Mercury-Redstone 4 was the fourth mission in the Mercury-Redstone series and the second U.S. manned suborbital spaceflight. The mission was essentially a repeat of Alan Shepard’s Freedom 7 flight. So why was it necessary to launch another sub-orbital mission? Why not proceed with an orbital flight to match the Soviet Vostok 1? Among other things the U.S. needed more space experience to corroborate the “Man-in-Space” concept. Also the Redstone was the only booster NASA had that was approved for manned launches. The Atlas booster was available but not ready. Atlas was capable of putting a Mercury Capsule into orbit, but it had been launched three times with unmanned capsules, and it had exploded on 2 of the 3 attempts.
The launch vehicle for the Luna E-1 series was a modified R7 named Vostok. The Vostok had three stages. The first and second stage were the standard R-7 which we covered in Episode 9. A 5.1 meter long by 2.4 meter diameter third stage was added to the top of the R-7. The third stage weighed 1472 kg and was capable of delivering 54.5 kiloNewtons or 12,252 lbs of thrust. This was the probes booster stage that gave it enough speed to escape Earth’s gravity.