Nuclear-powered rockets a revolution in spaceflight

ISRO has a nuclear rocket programme expressly meant to launch satellites. Last year, the first stage of an atomic-powered engine was successfully tested on India’s lunar mission, Chandrayaan-3.

A revolution in spaceflight is on as aerospace scientists prepare to test the first-ever nuclear rocket engine in space. Reports speak of major advances in a nuclear thermal propulsion (NTP) project jointly developed by the US Defense Advanced Research Projects Agency (DARPA) and aerospace giant Lockheed Martin. Called Demonstration Rocket for Agile Cislunar Operations (DRACO), the spacecraft’s original launch date — scheduled for 2027 — has now been advanced to 2025 or early 2026.

After launching on a conventional rocket, DRACO’s nuclear engine will be switched on after it reaches a 2,000 km high orbit around Earth — a perch from where it will take more than 300 years for DRACO to fall back to Earth, after safely exhausting its nuclear fuel. DRACO’s revised launch date has to do with NASA joining the project last year, and it wants to use NTP for its upcoming Moon and Mars missions.

Space engineers have long depended on chemical propulsion to power rockets. A mixture of fuel (comprising propellants like hydrogen) and liquid oxygen is ignited so that the expanding gas is ejected from the rocket, generating thrust. But chemical engines have a major downside when the fuel and oxidizer react and their chemical bonds get rearranged, the reaction releases only a limited amount of energy. Since the rocket’s power is limited to the amount of propellants it carries, this limits the thrust-to-weight ratio it can achieve for lifting payloads into orbit. This translates into huge costs every time a rocket launches a satellite or spacecraft into orbit. With chemical rockets and spacecraft driven by electric or solar-powered engines drawing closer to their limits of performance, nuclear propulsion has emerged as a viable alternative.

Space engineers are developing two types of nuclear reactors to power rockets: NTP and radioisotope thermoelectric generators (RTGs). RTGs do not provide propulsion, they merely generate nuclear electricity by harnessing the heat of radioactive decay. But this provides extremely low thrust making RTGs useful only for manoeuvring a spacecraft after launch. Famed interplanetary missions like Apollo, Pioneer, Voyager, and Cassini all had RTGs as their power source. In contrast, NTP is similar to chemical propulsion, the only difference being its use of nuclear fission (the process of splitting atoms) to produce an incredible amount of energy to propel the rocket.

A rocket driven by NTP carries a small fission reactor that creates tremendous heat to ignite the propellant gas, which is then let off explosively through a nozzle to create thrust. With nearly five times the propellant efficiency of chemical rockets, NTP can help astronauts reach, say, Mars in just 45 days instead of seven months on conventional rockets. This substantial cut in travel time also protects the astronauts from prolonged exposure to cosmic radiation and microgravity — a clear and present danger in outer space. Unlike nuclear-powered spacecraft whose fuel lasts interminably long, solar, and chemical systems simply do not have the fuel capacity or energy to operate in deep space.

The idea of nuclear-powered rockets is hardly new. Way back in the late 1940s, the US Air Force (USAF) began designing fission reactors for intercontinental ballistic missiles under its Project Rover. In 1959, NASA joined the project and, after elbowing out the USAF, changed the project’s goal to developing a nuclear rocket for space exploration. This led to the first atomic rocket engine: the 300-megawatt Kiwi-A. NASA, with the intention of launching a manned Mars mission aboard a nuclear-powered spacecraft by 1979, started the project’s next phase, the Nuclear Engine for Rocket Vehicle Application (NERVA). As it happened, however, after a flyable nuclear thermal engine was built in the late 1960s, NERVA was scrapped to make room for the Space Shuttle.

History need not repeat itself, and it is unlikely that the US military would let NASA ‘hijack’ the DRACO project, as had happened with Project Rover in 1959. DRACO was probably speeded up to help the US re-establish its leadership role in space that is contested by players such as China and Russia. Today, several spacefaring countries have their own nuclear propulsion programmes.

Chinese engineers have designed a mini ‘foldable’ reactor; on reaching orbit, it unfolds to power the spaceship. The Russians did develop a 500-kilowatt nuclear engine for Zeus, a spacecraft for carrying heavy cargo in space; but have now replaced it with another less powerful engine developed for the Sino-Russian International Lunar Research Station. The European Space Agency is making an atomic power plant, RocketRoll, whose prototype is expected by the end of the decade.

The Indian Space Research Organisation (ISRO) has a nuclear rocket programme expressly meant for launching satellites. Last year, the first stage of an atomic-powered engine — a radioisotope heating unit — was successfully tested on India’s lunar mission, Chandrayaan-3. It provides power to the propulsion module which is still in orbit around the Moon. ISRO sources say the agency is collaborating with the Bhabha Atomic Research Centre to build a 100-watt RTG.

Addressing safety concerns about the risks associated with nuclear-powered rockets — from the potential for launch accidents to the disposal of radioactive waste — is as challenging as the technology itself. Therefore, the next big leap for spaceflight depends on how well scientists negotiate this slippery slope.

(Prakash Chandra is former editor of the Indian Defence Review. He writes on aerospace and strategic affairs.)

Disclaimer: The views expressed above are the author's own. They do not necessarily reflect the views of DH.