Alameda Applied Sciences Corporation
3077 Teagarden Street, San Leandro, CA 94577; Tel: 510-483-4156; Fax: 510-483-8107
Variable Thrust/Specific Impulse Electrospray
Propulsion based on Ionic Liquid Electrolytes*
An STTR project
Alameda Applied Sciences Corporation, 3077 Teagarden Street, San Leandro, CA 94577,
USA
Yale University, 47 College Street – Suite 203
New Haven, CT 06510-3209
Abstract for
2012 Space Propulsion and Power Program Review
10-14 September 2012, Arlington VA
Research supported by AFOSR via STTR Contract FA9550-09-C-0178
1 . Problem addressed by this STTR project
Future Department of Defense space missions require precise, fine-positioning
capabilities combined with large maneuverability requirements. The purpose of this STTR is
to: a) identify propellants for electrospray propulsion able to cover, at high propulsion
efficiency, an unusually wide range of specific impulses, from several hundred seconds,
typical today of colloidal propulsion up to values of thousands of seconds, typical today of
purely ionic propulsion; and b) meet thrust requirements by microfabricated multiplexed
electrosprays.
There exists no electric micro-propulsion thruster that offers variable Isp (500-2500s)
with >50% efficiency and scalable thrust for small satellite applications.
1. Pure ionic thrusters based on ions of fixed mass/charge cannot span such a wide
specific impulse range at high propulsion efficiency and full power.
2. Electrospray propulsion, on the other hand, admits a wide range of mass/charge values
at relatively constant beam power. One can therefore use the full power available over
the whole range of specific impulse with a single propellant
2.
Our Objective
Our value proposition is to deliver (at TRL4 by Dec 2012) a thruster that may be adapted
to a wide variety of small satellite missions within 18 months (after Ph-III boost to TRL6).
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3 . Our Approach
3.1 Array Design and Fabrication
Our project suffered a setback at the onset with the loss of MEMS array fabrication
support from the ARL. Our contingency plan was to learn to fabricate arrays by ourselves. A
graduate student (Giovanni Lenguito) used the DRIE tool both at BNL and Yale facilities to
fabricate various capillary tip arrays in Si wafers. Figure 1 shows an image of a 7-tip array.
The pitch is large in this case as it was a prototype wafer for initial tests.
Fig. 1: Photograph of a 7-tip array
Fig. 2: Image of single tip; inset shows ~2µm
beads used to control flow impedance
Figure 2 shows an SEM image of a single tip of our capillary array design. The inset at
top right shows the capillary tips filled with ~2µm micro-beads designed to control flow
impedance and ensure flow stability. Figure 3 shows a 37-tip array (upwind) after 12 hours
of operation with Ethyl Ammonium Nitrate (EAN) propellant.
We designed a set of photo-masks necessary to fabricate the new nozzle arrays. In our
latest design, each nozzle is surrounded by a “moat” (Fig. 4), with multiple advantages: ease
the microfabrication process (less material removal), better uniformity of the electric field for
all nozzles (reduced edge effect) and the potential to handle temporary “flooding” due to
temporary operational instability and droplets flight reversal.
Fig. 37-tip MES (upwind) after
12hrs @ 7.4kV)
Fig. 4: “moat” design for Fig. 5: Drawing of extractor
each tip (see text for details
electrode for 631-tip array
(width of wafer is 1.5
inches)
We have designed and fabricated extractor and accelerator electrodes for 91-tip MES and
631-tip MES devices (Fig. 5). The extractor/accelerator dimensions are shown in Figure 6.
The current MES device, with geometric parameters listed in Fig. 6 is packaged in an
alumina case. The electrodes (nozzle array, extractor and accelerator) are separately bonded
to the alumina by means of epoxy (a circular bead of MEG-150 for the nozzle array, and
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ALAMEDA APPLIED SCIENCES CORPORATION
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Epotek 377 for extractor and accelerator). The 50m separation between nozzle array and
extractor is set by a PFA film; the bond is secured by clamping the alumina parts together
(Fig. 7).
