Phase II Final Report - NASA's Institute for Advanced Concepts

Planetary Exploration Using Biomimetics
An Entomopter for Flight on Mars
From the estimates of the thin film thermoelectric, sufficient power for the Entomopter's systems
should be available whenever the engine is operating. However, the engine will not be
operating continuously during the mission. In fact, operation time can be a small fraction of the
complete mission time. Therefore an auxiliary source of power would be needed. The best
choice for this power source is a rechargeable lithium battery similar to that used with the PV
system. The operational time of the thermoelectric is limited to 10 minutes over the 1 hour mission.
This would provide a total of 3.33 W-h of energy. If 4 W need to be available to the vehicle
while the thermoelectric is running, that leaves 16 W or 2.66 W-h available for storage. Therefore,
a battery would need to be used with a storage capacity of 2.43 W-h (3.1 W-h for the total
mission - 0.66 W-h provided by the thermoelectric and used during flight). Based on the lithium
battery specifications given in Table 3-25, the battery mass would be 0.018 kg, and the thermoelectric
conversion unit would be 0.02 kg. Also, a cooling system will be needed to keep the
back side of the thermoelectric cool in order to maintain the required temperature difference
across it. (This temperature difference is usually on the order of 200°C.) However, within the
cool atmosphere of Mars, the cooling system may be nothing more than some convective fins.
The mass breakdown for the thermoelectric system is given in Table 3-27. Based on these estimates,
the thermoelectric system would be about twice as heavy as the battery system. However,
if the mission profile is changed, this type of system may look more attractive and should continue
to be considered as a viable alternative to the PV system.
Table 3-27: Thermoelectric System Mass Estimate
System Component
Mass (kg)
Thermoelectric Unit 0.020
Battery (based on 100% capacity) 0.023
Cooling Fins 0.010
Contingency (50% for wiring, electronics, etc.) 0.043
Total System Mass 0.096
Another approach to using a thermoelectric is to use a radioisotope heat source instead of the
combustion-exhaust gasses. This would eliminate the need for a supplemental battery to provide
power when the engine is not running. A standard radioisotope heater unit (RHU) can be used as
a baseline for the heat source. The specifications of the RHU are given in Table 3-28. [9]
Table 3-28: Specifications for Radioisotope Heater Unit
System Component
Value
Isotope Material
Mass (Fuel Source)
Operating Temperature
Watts (thermal)
PU-238
3.02 gm
310 o K
1 Wth
222
Phase II Final Report