June 3, 2015

Vibration Testing Arkyd 6 at Planetary Resources

To get anything into space these days, you must first launch it on a rocket. A simple trip to visualize, but the real ride to space is incredibly intense.

Have you ever seen a rocket launch?  Have you ever HEARD a rocket launch? If so, you have definitely FELT a rocket launch.  If you haven’t had the experience, you should make every effort to see one in person as it will forever change the way you think about getting to space. It sounds and feels something like a bomb going off continuously, which is an accurate description because a rocket launch is essentially a controlled explosion – and if you are going to space today, you are sitting on top of it!

A spacecraft takes a lot of energy to get going fast enough to get into space, thus, it requires a rocket producing a lot of thrust. Rocket engines burn massive amounts of incredibly high-energy fuel to create that immense thrust. The result is a controlled explosion blowing highly supersonic exhaust gases out of the rocket nozzle. Those gases generate unbelievable levels of vibro-acoustic energy as the multiple shock waves hammer against the surrounding atmosphere. A large amount of that energy also resonates directly into the spacecraft through the rocket’s structure as well as from the atmosphere hammering back wildly at the rocket.

During launch, a spacecraft experiences extreme vibration. Because of this severe and chaotic environment, we at Planetary Resources need to design our spacecraft to be able to withstand the forces of launch, so once our spacecraft reaches space and begins its mission, it works!

To ensure that our spacecraft works once it reaches space, we can simulate and test it in the launch environment here on Earth, by going through what’s called a random vibration test. Right now, we are going through these tests with our next technology demonstration spacecraft, the Arkyd 6, planned for launch in December. The Arkyd 6 has much of the core technology that will be essential to fulfill our asteroid prospecting missions, including a mid-wave infrared sensor, 2nd-generation avionics and power systems, multi-band communications, attitude determination and control.

Prior to test, we attach accelerometers to various components and locations on the spacecraft structure. These sensors measure the acceleration that the particular components are experiencing as a result of the launch vibration levels. We then take the spacecraft out of its safer, normal environment in the clean room, pack it up and drive it over to the testing facility. There, we place the spacecraft on a shaker plate and apply the vibration levels to that plate using a gigantic electrodynamic shaker that operates much like a big speaker. The random vibration signals are sent through the speaker’s massive coils and into the plate where the energy hammers into the test fixture and the spacecraft simulating the launch. It’s a violent series of events, and the shaker is capable of being worse than a launch, so we do take precautions to make sure we don’t accidentally break the spacecraft.

The test runs are a series of steps where each time we increase the level of vibration by 3 dBs, which is doubling the vibration power level from the previous run. We finally reach the full launch vibration levels where the spacecraft dwells in this chaos for a full 60 seconds. After the full level run, we rotate the spacecraft 90 degrees and repeat the whole process until we have shaken the spacecraft in all 3 primary directions X, Y, and Z to simulate the fully random nature of the launch event. Surviving this test set with functioning hardware is the only way we can be confident that the spacecraft can survive the vibrations during launch.

Between each test, we analyze the data gathered by the accelerometers that we attached to various system components. In some cases, the particular accelerometer will show the component vibrating the same or much less than the launch input levels. In some, far more terrifying cases (at least for the engineer) the system is amplifying the launch vibration levels and the component is vibrating much more than the input. The pre-test analysis helps us design the hardware for this possibility and predicts this behavior to an extent, but there’s nothing like a good test to tell us what’s actually happening to the spacecraft and its various subsystems.

With all this crucial data in hand, we take the spacecraft back to its cleanroom home for a full checkout. We begin analyzing the data and make any necessary adjustments in the flight hardware to ensure our Arkyd makes it safely through launch and on to its prospecting mission. After all of this, we breathe a sigh of relief…until the real launch day. And on that day we’ll be confident that it will be ready for the ride to space.

Peter Illsley, Principal Mechanical and Thermal Engineer

Planetary Resources