On Apr 10, 8:29*am, Robert Clark wrote:
Note also that use of the rocket thrust from the X-33 would also allow
you to reach higher speeds say Mach 3+ before release. This would
allow greater payload, since less delta-V would need to be supplied by
the X-33 after release. The extra rocket propellant for the X-33
required for firing during the linked portion of the trip would be
carried in the carrier craft fuselage.

* Bob Clark

Since you need to reach Mach 25+ to orbit this saves you only 12% of
the speed requirement while imposing a requirement for an airframe
that can at least sustain that ground velocity while climbing.

A much better result can be had from a balloon-assisted launch system
where the objective is to start the rocket with as much altitude as
possible rather than speed.

http://academy.grc.nasa.gov/y2008/gr...-launch-system

-HJC