After the major setback of the recent failed testing of the AIS-gPPT3-1C V4 thruster, I needed to evaluate the data, and make some tough decisions on which direction to move. Here, I will present an overview regarding the state of developments with my efforts in open source propulsion, taking a look at where things will go moving forward from here. Some doors will be closed, others will be opened. Practical limitations, setbacks, and new paths will be addressed.
As you are aware by now, my primary focus has been on developing sub-1 Watt PPTs as the primary propulsion for PocketQubes, down to 1P systems. This has been done with the AIS-gPPT series. The thruster has evolved tremendously over the short period I have been working on it. It’s been quite a struggle, but the thruster has opened up many fantastic doors and opportunities. Lot’s of data, test results, and improvements have come from this effort. Despite this, practical limitations of the technology need to be openly addressed. While I did not achieve stable ignition for the V4 thruster, I did see a few spurious pulses. From my experience so far, and looking at all the testing/data until now, from the plume I do not need to measure the impulse-bit of the V4 to know where performance is trending. Despite my best efforts to improve performance, based on prior data and current trends of the design, I do not believe that a sub-Watt, sub-Joule PPT can reasonably scale to provide the necessary thrust and performance to act as the main propulsion for PocketQube satellites. While such a thruster still has potential for attitude control, based on my data, preliminary simulations by others in the community, and anticipated propulsion requirements for small-sats in LEO, the extreme power restrictions do no give enough room for needed thrust levels. The gPPT series thruster is a fantastic learning tool that could enjoy a lot of use in educational labs and with enthusiasts who want to get started with electric propulsion. It is really the stepping stone to show that EP can be done at home and open-source. However, practically speaking, in its current state, it cannot achieve performance needed for primary propulsion. At sub uN thrust levels, output is too limited, and does not look like it can scale unless available input power is increased to a higher minimum level. While I will look to refine and improve performance in low-power EP for PocketQubes and Cubesats, I will effectively be shifting my focus from this thruster class to several new areas of propulsion, and suspend development on the current line of gPPT series thrusters.
With this chapter coming to a close, though a bit hard, there are may exciting new areas to explore and doors to open. I will look at researching and potentially expanding into these next key areas:
- 1. Multi-Watt PPTs
- 2. Ionic Liquid Electrospray
- 3. Liquid-Metal FEEP
- 4. Water-Based RF Plasma Thrusters
It is clear that PPTs cannot provide the performance needed at this extreme low power and energy level. However, there is plenty of opportunity to expand into multi-watt units for larger PocketQubes and small Cubesats. This includes relying on the larger EMCO AG series 1.5W supply. This potential development will build off the prior gPPT thruster, and look to optimize performance for possible main propulsion at the 3P PocketQube level, while still keeping to an ultra-low cost, open source design. I will need to achieve thrust levels of at least close to 10uN for it to be viable at upper LEO levels.
The next propulsion, which is possibly the most exciting territory, is ionic liquid electrospray. Recent studies have shown that conventional CNC macro-structures can provide significant thrust levels via multi-site emission at very low power input, enough for main propulsion. For equivalent power, electrospray has shown to be able to provide higher thrust than PPT technology, and lends itself exceptionally well to scaling. Work is already underway to develop the first open-source AIS designs for an EMI-BF4 powered electrospray thruster. From the above mentioned technologies, I feel that ionic liquid electrospray has some of the greatest potential for a true main propulsion for PocketQubes that could reasonably be built at the maker level with conventional resources and relatively low cost. The key issues to address here are lifetime, HV switching, operation using only 1 emitter module alternated at higher rep rate, and proper filling of the porous emitter/reservoir. Some big challenges ahead, but based on available literature I think it can be done!
Next on the list is liquid metal FEEP. While the electronics are easier than ionic liquid electrospray, there are several key challenges as well. However, like its counterpart, it has great potential for low-power, high thrust performance for the PQ level. To attempt to eliminate the need for heaters at the PQ level, I will be first looking to explore Galinstan fuel. This presents some interesting challenges, since I have not seen reference to this alloy being used in FEEP thrusters in published literature. Another issue is the emitter and wetting. This presents some interesting technical challenges. While I cannot reasonably use porous tungsten, which is the conventional high performance emitter material now, there are commercially available solid micro-tip tungsten needles that could be used for simplified emitters. The single biggest issue however for a low power, low-cost DIY FEEP is the neutralizer. Thermionic and hollow cathode are immediately out. For low power, carbon nanotube field emission is used, though is currently out of reach for my capabilities. However, I have come across old Soviet research showing that liquid metal emitters can be used as high current electron sources. I do not know if this has been explored for FEEP before, but presents some very exciting possibilities for simplified, low-power neutralizers.
The last item on the potential thruster list is water based RF plasma thrusters. There is a lot of hype for water fueled electric propulsion technology, and water has some interesting benefits, especially from the open source DIY standpoint. RF plasma thrusters also put on quite a show! One thing about RF ionization is that it becomes easier to ionize gases at higher frequencies with less power for a given pressure range. Literature has shown that RF plasma thrusters can be ignited with power levels as low as 0.5W using 5-6GHz excitation. While not actually very practical at the PocketQube level like electrospray, a fully open source home-built water RF thruster is more about pushing the limits of the science at the maker level, exploring all types of EP, and getting other enthusiasts excited about the possibilities.
Finally, on the infrastructure side of status, my roughing pump is most likely shot. No roughing pump means no high vacuum. This presents a big hurdle going forward with any propulsion development, and I’m not in the position to get a replacement one immediately. I also realistically don’t have funds to support this level of development across the board myself, so for the time being I will have to just push out designs for the community. Even if I can’t build stuff immediately, I will be actively sharing new designs and research. And of course I wouldn’t have even come remotely this far without the continued support and encouragement of the community. Many thanks to all the friends I have made here on this journey! This is still only the beginning! Lots more thrusters to tackle!