Defense Tech Briefs – Variable-Thrust, Multiple-Start Hybrid Motor Solutions

Defense Tech Briefs – Variable-Thrust, Multiple-Start Hybrid Motor Solutions.

This article above refers to a press release from the Air Force Research Laboratory at Edwards Air Force Base, California, dated Friday, April 01, 2011. I couldn’t find the original press release at AFRL’s website. So, I’m hoping it is not an April Fool’s joke.

Missiles have always relied on solid rocket motors for their propulsion, but they have many drawbacks, for example, once started, they cannot be stopped. They are prone to explosions. Also, the burn pattern inside the motor can create lowered performance as the propellant is burnt away. Thus, there is a strong motivation for exploring hybrid rocket motor technologies.

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. 

I am not sure how specific impulse can be improved because missiles still have to carry their own oxidizer as well as the fuel, and the total mass of the oxidizer required to burn the fuel completely will not change even in liquid form.

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.

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

For space applications, chemical rockets are the only available option presently. The Space Shuttle also uses (as of this writing we have one last Shuttle launch left, Endeavour is now set to launch on May 16, 2011) a pair of solid rocket boosters. (The Challenger accident started with a flame out on the right solid booster rocket.) Thus, hybrid rocket motors could be very useful for future space applications.

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About propulsiontech

Propulsion technologist, aerospace engineer
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