Airship to Orbit [Was: Cities in the sky]

Hi all,

for a moment I was under the impression there is an "Airship to Orbit"
discussion going on.
But apparently it's just that some of you don't care about the message
subject line. :-(

Hi Andrew!

Andrew Goddard wrote:

>>>Ignoring the stuff on electrostatic lifters (as I can't see these as
>>>more than toys for the amateur scientist - but don't mind being proved
>>>wrong in the future ;-) )
>>>
>>>
>>>
>>Well, in my opinion that is the interesting part. Maybe we can get back
>>
>>
>>to that later.
>>(I may be wrong, but I would like to know why.)
>>
>>
>
>New thread on lifters required? Ok...fancy starting one off? Convince me
>they would work outside 1atm and someone's kitchen table!? ;-)
>
Well, I'm not *convinced* that this is the answer to the Airship to
Orbit puzzle.
It's only that it is my best guess, and I would like to ask for the
benefit of doubt for it.

Steam engines started as hopping pot caps on the kitchen stove and ended
up starting industrial revolution.

Ion engines work. Lifters work. Lifters are much more efficient because
they interact with air.
So a very large lifter-like structure could interact with a lot of air
molecules even at very high
altitudes.

>Yes, and we're looking for a range of speeds at the top end of which the
>gravitational forces are countered by rising centripetal forces. That
>is, the wings tend to become unnecessary (they don't need to provide
>lift) at the same time as the air pressure drops to make them
>unworkable. Which is kind of neat.
>
In fact I wonder very much about the V-wing-shape of the ATO upper stage.
What's the advantage of that (except that it looks cool on the demo slides)?

>[snipped maths, took me five minutes to go through it]
>
> = (e^(h))^(1/6)
>
>I think that's right. Oops! "I note your concerns." ;-)
>
>>My feeling is that a speed of around 5 to 6 km/s is most critical.
>>We still have most of the weight, but drag is already very high.
>>Convince me that it works for this speed, and I'll believe everything.
>>
>>
>
>Let me think about that some more.
>
Sure, take your time. ATO is still six years from now, not including
unexpected project delays ;*)

>>In fact I feel that energy is a critical point. Why do you say there is
>>
>>
>>(more than) enough?
>>
>>
>
>Solar energy is freely available at these altitudes - at least during
>daylight ;-). Given the slow acceleration to orbit there might be a
>valid case for using beamed microwaves from the Earth's surface, too.
>

Some months ago I tried to do a back of the envelope estimate about
solar energy,
assuming all of the ATO surface was covered with solar cells. 1.4 kW/m^2
solar constant,
about 10% conversion efficiency, guessing the mass of ATO etc. Knowning
how much
kinetic energy is necessary for orbit, I got something like a lower
bound estimate of
several days to collect this energy from the sun. This did *not* include
energy to fight drag.
Using solar energy may be possible, but it's a very critical issue.
(With a bit of steering capability ATO should be able to fly in sunlight
all of the time.)

It may even be the reason for JPAerospaces estimate of flight time to orbit.

Some roumors say that ATO is "burning off" its hydrogen while it
ascends. It could be
burned in fuel cells to create electricity. My problem with this idea is
that the rate of hydrogen
to burn changes very much during flight. In the beginning there is a lot
to burn and when
the buoyant part of the flight is over, there is almost nothing to burn,
but still several
energy hungry km/s of delta-v to go.

JPAerospace specifically said they do not plan to use beamed power.
I say it would be very difficult/expensive to build power beam sources
all around the world
or (more realistic, but still very expensive) a set of orbital power
stations / power relays.

Regrads,
Axel