I’ve searched and searched, and it’s probably out there but I can’t find the answer to this question.
The shuttle, and all other spacecraft today launch vertically. Looking at concepts of 2:nd generation shuttles they all launch vertically.
I figure there must be som major reason for doing this. Lauching horizontally must be much more efficent. You wouldn’t need nearly as powerful engines, and as much fuel. This reduces weight and cost. Also it would be more practical to use an ordinary airstrip than a launch tower, and you wouldn’t need crawler transporters (Shuttle), or trains (Russian way) to transport the spacecrafts/rockets.
How much more efficient would horizontal launch be?
To escape the earth’s gravity you need to do work on a body. I’m sure that the two powered methods do enjoy different efficiencies, but I wonder if they are vastly different?
Rockets have to get free of Earth’s gravity, fast, before they fall down again (rockets not being equipped with wings and engines like a jet), and the quickest way to do that, the shortest distance between two points being a straight line, is to go straight up.
Launching horizontally, so they’d cruise along like a jet plane, would require wings, and a different kind of engine in order to keep them “up”.
So, you’re basically asking, "Why don’t we just equip a Boeing 747 to cruise along, going gradually higher and higher until it’s in Earth orbit?’’ And I dunno the answer to that, but I bet the Rocket & Ballistics Geeks do, and they’ll be along any minute now, as soon as they’ve stocked up on Cheetos and rinsed out the pocket protectors…
Since they need to go up, it would be a waste of fuel to make the rocket go horizontally, then have to turn and go up. The least use of fuel is to accelerate as quickly as possible, within the constraints of what the rocket and its passengers can withstand.
Some launch vehicles do launch horizontally. The Pegasus rocket launches from the underside of a B-52 after being carried to altitude, so the B-52 can be considered a horizontal first stage for the Pegasus launch system. While this is only useful for fairly small payloads, the arrangement should answer your question: a vehicle designed for horizontal flight is completely different than a vehicle designed to enter orbit. You need lifting surfaces if you’re going to turn horizontal thrust into altitude. You need a lot of hardware that isn’t isn’t related directly to getting into orbit. If you want to lift the kinds of payloads that larger launch vehicles carry, your aircraft stage would have to be massive, with an enormous wingspan to support the weight. With large payloads, it’s easier to lift with thrust than lift with wings.
20 seconds into flight the Space Shuttle rolls and begins the non-vertical flight phase at about 78 degrees. At 60 miles altitude it is begins flattening out and then 20 minutes into flight it is basically flying in an orbital pattern (not increasing altitude). Friction is your enemy. Time means more fuel. Going vertical rather than horizontal give minimum time in the atmosphere, and a shorter burn time for on-board fuel needs.
To orbit, the Shuttle goes 17,500 mph around the earth, at an altitude of 135 to 600 miles.
It’s also a matter of structural efficiency. A vertical launching rocket’s structure only needs to support its weight in one direction - along the direction of thrust. A rocket designed to launch horizontally would need to support its weight in two directions - both along the axis of thrust, and perpendicular to that to keep from collapsing when it’s taxiing along the ground. That means more weight eaten up by structure, and less payload. When you consider that 80% or more of the weight of a launch vehicle is fuel, keeping the weight of the structure down is very important if you want to actually have a useful payload capacity.
The shuttle does have to worry about forces in two directions, but not to the degree a horizontally launched rocket will, since the shuttle only has to support its weight perpendicular to the thrust axis when it’s landing, when most of the weight of fuel is gone. And it is still a lot less efficient in terms of payload size to cost and total weight than a conventional rocket.
I think the simple answer is this–a horizontal launch is the shortest distance between two points. The two points are the distance between the center of the earth (or the center of gravity) and the point when the ship acheives escape velocity. Launching horizontally would mean the ship would have to travel a longer distance, and would require more fuel.
I’ll assume that’s a typo and you really mean that vertical launch is the shortest distance.
