a. The objective of the energy-balance method of global response
evaluation is to determine whether the target structure can absorb the
energy that is imparted to it without deforming excessively. This
method uses the principles of conservation of energy and
conservation of momentum, and requires that the energy absorption
capability (SE) of the target be greater than the kinetic energy
imparted to it (Ea). It recognizes that, since a significant portion of the
impact energy is dissipated in deforming the aircraft body, the
effective missile mass is less than the total mass of the aircraft but
more than the mass of the rigid components, such as the engines.
The effective missile mass for calculating the total
kinetic energy of impact (Ei) will depend on the mass of the engines
and the relative rigidity of the aircraft body.
b. To calculate Ei, the effective missile mass (m) can be conservatively
estimated based on Chelapati (Reference 10):
1. For small aircraft with airframes that are flexible relative to the
target structure, m=2 times the combined mass of the engines.
2. For large aircraft, m=8 times the combined mass of the engines.
However, m should not be less than 30 percent, nor more than
the total mass of the aircraft.
c. To calculate Ea, the effective target mass (Me) may conservatively be
taken as the mass of the target structure that is included within d/2 of
the periphery of the impact interface, where d is the thickness of the
target in the direction of missile travel. On the basis of the predicted
deformed shape of the target, less conservative but practical values
may be used if justified. However, Me need not be less than one-
tenth of the total mass of the target structure panel (Reference 10).