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I envision an airship, probably a balloon but maybe a long
tube made of carbon fiber or something of the sort. It
would operate in the upper atmosphere where things are
cold. Air enters the front and is heated by the black sides of
the tube. The hotter air has more pressure and shoots out
of
the back, providing propulsion.
This is a ramjet in that atmospheric air entering the front is
used as reaction mass out the back. But there is no
combustion here. Maybe there is a propeller in the front to
encourage air to go inside in the first place.
Solar Hot-air balloons
https://www.google....AQ&biw=1280&bih=595
1st linked to by DrCurry [2 fries shy of a happy meal, Mar 03 2014]
In the meantime...
http://en.wikipedia...a_Solar_Balloon.jpg
..if anybody feels like geeking out on the numbers behind solar balloonary [doctorremulac3, Mar 03 2014]
Solar balloon with draft jet drive
https://www.dropbox...riq/SOLAR%20JET.jpg
This is looking at the back. Air comes in the front bottom, heats up, shoots out that little notch on top. [doctorremulac3, Mar 05 2014]
Self-erecting solar tower
Self_20Erecting_20Solar_20Tower
All this blah blah and zentom beat me to it by 8 years. If I had a clearer idea what I was posting when I posted it maybe I would have turned this up. [bungston, Mar 11 2014]
[link]
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Maybe a little shill out front touting the hot naked air inside and the low cover charge ? |
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You might have to make use of the "pulse jet"
principle, to ensure the hot air only exits in one
direction. Ram jets only work when air is entering
the engine at supersonic speed. |
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That is right. Maybe globs of air could move Down the tube
thru chambers with connectors that open and close. 1 and
2 would connect and 3 and 4 would connect. Then 1 would
be open to the from, 2 and 3 would connect and 4 would
connect to 5. Like locks in a canal. |
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I thought high altitude for this would let it be heated more
readily by the sun (less convective cooling). I am not sure if
that is true for any altitude where there would be enough
gas mass to propel an aircraft. |
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If the air is hot on the solar side and cool on the shaded side, those two regions of air will move at different speeds. |
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If it is easy to leave the hot side, but just a few obstacles on the cool side. Maybe just a rough and smooth finish on inside of the tube. |
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Maybe paint black on top in the sun and white in the shade. |
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A saw tooth surface might cause the air to go slighty more in one direction, after picking up heat from the wall. |
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NASA must have a simulation program for air at those pressures and temps. |
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I agree with [Vernon] - ramjets are
counterintuitive beasts which rely on the weird
behaviour of gases at very high velocities. That's
not to say this is impossible, but it might be
impossible. |
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A pulse-jet would indeed be more feasible. |
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As for altitude and gas density - that's tricky too.
Less density means you can reach higher speeds
for any given thrust, so the tradeoff might work in
your favour (or against it - I don't know enough to
tell). |
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Make it a turbojet, with a lightweight propeller in the front drawing air in connected by shaft to a propeller in the exhaust stream, with the props blades canted appropriately to give it enough torque to power the front prop. |
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Put an axis on the tube, and spin it, to even out temperature differences, or just lots of bacofoil reflectors for the underside. |
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I like spinning it. It would be long and thin. I think
the whole thing should be black to maximize
heating. It is really more of a flying solar chimney
than anything to do with jets. The interior locks
compensate for the fact that it is not vertical. |
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//flying solar chimney// That's the ticket.
Turboshaft-style. Make it a vertical chimney,
propeller inside the chimney spun by the rising air
drives other props on the outer sides for horizontal
motion. |
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// //flying solar chimney// That's the ticket.// |
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I think that catching that upwards moving hot air
with a turbine then transferring it to propellers
wouldn't provide as much thrust as just blowing the
rising air out of the rear of the top, jet engine style.
Friction and transfer loss of the mechanism eh? The
simple jet thing at the top might work. |
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//The interior locks compensate for the fact that it is
not vertical.// |
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Why not make it vertical? Big long solar chimney, the
body an elongated torus, (whatever that's called)
heated air coming up through the center is directed
backwards once it hits the top propelling the thing
forward. The other thing you could do is have the
whole thing spinning using vanes lining the interior
duct then have a very lightweight propellor at the
top that's on a bendy thing so it droops down and
faces forward. No gears and very little friction.
