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Old November 14 2012, 11:43 PM   #327
gturner
Admiral
 
Location: Kentucky
Re: Envisioning the world of 2100

I'll add that 9 out of 10 aerospace or control engineers would agree that the horizontal orientation is easier to control than a vertical rocket steerred from the back, because it would be like hovering a Harrier instead of balancing a broomhandle. There's no question that horizontal touch-down is much easier, not even requiring a digital flight computer to control orientation, because you've got full pitch, roll, and yaw authority, no big instability due to gravity wanting to tip you over, and you've got a big fat landing gear spread to set down on.

The problem with rear steer is that if your nose tips left, you drift left, and have to shift the base left to stay balanced, and further left to kill your drift and return to your original position, and then you have to shift to the right to kill the horizontal motion. The control problem is so difficult that NASA didn't even attempt rear steering with the lunar module, instead keeping the engine pointed through the center of mass, using side-mounted thrusters to keep the vehicle's orientation locked to a vertical gyro, also to move it around in X and Y. By comparison, horizontal take-off is a no-brainer. Once you're up, there are no precision requirements for the pitch maneuver because there's nothing you can crash into, much like the way a submarine-launched ICBM can dance around all over the place right after it clears the water. Compared to clearing the tower with a rear-steering tail-sitter (and there are very strict limits on allowable launch-area winds because of that), it's like doing a bellyflop compared to a ballet.

The advantage of the tail-sitter is of course simplicity, at least at first blush, because you just light the motor and only have two axes to contol (ignoring roll). It's based on a missile, not an easily controllable flying machine with oddles of thrust. We do it the way we do because spaceflight developed from missile programs instead of aviation programs. Or as Admiral Gehman, who lead the Columbia investigation, summed up the fatal, underlying flaw in NASA culture, “We do it this way, because this is the way we’ve always done it.” As rockets became bigger and more unwieldy, we just threw money at ground support instead of suspecting that there might be a simpler option. The large tail-sitting rocket is simpler, but the large tail-sitting rocket system is not.
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