The rocket engine is the beating heart of space travel, and improving its efficiency is key to expanding our reach in the Solar System. In particular, the velocity of the exhaust gases must be maximized for optimal performance. It has always been understood that chemical rockets are not enough. To accelerate the exhaust to greater speeds, chemistry and its limitations must be ditched. One way to do this is by heating a stream of hydrogen with a high-temperature nuclear reactor and exhausting it out of a nozzle. This concept is known as Nuclear Thermal Rocket.
The beginnings
Concepts for nuclear propulsion for rockets and aircraft began floating around immediately after World War Two. Work on nuclear rockets started in 1955 when the Atomic Energy Commission gave the go-ahead to Los Alamos National Laboratory to commence research. The codename chosen was Project Rover.
Initially, the project was intended for military purposes. However, advances in chemical InterContinental Ballistic Missiles made the nuclear engines unnecessary. Thus, the focus shifted to civilian applications. The Sputnik Crisis in 1957 made a strong case for their utility. Project Rover became a joint NASA-AEC program in 1958 when the agency was created. The jointly-managed Space Nuclear Propulsion Office was created in August 1960 to oversee the project.
The site chosen for testing was Jackass Flats, inside the Nevada Test Site. Works on the facilities began in 1957, and over the years it expanded several times. Three engine test stands were built: Test Cell A and the bigger Test Cell C supported engines firing upwards, while the more complex Engine Test Stand 1 allowed downward firing in a reduced pressure environment, more closely resembling operational conditions.
Reactor assembly, disassembly, and maintenance took place in two buildings, E-MAD and R-MAD. These were built like massive hot cells, common in the nuclear industry. They had thick concrete walls, lead glass windows, and manipulator arms to allow technicians to work behind shielding. All buildings were connected by a small railroad. Several control bunkers and support facilities were also present.
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Kiwi
The first series of engines was named after the flightless bird since they weren’t meant to fly. Instead, their purpose was to demonstrate the concept of nuclear propulsion. The first design of the series was Kiwi A, with a thermal output of around 100 MW. These engines were quite small, having about as much thrust as RocketLab’s Rutherford engine. Three engines (Kiwi A, A’, and A3) were tested between July 1959 and October 1960.
All engines used gaseous hydrogen and a central island, containing control rods and heavy water. The latter provided both cooling and moderation (nuclear reaction enhancement) to reduce the need for uranium. The fuel used was uranium particles suspended in graphite plates for Kiwi A, while the other two engines also had a niobium carbide coating. Some fuel damage occurred, but they successfully proved the fundamental concept behind the nuclear rocket.
Next, the project aimed at increasing the power to 1000 MW with the Kiwi B series of engines. Engines with such power have thrust comparable to two RL-10s. Many design changes were implemented: the central island was deleted and new materials were employed. Control was switched from rods to drums, which can rotate to reveal sides that either absorb neutrons or reflect them to the core. Propellant was changed from gaseous hydrogen to liquid. The first series of tests took place from December 1961 to November 1962, with engines Kiwi B1A, B1B, and B4A. These suffered from several problems, including hydrogen fires, fuel elements breaking, and excessive vibrations.
Dealing with troubles
The latter problem required careful addressing. A series of “cold flow” tests with the reactor shut down revealed the cause. Hydrogen flowing in the gaps between the fuel elements caused the vibrations, and minor design changes fixed the problem. Between May and September 1964, the firings of Kiwi B4D and B4E reached full power with no significant issues. Other important features were demonstrated, such as the capability to restart a nuclear engine.
At Los Alamos, two reactors were run next to each other, showing that the neutrons escaping from one reactor affect the other only negligibly. Nuclear engines can thus be clustered to get more thrust. To investigate possible failure modes, mockup reactors were dropped from heights, and one was made to explode in a power excursion in January 1965. The latter test, known as Kiwi-TNT (Transient Nuclear Test), was conceived as a worst-case scenario accident. It showed that while accidents were possible, the energy releases were contained (in the context of rocketry) and contamination would not spread far out in significant quantities.
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NERVA
While Project Rover was underway, Aerojet and Westinghouse were selected to develop a flightworthy engine. This program was dubbed NERVA: Nuclear Engine for Rocket Vehicle Application. The design of the reactor was based on Kiwi B4, with the addition of the elements needed for autonomous flight, such as turbopumps.
