Taming Liquid Hydrogen

The giant spacecraft was sent aloft to conduct only one experiment. And after only four orbits, it disintegrated in flight. For all the brevity of its mission, though, the flight of the 29-ton SIV B vehicle last week was singularly important. It gave anxious earthbound scientists their first close look at the behavior of liquid hydrogen in space.

Liquid hydrogen is the most effective rocket fuel ever developed. Some 40% more powerful than the kerosene fuel used in the Saturn booster, it will place the Apollo spacecraft in earth orbit, then be used again in the same flight to hurl the spaceship toward the moon. The trouble is, its virtues are not bought cheaply. Its extreme volatility and the -- 423DEG F. temperature necessary to keep it in liquid form make it difficult to deal with both on the ground and in space. NASA spacemen had theorized that once weightless in orbit, liquid hydrogen would scatter around its fuel tank in an uneven mixture of liquid and gas. And unless liquid hydrogen can be kept at the bottom of the tank, it cannot reach the valves through which it is pumped to the combustion chamber.

Big Question. Under such conditions, how could an engine in orbit be restarted for a flight to the moon? Engineers speculated that if an Apollo vehicle were accelerated slightly, the inertia of the liquid hydrogen would force it toward the bottom of the tank. To provide that acceleration, they installed controllable, backward-pointing vents at the top of the SIV B's fuel tanks. They hoped that the gases gradually produced by the evaporating liquid fuel and oxidizer could be released into space to provide slight thrust and acceleration.

A big question remained: would the theory prove practical in space? To find out, NASA engineers installed television cameras inside the SIV B's hydrogen fuel tank. All through the acceleration of blastoff, and while the Saturn I first stage was pushing the SIV B aloft, the TV screens at the Houston control center showed the liquid hydrogen settled and calm on the bottom of the tank, its surface barely rippling. After the first stage had dropped away and the SIV B's engine was fired to insert it into orbit, the level of the liquid hydrogen could be seen dropping rapidly as fuel was consumed in the combustion chamber.

No Sloshing. Then, with the SIV B in orbit and its engine shut down, the TV screens showed a weird transformation in the fuel tank. Now weightless, globules of liquid hydrogen ripped loose from the churning surface and began to drift upward. Ground controllers immediately radioed signals that opened the SIV B's tank vents, allowing escaping gases to accelerate the vehicle slightly. On the screen, the globs could be seen obediently settling back to the surface. "It looks calm," the controllers reported. "It's behaving itself. There's no sloshing."

With its tank vents still spewing tiny jets of gas, the slowly accelerating SIV B was then put through a simulated engine restart. Valves at the bottom of the tank opened, allowing liquid hydrogen to flow into the combustion chamber. Clearly visible on TV, the dwindling fuel hugged the bottom of the tank, its surface calm.

NASA scientists were still not satisfied. During the SIV B's third orbit, they allowed the liquid hydrogen to run rampant by rapidly reducing pressure in the tank without increasing acceleration. A heavy cloud of hydrogen vapor billowed up, followed by large globs of liquid that floated around the tank and even covered the camera lens. Again the vents were opened. And again they produced acceleration that brought the tempest back under control. It was convincing evidence that liquid hydrogen has been tamed and no longer stands as an obstacle on the road to the moon.

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