Re: How does a blimp come down?

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.