Designing the truly ‘mission critical’ component
A unique, high specification valve, designed to control the flow of
liquid helium at near-absolute zero temperatures, will be at the heart of
the European Space Agency’s space observatory mission, which is due for
launch in 2007.
One of the tasks of the European Space Agency’s (ESA’s) planned space
observatory mission, which should be heading off into the depths of space
in little over six years time, will be to measure the minute levels of
infrared radiation emitted by distant stars and planets, to gain new
insights into the history, composition and eventual fate of star systems
and galaxies.
The mission satellite will carry an array of hypersensitive radiation
sensors to be deployed and aligned ready for their data-gathering role.
During operation it is vitally important to eliminate as much of the
sensors’ self-generated radiation as possible, to avoid drowning out the
tiny levels received from space. To achieve this they need to be kept at
an astonishingly low operating temperature of between 1 to 2oK - at the
threshold of absolute zero. The sensor coolant is liquid helium,
continuously controlled by a valve that has been specially designed and
constructed for this project by Norgren, working on behalf of liquid
helium equipment specialist, Linde and Etel, who developed the motor
operated actuator.
ESA’s specifications were demanding. The precision control valve not only
has to operate with liquid helium in the extreme temperatures and vacuum
of space, it also has to remain pressure tight during the preparation,
launch and deployment operations. In particular, the Agency wanted almost
zero loss or leakage of the vital liquid helium cargo; any loss of
coolant would result in a reduction of valuable operational time and
consequently the amount of data gathered. Lightness of weight and low
power consumption were also required but, above all, the valve had to be
100% reliable. A failed valve would be a failed mission.
The research and development was carried out at Norgren’s valve division
headquarters in Fellbach, near Stuttgart, Germany. This was formerly the
main site of Herion, which joined the Norgren Group in October 1997. The
prototype, which has now been successfully tested under extreme
conditions is based on a motorised valve with a precision metal to metal
seat arrangement of special geometric design. Particular attention was
paid to keeping the valve operating force low, thereby reducing the power
required from the motor actuator. A solenoid design was rejected at the
outset as the power consumption would have been too high and would have
increased the mass of the power supply plant. Ehrenfried Dittrich,
Norgren’s cryogenics specialist, picks up the story.
“Our technical expertise has been tested to the absolute limit by the
most demanding of operating environments - outer space. We had to fulfil
very special requirements in the use of cryostats to cool highly
sensitive instrumentation and it was essential that very low leakage
rates were achieved to make the most efficient use of the stored helium.
Other things we had to pay close attention to were a restricted power
budget, wear resistance (no plastics), weight and dimensions, as well as
leak-tightness under high acceleration loads. Above all, we had to
achieve absolute functional reliability - once the mission is in space,
there won’t be any opportunity for service or repair.