The modified J-47 turbojet

The first operating nuclear-powered jet engine was a General Electric modified J-47 turbojet. The first test of the design was a ground test of an assembly called the Heat Transfer Reactor Experiment no. 1. Later, a two-engine assembly was also successfully tested on the ground. The reactor was placed in a horizontal configuration and operated for more than 120 hours, including 65 continuous hours. [Source: Nuclear News, "Jet Engine Runs on Atomic Power", Oct-Nov 1960, p. 8

1946-1961 fission aircraft program

Source: Christopher E. Hamilton (Captain, USAF), Masters Thesis, "Design Study of Triggered Isomer Heat Exchanger-Combustion Hybrid Jet Engine for High Altitude Flight", AFIT/GAE/ENY/02-6 (Department of the Air Force, Air University, Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio), March 2002, pp. 1, 11-15

With the advent of nuclear fission in the 1940Õs and the rapid development of aerospace propulsion systems during that same time, it seemed that the idea of powering the engines of aircraft and spacecraft with nuclear energy was an ideal merger of these two research thrusts. Preliminary work was done on developing nuclear powered rockets, jet engines, and ramjet engines during the late 1940s and throughout the 1950s (1, 2, 3). Both the US and USSR conducted rigorous research and development programs in this field, but ultimately cancelled their respective programs, due to technical difficulties and growing safety concerns...

NEPA to ANP

The United States began its fission powered aircraft research in 1946 by authorizing the Fairchild Engine and Airplane Corporation to conduct a feasibility study for using nuclear energy for the propulsion of aircraft (NEPA). Preliminary research was done at Oak Ridge National Laboratory where other nuclear research was being accomplished. In 1948, the Atomic Energy Commission created a separate study group at Massachusetts Institute of Technology (MIT) to study the feasibility of nuclear propulsion for aircraft. This study concluded in a report, called the ÒLexington Report,Ó that nuclear propulsion was feasible and that it could be achieved in 15 years with a price tag of over one billion dollars. (2, 11)

Due to the findings of the Lexington group, the two separate research efforts were combined in 1950, into a more focused program called the Aircraft Nuclear Propulsion (ANP) program. This programÕs goals were to develop information on reactor materials and shielding, as well as creating designs for aircraft and power plants within a 3-5 year time span. In 1951, demonstration of nuclear flight was added to the list of goals. Throughout the 1950s, funding and priorities issues caused significant slowdown of achieving the set goals. Development occurred but at a much slower pace than originally thought.

Several projects developed within the ANP program including the Project Rover nuclear rocket, the Project Pluto nuclear ramjet, and the Snap nuclear auxiliary power system programs (2). The manned aircraft program remained the major focus of the ANP and several important research projects were contracted out. These projects included airframe development, jet engine and reactor design, and radiation shielding.

ANP Airframe Development

Airframe development began with early estimates pointing to the feasibility of a supersonic manned bomber. Technical issues quickly interfered with the concept, which resulted in the decision to convert a B-36 into a flying subsonic nuclear propulsion test-bed instead. The contract for this work was awarded to the Convair Division of General Dynamics Corporation, where the modified B-36 would be redesignated the X-6 (12). This aircraft was later used as a test-bed for shield development after the X-6 program was cancelled. A second contract was awarded to Lockheed Aircraft Corporation to investigate the feasibility of a transonic nuclear bomber that would fly below 5,000 ft (1.5km). (12:70)

The Lockheed study pointed out important concepts that the aircraft designer utilizing a nuclear propulsion system must be aware of (13). The first being that since the reactors of the time were immense, on the order of tens of thousands of pounds, it represented a highly concentrated weight in the aircraft. In a conventional aircraft, the weight of the fuel is normally carried in the wings, spreading the total weight of the aircraft throughout the structure. This cannot be accomplished in an all-nuclear aircraft, where the reactor and majority of the shield weight is located near engines. This would require intensive structural consideration in the design.

The second concept was that powerful radiation emanating from the reactor must be attenuated to acceptable levels. This led to the concept of divided shielding, where the shield is divided into a section around the reactor and a section around the crew compartment. This reduced the total weight of shielding while providing necessary radiation protection to the crew. The downside to divided shielding is that it allows for high radiation rates everywhere else in the airframe and into the environment.

