Air Ship To Space?

In article ,
Allen Meece wrote:
they [JPA] appear to need a positively miraculous L/D ratio to
make this thing work as described, and there's no hint of how they could
possibly do that.

Correct about the no hint. They just say the "ion engine for the orbiter will
be tested in the next five months."

The doubts about the aerodynamics are actually independent of the exact
engine type -- it's a generic problem that isn't sensitive to propulsion
details.

...Let's get
behind JPA and wish them success. So what if they can't or won't right now tell
us how to make an ATO, Airship To Orbit.

The concern is that they may not *know* how to make one: they may be
kidding themselves about basic feasibility. Indeed, it rather looks that
way. I wish them luck, and I remain open to the possibility that they've
found some clever loophole... but right now I wouldn't invest money in it.

It will be an affordable and wonderful thing
to slowly accelerate to orbit. Space will become closer to all of us when
this concept flies.

Correction: "It *would* be" and "*if* this concept flies".
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Jim Davis writes:

Henry Spencer wrote:

The doubts about the aerodynamics are actually independent of
the exact engine type -- it's a generic problem that isn't
sensitive to propulsion details.

But even if one ignores aerodynamics (ie, assume infinite L/D) the
scheme doesn't add up. Take the following statements from their
handout:

"The third part of the architecture is an airship/dynamic vehicle
that flies directly to orbit. In order to utilize the few molecules
of gas at extreme altitudes, this craft is big. The initial test
vehicle is 6,000 feet (over a mile) long. The airship uses buoyancy
to climb to 200,000 feet. From there it uses electric propulsion to
slowly accelerate. As it accelerate it dynamically climbs. Over
several days it reaches orbital velocity."

"Once in orbit, the airship is a spacecraft. With its
solar/electric propulsion, it can now proceed to any destination in
the solar system."

"The ion engine 120,000 foot flight test for the orbital airship
will be flown in the next five months."

The airship in orbit has a specific energy of 32,000,000 J/kg.
Taking several days to mean 4 days or 345,600 seconds that means
the power source has to supply 92.6 W per kg of airship in orbit.

Now using data from Larsen and Pranke an ISS 890 kg photovoltaic
blanket produces 28000 W for a specific power 31.5 W/kg.

So even if the airship were nothing but photovoltaic cells
converting electricity to kinetic energy at 100% efficiency it
couldn't possibly achieve orbit in several days even ignoring
atmospheric drag.

Surely the folks at JP Aerospace can do this simple calculation. So
what is going on here?


Among other things, the ISS 890 kg photovoltaic blanket is not the
latest and greatest word in photovoltaic cells. For that matter,
I'm pretty sure its mostly not solar cells at all, but support
structure and wiring harness and protective coating and whatnot.

There are credible solar power system designs in advanced development
with specific power levels of ~200 W/kg. If such a system could be
tightly integrated with the airship envelope, those numbers would
actually add up pretty well, with your 92.6 kW/kg for the overall
vehicle a not entirely unreasonable figure.


What's going on here is speculation at this point, but the numbers
start to fall apart not far beyond the solar power calculation you
did. Assuming basic competence on JP Aerospace's part and thus
looking for a minimum-number-of-discrete-miscalculations explanation,
the simplest hypothesis I can find is that they A: assumed that
COTS ion thrusters can be hooked directly to solar arrays, neglecting
the requisite power processing units that are the heaviest part of
the system, and B: used a specular rather than diffuse reflection
model for rarefied gas aerodynamics, leading to the false conclusion
that an arbitrarily skinny and low-alpha airfol can have an arbitrarily
high lift to drag ratio.

Those are actually two fairly common educated-amateur level mistakes
in electric propulsion and rarefied gas dynamics, respectively, and
if you grant those two mistakes I can almost make the numbers work
for the rest of the system.

Unfortunately, while direct-drive ion or plasma thrusters may be a
theoretical possibility, specular reflection simply does not describe
the way gas molecules behave in the relevant environment, period.


--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
*Chief Scientist & General Partner * -13th Rule of Acquisition *
*White Elephant Research, LLC * "There is no substitute *
* for success" *
*661-718-0955 or 661-275-6795 * -58th Rule of Acquisition *

John Schilling wrote:

Among other things, the ISS 890 kg photovoltaic blanket is not
the latest and greatest word in photovoltaic cells. For that
matter, I'm pretty sure its mostly not solar cells at all, but
support structure and wiring harness and protective coating and
whatnot.

