Nanosatellite Propulsion: You Don’t Need Much Power to do a Lot in LEO

But again, depending on the type of EP flown, you don’t actually need that much power to begin with, especially for simple maneuvers. Many types of EP have thrust to power ratios of 10uN/W and higher, so with a few Watts you can actually do quite a bit in LEO!

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When talking about nanosat power, there will always be trade-offs

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 third part of a five part interview series. If you’d like to start from the beginning of the series, you can click here

 You’ve touched on the power needs of EP many times throughout this interview. How does power and propellant work in electric nanosat thrusters? What power capacity is required for a nanosat to carry an electric propulsion system? What other additional systems are required to run an electric propulsion system? 

EP is such an incredibly diverse field, and as a result there are so many different ways to approach system design. Depending on the technology, there are a wide range of power requirements and fuel delivery that can be used, and even within the same technology class of thrusters, there are countless ways of power and fuel delivery that can be taken, some very conventional and well established, and others more experimental.

Electric power can vary massively. You have numerous different classes of thrusters, and EP power requirements can range from low voltage DC, high voltage DC, AC, RF (from the low MHz range up to GHz), and pulsed. Electric propulsion quite literally covers just about every form of electricity you can deliver!

In all cases however, power is converted from the typical bus voltage, ranging anywhere from 3.3V on the smallest of picosats to typically 24-28V on Cubesats, to higher voltages, ranging from hundreds of volts to thousands of volts to operate the thruster, depending on the thruster. Again, this can be delivered either as DC, RF, slowly alternating DC via polarity switching, or stored in some energy storage device, like capacitors and inductors, and delivered as intense pulses of hundreds to thousands of amps in very short intervals.

In terms of fuel delivery, this can also range tremendously. You have everything from solids, liquids, and gases. On the solid side a lot of these fuels are often seen in pulsed ablative thrusters. Common examples includes Teflon and other various plastics used in pulsed plasma thrusters (PPTs), to metals such as aluminum and titanium in vacuum arc thrusters (VATs). Alternatively, you have some solid fuels that are being explored for conventional Hall, gridded ion, and RF plasma, in the form of iodine, which can be stored as a solid and readily sublimated to a gas for feeding, or higher temperature solids, like Bismuth, which are vaporized at higher heat levels.

Liquid fuels are also used, though not the same as traditional chemical rockets that rely on combustion. Water is one of the more hyped ones explored, and can be used in numerous ways. There are thrusters that electrolyze the water as hydrogen and oxygen on demand to be burned like traditional chemical propulsion (kind of one of those grey areas where chemical and electrical systems cross). There are thrusters that heat the water to be used as steam. Water can also itself be used as fuel for things like plasma thrusters, where the water vapor is ionized by high frequency RF into a plasma.

Liquid however fuels really shine dominantly in the space of electrospray propulsion. There are three major forms of electrospray – colloidal, FEEP, and ILIS.

Colloidal is the oldest form, dating back many decades, where the exhaust consists of a charged droplet spray of fuel. This fuel is often some glycerol based liquid doped with either sodium iodine for positive ion generation, or sulfuric acid for negative ion generation.

FEEP, which stands for field emission electric propulsion, utilizes liquid metal fuels such as indium and gallium, where historically other metals such as Cesium have also been used. These fuels are almost always passively fed via capillary action, typically to tungsten emitters (usually porous sintered tungsten), resulting in an ion beam exhaust consisting of metal ion species (rather than droplets like in colloidal thrusters).

The most recent form of electrospray, and one that I am currently working on myself now, ILIS, which stands for ionic liquid ion source, relies on a unique liquid called ionic liquids, consisting of room temperature molten salts, where the liquids themselves are made up of ions or ion pairs. Due to the nature of these liquids, both positive and negative ions can be extracted from the same fuel, allowing for neutralization of the ion beam without the use of a conventional electron emitting neutralizer (which are needed for both prior mentioned types of electrospray and all other forms of ion thrusters.)

Finally, gases are the most common fuel for EP systems, consisting primarily of noble gases. Xenon is the single most widely used gas in all of EP, enjoying almost exclusive use in gridded ion and Hall Effect thruster. However, Xenon is quite expensive, and there is a lot of work looking into alternative fuels. For some thrusters, like RF plasma, literally any type of gas you can pass through the discharge chamber can be used, and even with conventional thrusters, numerous gases have been explored. Krypton, argon, nitrogen, hydrogen, helium – you name it, it has probably been done!

Power capacity largely depends on the individual nanosat that is being built. Some of the larger Cubesats can provide many tens of watts dedicated for propulsion, where the smallest picosats have less than a Watt of available power. Power capacity in relation to electric propulsion is also determined by the mission requirements, and what you can deliver to the EP system is also limited based on other payload power requirements.

