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

Planetary Exploration Using Biomimetics
An Entomopter for Flight on Mars
which is determined and controlled by a fuel metering valve, is directed to the RCM actuator.
The surface area of the actuator piston and its range and speed of throw, coupled with the pressure
of the gas generated in the reaction chamber, ultimately determine the horsepower or wattage
of the RCM system.
The design point offered in Table 3-2 represents a realistic operating configuration for Entomopter
on Mars. Here, a flapping frequency of 6 Hz over 75° will evolve 883W of power to
nominally allow 14m/s flight in the lower atmosphere when using 1.2 m wings. In addition, this
affords a useful lifting capacity of 1.5kg.
Using the 883W power output as a goal (about 1 HP), an RCM can be designed with the following
physical parameters:
• 5.08 cm (2 in) Piston Diameter
• 5.08 cm (2 in) Piston Throw
• 3.5 kg/cm2 (50 psi) working pressure (maintained constant by the fuel metering
through the reaction chamber)
Working in units of pounds and inches, this results in a piston force of 157 lbs/sec at 6 Hz, or
0.29 HP. Since the fourth generation RCM actuator uses a split piston that is double-acting, the
surface area is actually twice that of the single 2 inch face. This results in a power output of
852W. Variations due to friction losses and loading are easily accommodated by adjusting the
working pressure. 50 psi is a low value relative to the thousands of pounds of pressure that can
be evolved in a correctly designed RCM system. In fact, for the power output by the RCM actuator,
its weight is quite modest.
The RCM reaction chamber and actuator are the components that must withstand temperature
and pressure extremes. These extremes will be dictated by the fuels used and the design pressure
necessary to produce sufficient power for flight. Materials such as columbium coated titanium or
Inconel steel are suitable candidates for the design of these components. Titanium alloys such as
6AL-V-2Sn exhibit 150,000 psi yield strength while Inconel-718X yields at 100,000 psi. The
density of these materials is 0.164 lbs/in3 and 0.296 lbs/in3 respectively. Since RCM operation
as defined above requires only 50 psi, the strength of a cylinder made from materials such as
these is immense even with very thin walls. The addition of radial heat rejection fins adds significantly
to the strength of the pressure vessels while not contributing greatly to the weight. In the
final analysis, the thickness of the material will be dictated more by the desire for dimensional
stability than pressure containment.
For a split piston RCM actuator with a 2 inch diameter piston and 2 inch throw (per piston), the
overall actuator size would be approximately 4 inches (10.16 cm) in length (excluding actuation
shafts), and 2 inches in diameter plus the cylinder wall thickness and cooling fin diameter.
Because the Mars atmosphere lack density, larger cooling fins will be desirable, perhaps on the
order of 2 inches (5.08 cm) in radial length. To the 4 inch length of the actuator, one must add
the length of the plena at each end to accommodate ejectors as shown in Figure 2-1, and the gas
bearings with cam-follower wing cuffs to define the total length of the fuselage.
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Phase II Final Report