The National Aerospaceplane and its intended tech demonstrator the X-30 are well known in aerospace history circles because NASP was the American effort at developing a spaceplane capable of either hypersonic passenger travel or reaching orbit under its own power. However, little known are efforts by the Soviet Union, and later Russia, in the late 1980s and early 1990s to create their rival to the NASP. Tupolev and Mikoyan-Gurevich responded with the Tu-2000 and MiG-2000 SSTO spaceplane designs. The Tu-2000 would have been bigger than the NASP, while the Tu-2000 tech demonstrator would have similarly bigger than the X-30 but smaller than the SSTO Tu-2000. The MiG-2000, on the other hand, was similarly bigger than the NASP but was smaller than either the Tu-2000 tech demonstrator or SSTO Tu-2000.The Russian attempts at an equivalent to the X-30/NASP were all for naught, however. Even though Tupolev manufactured many components to be used in the Tu-2000 tech demonstrator, including cryogenic fuel tanks and nickel alloys, the Tu-2000 was axed in 1993 due to lack of funds.Info about the Tu-2000 and MiG-2000 can be found at:http://astronautix.com/t/tu-2000.htmlhttps://www.testpilot.ru/russia/mikoyan/mig/2000/mig2000_e.htmhttps://www.testpilot.ru/russia/tupolev/2000/2000.htm
"Slush hydrogen" at just above the melting point of hydrogen which is 14.01K or −259.14°C while providing 16-20% increased density over liquid hydrogen, must have had some unique challenges to its use.