|
The following Last Word questions and answers are drawn from this week's issue of New Scientist. A huge database of everyday scientific Q & A is also available on our website.
Atmospheric pressure would exert a huge force on the walls of the vacuum container, but some modern materials are very strong, and a lighter-than-air machine that can be controlled by removing air or allowing it back in would be useful. to keep the vessel light, the walls would have to be as thin as possible and its shape spherical. If the stresses in the walls could be accommodated, the vessel would become buoyant only if the mass of evacuated air was at least equal to the total weight of the vessel. These constraints suggest materials that have low densities and high strengths, such as composites, high-performance polymers and low-density metals. Composite materials can have high strengths but only in specific orientations, and they perform poorly in compression. High-performance polymers are light but their low strengths and visco-elastic behaviour would let them down. This leaves the low-density metals, such as titanium, aluminium and magnesium. Titanium has the best strength and density characteristics. The critical design parameter for the vessel would be the stress generated in its wall, which must be below the compressive failure stress of the material. Figures for compressive strengths of materials are hard to find, but ultimate compressive strengths commonly exceed the ultimate tensile strengths by up to a factor of 6. I thus assume 85 per cent of the ultimate tensile stress of titanium is a safe level of compressive stress. If a vessel were constructed from titanium (with a density of 4.5 grams per cubic centimetre and an ultimate tensile strength of 680 megapascals) then an evacuated spherical chamber with a radius of 22.2 metres and a wall 2 millimetres thick would just be buoyant in the atmosphere. Assuming a perfect vacuum, the mass of air ejected from the chamber would be 59.1 kilograms. But as the mass of the chamber would be 56.2 kilograms, that leaves a mere 2.9 kilograms for engines, fuel, passengers and luggage. Clearly, for a craft to be useful it would have to be larger. If the vessel had a radius of 155 metres and a wall 13.9 millimetres thick, then it would be able to lift a 1-tonne object off the ground. A lighter-than-air machine is therefore theoretically possible but very difficult to construct. TOBY HUTTON Dunlop Aviation, Coventry and Department of Materials Engineering, Nottingham University an aluminium shell 0.15 millimetres thick with a radius of 1 metre will just float if it contains a vacuum. The compressive stress in the shell will be 340 megapascals, which a good aluminium alloy can withstand. However, this thin shell would be unstable and would collapse at the slightest imperfection. To prevent this, stiffness is required. Using the stiffest currently available material, high-modulus carbon fibres (with an elastic modulus of 700 gigapascals, and assuming the elastic modulus is 250 gigapascals for a two-dimensional laminate), a self-buoyant shell 0.25 millimetres thick would still buckle at only about 1Ž20 of an atmosphere. This can be overcome by structural geometry. The simplest solution is a sandwich panel. Two 0.125-millimetre carbon-fibre skins separated by about 10 millimetres of light plastic honeycomb or foam would be stable at 1 atmosphere. Internal ribs or struts could achieve the same result. The real problem is the energy involved in dealing with a vacuum. Removing air to control buoyancy is easier said than done. To be competitive with helium airships, the vessel would have to operate at a minimum 85 per cent vacuum (because helium exerts a lift equal to 85 per cent of the weight of displaced air). To pump 1 kilogram of air per second from an 85 per cent vacuum requires about 160 kilowatts, about the total engine power available in a typical airship. An easier solution is to use air as ballast, and pump it into a helium-filled envelope. If taken to 15 per cent above atmospheric pressure (to give 15 per cent lift reduction), pumping in 1 kilogram per second of air requires only 12.5 kilowatts. There is also the problem of leakage and damage. A hole would let air in faster than helium would leak out of a low-pressure airship. And a small dent could cause a catastrophe. The collapse of a 6000-cubic-metre vacuum would release 600 megajoules--equal to 150 kilograms of TNT. ALAN SHERWOOD Aerospace Technologies of Australia Victoria having designed NASA's 1.13-million-cubic-metre scientific balloon, I have spent many hours considering the problem. Theoretically, one could build a lighter-than-air craft deriving lift from a vacuum but the performance enhancement of using a vacuum is not that great compared with using helium or hydrogen. Air has an average molecular weight of 29, helium 4 and hydrogen 2. The difference in molecular weight between air and the lifting gas (or vacuum) determines the actual lifting capability, not the molecular weight of the gas. So don't be surprised that hydrogen does not lift twice as much as helium--there is only an 8 per cent improvement in lift. Likewise, going from hydrogen to a theoretical vacuum would produce only a 7.5 per cent improvement in lift for an equivalent volume. THOMAS LEW Instrumental and Space Research Division San Antonio, Texas
the "boing" is caused by the degaussing circuitry used in all colour TVs and monitors. The TV tube steers three electron beams to a fine pattern of phosphor dots in the three primary colours on the screen. Just before they hit the screen, they pass through a perforated metal mask that only lets the beams through if they are heading for the right colour phosphor. Any errors, and the beam hits the wrong-coloured phosphor, resulting in distorted colours on the screen. Stray magnetic fields can divert the beams. Left to itself, the metalwork of the TV, especially the colour mask, becomes randomly magnetised. To prevent this, the TV has a large demagnetising coil which is operated each time you switch on. The circuitry sends a strong pulse of alternating current through the coil. The alternating field has to be quite strong to demagnetise the tube, and it rattles both the mask and any other ferrous metalwork in the TV, producing the "boing". DEREK POTTER Axminster Devon the "boing" only appears to come from the TV set. In reality, it's the sound of all the higher-level mental faculties in your brain closing down at once. IAN FRANK Ibaraki-ken Japan
|
© Copyright New Scientist, RBI Limited 2000