The first two tests, NRX A2 and A3, took place between September 1964 and May 1965. These validated the general design of the reactor. These tests were some of the first after vibration problems had been discovered with Kiwi B4A, and their success confirmed that the issue was solved. In March 1966, NRX-EST (Nuclear Rocket eXperimental – Engine System Test) ran a series of tests where propellant injection was performed by a turbopump powered by hot gas taken from the engine itself. Previous tests used a pump mounted on the test stand, but this would be impossible for a flight model.
Refining the design
Tests in June 1966 and December 1967 on the NRX A5 and A6 reactors showed that prolonged operation at full power was possible. The engines run for a total of 30 and 62 minutes, respectively. The last NERVA engine to be built and tested was NERVA XE PRIME, with XE standing for eXperimental Engine. This was very close to a flight configuration, having turbopumps and hydraulic actuators. The main difference was that some non-critical components were not radiation-hardened, so a shield was also included.
Unlike all other engines, tests took place at ETS-1, which simulated firing in reduced pressure and a downward orientation. Between December 1968 and September 1969, the engine was started multiple times, racking up almost two hours of operation. Once again, the tests were successful and showed that the construction of a flightworthy engine was feasible.
Phoebus
While NERVA was in development, Project Rover kept developing new experimental engines. Predicting that higher thrust engines would be needed, a goal for a 5000 MW reactor was put forward. The first step toward this was the Phoebus 1A and 1B engines. They were based on Kiwi 4, but featured attempts to increase the power density of the reactor. Tests took place in June 1965 and February 1967, with peak power outputs of 1100 MW and 1500 MW.
Both engines performed well, with the latter suffering from some corrosion. The former, however, ran out of fuel due to faulty instrumentation, overheated, and exploded. One-fifth of the core was ejected, while the rest melted together. The decontamination of the test cell took two months, and emergency propellant tanks were installed for later tests.
Phoebus 2A was the biggest nuclear rocket ever tested. It was fired between June and July 1968. The core was significantly larger, and tweaking the geometry of the cooling channels increased the power density. It reached a maximum thermal power of 4100 MW, which would yield about as much thrust as the J-2 engine. Even today, the biggest commercial reactors produce about as much heat. The engine performed well mechanically. However, the reactivity was found to be lower than expected.
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Pewee and Nuclear Furnace
Next, scientists turned to smaller nuclear rockets. These would be suitable for smaller tugs and scientific missions, and allowed for cheaper research. They dubbed it Pewee 1, like the small bird. The engine sported both traditional niobium carbide and innovative zirconium carbide fuel coatings. To help the small core react criticality, some zirconium hydride sleeves were fitted over structural components. The engine ran in December 1968, setting records for power density and temperature. In most Project Rover reactors hydrogen was heated to around 2400 K, but Pewee 1 reached 2750 K. The new zirconium coating was also found to perform much better. Funding issues and environmental concerns prevented further tests in the series.
The last reactor built by Project Rover was the Nuclear Furnace. This was a small 44 MW reactor meant to test fuel elements. It wasn’t meant to be a prototype for any flight engine, so it didn’t have a nozzle and used gaseous hydrogen. During test runs in June and July 1972 two new kinds of fuel elements were tested. One was a pure uranium-zirconium carbide, while the other was a “composite” uranium-zirconium carbide dispersed in a graphite matrix. The composite fuel elements achieved better corrosion resistance, while the pure carbide ones cracked due to poor handling of thermal stress. The tests were also the first ones to filter the exhausts to remove radionuclides.
The end
The downfall of the nuclear rocket program was its use case. They are best used to send large masses beyond Low Earth Orbit. The considerable mass of the reactor made it unsuitable for small spacecraft. No crewed Mars missions or lunar bases were funded, and NASA pursued the Space Shuttle instead. The nuclear engine was left without payloads to haul. Project Rover and NERVA were thus canceled in early 1973. Cleanup operations began and some buildings have since been demolished. As of August 2023, E-MAD and ETS-1 are still mostly intact. Three fatalities occurred in the program, none of which were nuclear-related.
However, spaceflight is picking up pace again. If the trend continues, the advantages of nuclear thermal propulsion will be greatly needed. Indeed, there are pretty good odds that we might soon see the first nuclear engine work in space. DARPA has tasked Lockheed Martin and BWXT with developing DRACO, a technology demonstration spacecraft for nuclear propulsion. It is slated to fly in 2027. The data acquired during Project Rover will surely be precious.
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