Pulse Detonation Engine

Rocket engines that work much like an automobile engine are being developed at NASA’s Marshall Space Flight Center in Huntsville, Ala. Pulse detonation rocket engines offer a lightweight, low-cost alternative for space transportation. Pulse detonation rocket engine technology is being developed for upper stages that boost satellites to higher orbits. The advanced propulsion technology could also be used for lunar and planetary Landers and excursion vehicles that require throttle control for gentle landings.

The engine operates on pulses, so controllers could dial in the frequency of the detonation in the "digital" engine to determine thrust. Pulse detonation rocket engines operate by injecting propellants into long cylinders that are open on one end and closed on the other. When gas fills a cylinder, an igniter—such as a spark plug—is activated. Fuel begins to burn and rapidly transitions to a detonation, or powered shock. The shock wave travels through the cylinder at 10 times the speed of sound, so combustion is completed before the gas has time to expand. The explosive pressure of the detonation pushes the exhaust out the open end of the cylinder, providing thrust to the vehicle.

A major advantage is that pulse detonation rocket engines boost the fuel and oxidizer to extremely high pressure without a turbo pump—an expensive part of conventional rocket engines. In a typical rocket engine, complex turbo pumps must push fuel and oxidizer into the engine chamber at an extremely high pressure of about 2,000 pounds per square inch or the fuel is blown back out.

The pulse mode of pulse detonation rocket engines allows the fuel to be injected at a low pressure of about 200 pounds per square inch. Marshall Engineers and industry partners United Technology Research Corp. of Tullahoma, Tenn. and Adroit Systems Inc. of Seattle have built small-scale pulse detonation rocket engines for ground testing. During about two years of laboratory testing, researchers have demonstrated that hydrogen and oxygen can be injected into a chamber and detonated more than 100 times per second. 

Pre-Compression and Detonation:

In the PDE the pre-compression is instead a result of interactions between the combustion and gas dynamic effects, i.e. the combustion is driving the shock wave, and the shock wave (through the increase in temperature across it) is necessary for the fast combustion to occur. In general, detonations are extremely complex phenomena, involving forward propagating as well as transversal shock waves, connected more or less tightly to the combustion complex during the propagation of the entity.
The biggest obstacles involved in the realization of an air breathing PDE are the initiation of the detonation and the high frequency by which the detonations have to be repeated. Of these two obstacles the initiation of the detonation is believed to be of a more fundamental character, since all physical events involved regarding the initiation are not thorough- ly understood. The detonation can be initiated in two ways; as a direct initiation where the detonation is initiated by a very powerful ignitor more or less immediately or as a Deflagration to Detonation Transition (DDT) where an ordinary flame (i.e. a deflagration) accelerates to a detonation in a much longer time span .

Typically, hundreds of joules are required to obtain a direct initiation of a detonation in a mixture of the most sensitive hydrocarbons and air, which prevents this method to be used in a PDE (if oxygen is used instead of air, these levels are drastically reduced). On the other hand, to ignite an ordinary flame requires reasonable amounts of energy, but the DDT requires lengths on the order of several meters to be completed, making also this method impractical to use in a PDE.
It is important to point out that there are additional difficulties when liquid fuels are used which generally make them substantially more difficult to detonate. A common method to circumvent these difficulties is to use a pre-detonator - a small tube or a fraction of the main chamber filled with a highly detonable mixture (typically the fuel and oxygen instead of air) - in which the detonation can be easily initiated.

The detonation from the pre-detonator is then supposed to be transmitted to the main chamber and initiate the detonation there. The extra component carried on board (e.g. oxygen) for use in the pre-detonator will lower the specific impulse of the engine, and it is essential to minimize the amount of this extra component.

Combustion Analysis:

While real gas effects are important considerations to the prediction of real PDE performance, it is instructive to examine thermodynamic cycle performance using perfect gas assumptions. Such an examination provides three benefits. First, the simplified relations provide an opportunity to understand the fundamental processes inherent in the production of thrust bythe PDE. Second, such an analysis provides the basis for evaluating the potential of the PDE relative to other cycles, most notably the Brayton cycle. Finally, a perfect gas analysis provides the 0framework for developing a thermodynamic cycle analysis for the prediction of realistic PDE performance.

The present work undertakes such a perfect gas analysis using a standard closed thermodynamic cycle. In the first sections, a thermodynamic cycle description is presented which allows prediction of PDE thrust performance. This cycle description is then modified to include the effects of inlet, combustor and nozzle efficiencies. The efinition of these efficiencies is based on standard component performance.

 

Any thermodynamic cycle analysis of the PDE must begin by examining the influence of detonative combustion relative to conventional deflagrative combustion. The classical approach to the detonative combustion analysis is to assume Chapman-Jouget detonation conditions after combustion.

The subsonic Chapman-Jouget solution represents the thermally choked ramjet. To insure consistent handling of the PDE and ramjet, this paper uses Rayleigh analysis for both cycles.

A comparison of the ideal gas Rayleigh process loss was made for deflagration and Chapman-Jouget detonation combustion, The comparison was made for a range of heat additions, represented here by the ratio of the increase in total temperature to the initial static temperature. Four different entrance Mach numbers were also considered. The figure of merit for the comparison is the ratio of the increase in entropy to specific heat at constant pressure. The results show that at the same heat addition and entrance Mach number, detonation is consistently a more efficient combustion process, as evidenced by the lower increase in entropy. This combustion process efficiency is one of the basic thermodynamic advantages of the PDE.