Nuclear Thermal Rocket: A Misunderstood Beast
Artist's impression of the Nuclear Engine for Rocket Vehicle Application (NERVA) spacecraft arriving in Mars orbit.Adrian Mann, Bisbos.com
Icarus Interstellar is a nonprofit corporation dedicated to accomplishing interstellar flight by 2100. The Center for Space Nuclear Research (CSNR) is a focus for research and development of advanced space nuclear systems, including power and propulsion systems, and radioisotope power generators. Icarus Interstellar have recently partnered with CSNR to bring you a series of articles aimed at exploring the potential uses of nuclear power for space propulsion and power generation for space missions.
Recently, an international marine life census concluded that over 200,000 organisms call Earth's oceans their home. Remarkably, a handful of those life forms happily exist at depths and temperatures where life was once thought to certainly perish.
Yet despite the fact that people have been intimately engaged with the seas for thousands of years, we still know very little about many of these creatures, discovering new ones on a routine basis. If even the darkest, coldest, and seemingly oxygen-depleted regions of our own world are capable of harboring life, perhaps life may be flourishing elsewhere in the solar system; for example, within the oceans of the Saturnian moon Titan.
Only recently have we begun to explore the surfaces of distant worlds, and as the 21st century unfolds, ambitious plans abound for undertaking such future ventures.
However, many of these plans are reliant upon technology that is realistically quite some time away from implementation, or even purely theoretical at this juncture.
Propulsion systems employing fusion power or matter/anti-matter collisions are all promising conduits for efficient and timely inter-solar and deep-space exploration, but are thus far undeveloped or unfeasible at the present time. Even the promise of ion propulsion, having successfully been demonstrated in a handful of robotic spacecraft (such as NASA's Dawn and the Japanese Hayabusa mission), does not offer an immediately viable, large-scale solution.
Yet as we aim beyond sending humans to Earth's moon, the risk for exploratory stagnation is high as traditional chemically-fueled rockets (utilized in every manned space vehicle from the Saturn V to the recently retired Space Shuttle) are unsuitable for long-distance, heavy payload missions.
A traditional chemical rocket trip to our moon takes about three days, covering some 238,000 miles. By comparison, Saturn's most promising lunar body resides nearly 4,000 times further away at over 790 million miles from Earth. However, despite the chasm between traditional and theoretical propulsion systems, deep space exploration need not grind to a halt as a stopgap propulsion system already exists.
Fully within the grasp of modern day technology and capable of fulfilling these immediate needs, are Nuclear Thermal Rockets (NTRs).
The Nuclear Thermal Rocket: A Misunderstood Beast
The idea of employing nuclear fission rockets for space exploration has been around for over half a century. In fact, President John F. Kennedy originally expressed his support of the government fission rocket project 'Rover/NERVA' as the step after the Apollo program during his famous Congressional moon speech in 1961.
During the mid-20th century, the NERVA program succeeded in proving, through ground-based testing, that NTR propulsion systems could provide a safe and efficient method for sending payloads greater than small exploratory probes into the outer reaches of our solar system. Unfortunately, little progress toward implementation has been made since then, as a cloud of misunderstanding slowly settled over the idea of utilizing nuclear technology within the space program.
Employing NTR rockets is a critical next-step toward sending human explorers or larger robotic laboratories to distant worlds such as Titan or Mars. Certainly, humanity has succeeded in launching probes to Saturn, Jupiter, and beyond with just chemical rockets, but the key limiting issue is mass.
The mass of those probes was significantly less than that inherent to, for example, a deep ocean Titan explorer. The power available by chemical rocket propulsion simply maxes out (and fuel weight becomes an issue), making such distant, heavy-voyages all but infeasible. Nuclear fission propulsion is ideally suited to provide for this need, as it is both superior to chemical rocket fuel and readily developable.
Traditional chemical rocket engines derive power from the resultant chemical reaction between fuel elements -- usually hydrogen and oxygen. However, the maximum amount of exhaust velocity generated by such an engine is severely limited due to the fact that the fuel is utilized to provide both the heat and the thrust. In contrast, a Nuclear Thermal Rocket maintains a separation between heat source and fuel, which allows for greater exhaust velocity to be achieved with less propellant.
Artist's impression of the Nuclear Engine for Rocket Vehicle Application (NERVA) spacecraft arriving in Venus orbit.Adrian Mann, Bisbos.com
Rather than relying on a chemical reaction between combustible elements, the NTR engine instead employs a two-part system for generating power.
First, a small, fully contained nuclear fission chamber utilizes a tiny amount of fissile material to generate heat -- much in the way a nuclear power plant operates. In the second part of the process, a lightweight element -- usually liquid hydrogen -- is forced around the reactor core where it is instantly heated. The resulting superheated hydrogen is then expelled at great velocities through the engine nozzle, generating over twice as much specific impulse (force) as a traditional rocket engine, permitting a rocket to travel further and at higher velocities.
In addition, because the process can be thought of as simply expelling a heated gas through a nozzle, many different gases could be utilized for propellant, potentially allowing such rockets to "refuel" by occasionally skimming the atmospheres of distant worlds to collect additional gas (a concept proposed by Robert Zubrin, president of The Mars Society, as a viable method for the long-term exploration of Titan's atmosphere).
Perhaps the most common misconception associated with fission-powered rockets involves the notion that radioactive material could be dispersed into Earth's atmosphere, or that a nuclear explosion may occur. Despite the inclusion of a fission reactor within the core of an NTR rocket engine, such a design is actually incredibly safe and can be utilized in several ways without outputting any radioactive exhaust material into Earth’s atmosphere.
One such control method would be to only utilize certain engines safely beyond the atmospheric layers of the Earth, thereby preventing any potential contamination (outer space is replete with radiation naturally, therefore any contributions from rocket propulsion are entirely inconsequential).
Another approach would be to utilize special materials with radiation absorbing properties that would completely contain the engine's radiation in orbit. An unproven but promising theoretical design involves enclosing the reactor entirely with a special quartz material which only permits the heat to escape in the form of intense ultraviolet light, passing harmlessly through the quartz shell to heat the hydrogen fuel, while all of the radioactive material is safely contained within the reactor.
Certain methods could permit ground based launches utilizing NTR engines, while other methods might employ chemical rockets to lift an NTR into orbit before engaging the fission engines.
The primary concern associated with any NTR rocket would involve a sub-orbital structural failure, however in such cases it would be highly improbable for any fissile material to escape as the reactor core is extremely robust, and only contains a very small amount of radioactive material in the first place.
In fact, an NTR engine was actually designed in the 1970’s for use with the original Space Shuttle program, as it generated twice the power of a conventional engine, but was eventually scrapped amid an increase in popular resistance to the use of nuclear technology.
Rather than relying on a potentially dangerous combustion reaction, such as that required in chemical rocket engines, in its simplest form an NTR engine functions by passing coolant over a hot surface, resulting in an exponentially safer engine that could ensure that Challenger-type accidents and the associated causes could be prevented.
Until more exotic propulsion methods are fully developed and tested, utilizing safe fission rocket engines is the most promising near-term method for delivering heavy cargo (i.e. humans, living habitats, larger robotic explorers) to more distant planetary systems. The chemical rockets of today and the last several decades are suitable for, at best, delivering humans to the moon and perhaps Mars under ideal circumstances.
However, to embark on a truly thorough mission to search for life in promising regions such as the moons Titan or Enceladus, fission powered rockets will be required, as they provide both a safe and efficient means for becoming one step closer to answering the question of whether or not we are alone in the universe.