If injected mass is a tpm being tracked using the

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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...
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