AS-203

• 1. Introduction

  ›

  AS-203 launched from Pad 37B at Cape Kennedy on the 5th of July 1966, carrying no astronauts and no spacecraft. In its place sat an aerodynamic nose cone, hiding a tank packed with 88 sensors and two television cameras pointed at something NASA needed to understand before it could send anyone to the Moon: what liquid hydrogen does in weightlessness.

  The engineers already knew that hydrogen would slosh. They had designed baffles to control it, heaters to keep fuel lines cold enough to allow an engine to restart in the void of space, and a careful system for keeping propellants positioned correctly in orbit. What they had not designed for was what would happen on the fourth orbit, during a pressure test that no one expected to destroy the vehicle. AS-203 would end in an explosion, and NASA would still call it a success.

• 2. A Stage That Had to Restart

  ›

  The central problem AS-203 was built to solve came from the architecture of the Apollo lunar mission. The Saturn V's third stage, called the S-IVB-500, had a specific and unusual job. It would fire once to push the spacecraft into a circular Earth parking orbit. Then, after a coast of perhaps a few hours, it had to fire again to send the crew toward the Moon.

  Restarting a rocket engine in orbit was not routine. Liquid hydrogen is cryogenic, which means it must be kept at extremely low temperatures to remain a liquid at all. After sitting in space during the parking orbit, the fuel lines and engine components could drift out of the temperature range needed for ignition. Engineers needed data on exactly how the hydrogen behaved in the tank: whether it pooled, how much control the anti-slosh measures gave, and whether temperatures could be maintained adequately.

  The flight vehicle used a slightly different version of the stage, the S-IVB-200, configured as the second stage of the Saturn IB. The lunar version, the S-IVB-500, would be a modified derivative. Data gathered on AS-203 would validate the design choices that engineers had already committed to for the Saturn V.

• 3. Preparing the Vehicle

  ›

  The S-IVB stage arrived at Cape Kennedy on the 6th of April 1966. The S-IB first stage followed six days later, and the Instrument Unit came two days after that. By the 19th of April, technicians had begun erecting the booster at Pad 37B.

  The preparation was not smooth. The same cracked solder joints in the printed-circuit boards that had troubled the earlier AS-201 mission surfaced again, requiring more than 8,000 boards to be replaced. The spring of 1966 also brought a scheduling shuffle: engineers decided to fly AS-203 before AS-202, because the command and service module intended for AS-202 was running late. AS-203, carrying no CSM at all, could proceed without waiting.

  The decision to omit the command and service module was deliberate and load-bearing for the experiment's validity. With no heavy payload on top, residual propellants would stay in the tanks rather than being expended countering the weight of the spacecraft. The liquid oxygen load was also reduced slightly so that the amount of leftover hydrogen would approximate what the Saturn V would carry into its Earth parking orbit.

• 4. Four Orbits

  ›

  The rocket lifted off on the first attempt on the 5th of July. The S-IVB and its Instrument Unit reached a 100 nautical mile circular orbit.

  During the first two orbits, the team ran through the primary design test objectives. The hydrogen behaved mostly as predicted. The anti-slosh measures gave sufficient control over the fuel's location, and engine temperatures stayed within the range needed for restart. That was the essential result: the restartable S-IVB concept worked as designed.

  With two orbits to spare, engineers moved to additional experiments aimed at gathering data for future cryogenic stage designs. One was a free-coast experiment, watching and controlling the small negative acceleration caused by atmospheric drag at orbital altitude. Another was a rapid depressurization test of the fuel tank. The third experiment would be the last thing AS-203 ever did.

• 5. The Pressure That Broke the Bulkhead

  ›

  The closed fuel tank pressurization test worked by closing the hydrogen tank's vents while letting the oxygen tank continue to vent freely. This created a pressure difference between the two tanks, which shared a common internal wall called the common bulkhead.

  Engineers had observed this bulkhead collapse in a ground test under similar conditions. The expectation on AS-203 was the same: a controlled rupture of the dividing wall. What happened instead was destruction of the entire stage. A pressure difference measured as high as 39.4 psi was recorded before the rupture occurred during a two-minute gap in communications, the loss of signal between the Manned Spacecraft Center and the Trinidad tracking station.

  When Trinidad's radar picked up the vehicle again, it was in multiple pieces. Telemetry was never re-acquired. NASA concluded that a spark or an impact had ignited the remaining propellants in the moment of rupture, triggering an explosion. The stage was gone.

• 6. Success, Despite the Explosion

  ›

  NASA classified AS-203 as a mission success. All primary objectives had been achieved before the destruction of the vehicle, and the data on hydrogen behavior in weightlessness was in hand.

  In September of 1966, Douglas Aircraft Company, which had built the S-IVB, formally declared that the design was ready for use on the Saturn V to send men to the Moon. That declaration rested directly on what the 88 sensors and two television cameras had recorded during the first two orbits. The explosion on orbit four was a loss of hardware, not a loss of knowledge.

  AS-203 was also the first launch of any Saturn IB from Pad 37B, a launch complex that would go on to support multiple Apollo missions in the years that followed.