No idea if this will really work, but it is a neat idea: Create a hybrid engine/motor that uses the earth’s air with hydrogen when in the atmophere, the switch over to liquid oxygen (and hydrogen) when leaving the atmosphere. Essentially switching from a jet to a rocket.
The benefits? A reusable single-stage jet/rocket for getting into space, only requiring 48 hours between flights. The spaceshuttle, for instance, needed new tanks and solid booster motors for each flight. Not sure, but I think the turn-around time was at least months.