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I guess it's not a very educated question, but I never quite understood why spacecrafts have to shoot up and can't just reach space by simply continuing an upwards ascent like an airplane.

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Because that stops working once you run out of atmosphere. –  Olin Lathrop Nov 14 '13 at 19:48

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rockets use newtons third law to take off this law is that for every action the is and equal and opposite reaction. if the rocket takes off like a plane the thrust will be used up too fast in addition when the fuel runs out the oxygen tanks fall of. so that is why rockets don't take off like planes

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Taking off like a plane would not use fuel too fast. There are other issues but this isn't one of them. –  Brandon Enright Nov 14 '13 at 20:06

The problem has to do with the presence of air. Planes need the air for 2 reasons: oxidizer for the fuel they carry to generate thrust and drag on the wings to produce lift. No air, no thrust, no lift, no flying. So obviously, you can only fly so high until you start running out of air. Rockets need to go higher than this and so they are built to do without air. They carry their own oxidizer+fuel combination (in whichever form, solid, liquid) and actually these propellants take up a lot of volume (see the huge tanks on the Space Shuttle). Which is a problem because the rocket has to go through the thick air and there, it generates a lot of drag and rockets don't need drag to fly ! So the rocket wants to go through the thick atmosphere as quickly as possible which means shooting up.

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The highest altitude recorded for a jet airplane is just over 100,000 ft in round numbers. To attain low earth orbit you would need to reach about 100 miles in altitude or about 520,000 ft. As you can see a jet would only take you about 1/5th of the way up and you would need to switch to a rocket propulsion due to the lack of oxygen. My guess is that switching from a regular airplane/jet to rocket propulsion is an additional complication which might make it more expensive then just using rocket propulsion.

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For what it's worth, even though a rocket starts its flight going straight up, once it has traveled through most of the atmosphere it soon starts to change its direction so that it spends most of its flight accelerating in the "around the earth" direction (i.e. basically horizontal).

Also, to reach orbit a vehicle either has to reach a high enough speed, or a high enough altitude, or a high enough combination of speed plus altitude. A gun firing a bullet horizontally at 20,000mph would do the trick. Or, dropping a ball from 20,000 miles altitude would also work. Neither of these are practical altitude/velocity combinations.

On to your question. There isn't any fundamental physical reason why an "airplane" can't fly into space. A relevant definition here of "airplane" would be a vehicle that:

  1. carries its fuel
  2. picks up air from the atmosphere as it flies, and
  3. has wings that can generate lift

Note that both an airplane and a rocket generate their thrust from exactly the same physical principle - by accelerating and ejecting some mass.

The reasons that you don't see an aircraft-like vehicle accelerate and climb its way into space are practical engineering reasons, not fundamental physical limitations. Here is a hypothetical "airplane to orbit" concept that the laws of physics would allow:

  • Take off from a runway, accelerate and climb to 500mph at 40,000 feet (so far, just like an ordinary transport aircraft)
  • holding 40,000 feet altitude, accelerate from 500mph to 20,000mph

Now here are some of the practical impediments to this actually being a buildable vehicle:

  • staying in the atmosphere like this, the faster the vehicle goes the more drag it will experience. The engine has to generate enough thrust to overcome the drag, plus a bit extra to accelerate the vehicle. Ordinary jet engines are powerful enough to fly an ordinary jet airplane at 500mph at 40,000 feet. To go 20,000mph is 40 times as fast, which will require 40*40 times as much thrust, which requires an engine that is 40*40*40 times as powerful. A particularly unkind law of physics says that the drag increases as the square of the velocity (actually this is a simplification that ignores shock waves, the reality is even more unkind!) and the power required increases as the cube of the velocity.
  • building a conventional airbreathing engine that operates from 500mph to 2,000mph is a major engineering feat (the SR-71 Blackbird can do this). Getting an airbreathing engine to operate at 4,000mph or 5000mph has not yet been achieved practically (look into current research on "scramjets"). Getting to 20,000mph is a long, long way further away than any of this.
  • Even if you could accelerate like this, you would face an insurmountable aerodynamic heating problem. Just like a spacecraft or meteor entering the atmosphere produces white-hot heat, your accelerating aircraft would encounter the same effect.
  • Chemical fuels (hydrocarbons, hydrogen, etc) have a certain amount of energy per unit mass, and engines convert some fraction of this energy into thrust (the amount depends various specifics but one thing you can count on for sure is that 1 Joule of fuel energy will always be converted into less than 1 Joule of propulsive energy (thrust times speed)). You quickly get silly results from your calculations - what if you need to burn a million pounds of fuel in order to put one pound of payload into orbit?

EDIT to add: more advanced discussions can be found at the "Selenian Boondocks" blog; start with the post "Part I : Air Launched SSTO" in the Orbital Access Methodologies section.

