Valveless Detonation Concepts for Space Exploration

The original goal as stated in the research training plan is provided: “The current study seeks to develop a valveless pulse detonation rocket engine for volume and weight constrained space applications desiring higher efficiency and performance through the use of properly tuned hydraulic circuits and hypergolic propellants. The challenges that face this work include the use of liquid propellants and the associated difficulties of rapidly injecting and mixing liquids in the highly transient detonation environment. The hypergolic propellants to be used include rocket grade hydrogen peroxide (RGHP) and a nontoxic hypergolic fuel, fuel-a, consisting of sodium borohydride in an industrial solvent, Triethylene glycol dimethyl ether (Triglyme). The use of hypergolic propellants in a detonation combustion engine aims to tackle the difficulties of repeated fast combustion ignition and detonation initiation. Within the research effort, a lumped parameter transient hydraulic model will be developed to aid in design of manifold and injector combinations. An experimental investigation into the performance of a lab-scale combustor under valveless mode operation will be conducted; it is expected that deviations from initial design choices will occur in order to better exploit the combustion phenomena produced. Lastly, a system study will be conducted to ascertain the overall performance benefit of the achieved unsteady combustion behavior relative to conventional liquid rocket systems.” The work completed over the course of the NASA Space Technology Fellowship (NSTRF) grant reflects the stated objectives. The research into an unsteady constant-volume rocket combustion device involved a standard progression of research in a cycle of concepts and designs to experiments and then analysis. The overall experience through opportunities afforded by the fellowship, however, provided an immensely diversified set of skills including experimental work with mixing and production of liquid hypergols Triglyme with sodium borohydride and the handling of hydrogen peroxide, numerical modeling of feed systems and injectors, design and fabrication of experimental rocket hardware and associated facility support, extensive training in designing and operating data acquisition and control equipment at Purdue and the Air-Force Research Laboratory at Wright-Patterson Air Force Base, investigations and understanding on instrumentation and measurement techniques, and articulation of completed work through conference papers and presentations. The following report provides a synopsis of the completed research work pertaining to the original NASA grant, but the supplemental tasks and skills gained through the research training fellowship should be recognized as a benefit of the program. Experimental operation of the rocket hardware covered a span of conditions. Due to a supply shortage of high purity industrial grade 98% H2O2 in the USA, the only available hydrogen peroxide concentrations available for testing the impinging jet injector hardware was 90.6%. Additionally some testing featured aged samples of the hypergolic fuel mixture of Triglyme and N aBH4. Despite the sharp decrease in concentration of hydrogen peroxide, the combustor functioned as expected without a nozzle. High amplitude pulse behavior presented itself through baseline pressurization conditions, and with new pressure measurements in the feed system, numerical predictions of feed line pressure amplitude response were confirmed. Experiments performed with optical chamber access showed the periodic spray and mixing characteristics suspected of the injector. The high amplitude pressure wave disrupted the injector and stopped flows momentarily, and subsequently with a relaxation in chamber pressure from the blowdown process, the injector flows restarted with the reforming of the impinging jet spray. This behavior can be observed in Fig. 4. Experimental operation of the combustor over a wide range of pressures from the baseline condition to 300% of the baseline pressure show a change in fundamental behavior. The plot of the data from the test campaign shown in Fig. 5 shows that pulse combustion frequency increases with supply pressure for the pressure fed system. The two trendlines correspond to the two batches of fuels tested. The plot reveals that limit cycle frequency has the greatest dependence on feed pressure with kinetics also providing some influence. The experiments involving adding nozzles of various area ratios to influence the blowdown process revealed that the instability is not outright suppressed. The combustion peak pressures decrease due to the presence of a mean chamber pressure during engine operation limiting propellant flow rates. The violent pulsed combustion response is altered, but the frequency dependence on feed pressure remains. At high feed pressure test cases, the combustor transitions into an acoustic instability dominated combustion response. The high amplitude 500 Hz pressure waves are suppressed and instead the majority of the spectral content is present at the 1/4 wave acoustic frequency of 2800 Hz. From the research conducted so far the conclusions regarding the combustion phenomena is that the frequency is tied to pressure through the injector produced velocity. It is highly suspected that the rate at which the injectors respond to a high pressure event are responsible for the behavior. The intermittent disruption to mass flow due to capacitative effects or compliance in the manifold determine the rate at which the mixing of hypergols and spray formation occurs. The ability to reproduce the behavior over various propellant combinations and injector designs lends credence to the fact that a new injector can be designed to take advantage of the behavior. The main challenge to the goal of producing an injector specifically designed for the observed behavior is to force propellants to participate in the constant-volume combustion process without being ejected from the chamber. The phenomena is likened to a highly repeatable hard start phenomena in liquid rocket engines. The final goal of obtaining increased performance measurements remains an objective of the research program, and concurrent work with unsteady combustion systems with members of the pressure gain community has provided insight.