Pioneer Rocketplane is developing the two stage (TSTO) partially reusable Pathfinder rocketplane to serve the small LEO and satellite constellation O&M (Operations & Maintenance) markets.   The Pathfinder uses a novel aerial refueling concept that significantly reduces the takeoff weight and thus the size and mass of the vehicle's wing.  The Pathfinder mission begins with the vehicle taking off and flying to 20,000 feet under its own power using two turbo-fan engines.  The vehicle then flies up-range to rendezvous with a 747 aerial tanker carrying the Pathfinder's  oxidizer.  The Pathfinder and tanker perform a mid-air refueling, similar to what is routinely done by  US military aircraft, to transfer 130,000 pounds of liquid oxygen to the Pathfinder.  Once the operation is complete, the Pathfinder will de-mate, fly clear of the 747, and ignite its single RD-120 rocket engine to climb to 70 miles altitude and Mach 15.  At this point, the Pathfinder is out of the Earth atmosphere, allowing it to open its payload bay doors and release the payload with a small upper stage capable of placing the satellite in the desired orbit.  The Pathfinder vehicle re-enters, ignites its two turbo-fan engines, and flies back to its operating base.