Tall towers
Aren't tall towers necessarily thick, heavy and expensive?
No. You can make a tall tower out of many short struts stabilized by cables.
This principle is illustrated by Snelson's needle tower (see image on the right).
The most efficient way to arrange the struts is normally in a column,
on top of each other - with lateral tensile stabilisation provided by
cable stays.
This principle is widely used by TV and radio mast designers (see
image on the left).
Notice how the TV mast is narrow, slender and light. It can be like
this because of the lateral support provided along its length by the
surrounding tensile network.
Suspension tower prototype
Just as suspension bridges enable exceptionally long clear spans, so "suspension towers" might enable exceptionally tall structures. Here is a prototype, illustrating the idea. It is like a suspension bridge but pointing straight up into the air. It is shaped like the Eiffel tower - and yet it is a tensile structure. It's shape mirrors the shape of the trunks of some trees. As a suspension bridge is to a cable-stayed bridge so this structure is to a classical cable-stayed radio mast.
Advantages
The longest suspension bridge is 2,023 meters. The longest cable-stayed bridge is 1,104 metres.
I think it is reasonable to expect similar dynamics to apply to suspension towers with lateral cable support. Having very long cable stays scales poorly.
Disadvantages
These kinds of tall towers need scaffolding to construct - more scaffolding than is required with a suspension bridge.
In a suspension bridge the catenary cable supports its own weight. In a suspension tower, much of the weight of the catenary cable must be born by the tower itself.
Most of the cable stays are near to horizontal. That means that they sag under their own weight. That makes it harder to tighten them and it makes them more "spongy". That is unlike a suspension bridge - where most of the cables are nearly vertical.
Applications
There may be applications for very tall structures like this. Currently, tall towers are used as radio masts, office space and tourist attractions. However, access to space could be another possible application. Towers could be used to raise jet fuel high into the atmosphere without employing rockets. This would largely bypass the pesky Tsiolkovsky rocket equation - potentially resulting in significant cost savings.
Here, a particular structure is proposed for payload launches. It is a space gun - or rail gun, or mass driver - with a horizontal accelerator leading to a tall tower over a mine shaft. A cable with a counterweight could be used to provide acceleration during the ascent. Use of electromagnetic acceleration is also a possibility.
As usual with such designs, the payload should be able to resist whatever acceleration forces are provided.
As some context: the Earth's equatorial bulge is about 21 kilometers (above sea level at the poles). Equatorial launches also get a speed boost from the Earth's rotation. The world's tallest mountain is around 9 kilometers above sea level. The Tibetian plateau is about 5 kilometers above sea level. The world's tallest tower approaches 1 kilometer. The atmosphere is often considered to be about 100 kilometers thick - but most of the mass of the atmosphere (90% of it) is concentrated in the lowest 1.5 kilometers. So: an extra 10 kilometer or so head start could help to get a spacecraft through a good fraction of the atmosphere.
Deccelerating objects from orbit may be another possible application. A grappling hook would attach them to a cable near to the top of the tower and then they could be decellerated gradually using a counterweight and landed slowly at ground level - probably at some distance from the tower.
This application could require a stronger tower with a greater ability to resist horizontal forces.
Potential
I feel as though humanity didn't really try to build any tall towers yet - since there's hasn't been a driving problem that requires it. Nobody needs office space or hotel rooms that are 2km above the ground. Similarly, tourists get a similar view regardless of whether they are 1km or 2km up. As for radio masts - much the same applies. A few smaller ones are as good as one extremely tall one.
However, providing materials with cost-effective access to orbit does appear to provide a driving problem where tall towers have real, viable applications.
Early prototype
Here is the first prototype I made:
References
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