Course: Space Mission and Launch Operations
Introduction
Newton defined three fundamental laws that govern the motion of an object. They are essential to the motion of a spacecraft and rocket engine performance. Based on Newton’s 2nd law and the momentum equation, rocket engine thrust is a physical quantity that is a function of the mass flow rate of the propellant, the exhaust velocity, the exhaust pressure, the exhaust area, and the ambient pressure. Based on Newton’s 3rd law, rocket engine thrust is a reaction force that opposes the spacecraft weight and drag forces, and acts in an opposite direction to the exhaust velocity. Thus, thrust is the force that propels the spacecraft upward or forward. When thrust is defined with respect to the mass flow rate of the propellant or specially the unit of propellant weight flow, such quantity is referred to as specific impulse. This is important because thrust and specific impulse are directly correlated to spaceflight durations.
Space exploration missions are currently restricted by the limitations of the common chemical and electrical propulsion systems. The magnitude of thrust and specific impulse generated by these systems are relatively low and lead to significantly extensive flight durations which present the problem that is microgravity and space radiations effects on human physiology (Miernik et. al, 2013). Achieving humans’ objectives of manned spaceflight and the development of human habitat across the solar system and the cosmos requires rocket engines capable of generating significantly more thrust than the common combustion-based and electrical propulsion systems. The most intriguing solution to achieve faster space travel is the development and usage of nuclear propulsion systems (Cassibry et al., 2015). Although a multitude of concept have been developed since the nuclear pulse concept presented by the Orion Project in the 1960’s, nuclear fusion systems exhibit the highest performance capabilities. Thus, what are the fundamentals characteristics of nuclear fusion propulsion systems?
Background
Space exploration missions’ capabilities are currently limited by the duration of spaceflight required to reach other solar and cosmic bodies. The most recent Perseverance and Tianmen-1 Mars probes averaged a flight duration of approximately 7 months (NASA, 2020). ESA’s Juice probe is expected to reach Jupiter vicinities after 7.6 years of space flight (ESA, 2021). NASA’s New Horizons probes reached Pluto vicinities after approximately 9 years of space flight. The problem of significantly extensive spaceflight durations is due to the limitations of the common chemical and electrical propulsion systems. While the combustion-based chemical systems are limited by the weight of fuel and propellant, and relatively low specific impulse, electrical systems are limited by low thrust (Anderson, 2016). Fuel cell engines essentially significantly improve system endurance capabilities and not thrust, specific impulse and maximum velocities. When considering practical solutions that operate with respect to the fundamental concept of accelerating a propellant mixture to generate thrust, nuclear propulsion systems are ideal because they generate both significantly higher thrust and specific impulse.
A multitude of nuclear propulsion concepts have been derived from the nuclear pulse principles pioneered by the Orion Project in the 1960’s. Unfortunately, progress with the promising nuclear pulse concept were halted by the Limited Test Ban Treaty of 1963 (Schmidt, Bonometti & Irvine, 2002). Although fission-based nuclear electric and nuclear thermal systems generate improve rocket engine performance parameters, their usage still correlate to relatively high spaceflight durations. Fission-based nuclear thermal engine reportedly generates a specific impulse of approximately Isp = 1000s and fission-based nuclear electric engine generate a specific power of approximately 100 W/kg. This is lower than the capabilities of fusion propulsion systems which can generate specific powers beyond 10 kw/kg (Cassibry et al., 2015)
Analysis
The fundamental concept of nuclear fusion propulsion is generally based on using heat generated from a nuclear reaction to accelerate a propellant mixture that is ejected through the rocket engine’s nozzle. In the case of fusion-based nuclear electric systems, thermal energy is first generated and then converted into electrical power. Table 1 displays standard performance parameters for multiple nuclear fusion propulsion concepts.
It is important to mention the Direct Fusion Drive concept which reportedly generate both thermal and electric power from a single fusion reactor. It is based on a field reverse configuration and generate specific power in the range of α = 0.75-1.25 kW/kg and specific impulse in the range of Isp= 8000- 12000 s (Gajeri, Aime & Kezerashvili, 2021).
