MichaelJP wrote:
This is one of the two big
technical advantages of air launch -- starting from even 30,000ft means
you're dealing with considerably thinner air, reducing the price tag
noticeably. (The other is also related to thinner air: rocket engines
are more efficient with less back pressure. The forward speed of the
aircraft is a relatively minor gain by comparison, unless it's a pretty
unusual aircraft.)

Thanks for the extra detail!



There's another advantage if you're using cryogenic propellants. The
propellants can be kept in insulated tankage within the carrier until
altitude is reached and the transferred into the LV. Since the
temperature is well subzero at altitude, there isn't water vapor around
to form ice on the tankage, so the weight and complexity of insulation
can be done away with.
Assuming you are using a Shuttle-style jettisonable ET, that a built-in
performance boost, as well as a cost savings on the ETs themselves.
Although a completely rreusable LV will have a TPS to take reentry
heating, and therefore will already have exterior insulation, the drop
tank solution makes for far easier design as far as vehicle weight goes.

Pat