LONG DIFFER SPACE TRAVEL

DIFFERENT PROPULSION OPTIONS FOR LONG DISTANCE SPACE TRAVEL

for
Jerrie Fiala
Technical Communication Instructor
Western Michigan University
Kalamazoo, MI

by
Ian Mackey
IME 1020 Student

November 5, 2013
TABLE OF CONTENTS

ABSTRACT …………………………………………………..........................

INTRODUCTION ………………………………………………..................

Definitions and Background ……………..…………...…….................. Purpose and Audience …………………………...…......….................... Sources ………………………………………………....…….................. Working Definitions …………………………......................................... Limitations …………………………………..……….….……...…......... Scope …………………………………………...…….………..................

COLLECTED DATA …………………........................................................

In Situ Based Propulsion ……………………….................…………… Creating Fuel on Mars…………………………………................ Metal Based Propulsion…………………….................................. Solar Thermochemical Propellant Synthesis ……….....................

Nuclear Propulsion……………………………………………............... Nuclear Power on Earth…………………....………….................. Nuclear Propulsion in Space………….....................…………...... Harness Energy with Regenerative Fuel
Cells................................ Negative Aspects of Nuclear Propulsion………………………....

CONCLUSION……………………………………….....................................

Summery of Findings……………………………………................…… Interpretation of Findings……………………………................……… Recommendations………………………………….....................………

REFERENCES………………………………………........................……......

3

4

4
4
4
5
5
5

6

6
6
7
8

9
9
11
12
14

16

16
16
16

18

ABSTRACT During the 1960’s, the United States impressed the world with the success of the Apollo missions and moon landings. Now, over 50 years later the moon is still the farthest destination man has ever reached, but it will not be for long. Space agencies across the globe are planning new missions, missions to send man to Mars. Within the next 20 years man is expected to land on the surface of the distant red planet (Guven & Velidi, 2011). To successfully send a man to Mars new propulsion techniques and technological improvements are needed to make the long distance mission possible. Some of the new propulsion techniques are the use of a nuclear powered thruster and In situ propellants. Along with these new techniques are the need for better radiation shields, heat resistant materials, and life support systems.

INTRODUCTION
Definitions and Backgrounds Everyday people are brought into this world faster than they are leaving and the increasing number of inhabitants on this planet is becoming a problem. As the population of Earth gets bigger the less room and resources there are for each individual person. It may not seem like a problem now, but in the near future population control will affect us all. The problem is that the human population is getting bigger, but the earth is not. The solution to this problem is in the stars. Space is incomprehensibly large, so the possible benefits of mastering space travel is limitless. Investing in space travel will potentially allow humans to be a multi-planetary species, mine resources from foreign planets, and get a better understanding of our universe (Sherwood, 2011).
Purpose and Audience The purpose of this paper is to examine and consider any potential propulsion systems that will make interplanetary travel possible. The two propulsion methods discussed in this paper are both realistic solutions to long distance space travel and should be applicable within the next 10 years. This paper was written mainly for engineers and applied science technicians, as the readers must have a basic understanding of physics and chemistry to fully understand the concepts described.
Sources
All of the sources and articles used in this paper were found through Western Michigan University’s online databases and Waldo Library’s interlibrary loan service.

Working Definitions There are numerous terms throughout this paper that readers may not recognize and are necessary for the full understanding of the concepts described. These include: 1. RFC – An acronym that stands for Regenerative Fuel Cells. A RFC is a type of fuel cell that allows multiple energy storages, meaning the cell can be reused after its’ energy is depleted (RFC). 2. Chemical Engine – A chemical engine is a device that allows for chemical reactions to take place. Can be as simple as a petri dish to as complex as a bacterial organism. 3. Endothermic – A reaction that requires an external energy source to react. 4. Exothermic – A reaction that gives off energy, usually in the form of heat 5. ISSP – an acronym for In Situ Propellant Production.
Limitations
This paper focuses on realistic propulsion systems that will be able to withstand and generate adequate energy for long distance space travel. The methods described in this paper do not take into consideration economical or financial aspects and emphasizes the mathematical, physical, and chemical feasibility of the concepts.
Scope
This essay will explain the problems of current spaceships propulsion systems, show the viability of two new propulsion methods, and give a general understanding of the physical and chemical properties of each new method.

