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HTP rocket motors

One of the lesser known rocket fuels is hydrogen peroxide. Only the British have used it to any extent, developing increasingly sophisticated motors in the period from the late 1940s to the early 1970s.

Hydrogen peroxide has the chemical formula H2O2. It is easily decomposed, and breaks down into water and oxygen:

2 H2O2 = 2 H2O + O2

A catalyst is used to make it decompose. One of the standard ways of preparing oxygen in a school laboratory is to drip a weak solution of hydrogen peroxide onto maganese (IV) oxide, MnO2.

The Germans used HTP to drive the turbine for the fuel pump of the V2/A4 rocket. Calcium permanganate was used, although this resulted in a very messy exhaust.

The British discovered that a silver plated nickel gauze could be used as a catalyst - the HTP was pumped through the gauze and decomposed.

It can be used as a monopropellant - that is, by itself - but this is not very effective. Since oxygen is produced in the decomposition, fuels such as kerosene can be burnt in addition.

One of the first HTP motors was built by the Rocket Propulsion Establishment, at Westcott in Buckinghamsire:

"The Gamma engine employs silver plated wire mesh as a catalyst. The catalyst has a life of about 2 hours whereas the engine has a life of 20hrs. Thus the design of motors is such as to permit the easy replacement of the catalyst. The motor is a simple combustion chamber with a flared outlet and having a restricted throat. HTP is pumped through the catalyst, where free oxygen and steam at around 500oC is produced. Kerosine is then pumped into the chamber in a finely divided spray, which is ignited by the heat of the steam. The temperature at the throat is of the order of 2300oC, while the temperature of the efflux is 1100oC. It will burn HTP alone but the thrust is halved." (December 1953)

HTP has a disadvantage that it is not the most energetic of rocket fuels. The effectiveness of rocket fuels can be measured by their Specific Impulse, or S.I. This can be thought of as the thrust produced per mass of fuel burned per second. In vacuum, the SI of HTP/kerosene can reach about 240, liquid oxygen/kerosene around 280, and oxygen/hydrogen over 400.

To offset this, it has features in its favour. Firstly, it is very dense. Its density is 1.48 times that of water; liquid hydrogen has a density of 0.08. This means that, weight for weight, liquid hydrogen occupies 17 times the volume, which means much bigger tanks and so extra weight. In addition, HTP is not cryogenic, and doesn't need the layers of inulation that liquid hydrogen tanks do. It does boil off on the launchstand either, which for cryogenic, fuels, can mean constant topping up of the tanks.

It was used a great deal on projects involving aircraft, and in service extensively with the R.A.F. From the crew point of view, this must have been by far the best choice of oxidant. All rocket fuels are hazardous, but HTP was easier to handle than most. It was not cryogenic. It did not give off any poisonous fumes. If any was spilled, it could quickly be washed away with a hose and water.

In the immediate post war period de Havilland, Napier, Armstrong Siddeley and the Rocket Propulsion Department or RPD (as it was then) all investigated the use of HTP. The next section illustrates some of the rocket motors they designed and produced.

Some British HTP Rocket Motors

Sprite rocket motor
The Sprite motor was designed for Rocket Assisted Take Off - in other words, it was fitted to aircraft such as the Comet airliners or the V bombers for a sorter and faster take off. It would have been jettisoned after take off. It was a monopropellant motor - that is, it used HTP only. A later version, the Super Sprite, burned HTP and kerosene.
Improvements in jet engines meant that although it was developed and tested , it never went into service.

Spectre rocket motor

A Spectre rocket motor produced by de Havilland. This is an excellent illustration, as it shows the double walled chamber [HTP would flow round this to cool the motor]. Judging by the appearance of the metal, it is made of stainless steel or nickel, and the discolouring shows that it has been fired. At the far right, at the top of the chamber, would be the catalyst pack [in this exhibit it has been removed] to decompose the HTP to steam and oxygen.
The Spectre was used in the SR 53 rocket interceptor.

Spectre rocket motor - HTP and kerosene

A de Havilland Double Spectre, as used in the early trials of Blue Steel.
[Both these photographs are taken from exhibits in the Science Museum Collection at Wroughton in Wiltshire.]

The Gamma 201 Black Knight engine bay, with its 4 chambers. Work in the last 40s and early 50s at the Rocket Propulsion Establishment (RPE) at Westcott in Buckinghamshire led to motors named Alpha, Beta and Gamma respectively. This Gamma 201 uses the chamber developed at RPE.
Each chamber could swing in one direction - radially (i.e., inwards and outwards) - and used in combination, could control the vehicle in any plane. The motor burned HTP and kerosine in the ratio 8.2 : 1.

This is a Gamma 301 motor, which powered the later versions of Black Knight.
The motor derived from the small chamber of the Stentor motor (see below). The original chamber from RPE was doubled walled, similar to the Spectre pictured above. However, this chamber was made from nickel tubes formed to shape and brazed together. HTP flowed down one tube and up another to cool the chamber.

Gamma rocket motor under test

A Gamma 301 engine on static test at Armstrong Siddeley. This shows the very clean flame produced by these motors due to the relatively low kerosene content of the mixture.
It had a much better fuel/oxidant ratio mixture control than the 201 and also a higher thrust. The later Black Knights had a lift off thrust of around 21,000lbs as against 16,400lbs for the 201 engine.

Gamma 8 rocket motor for the Black Arrow satellite laucher

The Gamma 8 which powered the first stage of the Black Arrow satellite launcher. 8 chambers were used for the first stage of Black Arrow, and two chambers with extended nozzle for vacuum were used for the second stage motor. This was something of a kludge: the Bristol Siddeley proposal for Black Arrow used 4 Stentor chambers - the Stentor gave 25,000lb thrust as against the Gamma's maximum of around 6000lb. However, this design was a good deal cheaper!

Gamma 2 rocket motor for the second stage of Black Arrow

And here is the second stage motor for Black Arrow. Note the extended nozzles for high altitude.

This is a Stentor motor as used in the Blue Steel air to ground missile. Blue Steel used a small and a large chamber; both for climb, but the small chamber for cruise. This chamber gave around 24 000lb thrust, and would have been ideal in any satellite launcher, but R.A.E. seemed wedded to the idea of the Gamma motor. However, Bristol Siddeley did propose a satellite launcher based on this motor.

Stentor production at Armstrong Siddeley at Anstey. This gives some idea of the size of the motor. The chamber top left reminds me irresistibly of a old fashioned wind up gramophone!

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