Long-distance fuel delivery gives Cluster II Spacecraft A Boost
20 March 2000A lot of down-to-Earth preparations have to take place before the ambitious Cluster II mission to study the magnetosphere can be launched into space this summer. One of the most difficult stages was completed at the weekend when a high security cargo of explosive fuel arrived under armed guard at the Baikonur Cosmodrome in Kazakhstan.
Sending four Cluster II spacecraft into highly elliptical polar orbits takes a lot of energy. Apart from a single 400 N main engine, which is used to manoeuvre the spacecraft into their operational orbits, each satellite also carries eight 10 N thrusters - four radial and four axial - for smaller orbital adjustments. Unfortunately, this means that fuel accounts for more than half the launch weight of each 1.2 tonne spacecraft.
Over the past three weeks, two containers loaded with tanks of 3.75 tonnes of fuel (enough for five satellites) have been en route to the launch site at Baikonur. Inside the containers were the two ingredients needed to propel the Cluster II spacecraft into their unusual operational orbits - monomethyl hydrazine (MMH), and mixed oxides of nitrogen (MON1).
Fortunately for everyone involved, although MMH and MON1 are so dangerous to anyone who is exposed to them that workers have to wear special protective suits and breathing apparatus, the two fuels are only explosive when they come into contact with each other. This special property means that these so-called hypergolic fuels are highly suited to spacecraft propulsion - simply open the valves, allow them to mix in the right amounts and stand back for ignition. Cut off the supply of fuel and the engine burn automatically shuts down.
The painstaking transfer of these noxious substances began in the U.K. port of Hull on 28 February. Over the next week and a half, they were shipped 2,500 km across the North Sea and the Baltic Sea to the Russian port of St Petersburg. There, in sub-zero temperatures, they were offloaded onto a container train for the week-long trek to Kazakhstan. Escorted all the way by two armed guards, the operation came to a successful conclusion on 19 March when the train finally pulled into the depot at the launch centre. It was then unloaded into the store room of the spacecraft integration area earlier today.
There the fuel will stay until, shortly before the Cluster II launches in mid-June and July, the four spacecraft will each be loaded with 650 kg of fuel in Baikonur's specially built Hazardous Processing Facility. Most of this fuel will be consumed soon after separation from the Soyuz-Fregat booster, during the six major manoeuvres required to reach their operational orbits. Each satellite's main engine will gradually adjust the orbit from a 64.80 inclination path, which varies in altitude between 200 km and 18,000 km, to the final 19,000 km - 119,000 km orbit which carries the spacecraft over the Earth's poles.