Fig. 6: Extractor/accelerator details: ID=10, Fig. 7: Clamped electrodes with grounded
OD=30, D2 =380, D3 =750, L12=50, L23=750, shield on the top
T2= T3=75 m
A grounded shield (see Fig. 7) is necessary to screen the electrodes, and prevent
secondary electrons, produced at the impact of the highly energetic droplet on the vacuum
chamber wall from reaching the connectors. Otherwise, the current recorded is biased by the
negative contribution of such electrons, as measured in preliminary experiments in which up
to a 20% spurious contribution to the current flowing in the extractor electrode was
measured.
3.2 Propellant selection
Several new propellants have been investigated as possible low-volatility substitutes for
our current best formamide-based electrospray propellant. They are all formed by mixtures of
the ionic liquids (ILs) EAN, EMI-N(CN)2, DMI-N(CN)2 with the high boiling point
(~280oC) neutral solvents sulfolane and tributyl phosphate (TBP). The TBP solutions have
been most extensively investigated because of the potential advantage of their low viscosities.
Unfortunately, due to imperfect miscibility with the polar ILs studied, no TBP mixture of low
viscosity has been found having the high electrical conductivity required to reach our goal of
high specific impulses. Further advances are still viable through somewhat less polar IL
additives. Although more viscous, the sulfolane mixtures are freely miscible with the ILs and
cover a broad range of promising electrical conductivities up to 3 S/m. We have begun
characterizing the propulsive performance of the rather large number of promising propellant
formulations possible with sulfolane-IL mixtures. The first and only one already extensively
tested as propellant is pure ethylammonium nitrate (EAN). Its propulsion efficiency ηp
reaches only up to 67%, smaller than the values up to 80% offered by formamide solutions.
This efficiency however is surprisingly high for a pure ionic liquid operating as a source of
nanodrops and is competitive with other electrical propulsion devices. All other propulsive
parameters of pure EAN are comparable to those of our best formamide-based propellant, so
EAN is a promising alternative propellant with low volatility.
3.3 Array MES tests
37-tip MES device test: We operated a 37-tip MES device with 2.01μm microbeads; the
propellant was EAN; the accelerator was grounded (V3=0 in Fig. 6). By keeping the chamber
pressure at <10-5 mbar, we could raise the voltage up to 7.4kV. We improved the data
acquisition system (DAQ) to record over time the currents flowing through the electrodes
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(Fig. 7) and recorded data at ~2.5Hz sampling rate, based on hardware (multimeters
TekPower TP4000ZC) limitations. This system does not have any transient voltage
suppressor so we experienced DAQ communication problems which resulted in loss of data
(dashed line in Fig.8). The current emitted at the nozzle array shows good stability
(fluctuations on the order of 1%). The current flowing through the accelerator is about 3% at
the highest flow rate (1% at lower flow rates). Droplets impinging on the accelerator appear
to fly back to the extractor, since the readings are symmetrical with respect to the abscissa
(zero current). The device operation became unstable only once (around t=6:20) with a
positive current flowing through the extractor as a clear sign of beam impact. However, this
unstable operation lasted only few minutes and the device recovered on its own without any
external interference. The device was tested in the range of total emitted currents from 14.2 to
23.0 µA, at an estimated total flow rate from 1.51 to 21.3 µL/h. This spans a 3-fold specific
impulse from 2100 to 710 s, which is lower than the 4-fold range of a single nozzle (from
2200 to 540 s) at 7.4kV. The thrust achieved ranges from 6.1 to 31 µN. The maximum power
consumed is 0.17 Watt, extrapolating to almost 3 Watt in a device with 631 emitters. Also in
this case we did not find any major sign of erosion, due to negligible current hitting the
electrodes, which also demonstrates the low-divergence of the beam.
Fig. 8: Operation of a 37-tip MES device; V1=7.4kV, V2=6V.
Fig. 9: XYZ motion stage
and ceramic holder for
multiple-tip MES thrusters
3.4 Future Plans
We are fabricating 91-tip MES and 631-tip MES devices for further testing. A more
robust assembly to handle various tip arrays with a 3-axis motion stage and laser drilled
electrodes (see Fig. 9) will be assembled and tested with 7-tip and 37-tip arrays soon.
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ALAMEDA APPLIED SCIENCES CORPORATION