COLLECTED DATA
In Situ Based Propulsion
Creating fuels on Mars One of the biggest contributors to making a manned mission to mars possible is the mass of the spacecraft. As the mass of the spacecraft decreases, so does the amount of fuel the ship needs to move across the solar system as according to Newton’s second law of motion. This law utilizes the concept of momentum. Momentum describes how strongly an object is moving; the two factors that define momentum are mass and velocity. This concept can be use to predict how an objects velocity will change if the objects mass is changed. The relationship between two different objects momentum can be described by the following equations. m1 v1=m2v2
(m representing an objects mass and v representing an objects velocity)
This equation shows us that an object of less mass, like a space ship, will travel faster than an object of higher mass when a constant force, or propulsion, is applied to the object (Crowell, 2013). So the key to making a manned mission to Mars possible is building a smaller spacecraft and minimizing the carrying load of the spacecraft. One idea scientists have come up with to minimize the carrying load of a spaceship is to eliminate the need to transport a large amount of fuel for a round trip. In situ resource technologies are a type of technology that can generate resources on the spot or in other words create fuel while on the surface of Mars. By using in situ propulsion technologies engineers can reduce the carrying load of a spacecraft by 20 to 45% (Wegeng & Pestak, 2010). This new type of technology can create fuel from many different processes such as refining metal powders and Solar Thermochemical processing.
Metal Based Propulsion All propulsion systems use the same idea of momentum that a thruster shoots off fuel particles in the opposite direction of a spaceships destination to create a force that will move the ship through a vacuum. Earth and other places that have an atmosphere use conventional methods of creating a net force on an object like using a turbine on an airplane, an engine in a car, or a propeller on a boat, but in the space none of these types of transportation will work. Space propulsions are different than conventional forms of transportation because the conventional forms make use of a present medium while space propulsions do not have a medium to use. A medium is a substance in which something currently exists in. In conventional transportation, the most common mediums are air, water and stone. In the vacuum of space there are no mediums to manipulate so spacecrafts use a different type of type of transportation called propulsions. Space propulsions utilize a fuel system that uses a thruster that shoot particles of fuel to into space to create a recoil force on the spaceship; the recoil force creates a net force on the spaceship accelerating it through space (Guven & Velidi, 2011). The most common and effective fuel source for rockets is a mixture of liquid oxygen and a hydrogen based compound. The problem with using a hydrogen based fuel for long distance space travel is that it is very difficult to make on a foreign planet. Gathering hydrogen on a foreign planet is not too difficult but changing gaseous hydrogen to liquid hydrogen takes a lot of work. Hydrogen starts to change from gas to liquid at around -423°F, and to get a compound at such a low temperature is very difficult to do, especially while on a different planet (Braeunig, 2008). The solution to this problem is to replace the hydrogen fuel with a more common metal one, like aluminum. While hydrogen fuel performs much better than aluminum based fuel, the metal based fuel has the advantage of being composed of a material that can be found on Earth, the Moon, Mars or even asteroids. This makes aluminum based fuel a generic fuel that can be created almost anywhere in our solar system. The aluminum propulsion system will work by having an inert, non-reacting, gas carry aluminum powder into a combustion chamber to burn with liquid oxygen. The resulting combustion reaction will produce a powerful flame that shoots the aluminum particles into space creating a net force that will move the spacecraft through space (Ismail, 2011).
Solar Thermochemical Propellant Synthesis Solar Thermochemical processing is a great way to create many different types of fuel on a foreign planet. This process uses concentrated solar energy to help accelerate certain endothermic chemical reactions that can manufacture propellants (Wegeng & Pestak, 2010). An endothermic reaction is a chemical reaction that needs external energy to react. In this case, solar power acts as the external energy. While solar thermochemical propellant processing is an effective way to create fuel, it is limited to the raw resources on a planets surface. Most fuels have a similar chemical makeup and the most basic building blocks of propellants are carbon dioxide (CO2) and water (H2O). Fortunately, both of these compounds can be easily obtain from the surface of mars; Mars’ atmosphere is about 95% carbon dioxide (Wegeng & Pestak, 2010) and majority of Martian soil is 2% water by mass. Visiting astronauts can gather carbon dioxide with a sorption pump. A sorption pump collects a mixture of Mars’ atmospheric gasses and expels unwanted gas while compressing carbon dioxide from 636 Pa (Mar’s average air pressure) to 100000 Pa. This process not only gathers pure carbon dioxide but also compresses it for easy storage (Wegeng & Pestak, 2010). Astronauts can gather water by simply heating Martian soil and collecting the escaping water vapor. With these two compounds collected, astronauts can use a chemical engine to create multiple solar propellants as shown in table 1. Source: Guven & Velidi, 2011.
Nuclear Propulsion
Nuclear Power on Earth Nuclear power is used all across the world because it is a great source of environmental friendly energy. As of 2011 there are 443 nuclear power plants in 47 different countries. Most people see nuclear power as very polluting process because of the byproduct of nuclear waste, but nuclear fission, the source of nuclear energy, is actually a very green process. A nuclear power plant usually works by encasing radioactive uranium pellets in a pressurized core that is then submerged in water. The uranium pellets naturally undergoes nuclear fission, a very exothermic reaction, in which the uranium atoms split releasing large amounts of energy in the form of heat. If left unattended the uranium core would get hotter and hotter until it would melt and destroy the reactor. Nuclear engineers prevent this by adding in control rods which manages how fast the uranium undergoes fission. The energy from the fission reactions will then heat the water surrounding the uranium core. The heated water slowly turns to steam which will build pressure and turn a turbine to create electrical energy (Brian & Lamb). As seen in figure 2 below, steam can be seen leaving a nuclear cooling tower.
Figure 2. A nuclear cooling tower
[pic]
Source: http://blog.heritage.org/tag/nuclear-waste-management/

