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I searched on the subject, thinking it could probably be cheaper to fly the spacecraft aboard a plane and then launch it from altitude. The only thing I found was this Wikipedia page, but it only enumerates advantages over ground launch without saying why it's not the traditional way of doing things. Knowing the X-15 was tested in the 50s, if it was the better option, it would be known since that time.

I think one the reason may be limitations on the payload for the plane to takeoff and fly to a sufficient altitude, but it shouldn't be a problem when sending astronauts on the ISS, for example. Any idea on this?

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marked as duplicate by Michael Kjörling, briligg, Vedant Chandra, TildalWave Jun 4 at 20:12

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Look up the Pegasus. Basically, air-launched ICBMs are the maximum possible payload (albeit prohibited by treaties). The Burlak project by the Russians was a bit larger but unsuccessful. –  Deer Hunter May 31 '14 at 5:39
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Because there is no plane that could do that. And even if there was one, you gain almost nothing from doing so.

The Saturn V rocket weighed almost 3 thousand tons before launch. The Space Shuttle weighed almost 2 thousand tons.

A Boeing 747 can "only" carry about 120 tons. That's something like 5% the weight of the space vehicles mentioned above. You'd need 20 Boeing planes to carry one fully fueled space vehicle.


But how about the altitude gain? The 747 could carry a payload up to 14 km altitude, or so.

By comparison, the orbit of the International Space Station is at 420 km. The Hubble Space Telescope is at 560 km altitude. There are no orbits below 160 km (because they would decay rapidly due to drag), and most are above 200 km. Geosynchronous satellites are at 40 thousand km above Earth.

The plane would only cover about 2...3% the orbital altitude, or even much less.

As you can see, the numbers just don't add up.


EDIT: The wiki page you mention refers to special designs, where the whole vehicle is made for takeoff as an airplane or balloon; the airplane is basically the first stage. But this is not a mainstream solution currently, for reasons indicated above. It may work for very small vehicles.

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Doesn't deal with drag in the lower atmosphere. –  Deer Hunter Jun 1 '14 at 3:25

SpaceShipOne

The picture above shows a picture of Space Ship One. on the jet powered 'carrier aircraft'/'mother ship' White Knight - that probably counts as an Aircraft.

However, I think these designs are only designed to reach low-earth orbit, not up high enough t carry a satellite, carry cargo to the ISS, etc

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Actually these design are (for passengers) only designed for sub-orbital trips into space, meaning they arc up into space, but don't make it into orbit. –  Dan 41 mins ago

Lots of good stuff on this topic in Wikipedia.: Air Launch to Orbit

Air Launch

A typical rocket spends the first few seconds going straight up (almost) to get out of the atmosphere. After that it spends almost all of its time accelerating to orbital velocity. Thus getting out of the atmosphere, while hard (rocket is heaviest at this point) is really only a very small portion of the process.

The mass you would need to carry to a higher altitude is so large as to exceed the capacity of the largest aircraft ever built, let alone a balloon.

Consider the case of StratoLaunch. They intend to build a carrier aircraft composed of parts from 2 Boeing 747 (in the top 3 of the biggest aircraft commercially available. A380, B747, and C5 Galaxy are probably the biggest). It will be the largest airplane (in mass, wing length, etc) flying if it succeeds.

Even then, it can only carry a scaled down version of a Falcon (The original payload was to be a SpaceX Falcon 9 but with 5 engines instead of 9 and a concomitant reduction fuel/oxidizer load and thus mass). So the biggest aircraft yet to be built could only carry a smaller version of a medium size booster.

The primary benefit of air launch is not extra mass, but rather launch constraints. If you launch from a fixed site, you have limited launch options to differing orbits, and inclinations. An aircraft can in theory fly to wherever is convenient to hit the right orbital parameters. (Assuming there is a big enough runway for something the size of Stratolaunch vehicle within flight range fully loaded).

Currently, there is the example of Virgin Galactic's SpaceShip Two that uses the White Knight carrier vehicle, whose payload to actual orbit (LauncherOne if they actually build it) would be in the 100 kilogram range.

Pegasus, launched beneath a Lockheed L-1011 (3 engine airplane), maxs out around 500-1000 kilos to orbit.

There was a European proposal to launch an orbital payload (in the 200Kilo range) from the top of an Airbus A300.

The scaling up, that would be required just does not work.

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Why hasn't any answer so far mentioned the Pegasus launcher? –  Vedant Chandra Jun 4 at 19:24
    
@VedantChandra Well, it was mentioned in a comment. Notice that this question is a year old. –  Michael Kjörling Jun 4 at 19:31