The Nuclear Engine for Rocket Vehicle Application (NERVA) was a U.S. nuclear thermal rocket engine development program that ran for roughly two decades. NERVA was a joint effort of the U.S. Atomic Energy Commission (AEC) and NASA, managed by the Space Nuclear Propulsion Office (SNPO) until both the program and the office ended at the end of 1972. NERVA demonstrated that nuclear thermal rocket engines were a feasible and reliable tool for space exploration, and at the end of 1968 SNPO certified that the latest NERVA engine, the NRX/XE, met the requirements for a human mission to Mars. Although NERVA engines were built and tested as much as possible with flight-certified components and the engine was deemed ready for integration into a spacecraft, much of the U.S. space program was cancelled by Congress before a crewed mission to Mars could take place.
Diagram of the NERVA nuclear rocket engine
|Country of origin||United States|
|Designer||Los Alamos Scientific Laboratory|
|Application||Upper stage engine|
|Thrust (vac.)||337 kilonewtons (76,000 lbf)|
|Chamber pressure||3,100 kilopascals (450 psi)|
|Isp (vac.)||825 seconds (8.09 km/s)|
|Burn time||1,200 seconds|
|Length||1.7 meters (5 ft 7 in)|
|Diameter||0.98 meters (3 ft 3 in)|
|Dry weight||12,300 kilograms (27,100 lb)|
NERVA was considered by the AEC, SNPO and NASA to be a highly successful program; it met or exceeded its program goals. Its principal objective was to "establish a technology base for nuclear rocket engine systems to be utilized in the design and development of propulsion systems for space mission application". Virtually all spacecraft concepts featuring nuclear thermal rockets use derivative designs from the NERVA NRX or Pewee.
During World War II, some scientists at the Manhattan Project's Los Alamos Laboratory where the first atomic bombs were designed, including Stan Ulam, Frederick Reines and Frederic de Hoffmann, speculated about the development of nuclear powered rockets. In 1946, Ulam and C. J. Everett wrote a paper in which they considered the using atomic bombs as a means of rocket propulsion. This would become the basis for Project Orion.
The public revelation of atomic energy at the end of the war generated a great deal of speculation, and in the United Kingdom, Val Cleaver, the chief engineer of the rocket division at De Havilland, and Leslie Shepard, a nuclear physicist at the University of Cambridge, independently considered the problem of nuclear rocket propulsion. They became collaborators, and in a series of papers published in the Journal of the British Interplanetary Society in 1948 and 1949, they outlined the design of a nuclear-powered rocket with a solid-core graphite heat exchanger. They reluctantly concluded that nuclear rockets were essential for deep space exploration, but not yet technically feasible.
In 1953, Robert W. Bussard, young physicist working on the Nuclear Energy for the Propulsion of Aircraft (NEPA) project at the Oak Ridge National Laboratory wrote a detailed study on "Nuclear Energy for Rocket Propulsion". He had read Cleaver and Shepard's work, that of the Chinese physicist Hsue-Shen Tsien, and a February 1952 report by engineers at Consolidated Vultee. Bussard's study had little impact at first, because only 29 copies were printed, and it was classified as Restricted Data, and therefore could only be read by someone with the required security clearance. In December 1953, it was published in Oak Ridge's Journal of Reactor Science and Technology. While still classified, this gave it a wider circulation. Darol Froman, the Deputy Director of the Los Alamos Scientific Laboratory (LASL), and Herbert York, the director of the University of California Radiation Laboratory at Livermore, were interested, and established committees to investigate nuclear rocket propulsion. Froman brought Bussard out to Los Alamos to assist for one week per month.
Bussard's study also attracted the attention of John von Neumann, and he formed an ad hoc committee on Nuclear Propulsion of Missiles. Mark Mills, the assistant director at Livermore was its chairman, and its other members were Norris Bradbury, Edward Teller, Herbert York, Abe Silverstein and Allan F. Donovan. After hearing input on various designs, the Mills committee recommended that development proceed, with the aim of producing a nuclear rocket upper stage for an intercontinental ballistic missile (ICBM). York created a new division at Livermore, and Bradbury created a new one called N Division at Los Alamos under the leadership of Raemer Schreiber, to pursue it. In March 1956, the Armed Forces Special Weapons Project (AFSWP) recommended allocating $100 million to the nuclear rocket engine project over three years for the two laboratories to conduct feasibility studies and construct of test facilities.
