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Lastly the system engineer should select those tpms

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Unformatted text preview: s that determine it, as top-level TPMs. At lower levels of the PBS, TPMs worth tracking can be identified through the functional and performance requirements levied on each individual system, segment, etc. (See sidebar on high-level TPMs.) In selecting TPMs, the system engineer should focus on those that can be objectively measured during the project life cycle. This measurement can be done directly by testing, or indirectly by a combination of testing and analysis. Analyses are often the only means available to determine some high-level TPMs such as system reliability, but the data used in such analyses should be based on demonstrated values to the maximum practical extent. These analyses can be performed using the same measurement methods or models used during trade studies. In TPM tracking, however, instead of using estimated (or desired) performance or technical attributes, the models are Examples of High -Level TPMs for Planetary Spacecraft and Launch Vehicles High-level technical performance measures ( TPMs) for planetary spacecraft include: • • • • • • • End-of-mission (EOM:) dry mass Injected mass (includes EOM dry mass, baseline mission plus reserve propellant, other consumables and upper stage adaptor mass) Consumables at EOM Power demand (relative to supply) Onboard data processing memory demand Onboard data processing throughput time Onboard data bus capacity Total pointing error. Mass and power demands by spacecraft subsystems and science instruments may be tracked separately as well. For launch vehicles, high -level TPMs include: • • • • • • • Total vehicle mass at launch Payload mass (at nominal altitude or orbit) Payload volume Injection accuracy Launch reliability In-flight reliability For reusable vehicles, percent of value recov ered For expendable vehicles, unit production cost at the nth unit. (See sidebar on Learning Curve Theory.) NASA Systems Engineering Handbook Management Issues in Systems Engineering exercised using demonstrated values. As the project life cycle proceeds through Phases C and D, the mea...
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