Nine Reasons for Mass Change
- Better definition of the design. As the design progresses beyond the proposal stage, the design criteria and requirements become better defined, and optimistic assumptions made during the proposal cannot be justified. These changes are generally early in the program but prior to drawing release.
- Out-of-scope changes. Mass can be affected when the customer adds or changes the requirements for the contracted vehicle beyond the scope of the original proposal.
- Redesign. The original component or subsystem design criteria may need to be changed because of repackaging, failure of a component during testing, effects of other subsystem changes, etc.
- Maturing component design. Mass estimates can be affected by improvements in mass analysis resulting from updates in drawings after original release.
- Error in previous estimate. The reason for change is an error in the mass calculations for an original or previous estimate, or for components added that were erroneously overlooked.
- Uncontrolled vendor changes. These involve mass changes reported by the vendor or supplier of hardware in excess of contractual requirements.
- Mass reduction activity. Mass can change as a result of official mass-reduction efforts.
- Measured versus calculated. Mass estimates can reflect the differences between the actual measured mass of components and the latest calculated value.
- Cost reduction, added mass. Sometimes, mass increases are incurred to save money, e.g., by swapping out expensive materials, or by eliminating elaborate machined parts and cutouts.
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