Space Engines of the Future
The Potential of Plasma Propulsion
Rocket propulsion is governed by Isaac Newton’s third law of motion; that for every action there has to be an equal and opposite reaction. When mass is ejected from the back of a rocket engine nozzle, the rocket recoils forward. However, every kilogram of mass ejected by a rocket at a given point in its trajectory has to be lifted to that point by the rocket itself. A kilogram of propellant used in orbit will require as much as ten kilograms of propellant to get it there in the first place.
With space launch costs reaching tens of thousands of dollars per kilogram, it’s important to get the most out of the least amount of fuel possible. Conservation of momentum tells us how best to achieve this. A kilogram of fuel expelled at a thousand metres per second will change the momentum of a spacecraft by ten times more than the same amount of fuel expelled at only a hundred meters per second. So the faster you make your exhaust, the more efficiently you use your fuel.
Whilst chemical rockets are the only practical method of launching vessels into space, once in space there are some innovative alternative propulsion systems. One way to achieve high exhaust velocities in space is to ionise your propellant using electrical energy and then accelerate these ions to high velocities before ejecting them out the back of the spacecraft. Such plasma based propulsion systems use propellant far more efficiently than conventional chemical rockets and have been used on recent space missions such as NASA’s DAWN and the ESA’s SMART-1.
A new plasma propulsion device is the Helicon Double Layer Thruster (HDLT) being developed by Professor Rod Boswell and Dr Christine Charles at ANU. As with all plasma thrusters, the principle is to eject charged particles, or plasma, at very high speeds. The innovation in the HDLT is the way the plasma is accelerated to these speeds.
The HDLT uses a phenomenon called an electric double layer, which is the electrostatic equivalent of a sheer drop. The plasma ions passing through the double layer experience a sudden and very forceful acceleration in the same way water does as it flows over a cliff. The same double layer physics are behind the awesome light show of the aurora. In this case, the charged particles of the solar wind enter the Earth’s atmosphere at the poles.
“The HDLT is a beautiful piece of physics because it is so simple and has an almost infinite lifetime. It doesn’t need any moving parts, any electrodes and is based purely on naturally occurring physical phenomena,” Dr Charles explains.
With conventional plasma thrusters the continuous ejection of positively charged particles creates a problem. Negative charging of the spacecraft. If left unchecked this would both create havoc with the onboard electrical systems and also strongly reduce the number of particles leaving the thrusters which results is poor performance. To overcome these problems, most electric thrusters have an electron ejecting charge neutralising system that neutralises the ions leaving the spacecraft in the exhaust. These systems are prone to failure and conventional ion thrusters usual have two or three neutralisers onboard, which adds to the mass of the spacecraft. However, one of the special features of the HDLT is that due to the unique configuration of the double layer both electrons and ions are ejected from the exhaust, which ensures that the plasma leaving is neutral and that the spacecraft doesn’t charge up.
Making a successful space propulsion system is a difficult business. Space engineers don’t have the luxury of flying one prototype thruster after another in orbit until they get the design just right. Such propulsion systems have to be designed using theory and computer simulations and then tested in labs on Earth. However, it’s impossible to create a vacuum test chamber that’s even remotely comparable with the vastness of space. And given that the magnetic field and ion exhausts of a plasma thruster extend over large distances, you can never be entirely sure that what you measure in the lab is going to be exactly what you get in space.
Michael West is undertaking his PhD testing the HDLT prototype in conditions that simulates the vacuum of space as closely as possible in a lab. “The main thing we’re aiming for is to immerse the entire thruster in vacuum, not just the exhaust port. If in addition, we can make the chamber as big as practical, we’re optimistic that we’re getting pretty close to actual space conditions.” He explains.
One of the most important parameters of any rocket engine is how much thrust it produces. With a chemical rocket this is relatively easy to measure because such engines produce a very large thrust for a very short time. You mount the engine on a jig, fire it up and measure how much force it exerts against a fixed mount. However with a plasma propulsion system it’s not so easy because they produce a very small thrust for a very long time.
“It’s a bit like paddling a canoe” Michael explains, “You can paddle like crazy until you’re worn out, then coast along with the momentum you’ve built up, and that’s like a chemical rocket. Or you can paddle gently all day, gradually increasing your momentum, which is more like electric propulsion.”
In the frictionless environment of space, a milliNewton of thrust applied for several months will slowly accelerate a 1000 kg space ship to huge speeds. But on Earth it’s very hard to measure this tiny thrust by just monitoring the force on the back of the thruster assembly. To get around this, Michael developed a simple but effective thrust measuring instrument. A silicon wafer is attached to the end of a rod, which is suspended on a pair of knife edges. This creates a pendulum-like structure. When the plasma exhaust from the thruster hits the wafer it displaces it from vertical – the larger the thrust, the greater the displacement. In order to measure the very small displacements, Michael uses a laser bounced of the back side of the wafer and directed to a CCD sensor some distance away. Using this device, he is able to measure thrusts of a few microNewtons – about the force an ant’s foot exerts on the ground.
The team are now using the thrust measurement system and simulation chamber to explore the possibility of operating the thruster in a super-bright high density mode, which generates about seven times the ion flux of normal operations. The group have collaborated with the European Space Agency during initial development and testing of the first HDLT prototype.
Michael explains, “Highly efficient electric propulsion systems like the HDLT are exactly what you need for manned flights to Mars. The idea is that a series of unmanned cargo craft use plasma propulsion to take a long, slow but super-efficient route to Mars. Because of the efficiency of the thruster the mass of cargo would be far higher than conventional rockets could carry. Once the supplies are all in place at Mars, you can send the astronauts on a conventionally powered express trip.“
When asked how he first became interested in space, Michael explains. “When I was in year nine I had a really enthusiastic science teacher, a real mad professor type!” He encouraged a group of students to enter a NASA sponsored competition to design an experiment for the Space Shuttle. Michael and a group of friends entered and won the competition. The prize was an expenses paid trip to see a Space Shuttle launch and a behind the scenes tour of NASA. “From that point on I was hooked,” he says, “I had to become a space scientist.” Proof positive that inspirational teachers make a huge difference to the lives of their students.