Rocket Motors

Rocket motors work by ejecting gases at high speed. From the physics point of view, the momentum given to the gases will be counteracted by an equal and opposite momentum given to the rocket. Rocket motors are designed to make this momentum change as large as possible.

A change in momentum implies a force, since force x time = change in momentum. The force is given by change in momentum per second (strictly speaking, rate of change of momentum). This is usually referred to as the rocket thrust, measured in newtons, or, in Britain in the 1950s and 1960s, in lb – a shorthand for pounds force.

Gases moving at high speed have kinetic energy, and in almost all rocket motors this kinetic energy comes from chemical energy. With solid fuel motors, the fuel and the oxidant are melted together and poured into a casing to cool and solidify. Almost all liquid fuel rockets need both a fuel and an oxidant. There are a few chemicals which can be used by themselves (referred to as monopropellants) – hydrazine (N2H4) and hydrogen peroxide (H202) are examples. They can be decomposed directly to gases (usually by means of a catalyst). The drawback is that they are not very energetic and tend to be used only for small control jets.

The most common rocket fuels are:

• hydrocarbons, referred to generically as ‘kerosene’ (some early British documents refer to ‘kerosine’), usually as some form of jet fuel.

• hydrazine or some related compound (usually UDMH – Unsymmetrical DiMethyl Hydrazine or (CH3)2N. NH2).

• liquid hydrogen.

Kerosene is cheap, easy to handle, not volatile and not poisonous. Hydrazine is easily storable, and it is mostly used in combination with dinitrogen tetroxide, N204. Both produce highly poisonous fumes, and dinitrogen tetroxide is also very corrosive. They ignite spontaneously on contact (i. e. they are hypergolic). The combination is often used in missiles which are left fuelled up on a long-term basis, or in upper stages of satellite launchers, particularly when a restart capability is needed.

Liquid hydrogen is the most energetic and effective fuel, although it suffers from two major drawbacks: it boils at -253 °C or 20 K, and has an extremely low density of 71 kg/m3 compared with 1,000 kg/m3 for water. Low density implies large tank volume and, as a consequence, extra weight.

Kerosene was the usual fuel of choice in the UK, with either liquid oxygen or HTP as the oxidants. Although a good deal of research and development was done on liquid hydrogen, including test firing of liquid hydrogen chambers, sadly no rocket stages were built using liquid hydrogen.

Common oxidants are liquid oxygen, and as mentioned, dinitrogen tetroxide in combination with hydrazine. However, Britain was to make extensive use of another oxidant, hydrogen peroxide (H2O2), and the way it was used was and still is unique. Hydrogen peroxide was used in the form of High Test Peroxide or HTP, a solution with 85% of hydrogen peroxide and 15% water. Hydrogen peroxide can be decomposed to steam and oxygen at a high temperature using a catalyst – nickel gauze plated with silver, the silver being the catalyst. In this way, the HTP could be used as a monopropellant, but it was much more efficient to inject a fuel such as kerosene into the hot gases to be burnt in the oxygen produced in the decomposition. HTP was also thought to be safer and easier to handle than liquid oxygen. In 1952, the decision was taken to use only HTP motors for all liquid propellant rockets used on, or in, aircraft1.

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