Meanwhile, engineers were looking at launch vehicles. The Atlas, used for Mercury, was not quite powerful enough for the heavier—8500 lb--, two man spacecraft. The Von Braun team, which had rushed the Redstone to the rescue for the Explorer satellite and Mercury suborbital flights, were now a part of NASA at the Marshall Spaceflight Center (MSC) in Huntsville, AL. They were working on a family of vehicles that were to lead to the moon rocket booster called Saturn. The first in the series was a gangly erector-set booster called the Saturn 1. It was to produce 1.5 million pounds of thrust, more than enough to boost the new spacecraft into orbit.
But there was no assurance Saturn 1 would be ready in time, so the rocket scientists looked back to Air Force ICBMs for their vehicle. By now, the Atlas, as the Strategic Air Command’s main missile deterrent, had been replaced by the Titan II. This was a two stage vehicle that used storable, hypergolic fuels. Hypergolic fuels are fuels that ignite when they come in contact with each other. Titan II used hydrazine and nitrogen tetroxide in both stages. This meant there was no question the second stage would ignite. The first stage’s two engines produced 430,000 lbs of thrust, almost 60,000 lbs more than Atlas and enough to boost the new spacecraft into orbit.
The storability of the fuels was also a plus for NASA. The vehicle could be fueled and not have to be unloaded and refueled through extended holds as with cryogenic (liquefied gas) fuels like liquid oxygen.
But the rocket scientists were still getting their on the job training. Flight testing of the Titan II did not proceed as desired. Flight tests by the Air Force revealed a unique and worrisome phenomenon for NASA—the POGO effect. Combustion deviations in the booster engines caused compression waves to travel through the vehicle. It was like the rocket was pulsing through its length. This was not a good situation for manned flight.
On top of that, the second stage engine showed instability on startup in ground tests. This had never happened in flight, but it was enough to concern NASA. MSC was put on the case.
The Huntsville team located a number of parts in the booster that had to be redesigned. Their main fix however was to add standpipes on the oxidizer lines and accumulators on the fuel lines to reduce the POGO effect.
Aerojet General, the engine manufacturer, meanwhile solved the instability problem. On Nov. 1, 1963, a Titan II with all the fixes lifted off from Cape Canaveral and posted a perfect flight. After that Titan II achieved a string of 11 flawless flights.
Meanwhile, NASA was completing the design of the new spacecraft, giving it a name, and tapping new astronauts to fly it.