I was using only the figure for the photovoltaic blanket. Larsen
and Pranke break down the solar array as follows:

Photovoltaic blanket 890 kg
Mast 330 kg
Gimbal 540 kg
Electrical equiptment 610 kg
Thermal control 730 kg
Misc. integration 610 kg
Total 3710 kg

There are credible solar power system designs in advanced
development with specific power levels of ~200 W/kg. If such a
system could be tightly integrated with the airship envelope,

Very, very tightly integrated. The solar cells are not only going
to convert sunlight to electricity but be load carrying structures
as well.

those numbers would actually add up pretty well, with your 92.6
kW/kg for the overall vehicle a not entirely unreasonable
figure.

No, it does not make sense even then. Grant them the 200 W/kg power
level. Assume the airship is made up of nothing but these cells
when it gets to orbit. This means it the it has to convert 46.3% of
the electric power to useful work to acheive orbit in 4 days. An
ion thruster operating at an Isp of 5000 s over a delta V of 8000
m/s only converts a maximum of 15% of the electric power to useful
work with about 15% of the initial mass as propellant. You can
improve the situation by lowering Isp but then your propellant
loads increase. At 1250 s you can theoretically get the required
46.3% but now 48% of the initial mass is propellant with the rest
of the airship made up of nothing but solar cells. It just doesn't
add up even ignoring the impossibly high L/D ratios required.

Those are actually two fairly common educated-amateur level
mistakes in electric propulsion and rarefied gas dynamics,
respectively, and if you grant those two mistakes I can almost
make the numbers work for the rest of the system.

I would be interested in seeing what you can come up with those
stipulations.

Jim Davis

Jim At 1250 s you can theoretically get the required 46.3% but now
Jim 48% of the initial mass is propellant with the rest of the
Jim airship made up of nothing but solar cells.

Not that I think the concept flies, but:

This airship wants to keep a particular airfoil shape as it rises.
Perhaps the fuel for the ion thrusters is the buoyant gas. If half
the initial mass is the buoyant gas, and they dump nearly all of
that as the atmospheric pressure drops off, then the change in size
of the airbag is diminished, which might be a good thing if you are
trying to keep the shape constant. You lose buoyancy as you dump
gas, of course, and as far as I understand you need a *lot* of
velocity before your lateral velocity generates much centripetal
acceleration on its own, so this doesn't balance out.

As far as I know, COTS ion thrusters use heavy elements like cesium
rather than light elements. Presumably lighter ions dropping across
the same electric field will get higher velocities, so I would
imagine that using hydrogen or helium as a fuel would involve higher
Isp and lower thrust. Lower thrust seems very bad for something
trying to punch out of the atmosphere, as you point out.

Maybe they use space mirrors to boost the available sunlight! A
gigantic airship might be a pretty good target for a LEO mirror,
and a 10x increase in sunlight density, especially if you
selectively target the more efficiently converted wavelengths,
will markedly improve the power-to-weight ratio of the solar cells.
But you'd need a lot of these mirrors to keep the power level up
as they zoomed by overhead.

Mirrors won't do squat for the power-to-weight of the ion thrusters
or power handling equipment though. Maybe it helps a little that
the gas in the bag is heated to high temperatures, but maybe that
just makes the bag material problem even more ridiculous than it
already is.

A more fundamental problem with this approach, is that the craft is
explicitly supposed to be *reusable*. If they exhaust any great
fraction of their lift gas on the way *up*, how do they get *back*?
They're going to need that H2 to keep from falling out of the sky
as they complete their re-entry.
Maybe not. The orbiter airship is a mile long and it only needed the gas to
float while it slowly accelerated to orbital speed. Coming back in, it is not
supposed to make a ground landing, [too big and fragile] but is going to make a
stall landing at the high altitude base from which it departed. This might be
possible with a little gas pressure and some aerodynamic flying, rather than
floating down to base after all the speed has been scrubbed. ??
^
//^\\
~~~ near space elevator ~~~~
~~~members.aol.com/beanstalkr/~~~