This preview shows page 1. Sign up to view the full content.
Unformatted text preview: role is to propose reasonable planned
profiles or margin requirements for approval by the
cognizant manager. The value of either of these
methods is that they allow management by exception -that is, only deviations from planned profiles or margins
below requirements signal potential future problems
requiring replanning. If this occurs, then new cost,
schedule, and/or technical changes should be proposed.
Technical changes may imply some new planned
profiles. This is illustrated for a hypothetical TPM in Figure 22(a). In this example, a significant dem- NASA Systems Engineering Handbook
Management Issues in Systems Engineering
onstrated variance (i.e., unanticipated growth) in the
TPM during design and development of the system
resulted in replanning at time t. The replanning took the
form of an increase in the allowed final value of the
TPM (the "alloAn Example of the Risk Management Method
for Tracking Spacecraft Mass
During Phases C and D, a spacecraft's injected
mass can be considered an uncertain quantity.
Estimates of each subsystem's and each
instrument's mass fare' however, made periodically
by the design engineers. These estimates change
and become more accurate as actual parts and
components are built and integrated into
subsystems and instruments. Injected mass can
also change during Phases C and D as the quantity
of propellant is fine-tuned to meet the mission
design requirements. Thus at each point during
development, the spacecraft's injected mass is
better represented as a probability distribution rather
than as a single point.
The mechanics of obtaining a probability
distribution for injected mass typically involve
making estimates of three points -- the lower and
upper bounds and the most likely injected mass
value. These three values can be combined into
parameters that completely define a probability
distribution like the one shown in the figure below The launch vehicle's "guaranteed" payload
capability, designated the "LV Specification,&q...
View Full Document