wrote:

That is to say, the first stage "reusable" space plane costs $126
million. We amortize it over 250 flights, assuming it is good for that
many flights. Dividing $126 million by 250, we find the cost of the
first stage is only about $500,000 per flight. Compare that to the $25
million cost of the disposable first stage!

What's the minimum time to perform those 250 missions using a single
reusable launcher? You cannot just divide the initial capital cost by
the number of flights to get the cost per flight, because you're
incurring an opportunity cost on the capital tied up in the launcher.

The cost per flight also has to include a component that reflects the
risk that the launcher will be lost before completing its 250 missions.
This might be covered by insurance, but either way it's a cost that has
to be included.

Sylvia.