As long as nuclear waste is properly disposed of, the process of making nuclear power has almost no negative environmental impacts.

When a nuclear power plant operates normally it creates large amounts of energy with almost zero carbon emissions, but when the nuclear plant has problems like a nuclear leak or a meltdown the effects can be disastrous. The two most famous nuclear disasters are the nuclear meltdown of Chernobyl, Ukraine and Fukushima Daiichi, Japan. The Chernobyl Nuclear plant showed the world a worst case scenario for a nuclear power. On April 5, 1986 the cooling system to one of the nuclear reactors at Chernobyl malfunctioned and caused a reactor to explode. The resulting explosion sent radioactive debris flying as far as Western Europe and released such high levels of radiation that more than 1000 square miles of land, known as the Chernobyl Exclusion Zone, was evacuated. This event happened over 25 years ago and the Exclusion Zone is still uninhabitable to this day and will be for another 20,000 years because of the deadly radiation levels. So while nuclear power is a great source of energy it is also a potential catastrophe if not closely monitored (Auyezov & Balmforth, 2011).

Nuclear Propulsion in Space

While the risks of nuclear power are great the potential benefits are greater. Nuclear propulsion on a spacecraft would not only be able to handle the energy requirements needed for the thrusters to send a spacecraft to mars, but it could also supply the energy for the life support system and electronics on board as well (Guven & Velidi, 2011).

Nuclear Propulsion uses the same basic ideas of a normal nuclear reactor with some minor changes. For a nuclear propulsion system, scientists would replace the solid uranium core with a gaseous uranium hexafluoride (UF6) core. The gaseous core is better for space travel because the energy output required of a nuclear reactor for a mission to mars is much greater than what a solid core could handle. When a solid core produces too much energy its gets too hot and melts causing a nuclear meltdown, but a gas core cannot melt and therefore it cannot cause a nuclear meltdown. A gas UF6 core can withstand temperatures up to 9000K while a solid uranium core could only withstand temperatures of 5000K (Guven & Velidi, 2011).

