BLACKSHIP ONE PRESENTS…

How Electric Propulsion Will Change Nanosatellite Mission Profiles & Applications

From extended lifetime with orbit keeping, to collision avoidance, orbital transfers, de-orbiting, precision pointing and attitude control, and formation flying, EP is particularly important for the nanosat field because it allows for new missions and capabilities for these small satellites, further expanding what these platforms can achieve, beyond just passively drifting through orbit.

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In the LEO range, really thrust on the order of only tens of uN is required for most things

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, that this is the first part of a five part interview series with Michael. At the bottom of this page, you’ll be able to navigate to the next page within this interview series. 

Let’s jump in! 

 Hello and thank you for joining us today to talk about Applied Ion Systems and the work you’re doing in the electric propulsion space. I must say, it’s a very inspiring project you have on your hands. Can you kick off the interview by giving us a 10,000 foot overview of what electric propulsion is and why it’s important specifically in the nanosat space? 

Thank you very much for having me and giving me the opportunity to discuss my work at AIS and electric propulsion in general!

To kick things off, at the most basic and fundamental level, electric propulsion (EP) is a means of propulsion where electrical energy is used to cause the acceleration of propellant for a spacecraft. Where chemical propulsion relies on combustion, chemical reactions, or acceleration of gases through conventional pressurized propellant delivery to some nozzle, EP relies on imparting energy to the fuel electrically. This can be anything from direct resistance heating to further accelerate gases (like in the case of resistojets), to direct heating of fuel through an electric arc (like arcjets and MPD drives), to electrostatic fields accelerating ionized fuel (gridded ion thrusters and electrospray), to ionization and heating via RF energy to generate a plasma for thrust (RF plasma thrusters), and other various forms such as pulsed ablation thrusters.

From extended lifetime with orbit keeping, to collision avoidance, orbital transfers, de-orbiting, precision pointing and attitude control, and formation flying, EP is particularly important for the nanosat field because it allows for new missions and capabilities for these small satellites, further expanding what these platforms can achieve, beyond just passively drifting through orbit.

In some cases, like in the PocketQube realm where I do a lot of my own work, there are currently no propulsion solutions at all for these picosats, and specifically for PocketQubes, no one has ever successfully demonstrated propulsion in orbit aboard such a small satellite before (hopefully this will change with the upcoming AMSAT-Spain GENESIS mission flying two AIS pulsed plasma thrusters this September!)

For this class of satellites, there are also many launch restrictions as secondary payloads that prohibit them from flying with any sort of pressurized or explosive fuel onboard, eliminating all forms of chemical propulsion, and even a few forms of EP. While still in the early stages for this class of satellites, this makes EP particularly crucial down the road. As nanosats and picosats advance further, there is an increasing need for propulsion capabilities to meet these new demands. Already Cubesats have proven invaluable tools in the commercial, educational, and scientific research realms, extending now from just low earth orbit to potentially interplanetary capabilities and beyond, and propulsion, especially EP, will be important for many of these missions.

 Can you give us a top level overview of how thrust is generated? What level of thrust is needed for most nanosat mission profiles?

In terms of EP, there are many ways thrust can be produced, but at the fundamental level, it follows the same principles as any propulsion system – for any action there is an equal and opposite reaction. Some form of exhaust is expelled out the thruster, generating some force which acts upon the spacecraft causing it to move. For EP, this is primarily done through the emission of charged particle beams (like in the case of ion thrusters), plasma (like RF plasma, pulsed plasma, etc), and electrically heated gas exhaust (in the case of resistojets and arcjets). Different methods of generating and accelerating exhaust can lead to very different performance, anywhere from moderate thrust and low efficiency, to very low thrust and extremely high efficiency. With the incredible array of EP technologies and fuels that can be run with EP, this makes the field extraordinarily diverse.

Required thrust is highly dependent on the mission itself, and there is no one universal solution or answer. However thrust is not the only consideration. In fact, EP is generally not primarily selected for its thrust levels, in comparison with other forms of propulsion like chemical, but rather for its efficiency and the advantages that come with this. There is however still a minimum thrust that is needed for any given mission, but for EP maneuvers are performed over long periods of time at very low thrust levels. Where chemical propulsion deals with thrust typically on the order of Newtons, EP generally ranges from micro-Newtons (uN) to the low milli-Newton (mN) range.

For a nanosat or picosat in low earth orbit (LEO), not much thrust is actually required. The level of thrust that is needed primarily is governed by the orbit in which the nanosat is located, the cross-sectional area of the satellite itself, and the time required to perform a maneuver. The lower the orbit, the more drag forces are encountered, which can increase very rapidly. However, at higher orbits, far less thrust is needed. Even the difference between 300km to 400km, and 400km to 500km can be massive in terms of drag forces. The larger the cross-section (including everything from the satellite itself to deployable like solar cell wings), the more drag force the satellite will encounter, also contributing to required thrust.

If we look specifically at nanosats and picosats (let’s say from a 6U Cubesat all the way down to a 1P PocketQube), in the LEO range, really thrust on the order of only tens of uN is required for most things. Obviously the higher the thrust, the faster the maneuver, but again, since thrust in EP is directly related to input power, and since power can be very limited on these small spacecraft, slower, longer burning maneuvers are the accepted trade-off for having propulsive capabilities when selecting EP if you do not need immediate maneuvers.

Often at the smaller scale, you also encounter EP systems that require very little fuel to operate for long periods of time. For things like station keeping, depending on the orbit and size, only a few uNs to a couple of tens of uNs are needed. Orbital transfers benefit from higher thrust, but if you can overcome the drag forces present then it becomes a factor of time. In this case, you need systems that are capable of providing enough delta-V to be able to meet these mission demands. Working with several other teams particularly in the picosat realm, even with something as small as a PocketQube, there are EP technologies that can in fact provide delta-V and thrust for more demanding maneuvers such as orbital transfers. The trick here now is just developing solid systems at this scale! For Cubesats however, this is less of an issue since you have significantly more space and power to work with in comparison, and there are already numerous Cubesat-compatible EP solutions that can meet many mission profiles in LEO and beyond.

⚠️This concludes the first part of our five part interview series. In the second interview within the series we talk about problems that propulsion systems face when they are miniaturized. Read the second 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.

Blackship One is a content marketing agency focused on helping space, robotics and hi-tech companies grow. Interested?  Learn more here.

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