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It seems a large amount of rocket fuel during launches is spent to get the mass moving; indeed according to QuickLaunch, Inc. it takes 40% of the rocket fuel to get to Mach 1.3. It seems as though the engines are firing quite a while before liftoff, and considering that the full launch weight of the space shuttle is 4.4 million pounds (~2 million kg).

Would it be feasible to add a counterweight system to help it get started? It seems as though one could be built with a structure a few hundred meters above the launch vehicle, which could significantly reduce the launch weight and get the upward motion started sooner.

What I'd imagine is four cables attached to the launch vehicle, running up to the structure and each having ~400,000kg weights attached. This would make the rockets only need to lift ~400,000kg for the first, say, 200m, which would lead to much greater acceleration for this span and it'd be to Mach 1.3 much sooner.

Is this too hard to make? Would it have any noticeable effect on the fuel requirements, or would it be negligible? Is the acceleration already near the limits of the astronauts' bodies? Or would it just be one other thing that could fail? The reason I ask it just because of the seemingly ludicrous amount of weight that needs to be launched.

Are there any other methods in the works to assist the launch besides rockets for manned vehicles? It doesn't seem space guns or sky ramps are ever planning on having humans in the launch vehicles.

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I'm aware the total energy would be the same, but lifting the counterweights could be powered by renewable resources like wind and solar, reduce the amount of fuel carried to start acceleration and thus lowering the weight and thus further reducing fuel (the Tsiolkovsky rocket equation), and could perhaps be economical across multiple launches. –  Ehryk Jun 25 '13 at 21:24
    
Further, I do refer to wikipedia. Since you seem to be of the opinion that this has been answered therein, could you provide a wikipedia link that properly addresses this question? –  Ehryk Jun 25 '13 at 21:26
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2 Answers

I think the main reason why this is not done is that the first stage in most rockets burns for several minutes. The acceleration from a counterweight cannot be higher than $g$ and for a falling distance of $200$m the total speed of $g\cdot t \approx 63$m/s is not significant enough to warrant such a huge engineering challenge.

On the other hand this looks very different if a higher acceleration is used, e.g. in a railgun or mass driver. Here the acceleration and terminal velocity is much higher but this is still only done on a research level.

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In addition liquid rocket engines run for a few seconds to get pumps upto speed and temperature. The engine gimbals are also used to balance the rocket as the support clamps are released –  Martin Beckett Sep 12 '12 at 15:14
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Using a pulley system or something you could achieve an acceleration greater than $g$ using a counterweight. (Probably not practical for a rocket, though) –  David Z Sep 12 '12 at 18:27
    
@DavidZaslavsky: Yes, I thought of that after I wrote the answer but a pulley for a rope that holds 400 tons is beyond my imagination and it does not really help as the kinetic energy is the same for the same counterweight. –  Alexander Sep 12 '12 at 20:08
    
How about four steel cables, each holding 100 tons? Some bridge reinforcement cables are rated for more than this. –  Ehryk Jun 25 '13 at 21:29
    
I should add that I wasn't counting on the acceleration from the counterweights to be of much importance in and of themselves; the 63 m/s velocity difference would pale in comparison (I think) to how fast the onboard rocket engines could accelerate the lessened (or non-zero) effective mass. Is there a way to calculate this? –  Ehryk Mar 19 at 23:51
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What if your counterweights were assisted by solid rockets pointed straight down so that a > g? I think then that the counterweight plus the initial propellant weight would have some additive quality to the launch equation that is not available to a launch that lifts all of its own propellant.

This might work particularly well for very large LVs on the Moon, such as for direct burns to other parts of the solar system, where low gravity permits very tall load-bearing structures and existing craters can be further dug out and used as blast containment pits when the counterweights impact the surface, free of air resistance considerations.

It might be used to yank very large rockets straight up from their subterranean, formerly pressurized assembly tubes, for example, and provide a half a minute or so for blast doors to close behind the LV. It may be advantageous for lifting a large LV high enough above facilities that the exhaust does not damage those facilities, or by manipulating weights, tensions, and thrusts, to toss the LV somewhat downrange from facilities so that a total failure does not fall back upon the launch facilities. A counterweight system wouldn't be a part of the rocket equation, but it could slightly improve the initial figures of the equation.

It would effectively be a zero-stage which supplies a modest initial boost for "free," in that none of the propellant mass or potential energy is actually contained within the launch vehicle itself. It seems the purpose of the mass in the counterweight system would be to keep acceleration within the tension limits of the thousands of meters of super-strong miracle string needed.

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