We live in an era of renewed space exploration,
where multiple agencies are planning to send astronauts to the Moon in the
coming years. This will be followed in the next decade with crewed missions to
Mars by NASA and China, who may be joined by other nations before long. These
and other missions that will take astronauts beyond Low Earth Orbit (LEO) and
the Earth-Moon system require new technologies, ranging from life support and
radiation shielding to power and propulsion. And when it comes to the latter,
Nuclear Thermal and Nuclear Electric Propulsion (NTP/NEP) is a top contender!
NASA and the Soviet space program spent decades
researching nuclear propulsion during the Space Race. A few years ago, NASA
reignited its nuclear program for the purpose of developing bimodal nuclear
propulsion – a two-part system consisting of an NTP and NEP element – that
could enable transits to Mars in 100 days. As part of the NASA Innovative
Advanced Concepts (NIAC) program for 2023, NASA selected a nuclear concept for
Phase I development. This new class of bimodal nuclear propulsion system uses a
“wave rotor topping cycle” and could reduce transit times to Mars to just 45
days.
The proposal, titled “Bimodal NTP/NEP with a Wave
Rotor Topping Cycle,” was put forward 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
selected by the NAIC this year for Phase I development, which includes a
$12,500 grant to assist in maturing the technology and methods involved. 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 technologies that have been thoroughly tested
and validated. For Nuclear-Thermal Propulsion (NTP), the cycle consists of a
nuclear reactor heating liquid hydrogen (LH2) propellant, turning it into
ionized hydrogen gas (plasma) that is then channeled through nozzles to
generate thrust. Several attempts have been made to build a test this
propulsion system, including Project Rover, a collaborative effort between the
U.S. Air Force and the Atomic Energy Commission (AEC) that launched in 1955.
In 1959, NASA took over from the USAF, and the
program entered a new phase dedicated to spaceflight applications. This
eventually led to the Nuclear Engine for Rocket Vehicle Application (NERVA), a
solid-core nuclear reactor that was successfully tested. With the closing of
the Apollo Era in 1973, the program’s funding was drastically reduced, leading
to its cancellation before any flight tests could be conducted. Meanwhile, the
Soviets developed their own NTP concept (RD-0410) between 1965 and 1980 and
conducted a single ground test before the program’s cancellation.
Nuclear-Electric Propulsion (NEP), on the other hand,
relies on a nuclear reactor to provide electricity to a Hall-Effect thruster
(ion engine), which generates an electromagnetic field that ionizes and
accelerates an inert gas (like xenon) to create thrust. Attempts to develop
this technology include NASA’s Nuclear Systems Initiative (NSI). Project
Prometheus (2003 to 2005). Both systems have considerable advantages over
conventional chemical propulsion, including a higher specific impulse (Isp)
rating, fuel efficiency, and virtually unlimited energy density.
While NEP concepts are distinguished for providing
more than 10,000 seconds of Isp, meaning they can maintain thrust for close to
three hours, the thrust level is quite low compared to conventional rockets and
NTP. The need for an electric power source, says Gosse, also raises the issue
of heat rejection in space – where thermal energy conversion is 30-40% under
ideal circumstances. And while NTP NERVA designs are the preferred method for
crewed missions to Mars and beyond, this method also has issues providing
adequate initial and final mass fractions for high delta-v missions.
New Class of Bimodal NTP/NEP with a Wave Rotor
Topping Cycle Enabling Fast Transit to Mars. Credit: Ryan Gosse
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This is why proposals that include both propulsion
methods (bimodal) are favored, as they would combine the advantages of both.
Gosse’s proposal calls for a bimodal design based on a solid core NERVA reactor
that would provide a specific impulse (Isp) of 900 seconds, twice the current
performance of chemical rockets. Gosse proposed cycle also includes a pressure
wave supercharger – or Wave Rotor (WR) – a technology used in internal
combustion engines that harnesses the pressure waves produced by reactions to
compress intake air.
When paired with an NTP engine, the WR would use
pressure created by the reactor’s heating of the LH2 fuel to compress the
reaction mass further. As Gosse promises, this will deliver thrust levels
comparable to that of a NERVA-class NTP concept but with an Isp of 1400-2000
seconds. When paired with a NEP cycle, said Gosse, thrust levels are enhanced
even further:
“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.”
Based on conventional propulsion technology, a
crewed mission to Mars could last up to three years. These missions would
launch every 26 months when Earth and Mars are at their closest (aka. a Mars
Opposition) and would spend a minimum of six to nine months in transit. A
transit of 45 days (six and a half weeks) would reduce the overall mission time
to months instead of years. This would significantly reduce the major risks
associated with missions to Mars, including radiation exposure, the time spent
in microgravity, and related health concerns.
Artist’s concept of a bimodal nuclear rocket making
the journey to the Moon, Mars, and other destinations in the Solar System.
Credit: NASA
In addition to propulsion, there are proposals for
new reactor designs that would provide a steady power supply for long-duration
surface missions where solar and wind power are not always available.
Examples include NASA’s Kilopower Reactor Using Sterling
Technology (KRUSTY) and the hybrid fission/fusion reactor selected for Phase I
development by NASA’s NAIC 2023 selection. These and other nuclear applications
could someday enable crewed missions to Mars and other locations in deep space,
perhaps sooner than we think!
Reference: NASA
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