Friday, January 19, 2007

Rockets are wrong

That Buck Rogers Stuff

As impressive as they are to watch, rockets are a dangerous and in the end inefficient means of getting to orbit.  Burning tons of liquefied oxygen and hydrogen and throwing away the rocket every time you want a satellite is not what your average beancounter would call sound economically.  Imagine if, to fly from New York to Los Angeles, you built a brand new 747, flew it across the country, and jumped out over LAX for a parachute landing and let the plane crash into the Pacific.  Getting a airline ticket would face a few more difficulties than just avoiding TSA’s watchlist.

This is a sound argument for reusable spaceships.  But it is an even better argument for taking a step away from rockets altogether.  Instead of rockets, why not have an elevator?  Walk through the doors, take a seat, and ride into space with as much fireworks and commotion as getting on the express elevator in the Empire State Building.  Building a physical structure that extends from the surface of the earth to orbit and beyond seems fantastical, but the idea actually has an extensive pedigree.

The idea for space elevators goes back to the misty dawn of the space age.  Russian space theorist Konstantine Tsiolkovsky first proposed the idea of an orbital tower in his 1895 paper “Day-dreams of Heaven and Earth.”

On the tower, as one climbed higher and higher up it, gravity would decrease gradually; and if it were constructed on the Earth’s equator and, therefore, rapidly rotated together with the earth, the gravitation would disappear not only because of the distance from the centre of the planet, but also from the centrifugal force that is increasing proportionately to that distance. The gravitational force drops ... but the centrifugal force operating in the reverse direction increases. On the earth the gravity is finally eliminated at the top of the tower, at an elevation of 5.5 radii of the Earth (36,000 km).

However, it was soon realized that no material could withstand the compressive stress of the weight of the tower.  Half a century and more down the road, another Russian, Yuri Artsutanov, proposed what we now think of as the space elevator.  Artsutanov suggested using a satellite in geostationary earth orbit (GEO) as a construction base, and extending a cable downward while simultaneously paying out a counterweight upwards to maintain the center of gravity in GEO.  Artsutanov also described using a tapered tether to reduce the stress on the cable.

Over the last several decades, many people have examined the idea.  Charles Sheffield and Arthur C. Clarke both used the idea as the central focus of their novels Fountains of Paradise and The Web Between the Worlds in the late seventies.  And more thorough research has established many of the engineering requirements for a working space elevator.  Most of these problems are solvable by a suitable application of engineering or politics – for example, building a working elevator car for the cable would be a straightforward, if difficult, application of the principles currently used in maglev trains.

But the biggest obstacle is the creation of a structural material for the elevator cable.  Our strongest materials until recently fell short of the required tensile strength by a large margin.  At a minimum, beanstalk cable material should have a tensile strength of 65 GPa (gigapascals, a measure of stress), and a density on the order of graphite.  (Too much weight, and it doesn’t matter how strong the cable is.) The strongest steel is at about 5 GPa.  Kevlar hits about the same, but is much lighter.  We’re off by at least an order of magnitude.  Quartz fibers and diamond filaments would reach up to the twenties.  But then, in the nineties, came carbon nanotubes.  Their theoretical tensile strength is in the range needed for a beanstalk. 

But, the strongest actual observed GPa was only in the fifties, and the tensile strength of a cable would likely be less than that of its nanotube components.  There are also difficulties with making bulk quatities of nanotubes and making them into suitable strands.  Cost is also a factor, as nanotubes run about $25 a gram.  But there is hope – carbon nanotubes have applications far beyond making space elevator cables, and someone, sometime, will for his own purposes invent a cable that is suitable for our beanstalk.

These developments in materials science put a working beanstalk in sight.  And one company has formed to pursue the creation of a space elevator.  I ran into Brian Dunbar of the Liftport Company in the comments section over at Murdoc Online, and asked him if he’d do an interview.  He graciously agreed, and below, part one of our interview:


Posted by Buckethead on 01/19/07 at 08:00 AM
That Buck Rogers Stuff • Permalink