Primary Power for Earth to Mars Transit



8.2 Primary Earth to Mars Transit Power and Surface Power

John Dankanich

8.2.1 Introduction

This section of power production deals with choosing the appropriate method of electrical energy generation. There are several methods to produce electrical energy for the duration of the surface stay on Mars as well as the transit to Mars from Earth. The primary focus in choosing an energy source is minimizing the cost, mass, volume, and risk associated with each system concept.

8.2.2 Primary Power for Earth to Mars Transit

One of the most prevalent features of any feasible mission to Mars is the use of in-situ resources. Not using what is already available to you is not only foolish, but is also impractical. With this thought it mind, we are using the nuclear thermal rockets (NTRs) remaining from the propulsion system. Both the Hab and ERA modules sent to Mars have three NTRs just begging to be used.

Each NTR has the capability of producing over 100 kilowatts of thermal energy while in an idle mode. The only question then, is how do you take the thermal energy from the NTRs and convert it into something useful. The technique is quite simple and more importantly proven. By using a thermionic power conversion system, the thermal output can be converted to usable electrical energy. The layout for an NTR operating in the dual mode to produce power is shown in figure 8.2.1. The dual mode concept is operating the NTR engine to produce enough thermal output for energy conversion without producing any thrust.

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Figure 8.2.1 NTR Operating in Dual Mode.1

8.2.3 Power Conversion for the NTR

If you name any space mission of relative duration, you can also name a space mission that incorporated the use of a thermo-electric power conversion system of some sort. A thermo-electric power converter operating at 15 percent efficiency would be capable of delivering all the necessary power for the Hab and ERA with just one operating NTR. Also, with the critical power requirements less than 10 kWe for life support and communications, we would still have a safe crew with the NTR systems operating at 20% of their nominal conditions.

The thermal output of the NTRs is focused through a single thermo-electric power converter providing an optimal thermal input of 300 kWt. This fact allows for a triple layer of redundancy with the energy source for the power converter. If an NTR suffers a complete failure and is unable to produce any thermal energy, the other two independent engines can still provide the energy input necessary for the power converter. In fact, the system can still meet the necessary energy requirements with the highly unlikely complete failure of two out of the three engines. This is a remarkable source of energy that is already available at a minimal cost, since the engines are already in place just waiting to be used.

There is of course some cost in the mass of thermo-electric power converter. The mass of the system will be approximately 1.5 tonnes. This mass includes the power converter system and all necessary connections. The reason the power converter is of such a large mass is that the system will be designed with its own redundancies. The typical thermo-electric power converter is shown below in figure 8.2.2.

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Figure 8.2.2 Basic Thermo-electric Power Conversion Process.1

The power converter will receive heat input from the NTR and then convert that heat into power using the Seebeck effect. The Seebeck effect occurs when two different metals are maintained at two different temperatures. As in figure 8.2.2, modern thermo-electric converters use p-type and n-type semiconductor materials to generate relatively large voltages per degree of temperature difference between the hot and cold plates.

The thermo-electric converter generates a current when a load is applied to the system. The power converter will operate at a maximum efficiency when the load applied to the system is equal to the internal resistance of the system. The resistance can easily be controlled by applied a shunt to the system. Matching the internal resistance to the external load is a common practice of all power systems and is demonstrated below.

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Figure 8.2.3 Thermal-electric Power Converter at Optimal Conditions.1

Another great benefit of the thermo-electric power conversion system is that an array of thermal couples can be placed in a web of parallel and series connections to allow additional redundancies within the system. If some of the thermal couples fail, the system still generates power. The redundancies minimize the chance of a short within the system, and therefore eliminate the chance of a total single failure mode.

8.2.4 Transmission of the Power

Perhaps the largest obstacle to overcome with using the NTR as a power source is getting the power from the NTR engines to the load several hundred meters away. The transmission line is deployed without any tension on the line, and so that the line does not tangle in the process.

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Figure 8.2.4 Transmission Line for Powering the Hab and ERA to Mars.

