WWII

The NB-36H was A Bold Experiment in Nuclear Aviation

The NB-36H, also known as the Nuclear Test Aircraft or ‘Crusader’, emerged as one of the most audacious ventures in aviation history.

During the 1950s, the United States embarked on this experimental project to explore the feasibility of nuclear-powered flight, a concept that promised virtually unlimited range and endurance for strategic bombers.

This aircraft, derived from the Convair B-36 Peacemaker, carried a nuclear reactor onboard, marking a significant milestone in aviation and nuclear engineering.

Conception

The conception of the NB-36H project stemmed from the strategic vision of the United States Air Force during the early Cold War period.



In an era characterised by intense competition and the looming threat of nuclear conflict, military strategists and aviation engineers sought to create an aircraft that could achieve unprecedented range and endurance.

 

This ambition aligned with the broader objective of maintaining a credible deterrent against potential adversaries. The idea of nuclear-powered flight, with its promise of virtually limitless range without the need for refuelling, became an attractive proposition.

The NB-36 was based on the Peacemaker.

Convair, a leading aerospace manufacturer, undertook the challenge of transforming this vision into reality. Engineers at Convair chose the B-36 Peacemaker as the foundation for this ambitious project.

The B-36, already renowned for its long-range capabilities, provided a robust platform capable of accommodating the substantial modifications required for housing a nuclear reactor.

Design

The design phase began with extensive theoretical studies and simulations to understand the implications of integrating a nuclear reactor into an aircraft. Engineers needed to address several critical challenges, including the safe containment of the reactor, effective radiation shielding for the crew, and the structural integrity of the modified airframe.

The decision to place the reactor behind the cockpit necessitated a complete redesign of the central fuselage section.

This redesigned section featured a specially constructed compartment for the reactor, equipped with reinforced structures to secure the heavy reactor unit. Convair engineers employed innovative materials and design techniques to ensure the compartment could withstand both the reactor’s weight and the stresses of flight.



They focused on creating a robust, vibration-resistant mounting system to keep the reactor stable under all flight conditions.

The reactor’s installation demanded meticulous planning to address the radiation hazards it posed. The engineers developed a sophisticated shielding system, incorporating layers of lead and polyethene, which effectively absorbed the reactor’s emitted radiation.

This shielding extended around the reactor compartment and included a specially designed crew compartment. The cockpit and crew areas were encased in a shell composed of lead and rubber, forming a barrier that protected the crew from gamma rays and neutrons.

The NB-36H in formation with a B-50 in 1955.

Cooling Requirements

Additionally, the design team had to consider the reactor’s cooling requirements. They selected an air-cooled reactor, as it offered a simpler and more reliable cooling mechanism compared to liquid-cooled alternatives.

This choice necessitated modifications to the aircraft’s airflow systems to ensure a consistent and adequate supply of air to maintain the reactor’s temperature within safe operational limits.

Throughout the design process, Convair engineers worked closely with nuclear physicists and safety experts to address potential risks and ensure the reactor’s safe operation during flight. They conducted extensive ground tests and simulations to validate their designs before the NB-36H took to the skies.

This collaborative effort between aerospace engineers and nuclear scientists underscored the interdisciplinary nature of the project, blending advanced aerospace engineering with cutting-edge nuclear technology.

The Reactor

The NB-36H’s nuclear reactor represented a groundbreaking technological achievement, central to the aircraft’s mission of exploring the feasibility of nuclear-powered flight.

Engineers selected an air-cooled reactor, a choice driven by the need for simplicity and reliability in the aircraft’s operational environment. This reactor, producing 1 megawatt of power, served primarily as a research tool rather than a propulsion source.

Its primary purpose was to test the integration of nuclear technology into an aircraft and to evaluate the effectiveness of various shielding methods.

The reactor’s integration into the NB-36H required meticulous planning and innovative engineering solutions. Engineers placed the reactor in a specially designed compartment within the aircraft’s fuselage, situated behind the cockpit.

The right nuclear engineer’s panel.

This location helped minimize radiation exposure to the crew while maintaining the aircraft’s centre of gravity. The compartment was structurally reinforced to secure the reactor, which weighed several tons, and to withstand the stresses of flight.



Cooling the reactor posed a significant challenge. The engineers opted for an air-cooled system to avoid the complexities associated with liquid cooling. This system relied on the aircraft’s airflow to dissipate heat generated by the reactor.

They modified the aircraft’s intake and exhaust systems to ensure a steady and adequate supply of cooling air, preventing the reactor from overheating during flight. The cooling system’s design was crucial for maintaining the reactor’s operational integrity and ensuring safe flight conditions.

