This week, the Miles Team Weekly Topic rotation once again revisits the topic of propulsion in our four topic rotation on subject matter relevant to the NASA Cube Quest Challenge.
Earlier this year, Miles Team sent a small three man team to Georgia Tech to prove that one of our team’s greatest competitive advantages, our team lead’s ConstantQ ion thruster concept, was as viable as the math had indicated.
This week’s topic will be about how a team of engineers goes about testing an engine concept with a vacuum chamber. Next week we will again cover the details surrounding satellite communication and what teams are choosing to utilize, and then we will end the second cycle by discussing a topic related to the competitors in the challenge.
Our team lead Wes, our vice team lead Frank, and our testing analyst/project manager Seth, traveled to Georgia Tech to test in one of the largest high vacuum chambers in the world that can mimic the high vacuum of deep space. Outside of testing how components, coatings, and materials behave in deep space, it creates an environment where minute forces can be measured. Most types of electric spacecraft propulsion thrust are measured in millinewtons. 10 millinewtons, or nM, translates to about 1 gram of force. These are very small forces.
A good parallel of how the vacuum is important for sensitivity is by the scenario of water in a swimming pool, if one removes the water molecules, then it is easier to walk along the bottom of a flat swimming pool. The concept is the same for testing in a vacuum chamber, the ion thruster can more easily move the thrust stand sensor pendulum if the ambient molecules present here in our atmosphere on Earth’s surface are removed.
So how can you tell if your thruster is working in the vacuum chamber? On a platform in the center of the vacuum chamber there is a machine, called a thrust stand, which is designed to be useful in a number of ways so as to test the minute forces coming from the concept thruster. In the case of Georgia Tech, the concept ion thruster sits on top of a very sensitive sensor pendulum that can measure very small changes in its pendulum’s position. Since the laboratory in which the Georgia Tech vacuum chamber resides is next to an active train track, there were times when we could not test due to train-induced seismic activity registering on the thrust stand instrumentation. The thrust stand is also built so that it is able to suspend all the wires and cables from above so as to reduces drag on the pendulum to an absolute minimum.
The fuel lines which supply the thruster are integrated in such a way that they move with the pendulum.
From the cable and fuel line suspension tower, there are innumerable cables that snake down the tower’s back side, opposite the direction of the thruster plume, near the bank vault sided door. The cables supplying the propellant wound over to the right hand wall where, on the outside of the chamber, there were three noble gas canisters, holding xenon, krypton, and argon that we used as the propellant to fuel our thruster and the mass flow control sensor station. Winding from the thrust stand tower over to the left side of the chamber were the electrical cables. These cables connect thermocouples, the high voltage supplies, spark initiators, as well as communication lines to control the integrated system, testing power input levels, fire rates, and spark frequencies. All of this was orchestrated from down stairs in the control room.
At the back of the control room, a wall-mounted switchboard panel controlled and monitored the vacuum chamber. Think Apollo 11 mission control aesthetics – rock solid and oh so cool. The control room had a window in the front that could look out at the six diffusion pumps that serviced the vacuum chamber with an electrical conduit opening above where the chamber and our sensors and thruster circuit control wiring exited the room. On the right side of the window, there was the master power supply and mass flow control station.
Seth controlled and monitored the mass flow and announced when the mass flow had stabilized so as to make sure the thrust stand had a control level to differentiate between cold gas flow and actual thrust from ion expulsion. Seth also controlled power to the boards while calling out amperages to the thruster to make sure the spark triggers were firing at their intended levels. To the left side of the window was the thrust stand sensor instrumentation station where the Georgia Tech graduate students would monitor and calibrate the thrust stand, as well as to mark points of note on the data output ticker scroll.
In front of the window, Frank operated an oscilloscope to track and verify the current to the thruster during firings. He also monitored the thermocouple readings to track the thruster board electronics temperature to measure heat dissipation for thermal management analysis. Right next to the thrust stand control station, Wes had his laptop set up to control the thruster firing sequence runs and to record the data. He also kept a watchful eye on the camera visuals from inside the vacuum chamber and to direct the experiment runs accordingly.
As it turned out, we ran the whole time we were at Georgia Tech and we ran hard. The days were 7 am to around 11 pm with the occasional 2, 3, or 4 am for 10 days of testing.
The result ended up having us successfully test a revolutionary new ion engine! We look forward to a return to Georgia Tech for another round of tests!