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SUNTAN - Hydrogen Fuel

For a hydrogen-fueled airplane, the very low temperature and density of liquid hydrogen pose special design problems for tanks, pumps, lines, instrumentation, and other components in the fuel system. The special requirements imposed by hydrogen are recognized immediately by all who consider such designs and, of course, received major attention by the men of the Skunk Works. The CL-400 design divided the hydrogen tankage into three sections; the forward tank had a capacity of 67 000 liters; aft, 54 000; and center (sump), 15 000. The two main tanks were kept at 2.3 atmospheres pressure and the sump tank slightly lower for fuel transfer. In the sump was a booster pump, built by Pesco Products, that supplied liquid hydrogen to the engines at a pressure of 4.4 atmospheres. The engines were mounted at the wing tips, which meant that the liquid hydrogen had to pass through a hot wing with surface temperatures up to 436 K. The design provided a vacuum-jacketed, insulated line for this purpose.

There were many unknowns in the design of the hydrogen tanks and other fuel components, and numerous experiments were conducted to obtain more information. These were done at Fort Robertson and included half-scale models of the sump tank, the vacuum-jacketed lines for carrying hydrogen from the tanks through the hot wings to the engines, booster pumps, valves, controls, and other components. These were tested in thermal environments simulating flight conditions. Later a full-scale sump pump was built and shipped to Pratt & Whitney for their use in engine testing.

Among the first concerns of Johnson and Rich were the fire and explosion hazards of hydrogen. Could it be handled as safely as gasoline? In his early visits to laboratories using liquid hydrogen, Rich inquired about fires and explosions, but obtained little information. The laboratories went to great lengths to avoid these problems. The only previous explosions Rich learned about were some minor ones Professor William Giauque experienced when oxygen crystals formed in a heat exchanger containing hydrogen. The paucity of information led Johnson and Rich to devise a series of experiments to determine for themselves the hazards of hydrogen fires and explosions. For this they turned to their only testing ground, Fort Robertson-less than a kilometer from the runways of the Burbank airport.

Tests were devised in which tanks containing liquid hydrogen under pressure were ruptured. In many cases, the hydrogen quickly escaped without ignition. The experimenters then provided a rocket squib (a small powder charge) to ignite the escaping, hydrogen. The resulting fireball quickly dissipated because of the rapid flame speed of hydrogen and its low density. Containers of hydrogen and gasoline were placed side by side and ruptured. When the hydrogen can was ruptured and ignited, the flame quickly dissipated -, but when the same thing was done with gasoline, the gasoline and flame stayed near the container and did much more damage. The gasoline fire was an order of magnitude more severe than the hydrogen fire. The experimenters tried to induce hydrogen to explode, with limited success. In 61 attempts, only two explosions occurred and in both, they had to mix oxygen with the hydrogen. Their largest explosion was produced by mixing a half liter of liquid oxygen with a similar volume of liquid hydrogen.

Johnson and Rich were convinced that, with proper care, liquid hydrogen could be handled quite safely and was a practical fuel - a conclusion that was amply verified by the space program in the 1960s. At the time, however, Johnson and Rich filmed their fire and explosion experiments to convince doubters.

The confidence of Johnson and Rich in hydrogen handling was not always shared by their hydrogen consultant, Russell Scott, who was often amazed at what he saw going on in the test areas of Fort Robertson.14 The facility, however, was well equipped with an explosion-proof electrical system, non-sparking safety tools, hydrogen sniffers or monitors, and other safety devices. In the three years of work and the handling of thousands of liters of liquid hydrogen, there was not a single accident caused by hydrogen. There was, however, one close call. In keeping with Kelly Johnson's philosophy of austerity, the ovens used for simulating hot wing temperatures of Mach 2.5 flight were made partially of wood. There were five such ovens, and early one morning, about 2 a.m., one of them caught fire.

The Skunk Works personnel, including Rich, were summoned because the fire department could not be called, for security reasons. At the time there were 2000 liters of liquid hydrogen stored in the area and Rich decided that the best course of action was to dump the liquid hydrogen on the ground. It was winter and very humid; the cold hydrogen quickly filled the revetment with fog about five feet thick. Rich and about two dozen other people were in the revetment and all they could see of each other were their heads, an eerie sight. Luckily, the hydrogen did not ignite.

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