Nothing to it
Question: With the advent of modern materials, I wonder if it would be
possible to create a lighter-than-air machine containing a vacuum?
Atmospheric pressure would exert a huge force on the walls of the container,
but a lighter-than-air machine that can be controlled by removing air or
allowing it back in would be useful.
Answer: 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.
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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
Answer: 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 1/20 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鈥攅qual to 150 kilograms of TNT.
Alan Sherwood
Aerospace Technologies of Australia
Victoria
Answer: Having designed NASA鈥檚 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鈥檛 be surprised that hydrogen does not lift twice as much as
helium鈥攖here 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
Interactive TV
Question: Just why do I hear a loud 鈥渂oing鈥� when I turn on my TV?
Answer: The 鈥渂oing鈥� 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 鈥渂oing鈥�.
Derek Potter
Axminster, Devon
Answer: The 鈥渂oing鈥� only appears to come from the TV set. In reality, it鈥檚
the sound of all the higher-level mental faculties in your brain closing down at
once.
Ian Frank
Ibaraki-ken, Japan