So either fire a rocket in space to circularize the orbit or reach more than Earth’s escape velocity 25,020 mph (11.186 km/s, 40,270 km/h) ~ Mach 32.6, due to some drag in air to thin for any kind of air breathing engine to work.
X-30 was aiming far lower ~Mach 20. Nuclear could make it more realistic than any form of chemical combustion. It might be physically possible using Hydrogen but you’re talking generating extreme thrust at vastly more extreme conditions than the space shuttle’s retry.
The highest jet record is 37km in MiG-25. The scramjet record is 33km. I found source that says the limit is 40km at Mach 15.
The NASA X-43A hit Mach 9.6 as an air breathing engine (test used a rocket at lower speeds for cost reasons) which in theory should be capable of ~65 km assuming it could survive a similar maneuver. Actual limits are heavily influenced by how much thrust you can generate while slowing down etc not just max velocity.
So yea no actually built air breathing aircraft can hit space, but it’s within the realm of possibility.
