Henry Spencer wrote:

The big reason why you might need tank insulation is if the tank holds
LH2, in which case you need to insulate to prevent liquid air from
condensing... and that'll happen even at subzero temperatures, so you
can't get away with leaving it off.


In the case of a carrier aircraft, the airstream should carry away any
liquid air on the tank.


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.


The gain is actually rather questionable, after you consider reentry --
the drop tank leaves behind a heavy, dense vehicle that makes a severe
reentry. At reentry time, it's *good* if lots of the volume inside the
TPS is empty tanks. The drop tank does make for far easier design if you
can "throw the TPS problem over the fence" to the materials team...

I'm really surprised that the small air-launched orbiter with giant drop
tank concept didn't get anywhere- both we and the Russians thought the
idea had enough merit to do designs of the concept:
http://www.buran.ru/htm/busfact.htm#maks-op
http://www.abo.fi/~mlindroo/SpaceLVs/Slides/sld053.htm

Pat