Planning+a+Trip+to+Mars+through+selection+of+a+propulsion+system

=**Introduction **=

toc Mars is the next major space achievement of the human race but the details of how still need to be planned. Everything from ship design to number of launches to number of crew to how long of a stay can be sustained all depends on which propulsion system is used to make the journey. Currently there is a planetary push to use renewable and green energy and this makes sense to apply to the next generation of space craft. There are plenty of alternatives to the traditional chemical based rocket such as nuclear-electric propulsion (NEP), Solar-electric propulsion (SEP), nuclear-thermal propulsion (NTP), and hybrid drives that use a combination of different technologies. Each of these different drives has advantages and limitations when compared to the others, making this decision increasingly difficult.

=**Traditional Chemical Rocket **=

The use of a traditional rocket is perhaps the fastest option that we have available today but using it for a mission to Mars proves to have many challenges.Due to the limited size of the current space shuttle a smaller crew of 3 would be necessary with a travel time of about 6 months and a stay of 120 days [1]. The biggest downfall of the traditional chemical rocket is that it would need a propellant refill for the trip back to earth [1]. This could be achieved be either sending a propellant tank to Mars orbit prior to the arrival of the manned mission or by harvesting a propellant from the Martian atmosphere [1, 12]. The first suggestion is unnecessarily expensive while the second suggestion would take precious time away from the landing team. The use of a traditional rocket would require between 4 and 6 launches depending on if the propellant tank is sent to Mars beforehand.

=**Nuclear Electric Propulsion **=

A Mars mission using nuclear-electric propulsion is being developed by a team at the Moscow Aviation Institute. In fact, they have made a nuclear power plant that is capable of producing 1MW of power. They are also working on a radio frequency ion thruster (RFIT) that is capable of being used in rows or blocks to produce more thrust per unit area[2]. Their system utilizes 64 ion thrusters in 8 blocks of 8 powered by a nuclear reactor and can make the trip to Mars in 11 months [2].The ship purposed can support a stay of 218 days and would require 3 separate launches to get all the necessary components into orbit [2]. The technology is still in development and cannot produce a launch before 2035. A propellant is required but it will not need to be refilled for the entirety of the mission [2]. When compared to the traditional rocket nuclear-electric propulsion can support a higher payload but has a slightly longer travel time [11]. Another reason that nuclear power is a more viable option is that there has recently been a discovery that allows the use of low-enriched uranium instead of high-enriched for use in nuclear reactors [5]. This is beneficial because of the danger of radiation from accidental leaks or the harvesting of nuclear material for nuclear weapons.

=**In development by NASA: VASIMR **=

VASIMR is the name and acronym of a new propulsion system being developed by NASA [8]. The Variable Specific Impulse Magnetoplasma Rock et is a hydrogen based propulsion system. The main advantage to hydrogen is it is everywhere in our solar system making it an abundant resource to use as a propellant. The VASIMR is basically a very large ion thruster that can be powered by either a nuclear reactor or by solar panels making this a versatile technology for the future that can be easily adapted to a hybrid propulsion system. It is purposed that a system such as this can make the journey to Mars in only 39 days [8]. However, VASIMR is not considered the best choice because it is still in development and may not be available for some time due to the extremely high power input needed and complex conversion system required to convert the nuclear power to electricity [14].

=**Nuclear-Thermal Propulsion **=

Nuclear-thermal propulsion is similar to NEP and the two could be beneficial to a hybrid system if used together [11]. The two systems could be combined to produce a ship with high thrust to weight and high output continuous thrust to shorten the travel time to Mars. The use of low enriched uranium also applies to NTP as well making this an exciting discovery for both the propulsion industry and for the rest of the world. The main benefit however, is the very high thrust to weight ratio of 3 to 10 making NTP capable of transporting extremely high payloads for a small vehicle [13]. The down side is a very low initial thrust. Perhaps NTP vehicles will become the tug boats of space but will probably not be the main power source for a manned mission to Mars due to the long travel time. NTP may also work well in a hybrid system similar to the NEP system.

=**Solar-Electric propulsion **=

Solar electric propulsion has many variables associated with the use; such as power output and performance degradation [10]. The greater the distance from the sun the less available solar energy there is [4]. In order to capture enough sunlight to create the energy needed the solar panels would have to be massive. This creates challenges for launching and landing the ship. Degradation of the performance happens over time with continuous use in a harsh environment like space. Due to this, fuel consumption and operation time need to be carefully monitored [10]. The travel time to Mars would be around 400 days, this means that the astronauts would have a round trip travel time of over 2.5 years [7]. This is dangerous for the astronauts to be in zero g for that long. Taking these factors into consideration solar electric propulsion is better for going toward the sun than away from it.

