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An artist’s illustration of a nuclear powered rocket. (DARPA)
As humanity moves closer and closer to leaving footprints on Mars, questions remain about how best to get there – and how to travel much further into the unknown. In this editorial, Dr. Mark Lewis argues that an early Cold War idea is enjoying a well-deserved renaissance: nuclear-powered rockets.
The 1965 edition of the World Book Encyclopedia I grew up with promised that the United States would land astronauts on Mars by 1986 using thermal nuclear rockets. As we can see, we are not there yet.
But nuclear power for space rockets was not science fiction even in the 1960s, and 60 years later, many programs are rediscovering this promising technology. In fact, one of the most exciting recent developments in space technology is a return to that vision, as nuclear power systems and nuclear rockets are once again the focus of NASA and the Department of Defense. .
In December last year, the White House issued Space Policy Directive 6, which established a national strategy to develop both nuclear space energy and nuclear propulsion. Despite the change of administration, the craze for nuclear power and propulsion seems to be maintained. The Defense Innovation Unit issued a call for tenders on September 9 inviting industry to provide information on a compact nuclear power plant that would be suitable for powering small and medium-sized spacecraft.
Another program already underway at the DOD’s strategic capabilities office is the Pele project, the objective of which is to develop a transportable nuclear reactor. While not specifically aimed at space applications, the lessons learned through Pele will help inform the development of space reactors.
Work is also continuing on the development of safe and reliable thermal nuclear rockets. To this end, the DARPA Demonstration Rocket for Cislunar Agile Operations (DRACO), whose objective is to demonstrate a nuclear thermal rocket in orbit, is doing well.
This is all very exciting, albeit significantly delayed, according to the naively optimistic predictions of my 1965 encyclopedia, which also stated that nuclear thermal engines would power Saturn V rockets in some of the subsequent Apollo missions. These claims had a factual basis; At the time of publication of this edition of the World Book, the US government had already been working on nuclear rockets for about a decade.
As part of the Kiwi program and the NERVA (Nuclear Engine for Rocket Vehicle Application) programs that followed, prototype engines were built and tested that provided significantly higher thrust per fuel consumption rate than any chemical engine built. before or since. In a series of ground tests, the nuclear engines were found to be reliable and restartable, and could operate for long periods of time, including nearly half an hour of continuous thrust.
In the late 1960s, as NASA’s plans for lunar exploration were scaled back, the NERVA program lost White House support and was finally canceled in 1973 before a nuclear heat engine flew into the air. space.
Nuclear reactors for power generation have also been a topic of great interest from the early days of space flight. Nuclear power systems can provide electrical power to spacecraft without tying them to solar panels, for which there are a number of intriguing applications for civil and military space. One of these applications is the use of nuclear energy to generate electricity to power electric propulsion systems.
The United States flew the SNAP nuclear reactor in 1965, then virtually abandoned the technology.
The Soviet Union had a much larger program, with a whole series of satellites powered by reactors. There were even plans to purchase Russian-built TOPAZ-II nuclear space reactors for various American spacecraft in the 1990s, and although several of these reactors were brought to the United States, the flight plans did not materialize. never materialized.
Nuclear versus. Chemistry: a physics lesson
The development of thermal nuclear rockets will enable a range of space missions that challenge even the best chemical rockets. To understand the value of thermal nuclear propulsion, it is instructive to compare the potential performance of thermal nuclear rockets to more traditional chemical systems.
In a chemical rocket, a chemical reaction between the fuel and the oxidant produces products which are also heated by the release of chemical bond energy; both the quantity of heat available and the properties of the exhaust mass are thus determined by the reagents chosen. In contrast, in a nuclear thermal rocket, a fission reactor is used to heat a working fluid, so that the heat source and the properties of the exhaust mass are essentially decoupled.
The thrust of a rocket per fuel weight consumption rate is known as the specific impulse, or Isp. All other things being equal, the increase in speed communicated to a spacecraft is directly proportional to Isp. As a general rule, for high thrust systems, the lower the molecular weight of the exhaust mass, the higher the Isp for a given amount of energy.
Because low molecular weight fuels such as hydrogen can be used in a nuclear thermal rocket without chemical stress, nuclear systems can provide Isp values ​​that are about double that of the best chemical systems.
A nuclear rocket could also offer the possibility of using fuel derived locally from other parts of the solar system, including the atmosphere of Mars.
Note that thermal nuclear rockets are different from electric nuclear engines that would use a reactor as a source of electrical power. Such electric motors have a significantly higher specific impulse than even nuclear thermal systems and could operate for very long periods of time but produce extremely low levels of thrust.
A better path to Mars
Nuclear rockets are therefore a proven technology that could enable an exciting range of possible future civil and military space missions. For example, they could reduce flight time to Mars compared to other proven propulsion concepts. This in turn would reduce the need for survival.
Thermal nuclear rockets could also allow wider launch windows to Mars and greater dropout opportunities on Earth. This means that nuclear propulsion could actually reduce the overall risk of radiation to a human crew and ultimately make missions safer compared to chemical or electric motors.
Fission reactors can be designed to survive a failed launch without the risk of accidental radiation leaks. Indeed, launching a nuclear reactor is arguably less risky than the radioisotope thermoelectric generators we are launching today.
For the Department of Defense, thermal nuclear rockets offer the potential for more maneuverable spacecraft, capable of avoiding threats or shifting orbits for more flexible mission operations in an increasingly contested space environment.
Like their terrestrial counterparts, the use of space nuclear systems has been constrained more by politics and perception than by technological readiness. If the United States is truly serious about a future that includes humans on Mars and regular operations in cislunar space, nuclear energy, and in particular its application to nuclear rocket propulsion, must be a priority. option.
To this end, current efforts in the development of thermal nuclear systems are indeed a wise and timely investment.
Dr Mark J. Lewis is the Executive Director of National Defense Industrial AssociationInstitute of Emerging Technologies (NDIA ETI). Prior to this role, Dr Lewis was Director of Defense Research and Engineering at the Department of Defense (DoD), overseeing technology modernization for all DoD departments and agencies, as well as Undersecretary Acting Defense Assistant for Research and Engineering.
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