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Rocket Science: Hybrid Rockets

What is a hybrid rocket, how does it work, and what are the advantages and disadvantages versus conventional solid and liquid rockets?

A hybrid rocket is a cross between a solid and liquid rocket, typically using a solid propellant grain and a liquid or gaseous oxidizer (though the reverse has also been tested). More formally, a hybrid rocket is one in which one propellant is stored in the fluid phase and the other propellant is stored in the solid phase1, pg.503.

Hybrids are similar to solid rockets, in that the fuel is in solid and thus storable form, but similar to liquid rockets in that the oxidizer must be drawn from a tank (either by pressure feed, pump feed, or a combination of the two) and combined with the fuel to sustain combustion. In a hybrid, this occurs at the head of the motor case, where gaseous oxidizer is injected into the motor case to sustain combustion along the receding face of the fuel grain, rather than being mixed upon injection into a combustion chamber with a liquid or gaseous fuel. As in a conventional solid motor, the combustion products and unconsumed oxidizer travel aft through passages in the fuel, heating and consuming the fuel as they go, finally passing through the familiar throat and expansion nozzle at the aft end to provide forward thrust to the vehicle. As in solid rockets, the aft nozzle can be either fixed for simple booster applications or “flexible” (steerable) to provide thrust-vector control.

Unlike a solid motor, the fuel grain in a hybrid rocket contains no oxidizer, and combustion is not sustainable without the presence of the oxidizer component. This makes hybrid fuel stable and thus safe to manufacture and transport without special precautions — the fuel can’t be ignited accidentally by rough handling or electrostatic discharges, for instance. Fuels tested include such quotidian materials as polyethylene (milk-jug plastic), polyvinyl chloride (PVC plastic), polymethyl methacrylate (Plexiglass), paraffin (candle wax), and polybutadiene (rubber) — the latter material (in combination with liquid oxygen) having similar chemical and performance characteristics to a typical LOx-kerosene system.1, pg.506 Hybrid fuel tends to be structurally stronger than typical solid rocket compositions, and therefore more resistant to cracking and debonding. The combustion products are typically non-toxic, though this can depend on which specific propellants are chosen and/or additives included to enhance performance. Specific impulse is typically higher than for standard solid motors.

Unlike a liquid rocket, a hybrid has fewer moving parts (for instance, one turbopump and pressurization system versus two), making it more reliable and less expensive. Because its fuel is in solid form, only one tanking operation is required before flight, and the related fuel handling and ground support hardware is reduced accordingly. While specific impulse is lower than for liquid rockets, “impulse density” (specific gravity of the propellants multiplied by the specific impulse)1, pg.388 is higher, resulting in a more compact overall system.

The primary benefits versus standard rocket types are that hybrids are throttleable on-demand over a wide range and can even be shut off and restarted (unlike solids), the solid fuel simplifies operations and reduces operations costs (versus liquids), and overall reliability is increased and system costs are decreased (versus both solids and liquids).

The primary disadvantages are lower combustion efficiency, a greater amount of residuals at cutoff/burnout, and scalability of the fuel grain geometry.

Hybrid rockets have so far only found application in small target missiles, sounding rockets, and high-power hobby rocketry.

More information on hybrid rockets:
1. George P. Sutton, Rocket Propulsion Elements (6th Ed.)
2. UK

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