May. 10th, 2015

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Recently Wikipedia's featured article of the day was about a bridge. This led me to read other articles about various kinds of bridges. They described many different kinds, some of which I had not been aware of.

But they omitted one type: The rocket bridge. They didn't even hint at the concept.

In a rocket bridge rocket engines are affixed to the edges of the deck at intervals, firing constantly. Their thrust supports the weight of the deck. Thus it has no need for piers or cables or stone arches or the like.

Rocket bridges are good for situations such as arbitrarily long spans over bottomless chasms where there is no place to put foundations for piers.

Since construction can proceed entirely from one end, they are also used where there is no way to get to the far side of the channel during construction. They are especially popular in dreamlands where no one knows whether the channel being bridged even has a far side.

If working space at the site is at a premium, sections of the bridge can be assembled elsewhere and fitted with temporary maneuvering controls to allow them to be flown to the construction site under their own power. There they can be joined to the already completed portion.

Note that no falsework is needed, at least for the main span. Construction techniques for the transitions to and from the ends of the bridge will depend on the type of terrain as well as other variables.

Since the rocket bridge is a common choice when it is not known what (if any) terrain awaits on the far side of the channel, it has become associated in the minds of many decision makers with last-minute design changes and the resulting high costs and construction delays. Some critics say that the only place this type of bridge makes sense is in comic books and cartoons.

Even its advocates admit that the design does have some disadvantages.

Since the rocket engines that support the bridge must fire continuously, noise can be a nuisance, especially if there are residential areas nearby. And since engine failure can lead to collapse, designs tend to be highly redundant, making this type of bridge relatively expensive. Operation and maintenance costs are also rather high.

Although in theory the span can be arbitrarily long, when spans get too long getting fuel to the engines becomes a problem. Pipelines are often used, but for long spans their weight uses up most of the load-carrying capacity of the bridge, leaving little available for actual traffic. Tanker trucks present similar problems. Trucks also have the added danger of possibly being delayed by traffic problems or labor disputes.

To get around the problems with trucks and pipelines some projects currently in the planning stages are considering mid-air refueling of the rocket engines from blimps or helicopters or other aircraft capable of hovering.

Some designers have proposed reducing fuel usage by adding lighter-than-air supports to the bridge structure. It is unclear whether anyone is seriously considering this alternative.

Public reaction to the design has been mixed.

Rocket engines tend to be noisy, and there have been complaints. People living near rocket bridges can't sleep, and as a result have been moving to places that don't have rocket bridges. Medical people say that exodus is a good thing because if people stayed near the bridges the noise from all those rocket engines running all the time could damage their hearing.

On the other hand, businesses located near rocket bridges are suffering because so many people have moved away.

Some suggest luring in deaf people to live near the bridges, but even those who can't hear the noise may feel the vibrations, and even if that doesn't bother them the vibration can cause structures to deteriorate faster than they otherwise would.

Others have suggested building noise-powered robots to live under the highways and actually make use of the noise. Some people, however, are concerned that such robots would eventually want to spread out and live elsewhere, and would create political pressure to build noisy rocket-supported elevated highways all over the world so the robots would have the noise to sustain them.

There are rumors of extremist groups plotting to sabotage the rockets to get some quiet. This has prompted officials to remind people that if the rockets were to stop, the bridges they support would fall down. This collapse is itself likely to be noisy, at least in the short term, and would cause considerable damage and quite a few injuries. So the government recommends that the rocket engines not be sabotaged, at least for now.

Stay tuned for further developments.
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