Question: When designing planes and different components - excluding safety - what is the most important factor which you consider?

Michael Carley answered on 16 Jun 2016:

One way or another, cost is usually the most important thing, after safety, but cost is a complicated issue.

For example, is it better to make something which is cheap but wears out quickly and has to be replaced often, or something more expensive which doesn’t wear out so fast? The first way makes the product cheaper for a customer to buy but makes it more expensive to use over its lifetime; the second is the other way round. On top of that, you have to include the cost of paying someone to change the component.

Airlines, for example, use a measure of cost based on flying one passenger a distance of one kilometre. That cost is made up of paying off the loan on the aircraft, fuel, maintenance, and paying the pilots, cabin crew and ground crew. The designer of the aircraft has to keep that total cost down as much as possible.

In aerospace, usually the most important way to keep cost down is to reduce weight because this saves on fuel to move the aircraft, but also frees up some space which the airline can use for passengers or cargo.

In space applications, weight is so important that you will pay almost any cost to reduce it. This is because it costs so much to get things into orbit (from $10,000 to $100,000 per kilogram).