Electrospray Propulsion Systems for Small Satellites

Douglas Spence, Eric Ehrbar, Nate Rosenblad, Nate Demmons, Tom Roy, Samuel Hoffman, Dan Williams, Vlad Hruby Busek Co. Inc. 11 Tech Circle, Natick, MA 01760; 508-655-5565

Chris Tocci ALFA, LLC 87 Plymouth St., Halifax, MA 02338

Distribution A: Approved for public release, Distribution Unlimited The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government

Propulsion systems for small spacecraft are not simple reductions in SWAP and propulsion capability from larger spacecraft. New ways of looking at the complete spacecraft system must be employed.

• Busek developed heuristic for evaluating capability of propulsion systems for spacecraft at the CubeSat level • Examples of selected propulsion technologies are presented for illustration and Busek’s different propulsion solutions for CubeSat- scale spacecraft • Brief overview of the DARPA Phoenix objectives and how the rocket equation heuristic, plus additional system considerations, led to selection of electrospray propulsion for Phase 1 development

• Challenge for very small spacecraft is not delta V but available mass and volume • Mass ratio (initial mass/final mass) has much greater influence than the specific impulse. 푚푓 • ∆푉 = −퐼푠푝푔0 ln 푚푖 • Propulsion engineers are “in love” with 퐼푠푝. • Provides a simple basis of comparison for various propulsion Illustration of effect of mass ratio technologies but how does this (initial mass/final mass) and Isp help select a useful propulsion upon ΔV system?

For small satellites Busek spans the spectrum:

• Electrospray (Pressure feed and Passive feed) • Micro pulsed plasma (for ACS and translation) • Microresistojet (MRJ) • RF Ion • Miniature green monopropellant chemical rocket

All are capable of (or nearly) a mass of 1kg and size 1000cm3, with some already at 500g and 500cm3, and may soon reach 100g and 100cm3

Consider the following Busek thrusters:

• Ionizes and accelerates droplets and/or ions from a liquid surface using strong electric field • Very high thrust/power and low absolute power requirements • Greatest ∆V potential capability of any propulsion technology for small spacecraft due to small thruster size • Electronics (PPU, DCIU, HSKP) advancing rapidly requiring only two 9 x 9 cm boards • First space qualified units delivered to NASA/JPL/ESA for ST-7 LISA Pathfinder • DARPA Phoenix Project Phase I

Unpressurized (wicking feed) electrospray thruster. Unit shown

is 7.5 x 6.5 x 1cm. ≈ 1.25cm≈ Heater

PPU/DCIU Board

Next-generation 100g/100cm3 with 100µN, 180N-s Impulse

• Operates by discharging an arc across the surface of Teflon at end of stick • No tank or feed system, only electronics and propellant rods • Small and easily distributed within a small volume envelope and at various thrust vectors for ACS or bundled for translation. • Exceptionally efficient propellant loading • Low efficiency but low power little as 2W • Low-cost and easy-to-integrate propulsion system • Selected for AFRL FalconSat - 3 mission launched in 2007 and still operational

• Operates by heating a gas • 1U system with primary propulsion and 8x cold gas ACS thrusters

2X Valves

1 cm RF Ion thruster Green monopropellant thruster. (thruster head only, also 3 and 7 cm) app. 1” dia. x 4” long

For small satellites Busek spans the spectrum - Producing ∆푉 ‘s (sec) (for 4kg SC)

• Electrospray (Pressure feed and Passive feed) 151, 76 • Micro pulsed plasma (for ACS and translation) 65, 63 • Microresistojet (MRJ) 60 • RF Ion 244 • Miniature green monopropellant chemical rocket 130

A comparison technique which includes mass (as a minimum) is required

Comparison of Propulsion Systems by Total • Get over “love affair” with Impulse vs. Wet Mass 퐼 . 2000 푠푝 1800 1600 • Wet Mass will be about the

s) 1400 - 1200 same as the Dry Mass. 1000 800 • The comparison of Total

600 Total Impulse (N Impulse Total 400 Impulse vs. wet mass 200 (initial mass) is instructive 0 for selection. 0 500 1000 1500 2000 Wet Mass (g) Electrospray (current 0.7 mN) Electrospray Next Generation (100 µN) µPPT (current) µPPT (next generation) RF Ion (100 µN) Mini Cold-Gas (10-50mN)

“The goal of the Phoenix program is to develop and demonstrate technologies to cooperatively harvest and re-use valuable components from retired, nonworking satellites in GEO and demonstrate the ability to create new space systems at greatly reduced cost. Phoenix seeks to demonstrate … robotically removing and re-using GEO- based space apertures and antennas from de-commissioned satellites in the graveyard or disposal orbit. The Phoenix program envisions developing a new class of very small ‘satlets,’ similar to nano satellites, which could be sent to the GEO region more economically as a “ride along” on a commercial satellite launch, and then attached to the antenna of a non-functional cooperating satellite robotically, essentially creating a new space system. ….”

Source: http://www.darpa.mil/our_work/tto/programs/phoenix.aspx

Busek initial proposed propulsion configuration

De-commissioned Satellite

Satlet = Cellularized Satellite

Radiation-tolerant 0-10kV 10W Electrospray PPU

≈ Ø2.5”

Thruster/ Feedsystem +Propellant

Proposed autonomous electrospray propulsion satlet, with reduced propellant loading but greater flexibility.

• Very different considerations must be applied to small spacecraft propulsion systems. Systems must be optimized for efficiency not simple reductions in SWAP. This implies new ways of looking at the complete spacecraft system where the initial mass is close to final mass. • A Busek developed heuristic can be applied for evaluating propulsion systems based on Total Impulse / kg. • Demonstrated Busek’s extensive work in small satellite propulsion technologies and Busek’s different propulsion solutions for CubeSat- scale spacecraft. • Electrospray thrusters may be the solution for the DARPA Phoenix mission to repurpose an antenna.