In the 1990s Mikoyan-Gurevich proposed a two-stage-to-orbit launch system (similar in concept to the Sanger II project designed in West Germant), the AKS (
aviatsionno-kosmicheskaya sistema=aerospace system), which comprised a hypersonic carrier aircraft powered by a combined turbojet/ramjet powerplant that would carry a small aerospaceplane piggyback. The carrier aircraft's turbojet would use T-6 grade kerosene at speeds up to Mach 3.5, before operating on liquid hydrogen between Mach 3.5 and Mach 6, and after achieving a parallax of 2,485 miles (4,000 km), the aerospaceplane would be released from the carrier aircraft into equatorial orbit. However, the AKS project did not progress beyond the design phase.
Link:
https://www.secretprojects.co.uk/threads/mikoyan-gurevitch-mig-aks.222/