Shielding

Radiation shielding constituted the most critical aspect of the reactor’s integration. The reactor emitted harmful gamma rays and neutrons, necessitating comprehensive shielding to protect the crew. Engineers devised a multi-layered shielding system combining lead and polyethene.

Lead, with its high density, effectively absorbed gamma rays, while polyethene, a hydrogen-rich material, proved effective against neutron radiation. The design team applied these materials strategically around the reactor compartment to maximize protection while minimizing additional weight.

Note the radiation warning symbol on the tail.

The shielding extended to the crew compartment, where engineers implemented a shell of lead and rubber around the cockpit and crew areas. This shell served as a secondary barrier, further reducing radiation exposure.

The design ensured that all critical areas where the crew operated were shielded, including the cockpit, navigation stations, and other control areas. Engineers paid particular attention to the seams and joints of the shielding materials to prevent any radiation leaks, ensuring a continuous and effective barrier.

Continual Radiation Testing

The complex interaction between the reactor and its shielding required extensive testing and validation. Engineers conducted numerous ground tests to measure radiation levels and assess the shielding’s effectiveness.

They simulated various flight conditions to evaluate how the reactor and shielding would perform in different scenarios. These tests informed adjustments and refinements to the shielding design, ensuring optimal protection before the aircraft commenced flight tests.

During the NB-36H’s test flights, engineers continuously monitored radiation levels throughout the aircraft. They installed a network of radiation detectors to provide real-time data on radiation exposure, allowing them to verify the shielding’s performance and identify any areas requiring further improvement.

The data collected from these flights was crucial in understanding the reactor’s behaviour in a flight environment and the effectiveness of the shielding in dynamic conditions.

Testing

The NB-36H embarked on its maiden flight in September 1955, marking the beginning of a rigorous series of test flights that would extend over the next two years.

These flights aimed to validate the aircraft’s design, evaluate the reactor’s performance, and ensure the effectiveness of the radiation shielding.

The comprehensive test program provided critical data and insights, shaping the future of nuclear-powered aviation research.

From its first flight, the NB-36H operated under close scrutiny. Engineers and scientists from Convair and the U.S. Air Force closely monitored every aspect of the aircraft’s performance. The initial flights focused on basic operational parameters, such as handling characteristics and structural integrity under the additional weight of the reactor and shielding.

These early flights confirmed that the aircraft could safely take off, fly, and land with the reactor on board, setting the stage for more intensive testing.

Detail of Convair NB-36H nose section. The aircraft has its original designation of XB-36H.

47 Flights

As the test program progressed, flights with the reactor active became more frequent. Engineers conducted a total of 47 test flights, accumulating significant operational experience with a nuclear reactor in an airborne environment.

The reactor operated for a combined total of 89 hours during these flights, providing ample data for analysis. Each flight followed stringent safety protocols, with contingency plans in place for reactor shutdowns or emergencies.

A key focus during these flights was the effectiveness of the radiation shielding. Engineers equipped the NB-36H with an array of radiation sensors strategically placed throughout the aircraft.

These sensors continuously monitored radiation levels, particularly in the crew compartment, to ensure the shielding performed as expected.

The real-time data collected allowed engineers to verify the integrity of the shielding and make any necessary adjustments.

The NB-36H’s test flights covered a range of operational scenarios to assess the reactor and shielding under various conditions. Engineers tested the aircraft at different altitudes, speeds, and flight manoeuvres to observe how these variables affected radiation levels and reactor performance.

They also simulated potential emergency situations, such as rapid descents and abrupt manoeuvres, to ensure the reactor remained secure and the shielding maintained its effectiveness.

Was it a Good Idea?

Throughout the test program, the NB-36H demonstrated that a nuclear reactor could be safely operated on an aircraft while effectively protecting the crew from radiation exposure.

The data collected provided valuable insights into the reactor’s thermal behaviour, structural impacts, and the dynamic performance of the shielding.

These findings informed subsequent designs and safety protocols for nuclear-powered aviation and other applications of airborne nuclear reactors.

The achievements of the NB-36H extended beyond its immediate technical successes. The program established foundational knowledge for future research into nuclear propulsion.

Although the concept of a nuclear-powered bomber did not come to fruition, the lessons learned from the NB-36H contributed to advancements in nuclear safety, reactor design, and materials science.

The project also highlighted the potential and challenges of integrating complex nuclear systems into mobile platforms.

Moreover, the NB-36H’s successful flights underscored the importance of interdisciplinary collaboration. The project brought together experts from aerospace engineering, nuclear physics, materials science, and safety engineering.

This collaborative approach proved essential in addressing the multifaceted challenges of nuclear aviation, fostering innovations that extended beyond the project itself.