Apollo AS 203 S IVB LH2 tank interior film from oribital coast

Apollo-Saturn video from inside a fuel tank while in orbit.

AS-203 Low Gravity Orbital Experiment

This is an unmanned test-flight of some components of the Apollo Moon rocket design, using a Saturn 1B booster to place the restartable third stage (S-IVB) in orbit. The test was launched and performed on July 5, 1966.

The camera view is from the top of the liquid hydrogen fuel tank of an S-IVB (Apollo 3rd stage). The rocket is in Earth orbit, and the video begins after the engine had been stopped and is allowed to coast.

Scientists wanted to know how liquid fuel behaved inside a fuel tank when in near weightlessness. This is important because to restart a liquid-fueled engine, the fuel must be at the bottom of the tank. That is assured when on a launch pad because the liquid is held to the bottom of the tank by gravity and, after launch, it is held down by the acceleration; but (as you will see), in orbit, the fuel will float freely toward the front of the rocket once the engine is shut-down.

To restart, the fuel must be moved to the rear where it can flow to the rocket motor. This is accomplished by venting some fuel out the rear of the craft (or by firing small forward ullage motors, i.e., "thruster rockets") causing the body of the rocket to accelerate slightly so that the free-floating blobs of liquid fuel within will settle to the rear.

Once the motor has been shut-down, the slight drag (which causes all bodies in low earth orbit to slow and eventually fall) allows the fuel inside the tank to drift forward, toward the camera end, and away from the motor end. We see the blobs of liquid hydrogen float toward the camera at that point. For a few seconds, it appears (but it is only an optical illusion) that the blobs of liquid hydrogen cover the camera view. That appearance is actually caused by temperature variations reflecting and refracting light.

Venting un-ignited fuel, causes the rocket to accelerate slightly which accelerates the tank so that the drifting globs within appear to "fall" toward the rear. When some of the liquid hydrogen touches the warmer material below it, it vaporizes, which can be seen as an appearance of steam moving toward the camera near the end of the film.