David Q. King

Preliminary Assessment of the Role of a Conducting Vacuum Chamber in the Hall Effect Thruster Electrical Circuit

The role of the electrically conductive vacuum chamber wall in the completion of the discharge circuit of a Hall effect thruster (HET) is experimentally investigated. The Aerojet Rocketdyne T-140 laboratory-model HET operating at a discharge voltage of 300 V, discharge current of 5.16 A, and anode flow rate of 5.80 mg/s serves as a representative HET test bed. The nominal facility operating pressure during thruster firings is 4.9 × 10 -6 Torr corrected for xenon. Two 0.91 m x 0.91 m square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the stainless steel vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along thruster centerline and 2.3 m radially outward centered on the exit plane. The plates are configured in three distinct electrical configurations with corresponding measurements: a) electrically grounded plates with measurements of currents to ground, b) electrically isolated plates with measurements of floating voltages, and c) isolated but electrically connected plates with measurements of the current conducted between them. The measurements are all taken simultaneously with the discharge current oscillations of the thruster at a sampling frequency of 100 MHz. Measurements of the current conducted to ground in the electrically grounded configuration reveal that the axial and radial plates collect ion currents that are 13.6% and 10.7% of the discharge current, respectively; the collected current is coupled to the discharge current oscillations but is smaller in magnitude and phase-delayed. In the electrically connected plate configuration, 5.5% of the average discharge current is observed to flow from the axial plate to the radial plate driven by a floating voltage difference of 7.6 V; this current is uncorrelated in time with the discharge current oscillations. These results indicate that the vacuum chamber conducts current and is a recombination site for a significant number of plume ions during HET operation.

Solar Electric Propulsion (SEP) Benefits for Near Term NASA Exploration

Aerojet Rocketdyne has completed mission analysis trade studies to support near-term NASA cislunar exploration mission concepts by comparing cargo delivery to the EarthMoon Lagrange points (EML1 or EML2) using a solar electric propulsion (SEP) stage (or “tug”) versus the most efficient all-chemical approach. The study examined the relationship between total delivered mass to the destination, trip time required, and power level at the SEP thrusters and focused on flight-proven performance regimes for thruster and solar array performance. The results show how SEP enables the delivery of significantly more total mass versus all-chemical approaches, with the benefits increasing as SEP trip time is allowed to increase. SEP solutions can actually deliver up to twice as much mass to EML2 versus a chemical solution or the same mass on a lower cost launch vehicle. Aerojet Rocketdyne also examined the relationships between transfer time, delivered cargo, and campaign costs versus chemical solutions, finding that incorporating SEP tugs in the campaign can reduce cost per delivered cargo mass (

STATUS OF ION PROPULSION SYSTEM DEVELOPMENT AT AEROJET REDMOND

/kg) by over 50% compared to chemical. The results showed that SEP tugs reduce the number of launch vehicles required for the campaign, which is by far the largest campaign cost driver. Overall, using SEP tugs for space exploration and cargo delivery dramatically increases mission flexibility over allchemical solutions, and can enable significant campaign savings.

Ion flux, energy, and charge-state measurements for the BPT-4000 Hall thruster

Aerojet General Corporation, Redmond has significantly increased its activity in the development of manufacturable ion propulsion systems in the past 2 years. Aerojet successfully fabricated and demonstrated a 30 cm ion thruster in 1994; completed a system design for a low mass 500 W-class ion system in 2000; and provided ion thruster firing and vibration testing support to two IMWG carbon-carbon grid programs from 2001 to present. Now, Aerojet is supporting three major high power ion system development programs with NEXT, NEXIS and HiPEP. Under NASAs Evolutionary Xenon Thruster (NEXT) program with NASA-GRC, Aerojet has designed, built, tested and delivered a breadboard Propellant Management System (PMS) and digital control system, as well as assisted in the assembly of a 40-cm Engineering Model thruster at GRC. In the upcoming Phase 2 NEXT program, Aerojet will design, fabricate and deliver two Prototype Model 40-cm thrusters; and design, fabricate, test and deliver an Engineering Model single string and three-string PMS. On the Nuclear Electric Xenon Ion System (NEXIS) program with JPL, Aerojet is currently designing a flightweight Development Model (DM) 65-cm thruster, and will fabricate, test and deliver three DM thrusters in 2004 and 2005. Finally, on the High Power Electric Propulsion (HiPEP) program with NASA-GRC, Aerojet has completed the preliminary design of a novel high-voltage feed system and will begin design and fabrication support of the thruster in the upcoming Phase 2 program. This paper provides an overview of Aerojet activity in all of these ion system areas.