Thirdly, since this propulsion system resulted in virtually unlimited flight endurance, several design challenges were introduced. The first is in calculating performance, where traditional methods take into account decreasing weight of the aircraft, due to fuel consumption. This lack of change in weight actually simplifies calculations, but the differences must be kept in mind during design work. Also, landing gears are normally designed to withhold the impact of about half the weight of the aircraft. This is not the case with the all-nuclear aircraft and therefore landing gear will have to be designed to withstand the full takeoff weight at landing.

Reactor and Engine Development

Development of the reactors and jet engines took two separate paths: a direct-cycle and an indirect-cycle system. Pratt & Whitney Aircraft Company was contracted, in 1953, to pursue the development of liquid-metal indirect cycle turbojet systems. The idea was to heat a liquid metal using the nuclear reactor and use the liquid metal to heat the air flow in a turbojet engine. Gains were made in reactor and heat exchanger designs, however the research never produced a test reactor (3).

The Direct-cycle program was run by General Electric and was extremely successful. In a direct cycle jet engine, the airflow in the engine is diverted after it leaves the compressor. It then enters the reactor, is heated directly, and then ducted back into the turbine section of the engine. In 1956, a ground test of a modified J-47 turbojet engine was operated by a nuclear reactor in what was referred to as the Heat Transfer Reactor Experiment No. 1 (HTRE-1) (14).

This program was continued with more rigorous experiments, HTRE-2 and Ð3, that validated the concept of utilizing a nuclear reactor to power one or more turbojet engines. The final configuration for HTRE-3 powered two turbojet engines and was of the size to fit within an aircraft even though it was not designed to be a flight test model.

In addition to proving the basic concept, it also showed that a chemical-nuclear system could be used in tandem. All three engines were in reality hybrid combustion-nuclear turbojets. Each modified J-47 engine kept its combustion section and utilized it in starting the engine until the reactor could be brought up to the correct temperature. The chemical fuel was throttled down until the reactor provided all of the heat, at which point the fuel was shut off and the combustion process ceased. (14: 98) While the HTRE series was very successful, a flight test model was never built.

ANP Shield Development

Radiation shield research was done both on the ground with the runs of the HTRE tests, as well as on board the Convair B-36 that was cut from the X-6 program. The aircraft was fitted with a one-megawatt reactor weighing 36,000 lbf (160.1 kN), for shield research. The aircraft completed 47 successful flights during the remainder of the ANP program. Unfortunately, shielding requirements for the envisioned manned bomber were prohibitive. (12: 69-73)

With a nuclear test ban treaty being worked on by the nuclear-capable nations and the technical hurdles slowing down the 3-5 year plan proposed in 1950 to a crawl, support for a nuclear powered aircraft dwindled. The ANP program was cancelled in 1961, despite the gains made. The technical hurdles that killed this nuclear fission powered program are important to any research dealing with nuclear powered flight.

Shielding weights were immense for the first aircraft design pursued. A low, fast, manned bomber was the goal throughout the NEPA/ANP program. This took advantage of the huge amounts of power available from fission reactions, but since radiation levels are directly related to the total power output of a reactor, radiation levels were also huge. Even after the decision was made to compromise the supersonic aircraft into a subsonic test vehicle, the goal was still a supersonic vehicle.

Materials, at the time, limited the heat that could be withstood by components of the engine. In addition, jet engines were relatively new and were inefficient compared to todayÕs standards. Even with the difficulties, progress was being made. With time and effort, there is little doubt that a working test vehicle would have been constructed and flown.

References cited in this excerpt:

[1] PerelÕMan, R. G. Soviet Nuclear Propulsion (Yadernyye Dviagateli). Washington, D.C.: Triumph Publishing Co., 1959.

[2] Keirn, Donald J. ÒThe USAF Nuclear Propulsion ProgramsÓ in Nuclear Flight: The United States Air Force Programs for Atomic Jets, Missiles, and Rockets. Ed. Kenneth F Gantz. New York: Duell, Sloan, and Pearce, 1960.

[3] Megazone. ÒThe Decay of the Atomic Powered Aircraft Program,Ó Undergraduate Research Paper from Worcester Polytechnic Institute, Worcester, MA, 1993.

[11] Subcommittee on Research and Development of the Joint Committee on Atomic Energy. First Session on the Aircraft Nuclear Propulsion Program. Hearing, 86th Congress, 1st Session, July 28, 1959. Washington: GPO, 1959.

[12] Miller, Jay. The X-Planes. Arlington, TX: Aerofax, Inc, 1988.

[13] Cleveland, F.A. and Johnson, Clarence L., ÒDesign of Air Frames for Nuclear PowerÓ, Aeronautical Engineering Review, 16:pg 48-57, June 1957.