EVEN IN AIRCRAFT, USING LIQUID H2 AT JUST 20° OVER ABSOLUTE ZERO, IT HAS THE DENSITY OF A POOL NOODLE AND THUS ITS TANKS ARE MASSIVE AND DRAGGY. INSULATION CRACKS CRYOPUMP IN AIR AND DRIP IT OFF AS A HIGHLY FLAMMABLE LIQUID. ENTRAINED AIR BECOMES A HIGH-EXPLOSIVE SLUSH. IT'S JUST AWFUL AS A FUEL.