Space: What Happened with Gemini 6

Another space mystery seemed close to solution last week. After painstaking analysis of hundreds of data-packed yards of magnetic tape, Air Force and NASA investigators offered a tentative explanation for the failure of an Agena rocket to soar into orbit as a target for the spacecraft Gemini 6. Looming unexpectedly out of the complex vocabulary of modern missilery, the Agena’s trouble sounded as old-fashioned as a Model-T. The Agena’s engine, said the scientific detectives, had backfired.

The Agena’s problems began 368 seconds after launch. At that moment, precisely on schedule, fuming nitric acid fuel began spraying into the rocket’s thrust chamber, followed a few milliseconds later by the oxidizer, unsymmetrical dimethylhydrazine. Somehow, too much fuel entered the chamber ahead of the oxidizer. The result was a “hard start” of the Agena’s engine, similar to the backfire that occurs when gasoline and air ignite prematurely in an automobile engine.

Investigators believe that the jolt was so severe that the Agena engine shut down and enough pressure built up in the fuel and oxidizer tanks to rupture them. After that, the Agena either broke up or was destroyed in a fiery blast of fuel and oxidizer from the burst tanks.

Though repeated ground tests of the Agena have been so successful that the odds are high against the occurrence of another hard start, NASA is not taking any chances. The rocket will be modified if it is still to be used as planned in the flight of Gemini 8. On that mission, Astronauts Neil Armstrong and David Scott are scheduled to restart the engine of an orbiting Agena after docking with it. Another backfire could destroy not only the Agena but Gemini 8 and its occupants as well.