Eger V. Murphree and Herbert Loper at the Atomic Energy Commission (AEC) were more cautious. The Atlas missile program was proceeding well, and if successful would have sufficient range to hit targets in most of the Soviet Union. At the same time, nuclear warheads were becoming smaller, lighter and more powerful. The case for a new technology that promised heavier payloads over longer distances therefore seemed weak. However, the nuclear rocket had acquired a political patron in Senator Clinton P. Anderson from New Mexico (where LASL was located), the deputy chairman of the United States Congress Joint Committee on Atomic Energy (JCAE), who was close to von Neumann, Bradbury and Ulam. He managed to secure funding.
All work on the nuclear rocket was consolidated at Los Alamos, where it was given the codename Project Rover; Livermore was assigned responsibility for development of the nuclear ramjet, which was codenamed Project Pluto. Project Rover was directed by an active duty USAF officer seconded to the AEC, Lieutenant Colonel Harold R. Schmidt. He was answerable to another seconded USAF officer, Colonel Jack L. Armstrong, who was also in charge of Pluto and the Systems for Nuclear Auxiliary Power (SNAP) projects.
In principle, the design of a nuclear thermal rocket engine is quite simple: a turbopump would force hydrogen through a nuclear reactor that would heat it to very high temperatures. Complicating factors were immediately apparent. The first was that a means had to be found of controlling reactor temperature and power output. The second was that a means had to be devised to hold the propellant. The only practical means of storing hydrogen was in liquid form, and this required temperatures below 20 K. The third was that the hydrogen would be heated to a temperature of around 2,500 K, and materials would be required that could both withstand such temperatures and resist corrosion by hydrogen.
Hydrogen was theoretically the best possible propellent, but in the 1950s it was expensive, and available only in small quantities. For the fuel, they considered plutonium-239, uranium-235 and uranium-233. Plutonium was rejected because it tends to form compounds, and could not reach temperatures as high as those achievable by uranium. Uranium-233 held the prospect of saving weight, but was not readily available. To control the reactor, the core was surrounded by control rods which were beryllium (a neutron moderator) on one side and boron (a neutron poison) on the other. The reactor's power output could be controlled rotating the rods.
LASL produced a series of design concepts, each with its own codename: Uncle Tom, Uncle Tung, Bloodhound and Shish. By 1955, it had settled on a 1,500 MW design called Old Black Joe. In 1956, this became the basis of 2,700 MW design intended to be the upper stage of an ICBM.
Nuclear reactors for Project Rover were built at LASL Technical Area 18 (TA-18), also known as the Pajarito Site. The reactors were tested at very low power before being shipped to Jackass Flats in the Nevada Test Site. Testing of fuel elements and other materials science was done by the LASL N Division at TA-46 using various ovens and later the Nuclear Furnace.
Work commenced on test facilities at Jackass Flats in mid-1957. All materials and supplies had to be brought in from Las Vegas. Test Cell A consisted of a farm of hydrogen gas bottles and a concrete wall 3 feet (0.91 m) thick to protect the electronic instrumentation from radiation from the reactor. The control room was located 2 miles (3.2 km) away. The plastic coating on the control cables was chewed by burrowing rodents and had to be replaced. The reactor was test fired with its plume in the air so that radioactive products could be safely dissipated.
The reactor maintenance and disassembly building (R-MAD) was in most respects a typical hot cell used by the nuclear industry, with thick concrete walls, lead glass viewing windows, and remote manipulation arms. It was exceptional only for its size: 250 feet (76 m) long, 140 feet (43 m) and 63 feet (19 m) high. This allowed the engine to be moved in and out on a railroad car. The "Jackass and Western Railroad", at it was light-heartedly described, was said to be the world's shortest and slowest railroad. There were two locomotives, the remotely controlled electric L-1, and the diesel/electric L-2, which was manually controlled, but had radiation shielding around the cab. Construction workers were housed in Mercury, Nevada. Later thirty trailers were brought to Jackass Flats to create a village named "Boyerville" after the supervisor, Keith Boyer. Construction work was completed in the fall of 1958.
