BLACKSHIP ONE PRESENTS…
The Truth is That Nanosats Still Have a Very High Failure Rate Upon Delivery to Orbit
Due to the rather secretive and stealth nature of the field, while general announcements are made for particular missions, very little extra details are given. In most cases, EP companies announce flights aboard a nanosat, but never actually reveal what happened after!
EP companies with many millions in funding that have yet to fly
Today, Blackship One, was fortunate enough to be able to talk with Michael Bretti, the founder of Applied Ion Systems, about his involvement in the electric propulsion space. We have an exiting interview planned for you today where Michael will walk us through what it’s like to work on advancing propulsion technology on a shoestring budget (see Michael’s Patreon page here).
Keep in mind, this is the fourth part of a five part interview series. If you’d like to start from the beginning of the series, you can click here.
What are common use cases or mission profiles for nanosats that use electric propulsion? Give us some interesting missions they have been used on?
Actually, most nanosats don’t fly any type of propulsion, and only a handful of Cubesats have flown with any EP to begin with.
Definitely in the picosat realm with PocketQubes, this is far, far rarer. I think besides the GENESIS mission with the gPPT3s, only one other PocketQube has ever flown with thrusters onboard (although communication was never established with the satellite so they were never tested or confirmed in orbit).
Unfortunately, the sad reality is that nanosats and picosats still have an extremely high failure rate upon delivery to orbit, and many never even operate right out of deployment. And due to the rather secretive and stealth nature of the field, while general announcements are made for particular missions, very little extra details are given. In most cases, EP companies announce flights aboard a nanosat, but never actually reveal what happened after!
Occasionally you will see a press release about a major success, but most of the time there is silence, though papers will pop up at EP conferences with some in orbit data from EP companies now and again. Since EP for the nanosat field is still relatively new, a lot of tests have been performed as basic demonstrations of the technology.
In fact, there are still plenty of EP companies who have been in the field for several years with many millions in funding that have yet to fly or effectively demonstrate their system in orbit.
Beyond just technology demos however, mission profiles can range from station keeping to orbit raising, deorbiting, and orbital debris management. I think EP is still new enough and emerging in the field of nanosats that we haven’t seen many unusual use cases. There are definitely lots of proposals over the next several years which will target their use with some nanosats beyond LEO, to the moon and even Mars! I think we can expect to see a lot of exciting things to come as the technology matures further and becomes more adopted at this scale.
What level of force and acceleration are your products able to generate? For example, at what speeds could a 1U Cubesat fly at and over what period of time of acceleration? How do propulsion requirements scale for 2U, 3U, 6U or 12U Cubesats?
Right now, focusing on the tiniest class of EP systems out there, my thrusters currently produce very low amounts of thrust, on the order of a couple of micro-Newtons and less. This has been mainly from me pushing the absolute limits of scaling and exploring what is truly feasible at the lowest power range. Moving on to my next generation thrusters however, which are already in the works, I am targeting substantially higher thrust, at the level of tens of micro-Newtons for single units, and close to 100 micro-Newtons for certain clusters.
When we talk about maneuvers and thrust with EP systems, since we are on the order of micro-Newtons to milli-Newtons, you don’t see instantaneous accelerations or changes like you would see with chemical propulsion. EP is meant for very slow burning, long transfers and maneuvers. Where chemical propulsion will change course and trajectory rapidly, on the order of minutes, EP is usually on the scale of many hours, days, or even weeks. Looking at the delta-Vs for some preliminary mission profiles that my thrusters will be used on for example we are talking about tens to a couple hundred of m/s delta-V budgets.
When your orbital velocities in LEO are in the range of like 6000-8000 m/s, the extra velocity you gain firing such tiny EP thrusters is pretty negligible. However, it is still enough to combat drag forces and do simple propulsive maneuvers.
In terms of scaling, propulsion requirements are highly dependent on the mission, such as the maneuver and the orbit. The size does play a role, and bigger sats will have much more power available, allowing for more propulsion capabilities, but it can really vary a lot. For example, for station keeping or attitude control, you don’t need nearly as much thrust or total impulse as you would for a larger orbital transfer. Other maneuvers like formation flying and collision avoidance may fall somewhere in between, but it is highly dependent on each satellite and mission itself. So you could have a big Cubesat at higher LEO that is only looking to do station keeping or fine attitude control, requiring minimal propulsion, or you could have a small Cubesat at very LEO levels looking to do a very long burn for a large orbital transfer, requiring much more demanding lifetime and thrust.
