Archive for the ‘Technical’ Category

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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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.)

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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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We’ve done some calculations to estimate the amount of solar power available to our satellite.  As you can imagine, it varies greatly with orientation.

There are 8 solar panels, each with 6 solar cells.  Each solar cell is 2.277 cm^2 and converts the 1,366 W/m^2 of solar radiation with 27% efficiency.  With perfect alignment to the sun, that gives 0.49 W for one of the 8 solar panels.  However, perfect alignment is rare and staying perfectly aligned would probably melt the solder holding the solar cells in place.  So, we consider several possible orientations in orbit and calculate the second-by-second orientation to the sun.

We consider first a “flat” orientation (aka “bullet” orientation) where the satellite’s nose is toward the sun at one pole, tail to the sun at the other pole, broadside to the sun at the equator, with the satellite long axis aligned to the orbit direction.  This orientation takes in 2,042 Joules during its half-orbit facing the sun, for an average of 0.75 W during the half-orbit.  Most power comes in near the equator diminishing near the poles.

Next we consider a “radial” orientation where the broadside is to the sun at each pole and the nose to the sun at the equator, always keeping a narrow end pointed toward Earth.  This orientation takes in 2,047 J for an average of 0.75 W.  No power comes in near the equator with most coming in near the poles.

Last, we consider a “sun seeker” orientation where the broadside is always facing the sun, likely combined with a “BBQ” roll along the long axis for cooling.  This orientation brings in 3,208 J, averaging 1.18 W, during its half-orbit.  Power is constant throughout.

Conversion of any light reflected from Earth will only increase these numbers.  We’ll likely revisit these as we do thermal calculations which must consider Earth visible and IR reflection.

What do these numbers mean for the mission?  Basically, with communications and CPU drawing a few watts, it means we can’t run everything at once.  We’ll have to run communications infrequently anyway according to international law – 10% duty cycle for transmissions.  This takes the 1 W of communication to 0.1 W on average, needing 0.2 W during the sun-side to ensure we’ve banked power for use on the dark side.  That leaves 0.15-0.25 W on average available for everything else.  We’ll have to design the system to bank energy, waking up for a second or two to check conditions and running experiments only when energy is available.  My best estimate right now is 5 minutes of engine operation every 9 hours.  We’ll know for sure after we build the onboard power supply.

The full calculations are here: TubeSat Power Estimates

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