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About your second to last point: Wouldn't the ordinary rockets face the same problem of heating? Why don't they look burning red when they are going up? –  timur Feb 27 '11 at 17:23
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@timur: They do have a heating problem, but by going up instead of across they don't stay in the atmosphere for long. The switch to across happens gradually, and mostly further up than that. –  dmckee Feb 27 '11 at 17:59
    
@dmckee: Thanks a lot for your reply. Maybe then it would help if spaceplanes go straight up from the point they start the second, more powerful engine. Or would it contradict the whole point of using air as much as possible during their ascent? –  timur Feb 27 '11 at 18:08
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@timur: It is not a stupid idea. Every time someone writes a report on inexpensive access to orbit there is a chapter on hybrid launch systems. But so far, the engineers keep coming back to rocket from the ground up. Looking forward, I think one of the private firms is really looking at a airplane for the first stage. –  dmckee Feb 27 '11 at 18:18
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@timur for a rocket the atmosphere is nothing but an obstacle so trajectories are chosen to get the rocket out of the atmosphere ASAP (i.e. go straight up til you clear most of it, then tip over). For an airbreathing craft it is a tradeoff, staying low in the atmosphere is good for your engine (more oxidizer, so more power can be produced) but bad for your airframe (more air means more drag). There's an even bigger problem though - the thrust-to-weight ratio of airbreathing engines turns out to usually be a bigger penalty than their improved fuel consumption –  Daniel Chisholm Feb 28 '11 at 15:35

I looked up this somewhat cheezy Disney clip from the 1950s. It features a spaceplane idea, but alas the plane takes off like a rocket.

http://www.youtube.com/watch?v=rBgkrhnThek

There are some reasons why you generally want rocket flight. In a vertical ascent you quickly clear the atmosphere. Within 90 seconds into rocket flight you are above 90% of the air. Air has friction and is generally not that much help in lifting you far. Aircraft can fly at a maximum of about 30,000m, and for orbital flight you have to go much further than that.

It is possible that ramjets or scramjets could be placed on a rocket. Once it reaches supersonic these could kick in and provide thrust without the need to carry an oxidant until you reach about 50.000m. The flight envelopes of modern combat aircraft reflect the safety limit. If you fire up the aferburners and go near vertical you will reach the “edge of space.” Of course the engines flame out and your descent will be violent and deadly.

The business of spaceplanes is largely a load of BS. These ideas keep surfacing now and then, and fortunately we have not yet spent billions on such a worthless program.

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that looks more like the space shuttle –  Emilio Pisanty May 27 '12 at 12:41

http://en.wikipedia.org/wiki/Pegasus_(rocket)
The Pegasus is a hybrid airplane-rocket system in actual current use.

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In short - A rocket works by exchanging momentum, AND at present airplanes simply do not have the ability to exchange enough momentum. In a rocket - Both the mass of the propellant as well as the high velocity of its exit from the engine system gives the rocket its momentum to propel itself into space. The propellant (fuel) attains its velocity by burning with an oxidizer in a high-pressure chamber creates a high energy exhaust which then funneled through a nozzle. This velocity, coupled with the right mass properties of the propellant, provides the power, or energy, required to get the vehicle into space. An airplane on the other hand requires air drag and lift to propel itself off of the ground... and when the atmosphere is to thin ... it cannot exchange momentum.

The US Airforce has been working with space planes - however due to the design they still need a ROCKET to lift them up into space. Pretty cool stuff - if you want some extra reading...

http://www.spaceflightnow.com/atlas/av026/status.html

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It's because spacecraft are not generally designed to immediately start returning to the Earth once its fuel has run out, but to have reached the escape velocity of 11.2 kilometers per second at this point. It's then able to orbit the Earth continuously without having to use further fuel in the ideal case, as an example.

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Aircraft rely on lift generated by interacting with the atmosphere and on using atmospheric oxygen to burn with fuel they carry.

Orbits aren't stable until you are high enough that there isn't enough atmosphere to interact with, and long before that the oxygen content drops too low to be useful.

So, to get to a stable orbit, you will need rockets eventually. Areospace types keep looking at hybrids (air breathing, lift generating while down low then switch to rockets as you get really high) because it does seem like a good idea, but so far the added complexity hasn't paid. Either the switching gear weighs too much, or the extra equipment adds too much risk of failure.

Maybe in the future.

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Still, spacecrafts could be launched from a plane where there's still oxygen, which could burn less rocket fuel. It would cost less and be less risky since there is no vertical launch pad, and the plane used would be reusable, which is not the case with the shuttle, where even rockets are disposed each time (I think) –  jokoon Apr 29 '12 at 10:46
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@jokoon: Spacecraft are launched from planes...and the Pegasus system happens to be the most expensive way to deliver payload to orbit in existence, on a cost per kg basis. Vertical launch pads aren't "risky", and you don't save anything by putting the pad support equipment on a giant aircraft that has to fly around before launch...and dump the rocket and payload somewhere if there's a technical issue preventing launch. Reuse of the plane means nothing...it's just an airborne launch pad (the altitude only matters for tiny rockets), and the vertical launch pads are reused as well. –  Christopher James Huff Dec 30 '12 at 2:31

The Wikipedia page on Spaceplanes should answer all the questions you've asked.

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