				AspireSpace
				The British Amateur Space / Rocket Programme

Hybrid Rocket Motors

The AspireSpace Hybrid Rocket Motors.

AspireSpace is developing a range of hybrid motors, ranging from small static test motors, up to large flight capable motors, for use in the larger ASPIRE rockets which will fly first to space, and then to orbit.

AspireSpace Hybrid Motor Static Test Firing (throttle tests) - Duration of test firing - 60 seconds.

The current range of AspireSpace hybrid motors are the:

H1 - This is the smallest of the Hybrid motors, and is used as a rapid static testing prototype motor, to evaluate GOX and eventually LOX as the oxidiser.
- Status : Static Testing
H2 - This is the first of the flight configuration Hybrid motors, developed by AspireSpace. The H2 hybrid motor is designed to be the workhorse hybrid motor, and will be phased in as the standard propulsion system for the ASRV rockets. Designed to test reusability and in-flight throttling, as well as optimum port diameter, regression rates and other empirical data, the H2 hybrid motor will be tested with a range of oxidisers.
- Status : Integration
H20 - The H20 hybrid motor is a 60,000 Newton seconds total Impulse Hybrid motor, using 20 kg of propellant. Initially developed as a reusable static test motor, to allow testing of throttling, port length, and grain configuration. It will in time be developed to flight configuration, for use as a high altitude motor, with the steel combustion chamber being replaced with carbon composite.
- Status : Component Construction
H1xx - The H1xx hybrid motor is a 330,000 Newton seconds total impulse Hybrid motor, using 100 kg + of propellant. The H1xx will be the first of the AspireSpace Cryogenic hybrid motors to be flight tested, as it will power the ASPIRE II rocket.
- Status : Design Frozen - awaiting funding
H1000 - The H1000 hybrid motor is planned to be a 3,000,000 Newton seconds total impulse Hybrid motor, using 1000 kg of propellant. The H1000 will be the largest of the AspireSpace hybrid motors, and will be used in the ASPIRE III sub-orbital rocket, and the AUSROC / ASPIRE orbital rocket. It is currently in the design stage. Its detailed design will be dependent on the results of the H100 static test motor firings, as well as the H2 and H20 motors.
- Status : Design Stage

Benefits of Hybrid Rocket Motors

Hybrid motors combine good features of both Solid and Liquid Fuelled rockets. They are controllable, semi storable, cheap and simple. They are based on the principle of a solid propellant and a liquid oxidiser, and have a number of features which make them ideal for use in a rocket such as ASPIRE II, namely:

Cheapness - Neither the oxidisers (Liquid Oxygen, Hydrogen Peroxide or Nitrogen Dioxide) or most polymers are particularly expensive, thus fuel costs are kept to a minimum. The flow lines are also considerably cheaper than those in a liquid fuelled rocket due to the simpler arrangement required (since you only have one liquid in the system, not two).

Simplicity - Although more complex than a solid rocket motor, a hybrid motor is considerably simpler to implement than a liquid fuelled engine, since there is only one set of plumbing required.

Safety - A hybrid rocket motor offers very safe usage characteristics over both a solid rocket motor and a liquid fuelled rocket engine. Combination of the two components, fuel and oxidiser will not cause an explosion, as can be the case with a liquid fuelled rocket engine, and the hybrid can be turned off at any stage of the flight or on launch. This is most certainly not possible with a solid rocket motor.

Environmentally Friendliness - The hybrid motors under development by AspireSpace burn their propellant at extremely high temperatures. The result of this is that the only major byproducts in the rocket's exhaust are Water (in the form of steam), Carbon Monoxide and Carbon Dioxide (similar to a car exhaust).
Hybrid Motors in general, use non toxic propellants and have fairly benign exhaust products, which are cleaner than those of solid fuelled rockets, and similar to those of most liquid fuelled engines. The exception to this is when maximum performance is required from a hybrid rocket motor, and every last drop of propellant is squeezed out of the motor, in which case the exhaust is less environmentally friendly.

Hybrid Rocket Motor Performance

It is commonly thought that hybrid motor performance, although reasonable, is not spectacular. This is not the case. Although the performance of most hybrid motors falls in the 200 - 300 seconds specific impulse range, it is possible to get specific impulse figures of 400 - 500 seconds, by using metallised fuels, such as Lithium/Lithium Hydride and Beryllium in a HTPB (Hydroxyl Terminated Poly Buta-diene) binder, combined with an oxidiser such as Fluorinated Liquid Oxygen. The drawback to these propellants though is that unlike the other hybrid propellants, they are much less environmentally friendly.

Typical Propellant combinations:

HTPB - Oxygen

Polythene - Oxygen

Perspex - Oxygen

HTPB / Magnesium - Oxygen

HTPB / Aluminium - Oxygen

HTPB - Hydrogen Peroxide

Polythene - Hydrogen Peroxide

HTPB - Nitrogen Dioxide

The Physics of Hybrid 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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