You’ve developed many different propulsion systems. I noticed that some of them are retired in favor of your new systems. What are the biggest technological advances you’ve personally made with your newest thrusters that have caused your older thrusters to become retired?
Right now, at this scale, the key advances between iterations are lifetime and reliability. I am not worried or focused too much on thrust or ISP numbers for early versions of systems, because this can always come later with optimization once systems are working well. In fact, in some cases, things like thrust have gone down, while the form factor and lifetime have improved significantly. But due to the extreme limitations I am working with, whether it be size, power, or even available manufacturing techniques or off the shelf components, lifetime and reliability have been the biggest challenges that have come with scaling, and making these systems useable and viable at this level.
For example, over the four first-gen PPTs I have made, I went from completely non-firing, to working only once, firing a few hundred times, to over a thousand. My newest gen PPT aims to push this even further, while reducing cost, making manufacturing even easier, and improving ignition reliability. For my ILIS thruster, moving forward my newest design will not only massively improve lifetime, but increase the thrust by several orders of magnitude, and allow for easier use with dedicated onboard processing and control.
Due to the speed I operate and iterate at, things are constantly evolving and changing at AIS. Sometimes changes happen so fast I can barely keep up updating everything everywhere myself! While I could always just shelve the work done on old systems, I feel that a large part of what I do is based on sharing all forms of progress. Everything is also connected in one way or another, and the systems are a continuous evolution.
Even for the systems that don’t fully work or don’t work well, I still release all of the specifications, details, and design files for them. Every system design and attempt is heavily documented, and even if they don’t work fully well or as expected, they can still provide valuable insights and learning experiences.
In addition, they also provide the backbone for others to be able to build off of, expand, improve upon, or just learn from and be inspired by.
What are your biggest challenges when it comes to boosting output and what are you doing to overcome those challenges?
This again goes back to fundamental scaling challenges of electric propulsion systems. Output is first and foremost determined by the power you can deliver. In some cases though, the reality is that you just need more power to get the performance boost you want.
For example, my first complete integrated EP system, the AIS-gPPT3-1C pulsed plasma thruster, operates at 0.5W max power, providing a tiny 0.22uN of thrust. While this can be used for fine attitude control, and at much higher orbits, for the general target for PocketQubes, which typically reside in the 300-400km range, this is just not enough to provide practical propulsion for main propulsion at this scale. This is just an inherent limit to the technology.
Thrust is related to stored energy in the capacitor and repetition rate. You can only charge a capacitor so fast though for given a given power level, and you can only have so large of a capacitor of the storage capacity and voltage required. So you have to trade-off charging time, capacitance, discharge energy, repetition rate to balance out performance. But when you are fundamentally limited in power, there is only so much you can do. The design can be optimized to produce a bit more thrust, but generally certain technologies follow general thrust trends, and in the case of this thruster, which stores a tiny 0.09J of energy, limited to 0.5W charging capacity on the satellite it was designed for, follows scaling trends that ultimately put limits to the performance it can achieve. This led to the conclusion that I would need to design a PPT system that had to require more power, as well as exploring other technologies, such as ILIS electrospray, with superior thrust to power scaling.
In this case, this fundamental constraint imposed on this very first system opened up development of a new PPT, the AIS-ePPT1, operating at 1.6W power, as well as the AIS-ILIS1 electrospray thruster operating at 1.6W max power, but with the technology potential for the electrospray thruster to provide over 100x the thrust for equivalent power of the PPTs. While this phases out PPTs for the smallest PocketQubes, it opens up potential opportunities for them to now fly with this other form of propulsion, which can provide far more output.
Now, focusing in specifically on optimizing output for the same thruster class, there are a lot of routes that can be taken. For example, after extensive testing with the ILIS1, I have just about maxed its output for the current geometry This has led me to start the next phase of development on the ILIS2, using a very different form of emitter which allows me to significantly increase thrust density by providing more emission points, as well as increasing field enhancement at these points to further boost emission over the original design. In this case, I am staying with the same technology class of electrospray thrusters, but optimizing performance of the thruster itself by working out better ion emitter geometries and extraction optics to improve current density and emission of the thruster. For ion thrusters, thrust is directly related to ion beam current, and ISP is related to accelerating voltage.
And finally, in scaling up these systems, for example the AIS-ILIS series from a PocketQube system to a larger Cubesat system, output increases in this case is the result of simply paralleling individual modules, which are already optimized for maximum output themselves, rather building a physically larger single thruster.
However, I would say that optimizing output at this scale is only part of the battle. In my experience, arguably the bigger challenge is increasing lifetime.