But again, depending on the type of EP flown, you don’t actually need that much power to begin with, especially for simple maneuvers. Many types of EP have thrust to power ratios of 10uN/W and higher, so with a few Watts you can actually do quite a bit in LEO!

For electric propulsion to truly be utilized and practical on a nanosat, there are definitely other systems that are needed, which I expanded upon earlier. The biggest and most obvious is power generation and management. Nanosat generate power in orbit though solar cells, which are used to charge onboard batteries. Besides just raw available power however, sometimes the power draw of other subsystems can limit what is used when, and in cases where tight power budgets are seen, sometimes you need to cycle on and off various systems in order to be able to run the EP system for the particular maneuver.

A second requirement is attitude control. It doesn’t do much good to have a thruster onboard if you can’t first point in the right direction to fire, unless you are doing a mission purely for the purposes of technology demonstration. This is often in the form of reaction wheels, which impart a torque on the spacecraft, and with several wheels oriented along the axis, can provide rotation of the satellite in three dimensions. Propulsion itself can be used for this purpose itself however, and one of the major advantages of EP is its ability to be used for very accurate and fine pointing and attitude control for satellites. This has been most famously demonstrated on the LISA Pathfinder mission, where several colloidal electrospray thrusters were used to keep extremely precise pointing accuracy.

Unfortunately, the need for attitude control adds significant requirements for satellites wanting to use EP. Not only do you need to provide power and control for the EP system, but you also need to provide power and control for the attitude control as well, regardless if using reaction wheels or EP. This adds extra cost, space, and complexity overhead for a mission. Especially for smaller nanosats and picosats in the PocketQube class, this can be very challenging from the power and available space perspective.

 How long are electric thrusters able to operate for on a nanosatellite? What will cause a thruster to stop working? What can be done to extend the life of a thruster? 

In terms of thruster lifetime, this also varies massively between different types of systems. In essence, you can have either a steady state system, where thrust is provided continuously, or systems that deliver thrust in short discrete intervals, in the form of pulses.

For steady state systems, you gauge operational lifetime typically in terms of hours. Even in this space however, it can greatly vary. For example, gridded ion thrusters and FEEP liquid metal electrospray have some of the longest continuous lifetime hours of any EP systems, tested literally to several tens of thousands of hours of operational life!

In the case of gridded ion thrusters, grid erosion mechanisms play a role in limiting lifetime, as well as neutralizer lifetime. In theory, for thrusters like RF plasma, which are electrodeless and do not rely on extraction electrodes or emitter like various ion thrusters, lifetime is technically only limited by propellant storage (assuming plasma erosion and thermal effects of the discharge chamber is well managed.) However, such thrusters have not been tested to the same lifetimes of ion thrusters, and since plasma thrusters have lower efficiency than ion thrusters, more fuel is required to operate for the same amount of time.

On the opposite end of the lifetime scale for continuous thrusters, certain technologies have lifetimes of hundreds of hours or less. For example, one of the biggest challenges for ILIS electrospray is in fact lifetime, currently limited to hundreds of hours, with a variety of failure mechanisms, including liquid pooling, liquid bridging, fuel degradation, and emitter degradation. For thrusters like arc jet or magnetoplasmadynamic (MPD), which are essentially just welding torches used as thrusters, lifetime is severely limited even further to much less operating time due to extreme and rapid erosion of the electrodes that are used to sustain high temperature electric arcs used to directly heat and ionize the thruster fuel gases.

For pulsed thrusters on the other hand, since thrust is produced in discrete steps, often on the order of small fraction of a second (microseconds to milliseconds), lifetime is more characterized in terms of number of pulses rather than hours of operation.

For pulsed thrusters, thrust is directly related to the number of pulses per second. Each pulse exerts a particular impulse bit, a discrete unit of force, which when multiplied by the repetition rate, gives thrust.

Repetition rates can widely vary depending on the technology. Generally pulsed plasma thrusters, which run primarily on solid plastic fuels like Teflon, are operated at only a few Hz.

Pulsed thrusters like vacuum arc thrusters, using solid metal fuel, can typically operate at higher repetition rates, on the order of several tens of Hz. There are also some more unconventional forms of pulsed thrusters that rely on pulsed laser ionization or ablation, which is also limited in repetition rate. There are also massive experimental pulsed thrusters that utilize fast gas injected fuel, delivery huge pulses of energy that can be operated on the level of once every few minutes of even hours.