Thus, nuclear fusion systems are an ideal solution for faster space travel because theoretical analysis have shown that a distant planet like Jupiter can be reached in 42 days as opposed to approximately 7.6 years with a rocket engine capable of generating a specific impulse in the range of Isp = 50000s and a specific power in the range of α = 100 KW/kg (Cassibry et al., 2015).
Recommendation & Conclusion
Successful unmanned missions across the solar system have prompted the question of whether manned missions to Mars and other cosmic bodies can be achieved. Given that one of the most significant hurdles to such an objective is the significantly high space flight durations, the nuclear fusion propulsion concept must be developed. First, significantly faster space travel mitigates human factor issues such as isolation, confinement, lack of movements and the effects of microgravity and space radiations. Second, ground launch issues such as spacecraft system size and weight, and environmental emissions can be mitigated by assembling the spacecraft in orbit and igniting the system from an earth orbit or position which present no environmental or radiation risks. Third, the feasibility of such a concept for real-life mission can be validated, verified, and evaluated through the configuration of unmanned spacecraft for the purpose of testing the reliability and survivability of nuclear fusion powered spacecraft.
Space environment threats such as orbital debris, asteroids and solar flux requires the development of human habitats across the solar system, other star systems and cosmic bodies. For example, solar evolution models demonstrated that life is unsustainable on earth. Within the next three to six billion years which is relatively short with respect to the universe timescale, solar flux will generate deadly temperature to earth’s biological life (Schroeder & Smith, 2018). This renders necessary space exploration efforts across the cosmos to ensure the survival of Earth’s biological species. As stated above, achieving this objective requires significantly faster space travel and one of the most attainable solution is the use of nuclear fusion powered spacecraft to improve the rate of space exploration by the human race. The Direct Fusion Drive and Colliding Beam Field Reverse Configuration are two nuclear fusion propulsion concepts that have demonstrated performance capabilities that can revolutionize the space exploration field.
References
Anderson, J.D. (2016), Introduction to Flight. McGraw-Hill Education, New-York, NY.
Cassibry, J., Cortez, R., Stanic, M., Watts, A., Seidler, W., Adams, R., Statham, G., &
Fabisinski, L. (2015). Case and Development Path for Fusion Propulsion. Journal of
Spacecraft and Rockets, 52(2), 595-612. https://doi.org/10.2514/1.A32782
ESA. (2021). JUICE. https://sci.esa.int/web/juice/home
Gajeri, M., Aime, P., & Kezerashvili, R. (2021). A Titan Mission Using the Direct Fusion
Drive. Acta Astronautica., 180, 429–438. https://doi.org/10.1016/j.actaastro.2020.12.013
Mattingly J.D. (2006). Elements of Propulsion: Gas Turbine and Rockets. Reston, Virginia:
American Institute of Aeronautics and Astronautics, Inc.
NASA. (2020, March 5). Mars Perseverance Mission Overview. Retrieved from
Miernik, J., Statham, G., Fabisinski, L., Maples, C. D., Adams, R., Polsgrove, T., Fincher, S.,
Cassibry, J., Cortez, R., Turner, M., & Percy, T. (2013). Z-Pinch Fusion-Based Nuclear
Propulsion. Acta Astronautica, 82(2), 173-182 doi.org/10.1016/j.actaastro.2012.02.012
Schmidt, G. R., Bonometti, J. A., & Irvine, C. A. (2002). Project Orion and Future Prospects for
Nuclear Pulse Propulsion. Journal of Propulsion and Power, 18(3), 497-504.
doi:10.2514/2.5969
Schroder, K.P., Smith, R. C. (2008, March 13). Distant Future of the Sun and Earth Revisited,
Monthly Notices of the Royal Astronomical Society, Volume 386, Issue 1, May 2008,
Pages 155–163, https://doi.org/10.1111/j.1365-2966.2008.13022.x
Song, J., Chen, Z., Zeng, Q., & Bai, Y. (2021). Analysis of Fusion Propulsion System for Earth-
to-Mars Mission. Fusion Engineering and Design, 164.
doi:10.1016/j.fusengdes.2021.112230
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