Word: liquidity
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Dates: during 1960-1969
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...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 - 423° 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...
...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...
...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...
...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...
...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...