Would it be economical to add a counterweight to rocket launches?

Question

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.

Answer 1

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.

Answer 2

Such a counterweight would provide force that accelerates the mass of the rocket; it wouldn't reduce the inertial mass of the rocket, so your proposed application of the rocket equation is not valid.

Counterweights massing 2400 tons would simply provide 2400*9.81 = 23,544kN additional thrust, bringing a Saturn V's acceleration to a little over 10m/s² immediately off the pad instead of 2m/s²; you'd hit 100m/s after 10 seconds instead of 20m/s, for a total ∆v gain of 80m/s. Nowhere near mach 5!

Conveniently, 10m/s² acceleration (of the rocket, upward) closely matches Earth's surface gravity 9.81m/s² (acceleration of the counterweights, downward), so you wouldn't need a complex pulley arrangement.

The total ∆v budget to orbit for Saturn V is about 9200m/s, so this method would provide not quite 1% of the needed oomph.

There's no problem with acceleration limits for crew or structure; the Saturn V stack peaks at just under 4g (~40m/s²), but that's at 1st-stage cutoff where the weight of the fuel is gone; right at takeoff the acceleration is very low, so that's a fine place to add additional force.

Answer 3

While this plan looks good on paper, there's a reason why it has never been implemented. Large hanging masses are dangerous and unstable. The structures necessary would require lots of protection from the rocket's exhaust, and much safety equipment to prevent the masses from swinging or even dropping accidentally. The structures necessary would clutter the launchpad area, which adds further danger - part of a rocket clipping a structure would never end well, and adding four more towers would quintuple the chance of that happening. Having a lot of mass attached directly to the rocket would add a lot of stress to the machine, which already is generally a relatively thin skin wrapped around a lot of fuel.

Basically, use Murphy's law - the more things you have, the greater the chance that something could go wrong. Not to mention the expense of erecting the rig in the first place - such an apparatus would cost at least tens of millions of dollars (and more likely hundreds of millions) - which is the cost of a few missions to Mars. Given the tight funding space exploration has today, it is unlikely that any rig of this sort will be built.

Answer 4

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.