Generally in all of these cases, nanosat level pulsed systems are typically rated at the level of millions of pulses. There are various factors that can affect lifetime. Electrode erosion is a large contributor, particularly for higher pulse energy systems. For certain thrusters like pulsed plasma thrusters that rely on plastic fuel, charring of the fuel can actually lead to shorting, due to the carbonization of the plastic, allowing for electricity to be conducted across the charred surface of the fuel, rendering it inoperable before fuel is depleted.

Electronics however can also be a major limiting factor, particularly for pulsed plasma thrusters, which rely on capacitive energy storage. Essentially in PPTs and VATs, energy is stored in some device (either capacitive or inductive), then rapidly discharged to vaporize, ionize, and accelerate the fuel as a plasma. These pulses are on the levels of hundreds to several thousands of amps for brief periods, which, especially in the case of PPTs using capacitive storage, can put a large strain on these parts, as well as switching elements, which can fail before the thruster. Often times, especially in the case of PPTs capacitor lifetime can limit total life more than the thruster or fuel amount itself. In addition, these large pulses can also damage nearby electronics.

Realistically though, are just so many ways for thrusters to fail, and studying failure mechanisms of EP is literally an entire field in itself. Each type of thruster has its own failure modes. Grid erosion, discharge chamber erosion, thermal stresses, and neutralizer erosion are common ones for more conventional high power thrusters.

Low power micro-EP can suffer from electronics malfunctions, erosion, emitter lifetime limits, and shorting due to fuel.

At least in my experience so far, I have often found that the electronics are just as difficult, if not more difficult than the actual thrusters themselves, accounting for a huge amount of failure modes, especially at the extremely small scale.

Many EP systems require high voltages, and when you couple high voltage with tiny component spacing, semiconductor electronics, high vacuum, and various different materials, lots of unexpected things can occur. Components breaking, board breakdowns and arcing, component degassing, some metal part or solder point being just slightly too sharp and slightly to close together, ionized fuel, charge imbalances – it is a mixture for many types of interesting and unexpected failures.

 What other propulsion systems are currently being used on nanosats and how do electric propulsion systems compare in terms of efficiency? 

Besides all the varieties of EP systems out there, there is also a whole field of chemical propulsion as well. Everything from cold gas thrusters, to warm gas, monoprop, and biprop. There are other forms of propulsion, like solar sails, that have also flown, most recently the LightSail 2 mission, which successfully changed the satellite’s orbit. Unfortunately, I don’t really have that much knowledge to comment on these types of systems to the depth that I can for EP in terms of use and prevalence in the field. However, there are many more satellites in general that have flown non-EP based chemical propulsion systems, and the chemical propulsion field seems to be much better established in terms of flight heritage and overall prevalence.

Efficiency, or specific impulse (ISP) in propulsion, tells us how effectively an engine or thruster uses fuel. ISP is typically given in units of seconds. On the subject of efficiency, chemical propulsion for space applications just cannot come close to the levels of efficiency of EP, based on the physics and the limitations of exhaust velocity through chemical means vs. electrical acceleration. Looking at numbers for ISP, we see chemical propulsion falls in the range of a few hundred seconds and less, where EP can range from several hundred seconds to many thousands of seconds. Conversely however, EP systems also cannot come close to the thrust levels of chemical systems due to the physics of operation as well. Whereas chemical propulsion can provide thrust levels of Newtons, EP on the nanosat scale is on the order of a couple of mN and less (usually in the uN range.)

There are always trade-offs though, especially in terms of thrust vs. efficiency, as well as mass, volume, complexity, and power requirements.

One key draw of EP is requiring significantly less fuel for equivalent missions. This has a benefit of mass reduction, however depending on the technology, EP and chemical can take up similar volumes for the total system, particularly at the higher power range of EP, though at the very low range for small Cubesats and PocketQubes you can reduce mass and size of EP quite significantly. Of course the trade-off here is the time required to complete a maneuver between chemical and electric. In a lot of applications around Earth orbit, cases could be made for either, and is highly dependent on mission requirements and limitations. However, when you start requiring deeper space missions, and more delta-V, EP dominates because the scaling of fuel required for an equivalent mission with chemical propulsion becomes far too impractical.

If you can afford the longer maneuver times, even in LEO and other Earth orbits, for equivalent delta-V EP will require far less fuel, which does add up in mass savings and volume. This becomes especially important for expanding nanosat and picosat missions to the moon and beyond. However, there is no one perfect solution for everything, and I believe that chemical and EP can be used effectively to complement each other, as each has its own inherent strengths.

⚠️This concludes the third part of our five part interview series. In the fourth interview within the series we’ll talk about common nanosat mission profiles and how EP will change what nanosats are capable of doing. Read the fourth 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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