At EBR-1, an attempt was made to build atomic powered bombers that could remain aloft for
extended periods, before ballistic missiles were perfected. The engines and aircraft needed to
carry them were so big that they would have required a 10 mile runway (hence the eastern Idaho
location). The flight crew would have received a fairly large dose of radiation, because good
shielding would have been too heavy to fly, and there were the obvious safety problems with
airborne reactors.

In 1955, the X-39 was run on a ground test stand in what was called the Heat Transfer Reactor
Experiment No. l (HTRE-l). Engineers tested a complete aircraft power plant consisting of a
reactor, a radiation shield, two X-39 engines, ducting, control parts and instrumentation.

In January 1956, the engines were operated successfully but, because there had been no attempt to
restrict the weight of the shielding, they would be to heavy to fly. Later in 1957, other cores
that were tested, HTRE-2 and -3, did reduce the weight somewhat. The HTRE-3 assembly produced
enough thrust to theoretically sustain a flight at 460 mph for about 30,000 miles. However
radiation levels were still a problem; at one point in the tests, controls failed and released
enough radioactivity to contaminate 1,500 acres.