Rocket propulsion

Daniel Jubb

There are three main types of rocket: solid propellant, liquid propellant and hybrid.

• A solid propellant rocket contains a solid fuel and solid oxidizer, which are premixed to form a propellant grain contained within a motor case. Solid propellant rockets are simple and reliable, however once ignited they cannot easily be stopped. This makes solids unsuitable for a land speed record car, which may need to slow down at any point during the run.
• A liquid propellant rocket generally has two separate tanks containing fuel and oxidizer, which are pumped or expelled under pressure into a combustion chamber where they mix and combust. A liquid mono-propellant uses a single liquid which is decomposed in a thrust chamber. Liquid propellant systems offer high performance, however they are complex and have a mass explosion hazard if the fuel and oxidizer are mixed.
• In a hybrid system the fuel is usually a solid contained within a combustion chamber and the oxidiser is a liquid, stored in a separate tank. Hybrid rockets can be shutdown easily and the propellants are relatively safe to handle, making them more suitable for use in a land speed record car. Hybrids are not as simple as solid propellant systems, but nor are they as complex as liquid propellant rocket engines.

After a detailed study of all these options, it was decided that a hybrid rocket was most suitable for BLOODHOUND SSC.

In the early stages of the project BLOODHOUND SSC was intended to be a pure rocket, while this offers several performance advantages, rockets are not as easy to control as jet engines and it was decided that a jet engine and a rocket provided the best solution for power and precision control.

All liquid rocket oxidizers are hazardous, the most common is liquid oxygen, it is a cryogenic liquid with a boiling point of -183°C it is difficult to handle and even minor contact can result in serious frostbite. Nitric Acid is another efficient, high density oxidizer, however it is toxic, corrosive and gives off choking fumes. HTP is non-toxic and although it can cause burns if it comes into contact with the skin, rapid washing with water can reduce the effect to a mild bleaching of the skin.

HTP was used in the UK space programme of the 1950’s, 60’s and 70’s, in rockets such as Black Knight and Black Arrow, which used liquid fuel engines burning kerosene. These programmes had an exceptional safety record. When HTP comes into contact with a catalyst it decomposes to steam and oxygen and generates heat. The combustion chambers used in Black Knight and Black Arrow had catalyst packs consisting of silver plated nickel gauzes, the HTP was fed into the packs and decomposed into steam and oxygen at around 600°C, atomised kerosene was injected into the decomposition products and ignited spontaneously with the hot oxygen. This removed the requirement for a separate ignition system. HTP can also be used as a monopropellant, simply decomposing HTP with a catalyst pack in a thrust chamber can provide thrust.

The hybrid rocket in BLOODHOUND SSC uses HTP as the oxidizer and a synthetic rubber Hydroxyl-Terminated Polybutadiene (HTPB) as the primary fuel. HTP is concentrated (86%) hydrogen peroxide H2O2, it is basically water with an extra oxygen atom. HTP is heavier than water, with a specific gravity of 1.35-1.4. It is a powerful oxidizing agent, however it is very sensitive to contamination and can be easily decomposed by many common substances. If HTP comes into contact with flammable substances it can cause spontaneous fire or explosion, great care must therefore be taken when handling HTP to ensure that it is only allowed to come into contact with clean, compatible materials. Personnel handling HTP must wear protective clothing.

The hybrid combustion chamber for BLOODHOUND SSC has a catalyst pack to decompose the HTP, the decomposition products then enter the fuel grain, which has a star shaped central conduit. The fuel automatically ignites in contact with the decomposition products, generating pressure. The combustion products then enter the nozzle which converts the high pressure, low velocity gas into high velocity low pressure gas generating up to 122 kN (27,500 lbs) of thrust. The chamber contains 181 kg (400 lbs) of fuel and can run for up to 20 seconds. The chamber is 45.7 cm (18-inches) in diameter and 4.27 meters (14 feet long). Development work has been conducted on a 15.2 cm (6-inch) diameter chamber.

Several firings have been conducted to test various aspects of chamber design, fuel grain configuration and catalyst material.

Static firing of the 15.2 cm rocket in the Mojave Desert

The principle design challenge for a hybrid rocket chamber is achieving efficient mixing.

Although high specific impulse figures are often quoted for hybrids, these are frequently the theoretical specific impulse assuming 100% combustion efficiency at the optimum mixture ratio, which is rarely achieved in practice.

The target specific impulse for the BLOODHOUND SSC chamber is 200 lb/sec/lb, this was achieved on the second 15.2cm firing and later firings have delivered over 220 lb/sec/lb.  

Sectioned view of the 15.2 cm hybrid combustion chamber

The work on the 15.2cm chamber also includes a research programme to create a full CFD model to enable a more complete understanding of how mixing, flow, heat transfer and chamber arrangement affect combustion efficiency. The combustion modelling is being undertaken in conjunction with the National Physical Laboratory (NPL) and Fluid Gravity Ltd.

The BLOODHOUND SSC vehicle will carry 963 kg (2,100 lbs) of HTP, this is stored in a lightweight aluminium tank at 1.65 Bar (25 lb/in2), the HTP is supplied to the chamber by a high speed pump. The pump is based on the large HTP pump used in the Stentor rocket engine, which powered the Blue Steel cruise missile of the 1960’s. The pump design was upgraded by the original designer and the new pump is 15% more efficient.

Sectioned view of the HTP Pump, which requires 620 hp to drive at full speed

In Blue Steel the pump was driven by a 50,000 rpm turbine powered by HTP decomposers, this was not considered suitable for use in BLOODHOUND SSC, so it was decided that the pump would be driven by a piston engine. The engine selected is a 4.5 litre V12 racing engine made by Menard Competition Technologies (MCT). The V12 also serves as the BLOODHOUND SSC auxiliary power unit providing electrical and hydraulic power, although the primary application is driving the HTP pump, which requires 620 shaft horsepower. The pump operates at 12,000 rpm and pumps 50.3 kg (111 pounds) of HTP per second, delivering it to the chamber at 75.8 Bar (1,100 lb/in2).

Pump test rig

The pump has been tested using water, initial tests at 5,000 rpm were carried out at MCT’s Leafield site using the hydraulic test cell (which is normally used for gearbox testing).Full speed tests using the V12 were carried out at MCT’s Kidlington site.

The HTP pump and V12 engine on the water test rig at MCT.

The tests were conducted by pumping water around a loop, this simulates the pump inlet condition and uses an orifice plate and varying pipe diameters to dissipate the pressure generated by the pump. A number of pressure transducers monitor the pump inlet and outlet pressures and the pressure either side of the orifice plate, thermocouples measure the inlet and waste water temperature. As virtually all the energy which goes into the rig is converted to heat, the temperature in the loop can increase rapidly if the loop is closed. A controlled amount of water is allowed to leave the rig through an orifice and cool water is supplied to the low pressure side of the loop by an external booster pump. The tests have confirmed that the pump met and exceeded the performance requirements.

The next stage of the programme will be the tests of the 45.7 cm (18-inch) combustion chamber. The initial firings will be part of the development programme. Before the hybrid rocket system is used in BLOODHOUND SSC it will undergo a safety and acceptance test programme, involving 10 full duration firings and testing beyond the operational limits set for running the car.