Among the small fleet of interplanetary spacecraft scattered around the Solar System is NASA’s Dawn mission to the largest asteroids Vesta and Ceres. What makes this spacecraft unique among currently operational interplanetary spacecraft is its high-efficiency ion propulsion system. Although this system only generates 90 millinewtons of thrust (equivalent to the weight of about two sheets of paper on Earth), it is an order of magnitude more efficient than conventional chemical propulsion systems. With its engine firing for months at a time, this ion propulsion system can change the velocity of the 1.2 metric ton Dawn spacecraft by 10 kilometers per second using only 425 kilograms of propellant. Without this ion propulsion system, Dawn’s mission not only to reach but also orbit two different asteroids in turn deep in the heart of the asteroid belt would be impractical. The success of Dawn’s ion propulsion system can be traced back to the first test of an ion engine in space which took place a half a century ago.

The Origins of SERT

The basic idea behind ion propulsion is fairly straightforward: a gas is ionized and the positively-charged ions are then accelerated by electrostatic or electromagnetic fields to high velocity to generate a reactive force or thrust. While the exhaust velocity can be huge (and hence the engine’s efficiency, or I sp , is very high), the drawback is that large amounts of electrical power are required to produce even small amounts of thrust – on the order of tens of kilowatts per newton of thrust. Taking into account the mass of the required electrical power generator, the thrust-to-mass ratio of ion engines is very poor compared to conventional chemical propulsion systems. But in the vacuum of space, ion propulsion systems operating for weeks or months at a time can easily outperform even the best hydrogen-oxygen rocket engine.

The first known published mention of ion propulsion was made in 1911 by the founding father of Russian cosmonautics, Konstatin Tsiolkovsky. American rocket pioneer Robert H. Goddard independently conceived of the idea of ion propulsion in 1906 and began his first laboratory experiments in 1916. With the beginning of the Space Age and the anticipated need for high-performance propulsion systems, work on developing working ion engines began in earnest.

While NASA had been performing ground-based ion engine tests at the Lewis Research Center (LRC) in Cleveland, Ohio since 1959 (now known as the Glenn Research Center after Ohio-native, astronaut John Glenn), it was quickly recognized that there were limitations on what could be done in a vacuum chamber. One of the key steps in ion engine operation is beam neutralization where electrons are injected into the departing ion stream to keep it and the engine electrically neutral. Without this key step, the ion engine and the spacecraft using it will build up a negative charge that will divert the flow of positively charged ions from the engine negating the thrust it produces.

Several methods had been developed and ground tested to neutralize the ion beam but there was still the possibility that electrons from the metallic walls of the vacuum test chamber or the thin plasma that filled the chamber during engine operation could be affecting the results. The only definitive means of verifying the effectiveness of the beam neutralization methods was to perform an experiment with an ion engine in the vacuum of space. In the summer of 1961, development of SERT (Space Electric Rocket Test) began under the direction of NASA’s Marshall Space Flight Center in Huntsville, Alabama with the primary goal of field testing beam neutralization. By the end of 1961, management had been transferred to the LRC with continuous work on building and launching SERT I starting in January 1963 after many months of preliminary study.

Two types of ion thrusters were to be tested on the SERT I mission. The first was a 19-centimeter electron-bombardment type engine built at LRC using mercury as a propellant. It would consume 1,400 watts of electrical power to generate 28 millinewtons of thrust with an I sp of 4,900 seconds. The second engine was a 10-centimeter contact-ionization type built by a division of Hughes Aircraft employing cesium as a propellant. This engine would consume 610 watts of electrical power to generate 5.6 millinewtons of thrust with an I sp of 8,050 seconds. While the thrust levels involved were tiny, these engines were up to 20 times more fuel efficient than the best chemical-based engines of that or even this era.

Given the modest goals of the SERT I test and the huge power demands of the ion engines to be tested, in the end it was decided to perform the test using a 170-kilogram spacecraft launched into a high suborbital trajectory employing the all-solid, four-stage Scout X-4. The Scout could provide up to an hour of flight time above an altitude of 250 nautical miles (463 kilometers) allowing a relatively simple bank of silver-zinc batteries with a mass of 34 kilograms to provide the power required for engine operation. This was more than adequate time to conduct the needed tests and was more cost effective than a longer orbital flight. The USAF was employing a similar approach with suborbital test flights of their ion engine development under Program 661A. Their first test flight, designated “Test Code A”, was launched on December 18, 1962 but failed due to a malfunction of the ion engine’s high voltage power supply.

Since the fourth stage of the Scout launch vehicle was spin-stabilized, the SERT I spacecraft was designed from the start to spin like a top. It consisted of a flat circular magnesium baseplate and pedestal with a sealed aluminum box on the top to house the batteries, power supplies, telemetry systems and command receiver. The ion thrusters were mounted on a pair of diametrically opposing arms that would deploy after separation from the Scout’s fourth stage.

