Ion drive. To the science fiction buff, it's an exotic means for propelling spacecraft.
But to National Aeronautics and Space Administration (NASA) researchers, electric rocket engines - ion drive - now have moved a step closer to becoming a practical means of propulsion in space.
Ironically, this success is attributed to a satellite - dubbed SERT-2 - that when shut down last year was listed in a catalog of space launches as ''considered unsuccessful.''
According to a recently issued summary of SERT-2's achievements, the space vehicle demonstrated ion drives as a versatile alternative to the chemical rockets now used universally for space flight.
Launched Feb. 4, 1970, the Space Electric Rocket Test (SERT) satellite was expected to operate for only a few years, if that. In fact, both of the small electric rocket engines to be tested failed before the experiment could be finished.
But unbeknown to the keepers of the chronicles, the satellite's controllers at the NASA-Lewis Research Center were able to revive one of the engines.
As project leader William Kerslake says, that engine ''was reliable and it lasted.'' By the time it was shut down April 19, 1981 - 11 years, 74 days after SERT-2 was launched - the capabilities of that engine had been thoroughly exercised.
Mr. Kerslake and his colleagues were working with a type of electric rocket known as an ion engine, in which electrically charged particles called ions are shot out of the rocket nozzle by electric forces.
Typically, such an engine develops a thrust many times weaker than a comparable chemical rocket. But because that thrust can be sustained for long periods - months or years, if need be - massive payloads can be accelerated to high speeds.
Ion rockets would not be used to launch payloads. But once a satellite or space ship were placed in low Earth orbit by conventional rocket or by the reusable shuttle, the ion drive would take over. Used this way, Kerslake says, ion rockets can place three to 10 times more payload in higher orbits or on interplanetary missions than can comparable chemical rockets.
What is more, ion rockets do not necessarily have to carry their own fuel. They can draw energy from solar cells, as did SERT-2. It carried the largest solar power supply the NASA had orbited up to 1970 - 33,000 solar cells in a 5 -foot-by-25-foot array. Ion rockets do, of course, have to carry propellant, the material that is vaporized to form the ions expelled through the rocket nozzle. In the case of SERT-2, this was mercury.
Rocket engineers in the United States and elsewhere have been studying ion drives for several decades. The first successful test of such an engine was made in 1964 by SERT-1 in a suborbital flight. SERT-2 was to be the first thorough orbital test. It carried two small test engines that developed 0.006 pounds thrust each. These both failed when lose bits of metal shorted out the engines' electric drives.
Although the experimenters were discouraged, Kerslake says, they took another look at the spacecraft a couple of years later. Most of its components were in working order.
So, he explains, ''We spun the craft to stabilize it. This shook loose the metal piece that was shorting out one of the engines. That engine then was reliable and lasted.''
Kerslake says his group has designed larger versions of the SERT engines that now are ready for final development as working rockets. That is a job for the NASA-Marshall Space Flight Center because Lewis is a center for research, not hardware development. However, this work has been postponed because of cutbacks in NASA funding. Despite the cuts, another test flight is scheduled on a US Air Force satellite planned for 1983.
Although Kerslake and his associates find the budget cuts as frustrating as the initial SERT-2 failure, he says they are continuing to develop the ion rocket concept. When such a rocket is needed badly enough to justify the money to produce it, he adds, the basic designs will be available.