Part II : 1950-1957
 The component and engine testing of hydrogen in the laboratory, essential as they were, did not answer an important question: Was it practical to use liquid hydrogen in an aircraft? Silverstein had been interested in finding this out from the beginning and his big opportunity came from a parallel interest by the Air Force.
In the fall of 1955, the power plant laboratory of Wright Field, headed by Col. Norman C. Appold, planned an experiment to determine the feasibility of flying an airplane fueled with liquid hydrogen. The bids for a contract-about $4 million a year for 3 years-were higher and longer than anticipated. Lt. Col. Harold Robbins. ARDC headquarters and former Air Force liaison at Lewis, suggested that the NACA be approached to do the work. Silverstein jumped at the opportunity. He promised to do the job in 12 months and with $1 million for special equipment. The agreement was  reached in December 1955, and Silverstein lost no time in getting started. He chose Paul Ordin to be the project manager, assisted by Donald Mulholland. The project staff was quickly selected and put to work on their new assignment.17
Although Silverstein was technical head of a laboratory with a complement close to 3000, it was characteristic of him to direct the project personally. He had a room in the basement of the administration building cleared for use by the project group. It was directly below his office and convenient for his close supervision. The project was classified secret and known as Project Bee.
The airplane selected for the project was the B-57B twin-engine bomber powered by Curtiss Wright J-65 turbojet engines. The basic plan was to equip the airplane with a hydrogen fuel system, independent of its regular fuel system, and modify one engine to operate on hydrogen as well as its regular fuel, which was JP-4 (kerosene). The airplane was to take off and climb on its regular fuel. After reaching level flight at about 16400 meters, the fuel on one engine was to be switched from JP-4 to hydrogen. When the hydrogen experiment was complete, the fuel flow would be switched back to JP-4 and the airplane would return to base under its normal operating conditions.
The project team, aided by others in the laboratory, began to design and test the various components for the flight system. A liquid hydrogen tank was designed for mounting beneath the tip of a wing. Two methods for pumping liquid hydrogen were selected. The first was to pressurize the hydrogen tank with helium, a simple and fast method but requiring a fairly heavy tank to withstand the pressure. The second was to employ a liquid-hydrogen pump, but this required time for development. Consequently the first tests were made with the pressurization system.
Earlier combustion experiments showed that gaseous hydrogen burned easily in the turbojet engine. To feed gaseous hydrogen to the airplane engine required some means for gasifying the liquid. A heat exchanger was designed and tested for this purpose. Ram air passed through it during flight to heat and gasify the liquid hydrogen.18
The dual fuel system and transition between the two fuels, JP-4 and gaseous hydrogen, called for an integrated control system, the key component of which was a flow regulator for the gaseous hydrogen. The speed of the engine was controlled by coupling the hydrogen flow regulator to the engine's JP-4 fuel control.19
The flights were the province of the laboratory's test pilots headed by William V. (Eb) Gough, Jr., the fourth Navy pilot to qualify in helicopters and the thirtieth in jets; he joined the NACA as a test pilot after the war. By early May, Gough had checked out on the B-57 at the Glenn L. Martin plant in Baltimore and the Air Force had ferried a B-57 to Cleveland for the experiments.20
Assisting Gough was Joseph S. Algranti, another test pilot, who would fly in in the rear seat and operate the special controls of the hydrogen fuel system. He participated in the ground testing of the system from the beginning of the project. A third test pilot served as back-up and was in charge of the ground control station.
The testing of the flight components required a considerable amount of liquid hydrogen-the problem that had plagued the rocket group at Lewis for a long time. The Air Force made available mobile hydrogen liquefaction equipment and tanks from the hydrogen bomb program. Glenn Hennings got the equipment in good working order and was soon producing liquid hydrogen for the various laboratory needs.21 In the first half of 1956, as part of another program, the Air Force let a contract  to build at Painesville, Ohio, a hydrogen liquefaction plant with a capacity of 680 kilograms per day. When this plant began production late in 1956, it supplied all of Lewis's hydrogen needs.
