Rosetta fuelled for the journey of a lifetime
17 December 2002Just as motor vehicles need fuelling before long journeys, so spacecraft require full tanks before they set off to visit other worlds. But whereas a typical car may carry 40 or 50 litres of petrol and then be refuelled after travelling a few hundred kilometres, there are no filling stations in space.
As a result, interplanetary explorers such as ESA's Rosetta comet chaser have to carry vast amounts of both fuel and oxidiser for their extended trek to other worlds.
In the case of Rosetta, the cube-shaped Orbiter has recently been filled with more than 1.7 tonnes of propellant for its 6.5 billion kilometre expedition to explore Comet Wirtanen. More than half of its weight on lift-off will be accounted for by 1060 kg of oxidiser (nitrogen tetroxide) and 660 kg of fuel (monomethyl hydrazine).(*)
Once Rosetta is boosted out of Earth's gravitational grasp by an Ariane 5 launcher, the spacecraft will rely on this supply for manoeuvring and attitude control throughout its 10.5 year space odyssey to the depths of the Solar System and back.
The task of transferring such a huge amount of propellant was entrusted to a team of six skilled engineers from Astrium-UK. Wearing protective suits, the team worked in shifts over three days to prepare the spacecraft for its January 2003 launch.
Team leader Chris Smith described the well-rehearsed procedure.
"We worked on two shifts for each propellant tank ? two men on each shift and two more in the control room," he explained. "Each shift lasted a maximum of 4.5 hours because that is the lifetime of our radio batteries."
"On shift one, we made all the connections between the tanks and started the filling operation. Later, we stopped the operation, closed the valves and handed over to our colleagues on the second shift to complete the fuelling. The men in the control room monitored the entire procedure by video, giving directions by radio to the fuelling team."
Every precaution was taken to ensure that there was no spillage or exposure to the toxic fuel.
"We have to wear special pressurised SCAPE suits. (SCAPE stands for Self-Contained Atmosphere Protective Equipment.) These are quite difficult to put on, so we always have a radio man and a safety officer standing by to assist us," said Smith.
"First, we put on a pair of 'long johns' that cover us up to the neck. After our oversocks and silk gloves are taped to the long johns, we put on a heavy waistcoat that carries our radio equipment. Next, we put on the rubber-impregnated suit. We squeeze the left leg in first, then the right leg, before curling into a ball and pushing the head into the top of the suit. Then the arms go in and the suit is zipped up. Finally, the helmet is fitted and the airline is connected."
"We breathe normal air, but the temperature of the suit is controlled by an air conditioner. The suit weighs about 40 pounds (18 kg) and the helmet is very heavy, so it presses on your shoulders and gives you a backache."
"We were transferring nearly 4000 pounds (1700 kg) of fuel - essentially a 4000 pound bomb - so while we were working, everyone was cleared from the site for a distance of one third of a mile (0.5 km) and blast walls were in place in the building. 'Sniffers' monitored the air in the room to detect any release of vapours."
So what was the outcome of this protracted procedure?
"It all went very smoothly," said Smith. "It is always difficult connecting the fuel lines to the spacecraft while wearing a SCAPE, but there was only one leaky valve and that was spotted immediately."
For a veteran of 10 ESA missions, isn't Smith ever concerned about working in such a hazardous environment?
"We always worry about the safety side, but if we didn't then it would be time to quit," he said. "Things can happen that we can't predict, but everyone on the team is confident in each other's abilities."
The success of this fuelling operation brings to an end a hectic three-month pre-launch campaign at the Kourou spaceport for the engineers from ESA and industry. The Rosetta comet chaser now rests inside building S5B awaiting the go-ahead for mating with its Ariane launcher. "This was the last major activity on the spacecraft, and the most dangerous," said Fleming Pedersen, the Rosetta System and Payload AIV Engineer. "We are now looking forward to 'babysitting' the spacecraft for a few weeks before the final launch preparations in January."
(*) Petrol or diesel fuel used by cars will burn in the presence of oxygen in the air. However, since there is no air (or oxygen) in space, Rosetta has to carry its own oxidiser, which enables the fuel to burn when it fires its 24 rocket thrusters.