Seems like you'd probably get some movement out of
it. |
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The suggested pulse jet thing might work, just very
slowly. I guess you'd open a shaft down the middle to
let air in, close both ends and let the air heat up
then open one end to squirt if forward. |
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This is one of those back-burner ideas I want to see made so much that make my fillings ache thinking about it.
Honestly.
Keeps me up at night sometimes. |
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It can be done though. All kinds of funky shapes and vortices and pressures involved. Very pretty. Massive implications when it finally gets built too. Floating airports and such...
Wish I was going to be alive long enough to see it real. (+) |
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Well, let's not get too excited. If you're designing a
system meant to move things you need to take into
consideration the useful energy you have available to
move the thing, in this case sunlight, then compare it
to the weight of the thing being moved, resistance /
friction the object will come up against etc. Then
you have to figure how well that energy going in is
going to stand up to the job at hand, in this case
pushing a heated gas envelope through the air. Then,
the least fun part, you need to compare the new
system to the old way of doing it. Our
solar jet would have to compare favorably to solar
panels supplying, 8 watts or whatever per square foot
and turning motorized propellers. |
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Solar panels and motors can be pretty light and I'd
guess they'd be the best way to harness the sun to
move an airship, but that doesn't mean a solar jet
powered airship isn't worth making. I just picture the
thing puttering along on a windless day moving at a
couple of miles an hour at best and sitting there
squirting forward a couple of inches every few
minutes at worst. There's a set amount of energy
you're getting per square foot of sunlight and heating
air to shoot it out the back probably isn't the most
efficient way to use it. Keep in mind, the more air
you heat up to shoot out the back, the larger the
heating chamber needs to be and the larger your ship
has to be. That causes your other end of the
equation, the numbers regarding ship itself, to have
more weight, more friction going through the air etc. |
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But I could be wrong, that's just my guess. That being
said, to make one of these things would be really
cool, and since that's the point anyway, I'd love to see
somebody pursue this. I suggest an airship with a very
long sleeve hanging down forming a chimney that
sucks air up, heats it and then hits an angle where it
meets the ship through which the heated air blows
out the back propelling the ship forward. I would
think such a chimney could be very light as the
heated air rising up through it would supply the
necessary structural stiffness to keep the chimney
open, a hoop at the bottom being the only stiff
structure necessary to keep the inlet at the bottom
open. |
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Another idea is to get big mylar parabolic reflectors
to focus energy on your "combustion chamber". That
would be a way to get a lot more sunlight into your
heating chamber with not a lot of weight. Of course,
massive mylar focusing reflectors make great wind
sails to so you unless you can figure some way to
mitigate that you've lost more than you've added. You
could have the bottom part of your envelope, which
is parabolic shaped, be mirrored and focusing on a
heating chamber running laterally through the center
of the thing while the top part is clear to let the sun
in. |
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Naa, chimney's the best way to do it I think. |
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//Well, let's not get too excited.//
I have to get a 'little' bit exited. I've built about fifty versions of this thing in my head already. I've also test flown/crashed several of them now and it will not only work, these systems working in conjunction will be able to suspend platforms indefinitely. |
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//If you're designing a system meant to move things you need to take into consideration the useful energy you have available to move the thing, in this case sunlight, then compare it to the weight of the thing being moved, resistance / friction the object will come up against etc. Then you have to figure how well that energy going in is going to stand up to the job at hand, in this case pushing a heated gas envelope through the air.//
Moving is the easy part. You then have forward momentum to force air in the direction you wish it to flow and wing camber to generate the extra lift lost by venting the heated gas.
Making platforms remain stationary without that forced directed air-flow is going to be the hard part, but I think I've got that figured out too. |
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// Then, the least fun part, you need to compare the new system to the old way of doing it. Our solar jet would have to compare favorably to solar panels supplying, 8 watts or whatever per square foot and turning motorized propellers. //
But... that's the funnest part of all. |
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You're just thinking about it wrong. That's the most fun part. All of the negatives become positives when turned on their heads.
