Electrospray emitter arrays can be assembled with different tanks according to varying mission needs. The image below shows the NASA MEP thruster tank, compared to the latest, fully scalable tank design on the left. These thrusters have been successfully tested for launch vibration, thermal cycling and static acceleration >12Gs, fully filled with propellant.

Fully scalable iEPS tank design compared to NASA S-iEPS tank design

MIT SPL integrates the MEMS based electrospray thruster technology for space based testing in different propulsion module configurations. The modular thruster design allows different propulsion module configurations to prioritize different aspects. These included attitude control featuring multiple individually commendable thrusters per module such as the modules shown below, and primary propulsion modules such as the NASA MEP iEPS module for high efficiency primary propulsion of a Cubesat. 

Flight propulsion modules of the iEPS thruster technology (Engineering modules shown).

early version	current version

The above images show the engineering unit of an early flight heritage module flown on two Cubesats, and a recent flight design scheduled for launch in fall 2016. These modules feature 8 thrusters each and are capable of attitude control, primary propulsion and self-neutralization.

The image below shows three single PCB propulsion modules developed for primary propulsion of Nanosatellites, targeting highest system compactness and overall efficiency. All images show the entire propulsion module including thrusters, propellant reservoirs and all necessary high voltage electronics.

S-iEPS propulsion module: A single axis, highly efficient and compact propulsion module developed during the NASA MEP program.

Electrospray thruster characterization

Electrospray emitter array and thruster characterization is performed using a variety of different techniques, to characterize the emission, including:

• Emitted/intercepted current versus applied emitter potential (IV): Determine voltage required to achieve necessary emission current. The ratio of the emitted current to the current intercepted in the extractor electrode determines the transmission efficiency of the system.
• Time-of-flight characterization (ToF): Mass sensitive beam diagnostics allows to determine the beam composition, and thus the average mass of the emitted beam. This allows indirect determination of the achieved specific impulse.
• Retarding Potential Analyzer (RPA): Studies the energy properties of the emitted particle beam, and allows calculation of the energy efficiency.
• Spatial Beam Scanning experiments: Scanning the spatial current distribution allows determining inefficiencies due to angular beam spreading.
• Extended duration firing tests: Measuring the propellant consumed during hundreds of hours of firing allows to indirectly determining the specific impulse, independently from ToF measurements.

Characterization results of full electrospray thruster arrays [1,2,3]

Typical iEPS IV curve showing small interception currents	Comparing IV curves for different emitter geometries
Typical angular beam distribution of emitted current	Energy distribution of emitted beam for different ionic liquids
Emitted current scanned over emitter array, from [4]

References

[1] D. Krejci, F. Mier-Hicks, R. Thomas, T. Haag, P. Lozano "Emission Characteristics of Passively Fed Electrospray Microthrusters with Propellant Reservoirs," Journal of Spacecraft and Rockets, Vol. 54, No. 2, 2017, pp. 447-458.

[2] D. Krejci, F. Mier-Hicks, C. Fucetola, P. Lozano, A. Hsu Schouten and F. Martel, "Design and characterization of a scalable ion electrospray propulsion system," IEPC-2015-149 34th INTERNATIONAL ELECTRIC PROPULSION CONFERENCE, Kobe, Japan.

[3] D. Krejci and P. Lozano, "Current Capabilities of Scalable Ionic Liquid Electrospray Thrusters for Nano-Satellites," 39th AAS Guidance & Control Conference, Breckenridge, CO.

[4]  C. Guerra-Garcia, D. Krejci and P. Lozano, "Spatial uniformity of the current emitted by an array of passively fed electrospray porous emitters," Journal of Physics D: Applied Physics, Vol. 49, No. 11, 115503 (12pp).