Having been a structures designer and having asked management "do
you
want it cheap or light" I have found out many things about costs. An
expensive
machined part will often be cheaper than a built up sheet metal part
when
you take into account the assembly costs of the sheet metal parts. The
flight structures are way cheaper than the systems (guidance, engines,
pressurization). Absolute Minimun Costs carries at least a 100% weight
increase. For a tank, you want to have minimal machining, which means
no
ribs and frames, which means fully monocoque structure and heavy
structure.
To further cut down on costs, no testing, which means a F.S. of 2 for
limit
vs a tested F.S. of 1.1 on limit. So as you well know, you have one
curve
for material cost (heavier vehicle means more material and with a fixed
material cost per pound, any pound of weight as cost) including bigger
engines, and another curve for machining and assembly and test costs vs
weight and where they cross, you have your cheapest vehicle.
- David Pearce
- Lockheed Martin Astronautics
David Pearce: This is in reply to your comment........ I appreciate
learning what you think of Minimum Cost Design even though it is in
disagreement......
I believe that if you were to check with experienced designers they
will
tell you that in most instances machined parts cost much more than ones
that can be fabricated from sheet metal. However, this is an
inapplicable
argument since the tradeoffs analyses between weight, cost and
reliability
should tell you whether a sheet metal or a machined part is
cost-optimum
for a particular application.......The MCD criteria applies to all
subsystems
including those you mentioned. I tried to explain this in the “Design
Examples”
which follow........I don’t know what work you have done to support
your
contention that a minimum cost vehicle will weigh twice as much. In my
experiences, I found the weight to be less than 25% more; please see
the
May 29th Column.........I believe you are subscribing to the contention
that the cost of hardware is a constant (as one might conclude from the
CER relationships commonly used) and that increasing hardware weight
increases
cost. Of course the MCD criteria obviates the use of CER relationships.
- Arthur Schnitt
- Consultant