The chimney effect need not be vertical.
Many horizontal chimneys in sequence with some geometric tweaking to super-heat individual pockets of air will produce thrust, and forcing them to directionally fight one another in groups will produce stable platforms with no planetary fuel source cost.
Enough excess solar energy can be stored throughout the day to keep the platforms aloft at night by focussing uv rays through underside apertures when the sun is down. |
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How much more cargo could be hauled in a single craft if this craft didn't need to ascend in order to gain cruising altitude?
Would these savings offset the cost of building new infrastructure over the long haul if it proves out? |
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//I've built about fifty versions of this thing in my head already. |
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Just how big is your head? |
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I only know that it's pretty echo-y in there. |
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My head has a surfeit of hot air if that helps. |
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Not sure there is enough energy in the sunlight
that would strike this to make useful work. |
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Also with a dependent tube of plastic sheeting
and a hoop at the bottom, if it really got flowing
then it could collapse the tube in on itself from
the bernoulli effect. One might need additional
hoops to prevent this. |
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I have a vision of a single balloon with eight of
these tubes below, zooming around the sky: an
ominous air squid. Of course glaring eyes would
be painted on the side. Must less useful that
2fries schemes. |
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It is annoying that one would still need some sort
of remote control to steer it. But if it is powered
by the sun there will not be fuel costs. Plus you
could have it descend on those whom you feel
might merit the gentle tentacled caress of the
rocket air squid. Then rise it back into the sky,
off on other mysterious errands. |
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//if it really got flowing then it could collapse the
tube in on itself from the bernoulli effect// |
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As much as I'm enamored with this idea, I'm afraid
that the tube collapsing would be among the least of
the design's problems. I'm thinking the air would rise
up at a couple of miles per hour at best and MAYBE
give you a little forward movement. |
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I will go on the record as guessing the best you could
hope for is something very slowly crawling around the
sky on a windless day. It wouldn't be good for much
more than entertainment but kites are entertaining
and they don't do much more than float around and
look pretty eh? |
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We should have a "Half Bakery To Do List" where
designs that have sparked some interest are listed
and people can pick and choose ones to actually
make. I think that may have happened with the
roving goldfish bowl. This might be another one. |
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I would love to see this crowd sourced and would contribute whatever I had to spare conceptually or monetarily to see this contraption actually built. |
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The air-squid design alone will muddle through the air and be able to travel following trade winds, but imagine much narrower, much more elongated squids laying side-by-each to form an enormous manta ray parafoil type of array.
Made entirely of industrial black plastic, when all exterior apertures are closed the air in the system alone is enough to generate lift for a payload as a massive solar-hot-air-balloon. |
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Opening forward and rearward facing vents would cause loss of lift and the parafoil shape would begin forward acceleration as the wing descends. The inflow of cold air from the front will pressurize the already heated air in the individual tubes and a series of baffle shapes within prevent backflow while creating back-eddies of increasingly super-heated air which must expand in order to escape but there is only one way for this air to go now because of the in-flow. The determining factor will be a ratio of individual tube diameter/length to pressure containment vs lift to weight.
(don't ask me to say that with numbers I can barely put it into words)
The rearward force of the heated air in addition to the lift generated by the shape of the wing itself will now be enough to overcome the loss of lift from venting the lifting gas and the whole thing will continue to increase in speed until drag or weather patterns interfere. |
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That's the Wright brothers version. Later versions using various materials and more intricate baffle designs and aerodynamic shapes will create very fast aircraft indeed. |
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The indefinitely suspended platforms are slightly down the road and through the woods from there. |
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2fries - I hope you have other current endeavors
that you can cultivate using the energy you seem
to have now! |
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Re baffle shapes - one could make it like the flaby
neck of a whoopie cushion. Pressurized air can
exit under pressure - otherwise the thing lies flat.
In the original conception of this scheme those
could serve as the locks between chambers. |
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The chimneys should not have hoops but rather
be flattened rectangles to maximize the surface
area / volume ratio and so maximize air heating. |
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Backing way off: a solar balloon. Is such a thing
possible? I am sure I have not seen one. Maybe
because it is so easy to heat air by other means
that people who want hot air balloons just fire up
the proane.
