All satellites so far have gone into elliptical orbits, rising considerably higher above the earth on one side than on the other. Some of the lopsided orbits have been intentional, to give information about thick layers of space. Others, although unwanted, were unavoidable. Once the original rockets had burned out, there was no power available to correct the orbit. Last week Lockheed Aircraft Corp. announced development of a rocket engine that can fire a second time, enabling ground controllers or an automatic mechanism to kick a straying satellite into a circular orbit by picking the moment to set off the second blast.
Manufactured for Lockheed by Bell Aircraft Corp., the Agena rocket has been used in its original single-shot version as the second stage of the Thor rocket, which has successfully injected six of the eight Discoverer satellites into polar orbits. The Agena is designed to ignite when high in the vacuum of space. This is not easy because few fuels will ignite in a vacuum. Bell gets around the problem by using hypergolic fuels (unsymmetrical dime-thylhydrazine and inhibited red fuming nitric acid) that ignite spontaneously as soon as they come in contact. After the first stage burns out, a small charge of solid propellant flares up and generates gas that runs a turbine and pumps the liquid fuels into the Agena's combustion chamber. They start burning immediately, giving 15,000 lbs. of thrust.
To make the Agena ignite a second time called for tricky engineering. A second charge of solid propellant to restart the turbopump was comparatively simple. But since the rocket would be fired while in orbit, when everything on a satellite is weightless, the fuel might be anywhere in the partly empty tanks—perhaps gathered in a ball in the center. To coax it into a position where the pumps could get hold of it, two small, solid-fuel rockets are fired, giving the main rocket a slight forward push. The fuel responds momentarily as if to gravitation, settles to the rear of the tanks and is duly pumped into the combustion chamber.
The restart version of the Agena has not yet done its tricks in space, but in ground tests it has performed well. When used in the second stage of a satellite launcher, it will use most of its fuel to make the satellite climb toward a high apogee on the far side of the earth. Left to itself, the satellite would descend again to the low point (perigee) where it first went into orbit. But at apogee the Agena will fire a second time, giving enough additional push to put the satellite on a high, near-circular orbit, and keep it there.