Paper aeroplanes propelled by ‘laser ablation’ have been demonstrated by scientists in Japan. Takashi Yabe and colleagues of the Tokyo Institute of Technology used a powerful laser and a new kind of ‘ablation target’ to resurrect the idea, which was first proposed in the 1970s as a method of powering spacecraft. The researchers have calculated that the technique could be scaled up to make small lightweight aircraft to monitor the atmosphere or volcanoes (T Yabe et al 2002 Appl. Phys. Lett. 80 4318).
When a laser beam hits an object, atoms can be ejected from its surface in a process known as laser ablation. These atoms depart with a certain momentum, and impart an equal and opposite momentum to the object. This led scientists to suggest that a laser on Earth could be used to propel a spacecraft with an ‘ablation target’ attached to it. In recent years, the potential of the technique has been proved by the launch of several miniature rockets, each weighing tens of grams.
Now Yabe’s team has shown that the technique could also be used to fly small aircraft. They successfully propelled two small aeroplanes – which were several centimetres long and weighed 0.1 and 0.2 grams – up to speeds of 1.4 m s-1.
The ablation targets on the aircraft were both made of a piece of aluminium foil 0.1 mm thick and with an area of several square millimetres. One target was coated with clear acrylic, and the other had a layer of water placed on it. Load cells on each plane recorded the push produced when a pulse from an yttrium-aluminium-garnet laser was focused onto the targets. Each pulse supplied 590 mJ of energy and lasted 5 ns.
According to the team, the two-layer structure of their targets is the key to the success of the aeroplanes. In simulations, they showed that in single-layer targets – like those used in used in earlier attempts – most of the laser energy is absorbed by gas atoms just above the surface of the target. This leaves just a few percent of the energy to drive the target, and only a small impulse is generated.
But in the layered targets, the simulations showed that the gas atoms ejected from the target get trapped between the layers and impart a much greater momentum to the target as they are subsequently forced out.
Yabe and co-workers found that their aeroplanes received impulses three times greater those received by aeroplanes with single-layered targets. But they say that they need to find a way to continually refresh the target – or make the whole aircraft from the ablation material – before the aeroplanes can be developed into a practical device. They even speculate that a laser could be used to periodically deform a shape-memory alloy on the aircraft, without the need for ablation.