Concurrent with the development of the flight system for supply and controlling hydrogen to the engine, a number of experiments were conducted with single turbojet combustors and full-scale engines using gaseous hydrogen as a fuel. The engine performance was high and insensitive to initial hydrogen temperature.22
In other research, hydrogen in a combustor 2/3 as long as a standard one, outperformed JP-4 and also operated at an altitude of 26000 meters-6000 higher than the limit for JP-4.23 This meant that a shorter engine was possible with hydrogen, with accompanying substantial savings in mass. In another investigation, a team led by William A. Fleming compared the altitude performance of two turbojet engines, one burning hydrogen and the other JP-4. The engines were single-spool, axial-flow types, developing 33-45 kilonewtons (7500-10000 lb thrust). Hydrogen provided stable operation to the limits of the test facility-about 27400 meters and Mach 0.8. In comparison, the same engine using JP-4 flamed out at altitudes 3000 to 4500 meters lower. Further, the specific fuel consumption (mass flow of fuel per hour divided by thrust) of hydrogen was 40 percent that of JP-4 fuel.24
Silverstein wanted a thorough check of the engine and control system, using both JP-4 and hydrogen fuels in the altitude wind tunnel before attempting flight. This was carried out by Harold R. Kaufman and associates, including test pilot Algranti. The hydrogen system consisted of a stainless steel, wing-tip fuel tank, a heat exchanger that utilized air passing through it to vaporize the liquid hydrogen, and a regulator to control the flow of hydrogen to the engine. The J-65 turbojet engine was modified by the addition of a hydrogen manifold and injection tubes. The modification did not change the engine's regular fuel system using JP-4. Kaufman reported that with JP-4 the maximum altitude for stable combustion was about 20000 meters and flame-out occurred at 23 000 meters. In contrast, hydrogen was stable to the limit of the facility at 27000 meters at flight-rated speed and temperature. The thrust was 2 to 4 percent higher, and specific fuel consumption was 60 to 70 percent lower, than with JP-4 fuel.25
In the simulated flight tests, 38 transitions were made from JP-4 fuel to hydrogen. Over three-fourths of these were satisfactory. The others had some engine speed variations, but they were so small and short in duration that the engineers believed there would have been no detrimental effect on aircraft performance. These satisfactory results in the altitude chamber cleared the way for testing the hydrogen system in the B-57.
The hydrogen fuel tank on the left wing of the airplane (figs. 21 and 22) was 6.2 meters long with a volume of 1.7 cubic meters. The stainless steel tank was designed for a pressure of 3.4 atmospheres and insulated by a 5-centimeter coat of plastic foam, covered by aluminum foil and encased in a fiberglass covering. On the opposite wing was the helium supply consisting of 24 fiberglass spheres charged to 200 atmospheres. The helium was used for pressurizing the hydrogen tank and for purging. A heat exchanger for vaporizing the liquid hydrogen, a flow regulator, and a manifold for feeding gaseous hydrogen to the engine comprised the rest of the hydrogen system.
As Christmas neared, pilots Gough and Algranti made a series of checkout flights without hydrogen, and finally the big day came. On 23 December 1956, Scotty...
