We are living in a new era of space exploration, with multiple agencies planning to send astronauts to the Moon in the coming years. This will be followed in the coming decade by crewed missions to Mars by NASA and China, which may soon be joined by other nations. These and other missions that will take astronauts beyond Low Earth Orbit (LEO) and the Earth-Moon system will require the development of new technologies ranging from life support and radiation shielding to power and propulsion. And, in this regard, Nuclear Thermal and Nuclear Electric Propulsion (NTP/NEP) is a top candidate!
During the Space Race, NASA and the Soviet space program spent decades researching nuclear propulsion. NASA restarted its nuclear program a few years ago with the goal of developing bimodal nuclear propulsion – a two-part system comprised of an NTP and a NEP element – that could enable 100-day transits to Mars. NASA selected a nuclear concept for Phase I development as part of the NASA Innovative Advanced Concepts (NIAC) program for 2023. This new type of bimodal nuclear propulsion system uses a “wave rotor topping cycle” and has the potential to reduce transit times to Mars to 45 days.
The proposal, titled “Bimodal NTP/NEP with a Wave Rotor Topping Cycle,” was submitted by Prof. Ryan Gosse, the Hypersonics Program Area Lead at the University of Florida and a member of the Florida Applied Research in Engineering (FLARE) team. Gosse’s proposal is one of 14 chosen by the NAIC this year for Phase I development, which includes a $12,500 grant to help with technology and method maturation. Other proposals included innovative sensors, instruments, manufacturing techniques, power systems, and more.
Nuclear propulsion essentially comes down to two concepts, both of which rely on thoroughly tested and validated technologies. The cycle for Nuclear-Thermal Propulsion (NTP) consists of a nuclear reactor heating liquid hydrogen (LH2) propellant, converting it to ionized hydrogen gas (plasma), which is then channeled through nozzles to generate thrust. Several attempts have been made to build and test this propulsion system, including Project Rover, a 1955 collaboration between the United States Air Force and the Atomic Energy Commission (AEC).
Nuclear-Electric Propulsion (NEP), on the other hand, uses a nuclear reactor to generate electricity for a Hall-Effect thruster (ion engine), which creates thrust by ionizing and accelerating an inert gas (such as xenon). NASA’s Nuclear Systems Initiative is one attempt to develop this technology (NSI). Prometheus Project (2003 to 2005). Both systems have significant advantages over traditional chemical propulsion, such as higher specific impulse (Isp) ratings, fuel efficiency, and virtually unlimited energy density.
“Coupled with an NEP cycle, the duty cycle Isp can further be increased (1800-4000 seconds) with minimal addition of dry mass. This bimodal design enables the fast transit for manned missions (45 days to Mars) and revolutionizes the deep space exploration of our solar system.”
A crewed mission to Mars could last up to three years using conventional propulsion technology. These missions would launch every 26 months when Earth and Mars are closest to each other (known as a Mars opposition) and would spend at least six to nine months in transit. A transit time of 45 days (six and a half weeks) would cut the overall mission time down to months rather than years. This would significantly reduce the major risks associated with Mars missions, such as radiation exposure, microgravity time, and related health concerns.
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