Super-efficient ‘electric engines’ set BepiColombo on its trajectory to Mercury

The QinetiQ-engineered Solar Electric Propulsion System (SEPS) on board BepiColombo, the joint ESA/JAXA spacecraft on a mission to Mercury, has now begun propelling the Mercury Transfer Module (MTM) to the heart of our solar system. The four QinetiQ T6 Gridded Ion Thrusters will power two science orbiters – the Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter – on their seven year journey to gather more knowledge about the least explored of our ‘neighbour’ planets.

The QinetiQ SEPS is the most powerful and high-performance electric propulsion system ever flown. Its super-efficient T6 thrusters enable the spacecraft to reach maximum speed with minimal fuel consumption, using electricity generated by solar panels to produce charged particles from xenon gas. A beam of these charged particles, or ions, is then expelled from the spacecraft to propel it forward. The thrusters will be used for interplanetary travel, running in pairs in order to share the workload. 

The BepiColombo spacecraft left Earth on 20 October aboard an Ariane 5 and the engines were first powered up on 16 November to undergo testing, the first time the engines had been fired in space. Each thruster was brought to life, warmed up, and then taken to full power individually – a force of 125 millinewtons (mN) – and then in pairs. This week saw the QinetiQ SEPS begin the first of the 22 planned ion thrust arcs that will steer BepiColombo on its interplanetary trajectory. 

The high-tech electric propulsion system is a significant space engineering achievement, which is making a major contribution to one of the most advanced and ambitious missions ever launched. Developed by a British-led multinational team, the consortium includes world-class engineers from Bradford Engineering in the Netherlands and CRISA in Spain, and worked with Airbus at Stevenage, responsible for integrating the engines into the spacecraft.

The SEPS is enabling ESA to address key mission challenges, including those presented by the Sun’s enormous gravitational pull. The system will provide the low thrust required for the spacecraft to continuously ‘brake’ against this pull during the journey to Mercury. In addition, the xenon gas burned by the thrusters requires 10 to 20 times less space on board than chemical fuel, freeing up capacity for more than double the scientific equipment to be transported to Mercury.

Peter Randall, QinetiQ’s Systems Engineer – Electric Propulsion, said: “The development of the SEPS system powering the mission to Mercury is a brilliant example of what British engineering can achieve, in collaboration with partners from Europe and the US. Together, we have changed the way we travel in space. We’re pleased to say that all four thrusters survived the launch and are operating perfectly in flight, performing well within expectations, and ready to start their important work.”

The spacecraft will arrive at its destination in late 2025, where it will gather data during a one-year nominal mission, with a possible one-year extension.

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The background:

QinetiQ has more than 50 years’ of heritage in researching and testing electric propulsion, beginning with the pioneering T1 ion thruster developed in the late 1960s. The first to fly in space was the T5 in 2001, followed by the highly successful first mission for ESA’s GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite in 2009, which ran for four years – 36,000 hours – using the same engine. A huge leap forward, the T6 is twice the size and diameter, and provides up to seven times the thrust. The QinetiQ team is continuing to develop the family of thrusters and is currently working on the T7.

The QinetiQ SEPS underwent rigorous engineering testing on the ground to prepare it for successful flight in space, with world class facilities replicating the conditions the engines will be exposed to on BepiColombo’s journey. This included:

·         Environmental testing in a vacuum chamber, where the system was subjected to extremes of temperature ranging from -140°C to +170°C.

·         A simulated launch, with a shaker table simulating the vibrations encountered on take-off.

·         Endurance testing to ensure the thrusters can meet the lifetime requirements for the seven year mission.

The technology:

The QinetiQ T6 ion engines are 22cm diameter, 4.5kW Kaufman-type ion thrusters, powered by solar energy from the spacecraft which they’ll use to accelerate xenon gas to an exhaust velocity exceeding 50km/s. This high propellant velocity allows the thruster to give a very high specific impulse (akin to miles per gallon for a car); around 10 times higher than traditional chemical rockets. In testing, the engine demonstrated it was capable of providing up to 145 mN of thrust at a specific impulse of 4,000 seconds and a total efficiency of 65%. These incredible exhaust speeds are key to the thruster’s performance, allowing the craft to reach astonishing speeds for minimal fuel consumption.

Other advantages of using an electric propulsion system include:

·         Reduced mass of the launch vehicle, resulting in lower launch cost

·         Increased payload capacity

·         Fine control of thrust levels, compared to the typical on/off setting of chemical rockets

·         Increased mission duration.

Only two thrusters are operated at one time, with two additional thrusters added for redundancy in line with the customer’s requirements.

QinetiQ also designed, built and qualified the electrical harness and xenon pipework required by the SEPS, and worked closely with other partners who provided the power processing unit that will take the power from the solar cells to the thrusters, and the flow control units which supply xenon from the propellant tanks.

The mission:

BepiColombo is a joint mission between the ESA and the Japan Aerospace Exploration Agency (JAXA), executed under the ESA’s leadership. The objective is to better understand the structure and evolution of Mercury, which is the least explored of all terrestrial planets due to its close proximity to the Sun. The spacecraft will arrive in late 2025, when it will gather data during a one-year nominal mission, with a possible one-year extension.

The mission comprises two spacecraft: the Mercury Planetary Orbiter (MPO) and the Mercury Magnetospheric Orbiter (MMO). The Airbus-built Mercury Transfer Module (MTM) will carry the orbiters to Mercury using a combination of solar electric propulsion and gravity assist flybys.

Travelling some nine billion kilometers in total, BepiColombo will take nine flybys at Earth, Venus and Mercury, looping around the Sun 18 times. By late 2025 the transfer module’s work will be done: it will separate, allowing the two science orbiters to be captured by Mercury’s gravity, studying the planet and its environment, along with its interaction with the solar wind, from complementary orbits.

The mission is named after the Italian mathematician and engineer Giuseppe (Bepi) Colombo (1920–84).