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taylorvich writes:

https://physicsworld.com/a/nuclear-powered-spacecraft-why-dreams-of-atomic-rockets-are-back-on/

Launching rockets into space with atomic bombs is a crazy idea that was thankfully discarded many decades ago. But as Richard Corfield discovers, the potential of using the energy from nuclear-powered engines to drive space travel is back on NASA’s agenda

In 1914 H G Wells published The World Set Free, a novel based on the notion that radium might one day power spaceships. Wells, who was familiar with the work of physicists such as Ernest Rutherford, knew that radium could produce heat and envisaged it being used to turn a turbine. The book might have been a work of fiction, but The World Set Free correctly foresaw the potential of what one might call “atomic spaceships”.

The idea of using nuclear energy for space travel took hold in the 1950s when the public – having witnessed the horrors of Hiroshima and Nagasaki – gradually became convinced of the utility of nuclear power for peaceful purposes. Thanks to programmes such as America’s Atoms for Peace, people began to see that nuclear power could be used for energy and transport. But perhaps the most radical application lay in spaceflight.

Among the strongest proponents of nuclear-powered space travel was the eminent mathematical physicist Freeman Dyson. In 1958 he took a year’s sabbatical from the Institute of Advanced Study in Princeton to work at General Atomics in San Diego on a project code-named Orion. The brainchild of Ted Taylor – a physicist who had worked on the Manhattan atomic-bomb project at Las Alamos – Project Orion aimed to build a 4000-tonne spaceship that would use 2600 nuclear bombs to propel it into space.

[...] Despite Project Orion ending, the lure of nuclear propulsion never really went away (see box “Nuclear space travel: a brief history”) and is now enjoying something of a resurgence. Rather than using atomic bombs, however, the idea is to transfer the energy from a nuclear fission reactor to a propellant fuel, which would be heated to roughly 2500 K and ejected via a nozzle in a process called “nuclear thermal propulsion” (NTP). Alternatively, the fission energy could ionize a gas that would be fired out of the back of the spacecraft – what’s known as “nuclear electric propulsion” (NEP).

So, is nuclear-powered space travel a realistic prospect and, if so, which technology will win out? [...] A nuclear spacecraft would instead use fission energy to heat a fuel (figure 1) – most likely cryogenically stored liquid hydrogen, which has a low molecular mass and high heat of combustion. “Nuclear propulsion, either electric or thermal, could extract more energy from a given mass of fuel than is possible via combustion-based propulsion,” says Dale Thomas, a former associate director at NASA’s Marshall Space Flight Center, now at the University of Alabama in Huntsville.

Thomas says that today’s most efficient chemical propulsion systems can achieve a specific impulse of about 465 seconds. NTP, in contrast, can have a specific impulse of almost 900 seconds due to the higher power density of nuclear reactions. Combined with a much higher thrust-to-weight ratio, NTP could get a rocket to Mars in just 500 days, rather than 900.

“The thrust-to-weight ratio is crucial because it determines the spacecraft’s ability to accelerate, which is especially critical during key mission phases, like escaping Earth’s gravity or manoeuvring in deep space,” says Mauro Augelli, head of launch systems at the UK Space Agency. “The specific impulse, on the other hand, is a measure of how effectively a rocket uses its propellant.”

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