Transfer to NASAEdit
By 1957, the Atlas missile project was proceeding well, and the need for a nuclear upper stage had all but disappeared. On 2 October 1957, the AEC proposed cutting its budget, but its timing was off. Two days later, the Soviet Union launched Sputnik 1, the first artificial satellite. This surprise success fired fears and imaginations around the world. It demonstrated that the Soviet Union had the capability to deliver nuclear weapons over intercontinental distances, and contested cherished American notions of military, economic and technological superiority. This precipitated the Sputnik crisis, and triggered the Space Race. President Dwight D. Eisenhower responded to by creating the National Aeronautics and Space Administration (NASA), which absorbed the National Advisory Committee for Aeronautics.
NACA had long been interested in nuclear technology. In 1951, it had begun exploring the possibility of acquiring its own nuclear reactor for the aircraft nuclear propulsion (ANP) project, and selected its Lewis Flight Propulsion Laboratory in Ohio to design, build and manage it. A site was chosen at the nearby Plum Brook Ordnance Works, NACA obtained approval from the AEC, and construction of the Plum Brook Reactor commenced in September 1956. Abe Silverstein, the director of Lewis, was particularly eager to acquire control of Project Rover.
Donald A. Quarles, the Deputy Secretary of Defense, met with T. Keith Glennan, the new administrator of NASA, and Hugh Dryden, his deputy on 20 August 1958, the day they after were sworn into office at the White House, and Rover was the first item on the agenda. Quarles was eager to transfer Rover to NASA, as the project no longer had a military purpose. Responsibility for the non-nuclear components of Project Rover was officially transferred from the United States Air Force (USAF) to NASA on 1 October 1958, the day NASA officially became operational and assumed responsibility for the U.S. civilian space program.
Space Nuclear Propulsion OfficeEdit
Project Rover became a joint NASA-AEC project. Silverstein appointed Harold Finger to oversee the nuclear rocket development.  On 29 August 1960, NASA created the Space Nuclear Propulsion Office (SNPO) to oversee the nuclear rocket project. Finger was appointed as it manager, with Milton Klein from AEC as his deputy. Finger was also the Director of Nuclear Systems in the NASA Office of Advanced Research and Technology. A formal "Agreement Between NASA and AEC on Management of Nuclear Rocket Engine Contracts" was signed by NASA Deputy Administrator Robert Seamans and AEC General Manager Alvin Luedecke on 1 February 1961. This was followed by an "Inter-Agency Agreement on the Program for the Development of Space Nuclear Rocket Propulsion (Project Rover)", which they signed on 28 July 1961. It soon became apparent that there were considerable cultural differences between NASA and AEC.
SNPO Headquarters was co-located with AEC Headquarters in Germantown, Maryland. Finger established branch offices at Albuquerque, New Mexico, (SNPO-A) to work liaise with LASL, and in Cleveland, Ohio, (SNPO-C) to coordinate with the Lewis Research Center, which was activated in October 1961. In June 1962, a branch was established at Las Vegas (SNPO-N) to manage the Nuclear Rocket Development Station at Jackass Flats. By the end of 1963, there were 13 NASA personnel at SNPO Headquarters, 59 at SNPO-C and 30 at SNPO-N. SNPO staff were a combination of NASA and AEC employees whose responsibilities included "program and resource planning and evaluation, the justification and distribution of program resources, the definition and control of overall program requirements, monitoring and reporting of progress and problems to NASA and AEC management, and the preparation of testimony to Congress." Finger called for bids from industry for the development of the nuclear engine for rocket vehicle application (NERVA) based upon the Kiwi engine. The award was scheduled for 1 March 1961, so the decision to proceed could be made by the incoming Kennedy administration. Eight companies submitted bids: Aerojet, Douglas, Glenn L. Martin , Lockheed, North American, Rocketdyne, Thiokol and Westinghouse. A joint NASA-AEC board evaluated the bids. It rated North American's bid as the best bid overall, but Westinghouse and Aerojet had superior bids for the reactor and engine respectively when they were considered separately. After Aerojet promised NASA administrator James E. Webb that it would put its best people on NERVA, Webb spoke to the selection board, and told them that while he did not wish to influence their decision, North American was deeply committed to Project Apollo, and the board might consider combining other bids. On 8 June, Webb announced that Aerojet and Westinghouse had been selected. Aerojet became the prime contractor, with Westinghouse as the principal subcontractor. Both companies recruited aggressively, and by 1963, Westinghouse had 1,100 staff working on NERVA.
