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 by Staff Writers Paris (ESA) Nov 26, 2018
In mid-December, twin discs will begin glowing blue on the underside of a minibus-sized spacecraft in deep space. At that moment Europe and Japan's BepiColombo mission will have just come a crucial step closer to Mercury. This week sees the in-flight commissioning and test firing of the four thrusters - with one or two firing at a time - of the Solar Electric Propulsion System that BepiColombo relies on to reach the innermost planet. This marks the first in-flight operation of the most powerful and highest-performance electric propulsion system flown on any space mission to date. Each thruster and its associated power processing and propellant flow control units will be tested to full power to check no ill-effects were incurred from launch, culminating in the first twin thruster operations - the configuration to be used throughout most of the mission. Their first routine firing is scheduled for the middle of next month, and the propulsion system will operate continuously for three months to optimise the spacecraft's trajectory for the long voyage to Mercury.
The voyage inward Like all objects in the Solar System, the spacecraft is in solar orbit, moving perpendicular to the pull of the Sun's gravity. BepiColombo therefore has to slow down through a series of braking manoeuvres and flybys, making it more susceptible to the Sun's gravity and letting it spiral closer to the heart of the Solar System. The thrust produced by the electric propulsion system serves to decelerate the spacecraft, or in some cases accelerates it to make its braking flybys more effective. No less than nine planetary flybys of Earth (once), Venus (twice) and Mercury itself (six times) are required to place the multi-module spacecraft in orbit around Mercury in seven years' time.
Space tug The high performance of the propulsion system, in terms of the amount of fuel the thrusters require, is critical. Inert xenon gas is fed in to the thrusters, where electrons are first stripped off the xenon atoms. The resulting electrically charged atoms, referred to as ions, are then focused and ejected out of the thrusters using a high voltage grid system at a velocity of 50 000 meters per second. This exhaust velocity is 15 times greater than conventional chemical rocket thrusters, allowing a dramatic reduction in the amount of propellant required to achieve the mission. "The propulsion system transforms electricity generated by the Mercury Transfer Module's twin 15 m-long solar arrays into thrust," explains ESA electric propulsion engineer Neil Wallace. "At full power, a thrust equivalent to the weight of three 1-euro coins is developed, meaning that the thrusters have to keep firing for long periods to be effective, but in the absence of any drag and assuming you are patient, the manoeuvres that are possible and the payload that can be carried are dramatic."
Electrifying spacecraft propulsion The first flight of their technology came with the 10 cm-diameter T5 thruster, a key element of ESA's 2009 gravity-mapping GOCE mission, where it allowed the satellite to orbit at the top of Earth's atmosphere for over three years, skimming through the diffuse atmosphere at the unprecedentedly low orbital altitude needed for the mission. The scaled-up T6 thrusters are 22 cm in diameter, the increase in size required for the higher thrust and lifetime requirements of the BepiColombo mission. And unlike GOCE's T5, these T6 thrusters are manoeuverable, courtesy of gimbal systems developed by RUAG Space in Austria. "They are clever mechanisms that complicate the system design a bit - all the electrical cables and pipes have to cross a moving boundary - but add a lot to performance," adds Neil. "They ensure the thrust vector of either a single or double engine firing crosses through the centre of gravity of the spacecraft, which changes over time as propellant is used up." Thruster operations are controlled using two Power Processing Units, the architecture of which are designed to support the firing of two T6s simultaneously even in the event of any system anomaly, guaranteeing the maximum thrust of 250 mN can be maintained.
Injecting intelligence The other key elements are propellant Flow Control Units, also overseen by the PPUs, and the high-voltage electrical harness. The FCUs ensure the correct flows of xenon gas are supplied to the thrusters and were developed by Bradford Engineering in the Netherlands to provide programmable flow rates. The various elements of the propulsion system have undergone individual and extensive performance and qualification testing ultimately concluding in a series of tests performed at QinetiQ's Farnborough site.
Bepicolombo magnetometer boom deployed Paris (ESA) Oct 29, 2018 The 2.5 m long boom carrying the magnetometer sensors onboard ESA's BepiColombo Mercury Planetary Orbiter (MPO) has been successfully deployed. The sensors are now prepared to measure the magnetic field on the way to Mercury. Following launch last weekend, and having completed the 'launch and early orbit phase' on Monday, confirming the spacecraft and systems were healthy and functioning now they are in space, attention has now turned to checking the suite of scientific instruments on the science ... read more
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