Hybrid rocket engines are promising propulsion systems which present appealing features such as safety, low cost, and environmental friendliness. On the other hand, certain issues hamper the development hoped for. The present paper discusses approaches addressing improvements to one of the most important among these issues: low fuel regression rate. To highlight the consequence of such an issue and to better understand the concepts proposed, fundamentals are summarized. Two approaches are presented (multiport grain and high mixture ratio) which aim at reducing negative effects without enhancing regression rate. Furthermore, fuel material changes and nonconventional geometries of grain and/or injector are presented as methods to increase fuel regression rate. Although most of these approaches are still at the laboratory or concept scale, many of them are promising.

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Approaches to Low Fuel Regression Rate in Hybrid Rocket Engines

http://thermophysics.ru/pdf_doc/AutoPlay/Docs/CollectionOfManuscripts/ECTP2005paper214.pdf

Analysis of the temperature distribution in a guarded hot plate apparatus for measuring thermal conductivity

The low regression rates of classic hybrid rocket fuels lead to large internal ports that limit potential applications. This experimental study investigated the increase in regression rate that results from adding a solid oxidizer and a catalyst to a hybrid fuel grain. The configuration is named a "mixed hybrid" hybrid to signify solid oxidizer and catalyst in the grain. A design of experiments approach guided fuel formulation to systematically control levels of ammonium perehlorate from 25% to 30%, ferric oxide from 0 to 5%, and hydroxyl-terminated polybutadiene from 70% to 75%. The 1.5-in. diam. port, 12-in. long center perforated grains were burned with gaseous oxygen at pressure levels from 150 to 550 psig and port flux levels from 0.1 to 0.4 lbm/s-in. 2 . The results show that the mixed hybrid propellants burn as a function of both pressure and mass flux. A grain formulation having 27.5 % ammonium perchlorate and 2.5% ferric oxide provided the maximum burning rate augmentation (447%) among the formulations tested.

Regression rates study of mixed hybrid propellants

The spray and breakup characteristics of a swirling liquid sheet were investigated by measuring the spray angle and breakup length as the axial Weber number We l was increased up to 1554 and the ambient gas pressure up to 4.0 MPa. As the We l and ambient gas density p increased, the disturbances on the annular liquid sheet surface were amplified by the increase of the aerodynamic forces, and thus the liquid sheet disintegrated from the injector exit. The measured spray angles according to the ambient gas density differed before and after the sheet broke up. Before the liquid sheet broke up, the spray angle was almost constant; however, once the liquid sheet started to break up, the spray angle decreased. As the ambient gas density and We l increased, the increasing aerodynamic force caused the breakup length to decrease. Finally, the measured breakup lengths according to the ambient gas density and We l were compared with the results of the linear instability theory. Considering the attenuation of sheet thickness in the linear instability theory, the corrected breakup length relation agreed well with our experimental results.

Effect of ambient gas density on spray characteristics of swirling liquid sheets

Heat transfer—A review of 2004 literature

Gelled RP-1 Nanophase Aluminum Propellant

The work presented here is a study of a digital method for determining the combustion bomb burning rate of a fuel-rich gas generator propellant sample using the ultrasonic pulse-echo technique. The advanced digital method, which places user defined limits on the search for the ultrasonic echo from the burning surface, is computationally faster than the previous cross correlation method, and is able to analyze data for this class of propellant that the previous cross correlation data reduction method could not. For the conditions investigated, the best-fit burning rate law at 800 psi from the ultrasonic technique and advanced cross correlation method is within 3 percent of an independent analysis of the same data, and is within 5 percent of the best-fit burning rate law found from parallel research of the same propellant in a motor configuration.

An Advanced Digital Cross Correlation Method for Solid Propellant Burning Rate Determination

A proof of principle test of a nitrous oxide/propane rocket concept was tested on the UAH campus. The project was funded by the Alabama Space Grant consortium and was a joint effort between the PRC and Micro Craft, Inc. The concept is attractive for small orbital maneuvering engines because of the propellants are commercially available and relatively easy to handle. Also because of the high vapor pressure of liquid nitrous oxide the system can be designed to be self-pressurizing, simplifying the overall system. The tests were accomplished at a sub-scale rocket test facility that has been constructed on the edge of the UAH campus. This facility can handle rocket firings up to 300 M The initial tests were at chamber pressures of 150 to 300 psi and had a C* efficiency of nearly 100% with a double counter swirling coaxial injector.

Marlow D. Moser

Papers

Gelled RP-1 Nanophase Aluminum Propellant

An Advanced Digital Cross Correlation Method for Solid Propellant Burning Rate Determination

Nitrous oxide/propane rocket test results

Use of Cross-Correlation Waveform Analysis to Determine Burning Rate Using Ultrasonic Technique

Internal Ballistics Model for a Mixed Hybrid Rocket Motor