In March 1961, President John F. Kennedy announced the cancellation of the aircraft nuclear propulsion project just as NASA's Plum Brook reactor was nearing completion, and for a time it seemed that NERVA would soon follow. NASA estimated its cost at $800 million (although AEC reckoned that it would be much less), and the Bureau of the Budget argued that NERVA made sense only in the context of a crewed lunar landing or flights further into the Solar System, to neither of which had the administration committed. Then, on 12 April, the Soviet Union launched Yuri Gagarin into orbit on Vostok 1, once again demonstrating their technological superiority. A few days later, Kennedy launched the disastrous Bay of Pigs Invasion of Cuba, resulting in yet another humiliation for the United States. On 25 May, he addressed a joint session of Congress. "First," he announced, "I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to the earth." He then went on to say: "Secondly, an additional 23 million dollars, together with 7 million dollars already available, will accelerate development of the Rover nuclear rocket. This gives promise of some day providing a means for even more exciting and ambitious exploration of space, perhaps beyond the moon, perhaps to the very end of the Solar system itself."
Reactor in-flight test (RIFT)Edit
The SNPO set an objective for NERVA of 99.7 percent reliability, meaning that the engine would fail to perform as designed only three times in every thousand starts. To achieve this, Aerojet and Westinghouse estimated that they would require 6 reactors, 28 engines and 6 reactor in-flight test (RIFT) flights. They planned for 42 tests, considerably less than the 60 tests that the SNPO had thought might be required. Unlike other aspects of NERVA, RIFT was solely a NASA responsibility. NASA delegated responsibility for RIFT to Wernher von Braun's Marshall Space Flight Center (MSFC) in Huntsville, Alabama. Von Braun created a Nuclear Vehicle Projects Office at MFSC, headed by Colonel Scott Fellows, a USAF officer who had worked on ANP. At the time, NASA was engaged in planning for the lunar landing mission that Kennedy had called for it to undertake. For this, it considered various booster concepts, including what became the Saturn family and the larger Nova. These were chemical rockets, although nuclear upper stages were also considered for Nova. In a 1960 paper, Schmidt proposed replacing the upper stages of a Saturn booster with nuclear NERVA stages. This would deliver the same performance as Nova, but for half the cost. He estimated that the cost of putting a pound of payload into lunar orbit as $l,600 for an all-chemical Saturn, $1,100 for Nova, and $700 for a chemical-nuclear Saturn. MSFC issued a study contract for a RIFT with NERVA as the upper stage of a Saturn C-3, but the C-3 was replaced soon after by the more powerful C-4 and ultimately the C-5, which became the Saturn V. Only in July 1962, after much debate, did NASA finally settle on lunar orbit rendezvous, which could be performed by Saturn V, and Nova was abandoned.
