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| Concept of a spacecraft enabled by nuclear thermal propulsion. Credits: NASA |
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.
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| New Class of Bimodal NTP/NEP with a Wave Rotor Topping Cycle Enabling Fast Transit to Mars. (Ryan Gosse) |
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 US$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 US 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 percent
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.
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 (1,800-4,000 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.
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!
This article was originally published by Universe Today. Read the
original article.