Why can’t we fly a plane in to space, what stops it from just flying higher and higher until we are in space?A plane flies because as it is propelled forward, the wings, which are shaped to make the air flow faster over the top of them than the bottom, generate lift.
image/text credit: Curious Droid
Just as we need air to breath, so the engines need oxygen to burn the fuel to create thrust to propel the plane forward. Jet Engine however can work at higher altitudes than people.
At 12,000 meters or around 40,000 feet, which is the upper limit of most modern airliners, the air density is about 18% of that at sea level.
Normal jet engines like those in the SR-71, have a maximum air speed limit of about Mach 3.5 or 2685mph. Beyond that the air pressure and temperature becomes too high for the compressors in the engine to work effectively.