May 15, 1957, marked the second flight of the JPL/Army Ballistic Missile Agency???s Re-Entry Test Vehicle stack. This re-entry test vehicle was the direct predecessor of the famous Explorer 1.

The Re-Entry Test Vehicle Project got its start in 1955. JPL Director William Pickering and Werner von Braun of the Army Ballistic Missile Agency???s Redstone Arsenal facility in Huntsville, Alabama, had proposed a ???Project Orbiter??? for the International Geophysical Year???s satellite competition. The proposed ???Orbiter??? stack consisted of a Redstone booster, a second stage built of eleven solid rocket motors scaled down from the Sergeant missile program, a third stage with three shrunken Sergeant motors, and a single-motor fourth stage for the orbital vehicle itself. But Project Orbiter lost to Project Vanguard from the Naval Research Laboratory that had promised a larger payload.

Instead, the Army decided to use the Orbiter stack to flight-test a new technology, ablative heat shields. In the 1950s, the Army was very interested in making itself the owner of the nation???s intercontinental ballistic missile capability, in competition with the Air Force. The Army Ballistic Missile Agency???s leadership knew that nuclear warheads re-entering the Earth???s atmosphere would have to be protected from extremely high temperatures, and both theoretical studies and laboratory tests had shown that glass-fiber based materials might accomplish this. They would remove heat simply by burning away.

JPL and Army Ballistic Missile Agency???s leaders formulated the Re-Entry Test Vehicle Project to prove that the lab tests reflected reality. JPLer Eberhart Rechtin devised a very-low power telemetry system called ???Microlock??? so that the flight of the vehicles could be tracked throughout the flight. And the nose cone assembly was designed to float, so that it could be fished out of the ocean and analyzed.

The first flight, September 20, 1956, actually used the Project Orbiter configuration???with the satellite vehicle???s fourth stage ???motor??? filled with sand instead of fuel???because that design was completed first. This test flight was completely successful, with the inert ???satellite??? crashing into the ocean 3350 miles from the launch site, demonstrating that the vehicle design and tracking system were sound.

The second flight, on May 15, 1957, was the first to fly the complete ablative technology demonstration. This one wasn???t so successful. The guidance package failed, and while the tracking network was able to follow the flight to impact and collect a complete set of telemetry data, the mock warhead could not be recovered. But the vehicle had reached a high enough velocity to show that the ablative material worked reasonably well???the transmitter kept working through impact. What JPL and the Army Ballistic Missile Agency lost in this test was the ability to determine how much of the glass-fiber material had eroded, which they still needed to know for efficient warhead design.

The final flight of the re-entry test vehicle series, August 8, 1957, succeeded brilliantly. The USS Escape recovered the nose cone 1160 miles down range. Analysis of the heat shield showed that only a small amount had been eroded away, confirming the design.

JPL and Army Ballistic Missile Agency conducted re-entry tests in intense secrecy. But President Eisenhower himself revealed it fairly quickly. On November 8, five days after the Soviet Union launched a dog named Laika into space, he gave a televised speech about the program, displaying the recovered nose cone from the third flight.