In November of 1961, Electro-Optical Systems was awarded a contract by the U. S. Air Force to develop a 8.9 mN, cesium-contact ionization IPS for three sub-orbital flight tests. The Electric Propulsion Space Tests were called Program 661A and were managed by the Air Force Space Systems Command in Los Angeles. The flight objectives were to demonstrate in-space operation of the cesium ion engine and to obtain accurate measurements of engine performance.

The cesium contact engine incorporated an ionizer array of 84 porous tungsten buttons. The power level, thrust, and specific impulse were 0.77 kW, 8.9 mN, and 7400 s, respectively, in this engine which had a beam extraction diameter of about 7 cm. The neutralizer was a wire filament which was not immersed in the ion beam. Power to the PPU was supplied by 56 V batteries. The longest ground test was 1230 hours.