For every action, there is a reaction: that is the principle on which all space rockets operate, blasting propellant in one direction to travel in the other. But one NASA engineer believes he could take us to the stars without any propellant at all.
Designed by David Burns at NASA’s Marshall Space Flight Center in Alabama, the “helical engine” exploits mass-altering effects known to occur at near-light speed. Burns has posted a paper describing the concept to NASA’s technical reports server.
It has been met with scepticism from some quarters, but Burns believes his concept is worth pursuing. “I’m comfortable with throwing it out there,” he says. “If someone says it doesn’t work, I’ll be the first to say, it was worth a shot.”
To get to grips with the principle of Burns’s engine, picture a box on a frictionless surface. Inside that box is a rod, along which a ring can slide. If a spring inside the box gives the ring a push, the ring will slide along the rod one way while the box will recoil in the other. When the ring reaches the end of the box, it will bounce backwards, and the box’s recoil direction will switch too. This is action-reaction – also known as Newton’s third law of motion – and in normal circumstances, it restricts the box to wiggling back and forth.
But, Burns asks, what if the ring’s mass is much greater when it slides in one direction than the other? Then it would give the box a greater kick at one end than the other. Action would exceed reaction and the box would accelerate forwards (see video below).
This mass changing isn’t prohibited by physics. Einstein’s theory of special relativity says that objects gain mass as they are driven towards the speed of light, an effect that must be accounted for in particle accelerators. In fact, a simplistic implementation of Burns’s concept would be to replace the ring with a circular particle accelerator, in which ions are swiftly accelerated to relativistic speed during one stroke, and decelerated during the other.
But Burns thinks it would make more sense to ditch the box and rod and employ the particle accelerator for the lateral as well as the circular movement – in which case, the accelerator would need to be shaped like a helix.
It would also need to be big – some 200 metres long and 12 metres in diameter – and powerful, requiring 165 megawatts of power to generate just 1 newton of thrust, which is about the same force you use to type on a keyboard. For that reason, the engine would only be able to reach meaningful speeds in the frictionless environment of space. “The engine itself would be able to get to 99 per cent the speed of light if you had enough time and power,” says Burns.
Propellant-less proposals aren’t new. In the late 1970s, Robert Cook, a US inventor, patented an engine that supposedly converted centrifugal force into linear motion. Then, in the early 2000s, British inventor Roger Shawyer proposed the EM drive, which he claimed could convert trapped microwaves into thrust. Neither concept has been successfully demonstrated and both are widely assumed to be impossible, due to violation of the conservation of momentum, a core physical law.
Martin Tajmar at the Dresden University of Technology in Germany, who has performed tests on the EM Drive, believes the helical engine will probably suffer the same problem. “All inertial propulsion systems – to my knowledge – never worked in a friction-free environment,” he says. This machine makes use of special relativity, unlike the others, which complicates the picture, he says, but “unfortunately there is always action-reaction”.
Burns has worked on his design in private, without any sponsorship from NASA, and he admits his concept is massively inefficient. However, he says there is potential to harvest much of the energy that the accelerator loses in heat and radiation. He also suggests ways that momentum could be conserved, such as in the spin of the accelerated ions.
“I know that it risks being right up there with the EM drive and cold fusion,” he says. “But you have to be prepared to be embarrassed. It is very difficult to invent something that is new under the sun and actually works.”