Europeans And Australians Make Space Propulsion
Breakthrough
The European Space Agency and the Australian National University
have successfully tested a new design of spacecraft ion engine that
dramatically improves performance over present thrusters and marks
a major step forward in space propulsion capability.
DS4G thruster firing during tests in the ESTEC Electric Propulsion
facility (CORONA vacuum chamber). (Credit: ESA)
Ion engines are a form of electric propulsion and work by
accelerating a beam of positively charged particles (or ions) away
from the spacecraft using an electric field. ESA is currently using
electric propulsion on its Moon mission, SMART-1. The new engine is
over ten times more fuel efficient than the one used on SMART-1.
Using a similar amount of propellant as SMART-1, with the right
power supply, a future spacecraft using our new engine design
wouldnt just reach the Moon, it would be able to leave the Solar
System entirely, says Dr Roger Walker of ESAs Advanced Concepts
Team, Research Fellow in Advanced Propulsion and Technical Manager
of the project.
The new experimental engine, called the Dual-Stage 4-Grid (DS4G)
ion thruster, was designed and built under a contract with ESA in
the extremely short time of four months by a dedicated team at the
Australian National University. The success of the DS4G prototype
shows what can be achieved with the passion and drive of a capable
and committed team. It was an incredible experience to work with
ESA to transform such an elegant idea into a record-breaking
reality, says Dr. Orson Sutherland, the engines designer and head
of the development team at the ANU. During November 2005, the DS4G
engine was tested for the first time in ESAs Electric Propulsion
Laboratory at ESTEC in the Netherlands, with support from Dr
Sutherland and ESA test engineers.
Traditional ion engines use three closely separated perforated
grids containing thousands of millimetre-sized holes attached to a
chamber containing a reservoir of the charged particles. The first
grid has thousands of volts applied, and the second grid operates
at low voltage. The voltage difference over the gap between the two
grids creates an electric field that acts to simultaneously extract
and accelerate the ions out of the chamber and into space in a
single step. The higher the voltage difference, the faster the ions
are expelled and the greater the fuel efficiency of the thruster.
However, at higher voltage differences approaching five thousand
volts (5kV), some of the ions collide with the second grid as they
are accelerated, thus eroding and damaging the grid and thereby
limiting its lifetime in space.
The DS4G ion engine utilises a different concept first proposed in
2001 by David Fearn, a pioneer of ion propulsion in the UK, which
solves this limitation by performing a two-stage process to
decouple the extraction and acceleration of ions using four grids.
In the first stage, the first two grids are closely spaced and both
are operated at very high voltage and a low voltage difference
between the two (3 kV) enables the ions to be safely extracted from
the chamber without hitting the grids. Then, in the second stage,
two more grids are positioned at a greater distance downstream
and operated at low voltages. The high voltage difference between
the two pairs of grids powerfully accelerates the extracted
ions.
The test model achieved voltage differences as high as 30kV and
produced an ion exhaust plume that travelled at 210,000 m/s, over
four times faster than state-of-the-art ion engine designs achieve.
This makes it four times more fuel efficient, and also enables an
engine design which is many times more compact than present
thrusters, allowing the design to be scaled up in size to operate
at high power and thrust. Due to the very high acceleration, the
ion exhaust plume was very narrow, diverging by only 3 degrees,
which is five times narrower than present systems. This reduces the
fuel needed to correct the orientation of spacecraft from small
uncertainties in the thrust direction.
There is of course still a great deal of work to be done before the
new engine design can fly in space. Working with our industrial
partners, the next challenge is to transition this promising new
engine design from laboratory experiment to spacecraft flight model
and properly define the new missions that it will enable, says
José Gonzalez del Amo, Head of Electric Propulsion at ESA. The
flight-suitable engines must then be tested: and for ion engines
this is a long process.
Since they must operate continuously in space for tens of thousands
of hours providing a small thrust, ground tests in a vacuum
facility must last several thousand hours to prove their
reliability. Only after all this could the first flight models be
launched.
Once ready, these engines will be able to propel spacecraft to the
outermost planets, the newly discovered planetoids beyond Pluto and
even further, into the unknown realm of interstellar space beyond
the Solar System. Closer to home, these supercharged ion engines
could figure prominently in the human exploration of space. With an
adequate supply of electrical power, a small cluster of larger,
high power versions of the new engine design would provide enough
thrust to propel a crewed spacecraft to Mars and back.
This is an ultra-ion engine. It has exceeded the current crop by
many times and opens up a whole new frontier of exploration
possibilities, says Dr Walker.