lasers, as they stand today, are capable of providing several watts of optical power at a variety of wavelengths, with efficiencies upwards of 50 to 60%. The presence of a solid fuel grain within the combustion chamber of a hybrid motor offers the advantage that optical power from a laser can be nearly completely absorbed by the fuel, heating it rapidly to the point of ignition when in the presence of an oxidizer. The goal of my research is to first demonstrate that laser ignition of hybrid motors is possible, and to then characterize the power requirements of obtaining reliable and repeatable ignition. Enabling restart capable hybrid rocket motors would allow this safer and more efficient propulsion technology to be used in a wide array of space applications, including CubeSat propulsion and outer solar system exploration.
Abstract: This paper discusses the potential of the hybrid rocket engine as a viable and attractive mode of propulsion for both space vehicles and missiles. Research and development work on this engine in other countries is presented and evaluated. The various advantages of a hybrid engine over solid and liquid engines and its problems are highlighted. It has been argued that because of the low technology needed in the development of the hybrid system, it constitutes a cost-and-time-effective propulsion system for several applications in space programmes as well as weapon systems. In support of this conclusion, experience on the developmental studies of a variable thrust 100 kg engine is presented. Some future possibilities for hybrid propulsion systems are cited.
Pub.: 01 May '79, Pinned: 28 Jun '17
Abstract: Outer planet exploration missions require significant propulsive capability, particularly to achieve orbit insertion. Missions to explore the moons of outer planets place even more demanding requirements on propulsion systems, since they involve multiple large ΔV manoeuvres. Hybrid rockets present a favourable alternative to conventional propulsion systems for many of these missions. They typically enjoy higher specific impulse than solids, can be throttled, stopped/restarted, and have more flexibility in their packaging configuration. Hybrids are more compact and easier to throttle than liquids and have similar performance levels. In order to investigate the suitability of these propulsion systems for exploration missions, this paper presents novel hybrid motor designs for two interplanetary missions. Hybrid propulsion systems for missions to Europa and Uranus are presented and compared to conventional in-space propulsion systems. The hybrid motor design for each of these missions is optimized across a range of parameters, including propellant selection, O/F ratio, nozzle area ratio, and chamber pressure. Details of the design process are described in order to provide guidance for researchers wishing to evaluate hybrid rocket motor designs for other missions and applications.
Pub.: 22 Jun '16, Pinned: 28 Jun '17