Soviet rocket designers had also long been fascinated by the concepts of Eugen Sanger and sketched out many variations on the space-plane theme. In 1962 the Mikoyan Design Bureau formally presented it's "50-50" concept. The name came from the 2 components of the system: a hypersonic mothership aircraft and its single-seat orbital space-plane cargo.

It was envisioned that the mothership would takeoff and when reaching 5.5 mach would release the spaceplane on a piggyback booster to attain orbit. While the mothership was never built, the Mig 105 Spiral (or "Lapot", meaning flat shoe) was constructed. It flew on at least 3 occassions after being airdropped from a Tupolev Tu-95 bomber.
Today the MiG 105 prototype sits in a muddy field at the Monino Soviet Air Museum outside Moscow. While 50-50 was never a good bet, the 105 design lived on in the BOR experimental shuttle and in the shape of NASA's HL-20 re-entry vehicle.