After several months of intensive research and brainstorming, I am ready to reveal the first official Applied Ion Systems ionic liquid electrospray thruster concept design for PocketQubes – the AIS-ILIS1! This is an exciting and major advance forward for the ultra-low cost high performance propulsion effort for PocketQube and small Cubesat class satellites.
Before we get into the design overview of this new thruster, let’s take a brief look at the background of the current state of electrospray thruster technology. The principles of electrospray can be very easily demonstrated in atmosphere, and is used in many types of manufacturing and scientific processes. Using a simple needle and syringe and high voltage, you can electrospray liquids in atmosphere yourself. I have done this for fun with a small needle and shellac solution. Electrospray can also be seen very commonly in everyday applications such as mass spectrometry as well as common household printers. Thruster applications increase the challenge and complexity due to the extreme environment, as well as operating requirements for high thrust densities.
Electrospray thrusters are extremely promising technology for PocketQube propulsion, but there are still many technical challenges to overcome. There are three major types of electrospray thrusters – colloidal, liquid metal FEEP, and ionic liquid ion spray (ILIS). All three have strengths and weaknesses, however ILIS is by far the most exciting in terms of low-cost, low power draw, high thrust density, and ease of manufacture at the PocketQube scale.
While both colloidal electrospray and FEEP require some sort of electron neutralizer (like traditional ion thrusters), a unique property of ILIS thrusters is that the fuel can generate positive/negative ions simply by reversing polarity, eliminating the need for neutralizers. These thrusters use a unique type of fuel known as room temperature molten salts. Room temperature molten salts are exactly what they sound like – salts in a liquid state at very low temperatures. A key advantage of such a fuel is the ability to generate both polarity ions from the solution. One of the most common fuels utilized is EMI-BF4, which can be readily purchased from specialty chemical suppliers. Additional advantages of this fuel are that it stays liquid at temperatures below -50C, is very stable at high temperatures of many hundreds of C, has good vacuum properties negligible vapor pressure), and is relatively safe. This means that power consumption can be reduced by eliminating the need for heaters as well as neutralizers, making this type of propulsion very efficient.
ILIS thrusters also rely on capillary feeding, eliminating the need for valves or pressurization. This further reduces size and feeding complexity, but introduces some unique challenges as well. As a result, manufacture of this type of thruster can be quite complex. Traditionally, such electrospray thrusters rely upon micro-machined arrays of emitters and extraction grids. This is done in a couple of primary ways – utilizing porous glass emitters, or hollow needle emitters. These can be fabricated at a density of hundreds of emitters per square cm. In either case, the basic principle is relatively simple. Capillary action feeds the propellant through the porous or hollow emitter substrate, where high field strengths between the emitter and extractor force a Taylor cone to form at the tip, causing both droplet and ion emission to occur. This basic principle of operation of high field gradients causing the formation of Taylor cones is common to all types of electrospray thrusters. However, each type operates in different modes. Colloidal, the oldest form of electrospray, operates mainly in droplet mode, whereas FEEP operate solely with pure ions. ILIS however has some interesting challenges, with the ability to operate in a mixed mode. Ideally, peak efficiency is attained when purely ionic emission occurs, in the pure ionic regime (PIR). However, in reality, some macroscopic droplet emission usually occurs as well, which is detrimental to performance of the thruster, and over time, can lead to failures caused by fuel pooling, electrode bridging, shorting, fuel carbonization due to arcing, etc. By slowly alternating between positive and negative drive voltages between two adjacent emitters, positive and negative ion emissions occur with ILIS thrusters, and charge buildup in both the fuel and on the spacecraft is prevented.
In the propulsion field, there are several major players for electrospray thruster technology. While colloidal has taken a backseat to FEEP and ILIS, it was the first electrospray thruster technology demonstrated in space, designed and built by Busek for the Lisa Pathfinder mission. FEEP is currently the dominant electrospray thruster in the market, being both highly mature with significant flight qualification and testing. The two major companies developing FEEP solutions includes Enpulsion and Morpheus Space. As of current updates, Enpulsion has flown and delivered the most electrospray thrusters of any company so far, boasting impressive performance capabilities and very long operating times.
ILIS thruster technology is the newest addition to electrospray, first appearing only a couple of decades ago through the research done at MIT. As a result, they are still in the early stages of development, with little available in the market. Currently, the two companies working on ILIS thruster solutions are Accion Systems and Ienai Space. Accion Systems in particular has gained increasing recognition in the field pioneering compact and high thrust density ILIS thruster technology. Accion Systems is a spin-off company from the pioneering work done at MIT with the original S-iEPS thruster, and has been in the market currently for about five years now. However, only a few of these thrusters have been delivered and tested in space, despite over a decade of prior testing and development. Ienai Space is very new to the market, with very little information on their technology. Having been established for only a year, it does not appear that they have yet developed a deployable commercial ILIS solution.
While electrospray technology is already well suited and scaled for Cubesats, there are no electrospray solutions specifically for PocketQubes in terms of power and size. Cost and accessibility are also key hurdles for this technology in general at the micro-satellite level. As mentioned before, ILIS electrospray has key benefits for PocketQubes in that it can be scaled low power, made compact, provide high thrust at high efficiency for its size, and have the ability to fire continuously. Manufacture at this level presents the biggest hurdle. Using conventional approaches of micro-machined emitters, such as those used by Accion Systems and Ienai Space, manufacturing cost and complexity increases significantly, requiring specialized processes such as laser ablation machining, which is not a technology readily available for myself for the Applied Ion Systems effort.
