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Time to dump the water and move into the future of airship ballast!
By changing the internal density from negative (slight vacuum) to positive (higher than external) would allow an airship to gain or lower altitude. This system would not provide total altitude control but work as an adjustment system.
This is for the rigid style airships (Graf, Hindenburg, Macon) where an internal balloon is used.
Basic operation is the internal balloons are inflated with helium to a zero point. The zero point is defined as a standard cruising altitude air density based upon the geographic zone of movement. Simply, the airship cruises at 5000 ft above the ground; if the ground is at 500 ft above sealevel then the density changes. This way no extra power draws would be required while cruising. The internal balloons are woven with "muscle wires" which expand or contract with an electrical charge. This would expand or contract a balloon plus or minus 10% of its size. As the wires compress, the volume decreases and the density increases, causing a lowered lift. If the wires expand then the volume increases and the density is lower, and more lift is generated. This would be enough to compensate for weight/wind shifts and internal temperature changes. Normally these compensations are made by dumping hundreds of gallons of water and then using condensers attached to engines to recover the water in flight. This system would remove this inefficient use of weight and fuel.
The interior balloons are woven from a carbon fiber/muscle wire/nylon mesh to allow for strength and weight as well as using a graphite coating to allow for lowered friction due to expansion or contraction. In order to seal the balloon against helium loss I would recommend a light custard lining. Custard ,known for its amazing physical properties, is unfortunately subject to gravity and would slide down and collect in a puddle (or a pudding). Therefore if we rotate the balloons as giant flywheels with just enough force/speed to hold the custard in place we could also use these as energy collection points and gyrostabilizers.
Allowing once again the ability to travel slowly around the world with a bit of room to stretch your legs on the way.
Shape memory alloys
Sometimes known as muscle wire [wiml, Sep 01 2006]
||Probably easier (or at least, lighter) to use normal balloonets and pump the lifting gas into or out of a small pressurized tank as needed. I've wondered why dirigibles didn't do this.
||What is this magic "muscle wire"?
||[ldischler]: A common term for wire made out of memory metal. See link. I've played with it a little. Unfortunately, it tends to be pretty power-hungry for the amount of work you get out of it.
||As I recall, memory alloys only go one way. If they get bent, you can heat them up and they return to the original orientation. But you can get them to back and forth electrically, can you? <edit> Okay, now I see there's a two-way material. Still, you'd have to run power through it continuously in at least one configuration...unless you could latch it somehow.
||Instead of having it flex two ways or require a constant voltage I was thinking of having two "threads" woven into the material. One would weave directly to itself causing the contraction while a second would be woven to the balloon support wires that hold the balloon in place within the shell. That way it would require no energy except when changing altitude.
||as for pumping the gas around, I like that idea but the pump are expensive, leaky, and inefficient- like my idea(grin).
||Further to wiml's comment
on using ballonets (in non-rigids) to
compress or decompress the helium in
the airship - you could do that by just
changing the air pressure in air filled
ballonets - normally set at very low
pressure in non rigids to just maintain
the envelope shape. Surely an envelope
could be made to take a little bit more
than almost zero psi ? I think this would
be rather better than the muscle wires
or carrying compression equipment to
compress lifting gas into tanks - just
dump the air in the ballonets when you
want more lift - the helium will expand
to give you more lift.
||One could incorporate the method used in hot air balloons: heat the lift gas. This would make you rise. To fall, you could wait for it to cool, or vent some lift gas. One must have a frame which can accomodate size changes, just as with this idea. Changes would not be as fast as with the muscle metal but this is a blimp, after all.
||Hot air balloons rise because the
higher temperature increases the
pressure of the lift air. Thus a
given sized envelope can hold a
lower quantity by weight, without
collapsing from external air
pressure. Lower weight in the
same volume = lower density =
displacement = bouyancy.
||Heating a fixed quantity by weight
of lift gas in a fixed volume
envelope would increase the
pressure, but the density would
stay the same, with no change in
||Unless the envelope was