As shown above the power cable will have a level in redundancy in sending two lines of DC power to the load. The system will operate at 100% even if one cable should break. The design also has the individual transmission lines wrapped in a helical fashion around a foam core. The purpose of this setup is to allow the chords to actually increase in length by compressing the forma core rather than adding tension to the cable. The final part of the cable will be an insulation layer to maintain an even temperature within the transmission line. There will be a slight heating affect within the lines due to the resistance of the cables.

The process of deploying the cable is as simple as deploying the tether. In fact, it is the same process. The transmission line is designed to fit with the diameter of the tether array. By placing the transmission line within the tether, the chance of the cable becoming tangled on deployment is minimized.

8.2.5 Miscellaneous Power Sources for the Surface of Mars

In the exploration of Mars, a limiting factor is the ability to produce power. Power is used for even the simplest tasks in our everyday lives. There is no doubt that a key element in exploring Mars will be the available power to the astronauts. Luckily, Mars has several viable options for power.

On the surface of Mars there are several possible power sources. Of course there are the obvious sources such as bringing batteries and fuel cells from Earth. Once again it is ignorant to believe that the best source of power would be to bring such massive sources of energy when there are so many other options. Wind power is certainly quite feasible on the surface of Mars. Because of the reduced gravity, structures could be built to place windmills to extreme heights where the wind speeds are considerable stronger. Even with an atmosphere one hundredth of that of Earth, there would be a practical and almost endless power source. The problem with using windmills is that it requires considerable effort from the astronauts to construct the devices. It is a source of power that would be far more practical after manufacturing techniques have been established on Mars.

Another great source of power is a geothermal power plant. Mars is known to have similar tectonic activity to Earth. The potential for geothermal energy exists as a great source of abundant power. An additional benefit to geothermal power is the useful byproduct of water. Again for our original mission we would have no way of knowing where the sources could be easily obtained. For the early missions to Mars, a power source is one that ultimately must come from the Earth and requires no human construction or previous exploration of the surface.

8.2.6 Solar Power on Mars

Solar power presents itself as the first feasible choice of power for an early mission to Mars. Solar cells are consistent in reliabilities above 99%. There are unfortunate drawbacks to using solar cells. The first obstacle, which is certainly not trivial, is the deployment of the arrays of cells without humans on the surface. Because the power source will be operating before any humans arrive, the power system must be fully autonomous. Unfolding the arrays needed to provide the power for the full mission would a marvelous engineering feat. The arrays would need to be roughly the size of a football field. The deployment would need to gently unfold the fragile arrays over an uneven and unexplored terrain. This would require immense structure and a good deal of luck in order to succeed.

Another problem with using solar cells is that there are dust storms on Mars that occur for seasons at a time. While the storms block us from seeing the surface there is a good chance that a high percentage of light would successfully reach the cells. When the light gets to the cells it may find that they are routinely covered with the Martian dust though, lowering their efficiencies. A routine sweeping of the cells could remedy this problem, but the cells would be operating for fifteen months before any human could sweep them off. Clearly another source needs to be found.

8.2.7 Nuclear Power on Mars

When the mission is analyzed for the most viable power source the two most important factors are the amount of power which needs to be produced and the duration the source needs to function. Our Mars mission requires approximately 100 kWe in order to accomplish its goals. The main driving force in requiring so much power is the mission requirement of producing the propellant for the return trip to the Earth before the astronauts leave home. It takes a considerable amount of power to produce the massive amounts of propellant in the fifteen months prior to the Hab launch, as well as the cryogenic storage of the Lox that is simultaneously produced. If we sum up the time of the propellant production as well as the surface stay of the astronauts, we realize we will need a power source for approximately four years. The answer is clear. As shown is the following figure, the feasible options are rather limited. Nuclear is the power source of Mars.

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Figure 8.2.5 Feasible power sources based on output and duration.12

The nuclear reactor for Mars is quite unlike the reactors used on Earth. Because there is a considerable cost in launching large masses, the reactor will be much lighter and smaller than any terrestrial nuclear power plant. In fact, the reactor is far closer in design to a reactor meant for space operation. This fact makes complete sense though, because it has the same driving constraints of a reduced mass and volume.