I can envision it: a mostly deflated, airtight black
sack. You would spread it out flat. It is heated by
the sun and the interior air expands, becoming
lighter relative to the exterior. After enough of
this goes on the thing becomes buoyant. |
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If you wanted to be pure, such a sack would
suspend the chimneys. It would sure be easier to
do that with a helium balloon and use the solar
power just for propulsion. |
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I actually drew this up and got something looking like
the wing of a 747 hanging down vertically. It's flat
side faces the sun and it's airfoil shape minimizes it's
forward facing surface area. It's wider top keeps it
floating upright and the air outlet nozzle at the top
front part can be moved to steer the thing. |
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Ooh! Can we see the drawing? Can we huh? Huh? Can we?! |
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I'm taking a kick at a couple of other cats [bungston]. If I can actually clip one or more of them it will have only been to eventually try and fund the building of this here idea.
On a smaller scale the one-man versions of these things are going to make for a very safe, extremely cool past-time.
The money would be in advertising unfortunately. It is going to be almost impossible not to watch these things flying around especially when illuminated at dusk.
It will be prohibitively expensive to own a custom kit, but flying a Cola or Harley billboard would decrease the price for the enthusiast substantially I bet. |
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Solar-hot-air balloons are awesome. The envelope doesn't need to be air tight either. [link] |
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I'll neaten it up and post it for review/comment. |
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I like the clear balloon with the black central structure. I
bet that would capture more heat than a plain black
balloon. |
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Solar balloons are so possible as to be old hat. |
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Because they work when open at the bottom, and
when relatively small and made of garbage bags
and sticky tape, a large, well-made one could be
fitted with a horizontal nozzle near the top to
provide thrust; just make the nozzle small enough
that it still
floats. So a dirigible, passive solar hot air balloon
seems trivial, and I'd be surprised if it hasn't been
done. |
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But it won't be anything like a ramjet. |
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It is not like a ramjet. I have corrected the title. |
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Thinking about black-in-clear solar balloons and doctorremulacs dangling plane wing design. I like because the thermal reservoir can be made of something more durable than balloon plastic. |
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I envision the wing balloon to be clear and thin, like a dry cleaner bag. The question is what to optimally make the black interior from. I think something with low specific heat would be best because we want the thing to give up that energy to the air around it. One could use silver foil which can be made very thin and also easy to keep black. It would, however, be a mess if the propulsion chimney got crumpled up on collision or landing. |
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If it is not going to get that hot in there you could use plastic. Given that there would be air passing by and that this will increase in velocity as it gets hotter, maybe the temperature will autoregulate. |
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About 50 feet tall, made out of trash bags. |
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That is a badass looking solar Sword of Damocles you depict, doc. |
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Once upon a time someone posted
/C'mon, bungston, at least do the math. How many leafblowers would you need to lift, say, a leafblower?/ |
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Now I wonder if the Sword could lift itself, with ducts pointed down. It is the leafblowers all over again. My math cannot get me far I estimate 25 m2 of surface area. I find summer sun cited to have 600 watts / m2 / hour. The part I cannot do is figure how that energy translates into heating of air, and the heating of air into work. |
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Once upon a time lurch stepped up with a phenomenal chalk talk on rotational inertia (for a proposed Pinewood Derby car). Lurch I feel bad about tapping you again for a seminar but if it is only every 5 years that is not so bad. |
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Nice sketch; but to make a proof-of-concept
prototype, I'd just make a boring solar balloon like
the one in the second link. Once you've got it to
work, make a small nozzle - such as a short piece of
drinking straw through a hole in the top, taped down
so it points horizontally. It should still fly, while
producing (feeble) thrust. On a very calm day, you
might even see it move. |
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If you had only a hole at the top it would simply
deflate. You need a separate chamber with a hole at
the bottom and at the top through which air is
replenished. |
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However upon further review of this design, I
estimate that
the
top speed under ideal conditions would be "some
perceptible movement" at best. |
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[bungston] - unfortunately (well, not really) I'm currently employed,
lacking a bit of free time. But I will haul off and call the lot of you a
bunch of pessimists... |
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You said something about 600 watts per square meter. The ideal
uncorrected value at Earth's distance from the sun is a little over 1300
w/m², so I'm willing to go with your number down at the bottom of the
atmosphere. |
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So we take a black plastic bag (10% reflective) that's 2 square meters
of lit surface - it's going to absorb over a kilowatt (about 1œ hp). |
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Then it has to either shed the power, or move. A horse-and-a-half on
a plastic bag doesn't sound like "barely perceptible" to me... |
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Very cool rendering [doc]. |
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//that's 2 square meters of lit surface - it's going to
absorb over a kilowatt (about 1œ hp).// |
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That's fine, but it's the mechanism to turn that
energy into forward motion that we're critiquing
here. Jets work by taking your reaction mass, air
mixed with burning fuel, and imparting incredibly
high velocity to it through compression and burning.