However, there have been very recent papers on ILIS electrospray with exciting implications for ILIS technology that makes it potentially radically more accessible. In recent years, it has been shown that multiple site field emission can occur in macroscopic sintered glass emitters through the use of conventional CNC machining. This means essentially means that ILIS technology has now entered the realm of conventional machining methods. Conventional micro-machined ILIS generally uses micro-tips with radii of a few micrometers. Macro-scale CNC machined extractors however can have tips ranging from many tens of micrometers all the way up to hundreds of micrometers. In fact, a Chinese team recently published the results of a flat emitter with an emission edge 0.9mm thick! Extraction grids can also be made as larger, more conventional apertures in thin sheet stock, precision cut using common water jet technology. With standard manufacturing techniques at the macroscopic level apparently use-able for electrospray, this blows open new doors for advanced propulsion accessibility!
In addition to increased manufacturability and reduced cost, there are other exciting implications for macro-scale ILIS thrusters over conventional micro-machined ones. Based on current published data, both for experimental systems and commercial products, macro-scale multi-site emission ILIS thrusters can achieve the same thrust to power ratios (assuming PIR mode of operation) as conventional micro-machined arrays (like those from Accion Systems) at radically higher ISP. While lifetime remains a key challenge, development is recent enough that significant thruster optimization has not been achieved. Once solved however, this technology has significant potential to be a major disrupter in the micro-propulsion field. So far in literature, direct performance comparisons in published data between macro and micro ILIS has not been explicitly addressed. Based on preliminary data however, it is evident that macro-scale ILIS has potential to outperform current micro-machined tech. Published data also shows that micro-machined ILIS currently operates in a mixed mode regime, as opposed to the more favorable PIR mode. While this increases thrust density (due to the acceleration of larger droplets), efficiency suffers (which can be seen through calculation of theoretical thrust and ISP and comparing with published data.) Due to the larger surfaces, and lower field enhancement of the tips, macro-scale ILIS naturally operates in a much higher mode of PIR, due to use of higher extraction voltages. Micro-machined ILIS has a benefit of operating at much lower voltages, due to higher field enhancement at the emitter tips, resulting in lower turn-on voltage, and reduced emitter-extractor gap spacing, but as a result, has issues with reduced PIR mode operation, as well as failures resulting in bridging, pooling, and shorting.
Coupling impressive performance data with radically simplified manufacturing and lower cost, at present macro-scale ILIS holds advantages over micro-machined arrays, and is the direction I believe ILIS electrospray should trend towards for truly low cost, higher performance thrusters. Based on published performance so far, it would be relatively do-able to build a low cost thruster in the 30uN/7500s to 50uN/3200s range for 1.5W of input power. Further room in optimization from initial performance data should be able to improve this further. As such, I will be directing my full efforts and resources at present into the development of macro-scale ILIS thrusters for both PocketQubes and Cubesats. Such a thruster, if successful, could allow for significant propulsion performance at unprecedented cost reduction.
Now, let’s take a look at the new Applied Ion System concept design! The AIS-ILIS1 utilizes a single macroscale porous glass ridge emitter for multi-site emission. A porous glass reservoir is used to store the propellant for passive capillary feeding, mounted in PEEK or plastic housing. A distal electrode makes contact between the reservoir and board via wave spring, which compresses the parts together while providing integral electrical connections. By making the board a part of the thruster tank assembly, connections and assembly are greatly simplified. While a slit emitter is used for this first prototype, the design allows for any macro-scale emitter variation by simply changing the emitter and extractor. Depending on performance, multi-ridges, single spikes, and spike arrays can also be explored. A single ridge allows me to experiment with a higher thrust output over a single spike design while retaining manufacturing simplicity. Actual geometry needs to be finalized for the emitter and extractor, but additional field simulations will be presented once completed. EMI-BF4 will be used as propellant. I will also be exploring the use of a single emitter block vs the typical dual block to reduce size, and explore feasibility of single module neutralization for a simplified thruster module suitable for integration with PocketQubes, and scale-able for Cubesat applications.
Switching was the biggest challenge after solving the emitter manufacture. With conventional ILIS, lower voltages allow for relaxed switching requirements, where lower voltage components can be used for alternating between positive and negative polarities. However, at the expected voltages of up to 3kV or higher, in addition to space and power restrictions, switching technology no longer becomes feasible. High voltage MOSFETs, IGBTs, high voltage relays, and high voltage opto-isolators were considered, but decided to not be suitable for the extreme restrictions of the module. Instead, two micro HV modules will be directly switched on and off the single emitter block to provide alternating power. For this thruster, two Pico Electronics supplies will be directly alternated on and off. The total size of this module comes out to 45x45x17mm, utilizing a 25mm diameter, 4mm thick standard porous glass filter disk for the emitter and reservoir, with plenty of space to increase the reservoir for scaling up. Max power draw is up to 1.67W.
Shown below includes data provided by Pico Electronics for the HV supply to be used with the electrospray thruster. Green represent 5V in, and purple represents 3kV out. For full load conditions, the turn-on rise time is 7ms, fall time is 25ms. For minimal 10% load however, fall time increases to 175ms. From left to right – rise time full load, fall time full load, fall time 10% load. For the thruster to work at this low power and small size, it is crucial that direct switching of these supplies work. I haven’t found any reference in literature if there are required rise/fall times for electrospray, so this could be pushing some interesting territory.
With the advent of the AIS-ILIS1, this represents the biggest and most important development initiative to date at Applied Ion Systems. Since the system is relatively very low cost, and designed specifically for ease of manufacture and assembly, I should be able to bang out the first prototypes quite rapidly for testing. Should I get even any ion emission from this first prototype, it would be a fundamental shift in micro-propulsion technology accessibility for micro-satellites. Lots of exciting developments to come, this is only the beginning!