Recovering stages from far down range is likely to be on the agenda for several companies in the next decade or so. RTLS for the boosters or a first stage is a different proposition than collecting something from Mach several and a couple of thousand miles down range or so. The more performance that can be obtained from a stage that is economically recovered and reused, the less performance required from the upper stage. The less upper stage performance required, the less it should cost if expendable, or the more margin is available for recovery if reusable. A dense fuel upper stage released at Mach 17 should require a mass ratio of about 2 to reach LEO.
I vaguely remember discussion of an air captured stage from years ago and have sketched it out as I remember it.
On the left is the stage vertically ascending with some small fins to help stabilize it along with normal thrust vectoring. An aerospike is a prerequisite for this technique. If not for the other use in recovery, the fins would be hard to justify economically. On the right is the stage reentering with the fins operating as wings just large enough to enable a high speed glide. The tail surfaces fold out for the reentry. The fins/wings have a loading of 500 pounds per square foot to enable a glide at about 400 knots.
If this stage had a million pounds of sea level thrust and a mass of a million pounds, it should have an empty weight of about 50,000 pounds. The wing area would be 100 square feet with tail surfaces about a quarter of that. Fin/wing span would be about 16 feet plus the diameter of the stage. Mass of aero surfaces and controls estimated at 2,000 pounds.
This middle stage or parallel stage would reenter with the aerospike in the nose position absorbing the most of the heat and supplying the mass in the nose for stability. The blunt back end which was the stage front end before the upper stages will be high drag and a further stabilizing force.
A high speed tow aircraft hooks up to the gliding stage at fairly low altitude and tows it back to the launch site. Since the fin/wings on the LV are far too small to land, and there is no landing gear either, the means of safely collecting the LV from this point I don’t recall except that a lot of the ideas were way out there. It may have been a usenet discussion from the late 90s.
Does anyone remember the discussion? Has anyone worked out details that you know of?