AS-203

The purpose of flight AS-203 was to study the zero-gravity behavior of fuel - liquid hydrogen in the second stage of S-IVB-200. It was planned to test this stage of Saturn-1B in this flight for on-off in orbit around the Earth, since in lunar expeditions it will be the third stage of Saturn-5 (in version S-IVB-500) and for launch to the moon with the Apollo spacecraft its engine must restart in the reference orbit. To ensure that the engine starts in zero gravity, measures were taken to control the position of liquid hydrogen in the fuel tank. In order to have the necessary amount of fuel in the second-stage tanks in orbit, we had to abandon the payload - only the head fairing was installed. In addition, when refueling the second stage, the amount of oxidizing agent (liquid oxygen) was slightly reduced so that the hydrogen-oxygen ratio corresponded to the situation with the third stage of Saturn-5 in the reference orbit ^[1] .

The tank was equipped with 88 sensors and two television cameras to record fuel behavior. One of the cameras broke before launch, the other transmitted images to Earth ^[2] . In flight, a new type of instrument compartment was tested for the first time, which was used in all Saturn missile launches from launch pad No. 37B (also at the first launch of Saturn-1B - AS-201 ).

In the spring of 1966, it was decided to launch AS-203 earlier than AS-202 , since the ship was not ready for AS-202 flight. On April 6, 1966, the second stage of S-IVB arrived at Cape Canaveral, the first - six days later, the instrument compartment - two days later. On April 19, technicians began installing the launch vehicle on the launch pad at position 37B. During testing, we again encountered problems that seemed to be fixed after the AS-201 flight: due to poor soldering, we had to replace more than 8,000 rations on the printed circuit boards. In June 1966, it was possible to simultaneously see three Saturn rockets at different launching sites of Cape Canaveral: Saturn-5 technological prototype was on platform 39A, Saturn-1B rocket for flight AS-202 was located on platform 34, and Saturn-1B for flight AS-203 - on site 37B.

The rocket launched from the first attempt on July 5, 1966. The second stage and the instrument compartment entered a circular orbit 190 km high. The main tasks of the S-IVB flight were solved in the first two turns. Hydrogen, as expected, behaved predictably, its position in the tank and engine temperature were controlled, and the engine was restarted successfully. The following two turns were used for additional experiments: the study of the free movement of fuel in the tanks with a slight negative acceleration due to aerodynamic braking of the stage, and the test of the fuel tanks for fast depressurization and for sealing.

The experiment to close the valve of the liquid oxygen tank led to an increase in pressure in the hydrogen tank. After depressurizing the oxygen tank when the valve was closed, oxygen continued to evaporate. It is likely that with a pressure drop between the tanks of 39.4 psi (272 kPa) the bulkhead separating them could not stand it, as was the case in the ground test. The destruction, apparently, occurred when the stage flew over the zone of a two-minute loss of communication between the Control Center and the tracking station on Trinidad. The radar image from the Trinidad station showed a large number of debris instead of the S-IVB, communication could not be established. NASA concluded that a spark or pressure ignited the fuel, resulting in an explosion.

Despite the destruction of tanks, the flight was recognized as successful. All the main goals were achieved and the design concept of the 500th version of the S-IVB stage was confirmed. In September, Douglas, the developer of S-IVB, announced that the stage was ready for use and could be used on Saturn 5 to fly a man to the moon.

• AS-203 - Start (video)