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There's a reason why designs for reusable space launchers use a vertical launch configuration even when they show a horizontal landing. The take-off weight is much, much greater than the landing weight. For example, the Space Shuttle lifts off at around 6 million pounds, and lands at about 1/4 million pounds.
So designing "take-off gear" and a runway to support a horizontal take-off is a considerably greater challenge than designing for vertical take-off and horizontal landing. Consider the landing gear required for a 747-400, which has a maximum takeoff gross weight of 875,000 pounds. (The image shown here is actually the VC-25A presidential carrier aircraft, also known as Air Force One. It is based on the 747-100 airframe. See the Boeing web details on the 747-400.) The aircraft requires a huge array of tires to spread its weight, to avoid damaging the concrete surface of airport runways.
That landing gear and the mechanisms to operate it add a tremendous amount of weight to the aircraft. We can tolerate the weight in a 747-400 because of the high efficiency inherent in jet transports, but when we are pay our dues to the rocket equation in the design of spacecraft, inert weight is our solemn enemy.
For another comparison, take a look at the Shuttle landing gear. This
is a smaller image of the STS-67 landing. The
204k, 1280x1020 version is available from the NASA web.
The image of the STS-49 landing is just as large, but the file is only 125K.
Obviously, the gear required for a quarter-million-pound Shuttle is trivial compared to a million-pound 747. Extrapolate that to a vehicle that weighs 10 times as much as a 747, and you can see that we'll need to build specialized runways and carry an enormous gear weight if we take off horizontally with the full fuel load. Back in the early Seventies, in Boeing's exploratory design group, we got into some really exotic combinations of dual runways and gazillions of landing gear trucks. That was for the Arctic Resources Aircraft, a conceptual design for a 3-million-pound oil tanker. Engineering analysis showed it was possible to get that behemoth off the ground, but we weren't talking about accelerating all that hardware to orbital velocity.
There are alternatives to the full-weight horizontal takeoff besides a straight vertical take-off. Mitchell Burnside Clapp suggests the stage-and-half concept, where you fuel the spacecraft after launch. See Daniel Risacher's Black Horse page for more on that one.
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