The RIFT vehicle would consist of an S-IC first stage, a dummy S-II middle stage filled with water, and an S-N (Saturn-Nuclear) NERVA upper stage. For an actual mission, a real S-II stage would be used. The S-N stage was to be built by Lockheed in a dirigible hangar NASA acquired at Moffet Field in Sunnyvale, California, and assembled at NASA's Mississippi Test Facility. The SNPO planned to build ten S-N stages, six for ground tests and four for flight tests. Launches were to take place from Cape Canaveral. NERVA engines would be transported by road in shockproof, watertight containers, with the control rods locked in place and nuclear poison wires in the core. Since it would not be radioactive, it could be safely transported and mated to the lower stages without shielding in the Vehicle Assembly Building (VAB). The RIFT test vehicle would be 364 feet (111 m) tall, about the tame as the Saturn V; the Saturn C-5N mission configuration would be larger still, at 393 feet (120 m) tall, but the 525-foot (160 m) VAB could easily accommodate it. In flight, the poison wires would be pulled and the reactor started 75 miles (121 km) above the Atlantic Ocean. The engine would fire for 1,300 seconds, boosting the it to 300 miles (480 km). It would then be shut down, and the reactor cooling before impacting the Atlantic 2,000 miles (3,200 km) downrange. NERVA would be regarded as mission ready after four successful tests.
The main bottleneck in the NERVA program was the test facilities at Jackass Flats. Test Cell C was supposed to be complete in 1960, but NASA and AEC did not request funds for additional construction in 1960, although Anderson provided them anyway. Then there were construction delays, forcing Anderson to personally intervene. Then, in August 1961, the Soviet Union ended the nuclear test moratorium that had been in place since November 1958, so Kennedy resumed U.S. testing in September. With a second crash program at the Nevada Test site, labor became scarce, and there was a strike. When that ended, the workers had to come to grips with the difficulties of dealing with hydrogen, which could leak through microscopic holes that would contain other fluids. On 7 November 1961, a minor accident caused a violent hydrogen release. The complex finally became operational in 1964. SNPO envisaged the construction of a gargantuan 20,000 MW nuclear rocket engine, so Boyer had the Chicago Bridge & Iron Company construct two gigantic 1,900,000-litre (500,000 US gal) cryogenic storage dewars. An engine maintenance and disassembly building (E-MAD) was added. It was larger than a football field, with thick concrete walls and shield bays where engines could be assembled and disassembled. There was also an engine test stand (ETS-1); two more were planned.
The first phase of Project Rover, Kiwi was named after the New Zealand kiwi bird. A kiwi cannot fly, and the Kiwi rocket engines were not intended to do so either. Their function was to verify the design, and test the behavior of the materials used. The Kiwi program developed of a series of non-flyable test nuclear engines, with the primary focus on improving the technology of hydrogen-cooled reactors. In the Kiwi A series of tests conducted between July 1959 and October 1960, three reactors were built and tested. Kiwi A was considered a success as a proof of concept for nuclear rocket engines. It demonstrated that hydrogen could be heated in a nuclear reactor to the temperatures required for space propulsion, and that the reactor could be controlled.
The next step was the Kiwi B series of tests, which commenced with Kiwi B1A on 7 December 1961. This was a development of the Kiwi A engine, with a series of improvements. The second test in the series, Kiwi B1B on 1 September 1962, resulted in extreme structural damage to the reactor, with fuel module components being ejected as it was ramped up to full power. The subsequent full-power Kiwi B4A test on 30 November 1962, along with a series of cold flow tests revealed that the problem was vibrations induced as the hydrogen was heated when the reactor was brought to full power that shook the reactor apart (rather than when it was running at full power). Unlike a chemical engine that would likely have blown up after suffering catastrophic damage, the nuclear rocket engine remained stable and controllable even when tested to destruction. The tests demonstrated that a nuclear rocket engine would be rugged and reliable in space.
Kennedy visited Los Alamos on 7 December 1962 for a briefing on Project Rover. It was the first time a president had visited a nuclear weapons laboratory. He brought with him a large entourage that included Lyndon Johnson, McGeorge Bundy, Jerome Wiesner, Harold Brown, Donald Hornig, Glenn Seaborg, Robert Seamans, Harold Finger and Clinton Anderson. The next day, the flew to Jackass Flats, making Kennedy the only president to ever visit a nuclear test site. Project Rover had received $187 million in 1962, and AEC and NASA were asking for another $360 million in 1963. Kennedy drew attention to his administration's budgetary difficulties, and asked what the relationship was between Project Rover and Apollo. Finger replied that it was an insurance policy, and could be used in the later Apollo or post-Apollo missions, such as a base on the Moon or a mission to Mars. Weisner, supported by Brown and Hornig, argued that if a Mars mission could not occur before the 1980s, then RIFT could be postponed to the 1970s. Seamans noted that such an attitude had resulted in the Sputnik crisis and a loss of American prestige and influence.
