Archive for August, 2013

Going to Small Sat

Tomorrow begins the Small Sat conference in Logan Utah. It is 6 days immersed in space science, technology, policy, and inspiration. The format looks to be a new presentation every 15 minutes! Of special interest is a presentation about in-orbit thrust measurements on the CanX-2 satellite. It will be interesting to see how their technique compares to the FRETS1 spin technique. There are also several presentations on high voltage for pulsed plasma thrusters. Since FRETS1 uses elements of a pulsed plasma thruster to make its plasma, these should be very useful sessions. Watch for tweets (@iondragonfly) during the conference.


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Vacuum Chamber Testing

Tests have begun in a small vacuum chamber. We’re getting the expected 5 sparks/second from our Emco high voltage supplies, building to around 5000 volts before the spark. This is the fallback failure mode for the high voltage supply which is redundant and normally able to do the mission rated 10 sparks/second.

We’re still doing ablation tests on closed cell foams, trying to get the most plasma. Several new ideas have come up for getting the spark to better contact the foam. As such, we’ve started a new version of the 3D printed engine. The propulsion principles are the same but we think the plasma production system can be significantly improved.

With vacuum testing comes the need for data collection in the vacuum chamber. A lot of people do their collection outside the chamber, passing signals through interconnects in the chamber wall. Don Smith has come up with an interesting way to gather pulse data at 16MHz without taxing the engine’s microprocessor. It’s all small enough to fit not only within the chamber, but within the satellite. After we build and blow up the first couple we’ll open source the data collector on Upverter.

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The engine used by the FRETS1 satellite is patent pending and has been 3D printed! The patent is pending in the US and was filed under the older first-to-invent rules. Because it is not yet an international patent, images and descriptions will be limited to those that don’t reveal enough information to reproduce the engine. Perhaps a future post will expound on how the US ITAR law prevents me from disclosing this information anyway.

Onto something more fun: 3D printing. The engine is pretty small and requires some interesting interior detail for experimentation. Things like embedded wires, high voltage guards, mounting holes, and sockets for sensors are all required. And it all has to be made from a good strong dielectric that can withstand the electric field stress required for the engine to be useful. Acrylic is my choice as it is plenty strong and still clear enough to see what is occurring within the chamber. The first plans were to laser cut pieces and use plastic cement to weld them together.

Then I found that TechShop offers 3D printing in acrylic and wax. In this process, voids are filled with wax, avoiding the use of supporting struts that would have to be cut away. Instead, the part is placed in hot oil and the wax drains away. Some of our wire slots are 0.5 mm diameter and the wax didn’t fully drain. To get the wire into these slots, we heated a test wire and worked it in until the wax melted and pulled out.

Draft 1 of the engine is show below, near a US quarter coin for size comparison. There are plenty of mounting holes, wire slots, and holes for sensors. OpenSCAD was used to create the model. I must say it was quite nice to script an engine design.

3D printed acrylic

FRETS1’s ion engine – 3D printed acrylic

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