Before NASA launched their first human into space, they needed to prove that the Mercury capsule’s launch escape system would function properly at different altitudes and speeds. They were in a rush, and NASA envisioned many tests, so whatever test vehicle that would be used needed to be cheap. This was accomplished by using as many existing components as possible. 

Engineers designed a booster called the Little Joe, using four existing solid rocket motors taken from a military missile, the MGM-29 Sergeant, and a new fuselage.  The Little Joe was later scaled up into a Little Joe II for Apollo capsules. To keep costs low, it would not have an active guidance system. That meant that after the rocket ignited, there was no way to control it. The rocket was put on an angled launch rail and kept stable in flight by canted nozzles on the rockets and four large fins. The basic rocket could be augmented with four other boosters, and all eight fired in different sequences for different amounts of thrust.

By February 1959, the final design for the rocket was almost completed. Just seven months later in September, the first launch consisted of a booster, a mockup capsule, and a live launch escape system. This first launch was not successful, but they proved the launch escape system worked! The launch escape tower fired 35 minutes before launch and carried the capsule away from both the pad and the booster. No one on the ground was injured. The error was traced back to poor wiring on the launch escape system.

NASA realized that launching the booster with an active launch escape system on the first flight was too ambitious, so the second launch — Little Joe 6 — carried only the booster, with mockup versions of both the capsule and launch escape system. That launch was successful.

The next launch, Little Joe 2, had a bit more going on. The technical goals of Little Joe 2 were to escape the rocket at high Mach and measure the forces on the capsule, including pressure and heating from a space re-entry. It would also carry all the biomedical experiments that doctors at the School of Aviation Medicine could think of to fly on a high-powered rocket. The main passenger was a live rhesus monkey named Sam, an acronym for the School of Aviation Medicine. Also on board were some barley seeds, cultures of the mold Neurospora crassa, rat nerve cells, and packets of insects.

Little Joe 2 launched on December 4, 1959, at noon local time. Booster burnout was at 30 kilometers, and momentum, as well as the firing of the escape tower, carried the capsule to 85 kilometers. That altitude was only half of what was planned due to a windage error, or in other words, how wind affects the flight of the projectile.

All test objectives were completed, but the overall result was disappointing. The engineers still did not get their test of the launch escape system at maximum dynamic pressure. The biologists hardly learned anything new, though they did demonstrate the life support system in the capsule. Robert Gilruth, head of the Space Task Group at NASA told the press of “the relatively minor role of this particular task in the context of the total Mercury program.”

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Little Joe (Astronautix)