Space Systems

At present there exists no single propulsion technology capable of meeting both orbit-raising and stationkeeping requirements for communications satellites. Contemporary satellites thus carry two completely separate propulsion systems and incur the mass penalty associated with each. It is conceivable that a single propellant could be used for both a high-thrust chemical thruster that performs orbit raising as well as a high-specific-impulse electric thruster to perform stationkeeping. In this case the mass burden on the spacecraft bus would be significantly reduced simply by eliminating one of the large propellant storage vessels. Although no single propellant has been identified yet, recent developments suggest that it is possible to synthesize custom nanostructured propellants that could be used in both chemical combustion-based thrusters as well as electrostatic thrusters.

The goal of work proposed here is to fabricate and implement an experimental test bed that is capable of fully characterizing the electrostatic acceleration and chemical combustion of nanostructured propellants for spacecraft propulsion. The instruments will be developed through collaboration between Michigan Technological University and the University of Maryland. The proposed test-bed represents the first-ever facility devoted to combustion and electrospray research on nanostructured colloidal propellants.

The investigators on this project together are presently engaged in DoD research with total funding exceeding $4M. Agencies supporting this research include Office of Naval Research, Air Force Office of Scientific Research, and the Defense Threat Reduction Agency. All of this work is devoted to studies, in one form or another, of the behavior of nano-structured propellants for either chemical or electric propulsion. The test equipment proposed here will be directly relevant to these on-going activities.

The investigation of a low cost SEP microsatellite capable of launching within an ESP A class rideshare opportunity. The research is focused is on optimizing the low-thrust trajectory to minimize satellite hardware and operation costs while maintaining the ESPA class requirements. In particular, the solar array and mission operation costs as a function of array power and inclination of the spacecraft orbit as it transits the radiation belts. As power increases, trip time and thus radiation exposure and operation costs decrease. However, large solar arrays increase cost and the mass reduces payload capability. Also, varying inclinations follow different trajectories through the radiation belts as they reach their final orbits. Trajectories that start at a higher inclination have a longer path, but pass through lower intensity radiation bands. This can decrease the size and costs of the solar arrays, but requires larger Δ Vs and reduces payload capabilities

Tasks and Deliverables:

1. Provide a trajectory optimization of the SolRider vehicle to minimize cost of the following orbit transfers:

2. Analysis should consider the following variables/factors. Values are provided in the Solar Rider specification.

a. Radiation degradation of solar arrays

b. Solar Array $/W

c. Solar Array kg/W

d. Propellant & Tank Mass

e. Mission operations cost: $/day

f. Thruster shut down and start up due to eclipse

g. Atmospheric Drag

3. Deliverables:

a. For each transfer orbit:

i. Normalized Cost vs. Array Power

ii. Trip Time vs. A1Tay Power

iii. Payload Mass v. Array Power

iv. Radiation Degradation v. Array Power

b. For the LEO to GEO transfer orbit

i. Normalized Cost v. Initial Inclination

ii. Trip Time v. Initial Inclination

iii.. Payload Mass v. Initial Inclination

iv. Radiation Degradation v. Initial Inclination

c. Input files

d. Raw Output Data

This five year research program consists of research in three areas: electrospray propulsion, field-reversed configuration devices, and non-invasive plasma optical diagnostics.

1. Electrospray Propulsion

MTU has pioneered a unique micromachining technique for fabricating all-metal electrospray structures for space propulsion. Unlike devices built using silicon MEMS protocols, the all-metal emitters are suitable for use with reactive propellants such as AF315. The all-metal structures can also tolerate very high temperatures. The electrode needles in this array are solid tungsten, while the substrate is solid molybdenum. MTU will investigate the use of externally wetted emitters - as well as internally wetted capillaries for use as electrospray sources for space propulsion. Research will focus on the use of AF315 as a propellant. Investigations will address fabrication challenges, device performance, plume characterization, and spacecraft interaction.

2. Field-Reversed Configurations

The FRC presents a unique plasma geometry that is well suited for plasma propulsion.

