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Final Design and Molflow Simulations of a Micro Propulsion Testing Chamber

Posted: Tue Dec 25, 2018 7:36 pm
by Michael Bretti
This is just brief overview of the second of my three major vacuum chambers used with my modular pumping station that I have recently posted about. This also includes an overview of the Molflow simulations used to simulating ultimate pressures of the chamber during pumpdown for various levels of conditioning. This is the first time I have shared the design of this chamber here on the forums. The final design for the chamber is as follows:

Micro Propulsion Test Chamber.jpg

The chamber is made from two 6" conflat tees. The engine input (right side) features a 6-pin high voltage feedthrough. The diagnostic input (left side) features a 9-pin feedthrough for a wide variety of propulsion, plasma, and ion diagnostics. In addition, the chamber also features a large optically clear 6″ conflat viewport for direct and full viewing of the engine in operation, placed out of the direct line of engine firing to minimize deposition on the viewport surface. The 6" conflat pumping tee also includes an additional 2.75" conflat port for a high vacuum gauge for pressure monitoring. The full chamber and pumping assembly on the Integrated High Vacuum Test Stand is shown below:

Micro Propulsion Test Chamber Full System Assembly.jpg

This chamber will be used for leading the testing and development of open source technologies for a wide variety of micro propulsion engines for use with small satellite applications such as Cubesats and Picosats, focusing primarily on pulsed plasma thrusters and similar technologies. The project page with full engineering specs, calculations, and CAD files can be found here:

http://appliedionsystems.com/portfolio/ ... g-chamber/

The build pictures page shows the design and assembly of the chamber. I have all the components to assemble the chamber now except for the conflat gaskets and bolts:

http://appliedionsystems.com/micro-prop ... -pictures/

Finally, a brief overview of the Molflow simulations for the chamber are presented below. As in the previous two Molflow simulation results posts, I simulated three different conditions for chamber preparation: unbaked and pumped 1 hr, unbaked and pumped >24 hours, and baked and pumped >24 hours. The results for the first simulation will be briefly discussed. The Molflow simulation page with the full model preparation, input parameters, results, discussion, and Moflow files can be found here:

http://appliedionsystems.com/portfolio/ ... g-chamber/

The first result looks at the average molecular trajectories of pumped particles in the system:

Micro Propulsion Testing Chamber - Molecular Trajectories.jpg

With an inlet speed of the diffusion pump at 600 l/s, the resulting effective speed at the pumping port inlet of the chamber is 342 l/s.

The next results looks at the pressure mapping of the entire chamber and pumping assembly for the chamber unbaked and only pumped for 1 hour:

Micro Propulsion Test Chamber - Unbaked Pumped 1hr Pressure Mapping.jpg
Micro Propulsion Test Chamber - Unbaked Pumped 1hr Texture Scale.jpg

As can be seen from the results, due to the large internal geometry with full metal seals in the stainless steel chamber, a very uniform pressure gradient is observed throughout the whole assembly. As explored in previously posted simulation results, although I am using several viton gaskets around the baffle and chamber inlet, due to the high throughput and pumping speed of the system, this compensates for the outgassing load from the gaskets allowing for good resulting pressure. It can be seen that, assuming no leaks and minimal backstreaming from the pump, a pressure in the 10^-6 mbar range can be achieved with the system for outgassing loads of materials only pumped for about an hour. This meets the design criteria for the chamber, where the ultimate pressure must be at the 10^-6 mbar region, with an operating pressure of around 10^-5 mbar. Therefore, with minimal pumping preparation, target pressures can be achieved relatively quickly, making it convenient to make fast changes and modifications to the test engine as needed.

Finally, by applying a transparent measurement facet through the chamber, the chamber pressure can be plotted. The results are as follow. X-axis is arbitrary units, where Molflow divides the measurement facet into 100 equal length segments. Y-axis is mbar. The curve from left to right follows the facet from left to right across the chamber, from the pumping port and instrumentation input cross (left) to the engine input and viewing cross (right).

Micro Propulsion Test Chamber - Unbaked Pumped 1hr Pressure Profile Facet.jpg
Micro Propulsion Test Chamber - Unbaked Pumped 1hr Pressure Profile Plot.jpg

A slight dip is seen around point 20 on the x-axis, representing the point in the chamber that is directly above the pumping port. Likewise, the peak pressure in the chamber occurs between points 70 and 80, which is the area directly in front of the viewport, which exhibits higher outgassing load from the glass than the stainless steel. A final pressure of around 3.5-3.7 x 10^-6 mbar is expected during pumpdown. With proper extended pumping and baking, the ultimate pressure can be expected to drop into the mid 10^-8 mbar region under ideal conditions.

These simulations will be revisited after my first engine designs are complete, where I will simulate dynamic loading of the system due to operation of the engine, and the resulting pressure that is established, as well as the maximum engine output that can be maintained for the target operating pressure.