Validating ion engines for interplanetary travel

Curiosity led exploration of space has driven the development of over 2000 technologies including integrated circuits, lightweight insulation and automated credit card transactions. However, overcoming this harsh environment requires state-of-the art technology which presents its own new measurement challenges. This is especially true for interplanetary travel where high vehicle fuel efficiency is required for the long distances involved, maximising mission payloads and the amount of science that can be performed.

Challenge

Space exploration has provided insights into the formation of
the solar system and processes underway on Earth. Missions to
Venus, for example, have informed us on the greenhouse effect
and the dangers of chlorofluorocarbons (CFCs). Ion engines, a
form of electric propulsion, may be preferable to chemical rockets
for such missions. Being more efficient they require less fuel thus
smaller, lower-cost launch vehicles can be used, and payloads
maximised. One common form of ion engine employs ‘gridded’
thrusters in which propellant atoms are bombarded by electrons
to generate positively charged ions. The potential difference
between two plates at the rear of the engine, the screen and
accelerator grids, focus these particles and accelerate them to
extremely high velocities to provide thrust. Key to efficiency is
the separation (< 1 mm) between these plates, but pre-launch
measurement of this parameter has proven challenging. During
operation grid plates can exceed 400 ºC and only mathematical
models existed to inform on effects such as thermal expansion
on plate separation. In addition, engines are tested in vacuum
chambers which are large to minimise interactions between
the thruster being tested and the walls of the chamber in order
to reproduce as closely as possible the conditions in space. No
practical methodology existed to accurately measure changes in
grid plate separation down to the required resolution, in vacuo
and from outside of the vacuum chamber, over 6 m from the
thruster.

Solution

The EMRP project Large Volume Metrology in Industry successfully
tackled the most critical needs expressed by a range of users of
Large Volume Metrology, including novel ways of measurement
compensation for thermal and refractive index distortions. During
the project’s lifetime a system based on ‘divergent beam frequency
scanning interferometry’ (FSI) was developed. FSI uses a tunable
laser to provide highly accurate distance measurements with the
potential to track multiple targets with an accuracy in the μm
range. After successfully demonstrating an accuracy of FSI of 50
μm over small (1 m3) volumes it was then developed further into
a longer-range system. This demonstrated an accuracy of 100 μm
over a range of 10 m. As well as improving the measuring range
using spatial light modulators, which also allowed beam shaping
and steering, a novel dual-wavelength technique allowed the
system to compensate for the presence of vibration.

Impact

QinetiQ is a world leader in defence, technology and security and,
since development of their ground-breaking T1 ion thruster in
the sixties, it is also a world leader in electric propulsion systems.
Using a modified FSI system NPL, who developed the method
in the EMRP project, was able to measure the alignment of the
grid plates in QinetiQ’s innovative, 22 cm-diameter, T6 ion engine
with < 100 μm uncertainty, validating QinetiQ’s mathematical
models. These engines are now in use in ESA and JAXA’s 2018
BepiColombo mission to Mercury. NPL’s contribution has been
instrumental in giving ESA the confidence required to use these
engines and it is the first-time electric propulsion has been
used to travel to one of the inner planets. QinetiQ’s thrusters
will minimise fuel consumption and hence help address key challenge such as the enormous amount of energy required to

brake the craft at its destination whilst maximising the quantity of
scientific equipment being transported. Once the BepiColombo
mission arrives at the planet in late 2025 it will help reveal
information on the composition and history of Mercury and
about the formation of the inner planets in general, including
Earth.

New low-cost, large volume measurements for industry

he EMRP Large Volume Metrology in Industry (Luminar) developed three approaches to compensate for measurement errors associated with industrial conditions, including refractive index effects in optical measurements caused by environmental variations. Prototypes and novel systems based on optical technologies capable of measuring distances covering 10 metre and 50 metre ranges with micrometre precision were built and validated. These were assessed at a 50-metre tape bench facility which was upgraded with additional temperature, pressure and humidity sensors to enable provision of conditions similar to industrial environments. Performance of the new techniques were then assessed under industrial conditions. As well as submitting five patents a new calibration service using tracking interferometry was established for large component measuring machines. A novel multi-target measuring system also developed will help solve problems in large science (CERN, ESRF) facilities where the performance of existing large volume metrology tools is insufficient for next generation accelerators.

  • Category
  • EMRP,
  • Industry,
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