Advances in Electric Propulsion

By Michael Arnold

Rocket Propulsion ASEN 5053 Prof Lakshmi Kantha NASA has made many advances in electric propulsion within recent history. NASA’s use of electric propulsion began with the flying of Deep Space 1. This marked the first use of an electric propulsion engine on a NASA mission as the primary means of propulsion. The lessons learned from the Deep Space 1 mission were then applied to a more advanced ion engine called NEXT. Then NASA decided to begin work on Hall thrusters for use on outer solar missions. The introduction of the JIMO mission also added to the development of two new ion thrusters that use very high power. It has been an interesting time in the world of electric propulsion at NASA.

Deep Space 1 was the first mission flown by NASA that uses electric propulsion as the primary drive system. It was launched on October 24, 1998 and turned off on December 18, 2001. This mission was designed as a proof of concept for many new technologies including the NASA Solar Electric Propulsion Technology Readiness

(NSTAR) electrostatic ion thruster. The thruster provided between 19 and 92.7mN of thrust using a 30 cm grid ion engine. The input power was between 423 and 2288 W with an Isp between 1814 and 3127s. The mission flew as expected with no major errors which helped pave the way for the advancement of electric propulsion in the United

States. Images of the NSTAR engine can be seen below including one of the testing of the engine.

Figure 1:NSTAR During Testing

Figure 2: NSTAR before launch

Nasa’s Evolutionary Xenon Thruster (NEXT) was started in 2002 and was originally planned as a part of the SEP mission to Titan. After the mission was cancelled by NASA, the project was continued in order to explore the lessons learned from the

Deep Space 1 mission and develop an ion engine capable of a higher input power, higher thrust capability, greater throttle range, and higher throughput. The development of the engine is being lea by the NASA Glenn Research Center with the assistance of Aerojet, the Jet Propulsions Laboratory (JPL), and L3 Elctron Technologies.

In order to accomplish the requirements, Aerojet and NASA engineers designed a

40cm ion system that operates on xenon with an input power between 0.54 and 6.9 kW with a max Isp of 4170s. The thruster uses 2-grid ion optics with a 36 cm beam diameter.

2 Phase 1 of the project consisted of demonstrating component and system technology at a breadboard level. Phase 1 was successfully completed and phase 2 was begun in October of 2003. Phase 2 consists of developing flight-like engineering components that are tested to validate the technology and approach. Images of the NEXT thruster can be seen below.

The image on the left is of the thruster during assembly while the image on the right is of the thruster during testing.

Figure 3: NEXT After assembly

Figure 4: NEXT During testing

3 The High Voltage Hall Accelerator (HiVHAC) Development Project was started in May of 2003 after an evaluation of the NSTAR and NEXT projects compared to Hall thrusters revealed that the use of a Hall thruster could reduce the travel time or increase the payload of deep space missions to Saturn and Neptune. HiVHAC was initially a six to eight kW option for the NEXT mission, but after a review of the project determined that low power engines would be better suited for NASA missions, it was redirected to a three kW Hall thruster design for deep space missions.

The system was designed by scientists at Aerojet in Redmond, WA and the

NASA Glenn Research Center in Cleveland. The main focuses of the design were to increase efficiency at low power and minimize the recurring costs. The increase in efficiency came from the development of an optimized magnetic circuit, minimizing the power consumption of the electromagnets, and reducing the amount of propellant needed for operating the hollow cathode. The recurring costs were reduced by decreasing the amount of parts and making the parts that were needed with simplified manufacturing.

The final design of the thruster called for a 77mm outer discharge channel diameter which gave the engine the designation NASA-77M. The 6.35mm cathode used was based on the NEXT neutralizer cathode with modifications for low power usage. Images of the

HiVHAC thruster can be seen below. The image on the left is of the thruster on its test stand while the image on the right is of the thruster being tested.

Figure 5: HiVHAC In testing chamber

Figure 6: HiVHAC During testing

HiVHAC was constructed by Aerojet and delivered to the NASA Glenn Research

Center in April of 2005. Testing was then conducted on HiVHAC in a vacuum chamber to simulate the space environment. The performance testing was completed in August of

2005 in which the system was run at varying input powers ranging from 200 to 2900 watts. The resulting specific impulse of the engine was found to range between 1,000 and

2,800 seconds which met the design requirement of 2200 to 2800 seconds. The next stage of testing has begun on HiVHAC which will test the long duration of the spacecraft.

The proposal of the Jupiter Icy Moon Orbiter (JIMO) in 2002 lead to the development of two ion propulsion engines by NASA with requirements of a greater than

20 kW engine with a specific impulse between 6000 and 9000 seconds that is capable of

5 greater than 2000 kg throughput of propellant. This mission proposed the use of a nuclear reactor in order to power the engine so the power requirements could be achieved.

The first engine was the High Power Propulsion Program (HiPEP) lead by the

Glenn Research Center. The HiPEP engine has a square configuration in order to allow for straightforward scaling. The engine grid was flat and made from pyrolytic graphite. It is designed to operate with a current density of 1mA.cm2 at a specific impulse of 7500s.

The engine must be able withstand a 2000 hour wear test. This test was completed in the summer of 2005. The average specific impulse was found to be 7650 s, with an average beam power of 20.8 kW, and average beam current of 3.6 A. The wear on the ion optics was seen to agree with the pre-test predictions. The images below show the HiPEP thruster prior to the wear test. They also show the advance of wear on the optics during testing.

Figure 7: HiPEP Thruster Before Testing

Figure 8: Downstream surface of accelerator before testing

Figure 9: Downstream surface of accelerator after testing

The second engine developed was the Nuclear Electric Xenon Ion

System(NEXIS). It was designed to operate with power levels between 16 and 20 kW, impulses between 6000 and 7500s, a current density between 1.6 and 1.7 mA/cm2, and have a burn time of up to ten years. The greatest difference between the two thrusters was in their shape. While HiPEP used a rectangular thruster, NEXIS used a more traditional circular thruster. The grid was circular, with a curved surface, 65cm diameter, and manufactured from carbon-carbon. The assembled engine can be seen in the figure below.

Figure 10: Assembled NEXIS thruster

The engine must also be able to with stand a 2000 hour wear test. This was completed in August of 2005. The specific impulse was found to be between 6040 and

7590s with a thrust between 403 and 473 mN. The total efficiency was found to be between 85.2% and 85.6%. The beam current was between 4.00 and 4.30 A. These results show that the thruster satisfied the requirements of the mission. The picture below shows the NEXIS engine being tested.

Figure 11: Testing of NEXIS engine

8 These two projects have later been combined into one large project so that the best of both system can be utilized. It is unfortunate that the JIMO mission was cancelled due to problems finding a launch vehicle capable of lifting the mission to space. The advances made in meeting the propulsion requirements seem to have made this mission feasible once it is in orbit.

SMART-1 was launched by the European space agency on September 27, 2003 and sent to the moon. SMART-1 uses a Hall effect thruster with a specific impulse of

1640s. The engine uses less than 500 W of power. It is currently in orbit around the moon and has demonstrated Europe’s advantage to the US in the field of electric propulsion.

NASA is making clear progress in the field of electric propulsion. With all of the new engines that are currently under development it should not take long for the United

States to take the lead in this field. NASA’s ability to learn from its previous missions and apply them to new needs is something that NASA has seemed to lack in the past. The introduction of Hall thrusters should make us competitive to the rest of the world. These truly are interesting times in the world of electric propulsion.