From: schillin@spock.usc.edu
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.
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