Performance Projections for the AIS-ILIS1 Ionic Liquid Electrospray Thruster

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After extensive analysis of data and trends for macro-ILIS thruster technology based on current literature, I believe I have established projected performance for the AIS-ILIS1. The following post will overview this performance projection based on research in macro-ILIS technology, and ultimately compare performance with current ILIS thrusters on the market. For the ILIS1, the expected performance is as follows:

  • Expected Thrust22uN
  • Thrust/Power Ratio14uN/W
  • ISP>3500s

For thrust there are several key considerations. Thrust of ILIS1 is limited due to on-board power from the supply (3.5kV, 350uA). However, it has enough use-able emitter area to support significantly higher beam currents, and hence higher thrust, coupled with a larger supply. There are only a few macro-ILIS thrusters with full measurements in literature, however they cover a wide range of topologies. Despite these differences, looking at beam current to thrust, there is a linear trend to greater than 99% for establishing ILIS1 performance on this trendline:

Thrust vs Beam Current Performance for Macro-ILIS Thrusters

If we look at ideal thrust for given parameters of 350uA at final loaded voltage of 3kV, the max theoretical thrust from the ILIS1 is 27uN, resulting in relatively realistic predicted losses due to inefficiencies at 18.5%. Predicted losses from literature numbers vary 3-30%. Thrust to power is related to the total input power and projected thrust performance. At 1.6W in, there are hard limits on total current delivered, where some losses will occur in emission and transport. This number can improve however depending if losses can be reduced.

Finally we get to ISP. For ILIS based thrusters in general, this appears to be the most tricky parameter. This is due to the complexity of the fuel, consisting of two different ion species: +EMI and -BF4. Further complicating this is various emission regimes (mixed mode or pure ionic) on top of polydispersivity of the fuel, which is seen as the fraction of emission based on ionization levels. EMI/BF4 also have different atomic masses, which in themselves creates varied ISP. Polydispersivity results in the biggest efficiency losses for ILIS-based thruster technology. Singly ionized monomers result in ISP levels in the 7000-8500s for the expected operating voltages. However, dimer emissions, consisting of two attached monomers, results in lower ISP in the 3000-4500s range. Based on ToF measurements and general performance trends, macro-ILIS operates in PIR mode, where more conventional micro-ILIS operates in a mixed droplet/ion mode. For macro-ILIS, emissions are a distribution of monomers/dimers, with a fraction of trimers, etc. Factoring in real losses due to polydispersivity and other beam inefficiencies, for the given fuel and expected nominal operating voltage of 3-3.5kv, based on measurements and predictions so far, average ISP should not be less than 3500s, likely in the 3500-4500s range.

Currently in macro-ILIS literature, mostly only ideal case ISP predictions have been presented, with preliminary ToF measurements. However, more in depth characterization of micro-ILIS ISP has been performed on the MIT S-iEPS thruster, which directly evolved into Accion Systems TILE thruster. In fact, right now, the only completely developed ILIS thruster on the market for comparison is the Accion Systems TILE. Literature indicates the TILE blocks are the blocks from the S-iEPS development, packaged as sets of 4 vs 8, with changes to layout and improvements to lifetime. For the TILE-50:

  • Thrust = 50uN
  • Power = 1.5W
  • Thrust/Power = 33uN/W
  • ISP = 1250s

For the expected ILIS1:

  • Thrust = 22uN
  • Power = 1.6W
  • Thrust/Power = 14uN/W
  • ISP = >3500s

Data in literature shows TILE lifetimes at around 800 hours, a vast improvement over the initial starting S-iEPS lifetime of about 100 hours. Currently, macro-ILIS has only been tested to a few tens of hours at most. The first major goal after ignition testing will be to hit operating lifetimes at the 100 hour mark for the ILIS1. Should it successfully fire, the ILIS1 would be, as far as I am aware, the most complete macro-ILIS system ever built to date, and the first fully integrated macro-ILIS thruster module ever built. Achieving the 100 hour mark would also be a monumental milestone, on target to match work at MIT from home on a budget of only a few thousand total (for everything I have done up to date combined), developing the thruster in a matter of months, vs. now several tens of millions of dollars invested and over a decade of prior research!