=**Using a Solar Sail to offset fuel consumption **=

The use of a solar sail can provide many benefits to a Mars mission. The sail works on solar pressure alone so no light capturing and converting is necessary. Although the travel time can increase with the use of a solar sail the fuel consumption reduces proportionally all the way to 0 [6].This saves fuel which directly translates to saved money when used with a nuclear electric drive. The main benefit of a solar sail is that it can be used to propel the ship between Low-Earth orbit and High-Earth orbit with the use of a spiraling maneuver [9]. This maneuver can be performed by an unmanned craft providing a shorter travel time due to a closer starting position and it would allow for more fuel consumption on the trip which would in turn shorten the travel time to High-Mars orbit for the astronauts. This is appealing because of less time spent in zero g for the astronauts.

=**Why NEP with a Solar Sail **=

<span style="font-family: Arial,sans-serif; font-size: 10pt;">The use of NEP with a solar sail can benefit the mission by saving fuel and money. The required number of launches is only 3 or 4, there is no refueling needed for the duration of the mission and when the spiral maneuver is performed with an unmanned ship the travel time for the astronauts is reduced. Compared to the other options NEP has a greater reusability factor and with the development of new ion thrusters and a 1 MW reactor the capabilities have also increased. This technology is purposed to be ready for a launch by the year 2035 [2]. In the meantime, construction of the ship could be started in Low-Earth orbit giving the next 18 years to spread out the 3 launches required to get all the necessary pieces into orbit and to finish and perfect the propulsion system.

=**<span style="font-family: Arial,sans-serif; font-size: 10pt;">Conclusion **=

<span style="font-family: Arial,sans-serif; font-size: 10pt;">Further research needs to be done to achieve this goal and the next step is to select a propulsion system and plan the mission around that system. Taking into account the travel time, length of the stay on the surface, the cost and above all the safety of the astronauts. If this mission is planned and executed properly it will become the new blue print of space travel for future generations. As Neil deGrasse Tyson has said “space exploration is a force of nature unto itself that no other force in society can rival” [15].

=**<span style="font-family: Arial,sans-serif; font-size: 10pt;">References **=

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<span style="font-family: Arial,sans-serif; font-size: 10pt;">[2] H. W. Loeb, V. G. Petukhov, G. A. Popov, and A. I. Mogulkin, “A Realistic Concept of a Manned Mars Mission with Nuclear–Electric Propulsion,” //Acta Astronautica//, vol. <span style="font-family: Arial,sans-serif; font-size: 10pt;">116, no. Supplement C, pp. 299–306, Nov. 2015.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[3] F. Winterberg, “To Mars in Weeks by Thermonuclear Microbomb Propulsion,” //Journal of Propulsion and Power//, vol. 30, no. 6, pp. 1480–1484, 2014.

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<span style="font-family: Arial,sans-serif; font-size: 10pt;">[5] M. G. M. Houts, “Low-Enriched Uranium Nuclear Thermal Propulsion Systems,” presented at the Annual AAS Guidance, Navigation and Control Conference, 2-7 Feb. 2017, United States, 2017.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[6] S. Gong, J. Li, and F. Jiang, “Interplanetary Trajectory Design for a Hybrid Propulsion System,” //Aerospace Science and Technology//, vol. 45, no. Supplement C, pp. 104–113, Sep. 2015.

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<span style="font-family: Arial,sans-serif; font-size: 10pt;">[8] “NASA - Propulsion Systems of the Future.” [Online]. Available: https://www.nasa.gov/vision/space/travelinginspace/future_propulsion.html. [Accessed: 07-Oct-2017].

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[9] N. Sullo, A. Peloni, and M. Ceriotti, “Low-Thrust to Solar-Sail Trajectories: A Homotopic Approach,” //Journal of Guidance, Control, and Dynamics//, pp. 1–11, Aug. 2017.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[10] P. Zhang, J. Li, and S. Gong, “Fuel-Optimal Trajectory Design Using Solar Electric Propulsion Under Power Constraints and Performance Degradation,” //Sci. China Phys. Mech. Astron.//, vol. 57, no. 6, pp. 1090–1097, Jun. 2014.

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<span style="font-family: Arial,sans-serif; font-size: 10pt;">[12] Zubrin Robert M., Muscatello Anthony C., and Berggren Mark, “Integrated Mars In Situ Propellant Production System,” //Journal of Aerospace Engineering//, vol. 26, no. 1, pp. 43–56, Jan. 2013.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[13] S. K. C. Borowski, “Nuclear Thermal Rocket/Vehicle Design Options for Future NASA Missions to the Moon and Mars,” presented at the Space Programs and Technologies Conference and Exhibit, Huntsville, AL, United States, 1995.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[14] “Variable Specific Impulse Magnetoplasma Rocket,” //Wikipedia//. 02-Dec-2017.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">[15] Tyson, N. and Lang, A. //Space chronicles//. Grand Haven, MI: Brilliance Audio 2015.