HTP fuel could improve safety of satellite thruster systems

UK engineers are turning to a rocket fuel used by the British space programme in the 1960s to create safer satellite thruster systems.

A team at Astrium is leading the development of thruster technology that runs on the same propellant as British Black Arrow rockets — and that being used by the Bloodhound land-speed record team — as a replacement for the more dangerous hydrazine, which could be banned under incoming EU law.

The new thruster will enable satellites to reposition themselves into different orbits using a combination of solid fuel and a liquid oxidiser, which will also be used on its own to power smaller thrusters that control the craft’s attitude (inclination).

Although this oxidiser, high-test peroxide (HTP), is hazardous, it is a safer alternative to the highly toxic and unstable hydrazine, said Astrium’s propulsion manager, Graham Viney, who is leading the Technology Strategy Board co-funded project.

‘Hydrazine vapours burn internally so it’s not so good for humans,’ he told The Engineer. ‘With significant exposure, it can be considered lethal. Having said that, it’s excellent for providing the thrust we need for the spacecraft.’

Both hydrazine and HTP were first used as rocket fuels by the Germans during the Second World War and later became spacecraft propellants, but HTP was largely confined to experiments by the UK while hydrazine became much more widely used.

Now the Astrium team hopes to draw on Britain’s heritage of HTP to develop thrusters that can equal or perhaps even exceed those fuelled by hydrazine, which could be outlawed under the EU’s Registration, Evaluation, Authorisation and Restriction of Chemical Substances (REACH) legislation in the next few years.

‘A door has opened on opportunities with this following some of the proposals of the catalysts that could be used,’ said Viney.

‘The first step is to establish the thruster’s design, get it firing, get the technology working and then there’s a number of avenues of development, one of those being the catalyst and one being the concentration of the peroxide used.’

The challenges for the team include determining the right materials to use in the thruster design, partly because some materials used to make smaller parts such as seals and valves have changed in the 30 years since HTP was last widely used and may not be compatible with the fuel.

Another issue is storing the two fuel components, including the solid rubber hydroxy-terminated polybutadiene (HTPB), and understanding how they react in order to build the most efficient system within the physical constraints of a satellite spacecraft.

‘Previous craft may have had to think about two liquids on board; how to split those among tanks,’ said Viney.

‘This is why we need a solid and a liquid and it’s got to react in a certain way. The material solid is fixed — you can’t change it — so that needs to be considered… It needs to be mastered; it’s not just “let’s throw a bit of rubber and liquid in”.’

Using solid HTPB in a hybrid rocket system does have precedents, not only in Bloodhound — which recently conducted its first UK rocket test — but also in the private spacecraft SpaceShipOne flown in 2004 when it was burnt with nitrous oxide.

Although it is being designed with satellite spacecraft in mind, the thruster could also be used for other applications such as planetary landers, said senior propulsion engineer Jonathan Wynn.

‘The technology we’re looking at is highly scalable in nature, ranging from a small 1N thruster that you might find on an Earth observation satellite, through to the type that we’ll be testing in the coming months, which will be of a 10N variety — which is more to do with attitude control for geostationary satellites.’

Astrium is hoping to complete its first prototype thruster by the end of the month and to conduct tests in November.

NASA is also investigating replacements for hydrazine and has selected Ball Aerospace & Technologies of Colorado in the US to develop a technology demonstrator, leading to a flown mission in three years’ time.