Another change to scientist would have to make for a nuclear propulsion system is the utilization of the heat. Instead of having the fission reaction heat water, the exothermic reaction will transfer the heat energy into gaseous hydrogen molocules (H2). Hydrogen gas will be released into the gaseous core which already contains UF6. The UF6 molecules will undergo fission and the heat byproduct will be transferred into the hydrogen molecules as according to the equation below (Guven & Velidi, 2011).
UF6 + H2 + n ( 92Kr + 142Ba + 2.7n + 2HF + F4
The heat transferred into the hydrogen molecules will cause them to have a greater agitation (movement) and increase the pressure of the nuclear core. When the pressure becomes too great the excited hydrogen molecules will leave the core though a nozzle or exhaust. The flow of hydrogen molecules exiting the exhaust will create a net force on the spaceship, moving it in the opposite direction of the exhaust stream (Guven & Velidi, 2011). Figure 3 below shows how gaseous core reactor is set up.
Source: Guven & Velidi, 2011.
[pic]
Harness Extra Energy with Regenerative Fuel Cells One of the greatest benefits of a nuclear propulsion system is that it not only creates enough energy for the propulsion system of the spacecraft, but also for the life support and general electronics on board. The nuclear core could accomplish this by creating a second exhaust to let a small amount of heat be used as a thermionic source for electricity (Guven & Velidi, 2011). The heat could be used to change water to steam and turn a small turbine to create electricity. Nuclear fission can produce large amounts of energy, but it has no way to store the released energy for future use.
The solution to this problem is using a regenerative fuel cell (RFC) to store the unused energy. A RFC is ideal for space travel because it is a reusable electrical storage device, has a high energy capacity, and has a chemical byproduct of water. RFC utilize the concept of water electrolysis. Electrolysis is a simple endothermic chemical reaction that adds an electrical current to a container of water to separate the compound into its elemental forms. Figure 4 below shows a concept design of a RFC
cell. Figure 4. Concept of a Regenerative Fuel Cell [pic] Source: Sone, 2011.
The separated elements pass through a polymer electrolyte membrane layer that keeps the elements divided until the energy is need. When the energy is needed, simply remove the polymer electrolyte membrane, combine the elements and the reverse electrolysis reaction will create a flow of electrons, or electricity (Sone, 2011). The chemical formula for this process is shown below (Zubrin, Muscatello, & Berggren, 2013).
2H2O ( 2H2 + O2 (∆H=484 kJ)
2H2 + O2 ( 2H2O (∆H=-484 kJ)
Negative Aspects of Nuclear Propulsion Nuclear power is an excellent source of power when it is functioning correctly, but nuclear fission is a very unpredictable source of energy. The major problem with nuclear fission is that it gives off high levels of radiation. Radiation is the decaying of an atomic nucleus causing ionizing particles to be released. When ionizing particles comes in contact with humans, or any organic tissue, they can cause radiation poisoning. The effects and damage caused by radiation poisoning depends on the amount of radiation exposure. High levels of radiation exposure usually only occur on two occasions, a nuclear meltdown and an explosion of an atomic bomb. The only recorded malicious use of an atomic bomb happened at the end of World War II when the United States military dropped two atomic bombs on Japan. These bombs killed attributed to over 200,000 deaths within the first few months after the blast, and the high radiation levels killed an additional 100,000 people over the course of the following decade. Nuclear meltdowns are pretty uncommon, but when they do happen the results are even more destructive than an atomic bomb. The nuclear meltdown of the Chernobyl nuclear plant is proof of this. High levels of radiation exposure have the potential to kill victims instantly, but survivors of severe radiation poisoning still live with very deadly side effects. The most common side effects of high radiation exposure are higher rates of leukemia and cancer of the thyroid, breast, lung, stomach, and salivary glands (Dower, 1995). While high levels of radiation are more dangerous than lower levels, lower levels of radiation are of more concern because they are much more common. Low levels of radiation exposure are a major concern for nuclear propulsion because of its space requirements. Most nuclear power plants are massive, covering acres of land, but the problem nuclear engineers’ face is building a nuclear reactor on a spaceship with limited space. Nuclear power plants are so big because multiple precautions and safeguards are in place to minimize radiation from escaping the reactor. The reactor on a spaceship will have significantly less radiation shielding than a full size nuclear plant, so the risk for minor radiation poisoning is much greater on a spacecraft than at a reactor on earth. Scientists are still trying to find a happy medium between a small enough reactor to fit on a spaceship, but big enough to still adequately shield radiation. Lack of radiation protection can lead to low levels of radiation exposure which is known to cause nausea, bone marrow nodes, headache, lower white blood cell count, and Hemophilia (Nordqvist, 2011). To make nuclear propulsion possible, advanced radiation shielding technologies are needed to minimize radiation exposure for traveling astronauts.

