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| Fig. 1: Heated gases ejected from propulsion engine (Source: Wikimedia Commons) |
Recently, researchers and investors have once again shifted focus to space travel and exploration. Space flight, however, is limited by the scarcity and inefficiency of the chemical fuels used to power large spacecraft. This has led many to turn to nuclear energy as a potential replacement for chemical fuel in spacecraft. [1]
Space travel consists of 2 main energy-expensive procedures: lift-off and acceleration. [2] Lift-off is the ejection of the spacecraft from the Earth's atmosphere. Once out of the Earth's atmosphere, spacecraft utilize propulsion devices to speed up and direct the flight path. This constant acceleration makes up the majority of the flight's energy consumption. [3] Modern rockets create propulsion through the burning of chemical fuels. They make use of Newton's 3rd Law of Motion. Through chemical reactions, the fuel is burnt to heat gas. The gas is then ejected. [1] A traditional chemical fuel propulsion rocket can be seen in Fig. 1. The equal and opposite force created by the high-energy gas particles results in impulse for the spacecraft. This allows the spacecraft to accelerate. This can be seen through the example of the Italian rocket, Vega, which uses chemical propulsion rockets that consume around 320 to 360 grams per second to generate a single kilonewton of thrust. In the case where the rockets used liquid fuel, the ejected exhaust carried an effective velocity on the scale of 3000 meters per second. [4]
Nuclear pulse propulsion is the most straightforward method of using nuclear energy as a source of impulse. As the name suggests, this method involves using the energy output from repeated nuclear reactions to accelerate the spacecraft. This can be done through controlled, pulsed nuclear explosions. [5] Similar to chemical propulsion, these explosions heat and eject high-velocity particles. The impulse generated by each nuclear "pulse" is dependent on the velocity of the ejected particles. Because nuclear reactions output more heat than chemical reactions, the impulse created by nuclear pulse propulsion is greater than that created by chemical propulsion. [6] In the case of one of the rockets used by Project Orion, one of the first nuclear pulse propulsion spacecraft, the ejected exhaust had an effective velocity on the scale of 1.9 × 104 m s-1 to 3.1 × 104 m s-1. [7] The time, in seconds, that a propulsion engine can accelerate its mass by 10 meters per square second is known as the specific impulse of the propellant. Project Orion generated a specific impulse ten times greater than that generated by the chemical rockets used by Vega. In theory, nuclear pulse propulsion can appear to be a great replacement for chemical propulsion. However, the byproducts of nuclear reactions are hazardous. The repeated explosions create fallout that enters the Earth's atmosphere, where it can be harmful to living organisms. [8] Therefore, further research will need to be done to make nuclear pulse propulsion a replacement for chemical propulsion.
While it is easier and more intuitive to create propulsion using particles with mass, it is also possible to create impulse using light. Although light is massless, it carries momentum and kinetic energy in the form of photons. [9] The theory of blackbody radiation explains that these energetic photons can be created using nuclear energy. Using a nuclear reactor, plasma is heated to high temperatures, at which they emit high-velocity photons. [10] These photons can be directed and ejected from the spacecraft to accelerate. The momentum these photons carry is then converted to impulse, again, following Newton's 3rd law of motion. Because photons are light, the effective velocity of an ideal photonic propulsion rocket is about 3.0 × 108 m s-1. This would generate a specific impulse of around 3.0 × 107 s or 10,000 times greater than the specific impulse of the rockets used by Project Orion. Using a highly reflective mirror to direct all emitted light in one direction, a 1 GW generator can create 3.33 newtons of thrust. [10] This is vastly lower than the 2450 newtons of thrust generated by chemical propulsion engines such as that used by Vega. [4] However, utilizing this technology, in conjunction with light-weight sails designed to collect photon momentum, can potentially accelerate using less power than chemical propulsion engines. [3] Nuclear photonic propulsion can be less dangerous than nuclear pulse propulsion. Because the impulse is created by ejecting photons, which are harmless, photonic propulsion eliminates the threat of nuclear fallout that makes nuclear pulse propulsion unsafe. [9]
As with all nuclear technology, nuclear propulsion is still highly theoretical. Further research and development of photon sails can make photonic propulsion more energy efficient.[10] Advancements in the containment of high-temperature plasmas and the control of hazardous radioactive fallout pave the way for the possibility of nuclear reactors aboard spacecraft. [1]
© Gabriel Ruiz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. Charles, "Plasmas for Spacecraft Propulsion," J. Phys. D 42 163001 (2009.
[2] M. Fukunari et al., "Replacement of Chemical Rocket Launches by Beamed Energy Propulsion," Appl. Opt. 53 I16 (2014).
[3] H. Alfvén, "Spacecraft Propulsion: New Methods," Science 176, 167 (1972).
[4] S. Bianchi et al., "VEGA, the European Small Launcher: Development Status, Future Perspectives, and Applications," Acta Astronaut. 63, 416 (2008).
[5] J. C. Nance, "Nuclear Pulse Propulsion," IEEE Trans. Nucl. Sci. 12, 177 (1965).
[6] R. S. Cooper, "Nuclear Propulsion for Space Vehicles," Annu. Rev. Nucl. Sci. 18, 203 (1968).
[7] G. R.Schmidt, J. A. Bonometti, and C. A Irvine, "Project Orion and Future Prospects for Nuclear Pulse Propulsion," J. Propuls. Power 18, 497 (2002).
[8] J. L. Ryan, "Ionizing Radiation: the Good, the Bad, and the Ugly," J. Invest. Dermatol. 132 985 (2012).
[9] I. Levchenko et al., "Prospects and Physical Mechanisms for Photonic Space Propulsion," Nat. Photonics 12 649 (2018).
[10] N. Rajalakshmi and S. Srivarshini, "Fuel Effective Photonic Propulsion," IOP Conf. Ser.: Mater. Sci. Eng. 234, 012005 (2017).
