HackSpace: Let’s get something out of the way early on. When I see ‘space’, I think ‘rockets’. But the things you make aren’t rockets, are they?
Michael Bretti: No, not at all. They follow similar basic principles: you throw something out the back end, and you move something in space, but they’re very different mechanics.
I’ve always been fascinated with ion beam, particle beam stuff, plasma physics, things like that. I’ve worked on a bunch of projects in atmospheric conditions, because you don’t need any special vacuum hardware to run the stuff for that.
And I always wanted to get into vacuum-based plasma physics, and those types of things. So a couple of years ago, I finally bit the bullet. I sat down and did proper research and design for a vacuum chamber, all set up and configured, and I took a year to become kind of an expert in the vacuum engineering side of things, so I’d put things together and know that it would just work, and I’d know what I was doing.
That took a lot of time scrounging and locating materials. One of the nice things about being in the US is our surplus: the surplus of components and stuff like this on eBay is very, very prolific. You can go on eBay, and you can buy any type of scientific equipment you want. So there’s a lot of surplus vacuums, hardware, and stuff like that floating around, and other surplus vacuum shops in the US.
So, I spent a year doing that stuff, getting a vacuum system put together. Originally I had no intention of actually doing electric propulsion: it was for another project. I had been really fascinated with particle physics. So I had been working towards designing a very high-power pulsed electron gun.
So huge system for injecting kiloamp level beams at, you know, tens of nanoseconds wide, really, really intense pulse things. There’s an issue with that, because when you have very high voltage, high-power electron beams, you have a lot of X-rays. And that’s a huge safety issue. So while I was trying to figure out the shielding and everything for that, I had some spare leftover stuff.
I’ve always been fascinated with propulsion things. Like, back in high school, I remember looking up home builds of turbojet engines and pulse jets and stuff like that, that people have done. And electric propulsion allowed me to combine all my interests in one thing, so plasma physics, particle beam physics, vacuum physics, high voltage, pulsed power propulsion, all into one category.
So I figured, you know, ion and plasma thrusters are pretty cool. Let’s see if, on the side, I can play around with this stuff. So I put together that chamber, and I started designing and playing around with thrusters and posting about it on Twitter and stuff. Eventually, from there, people like Jo Hinchliffe took notice and reached out and connected me to people especially in the PocketQube [satellite] community where there’s almost no propulsion work at that scale.
So it just took off from there. I started focusing and tailoring my effort towards really, really tiny, really low-power systems and developing specifically for the community. And, eventually, I developed my first full thruster system. I actually have it still here. Let’s grab that. So this is the first full thruster I ever built.
HS: That’s tiny. I’ve seen the image of it before but I had no idea it was that small until seeing you holding it.
MB It’s tiny because, when you’re talking about PocketQube stuff, you’re talking about a cross-section of five by five centimetres, and then a single cube is five centimetres deep. So super, super tiny systems.
I built them specifically for PocketQubes, and I delivered them to an AMSAT Spain group. Unfortunately, that rocket flew recently, just last year, but it failed to reach orbit. So we lost the two units, unfortunately.
But this one actually did make it to orbit last March aboard the Care Weather Technologies CubeSat that was launched on a Rocket Labs rocket.
And you can download all the design files for this thruster. So the PCB, the mechanical [data], everything for it.
HS: Isn’t that quite unusual, for space hardware to be open-source?
MB: Very much so. I mean, there are definitely people doing open-source PocketQube stuff. And there’s the Open Source CubeSat Workshop that Libre Space Foundation hosts every year. But in terms of propulsion, it’s completely unheard of. And really, at this scale, outside of well-funded research facilities like MIT or NASA, or multimillion-dollar startups, you don’t see this stuff at home.
So, I’ve been embracing the open approach, and it’s really allowed me to create a unique niche for myself. I’m doing something different than anyone else is doing, and ultimately making it more understandable for people. For accessibility to mean anything, it’s not just about making things cheaper, or even necessarily easier to manufacture or open source. It’s also important to provide resources and show people that, you know, this stuff isn’t black magic. When NASA publishes articles and stuff about ‘next-generation ion thrusters’ and things like that, it’s always spun in a way that makes it seem so sci-fi. ‘This is state-of-the-art; this is super-futuristic.’ But it’s really not: this stuff has been around for, like, 60, 70 years. It’s all really, really old technology.
I mean, this thruster here, this uses Teflon fuel. The circuit is literally just like a camera flash circuit. It’s really, really, simple. And all the systems I’m doing – anything from pulse plasma thrusters to Hall thrusters, and stuff like that. The fundamentals of the stuff are really, really simple, and I want to show people the behind the scenes of how it’s done. It’s a lot of failures and a lot of struggles and some pretty cool successes.
