In 1949, the program ran a series of tests, known as the Heat Transfer Reactor Experiment (HTRE), involving three types of reactors, with the purpose of determining the most efficient method of transferring energy from the reactor. After an extensive trial series, the HTRE-3 emerged as the selected transfer system. The HTRE-3 was a Direct-Cycle Configuration. In a direct cycle system, the air entered the engine through the compressor of the turbojet, it then moved to a plenum intake that directs the air to the core of the reactor. At this point the air, serving as the reactor coolant, is super-heated as it travels through the core. After that stage, it goes to another plenum intake; from there the air is directed to the turbine section of the engine and eventually to the tailpipe. This configuration allowed the aircraft engine to start on chemical power and then switch to nuclear heat as soon as the core reached optimized operational temperatures, thus providing the proposed aircraft the ability to take-off and land on conventional power. Another system considered was the Indirect-Cycle Configuration. In this configuration, the air did not go through the reactor core, air instead passed through a heat exchanger. The heat generated by the reactor is carried by liquid metal or highly pressurized water, to the heat exchanger where the air is, thus heating the air in its way to the turbine. Engineers preferred the direct-cycle approach due to the fact that was simpler to produce; program managers preferred the idea because its development time was relatively short compared to the indirect system.

After establishing the parameters for the power plant and the transfer mechanism, engineers commenced work on the shielding for the crew and aircraft avionic systems. Initial plans called for the shielding of the reactor by massive layers of cadmium, paraffin wax, beryllium oxide, and steel. The idea behind this setting was that the more protection the reactor have, the less shielding the crew cabin would require. Technically, this was a sound approach, but in a rapidly functioning environment such as an aircraft setting, this shielding proved to be ineffective. For this reason it was decided to implement what is known as Shadow Shielding Concept. In shadow shielding, the layers of protection would be equally divided between the reactor and the crew cabin. Shadow Shielding would also provide a more robust protection for the aircraft’s avionics systems. An added plus from the implementation of this system was the reduction in the weight of the aircraft due to the distribution of the shield.