In January 1963, Anderson became chairman of the United States Senate Committee on Aeronautical and Space Sciences. He met privately with Kennedy, who agreed to request a supplemental appropriation for RIFT if a "quick fix" to the Kiwi vibration problem that Seaborg promised could be implemented. In the meantime, Finger called a meeting. He declared that there would be no "quick fix". He criticised LASL's management structure and called for LASL to adopt a project management structure. He wanted the case of the vibration problems thoroughly investigated, and the cause definitely known before corrective action was taken. Three SNPO staff (known at LASL as the "three blind mice") were assigned to LASL to ensure that his instructions were carried out. Finger assembled a team of vibration specialists from other NASA centers, and along with staff from LASL, Aerohet and Westinghouse, conducted a series of "cold flow" reactor tests using fuel elements without fissionable material. RIFT was cancelled in December 1963. Although its reinstatement was frequently discussed, it never occurred.
A series of minor design changes were made to address the vibration problem. In the Kiwi B4D test on 13 May 1964, the reactor was automatically started and briefly run at full power with no vibration problems. This was followed by the Kiwi B4E test on 28 August in which the reactor was operated for twelve minutes, eight of which were at full power. On 10 September, Kiwi B4E was restarted, and run at full power for two and a half minutes, demonstrating the ability of a nuclear rocket engine to be shut down and restarted. In September, tests were conducted with a Kiwi B4 engine and PARKA, a Kiwi reactor used for testing at Los Alamos. The two reactors were run 4.9 metres (16 ft), 2.7 metres (9 ft) and 1.8 metres (6 ft) apart, and measurements taken of reactivity. These tests showed that neutrons produced by one reactor did indeed cause fissions in another, but that the effect was negligible: 3, 12 and 24 cents respectively. The tests demonstrated that nuclear rocket engines can be clustered, just as chemical ones often are.
SNPO chose the 330-kilonewton (75,000 lbf) Kiwi-B4 nuclear thermal rocket design (with a specific impulse of 825 seconds) as the baseline for the NERVA NRX (Nuclear Rocket Experimental). Whereas Kiwi was a proof of concept, NERVA NRX was a prototype of a complete engine. That meant that it would need actuators to turn the drums and start the engine, gimbals to control its movement, a nozzle cooled by liquid hydrogen, and shielding to protect the engine, payload and crew from radiation. Westinghouse modified the cores to make them more robust for flight conditions. Some research and development was still required. The available temperature sensors were accurate only up to 1,710° C, far below what was required. New sensors were developed that were accurate to 2,376° C , even in a high-radiation environment. Aerojet and Westinghouse attempted to theoretically predict the performance of each component. This was then compared to the actual test performance. Over time, the two converged as more was understood. By 1972, the performance of a NERVA engine under most conditions could be accurately forecast.
The first test of a NERVA engine was of NERVA A2 on 24 September 1964. Aerojet and Westinghouse cautiously increased the power incrementally, to 2 MW, 570 MW, 940 MW, running for a minute or two at each level to check the instruments, before finally increasing to full power at 1,096 MW. The reactor ran flawlessly, and only had to be shut down after 40 seconds because the hydrogen was running out. The test demonstrated that NERVA had the designed specific impulse of 811 seconds (7.95 km/s); solid-propellant rockets have a maximum impulse of around 300 seconds (2.9 km/s) while chemical rockets with liquid propellant can seldom achieve more than 450 seconds (4.4 km/s). Executives at Aerojet and Westinghouse were so pleased they took out a full-page ad in the Wall Street Journal with a picture of the test and the caption: "On to Mars!" The reactor was restarted on 15 October. Originally this was intended to test the nozzle, but that was dropped as it was close to its design maximum of 2,000 C. Instead, the turbopump was tested. The engine was powered up to 40 MW, the control drums were locked in place, and the turbopump was used to keep the power steady at 40 MW. It worked perfectly. The computer simulations had been correct, and the whole project was ahead of schedule.