 ... Simpkinson made the final check of instruments and the B-57 was fueled with JP-4. It was then towed to a remote site for loading liquid hydrogen. The vent of the tank was connected by pipe to a discharge area well away from the airplane and the system purged with helium. After countdown, 94 kilograms of liquid hydrogen were loaded into the wingtip tank. The ground crew left the vent-pipe system connected until Gough started the plane's engines on JP-4. At that time, Algranti closed the vent valve, the ground crew disconnected the vent line, and Gough began to taxi. He was accompanied by an Air Force chase plane equipped with a camera.26
As the B-57 taxied into position for take-off, Algranti was maintaining the pressure in the liquid hydrogen tank. With the vent valve closed, the vaporization of a small amount of hydrogen caused the pressure in the gas pocket above the liquid hydrogen to rise. The vaporization was caused by heat leakage through the insulation, which is unavoidable in a practical installation. From ground testing, Algranti knew that the pressure would rise from 1 to 3.5 atmospheres in about five minutes, and he had to manually vent the tank when the pressure began to rise above 3.5 atmospheres. While taxiing, he noticed that the rate of pressure rise was considerably slower than in ground tests; the instrument records indicated that sloshing and agitation of the hydrogen during taxiing slowed the pressure rise by a factor of two. During takeoff, the tank pressure dropped sharply from agitation. Once airborne, however, the agitation ceased and the pressure began to rise at about the same rate as in the stationary tests. This phenomenon was caused by thermal gradients and stratification of liquid hydrogen and its vapor and was the subject of detailed investigation later.
The takeoff and climb to the cruising altitude of 15 200 meters took almost an hour, and during that time, Algranti vented the tank 8 times to keep the pressure within limits. This resulted in a loss of about 16 percent of the hydrogen. On signal, Algranti made the transition from JP-4 to hydrogen. The engine responded by overspeeding and vibrating hard. The startled pilots quickly shut it down, purged the lines, and jettisoned the liquid hydrogen in the wing tank. The B-57 was difficult to fly on one engine, but Gough's training included this contingency. The experiment had taken place over Lake Erie and the weather had deteriorated. Gough dismissed the chase plane, but the pilot elected to accompany him back to the Cleveland airport. The two landed side by side on dual runways in a light rain.
Although the first flight was unsuccessful in operating the engine with hydrogen for an extended period, it was successful in showing that hydrogen could be handled and jettisoned safely. In addition, data were obtained on the phenomenon of hydrogen thermal stratification in the tanks.
The second flight was also only partially successful. The transition from JP-4 to hydrogen was made successfully, but insufficient hydrogen flow prevented satisfactory high-speed engine operation. Again, the bulk of the hydrogen was jettisoned without incident. The jettisoning took less than 3 minutes, with the hydrogen forming a dense plume which vanished about 6 meters aft of the tank.
On 13 February 1957, the first of three successful flights was made and the fuel system worked well.27 The transition to hydrogen was made in two steps. The hydrogen lines were first purged, then the engine was operated on JP-4 and gaseous hydrogen simultaneously. After two minutes of operations on the mixture, Algranti switched to hydrogen alone. The transition was relatively smooth and there was no appreciable  change in engine speed or tailpipe temperature. The engine ran for about 20 minutes on hydrogen. The pilots found that the engine responded well to throttle changes when using hydrogen. When the supply was almost exhausted, the speed began to drop. As this became apparent, Algranti switched back to JP-4 and the engine accelerated smoothly to its operating speed. The engine burning hydrogen had produced a dense and persistent condensation trail, while the other engine operating on JP-4 left no trail.
On 26 April, Silverstein held a special conference to report what had been learned by the Bee project using hydrogen in flight. The 175 attendees heard 7 papers by 19 members of the project team. They covered hydrogen consumption, fueling problems, airplane tankage, airplane fuel system, and the flight experiments. The results were also given in a series of research reports published later.28
The first series of flights of the hydrogen-fueled B-57 was made with a helium pressurization system to force the liquid hydrogen from the wing-tip tank to the engines. This required a fairly heavy tank to withstand the pressure. Later, a liquid-hydrogen pump was developed which permitted a reduction in tank weight that more than offset the weight of the pump. Arnold Bierman and Robert Kohl developed the five-cylinder piston pump, driven by a hydraulic motor, for installation in the wing-tip liquid-hydrogen tank.29
Flight experiments with the pump extended into 1959. Three successful flights were made. Although the pump speed and discharge pressure varied, the hydrogen regulator maintained a constant engine speed during operation with hydrogen. All the transitions from JP-4 to hydrogen, burning hydrogen, and transition back to JP-4 were made without incident. The feasibility of using liquid hydrogen in flight had been thoroughly demonstrated.30