Colleen Marrese-Reading

The VHITAL Program to Demonstrate the Performance and Lifetime of a Bismuth-Fueled Very High Isp Hall Thruster

In the Very High Isp Thruster with Anode Layer (VHITAL) Program the performance, plume and lifetime capability of the radiatively-cooled two stage, bismuth-fueled VHITAL-160 will be characterized in the US and Russia.

Emission statistics and the characterization of array current

The current from an array of field emitters is often dominated by a subset of tips. The impact of processes such as sputtering damage on the evolution of array current can be estimated based on a hyperbolic tip model if the distribution of the emitters in both work function and apex radius can be approximated. In this work, we describe the characterization of emission under the assumptions that (i) the emission site radii are log-normal distributed and (ii) that the degree to which work-function-increasing adsorbates are present is related to background pressure. These assumptions generate statistical factors that may be used to generalize a single tip theory to array current estimates via the statistical hyperbolic model. A derivation of the statistical factors are given and the methodology of the hyperbolic model’s application explained. The methodology is then applied to analyze experimental data for molybdenum field emitters coated with ZrC under development for spacecraft electric propulsion and elec...

Integrated Liquid Bismuth Propellant Feed System

A prototype bismuth propellant feed and control system was constructed and operated in conjunction with a propellant vaporizer. An electromagnetic pump was used in this system to provide fine control of the hydrostatic pressure, and a new type of in-line flow sensor was developed in an attempt to provide an accurate, real-time measurement of the mass flow rate. High-temperature material compatibility was a driving design requirement for the pump and flow sensor, leading to the selection of Macor for the main body of both components. Post-test inspections of both components revealed no degradation of the material. In separate proof-of-concept experiments, the pump produced a linear pressure rise as a function of current that compared favorably with theoretical pump pressure predictions, with a pressure of 10 kPa at 30 A. Preliminary flow sensor measurements have been made at a bismuth flow rate of 6 mg/s ± 6%. A real-time controller was successfully used to control the entire system, simultaneously monitoring all power supplies and performing data acquisition duties.

The Development of a Bismuth Feed System for the Very High Isp Thruster with Anode Layer (VHITAL) Program

A bismuth feed system is under development for the VHITAL Program to deliver 8-12 mg/s of bismuth vapor to the VHITAL-160 with continuous flow rate monitoring and flight-compatible concept components. The system components include a bismuth reservoir, an electro-magnetic pump for bismuth liquid pressure control, a hot-spot flow sensor to measure the flow rates in the liquid phase of the feed system and a vaporizer to control vapor flow rates to the thruster. The feed system components have been integrated and together demonstrated >7 mg/s at ~350 W.

Colloid thruster propellant stability after radiation exposure

We present a characterization of the stability characteristics of the ionic fluid EMI-Im to the operation of colloid microthrusters in gamma radiation environments.

Mission enabling and enhancing spacecraft capabilities with microNewton electric propulsion

The capability to significantly improve current spacecraft pointing, precision orbit maintenance and disturbance mitigation were considered using precision, quiescent microNewton electric propulsion systems.1,2 The results of the analyses showed that electric propulsion systems operating in the microNewton to hundreds of microNewtons thrust range can offer significant improvements over state-of-the-art mission capabilities to enable 30 m Earth-fixed orbital tubes, constellation spacecraft position control to within nanometers and exoplanet observatory pointing with 0.1 milliarcsecond precision. Electric propulsion thrust characteristics required to support these capabilities are discussed.

Spacecraft propulsion applications for field emission cathodes

Many small-scale spacecraft propulsion systems under development could benefit from a compatible FE cathode. These thrusters challenge the performance of FE cathodes because of the plasma environment they create could significantly limit the cathode lifetime if it does not have the proper configuration and materials. The thruster current requirements and cathode integration challenges are discussed in this article. Cathode configurations are recommended to improve the cathode and thruster compatibility. Preliminary results are presented from the first successful test of an electric thruster with a Spindt-type FEA cathode neutralizer.

Orion spacecraft nominal and contingency earth landing retro rocket system options

Retro rocket Earth landing system architectures for the Orion crew module were developed for the Constellation Program during the Landing System Advanced Development Project design trades. The architectures include both variable and fixed impulse base-mounted solid-propellant retro rocket motors for vertical and horizontal landing velocity control. Monte Carlo performance analyses were conducted, detailed motors were designed and support system configurations were developed for mass and performance characterization. Preliminary penetration schemes for blow-out ports through the TPS were designed for the motor exhaust plumes. The requirements, configurations, elements, mass and performance of the retro rocket landing system architectures for multiple landing scenarios are discussed.

Retro rocket plume actuated heat shield exhaust ports

A preliminary scheme was developed for base-mounted solid-propellant retro rocket motors to self-penetrate the Orion Crew Module heat shield for configurations with the heat shield retained during landings on Earth. In this system the motors propel impactors into structural push plates, which in turn push through the heat shield ablator material. The push plates are sized such that the port created in the ablator material is large enough to provide adequate flow area for the motor exhaust plume. The push plate thickness is sized to assure structural integrity behind the ablative thermal protection material. The concept feasibility was demonstrated and the performance was characterized using a gas gun to launch representative impactors into heat shield targets with push plates. The tests were conducted using targets equipped with Fiberform® and PICA as the heat shield ablator material layer. The PICA penetration event times were estimated to be less than 30 ms from the start of motor ignition. The mass of the system (not including motors) was estimated to be less than 2.3 kg (5 lbm) per motor. The configuration and demonstrations are discussed in this paper.

Plasma Potential Measurements in the Plume of a Colloid Micro-Newton Thruster

The plume produced by a single-emitter colloid micro-newton thruster was interrogated for potential field characteristics using an emissive probe. Plasma potential distributions in the plumes were characterized for 2-4 kV beam energies with various thruster electrode potentials and beam target potentials. A two-dimensional translation stage allowed both radial and angular profiles to be obtained. The results show that beam targets can significantly affect the plasma potential measurements in these low charge density plumes. It is shown that secondary electrons from the beam target will partially neutralize the positive charge density in the plume unless they are suppressed. The measurements also showed that biasing the outer beam target grid to 100 V significantly broadened the beam potential profile. The presence of a low-energy ion population is suggested to explain the results from varying beam target screen potentials. The measured plasma potential profiles indicate potentials between 0-30 V within an unneutralized beam that maintains a constant expansion angle past 10 cm downstream of the thruster head.