Since measuring the tiny amount of linear acceleration produced from the firing of the ion engines would be very difficult during the relatively short test, the designers instead directed the engines to be parallel to the spacecraft’s spin direction. This configuration would alter the 85 RPM spin rate of the spacecraft allowing the more easily measured angular acceleration to be detected. Combined with an accurate measurement of the spacecraft’s moment of inertia from ground tests, the thrust of the ion engines as they fired one at a time could be accurately calculated. Three independent methods of measuring the spin rate of the test craft were employed on the SERT I mission. Two used solar cells to sense the passage of the Sun through the detectors’ field of view as the spacecraft spun while the third employed an accelerometer to sense changes in the centrifugal force as the rotation rate increased.

The Flight of SERT I

Scout X-4 number S124R lifted off on July 20, 1964 at 5:53:15 AM EDT from Launch Area 3A on Wallops Island, Virginia carrying SERT I (as a side note, this was the first launch from the then-new LA-3A). All four stages of the Scout operated as intended and SERT I separated from the last stage 4 minutes and 38 seconds after launch. Seconds later, the pair of arms holding the ion engines were successfully deployed. The first engine to be tested was the contact-ionization thruster system built by Hughes. Unfortunately high-voltage breakdowns occurred as the voltage ramped up causing the power supply to reset automatically before the required operating conditions were reached. After each reset, the power supply would shutdown then turn itself back on after one second followed almost immediately by another high-voltage breakdown. Believing that the problem was related to outgassing that would subside with time, attempts to start the engine ceased on ground command after 9 minutes and 11 seconds of attempted operation.

Next, the LRC-built electron-bombardment type thruster was powered up 13 minutes and 51 seconds after launch. About four minutes later, the engine started producing measurable levels of thrust. Over the next 16 minutes, the ion thruster was put through it paces under a range of operating conditions to measure the performance of the engine and the beam neutralizer system. When the beam neutralizer was shut down on ground command, the measured thrust of the ion engine dropped to zero as expected. And when the system was reactivated, the thrust level returned to normal indicating that the beam neutralizer was working as intended. At 36 minutes and 51 seconds after launch, the electron-bombardment thruster was commanded off after its series of successful tests.

Afterwards, a second attempt was made to start the uncooperative contact-ionization thruster. But once again, high-voltage breakdowns followed by automatic resets prevented the engine from operating and the start attempts ceased upon ground command after about two minutes. At 38 minutes and 42 seconds after launch, the electron-bombardment engine was restarted for additional tests. The engine was shutdown 46 minutes and 58 seconds and the mission officially completed after launch having reached a peak altitude of 1,004 kilometers during the flight. The SERT I payload was destroyed upon reentry over the Atlantic Ocean. With the end of its mission, SERT I became the first successful test of an ion engine in space beating out the second test of the USAF’s Program 661A, known as Test Code B, which was launched on a suborbital flight less than six weeks later on August 29, 1964.

While the electron-bombardment engine was producing thrust, the spin rate of SERT I had increased from 85 to 94 RPM confirming that the engine had performed as predicted. During a total of 31 minutes of powered operation, the thruster experienced 53 high-voltage recycle events from which it successfully recovered each time. These recycling events were largely expected to occur as various operating modes were tested and measurements of the ion beam were made just as had happened during ground testing of ion engines. One of the secondary objectives of the mission was also met with no electromagnetic interference effects noted on spacecraft systems during engine operation.

While the SERT I mission was largely successful and proved that the technique developed for ion beam neutralization worked, this was but the first small step towards building reliable ion engines that could operate for long periods of time in space. It would take another third of a century of research and development with another eight space-based tests (including a SERT II orbital test launched in 1970) and five major ground-based research programs before the technology was mature enough for the U.S. to deploy its first operational ion engine.

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“The First Nuclear Reactor in Orbit”, Drew Ex Machina, April 3, 2015 [Post]

General References

Ronald J. Cybulski et al., “Results for SERT I Ion Rocket Flight Test”, NASA Technical Note TN D-2718, March 1965

Harold Gold, “A Spacecraft for Ion Thrustor Flight Tests”, NASA Technical Memorandum TM X-52008, 1964

Harold Gold et al., “Description and Operation of Spacecraft in SERT I Ion Thrustor Flight Test”, NASA Technical Memorandum TM X-1077, March 1965

William C. Nieberding and Robert R. Lovell, “Thrust Measurement of SERT I Ion Thrustors”, NASA Technical Note TN D-3407, April 1966

James S. Sovey, Vincent K. Rawlin and Michael J. Patterson, “Ion Propulsion Development Projects in the U.S.: Space Electric Rocket Test I to Deep Space 1”, Journal of Propulsion and Power, Vol. 17, No. 3, pp. 517-526, May-June 2001