Why can’t we fly a plane into 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 create more lift and when the lift is greater than the weight of the plane, it will climb up into 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 into 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 into 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 lift. 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.
Lack of oxygen
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 Engines, 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 into 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 kp/h 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. The X-43 is currently the fastest free-flying air-breathing aircraft in the world having flown at Mach 9.6 or 7310 mph in November of 2004.
SCRAMJET’s do away with the turbine compressors of a jet engine so they have no moving parts, instead they use shockwaves in the engine to compress and raise the temperature of the air in the engine to burn the fuel and create thrust and in theory they could fly at up to Mach 20.
The problem is that they wont work at speeds of less than around Mach 5, so they have to be brought up to speed by a rocket engine booster before they can operate, which is how the NASA X-43 worked. They also don’t work in space because there is no air with oxygen in it to combust the fuel.
So, This is why space vehicles are launched by rockets. Rockets can have much more power, and can operate from a speed of zero on the Launchpad to Mach 33 and beyond which is the escape velocity of earth.
One of the earliest experimental space planes was the North American X-15, which reached a height of 353,000 feet or 107,000 meters in 1963 and was powered by a liquid rocket engine… but had to carried upto 45,000 feet attached to the underside of a B52 bomber before being released.
Then of course we have had the Space Shuttle, the Soviet version of the space shuttle the “Buran”, SpaceShipOne and the Boeing X-37, all of which were examples of spaceplanes but are really just rocket powered gliders.
Rockets differ from jet engines because they bring along their own oxygen to burn the fuel and don’t rely on atmospheric oxygen. This means that they work in space just as well as the in the atmosphere. The problem with rockets is that because the need to bring the oxidizer with them it makes them very heavy.
Look at the space shuttle, the external fuel and the tank to hold it, along with the two solid rocket boosters weighted 1,940 metric tonnes at lift off and that’s without the space shuttle, all of which has to be carried along with the shuttle to the edge of space where they are then jettisoned. The maximum payload the shuttle could deliver in to low earth orbit was 27.5 metric tonnes, which as a payload fraction, is just 1.3% of the total take-off weight.
Rockets, however, can create a huge amount of power, so they can achieve the speed that is needed to escape the pull of earths gravity and go in to orbit or beyond.
But what of the future, will we ever get planes that can take-off from an aircraft runway and fly in to space and then return back to a runway. There are still considerable technical issues to overcome but one design which looks promising is The Skylon.
This is an SSTO or Single Stage To Orbit design, meaning that unlike a rocket, it stays in one piece, rather than having a separate main booster stage which detaches and returns to earth and smaller second stage which goes on in to orbit. The key to the Skylon working, are the SABRE or Synergetic Air Breathing Rocket Engines.
These are a kind of hybrid jet-rocket engine, which can take off like a normal jet engine and breathes air up to 93,000 feet at a speed of up to Mach 5.4, when it then switches to rocket mode to fly in to space at up to 800km or 500 miles.
It would then re-enter the earth’s atmosphere and return and land as an air-breathing plane to be checked and refuelled and ready for another launch.
Because it uses more efficient engines and the lift of the wings, it would use only 20% of the fuel compared to a conventional rocket.
It still need to bring its oxidiser for the rocket portion of the journey but that a lot less than would be required for a normal rocket. This allows a larger payload when compared to the total weight of around 5.5% to the shuttles 1.3%.
Unmanned test flights of the Skylon could happening by 2025 if all goes well…. but potentially large fly in the ointment is the recent advances in reusable rockets like the SpaceX Falcon f9r and the Blue Origin new Shepard, these could make the development costs of the skylon look expensive for satellite deployment and supplying of the International Space Station.
One thing that could come out of it though, is a rocket less version of the SABRE engine which could make hypersonic air travel a more viable option than a SCRAMJET engine.
Only Time will time but this is an exciting time for both the future of air and space travel, so we may yet see a plane that can fly into space. So as always Thanks for watching and please subscribe, rate and share.
Thanks for the informed, well-balanced article. “Winging into space” using hypersonic air-breathing vehicles is an evocative, romantic concept. It has long been a dream. Unfortunately real-world studies and developmental projects have shown it’s vastly more difficult than first envisioned. It is getting to orbit “the hard way”. The history of these efforts is described in the free on-line NASA book “Facing the Heat Barrier” by T.A. Heppenheimer:
Part 1: history.nasa.gov/sp4232-part1.pdf
Part 2: history.nasa.gov/sp4232-part2.pdf
Part 3: history.nasa.gov/sp4232-part3.pdf