Nuclear Propulsion

Oak Ridge, Project SIGN and the hunt for radioactive UFOs

The Manhattan Project had considered the possibility of powering ships and aircraft with nuclear energy, but with most of its creative energy focused on designing bombs, it did little serious work on non-weapon applications during the war.

In order to begin to understand the issues resulting from placing a reactor in an airplane, and to help uncover unanticipated problems, in the spring of 1946 the Army Air Force established a project called NEPA -- Nuclear Energy for the Propulsion of Aircraft -- under contract to Fairchild Engine and Airplane Corporation. Known more for its stodgy cargo planes and trainers than for prowess in radically advanced technology, Fairchild seemed ill-suited for the project. The company produced many reports and conducted crude experiments -- placing radium in the bomb bay of a B-29 and measuring the radiation field in the cockpit with geiger counters, and so on -- but made little real progress toward building an actual nuclear-powered airplane.

In 1947, at the height of the flying saucer wave of early July, a sensational article on an alleged "atomic-powered" Russian saucer appeared in the Los Angeles Examiner. According to the story, a "top-flight nuclear physicist" in Los Angeles had received a mysterious letter containing a description of the Soviet saucer from someone associated with an officer of a Russian oil tanker which was docked in Los Angeles harbor. The letter claimed to describe a "kidney-shaped" aircraft, just eighteen inches thick, with a highly polished surface. The pilot was said to lay prone in a cockpit that was cooled against the heat produced by the saucer's tremendous speed. "The lifting force is an entirely new principle, found about 10 years ago among the unpublished papers of a Russian chemist," the story claimed. "Energy is required only for climbing, but no energy is needed for support when the airplane goes along the earth's gravitational contour lines." The letter was supposedly turned over to the FBI.

For a revealing look at the continued poor quality of US intelligence on the real Soviet nuclear aircraft project a decade later, and an example of the kind of feverish speculation that sometimes overcame US analysts who were under pressure to prevent repeats of surprises like the unanticipated launch of Sputnik 1, see The Soviet "Nuclear-Powered Bomber" Fiasco of 1958

While the 1947 Examiner "Red Atomic Disk" story had more in common with contemporary pulp science fiction tales than with genuine nuclear aircraft concepts, concerns about real Soviet nuclear-powered aircraft, and the problems inherent in transporting American bombs to Soviet targets, were beginning to stimulate serious nuclear propulsion development efforts in the US.

NEPA project logo, July 1947 (DOE)

In 1948, an MIT summer study conference called the "Lexington Project" carried out the first thorough engineering survey of the problems of applying nuclear reactors to aircraft propulsion. It concluded in its final report ("LEX P-1") that the idea was technically achievable on a practical basis (although it would be far from easy and it might take 15 years to produce a flying aircraft), and that two different approaches to the goal seemed apparent. These dealt with the "cycles" used by the jet engines -- "open" or "closed" thermodynamic cycles. In both cases, the heat of the reactor was transferred to the propulsion engine by some working medium. In the open cycle, the turbojet engine's air was the medium, and would simply flow through the reactor, which functioned as a replacement for the combustion chamber where fuel was burned in an ordinary engine. In a closed cycle, a fluid of some type, possibly molten metal, would circulate between the reactor core and the turbine. While vastly more complex in engineering terms, the closed cycle eliminated air as a heat transfer medium and promised much higher engine efficiencies.

Because of these problems, one of the ideas considered by the Lexington group was to simply eliminate the crew from the nuclear airplane, make it into a flying atomic tugboat, and let it tow a more conventional airplane -- safely distanced from the reactor by a long cable -- to the vicinity of a target.

Northrop concept for nuclear-powered bomber with six air-launchable defensive "parasite fighters," c. 1952

Other researchers realized that some of these shielding problems could be circumvented through the use of hot radioisotopes -- essentially a form of nuclear waste -- rather than a nuclear reactor, as a concentrated heat source for nuclear engines. These radical engine concepts may have been what Project SIGN's engineers had in mind when they theorized about advanced atomic engines for man-made saucers.

See: Radioisotope-powered nuclear aircraft

On April 27, 1949, a conference was held at Oak Ridge National Laboratory, Tennessee (ORNL), between NEPA contractors, the Air Force, and AEC personnel, with the intent to plan Oak Ridge participation in the NEPA project. The ORNL NEPA effort was called the Aircraft Nuclear Propulsion project (ANP) and was established under Air Materiel Command project officer Lt Col Clyde D. Gasser

See: NEPA History 1

While many types of engines were studied, including giant propellers driven by steam turbines, as of 1951-2 the Air Force's vision of an operational nuclear-powered bomber centered on the mammoth Convair YB-60, a swept-wing, turbojet-powered modification of the B-36 strategic bomber. It was a far cry from Newsweek's compact 1945 flying wing. NEPA's July 1947 report had set its sights on the goal of building an aircraft in the 300,000 lb gross weight range, with a speed of 515 mph at 35,000 ft and a weapon load of 12,000 lb. The B-60 closely followed those criteria.

The nuclear powerplant designed for the B-60 was the General Electric P-1, an open-cycle, air-cooled reactor with a thermal output probably in the 50 megawatt range, which was married to four powerful GE J53 turbojet engines by a complex tangle of air ducts. Air from the engine compressors would flow directly through the reactor core, where it would be heated to some 2,000 degrees F before being returned to the engine turbine sections.