When you are working on such small scales for propulsion, although output is reduced from larger and higher power systems, lifetime becomes a significant challenge due to size, component selection and availability, and other constraints. Right now, the AIS-gPPT3-1C has a lifetime of about 1200 shots before it dies.
The thruster itself exhibits almost no erosion or fuel depletion at the low energies per pulse. Rather, the onboard pulse capacitors die far before anything else on the thruster does. At this scale, it is incredibly hard to find pulse capacitors at the required capacitance, voltage, and size needed, in addition to being vacuum rated and rated for high intensity repetitive pulsed operation. Hopefully the new ePPT1 will solve this. Increasing the size a bit, I can leverage a different capacitor technology, which could provide longer operating lifetimes. For the ILIS1, lifetime is a fundamental issue of ILIS technology to begin with, and there will be a lot of working going forward in mitigating these issues, which include shorting due to liquid bridging, fuel depredation, and emitter degradation over time, that hopefully the ILIS2 and future iterations will solve. These are major issues that even multi-million dollar labs still face.
When you look into the future and see your next round of products, what do you see the major advances will be? What technological or knowledge gaps are stopping you from getting to that point now?
At this scale, I think the biggest and most fundamental advances first and foremost will be improving lifetime and reliability of these tiny micro-thrusters, above all else. Once they are working for reasonable periods of time reliably, then it becomes easier to improve and fine-tune performance. Particularly for ILIS, since it has so much incredible potential in the field, unlocking higher lifetime will allow it to be an extremely competitive technology in the field. Since I am already aiming to offer the lowest cost EP systems out there, having reliable lifetime and solid operation will open them up for many use cases with many nanosat teams, as well as even educational tools and demo payloads.
Even in the case where thrust is minimal, such systems can still be employed for fine attitude control or pointing. Specifically, major near-term advances for my PPT line will be lifetime. For ILIS, these advances will be lifetime, thrust, fuel capacity, and expanded modules, everything from attitude control versions to large Cubesat clusters.
Other advances will also be tackling new forms of EP. In addition to PPTs and ILIS for both PocketQubes and Cubesats, I am currently working on developing other technologies, such as my own line of liquid metal FEEP thrusters for the Cubesat scale, as well as my new Io Series, which consist of a highly modular platform to support both solid iodine fueled RF gridded ion and RF plasma thrusters for the Cubesat scale
My philosophy in engineering, designing, and making things throughout my entire life, even back to grade school, stems from the simple concept asking myself how can I build these cool technologies myself at home with little money and resources? This is the driving force in everything I do, and is a mentality I have cultivated for years.
I am an extremely hands on learner, and a constantly thinking about ways to build things in the simplest and most effective manner possible. When I look at these technologies, I don’t really think there is anything outside the realm of possibility for me to tackle, even currently developing and testing them at home.
I am constantly learning and improving my knowledge set, and because of how I approach problems in the field, I am often forced to look far beyond just immediate or conventional knowledge for a particular thruster, and expand to many, many other fields.
All EP systems draw upon well-established technologies and principles in many other fields of research and industrial applications. Pulsed plasma thrusters are just flash camera circuits that ablate fuel rather than trigger a xenon flash tube.
RF plasma thrusters are just inductively coupled plasma torches. Arcjets and MPD thrusters are just welders. Gridded ion thrusters are just ion guns used in deposition sciences and the particle accelerator field, hall thrusters are similar to anode layer ion sources used in deposition sciences, and electrospray thrusters use the same principles and techniques found in printing, focused ion beam milling, and chemistry analytics sciences.
Everything is connected in some way or another, and when you see that EP systems are nothing more than basic fundamental technologies just packaged compactly and strapped to the backs of satellites to make them move, you can draw upon new ideas and approaches from other related fields.
There are some technical challenges, like lifetime issues for ILIS, some fabrication and fueling technicalities for FEEP, or material compatibility problems when running a corrosive fuel such as iodine, but I do not see these as stopping me from developing these systems. I prototype and iterate very fast, and at this point funding is the sole bottleneck for development.
Despite how low cost and efficiently I can develop, tackling several thruster systems already at only a few hundred dollars a month currently, there is only so much I can do on that budget, and I have to make choices and prioritize which technologies I explore and develop currently. My goal isn’t really to reinvent the wheel, but rather, approach EP development from a fundamental and radically different way.
⚠️This concludes the fourth part of our five part interview series. In the fifth interview within the series we’ll talk about Michael’s decision to open source his space company. Read the fifth part of the interview here.
If you’ve enjoyed this interview series so far and would like to help contribute to this open source space project, you can do so here.
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