Design Criteria

Power is an essential subsystem of the Epsilon Eridani Space Probe (EESP). If the craft makes it to Epsilon Eridani without any power, then the team just launched a very expensive rock. Without power there is no station keeping, instrumentation, or transmission. The power scheme must be able to operate in deep space temperatures, have a long enough lifetime to reach the destination, and provide enough power once the craft reaches Epsilon Eridani. Current generation space power systems include batteries, solar panels, radioisotope thermal generators, and nuclear power.

As the craft must travel at least 10 light years without a significant radiative power source, solar panels would not be a viable option on this mission. Batteries as a power source would also not be viable on this mission given the weight of a battery system for such a long trip. Furthermore, batteries require an external heating source to keep them functioning in the cold temperatures of deep space. Nuclear power reactors are a viable option, as repurposed submarine nuclear reactors have a high power density. However, given the amount of power required for the journey (less than 1 kW at the destination), launching a nuclear reactor is not the most efficient power scheme in terms of cost, materials, and weight.

Proposed Power Scheme

The proposed Interstellar Data Express EESP power scheme is the use of a radioisotope thermal generator (RTG) using 238Pu as the fuel source used. This was chosen because it is a thoroughly tested and reliable power system and could supply enough power once the craft reaches its destination. The RTG assembly on the Voyager spacecraft is shown in Figure 1.^[1]

Typical radioisotope thermal generators use thermocouples to convert thermal energy released from the radioactive fuel into electrical energy. Thermocouples operate on the principle of the Seebeck Effect, whereby a an electric current flows when two dissimilar metals at different temperatures form a series circuit.^[2]

Efficiency

Thermocouples are simple in nature and thus reliable; however, their conversion efficiency is very low. Typical RTG's rarely reach upwards of 7% efficiency. Other methods of converting the thermal heat into electrical energy is to use a thermionic converter instead of thermocouples. A thermionic converter operates by boiling electrons from a hot surface across an electrode gap to a cooler collection surface, as shown in Figure 2.^[3]

The cathode and anodes of the thermionic converter must be kept at high, but distinctly different temperatures (~1800K and ~1000K respectively) to work properly.^[3] Though these converters operate at higher temperatures and are bound by the Carnot efficiency, thermal conversion can still reach up to 20%. It is assumed that the EESP and its hardware will be able to withstand the higher temperature requirement of the thermionic converter. In fact, design proposals for thermionic converters built for space applications are available, as shown in Figure 3.^[3]

With increased efficiency, less fuel is required for the same power output, and less fuel means less weight. Assuming the implementation of the best thermionic converter RTG yielding an efficiency of 20%, this means that five times the power output in fuel is required. A kilogram of 238Pu can generate 570 W_t of energy^[4] which is equivalent to 114 W_e at 20% efficiency.

Power Calculations

The half-life of 238Pu is 87.7 years. Given the proposed inertial confinement fusion drive propulsion scheme, the estimated total transit time to enter orbit with Epsilon Eridani is around 93.2 years. Given an estimated power usage (including communication transmit power, digitial signals processing, camera functionality, and all other miscellaneous power sinks) of 500 W max, the estimated power that we need initially is given below:

N_t = N₀ * 0.5^{(t / hl) [5]}

Here "hl" is the half-life of the radioactive isotope, "t" is the time in years, "initN" is the amount of power available initially, and "remN" is the amount of power remaining after t time has elapsed. Calculations with the given parameters yield a required initial power of 1044.2 W.

Required Fuel

Implementing a RTG with a thermionic converter, it is assumed that 1 kilogram of Plutonium-238 can produce 114 W of usable electrical energy. Given that we need to send 1044.2 W of power, this is equivalent to 9.16 kg of 238-Pu. This is of course a trivial amount of fuel compared with the weight of the propulsion system, fuel, and, communication devices.

References

1. Voyager, The Interstellar Mission: Radioisotope Thermoelectric Generators
2. Seebeck Effect
3. Thermionic Energy Conversion
4. Los Alamos made material for plutonium-powered pumper
5. Exponential Decay and Half-Life