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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.

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.

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

Launch provider news

Our launch provider, Interorbital Systems, recently appeared on The Space Show. The podcast is here. (Rules say we can’t really transcribe it, so you’ll have to listen.)

Key news includes a change to the launch location, another upcoming engine test, options to do suborbital testing (including the prized ability to leave the launch rocket), modular rocket motors, rocket motor tradeoffs, and insider information about the politics of rocket building. A trivia question: How much does a launch license cost?

I am particularly intrigued by the idea of using a suborbital rocket to test the ion engine. A suborbital flight provides 10-15 minutes of microgravity in outer space, which should be plenty to do testing without the hassle of power conservation needed for longer, solar-powered missions. All other providers I’ve seen will not eject payload, meaning we couldn’t test the effectiveness of an engine. Sure, we won’t get the engine back, but maybe we could turn that into a feature, learning limits for the longer orbital mission by running the engine destructively.

Our presentation with mission details is now on SlideShare at:

http://www.slideshare.net/wes_faler/frets1-satellite

Interorbital, our launch provider, is hard at work on their launch vehicle.  They had a very successful test of their new rocket motor and posted a video on YouTube:

http://www.youtube.com/watch?v=It-Q-lhDlbg&feature=youtu.be

and a nice high-res photo of the burn:

http://www.interorbital.com/Main%20Engine%20Test%20Photo%20Large_1.htm

We’re preparing for the upcoming Tampa Mini-Maker Faire in March, 2013.  I’m quite anxious to meet my new maker neighbors!

Choosing an engine design

It’s time to make some final choices on the engine design.  Lacking time and money to build every engine design our imagination conceives, we’ve run a number of simulations. Each simulation is a thin 2D slice of an engine – much like a slice in a loaf of bread – presented with a burst of plasma.  Each simulation takes about a week on an NVidia Tesla GPU and we use a combination of Processing and R to visualize the results.  Sorry, we can’t share the visualizations yet as doing so would affect a patent.

Many scenarios were tried in short runs and the seven most promising were simulated in detail.  Technically, an additional “null” case was also simulated.  In an ion engine, the “null” case is a simple thermal expansion of the plasma without any other effects other than exit nozzle geometry.  This basic thermal expansion produced much lower momentum than the 7 designs simulated.  That brings up a key performance indicator of an engine: the total momentum it imparts to the spacecraft.  As a pulsed design, each pulse imparts a bit of momentum.  The time average of this momentum is the engine’s average force.  Let’s look at the total momentum for each design choice:

Total momentum imparted by each engine design.

Interpreting this is pretty interesting.  Models D and F go strongly negative, very strongly negative.  This makes them unacceptable.  Why?  Negative momentum means movement backwards, towards the exhaust plume – an unsustainable situation.  This leaves models C and E.  Both converge to the same total momentum, though E converges sooner because it exhausts its plasma burst sooner.

There is another source of data to help make the choice.  We can also look at the momentum on the charged plates within the engine.  These plates contribute their momentum to the above totals.

Momentum due to charged plates

Model C’s charged plates have negative momentum, meaning it is gradually eroding the overall momentum gain from the engine.  Model E gives the same total momentum and never backtracks on its gains, making model E the chosen design.

Now, just what can we expect from a model E engine?  It creates 1.75e-9 kg m/s of momentum every 3.52e-5 seconds.  From the simulation, each slice is 1e-4 meters (0.1 mm) and the total engine has 100 of these slices.  The engine’s force is then (1.75e-9 / 3.52e-5) * 100 = 4.97e-5 N, or about 5 mN.  A 5 mN thrust is quite reasonable, and seems quite nice in a 1 cm engine.

What affect will this engine have upon the satellite? We’ll mount the engine 2.5 cm off center of the long axis so that it’s thrust will spin the satellite.  The spin rate will increase with each plasma burst and eventually get out of the noise band of gyroscope sensors.  (We don’t use accelerometers to sense thrust because it is so low that we fear any accelerometer sensitive enough to detect the thrust would not survive the shock of launch.)  The 750 gram TubeSat’s angular momentum vectors [Ixx, Iyy, Izz] are estimated to be [7.5938E-04, 1.3878E-03, 1.3878E-03].  This makes the angular momentum change per pulse to be (1.75e-9 * 100 * 0.025) / 1.3878E-03 = 3.15e-6 radians/second/pulse = 1.81e-4 degrees/second/pulse.  So, in 5,535 pulses the satellite will have added 1 degree/second to its rotation.  Its forward velocity will have also changed by ((1.75e-9 * 100) / 0.750) * 5535 = 1.29e-3 m/s.  With 75 mm^3 of fuel available, we estimate 4958 degrees/second for rotation and 6.4 m/s final delta V.  (Clearly, 4958 degrees/second is far too fast, but that’s another post.)

Closed Cell Foams

Propellant.  Engines need it and our ion engine is no exception.  Ours is an electrostatic ion engine that accelerates both ions and electrons, using lasers to boost the plasma flow rate through a novel nozzle.  The question then becomes, how do we get the raw material for a plasma?  The plan so far has been to use a small cylinder of compressed gas – perhaps an 8 gram nitrous oxide cylinder commonly used to make whipped cream to order.  The gas is stable and easy to obtain.  The only projected difficulty was metering the gas flow in orbit as most valves are simply too large physically for our satellite.

That was the plan until we looked back at our ConOps documentation.  After we ship our satellite to the launch provider, we can’t expect them to do substantial technical work installing our gas cylinder and checking flows immediately prior to launch.  We have to plan that they will only remove our “Remove Before Flight” pins and that our satellite could spend a month in desert temperatures.  It doesn’t seem practical to presume o-rings, screw threads, and a homemade micro valve will stand up to those temperature swings without leaking.

Our new idea: use a closed cell foam filled with pressurized nitrogen, vaporizing the supporting plastic and releasing the enclosed nitrogen using a spark system similar to that used by Pulsed Plasma Thrusters (PPTs).  PPTs create an arc across the face of a solid Teflon bar, turning a few micrograms of Teflon into plasma.  The plasma moves along the PPT’s cathode and anode by Lorentz forces, much the same way that a rail gun accelerates its conducting projectile.

We won’t be building a PPT of course, but will use the inspiration of eroding a fuel bar, making some plastic-based plasma and nitrogen-based plasma from a temperature stable storage media.

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