Why can’t we fly a plane in to space, what stops it from just flying higher and higher until we are in space?
Well, there a several issues but assuming we are in something like a normal jet airliner, then one of the main problems is the air, or lack of it as we get closer to 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. As the plane goes faster the wings creates more lift and when the lift is greater than the weight of the plane, it will climb up in to the air.
For our plane to continue to climb it needs more speed to increase the lift. If you throttle back on the speed a bit, the plane will settle in to level flight and if you decrease the speed the plane will start to fall as lift from the wings is not enough to overcome the weight of the plane.
As our plane climbs higher and higher in to the atmosphere, the air becomes less and less dense, so the plane has to fly faster to create more lift until eventually it reaches an altitude where the engines cease to function correctly because of the lack of oxygen or the air is too thin to create enough lift.
Now, This is a greatly simplified way of looking at this because as you approach the speed of sound or Mach 1, which also changes with altitude and if your plane has quite straight wings, the airflow over the wing can become unstable and it loses li ft. This unstable airflow can also shake the control surfaces, that’s the flaps on the wings that go up and down, so violently it could break them and you then lose control of the plane. That’s why supersonic or hypersonic planes have highly swept back and often delta shaped wings like Concorde and the space shuttle.
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. We humans have a limit of about 8000 meters or around 26,000 feet, above this is what climbers call the “death zone” where there is not oxygen for humans to survive for sustained periods.
The summit of Mount Everest is 29,000 feet high and the air density there is about 33% of that at sea level. This means that with each breath you take, you are getting only 33% of the oxygen. If you were to stay at this altitude without additional oxygen you would suffer a condition called “Hypoxia” where due to the lack of oxygen, the body to slowly shuts down and dies and is the cause of most the 200+ deaths that have occurred on Mount Everest.
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. If you were in a plane that had a rapid decompression at 40,000 feet, you would have about 5-10 seconds to get your emergency oxygen mask on before you became unconscious.
Concorde flew at 60,000 feet or 18,300 meters and where the air density is just 7% of that at sea level. To achieve this height, it had to travel at Mach 2, twice the speed of sound or 1350 mph.
The highest-flying jet plane in level flight was the Lockheed SR-71 Blackbird with a height of 85,069 or 25,929 meters and where the air density is just 2% of that at sea level. At that height, it’s travelling at around Mach 3.2, or 2190 mph.
The SR-71 pilots had to wear a full pressure suit with its own oxygen supply in case of a cockpit depressurization or emergency ejection. This put to the test, when in 1966 an SR-71 piloted by Bill Weaver disintegrated at Mach 3.1 at an altitude of 78,000 feet, as it was performing a test flight to optimise it’s performance.
At that altitude, your blood will boil in a similar way to when you open a bottle of fizzy drink as the nitrogen in your blood turns to gas in the low-pressure atmosphere. The pressure suit worked and Weaver survived the decent from 78,000 feet but tragically the navigator, Jim Zwayer, died of a broken neck as a result the breakup of the plane.
Now while you would think that the SR-71 is fast, to get in to space you need to reach what is known as “escape velocity”. This is where you are travelling faster than gravity is pulling you back to earth and that speed is 25,020 mph or 40,270 kmph and If that wasn’t a problem then there is also the recognised altitude of where space starts which is 328,000 feet or 100,000 meters, well over 3 times the highest flight of the SR-71.
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.
For hypersonic speeds, experimental unmanned aircraft like the NASA X-43 use a SCRAMJET engine.
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