Because the FRC plasmoid is not linked to the structural magnetic flux, the plasma is not attached to the open field lines and can be ejected from the formation region as a self-contained entity. Furthermore, FRCs result from inductive "electrodeless" formation, avoiding the failure mode imposed by electrode erosion that is common to contemporary plasma thrusters. Motivated by the surprising stability of FRCs and their ability to translate while remaining coherent, MTU and the Air Force Research Laboratory (AFRL) at Edwards AFB will collaborate on an investigation into the feasibility of FRC space propulsion. MTU has designed and built a unique co-axial FRC mounted to a large expansion chamber that will be used for MACEEP research.

The goal of MACEEP research will be to determine the optimal physical and electrical configuration of a coaxial FRC for space propulsion and to characterize the conversion of stored electrical energy into plasmoid kinetic energy during translation and ejection. MTU researchers will employ a number of diagnostic techniques - to include magnetic field probes, electrostatic probes, and optical diagnostics - to quantify the energy conversion process during FRC formation and ejection. Thrust will be estimated from exhaust plume properties.

3. Non-invasive Optical Diagnostics

MTU is currently developing an optical diagnostic technique that can, for the first time, obtain direct measurements of electron density and electron energy distribution function (EEDF) within the discharge chamber and near-field plume of a Hall thruster. The MTU technique uses a

1,000-mJ-per-pulse Nd-YAG laser to induce laser Thomson scattering (LTS) from the free electrons in a plasma. The scattered radiation is measured using a triple-grating spectrograph and electron-multiplied CCD camera with single-photon detection capability. The scattered spectra can directly provide density and EEDF without perturbing the plasma. Researchers will use MACEEP funding to demonstrate this technique and apply the measurement to Hall thrusters and FRC plasmas. It is anticipated that knowledge gained from LTS measurements will provide insight into Hall thruster cross-field mobility as well as FRC internal energy storage and conversion.

The term "electrospray" refers to the charged droplets and molecular ions that are emitted from the meniscus of a conducting liquid due to a strong electric field. The applied electric field induces a layer of surface charge on the liquid resulting in an electrostatic stress on the surface; if the stress is strong enough to overcome liquid surface tension the result is an ejected beam of charged particles. In 2013 Meyer and King demonstrated a completely new mechanism of electrospray that is fundamentally different from anything observed to date. This new electrospray was enabled by a unique and exotic fluid that was synthesized for the first time by Jain and Hawkett in 2011. This new fluid is called an ionic liquid ferrofluid (ILFF), which is a superparamagnetic, electrically conductive, room-temperature molten salt. Because the ILFF is superparamagnetic it can be stressed by magnetic fields; because the ILFF is also electrically conductive it can be stressed by electric fields. Meyer and King induced a magnetostatic instability in the ILFF surface known as a Rosensweig instability, which caused a regular array of static fluid spikes in the free surface. An applied electric field then further stressed and deformed the magnetically formed peaks causing their amplitude to increase and tip radius to decrease. A sufficiently strong electric field was shown to cause electrospray emission from the tip of each spike in the array.

This combined magneto-electrostatic instability has never before been observed and hence no description is available. The phenomenon was observed in two different fluids and under various values of applied electric field, but systematic investigation of the effect has not yet been addressed. This document proposes an integrated experimental, theoretical, and numerical investigation of electrospray from Rosensweig-Taylor instabilities that is designed to uncover the governing processes in this new phenomenon. The goal of work proposed here is to quantify how the combination of electric and magnetic surface stress components affect electrospray behavior in a superparamagnetic liquid. A two year research effort is proposed to

• Design and fabricate an apparatus that isolates a single Rosensweig-Taylor tip and permits controlled variation of both electric and magnetic field

• Measure rheology, surface tension, magnetic nanoparticle concentration, and electric conductivity for each ionic liquid ferrofluid tested

• Measure the critical voltage required to induce electrospray from a magneto-electrostatic instability as a function of (I) fluid magnetization, (2) magnetic field strength, (3) and fluid conductivity

• Establish the relationship between applied voltage, spray current, and total spray mass flow as a function of magnetic field strength

• Measure the angular divergence of the emitted electrospray beam as a function of electric and magnetic field strength

• Numerically model the fluid emission site using molecular dynamics simulations

• Measure the mass-to-charge ratio of emitted products using mass spectrometry

• Attempt a theoretical description of the static instability as a function of fluid parameters and electric/magnetic field strength.