One of the major and primary goals of Applied Ion Systems is to explore and develop low-cost, reliable, open-source electric propulsion for CubeSat and small satellite applications, and show how such engine design and testing could be approached at a cost level orders of magnitude lower than seen in conventional academic or industry research labs. Having become more involved with the community, and talking with more people in the field, both enthusiasts as well as professionals in the industry, there appears to currently be little to almost no open source, low-cost CubeSat propulsion testing and development done in the maker community, or in home-based systems (completely separate of any grants or official research funding.) As such, the efforts here on high vacuum testing and design of these propulsion systems at Applied Ion Systems may represent one of the first true, independent, home-based maker-led efforts into this field.
Looking at the history of electric propulsion for space, you will find that it is a quite old field – the first EP system actually flown was a pulsed plasma thruster on the Zond II in 1964. Since then, numerous electric propulsion modules have been flown in space, and are widely used on commercial satellites, and have even been used on CubeSats. However, all of the current EP in use are primarily based on high critical, highly funded systems – either for NASA, military, government, or other large companies. The CubeSat community has rapidly expended, with many student groups and even open-source communities gaining access to space through small satellites. However, to date, none of these open source, amateur, or maker based CubeSats have flown with electric propulsion. This means that once the satellite is launched, it will start on a decaying orbit until it burns up in the atmosphere. Considering the cost of building and launching a CubeSat already ranges in the tens of thousands of dollar range, with such a high investment for these typically low funded student or maker groups, this is a major investment that can greatly benefit from increased flight duration and corrections provided by EP.
If we do some market research analysis on the field, there are numerous companies, both established and recent startups, pursuing advances and developments in this field. However, looking at the current funding structure of these companies, we can see that the entirety of funding comes from government, military, NASA, or research grants – millions of dollars in contracts are awarded to start ups to pursue R&D and contracts to develop the latest and greatest EP. While the eventual goal will inevitably be to reduce the cost of EP and target a wider market as the CubeSat market grows, currently there is little incentive to pursue ultra-low cost (under $1k range) propulsion targeted to student and hobbyist groups. Current prices for propulsion systems run anywhere from around $25k (starting price without any engineering or customization) to several $100k. For these independent and open-source groups, the cost for a propulsion module alone is more than the entire satellite and launch. In addition, while EP is field many decades old, and information on the technology is widely accessible, current EP efforts in the market are tightly guarded secrets. While there is a rapidly growing open-source CubeSat community, covering everything from hardware, to software, communication, sensors, modules, etc, there is still very little to no work on affordable EP for this group.
If you have been following the progress of my work here and on social media, you will see that I have been rapidly gearing up for some major and exciting developments in this goal. Last Friday, February 22, 2019, I revealed the first prototype concept CAD model render for the first open source propulsion system currently in development at Applied Ion Systems: the AIS-uPPT1:
This thruster is the first thruster effort I will be exploring. The AIS-uPPT1 is a small pulsed plasma thruster that utilizes solid Teflon propellant. One of the main goals of this prototype is to create a pulsed plasma thruster that can be built with almost no machining, using common, off-the-shelf parts. The main electrodes are made from 316 stainless steel. While copper would be ideal from an electrical standpoint, stainless steel was chosen do to the much wider availability of diameters, reducing the meed to custom machine the electrodes to size. The thruster is made up of two modules – the actual thruster assembly itself, and the socket for electrical connections. The socket was inspired by from my experience in pulsed power based on tube sockets I have seen. Beryllium copper fingerstock is soldered to properly sized large diameter holes in the connection PCBs to tightly fit against the electrodes. The socket is bolted together with 316 stainless steel 6-32 hardware, using Teflon spacers for insulators, and the bolts themselves and ground connections. On the back of the socket are the main discharge capacitors which are connected between the anode and the cathode. Not shown are the solder points for the trigger electrode. The entire assembly is press-fit together.
Another unique feature about this thruster is the approach I am exploring for the ignition electrode. On almost all solid-fuel based PPTs, the igniter is either an aerospace-grade spark plug, or a small igniter pin. Some topologies feature a semiconductor material like silicon between the ignition pin and ground to help initiate discharge. In this thruster, the igniter is coaxial and tightly spaced relative to the cathode. The outer electrode is the cathode, the second electrode is the igniter, and the inner electrode is the anode, making a triaxial configuration. Since spacing is so close between the cathode and ignition electrode, several layers of Kapton is used to provide insulation and spacing between them. Once the spacing and final CAD models are complete, I will release additional cross-sectional views to better show the electrode and connection geometry.
I wanted to get away from conventional igniters for several reasons. The first is that by utilizing a very large surface area ignition surface, as opposed to a point source, lifetime of the igniter could possibly be significantly extended, which is one of the primary limitations of current PPTs. In addition, the large coaxial configuration may provide more even vaporization and ionization of the Teflon fuel. Finally, such a triaxial configuration, if proven successful, represents a more streamlined approach towards PPT design.
In terms of size, this first prototype is quite compact. The outer diameter of the cathode is 1″, while the socket PCBs are about 2″ in diameter. The total length of the assembly is only 1.5″ long. The fuel rod is a Teflon rod 0.75″ in diameter, bored out in the center to 0.25″ to accommodate the anode, and is 1.125″ long. In terms of cost, rough initial estimates for parts are in the $100-$200, with plenty of material left-over for several thrusters.
Currently, I have already ordered and received most of the hardware for this prototype thruster build, and will be finalizing the design an starting construction in the next few weeks. With the completion of the Integrated High Vacuum Test Stand and most of the Micro Propulsion Testing Chamber built and ready, all that remains is to build the engine, mount it in the chamber, and proceed to first ignition testing. If everything goes well, the AIS-uPPT1 may see first firing within the next month. Lot’s of exciting things in the works here to come!