While a terrestrial based nuclear power plant can have an inner tank mass approaching 100 tonnes alone, we will use an entire system of approximately three tonnes. The numbers seem almost unfeasible to think terrestrial systems would be such a waste of material, but pouring several tonnes of concrete is relatively cheap with respect the large amounts of energy that is generated and sold to paying customers.

The nuclear reactor for the PERForM mission will only stand two meters tall, four meters after the radiators are extended up, approximately 3 meters wide, and 4 meters long. The radiators are a drastic difference than those seen in space nuclear reactors because of the decreased size. While we will only have a few dozen meters of surface to radiate heat, a typical reactor in space would generally require over 100 square meters. The reason for the reduced area requirement is of course that we are not in space. The planet has an atmosphere, and a cold one. The gentle breeze passing over the radiators will provide convective heat exchange considerably more than a pure radiating system. The reactor for this mission will look similar to the one below.

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Figure 8.2.6 100 kWe Power Plant for Mars Surface Power

Again, the driving force in designing a reactor is the mass of the system. The reactor itself has several unique features to minimize the mass. One special feature of this reactor is that the fuel source is a Uranium 235, Zirconium dispersion fuel matrix. Purdue University and The Argonne National Laboratory are currently researching this fuel concept. This type of fuel source shows considerable benefits including proliferation resistance, severe accident mitigation, a low fuel failure rate, lower internal fuel temperatures, and an improved waste stability.

8.2.8 Radiation Shielding

The overall breakdown of mass distribution is shown in figure 8.2.7. The largest source of mass for the system is the radiation shielding. The major drawback in using a nuclear reactor on a human mission is that the reactor must be designed to shield the harmful radiation from the equipment near it, as well as the humans.

Figure 8.2.7: Mass breakdown of the nuclear system.

The shield developed was designed using a program called Microshield. The program takes the core reactivity and analyzes its effects after penetrating various layers of shielding. The shield itself is not composed of large amounts of lead as would be expected on a terrestrial plant. The shielding only has a small amount of heavy metal including beryllium in order to shield the gamma rays. The bulk of the shielding is composed of lithium hydride. The LiH is ideal for our mission because it has a large amount of hydrogen atoms, which attenuates the neutron radiation, and it has a low density. Unfortunately the volume of LiH is non-trivial and does add considerable size to the system.

The limiting factor in optimizing the shield is how much radiation various materials can withstand. Figure 8.2.8 shows the threshold of radiation before damage will occur for various materials. The figure clearly shows that the object most affected by the radiation is obviously humans. However, humans can be kept a certain distance from the reactor itself. Because of the inverse square law of radiation fall off, the humans will be safe as long as they do not spend considerable amounts of time near the reactor. Electronics and other equipment though, need to be placed very close to the reactor since it is fully autonomous and contains its own energy converter.

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Figure 8.2.8 Limiting constraints on radiation for various materials.1

The shield layout is also important in reducing the mass of the system. Typical space reactors are designed with a shadow shield because the reactor can always be kept from facing the astronauts. Because we would like to be able to explore the planet, a shadow shield is not the best option. Another option is a 4( shield. As the name suggests it would decrease the radiation effects in all directions. The 4( shield though has a substantial mass penalty, so we use a preferential 4( shield in order to allow exploration near the reactor if necessary and still reduce the overall mass of the system. If it is imperative to spend considerable time near the reactor, the reactor could be rotated since it will be on wheels from its original deployment. The various shield layouts mentioned are shown below.

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Figure 8.2.9 Various shield geometries for space nuclear reactors.1

The shield layout and core are shown in figure 8.2.10. The basic layout is a core with a reflector to increase the fission process followed by several layers of shielding and insulation. The plant will exchange heat with its power converter thought the use of heat pipes to maintain an entirely passive system. Because the core is made of a zirconium dispersion fuel the heat transfer is high and the heat pipes can be placed within the core and increase its efficiency.

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Figure 8.2.10 Layout of the reactor.

