Space Slingshot: Impossible or Obvious?
Back in 1687, Issac Newton proposed the idea of an orbital cannon which could fire a projectile so fast, it would head into space – and never come down. Since then, people have proposed all sorts of different ground-based space launchers: superguns, railguns, laser launchers, levitating magnetic sleds, you name it.
My article in this month's BBC Focusmagazine explores one of the most interesting and overlooked ideas: the Slingatron. Put an ice cube in a frying pan, move the pan in small circles, and you'll get the idea. The ice starts going round the outside of the pan at high speed. This is the same basic principle as the Slingatron, which could also be compared to a giant slingshot. Make one big enough, and you could launch a payload into orbit.
Every time the idea comes up, people insist that it's impossible. Not so, says Slingatron designer Dr. Derek Tidman, now President of Advanced Launch Corporation.
Slingatron could be built using existing materials. He says he has done the engineering calculations and arrived at a figure needed for the stength of the material at the point of maximum strain for an orbital-capable Slingatron. It's within the limits of what modern steel can reach.
Another objection is that "any conceivable payload would be incinerated in a few seconds." This applies equally to all ground-based launchers, but is contradicted by various studies. The payload will need some serious thermal protection, but the short duration of the transit through the atmosphere means heating is less severe that with proposed hypersonic aircraft.
"The friction on the payload inside the Slingatron would be too great." Much of the work so far has been in firing projectiles down a curved track, and this has shown that a projectile coated with Lexan or a similar material does not suffer excessive friction. The coating is vaporized and the projectile rides on a cushion of gas.
DARPA did explore the Slingatron, and went so far as building a small prototype, but found it unsatisfactory. Dr. Tidman believes that the problem with DARPA's Slingatron was a simple one, and that the design simply needed to be 'debugged'. (He was also advised that if he wanted real funding, he should be looking at a small version urban combat; he declined).
Some of the barriers to the Slingatron's progress seems to be political. Various people and companies have been working for years on high-power railguns and do not like to see their work jeopardised by competition. "One senior railgun researcher, speaking at a meeting I did not attend, expressed his opinion that the Slingatron violated the laws of physics," said Dr. Tidman. No explanation of the impossibility was forthcoming, but the audience apparently accepted the statement and the damage was done.
Perhaps the problem is the sheer simplicity of the Slingatron's design – people don't believe that something so simple can work better high-tech devices like the railgun.
Dr. Tidman thinks it's just a matter of oversight that the Slingatron was not developed before. It might have been big and clunky, but a steampunk Slingatron might have outgunned the heaviest artillery a hundred years ago.
Maybe the nearest historic parallel is perhaps in siege engines. The Romans used a device called an onager to hurl rocks, which relied on the relatively sophisticated use of animal sinews as springs. Centuries later, in the less sophisticated medieval era the dominant siege engine was the trebuchet , a simpler device using a counterweight - which was actually far more powerful.
Tidman is now working on a textbook which will describe the mechanics of the Slingatron and should show how easily the apparent difficulties can be overcome.
Perhaps the Slingatron really is a non-starter, and it would take an engineer to assess it properly. But if you look at the website you will see that this is an idea that has been thoroughly developed.
Dr. Tidman is talking to a number of people about advancing the project, so maybe further prototype Slingatrons will be built. But the risk is that the 'impossible' label will stick and the idea will be lost.