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

Chapter 3.0 Vehicle Design
3.4 Reciprocating Chemical Muscle
As shown in Figure 3-139, the third
generation RCM used external
mechanical spool valve actuation and
could be throttled by pressure regulation
(either through fuel decomposition
rate at the input, or by modulating back
pressure on the exhaust port). The single
piston design of the third generation
device also produced a longitudinal
vibration (in the direction of the piston
motion) at the fundamental reciprocation
rate. Although not appreciable in
the 2.5X version, this vibration would
become significant as the RCM cylinder
housing was sculpted to remove
mass as downsizing continued in latter
generations.
Figure 3-139: Third-generation Reciprocating
Chemical Muscle
The vibration of the RCM along its longitudinal axis might not have been a significant problem
in the Entomopter while in flight, (especially at a 30 Hz reciprocation rate), but it was realized
that there are many other applications for the RCM for which vibration might not be acceptable,
especially were a number of RCM devices used simultaneously in the same platform. Noncoherent
actuation of numerous RCM actuators would result in random canceling vibration, but
were free running units to drift in phase, coherent operation might momentarily result in the
vibrational forces adding constructively. The solution was to make changes in the fourth generation
RCM to eliminate vibration altogether while making other performance improvements.
As it turns out, this is also an important consideration for the Mars Entomopter too. The size of
the RCM for the Mars Entomopter will be scaled up from the first generation unit in order to
provide enough power to flap the longer wing (approximately 1m) that will be used for flight in
the lower Mars atmosphere. The larger the RCM, the greater the inertias developed internally. It
is therefore even more advantageous to design the RCM to be reactionless, that is, to have canceling
inertias and no vibration.
3.4.1 Fourth-generation Design Goals
The fourth-generation RCM development began with an analysis of methods by which the
mechanism could be simplified as it is reduced in size. Of principal concern was the vibration
inherent in the third generation design. It was felt that the features of the RCM contributing to
vibration must be eliminated prior to further size reduction. A new concept using pneumatic
shuttle valve actuation was considered because this would eliminate all external components
such as the strike plates shown in the third generation RCM depicted in Figure 3-139. Also, this
new concept offered the possibility of eliminating any mechanical spool valve actuation internally
(thereby reducing parts count and complexity. The new concept employed gas pressure
increase during static end-of-stroke conditions to actuate a reversing spool valve.
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