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Vertical takeoff requires disposable rockets (would it a satellite rocket), which is a money loss, and also a lot of fuel, because initial velocity is zero. Also vertical takeoff seems risky, involves huge pieces of equipments, launch pads, to diminish risk.

Horizontal takeoff are done with a reusable aircraft, like a modified 747. Initial velocity not being zero, there are much less risk, and the fuel spent in a 747 is much less expensive than a disposable rocket.

So, Why are there more vertical takeoff than horizontal for spacecrafts?

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semi-on topic: en.wikipedia.org/wiki/Space_elevator –  Andrew Apr 30 '12 at 1:32
    
I'd like to point out that, if you consider the entire launch procedure in both cases, they both start with zero initial velocity; and that one difference between a vertical and a horizontal takeoff is that the vertical one is pointed toward the destination (i.e. space) while the horizontal one takes a roundabout route, flying sideways first instead of up right away. –  Asher Feb 5 at 15:24
    
True, the amount of energy spent might be higher with the horizontal takeoff. But a horizontal takeoff seems like a "sprint", it requires burning a lot of energy in a short time, requiring very specific engines, meaning more complexity. Getting in space was to develop the techs, but at some point, if you want to make it more mainstream and economically possible, you have to make compromises. –  jokoon Feb 5 at 19:33

5 Answers 5

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A 747 - can get you to around 35,000 feet. Still very much within the atmosphere.

So what do you do then? Launching a rocket from that point still requires an awful lot of kit, so while you have reduced your propellant requirements a little, the 747 still has to carry a launch platform, so you're not really getting anything out of this.

New technologies, such as that used by Virgin Galactic is managing to make this work, hopefully, with a hybrid model that does fly up to around 50,000 feet before launching the spacecraft section, but this is very new.

enter image description here

So the simple answer is - it used to require vertical rocket launches, and all the associated paraphernalia, but modern technology is moving towards fully reusable methods such as this.

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I'm sure it's not that much new... there was other kind of projects like this... –  jokoon Apr 28 '12 at 19:42
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Yes, but they couldn't do it successfully, which is why it is only recently beginning to work. –  Rory Alsop Apr 28 '12 at 20:00
    
It's worth pointing out that the craft you've pictured (SpaceShipTwo) is a sub-orbital one. It's much harder to launch orbital craft that way, although Virgin Galactic say they're working on it. I suspect it will only be for small payloads, at least with this generation of technology. –  Nathaniel Dec 20 '12 at 11:52
    
True - it's all about progress, and this is a step. –  Rory Alsop Dec 20 '12 at 12:21
    
it crashed recently. sad because this solution seemed much more elegant than a vertical launch. –  jokoon Feb 5 at 19:25

Short answer: payload!

To launch a reasonable payload you ill need a colossal airplane (and maintenace, runway, operation, etc)

There are the pegasus system using it and you can see he got a very limited payload

http://en.wikipedia.org/wiki/Pegasus_%28rocket%29

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Any system that has enough ISP to get itself out of the atmosphere has enough ISP to not need wings for lift. Thus, vertical takeoff. See this related question for a discussion of why the ISP of jet engines is so much lower than for rocket engines.

Note also that just a few weeks ago NusSTAR was launched from a Pegasus rocket on a modified 747. This is a relatively lightweight cargo and thus was fit for what the 747 could carry.

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Sabre engine-powered skylon aircrafts are the only horizontal takeoff spacecraft in the horizon that will be able to go into LEO and back in a single stage.

I expect that we will see a prototype of this aircraft doing its first test flights in the next 10 years, but there are many financial and political roadblocks that need to be surmounted first

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one more corporate video: youtube.com/watch?v=0UL4rgq6glE –  lurscher Dec 20 '12 at 16:32

A 747 moves at approximately 1,000 km/h, a satellite in orbit travels at 28,000 km/h. So, after your rocket is released from the 747 it still needs enough fuel to accelerate a further 27,000km/h. That requires a lot more fuel than the 747 is capable of carrying. Remember that the shuttle lift-off weight is about 2,000 tons - far more than the 747 can carry.

In addition, you need to get above the atmosphere. Going 28,000 km/h at 10,000 m will burn up any spacecraft, so you need to get up very high before you start going at extreme speeds.

It is possible to reach orbit after horizontal take-off, but we're not there yet

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So the fuel saving is not worth a horizontal takeoff, if I understand well. I thought escaping earth's gravity required a lot of energy, and that's why I'm thinking about planes... –  jokoon Apr 30 '12 at 17:26
    
Apart from the fact that you can't escape earth's gravity, yes it takes lots of energy to get into orbit. Horizontal take-off would imply using jet engines, hence using oxygen from the air; that will save quite a bit in take-off weight. Nevertheless, it's still not really feasible. Skylon claim to be ready to unveil a space plane, but I'll need some convincing. –  hdhondt May 1 '12 at 0:14
    
What I was thinking was like any vehicle, the energy required to make a vehicle move from no speed at all to small speed was the greatest energy taker. –  jokoon May 1 '12 at 10:37
    
It's the change-of-speed (delta-V) that determines the energy required. To change from 0 to 100 km/h takes the same amount of energy as the change from 10,000 to 10,100. The current speed of the vehicle has nothing to do with the energy required - except for the fact that a rocket (or jet) engine has highest efficiency when it is moving at the same speed as the exhaust gases, so the exhaust becomes stationary when exiting the engine. –  hdhondt May 3 '12 at 23:28

protected by Qmechanic Feb 5 at 13:33

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