Interceptor boost propulsion has traditionally been dominated by solid rocket systems due to their responsiveness and high mass fraction capabilities. However, increased costs of handling these flammable motors, the need to evolve more and more insensitive munitions, the static thrust profile created at propellant casting, and the inherent performance limitations of solid propellants have motivated the interest in exploring alternate technologies.

A representative model of the Hybrid Motor Stack-Up shows a modular fuel grain housing and combustion chamber designed to accommodate a range of fuel grain lengths.
Hybrid propulsion devices have the potential to substantially increase the payload/range characteristics of modern missile systems due to a substantial advantage in specific impulse as compared to current solid propellant devices. With the use of a variable position valve to control instantaneous oxidizer flow, additional energy management features can easily be incorporated in a hybrid system to limit a wide range of thrust variation/throttling. Further, the use of a hybrid system, with a liquid oxidizer and solid fuel, provides substantial safety and handling advantages that translate to reduced operations costs and limit exposure for military personnel.

The objectives of this work were to assess throttling capabilities and novel fuel concepts for hybrid motors. Experimental studies were conducted using 90% hydrogen peroxide (HP) with a variety of unique fuels in both direct injection and catalytic bed injection approaches. Performance efficiencies ranged from 91% to 100%, and the combustion in all tests was smooth with the highest level of combustion roughness reaching only 0.6% of the steady-state pressure.

These hybrid motor tests also displayed the expected connection between the oxidizer flux level and the time required to ignite the fuel grain, with higher flux levels resulting in lower ignition delays. Substantial throttling capabilities were demonstrated. Throttle-down tests analogous to a powered vertical landing exhibited a 10:1 throttling ratio with stable combustion across the entire range.

Boost/Sustain/Boost thrust profiles representative of tactical solid rocket motors were tested with 75%, 50%, and lower sustain-to-boost chamber pressure ratios with rapid throttle-up achieved following the sustain period. To add multiple-start capability to a hybrid motor without reliance on a catalyst bed or separate ignition system, fuel grains catalytic with the oxidizer were investigated. Test fires of these fuel grains in the hybrid motor test article exhibited regression rates 2.5 times higher than the highest regression rates realized with the uncatalyzed polyethylene fuel grains.

Two test series were conducted as part of this effort. In the first series of tests, a catalyst bed was placed upstream of the fuel section to directly feed decomposed HP gases to the fuel. The high decomposition temperature of rocket-grade HP permits for direct ignition of the fuel grain in this instance without the use of a separate ignition source. In the second series of tests, catalytic fuel grains were manufactured such that ignition could be attained with direct injection of liquid HP into the combustion chamber. Both approaches yielded successful ignition and reliable combustion.

Experimental studies successfully demonstrated restartability and throttleability for hybrid rockets utilizing hydrogen peroxide as an oxidizer. Both catalytic bed and catalytic fuel grain alternatives produced excellent combustion characteristics. Testing has also verified that a slow throttle-up from the nominal flow rate is possible to maintain the optimum mixture ratio, providing higher specific impulse performance as the fuel grain burns back.

This work was done by B. Austin of IN Space LLC; S. Heister, E. Dambach, and S. Meyer of Purdue University; and E. Wernimont of General Kinetics for the Air Force Research Laboratory. AFRL-0186


This Brief includes a Technical Support Package (TSP).
Variable Thrust, Multiple Start Hybrid Motor Solutions for Missile and Space Applications

(reference AFRL-0186) is currently available for download from the TSP library.

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This article first appeared in the April, 2011 issue of Defense Tech Briefs Magazine.

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