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7.8: Evaluate Trade-offs

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    142404

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    7.8 Step 5 — Evaluate Trade-offs

    Trade-offs are the core of design. You rarely improve everything simultaneously: thicker bracket → more strength but more mass; lower resistance → more current but more power; shallower cable angle → lower structure but higher tension.

    A decision matrix is the standard engineering tool for comparing alternatives against multiple criteria. It makes evaluation explicit, transparent, and reproducible — rather than based on gut feeling.

    Building a Decision Matrix

    Steps: (1) List the criteria that matter. (2) Assign weights that sum to 100%. (3) Score each alternative on each criterion (1 = worst, 5 = best). (4) Compute weighted total = Σ(weight × score). (5) The highest weighted total wins — subject to re-examining the weights.

    For the bracket:

    Weighted decision matrix for bracket design alternatives.
    Criterion Weight Alt A Alt B Alt C
    Low mass 40% 2 → 0.80 4 → 1.60 5 → 2.00
    Manufacturing simplicity 30% 5 → 1.50 3 → 0.90 2 → 0.60
    Safety margin 30% 5 → 1.50 4 → 1.20 3 → 0.90
    Weighted Total 100% 3.80 3.70 3.50

    With these weights, Alternative A wins — but notice: if mass reduction were weighted 60% instead of 40%, Alternative B or C would win. The decision matrix makes this sensitivity visible. That is why the weights must be set deliberately, not arbitrarily.

    ⚠ Watch Out — "The safe choice" is not automatically the right choice

    A common student instinct is to pick the design with the highest safety factor, reasoning that "more safe is always better." But over-engineering has real costs: mass, material, cost, size. In aerospace, unnecessary mass is itself a safety problem — it reduces fuel margin and payload capacity. The design goal is to meet the safety requirement with an appropriate margin, not to maximize it.


    This page titled 7.8: Evaluate Trade-offs was last modified on Thu, 24 Sep 2026 17:35:44 GMT and is shared under a CC BY-NC license and was authored, remixed, and/or curated by .

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