All IST ground based launch systems are designed on the basis that there will be plentiful supplies of cheap rocket fuel available to them in LEO. For a vehicle which returns from orbit to the ground, this fuel can be used by its own engines or via an attachable deceleration module to reduce its orbital velocity to close to zero, prior to reentering the Earth's atmosphere. This avoids the need for heat resistant materials on the external surfaces of the craft and for stronger internal structural components to cope with stresses and strains of sustained aerobraking.
A craft which aerobrakes from full orbital velocity down to a low mach number velocity in the troposphere, undergoes severe heat and turbulent air flow stress at the most intense phase of such a manoeuvre. Reusable craft which are to undergo this procedure on every flight, have to be strong enough not just to survive the manoeuvre, but to do so repeatedly without any significant weakening of their structures. The larger a craft is the more significant the stresses become, as the surface area of the craft is smaller in proportion to its weight. IST craft are not currently designed to lift more than 15 tons to LEO. To achieve this the orbital space plane component of the system has an empty weight of about 10 tons, which is quite light for such a vehicle. It is still the case, that such craft can operate much more efficiently in the long run by not using aerobrake methods for reentry.
IST currently has a number of designs of ground launch systems which use the same general formula. A phase 1 lifter vehicle takes off from the ground like an airliner under turbojet power, carrying the phase 2 spaceplane on its back. Once it has achieved a height of about 40,000 ft on the correct orbit alignment, it accelerates to 2 km/s orbital velocity using rocket engines. At apogee, it releases the spaceplane which accelerates to about 7 km/s orbital velocity. At apogee, the phase 2 vehicle achieves a rendezvous with the phase 3 vehicles which aerobrake from orbit. The phase 3 vehicles collect the cargo from the phase 3 spaceplane and transfer enough fuel to it to allow it reduce its speed to a low orbital velocity prior to reentry. The phase 3 vehicles return to orbit with the payload cargo. The principal of this formula is to eliminate the more sensitive lower orbital velocity rendezvous used in the pipeline launch system, whilst ensuring that no vehicle experiences aerobrake stresses of even 1/16th that of full orbital velocity aerobaking.
The formula is scalable with current focus on systems that lift 5, 10 and 15 tons respectively to LEO.