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Factors of safety are used to account for unknowns in the loadings to which the vehicle will be subjected, for unknowns in the environment in which the vehicle will operate, and for variations in strength or performance of the vehicle and its components. The magnitude of factors of safety effect reliability which in turn can be related to cost and loss of human life. Analytical procedures for deriving factors of safety have been proposed, but it is believed that the practice of using traditional factors will continue as long as there are advanced missions and innovations in technology. Whenever feasible, derived relationships between factors of safety and cost, and factors of safety and reliability would represent powerful design tools.
Aircraft are designed essentially to the minimum weight criteria. Many design iterations have shown that its use results in a minimum cost vehicle. The criteria, however, may sometimes be constrained by such factors as sales price and modified by operational considerations such as ease of refueling. In certain designs, minimum weight, like reliability, is a convenient substitute criteria for cost. Very often it pays to spend large sums to advance the state-of-the-art in minimizing weight. Of course the least costly way of manufacturing minimum cost hardware is generally sought and employed. On rare occasions during detail design, weight is traded for cost, using an arbitrary or "calculated" value of a pound of weight. (Example: An unnecessary pound of weight can be removed from a fuselage bulkhead by more intricate machining at a cost of $300 per airplane. If this cost is less than the value of a pound of weight, the more intricate machining is approved.)
Designing aircraft to a substitute criteria and subjugating costs has been quite convenient; however, it has had negative consequences. Designers have become better at estimating weight than cost. Although there has been a history of weight growth, cost growth has often been much larger. There are other hardware systems for which reliability or life is the substitute design criteria, and neither weight or cost is justifiably given much consideration.
There is another design criteria concept worth noting that may be useful in the design of small components of an aerospace system. It is impractical to proof or static test complete civil engineering structures such as buildings and bridges. (Some suspension bridges are now model tested in a wind tunnel as a consequence of the failure of the Tacoma bridge due to flutter.) Instead, large factors of safety are used to insure structural integrity.
At the time these observations were made (1960-62) there were no published analyses that proved that aircraft, ballistic missiles, space launch vehicles or space payloads should be designed to the minimum weight criteria. (No literature search on this design aspect has been conducted since then; however, it appears that only small deviations from the minimum weight design criteria have been proposed.) The apparent reasons for the universal adoption of the criteria have been somewhat different for each vehicle type, although the designers felt assured that they were designing minimum cost vehicles.
In the design of most aircraft the desire to attain maximum performance and payload for available propulsive power led to the minimization of hardware weight. In addition, since typical aircraft growth factors range from 10 to 20, each pound of weight saved represented a saving of 10 to 20 pounds of which a large fraction was hardware weight. Commercial transports had the added impetus to reduce fuel costs for the expected life of the airplane. Under the usual assumption that hardware costs in $/lb were essentially fixed, smaller aircraft meant lower fabrication costs.
In designing our first intercontinental ballistic missiles (about 1955 to 1960) more emphasis was placed on minimizing weight in order to maximize payload weight and range. In one instance, there was the added constraint of vehicle size to facilitate transportability. However, there is no evidence that the designers had the slightest suspicion that the minimum weight criteria may not apply for this vehicle type.
The early space launch vehicles were adaptations of ballistic missile stages. It did not take long before payload requirements became too ambitious; this resulted in extremes in payload weight minimization and sophistication of structure and components, and low reliability. Meaningful cost constraints were lacking.
During 1960, '61 and '62, there were six publications known to me that discussed deviations from the minimum weight design criteria. None of the papers disputed the minimum cost design criteria, but they did employ limited cost-weight tradeoffs. They discussed the following:
There are several perceptions upon which the MCD criteria is based:
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you have any experience trading Minimum Weight vs. Minimum Cost Design?
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