CONCLUSION
Summery
By using either nuclear or in situ propulsion technologies, a long distance manned space mission is very possible. Nuclear propulsions utilize the power of a uranium hexafluoride core reactor to heat gaseous hydrogen fuel to move a spaceship through space. While nuclear propulsion is a great source of energy it is also a very dangerous system. Radiation levels are a major concern for nuclear propulsions and advanced radiation shielding is needed to make this system possible. In situ technologies allows for a spaceship to have a conventional propulsion system, but instead of bringing a large amount of fuel in situ technology lets astronauts create fuel on the surface of foreign planet through either metal powder fuel or thermochemical processing.
Interpretations of Findings While both methods for space propulsions are possible, in situ technologies is a more realistic system right now. Engineers have all of the components for in situ propulsions already built, tested and ready for action. Nuclear propulsion is theoretically possible right now, but a nuclear powered spaceship has never been built before. Engineers building a nuclear spaceship would not only have to worry about radiation levels, space requirements and weight limits, but also controlling nuclear fission reactions in a zero gravity environment. Problems occur in nuclear power plants on Earth, so one can expect an experimental nuclear reactor operating in a zero gravity environment to have problems (Guven & Velidi, 2011).
Recommendations
The next step in space propulsions should be to become more familiar with nuclear propulsions. If the problems of nuclear propulsions are overcome, it would be the best solution for long distance space travel. Nuclear propulsion has a greater energy output than any other propulsion system, eliminates the need to refuel a spacecraft, and provides the rest of spacecraft with electrical energy. Scientists and engineers need to research and experiment with nuclear propulsions so they can solve any current and future problems that would impede the construction of a functional nuclear spaceship.

References Auyezov, O., & Balmforth, R. (2011 March 15). In Chernobyl, a disaster persists. Thomsonreuters.com, retrieved November 5, 2012, from http://graphics.thomsonreuters.com/specials/Chernobyl.pdf

Braeunig, R. (2008). Basics of Space Flight: Rocket Propellants. Rocket and Space Technology, 4, 1-7.

Brian, M. & Lamb, R. How nuclear power works. science.howstuffworks.com. retrieved November 5, 2012, from http://science.howstuffworks.com/nuclear-power

Crowell, B. (2013). Chapter 14, conservation of momentum. www.lightandmatter.com, retrieved November 5, 2013 from http://www.lightandmatter.com/html_books/lm/ch14/ch14.html

Dower, J. (1995). Hiroshima, Nagasaki, and the politics of memory. Technology Review, 98(6), 48-51.

Guven, U., & Velidi, G. (2011). Nuclear propulsion in spacecraft as a unique solution for a mars mission. 62nd International Astronautical Congress, 2, 1200-1206.

Ismail, A. (2011). The potential of aluminum metal powder as a fuel for space propulsion systems. 62nd International Astronautical Congress, 10, 7904-7918.

Nordqvist, C. (2011 March 19). What are the effects of radiation on humans? what is radiaition poisoning? medicalnewstoday.com, retrieved November 5, 2013, from http://www.medicalnewstoday.com/articles/219615.php

Sherwood, B. (2011 January 31). Comparing future options for human space flight. Acta Astronautica, 69, 346-353.

Sone, Y. (2011, November 4). A 100-W class regenerative fuel cell system for lunar and planetary missions. Journal of Power Sources, 196, 9076-9080.

Wegeng, R., & Pestak, C. (2010 September 2). Solar thermochemical processing for the production of propellants on mars and fuels for the earth. AIAA Space Conference & Exposition,1, 3-16

Zubrin, R., Muscatello, A., & Berggren, M. (2013 January). Integrated mars in situ propellant system. Journal of Aerospace Engineering, 26, 43.