8.2.9 Power Conversion

Again the power conversion for this system is thermo-electric. It is able to produce DC power to the required loads and maintains its high reliability. Other power conversion systems seem ideal because of higher efficiencies such as a Brayton or Stirling engine, but the risk of moving parts is preferably avoided.

As shown below the system will use a thermionic power converter. A thermionic converter is a system that essentially boils electrons from a hot emitter surface across a small interelectrode gap to a cooler collector surface. Again, the main driving force in choosing a direct power converter is the lack of moving parts. The entire system is passive and self-contained to increase the overall reliability of the system. Direct energy converters are practical up to approximately 300 kWe. As the need for such large amounts of power occurs the increased efficiencies associated with active systems dominates the reliability factor.

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Figure 8.2.11 Thermionic converter for direct power conversion.1

8.2.10 Deployment and Operation:

The last thing to note is how the entire system comes together and is put into use. The reactor will initially land on the surface of Mars fifteen months prior to the Hab launch. The reactor is kept on a set of wheels that can me driven away from the landing site with the propellant producing facilities to a safe distance from any other electronics. The reactor will ideally be placed in a depression to add shielding around the reactor, though this is not imperative. The reactor will then power itself up and start the propellant production process. The system will power the production of propellant until that process is finished. After approximately eighteen months of operation the Hab will arrive and be connected to the reactor.

It is clear that while governments on Earth may limit the use of nuclear power, it is the source for Mars. Without nuclear power there could not be any feasible mission to explore Mars.

8.2.11 Acknowledgment

I would like to thank the Purdue University Nuclear Engineering Department for their advise on the design of the Mars surface reactor. I would especially like to thank Scott Kiff for his individual efforts.

8.2.12 References

1. Angelo, Joseph A., and Buden, David, “Space Nuclear Power,” Orbit Book Company, Florida, 1985.

2. McDeavitt, S. M., Downar, T. J., Hash, M. C., Revankar, S., Solomon, A. A., Xu, Y., Hebden, A. S., Walker, D., Robey, W., Xiu, J., “Development of a Mixed Oxide Cermet Dispersion Fuel Using (Th,U)O2 in a Zirconium Metal Matrix,” Winter ANS Meeting, Washington D. C., 2000.

3. El-Genk, M. S., Hoover, M. D., “Transactions of the 5th Symposium in Space Nuclear Power Systems,” Albuquerque, New Mexico, 1988.

4. Chilton, Arthur B., Shultis, J. Kenneth, Faw, Richard E., “Principles of Radiation Shielding,” Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1984.

5. Hyder, A. K., Wiley, R. L., Halpert, G., Flood, D. J., Sabripour, S., “Spacecraft Power Technologies,” Imperial College Press, London, 2000.

6. Sanford, F., “Electric Distribution Fundamentals,” 1st ed., McGraw-Hill Book Company, I nc., New York, 1940.

7. ASME, “Damage Assessment, Reliability, and Life Prediction of Power Plant Components,” edited by Pangborn, R. N., Narayanan, T. V., Means, K. H., Bond, C. B., PVP Vol. 193, NDE Vol. 8, New York, 1990.

8. Mason, Lee S., Bloomfield, Harvey S., Hainley, Donald C., “SP-100 Power System Conceptual Design for Lunar Base Applications,” , 1999.

9. Grover, M. R., Odell, E. H., Smith-Briton, S. L., Warwick, R. W., Bruckner, A. P., “ARES Explore: A Study of Human Mars Exploration Alternatives Using In-situ Propellant Production and Current Technology,” , 1996.

10. Renewable Energy Concepts Incorporated, “Renewable Energy Concepts,” , 2000.

11. Fuel Cells 2000, “On-Line Fuel Cell Information Center,” , 2001.

12. Mayer, A. J. W., “Power Sources for Lunar Bases,” , 2001.

13. Department of Energy, “Data for Nuclear Power Plants,” , 1997.

14. HobbySpace, “Life in Space,” , 2001.

15. Mogami, “Wire Gauge Calculations,” , 2001.

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