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| The British Amateur Space / Rocket Programme |
| THE PHYSICS OF HYBRID ROCKET MOTORS |
Hybrid motor propellant is normally composed of a solid fuel
and a liquid oxidiser. The solid fuel is generally referred to as the
grain. This fuel grain is placed in the combustion chamber. In this
sense the hybrid resembles a solid rocket motor, which also has a
solid fuel grain in a combustion chamber. The solid fuel motor
however combines fuel and oxidiser together in an explosive mixture. The fuel grain has either 1 or multiple channels along which
oxidiser can flow, thus enabling the reaction that allows the hybrid
to work. These channels are called ports. The liquid oxidiser is
contained ina separate pressure vessel, which is connected to the
combustion chamber and solid fuel grain, via a flow line, throttle
valve and injector head assembly. At ignition, the liquid oxidiser flows
through to the head of the combustion chamber. The liquid
oxidiser is converted in the injector head, into a mixture of
liquid droplets and gaseous oxidiser, in a fine spray. Following ignition, the temperature and the
pressure in the combustion chamber rise to a point where the solid
fuel sublimes to the vapour phase, it is only then that the
conditions are sufficient for the solid fuel to burn vigorously.
The actual burning occurs at the boundary layer or interface between the vapourised
the fuel grain and the gaseous oxidiser. This interface is called the flame
sheet. The flame sheet is maintained by the flow of oxidiser entering
the combustion chamber and the vapourised fuel.
The heat generated by the flame sheet produces the exhaust, while a
small portion of the heat generated, vapourises more fuel to continue
the reaction.
The exhaust products then pass through the nozzle throat where they
undergo expansion at supersonic speeds.
The mass flow rate of the liquid oxidiser into the solid fuel
grain / combustion chamber can be varied, thus allowing the hybrid
motor to be throttled. This is a great benefit over a solid fuel
motor, and in combination with a hypergolic ignition system, even
allows start-stop-start operation of the hybrid motor. Throttling
operations are also useful for static testing, since unlike a solid
motor, a hybrid can be stopped, and then the fuel grain can be
examined after different burn times, to determine the efficency.
| REFERENCES |
Rocket Propulsion Elements, George Sutton, 1992, 6th edition, John Wiley and Sons.
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