Why rockets cannot be launched from fast planes
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The fastest airplane ever is the SR-71 blackbird.
Its maximum speed was 3529 km/ hr (980 m/sec). It 
could reach an altitude of 26 000 meter.
Its take off weight was 78 000 kg (empty 36 000 kg)

The biggest airplane ever is the Antonov-225 Mriya.
It has a maximum take off weight of 640 000 kg. 
(empty weight 285 000 kg.) 

Using a numerical simulation, taking into account earth
rotation, Saturn5 reaches 980 + 462 = 1442 m/sec at an altitude
of 32 000 meter, and with a remaining mass of 1 300 000 kg.

We take this as starting point for an in-flight launch
of a heavy rocket (that will eventually bring 140 000 kg in
orbit). 

We see that we saved 55% of fuel. 
In order to obtain this for a rocket with size of Saturn5, 
one needs a plane that can lift 1 300 000 kg 
(3.6 times load of Antonov 225, 30 times load of SR-71)  
to an altitude of 32 km, reaching the speed of the SR-71 at this 
altitude.

Note that my calculations are very generous. I didn't consider direction,
and I overestimated useful load of the planes involved.
The real numbers are probably much worse. It is clear that 
launching from planes is (1) not worth the effort, and (2) probably 
impossible. 

This doesn't exclude the possibility that small rockets can be launched
into orbit from planes, but not for the reason of saving fuel.
It may be an advantage to be flexible in launching site and time,
and not to ask permission to build a launching site.