|MadSci Network: Physics|
There are a couple of ways that blimps can change height. One method is to dump helium to change the buoyancy. The blimp could also pump the helium into a high pressure reserve instead of dumping it, keeping the helium on board. Most blimps will instead use slightly less helium than needed for lift, and run their engines to make up for the lack. This way all that is needed to change height is to angle the engines differently, so losing helium is not an issue. It is very difficult to keep helium in one place (helium will leak out of nearly anything), so supplies of helium are necessary to keep a blimp running, but a blimp could land and take off again without having to refuel right away. You might also want to look at the Airship and Blimp Resources page for more information about blimps, including how to build your own.
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