The next test was of NERVA A3 on 23 April 1965. This test was intended to verify that the engine could be run and restarted at full power. The engine was operated for eight minutes, three and a half of them at full power, before the instruments indicated that too much hydrogen was going into the engine. This was later determined to be caused by A scram was ordered, but a coolant line became clogged. Power increased to 1,165 MW before the line unclogged, and the engine shut down gracefully. There were fears for integrity of the tie rods that held the fuel clusters together. They were supposed to operate at 200° C, with a maximum of 378° C. The sensors recorded that they had reached 822° C, which was their own maximum. Laboratory tests later confirmed that they might have reached 1100° . They was also what appeared to be a hole in the nozzle, but this turned out to be soot. The robust engine was undamaged, so the test continued, and the engine was run at for thirteen minutes at 1,072 MW. Once again, the test time was limited only by the available hydrogen.
Testing of NASA's NERVA NRX/EST (Engine System Test) commenced on 3 February 1966. The objectives were:
- Demonstrate the feasibility of starting and restarting the engine without an external power source.
- Evaluate the control system characteristics (stability and control mode) during startup, shutdown, cooldown and restart for a variety of initial conditions.
- Investigate the system stability over a broad operating range.
- Investigate the endurance capability of the engine components, especially the reactor, during transient and steady state operation with multiple restarts.
The NRX/EST was run at intermediate power levels on 3 and 11 February, with a full power (1055 MW) tes on 3 March, followed by engine duration tests on 16 and 25 March. The engine was started eleven times. All test objectives were successfully accomplished, and NRX/EST operated for nearly two hours, including 28 minutes at full power. It exceeded the operating time of previous Kiwi reactors by nearly a factor of two.
The next objective was to run the reactors for an extended length of time. The NRX A5 was started up on 8 June 1966, and run at full power for fifteen and a half minutes. During cooldown, a bird landed on the nozzle and was aphyxiated by the nitrogen or heloum gas, dropoping onto the core. It was feared that it might block the propellant lines or create uneven heating before being blown out again when the engine was restarted, so the Westinghourse engineers rigged a television camera and a vacuum hose, and were able to remove the bird while safely behind a concrete wall. The engine was restarted on 23 June and run at full power for another fourteen and a half minutes. Although there was severe corrosion, resulting in about $2.20 of reactivity lost, the engine could still have been restarted, but the engineers wanted to examine the core.
An hour was now set as the goal for the NRX A6 test. This was beyond the capacity of Test Cell A, so testing now moved to Test Cell C with its giant dewars. NRX A5 was therefore the last test to use Rest Cell A. After shutdown on 7 December 1966 after 75 seconds caused by a faulty electrical component, and then a postponement due to inclement weather, NRX A6 was started up on 15 December. It ran at full power (1125 MW) with a chamber temperature of over 2,000°nbsp;C and pressure of 4,089 kilopascals (593.1 psi), and a flow rate of 32.7 kilograms per second (4,330 lb/min). It took 75.3 hours to cool the reactor with liquid nitrogen. On examination, it was found that the beryllium reflector had cracked due to thermal stress. The test caused the abandonment of plans to build a more powerful NERVA II engine. If more thrust was requiured, a NERVA I engine could be run longer, or clustered.
The second NERVA engine, the NERVA XE, was designed to come as close as possible to a complete flight system, even to the point of using a flight-design turbopump. Components that would not affect system performance were allowed to be selected from what was available at Jackass Flats, Nevada to save money and time, and a radiation shield was added to protect external components. The engine was reoriented to fire downward into a reduced-pressure compartment to partially simulate firing in a vacuum, using the new ETS-1.
The objectives also included testing the use of the new facility at Jackass Flats for flight engine qualification and acceptance. Total run time was 115 minutes, including 28 starts. NASA and SNPO felt that the test "confirmed that a nuclear rocket engine was suitable for space flight application and was able to operate at a specific impulse twice that of chemical rocket system[s]." The engine was deemed adequate for Mars missions being planned by NASA. The facility was also deemed adequate for flight qualification and acceptance of rocket engines from the two contractors.
