Some time ago someone wrote question about General Electric X-211 nuclear engines. Here is the answer.
Under the ANP program the General Electric Co., at Evendale, Cincinnati was issued a contract to develop a direct-cycle turbojet, and Pratt & Whitney
Aircraft Division of United Aircraft Corp. was authorized to study an indirect cycle and work was started at the Connecticut Aircraft Nuclear Engine
Laboratory (CANEL). In the direct air cycle air enters through the compressor stage of one or more turbojets. From there the air passes through a
plenum an is directed through the reactor core. The air, acting as the reactor coolant, is rapidly heated as it travels through the core. After
passing through the reactor the air passes through another plenum and is directed to the turbine section of the turbojet(s) and from there out through
the tailpipe. An indirect system is very similar, except that the air does not pass through the reactor itself. After passing through the compressor
the air passes through a heat exchanger. The heat generated by the reactor is carried by a working fluid to this heat exchanger. The air then passes
through the turbine and out the tailpipe as above. The working fluid in the indirect cycle is usually a dense fluid, such as a liquid metal, or highly
pressurized water. This allows more heat energy to be transfer, thereby increasing the efficiency of the system.
Originally the ANP program was to develop an indirect cycle, single reactor propulsion system. However, a petition by General Electric to the
government allowed them to develop the direct cycle system. GE claimed that the direct cycle was simpler and therefore would have a shorter
development time. For the indirect cycle system, Pratt & Whitney developed the super-critical water reactor, in which the working fluid is water
heated to 1,500 degrees fahrenheit, but kept in a liquid state by pressurizing to 5,000psi. This avoided the problems of using a liquid metal working
fluid. However, progress on indirect cycle engine went much slower that it did with the GE HTREs. P&W never ran a practical test system. In fact their
work was limited to component testing. In addition to work on the super-critical water reactor P&W worked with liquid metal coolant designs. It was
the latter that received the most attention. The two major designs were a solid core reactor, in which the liquid metal circulated through a solid
reactor core, and a circulating-fuel design, in which fuel was mixed with the coolant and critical mass was achieved as the coolant circulated through
a central core. After the circulating-fuel design showed promise, work on the super-critical reactor was halted. P&W did accomplish a great deal on
the design of liquid metal cooling loops, corrosion prevention, and heat exchanger design. However, P&W work at CANEL never led to a test reactor,
much less one which was flight ready. In the long run the indirect cycle showed more promise, but it also required a great deal more developmental
The engine chosen for X-6 was the General Electric J53 turbojet. At the time the J53 was a conventional turbojet in the planning stage. The J53 was a
high- performance design and it was felt that conversion to nuclear power would present no more difficulty than any other design then in use. In the
early stages of the program, before GE's petition, it was planned to connect the J53 to a liquid-metal reactor for use on the X-6. The original
propulsion system was to have weighed 165,000 pounds. This was composed of a 10,000 pound reactor, 60,000 pounds of reactor shielding, 37,000 pounds
of crew shielding, and a total engine weight of 18,000 pounds plus an additional 40,000 pounds for ducts and accessories. After experiencing
development problems with the J53, GE resorted to the J47 as the powerplant. J47s converted for nuclear testing were called X-39s or X-211s.
General Electric ran a series of very successful experiments using the direct cycle concept. These were referred to as the Heat Transfer Reactor
Experiment (HTRE) series. The series involved three reactors, HTRE-1 through HTRE-3. HTRE-1 became HTRE-2 at the conclusion of its test program.
HTRE-1 (and therefore HTRE-2) successfully ran one X-39 (modified J-47) solely under nuclear power. HTRE-3 was the closest to a flight article the
program came. It was solid moderated, as opposed to the earlier reactors which were water moderated, and it powered two X-211s at higher power levels.
HTRE-3 was limited by the two turbojets, but it could have powered larger jets at even higher power levels. The system was called XMA-1. HTRE-1 was
principally a proof of concept reactor. HTRE-1 achieved a number of full-power runs that demonstrated conclusively the feasibility of operating a jet
engine on nuclear power. HTRE-2 was simply HTRE-1 modified to test advanced reactor sections in a central hexagonal chamber. In this way new reactor
designs could be tested without the need to build a totally new reactor from scratch. The experience gained from HTRE-1 and HTRE-2 was used in the
construction of HTRE-3. HTRE-3 was the final test item designed to prove the feasibility of producing an actual aircraft powerplant. The design and
testing of HTRE-3 has advanced the direct-cycle program beyond the question of feasibility to the problems of engineering optimization.
All three of the HTRE reactors were of the standard direct cycle configuration, with the addition of a chemical combustor just upstream from the
turbines. This combustor allowed the jets to be started on chemical power and then be switched over to atomic heat as the reactor was brought up to
operating temperatures. The operational system may have also utilized a chemical combustor for use during takeoff and landing, and possibly target
penetration, when the reactors relatively slow response time could be a disadvantage. The HTRE either met or exceeded their goals, but although all
had reactor cores of roughly the size needed to fit into an aircraft, none of the HTREs were designed to be a prototype of a flight system; the series
showed that it then appeared "possible and practical with the technology in hand to build a flyable reactor of the same materials as HTRE-3 and
similar in physical size." Despite the fact that HTRE-3 didn't produce the power that would have been needed for flight, that was mainly because it
was not an optimized design; it was designed simply as a research reactor, to prove the concepts needed for a flight article. At the end of the HTRE
run the probability of flying a reactor seemed high. The test runs showed that a reactor using the same materials as HTRE-3, and which could power a
gas-turbine powerplant, could have been built at that time.
In april 1959 GE stated that studies indicated that the basic XMA-1 power plant was suitable for the CAMAL mission. Studies by both Convair and
Lockheed on the CAMAL airplane based on design objectives for the XMA-1C (Expected to use an advance fuel element of iron-chrome-aluminum or ceramic
material. The turbine inlet temperature was expected to be 1700 degrees F, producing about 42 000 pounds of thrust at static sea level conditions.)
power plant indicated the possibility of attaining such an airplane. GE proposed that, after the airplane had been checked out on chemical power
plants, the XMA-1A (Planned to operate with nichrome fuel elements at a turbine inlet temperature of about 1500 degrees F, producing about 26 000
poundss of thrust at static sea level conditions) would first be tested, to be followed by testing of the XMA-1C power plant. As a consequence of a
program reorientation in July 1959, work on the XMA-1A powerplant was canceled in August 1959.