In article ,
Pat Flannery wrote:
This is one of the two big technical advantages of air launch...

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.

You don't really need insulation against ice anyway, unless you've been
stupid enough to put something fragile downstream of the tank surfaces.
Just let it fall off after engine ignition, as the Saturn V did.

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.

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...
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