Loss of political support and cancellationEdit
At the time of the NERVA NRX/EST test, NASA's plans for NERVA included a visit to Mars by 1978, a permanent lunar base by 1981, and deep space probes to Jupiter, Saturn, and the outer planets. NERVA rockets would be used for nuclear "tugs" designed to take payloads from Low Earth Orbit (LEO) to larger orbits as a component of the later-named Space Transportation System, resupply several space stations in various orbits around the Earth and Moon, and support a permanent lunar base. The NERVA rocket would also be a nuclear-powered upper stage for the Saturn rocket, which would allow the upgraded Saturn to launch much larger payloads of up to 340,000 lb (150,000 kg) to LEO.
NERVA rockets had progressed rapidly to the point where they could run for hours, limited in run time by the size of the liquid hydrogen propellant tanks at the Jackass Flats test site. They also climbed in power density. Although the engine, turbine and liquid hydrogen tank were never physically assembled together, the NERVA was deemed ready to design into a working vehicle by NASA, creating a small political crisis in Congress because of the danger a Mars exploration program presented to the national budget.
The Mars mission became NERVA's downfall. Members of Congress in both political parties judged that a crewed mission to Mars would be a tacit commitment for the United States to decades more of the expensive Space Race. Crewed Mars missions were enabled by nuclear rockets; therefore, if NERVA could be discontinued the Space Race might wind down and the budget would be saved. Each year the RIFT was delayed and the goals for NERVA were set higher. Ultimately, RIFT was never authorized, and although NERVA had many successful tests and powerful Congressional backing, it never left the ground.
Clinton P. Anderson, the New Mexico senator who had protected the program, had become severely ill. Lyndon B. Johnson, another powerful advocate of human space exploration, had decided not to run for a second term and was considerably weakened. NASA program funding was somewhat reduced by Congress for the 1969 budget, shutting down the Saturn rocket production line and cancelling Apollo missions after Apollo 17. Without the Saturn rocket to carry the NERVA to orbit, Los Alamos continued the Rover Program for a few more years with Pewee and the Nuclear Furnace as funding was cut to lower and lower levels by Congress, but it was disbanded by 1972.
After 17 years of research and development, Projects Nova and NERVA had spent about $1.4 billion.
In 1983, the Strategic Defense Initiative ("Star Wars") identified missions that could benefit from rockets that more powerful than chemical rockets, and some that could only be undertaken by more powerful rockets. A nuclear propulsion project, SP-100, was created in February 1983 with the aim of developing a 100 KW nuclear rocket system. The concept incorporated a pebble-bed reactor, a concept developed by James R. Powell at the Brookhaven National Laboratory, which promised higher temperatures and improved performance over NERVA. From 1987 to 1991 it was funded as a secret project codenamed Project Timber Wind, which spent $139 million. The proposed rocket was later expanded into a larger design after the project was transferred to the Space Nuclear Thermal Propulsion (SNTP) program at the Air Force Phillips Laboratory in October 1991. NASA conducted studies as part of its Space Exploration Initiative (SEI) but felt that SNTP offered insufficient improvement over NERVA, and was not required by any SEI missions. The SNTP program was terminated in January 1994. About $200 million was spent.
An engine for interplanetary travel from Earth orbit to Mars orbit, and back, was studied in 2013 at Marshall Space Flight Center with a focus on nuclear thermal rocket (NTR) engines. NTRs are at least twice as efficient as the most advanced chemical engines, allowing quicker transfer time and increased cargo capacity. The shorter flight duration, estimated at 3–4 months with NTR engines, compared to 8–9 months using chemical engines, would reduce crew exposure to potentially harmful and difficult to shield cosmic rays. NTR engines, such as the Pewee of Project Rover, were selected in the Mars Design Reference Architecture (DRA). On 22 May 2019, Congress approved $125 million in funding for the development of nuclear thermal propulsion rockets.
- Koenig 1986, p. 31.
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