It's a lot of stuff leaving that tailpipe at hundreds of
miles per hour kicking everything forward. At least
with the thing I drew, we're talking maybe ten fifteen
miles per hour exhaust velocity at best. Depending on
the weight of that air coming out and kicking the
whole thing forward, we're probably talking... two
miles per hour at the most. I base that on absolutely
nothing but wild speculation. |
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I would say the best way to harness that energy is
with paper thin solar panels and very lightweight
motors turning very lightweight propellers, but that's
not interesting. The jet drive is. |
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That being said, somebody just needs to invest in
some garbage bags and tape and make one of these. |
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I spent a long weekend once heat-sealing a 50 ft. roll of black plastic film with one of those kitchen vac'n'seal units to figure out a lift to weight ratio.
I underestimated the wind above the roof of my place though and all the neighbours saw this gigantic black tentacle snag on my roof and go writhing out over the front yard like a slow motion inflatable-flailing-tube-dude. |
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//If you had only a hole at the top it would simply
deflate.// |
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Yes, but notice that the balloon in the second link
has a large hole in the bottom. It remains inflated
for the same reason common gas-powered hot air
balloons remain inflated - even if there is a small
amount of air loss from the upper part of the
balloon. A tiny pinhole near the top would make
very little difference; a huge hole would cause
rapid failure; somewhere in between, you will get
maximum thrust without losing lift. |
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Well, a compressed gas balloon wouldn't remain
inflated with a hole in it and the hot air doesn't
escape from hole at the
bottom of hot air balloons, solar or otherwise
because hot air floats upwards. There might be
enough coming through the bottom opening to
replenish the air leaving the top, but that air would
be cool and it would effect the buoyancy of the
balloon. Having a separate channel would mitigate
this effect. |
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That being said, the speculation part of this is
probably done, somebody just has to pony up the five
bucks to buy some garbage bags, tape and
the afternoon at the park on a summer day to test
this out. Sounds like a fun day out actually. If I
ever get around to this I'll post
pictures. |
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/ the speculation part of this is probably done / |
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Now, now. This armchair is so comfy. |
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I have been thinking about cleaners bags. I bet they come as
a long clear tube. They are lighter weight than trashbag. A
flying solar chimney could be a long clear tube with a piece of
black paper in the middle. Better - a tube of expanded black
paper. |
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It would need to either be hoisted by helium balloons or a
kite, or hung from a high point to get the draft started. I am
thinking the jet ducts at the top should be plastic. |
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Well, if you want to armchair it, all this stuff is computer modelable. The main number you'd need to find is how much draft you're getting up through that tube. Actually a computer would probably be overkill, a pencil and paper should do fine. |
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That being said, it's nice to actually get out and get some fresh air from time to time. |
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Ok, go time. Here's my design. |
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Standard garbage bag hot air balloon but with a
twist. |
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I'm going to take a bunch of cardboard tubes and
tape them together in an inverted L shape. The foot
of the L forming the hot air exhaust outlet. I'll then
form the bags around this mold and tape them so
there's a channel running up the side. So picture a
standard solar balloon with a tube formed into one
side. If it's a 2 foot wide balloon, the the tube will be
about 6 to 8 inches. Then put some extra tape at the
opening at the bottom and the outlet at the top to
hold the thing open, fill 'er up, set it in the hot sun
and watch what happens. |
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