HS: Forgive me if it takes a while for me to grasp what ion propulsion systems are. So, there’s an amount of a fuel source, such as Teflon, like you mentioned earlier, or something called adamantine, which sounds cool; and then there’s some sort of electric field around it, and then that fires ions from the fuel source out into the void of space.
MB: Yes – for one of the throttle thrusters that I’m working on, adamantine is the fuel source for that particular thruster.
Essentially, for all electric propulsion, regardless of the fuel type, you have some sort of fuel that you apply electricity to. So for the pulsed plasma thruster, for example, the way that’s done is you have an ignition spark inside, and that little ignition spark ablates a little bit of Teflon in there, and it ionises in there and expands into a little gas.
And then that connects between an anode and a cathode where there’s a capacitor that charges up to really high voltage. And that allows you to dump electricity through that in a bigger arc and accelerates it out. For the adamantane fuel, you have the solid fuel that you apply heat to.
So it sublimates into gas. And then you will have again, an anode and a cathode and a little magnetic field with that, and you apply high voltage to that again, and that discharge creates the plasma, and the interaction with the magnetic field allows the ions to get accelerated out. So in all cases, for electric propulsion, you’re essentially taking some fuel – doesn’t matter if it’s solid, liquid, or gas – applying electricity to that fuel in a vacuum, to create a plasma, and then either plasma or ions are accelerated out, depending on the type of thruster you’re doing.
HS: A rocket uses a combustion effect, which is heat and expansion, which creates force. This is plasma, which is something that I’ve heard the word, but don’t really understand what it is…
MB: Essentially, going from the gas, you’re adding more and more energy into it, and you’re ionising it, so you’re ripping away electrons. And you’re creating the next state of matter, essentially, where you have this electrically charged gas. And different systems can do different things with it. So you can have just a bulk plasma exhaust, so you’re just firing out the plasma, which is a mix of ions and electrons, and everything all just kind of spits out.
And then for other stuff, like the adamantane thruster, or even some of the other things that I’ve worked on, [those use] room temperature, molten salts for the fuel, that is sprayed out as an ion. So you have a plasma, and then from that plasma, you pull out the ions and accelerate them out as a beam.
So the mechanisms are different, but a lot of it is very related across the field.
HS: I guess if the stuff that you’re firing out is just the small particles of electrons or ions, there’s not very much mass coming out from the fuel?
MB: Very, very tiny mass. And actually, in some cases, it’s so little that I can’t even measure it, even after firing the system. The pulsed plasma thruster uses so little fuel, that if I weighed it before and after, I wouldn’t even be able to tell [that it’s used any fuel].
That’s partly because this thing has a really limited lifetime; the capacitor fails catastrophically after about 500 shots. Some of the other stuff, like the adamantane, you can definitely measure how that sublimates down, you can check the level very easily. And some of the new stuff where I’m using bismuth fuel, it has such a high ion erosion rate, the rate at which it produces ions is so high that it’s very, very measurable. Whereas if I use something like titanium for the same type of thruster, it’s almost negligible. So different fuels in different thrusters will have different kinds of rates.
But essentially, that’s the trade-off with electric propulsion: you have really high exhaust velocities, so tens of 1000s of metres per second exhaust velocity, but the mass is so tiny, that your thrust is really, really, really small. So we’re talking about micronewtons of thrust at this scale.
HS: How about battery life for the electric pulse that you send through the fuel sources? Is that a limiting factor?
MB: The biggest single thing in electric propulsion that really governs the performance and essentially what you can do – and the scaling and what technology would be suitable for particular missions – a lot comes down to the power that’s available. So all that power comes from the onboard battery supply on the satellites themselves, which are charged through solar panels that are mounted to the satellites. So really, as long as you have enough power to run the thruster, then the thruster will run, you know, assuming you have fuel and there are no other failure mechanisms in the electronics.
For this thruster, for example, it just takes the five volts from the satellite bus, turns it into 2000 volts with this tiny, high voltage supply, and then you have the rest of the circuitry on there that applies it. All the power comes from the battery on the satellite, and then that’s converted into various voltages on board on the thruster.
HS: Are you pleased with how the work’s going so far?
MB: Yeah. I think, when I first started, I had no idea what I was doing. So this is the first thruster I ever built. This is another Teflon pulsed plasma thruster; I had no idea what I was doing. I got the inspiration from old [vacuum] tube-based sockets. So you have a thruster with different electrodes that plugs into the PCB. I applied high voltage over the outside of the chamber. And if it didn’t fire at all, I had horrible arcing in the chamber. I had a lot of electronics failures; it was disastrous. And then I reworked that and played around with a different design. And with that one, the thruster fired only once – I captured it on camera; it was just a single pulse.
