NASA’s SERT II: ‘A Proper Spot Success Epic’

The Long Road from Theory to Orbit

The concept of electric propulsion dates to the early twentieth century, but practical application awaited the space age. Unlike chemical rockets that produce high thrust for short durations, ion thrusters generate tiny amounts of thrust by accelerating charged particles to extreme velocities. Over months or years of continuous operation, they can propel spacecraft to very high speeds while consuming a fraction of the propellant required by conventional systems.

In the late 1950s, researchers at NASA Lewis—now NASA Glenn—began systematic investigation of electrical propulsion types. The work accelerated in the early 1960s with the establishment of the Electromagnetic Propulsion Division and construction of advanced vacuum facilities, notably the Electric Propulsion and Energy Laboratory (EPPL). Harold Kaufman's electron bombardment ion engine, using liquid mercury as propellant, emerged as the leading candidate.

SERT I: The First Proof in Flight

Before SERT II could attempt extended orbital operations, the thruster needed validation in the space environment. On 20 July 1964, a suborbital SERT I flight lasting 50 minutes carried Kaufman's engine to space. It became the first ion thruster to operate outside Earth's atmosphere, confirming basic functionality but leaving critical questions unanswered: could the system survive months of continuous operation? How would it interact with other spacecraft systems? Would solar arrays degrade under sustained thrusting?

SERT II: A Complete Mission Under One Roof

Approved in August 1966, SERT II represented a rarity in spaceflight: an entire mission—launch vehicle, propulsion system, spacecraft, and control center—managed by a single NASA center. Lewis engineers developed both the ion thruster system and the spacecraft bus simultaneously, a Thorad-Agena rocket configured for a circular polar orbit that provided optimal solar illumination for the 5-by-40-foot solar array, the largest NASA had flown to that date.

Mission Manager Raymond Rulis described the launch as "one of the smoothest operations I've seen." The spacecraft reached its intended orbit, and on 14 February 1970, controllers activated the primary thruster for its planned six-month operational test.

Early Setbacks, Enduring Success

The primary thruster operated for five months before an electrical short in the accelerator grid caused failure on 22 July 1970. The backup thruster was activated two days later and ran for three-and-a-half months before suffering a similar failure in mid-October. Neither thruster achieved the full six-month target, but both operated long enough to validate ground-test data from the EPPL vacuum tanks.

Crucially, the mission demonstrated that thruster operation had no adverse effects on spacecraft systems or solar arrays—a significant concern at the time. The mission might have ended there, but Lewis engineers had other ideas.

Eleven Years of Unexpected Science

In 1973, controllers reactivated SERT II to demonstrate cathode restarting capability. The following year, they diagnosed and resolved an electrical short in one thruster. During periods of intermittent sunlight, operators proved the thruster could restart with less than an hour of power available. When the spacecraft returned to full sunlight in 1979, the team conducted over 500 restarts, accumulating 18,000 hours of operation before propellant exhaustion in spring 1981.

Over eleven years, SERT II delivered data on hundreds of thruster restarts, restarts after shutdowns lasting up to 18 months, ion beam neutralisation between thrusters, and the discovery of a previously unknown plasma thrust mode. An auxiliary micrometeoroid experiment on the solar mirrors provided a bonus: in 1990, after twenty years in orbit, researchers confirmed no degradation of the mirrors' optical properties.

Direct Lineage to Modern Missions

Technological elements of the SERT II thruster system were incorporated into subsequent generations of ion thrusters. By the time SERT II was terminated, Lewis was already ground-testing thrusters twice the size. The center continued leading NASA's electric propulsion efforts, developing the NASA Evolutionary Xenon Thruster (NEXT) family that powered the Deep Space 1 and Dawn missions.

Today, that lineage extends to the Artemis programme. NASA Glenn recently tested the Advanced Electric Propulsion System (AEPS) thrusters that will power the Power and Propulsion Element of Gateway, the lunar-orbiting space station central to sustained Moon exploration. Each AEPS thruster delivers 12 kilowatts—orders of magnitude beyond SERT II's modest output, but built on the same fundamental principles validated in 1970.

Commercial and Strategic Implications

The SERT II legacy extends beyond NASA. Commercial satellite operators now routinely use electric propulsion for station-keeping and orbit raising, reducing launch mass and cost. Boeing's 702SP platform, SpaceX's Starlink satellites, and OneWeb's constellation all employ ion or Hall-effect thrusters descended from the technology SERT II proved viable.

For procurement and supply-chain professionals, the electric propulsion market represents a growing sector. The global electric propulsion satellite market was valued at approximately $7.3 billion in 2023 and is projected to exceed $12 billion by 2030. Component suppliers—xenon feed systems, power processing units, thruster gimbal mechanisms—are increasing demand as both government and commercial operators adopt electric propulsion for missions ranging from geostationary telecommunications to interplanetary science.

What this means

SERT II's story is one of engineering persistence rewarded. What began as a six-month technology demonstration became an eleven-year orbital laboratory, yielding insights that continue to shape spacecraft design five decades later. As NASA's Artemis programme prepares to establish a sustained human presence at the Moon, the thrusters that will power the Gateway station trace their heritage directly to the two mercury ion engines that fired for the first time on 14 February 1970. For anyone tracking aerospace technology trends, supply-chain developments, or the evolving economics of space access, SERT II remains the foundational case study: the moment electric propulsion proved it belonged in space.