However, it’s not necessarily a shortest-distance argument. If you want to drive your car up Pike’s Peak, do you drive straight up the steepest slope or use the road that has switchbacks? It depends on how your vehicle is designed. Aircraft with wings are very efficient at turning thrust into altitude using wings as lifting surfaces. It takes a lot more fuel to push a vehicle straight up than it does to push it sideways fast enough for the wings to provide lift. All things considered, it is quite efficient to launch vertically and use thrust for lift if you’re headed for orbit, but there’s a lot more to consider than just shortest distance travelled.
Also, the shuttle does not ever reach escape velocity, just orbital velocity which is somewhat lower. Escape velocity is the velocity at which you will escape the gravity well with no additional thrust (except to offset drag, if you’re in the atmosphere). It is perfectly possible to crawl into orbit with continual thrust rather than using one big push to reach escape velocity, and that’s what all launch vehicles do.
The reason may be nothing more than a bureaucratic decision. As Micco mentions the Pegasus launches as an aircraft but the X-15 program was working toward that in the sixties. It was never meant achieve orbital flight but flew high enough that some of the air force pilots recieved astronaut wings.
Among the reasons you can’t do it with a 747 is of course that jet engines don’t work in space. I’m sure you knew that but attaching rocket engines wouldn’t be a solution either as the airframe wouldn’t handle the thrust needed to achieve ballistic flight out of the atmosphere. This type of flight requires a very high thrust to weight ratio though not as high as pure vertical launch and very high airspeeds while still in the atmosphere, both of which put extreme stress on the airframe. The X-15 achieved speeds of mach 6.7 so the airframe was of course strong and pointy. Very, very pointy.
Backing up AndrewL and UncleBill, this site mentions that the shuttle’s rollover is executed for a number of reasons, but one of the important ones is to reduce increasing stress on the Orbiter’s wings and tail as its velocity increases through the lower atmosphere. The shuttle reaches its maximum dynamic pressure, intended to be kept below about 580 pounds per square foot, about one minute into the launch. During that period the main engines actually throttle back slightly until it passes through the denser part of the atmosphere.
I think that means the Orbiter would have to be designed quite differently (read: stronger and therefore heavier) in order for it to survive the stresses of a level takeoff. The atmospheric stresses on a launch are simply too great to be used profitably, so it’s best to just plow through the lower atmosphere as directly as possible.
Getting into orbit is not simply a matter of getting up high. A substantial horizontal velocity is required to get into a decent orbit other wise, no matter how far up you go, you will simply fall back to earth.
I once asked this very question of a real life rocket scientist. He explained that the main reason for a vertical lauch was to punch through the thick atmosphere as quickly and by the shortest route possible. A significant proportion of a rockets fuel is spent overcomming atmospheric drag - I guess it must be even worse for the shuttle that for a conventional rocket.
A horizontal component is important. It is for this reason that most rocket launches take place as near to the equator as possible - to get the benifit of the Earth’s spin.
Handy, horizontal launch doesn’t mean absolutely horizontal, just more horizontal then vertical. If I have a powerful enough cannon I can shoot a cannonball into orbit. Obviously if I aim it parallel to the surface of the earth the ball will hit the ground at some distance. If I shoot it straight up it will come straight back down on me. Aim it at the correct angle and with enough power it will achieve orbit.
That’s a non-workable solution so instead we build a rocket plane and drop it off a B-52 at altitude. Rocket ace lights the engines and pulls back on the stick, putting the plane into a into a steep climb. At the beginning of the flight it’s flying like any normal airplane with wings providing lift. Eventually there is not enough atmosphere and it’s flying like any other launch vehicle making a transition to orbit.
I imagine part of the answer is political. Airplanes need to get clearance over foreign countries… up to a certain height. By going straight up rather than across, you effectively eliminate the air rights issue. (Though I recall that in the Heinlein story, The Man Who Sold The Moon, he secures the rights from all the countries he will be travelling over.) This works for jettisoning spent booster rockets too. Much better to do that a few miles downrange than way out at sea where the Russians might get to them first…
