Joe Strout wrote:

But airships (whether filled with hydrogen or helium) are.
Big ones can have a quite impressive lift capacity, too.

The problem is how to bring the balloon or the airship
back to the earth -- you would have to release lots of
expensive hydrogen. The cost of making hydrogen is about
0.7 $/kg, but the cost of liquefying and transporting hydrogen
from the oil refinery to the user raises the cost to about
3 $/kg. Hydrogen, like chlorine, is a destroyer of the ozone
layer. If the rocket weighs 10 tons, you would spend about
$50,000 on the hydrogen alone. Helium is even more expensive.

Did you consider a power source on the ground beaming power
to the helicopters in the form of lasers or microwaves?

Leik N. Myrabo experimented with this idea some 20 years ago.
It works, and it is not very expensive. The microwave
electronics would cost about $100 per 1kg of the rocket weight.
(Batteries cost about $200 per 1kg of the rocket weight.)
I did not mention microwaves because the safety concerns would
drive up the cost.

The cost of electric motors is only about $30 per 1kg of the
rocket weight.