content long 24-March-2019 03:24:49

Engineering

Electrical Power

Most of the power needed to propel Mars Express from Earth to Mars is provided by the four stage Soyuz-Fregat launcher which will separate from the spacecraft after placing it on a Mars-bound trajectory. The spacecraft uses its on-board means of propulsion solely for orbit corrections.

The main engine, an off-the-shelf item attached to the underside of the spacecraft bus, is capable of delivering a force of 400 N. It uses a mixture of two propellants which are contained in two tanks each with 267 litre capacity. Fuel is fed into the engine using pressurised helium from a 35 litre tank.

"The main engine is pretty powerful," says Rudi Schmidt, Mars Express project manager at ESA's technical centre, ESTEC in Noordwijk, the Netherlands. "It can propel the spacecraft a long way. It's used to decelerate the spacecraft to go into orbit around Mars. By the time Mars Express gets to its final orbit, most of the propellant is used up."

Corrections to the spacecraft's trajectory en route for Mars will be achieved by firing two or more of the eight 10 N attitude thrusters which are attached to each corner of the spacecraft bus and are fuelled by the same bi-propellant mixture as the main engine. The attitude thrusters are being developed for the Cluster mission which puts similar demands and constraints on spacecraft design. "The attitude thrusters are also back-up," says Schmidt. "They could do the job of the main engine if they had to, although we would not be able to reach the same final orbit."

Electrical power is provided by the spacecraft's solar panels which are folded against its body during launch and deploy shortly after the launcher housing has been jettisoned.The panels are mounted on a drive mechanism, also under development for the Rosetta mission, which tilts them forwards and backwards to catch most sunlight. The panels themselves are off-the-shelf technology. Their surface area, 11 m2, is larger than those used on near-Earth orbiting satellites to compensate for the drop in sunlight intensity at Mars.

When the spacecraft's view of the Sun is obscured during a solar eclipse, an innovative lithium-ion battery (67.5 Ah), previously charged up by the solar panels, will take over the power supply. 1400 eclipses, lasting up to 90 minutes, are expected during the nominal mission's lifetime. They occur when the spacecraft is in polar orbit around Mars and the red planet obscures its view of the Sun. When Mars is at its maximum distance from the Sun (aphelion), the solar panels will be capable of delivering 650 Watts which is more than enough to meet the mission's maximum requirement of 500 Watts, just half that of a single bar 1 kW electric fire!


Last Update: 26 October 2017

For further information please contact: SciTech.editorial@esa.int

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