Piercing the Silent Curtain

Abstract

Scientists predicted it, missile designers were plagued by it, astronauts have had to live with it, and it is likely to continue making trouble for years to come. But space researchers are fighting back, looking for a way to puncture the wall of silence that temporarily cuts off communications to and from spacecraft and missiles reentering earth's atmosphere at high speed. The blackout was anticipated as long ago as the early 1950's in research that was to lead to the intercontinental ballistic missile program. When an object plunges through earth's atmosphere at speeds greater than about 10 times the speed of sound, the investigators theorized, the heat of friction should produce a layer of ionized gas or plasma surrounding the forward parts of the object's skin. This curtain could completely block off radio transmissions in both directions, leaving the object cut off from the ground for a possibly critical few moments. Early ICBM tests and sounding rocket flights proved the theory rightand turned up another problem. When a rocket has been launched, as the surrounding atmospheric pressure decreases with altitude, the hot plume of the rocket's exhaust expands to many times its original size. As with the reentry sheath, the ionized exhaust gases can block off signals to and from the ground below. The rocket exhaust blackout problem has since been circumvented by stuch methods as relaying ground communications up from a point well away from the launch site, to enable transmissions to reach around the plume. But it was during just such a blackout that a phenomenon occurred which has since become the basis for much of the research into remedying the more troublesome reentry blackout. On July 1, 1960, the National Aeronautics and Space Administration launched the first of its Scout rockets, which have since become the agency's most popular workhorse boosters. As usual, the exhaust plume soon began to blank out the telemetry signals from the vehicle. Then several observers noticed that whenever the rocket's hydrogen peroxide control jets fired into the plume, the strength of the signal was partially restored. Some of them thought their equipment was wrong, says Theo E. Sims, who was one of the original observers