I destroyed all of my electronics during the process, pushing the thing way, way, way too hard. So that was not fun. But it led to the thought that, you know, maybe I can actually do this. I got one shot; maybe I can get two next time, or ten. And it’s really grown from there.
What’s been so amazing has been the support from the community. When I started out, I was, you know, completely unknown, just posting random stuff on Twitter, trying to connect with people. And I had no intention of starting growing this into a startup on the side, or even trying to expand further. I never even considered the possibility of this stuff even going into space; I was just playing with it in the basement. So the fact that I’ve gotten something into space, and I have a lot of people I’m working with, developing a number of systems for a number of people and expanding to tons and tons of different technologies across the board at various power levels…and with all the support of the community, it’s really blown me away. I would never have imagined that just playing with this stuff on the side would open up so many opportunities.
HS: When you say community involvement, community support, does that mean that you get a lot of input and help from other scientists?
MB: I do get input. But it’s challenging because on one hand, I’m also learning as I go, so I’m experimenting with things like that and doing a lot of experimentation on the side, and it’s still an area where you have people in industry who are doing this, and there are tons of companies and researchers all over the world doing this stuff, but outside of that, the maker community, with the hobbyist community, people know the high-level fundamentals.
But actually, the nitty-gritty details and the mechanisms and stuff, that’s what I’m trying to bring to the community itself, through all these experiments and this open development and research. I want to show people from the ground level how it’s done. So I think it’s tough for people to give input, just because it’s a field that you have to really spend a lot of time looking into and looking at the details. I’m also trying some really weird things because my budget is so limited, so I have to play around with some unusual configurations.
But in terms of support in general, like, all the encouragement, and obviously with Patreon, for example, it’s helped me do a lot of research. All the encouragement from makers, from hobbyists, from the satellite people. Because there are tons of failures. And there are periods where it’s just failure after failure after failure. And it really wears on you. The community don’t let me give up when it gets really tough.
For example, when I first started with my vacuum system, I had a cheap refrigeration pump for one of the primary pumps. And that failed, and I didn’t have the budget to keep going. So some members of the community [it was actually organised by Bruce Perens] started a GoFundMe to raise funds for a scientific-grade pump. And that’s been running ever since and supporting a lot of development. I have a lot of trouble asking for help. I never asked for help, just because I really struggle with that. I think people have been really inspired by this stuff, and what I’ve been doing on the side, so they’ve been willing to pitch in and see how far this development can be pushed.
HS: What are you working on right now?
MB: I’m working on another pulse thruster – it’s called a vacuum arc thruster. So that uses the solid metal fuel that I was discussing earlier, working specifically with bismuth. It’s about the same size as the image below, but it uses a very different mechanism. It’s actually a little bit weird because, unlike that one, which uses about 2000 volts, this one generates the plasma at very low voltages, so 40 volts or something on the input. And it’s very weird, you have this graphite layer on a ceramic washer that you apply a pulse to.
It causes these little things called cathode spots to form. So it explodes out electrons and gas and graphite from that layer and causes a plasma flash-over to arc between the electrodes. It’s a really cool little device. Again, it’s very good for scaling, so it can be made really small, really low power for these tiny PocketQubes. But it can also be scaled-up for big CubeSats and other stuff pretty readily. I’ve been working on a tiny system. So for a single one, I actually have boards in that I’ll hopefully be running within the next week or two, firing three of them simultaneously in a vacuum just to start doing some lifetime qualification of them.
I’ve been doing a lot of pendulum tests. So I made a tiny ballistic pendulum with a super-thin piece of Kapton [a heat-resistant, electrical insulating polymer]. You put the thruster right in front, and you fire it. And by knowing the displacement and the mass and everything, you can calculate the force generated on that. There are some issues because with a classical ballistic pendulum, if you fire like a bullet into a block, it sticks, so it transfers all the momentum. With the plasma, you have particles that hit the target, and there’s a ton that bounce off. There’s some that stick; there’s some that scatter. So the readings are really inaccurate with a flat pendulum, which is what I’ve been using.
I’ll be changing to a conical shape – there was a research paper done in the 1980s where they used a cone shape facing the thruster, so when the particles hit, they bounce off radially, and that cancels out any additional momentum. I’m transferring the pendulum so you can get more accurate readings. So I’ll be kind of playing around with that pendulum.
And then, also, I recently got some requests to see if I can do a much higher power system. So the small system runs at about 2.5 watts – really, really low power, it’s about 10 micronewtons of thrust, in that ballpark range. And I’m working on a much bigger system, that would be a 50-watt class system. So much, much bigger. Really big plasma plumes coming out of that, and probably looking at 500 to 1000 micronewtons of thrust. So it’s a much bigger scale. I’m focusing on this particular class of the vacuum arc thruster category, just because there’s a lot of potential, and it’s been one of my best firing systems yet.