A prototype GE X39 nuclear-heated turbojet, based on the J47 used in the B-47 bomber, shows air duct scrolls for connection to the reactor (arrows). This engine probably would have been used for early flight testing of the nuclear propulsion concept

The P-1 reactor/X39 engine complex would have been flight tested aboard a highly modified B-36 bomber known as the X-6. The P-1 would have been installed in the X-6's bomb bay for flight and removed shortly after landing by taxying the plane over a huge pit equipped with an elevator that would lower the engine into a shielded isolation area to reduce irradiation of the airframe, crew and ground personnel. The model (below) shows the four X39 pods protruding from the belly of the X-6.

When the P-1 engine was applied to the actual B-60 production nuclear-powered bomber, the reactor/J53 engine complex apparently would have been installed in the aft bomb bay area of the fuselage, as far as possible from the crew compartment. The arrows, below, indicate the probable location of the engine assembly. (The B-60 flew numerous times powered by conventional turbojets, but never with the nuclear engines). Heavy shielding was planned, consisting of tanks of a water-boron solution (boron-10 isotope is an excellent neutron absorber) and layers of metal. The crew compartment itself would have had additional heavy shielding. This "divided shielding" concept was considered one of the major technical breakthroughs of the early NEPA project.

The aircraft probably would have retained its conventional jet engines for use during takeoff and landing. Once airborne and away from populated areas, the crew would bring the reactor up to power and the aircraft would cruise on nuclear heat. The intention was to create an aircraft with nonstop around-the-world range, endurance measured in days, and near sonic speed, all while carrying a heavy nuclear weapon payload.

See: Project SIGN and radioactive UFOs

Nuclear Aircraft Powerplant Experiments

INEEL (established in 1949 as the National Reactor Testing Station) Test Area North (above) was the site of Heat Transfer Reactor Experiments -- "HTRE" -- aircraft nuclear engine prototype ground-based testing, which began in 1955. The nuclear-powered aircraft prototype would have been flown from here. A huge hangar was constructed (black building, center right), but not the fifteen thousand foot runway that a ponderous nuclear plane would have required. A multi-mile runway for the X-6 was also considered at Edwards Air Force Base, California, running between Muroc dry lake and Rosamond dry lake, but it too was never built.

A special hangar for the plane, and associated maintenance facilities, were built with enormously thick, nuclear-shielded walls and bays. General Electric, the program contractor, planned to equip the engine maintenance facilities with closed-circuit television systems and remote manipulator arms to allow technicians to work on the aircraft and its powerplant without direct exposure to the intense radiation field that would persist even after the reactor was shut down. Since the turbojets essentially functioned as the cooling system for the reactor, they would have to be run at high power settings even after shutdown of the reactor in order to maintain cooling airflow through the still-hot core. After an initial cooldown period, ground cooling systems would be connected to the reactor and the engines could be shut down as the P-1 was extracted from the airplane and placed in its shielded storage bay.

One of the GE HTRE reactor testbeds at INEL. © 1996, Bureau of Atomic Tourism.

The initial HTRE engine experiments were intended to prove out the engineering and operational concepts for a nuclear bomber powerplant, but without the restrictions on weight and size that an airplane powerplant would demand. These early assemblies were gigantic monstrosities weighing at least a hundred thousand pounds, and were built on railcars which would move them to remote test locations far from their assembly, maintenance and control facilities. When the engineering aspects of the designs were proven, the next step would be to miniaturize the designs while increasing their power output, with the goal of producing a final, operational version of the design that would a fraction of the size and weight while producing much greater thermal output. This was to be done in stages over a several year period.

General Electric began HTRE-1 test runs in 1955 and the reactor successfully powered the X39 engines the following year, although the massive contraption was far from a practical aircraft powerplant. The improved HTRE-II of 1958 had a rated reactor power output of about 10 Megawatts. HTRE-III continuted the test program until 1961.

The giant HTRE assemblies were pushed by locomotive to a remote site in Test Area North for powered testing. They remain radioactive to this day. HTRE-II was shielded with mercury.

Nuclear Flight Testing

To serve as an airborne reactor testbed, Convair rebuilt a B-36 in 1955 as a special model called the NB-36H Crusader. The aircraft was equipped with massive shielding and air-cooling ducting (arrows) to support a small nuclear reactor in the aft fuselage. The NB-36H was essentially similar to the earlier X-6 concept, but its reactor was far less powerful and was not capable of propelling the aircraft.

A heavily-shielded, multi-ton sealed crew capsule, equipped with a bank-vault-like hatch and foot-thick leaded-glass windows, was installed in the forward fuselage.

ASTR

The reactor used in these tests was known as ASTR - the Aircraft Shield Test Reactor. It was air-cooled and produced a nominal 1 megawatt output. ASTR was installed in the NB-36H aft bomb bay in the approximate location of the full-scale P-1 reactor in order to simulate the radiation field and operational techniques of the larger propulsion reactor.

The NB-36 was based at the Convair plant at Carswell AFB, Ft Worth, Texas. On reactor test missions, the aircraft would fly west to the White Sands, New Mexico area where the ASTR would be brought up to power and experiments would begin.

In all, 47 flights were made with the reactor aboard and operating between September 1955 and March 1957. All flights were followed by an instrumented B-50 bomber carrying a crew of specially-trained troops. In event of a crash of the NB-36 they were to parachute to the ground, cordon off the area, and work with local emergency officials to cope with a radiological disaster.