Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
Clearly this is a metaphor for something.

idea: add, search, annotate, link, view, overview, recent, by name, random

meta: news, help, about, links, report a problem

account: browse anonymously, or get an account and write.



Flying solar chimney

Self propelled solar balloon.
  [vote for,

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.

bungston, Feb 23 2014

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]


       Maybe a little shill out front touting the hot naked air inside and the low cover charge ?
popbottle, Feb 23 2014

       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.
Vernon, Feb 23 2014

       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.   

       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.
bungston, Feb 23 2014

       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.   

       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.   

       Maybe paint black on top in the sun and white in the shade.   

       A saw tooth surface might cause the air to go slighty more in one direction, after picking up heat from the wall.   

       NASA must have a simulation program for air at those pressures and temps.
popbottle, Feb 23 2014

       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.   

       A pulse-jet would indeed be more feasible.   

       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).
MaxwellBuchanan, Feb 23 2014

       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.
Kansan101, Feb 23 2014

       Put an axis on the tube, and spin it, to even out temperature differences, or just lots of bacofoil reflectors for the underside.
not_morrison_rm, Feb 23 2014

       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.
bungston, Feb 23 2014

       Neat idea.
doctorremulac3, Feb 23 2014

       //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.
lurch, Feb 24 2014

       // //flying solar chimney// That's the ticket.//   

       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.   

       //The interior locks compensate for the fact that it is not vertical.//   

       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.   

       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.
doctorremulac3, Feb 24 2014

       This is one of those back-burner ideas I want to see made so much that make my fillings ache thinking about it.
Keeps me up at night sometimes.

       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. (+)

       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.   

       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.   

       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.   

       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.   

       Naa, chimney's the best way to do it I think.
doctorremulac3, Feb 28 2014

       //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.

       //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.

       // 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.

       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.

       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?

       //I've built about fifty versions of this thing in my head already.   

       Just how big is your head?
not_morrison_rm, Mar 01 2014

       I only know that it's pretty echo-y in there.   

       My head has a surfeit of hot air if that helps.
AusCan531, Mar 01 2014

       Not sure there is enough energy in the sunlight that would strike this to make useful work.   

       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.   

       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.   

       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.
bungston, Mar 01 2014

       //if it really got flowing then it could collapse the tube in on itself from the bernoulli effect//   

       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.   

       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?   

       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.
doctorremulac3, Mar 01 2014

       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.   

       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.

       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.

       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.   

       The indefinitely suspended platforms are slightly down the road and through the woods from there.   

       2fries - I hope you have other current endeavors that you can cultivate using the energy you seem to have now!   

       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.   

       The chimneys should not have hoops but rather be flattened rectangles to maximize the surface area / volume ratio and so maximize air heating.   

       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.   

       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.
bungston, Mar 02 2014

       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.
doctorremulac3, Mar 03 2014

       Ooh! Can we see the drawing? Can we huh? Huh? Can we?!   

       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.

       Solar-hot-air balloons are awesome. The envelope doesn't need to be air tight either. [link]   

       I'll neaten it up and post it for review/comment.
doctorremulac3, Mar 03 2014

       I like the clear balloon with the black central structure. I bet that would capture more heat than a plain black balloon.
bungston, Mar 03 2014

       Solar balloons are so possible as to be old hat.   

       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.   

       But it won't be anything like a ramjet.
spidermother, Mar 04 2014

       It is not like a ramjet. I have corrected the title.   

       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.   

       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.   

       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.
bungston, Mar 04 2014

       Did a sketch. See link.   

       About 50 feet tall, made out of trash bags.
doctorremulac3, Mar 05 2014

       That is a badass looking solar Sword of Damocles you depict, doc.   

       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?/   

       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.   

       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.
bungston, Mar 05 2014

       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.
spidermother, Mar 06 2014

       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.   

       However upon further review of this design, I estimate that the top speed under ideal conditions would be "some perceptible movement" at best.
doctorremulac3, Mar 06 2014

       [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...   


       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.   

       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).   

       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...
lurch, Mar 06 2014

       Very cool rendering [doc].   

       Thank you 2 frys.   

       //that's 2 square meters of lit surface - it's going to absorb over a kilowatt (about 1œ hp).//   

       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.   

       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.   

       That being said, somebody just needs to invest in some garbage bags and tape and make one of these.
doctorremulac3, Mar 06 2014

       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.


       //If you had only a hole at the top it would simply deflate.//   

       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.
spidermother, Mar 07 2014

       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.   

       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.
doctorremulac3, Mar 07 2014

       / the speculation part of this is probably done /   

       Now, now. This armchair is so comfy.   

       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.   

       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.
bungston, Mar 09 2014

       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.   

       That being said, it's nice to actually get out and get some fresh air from time to time.
doctorremulac3, Mar 09 2014

       Ok, go time. Here's my design.   

       Standard garbage bag hot air balloon but with a twist.   

       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.
doctorremulac3, Mar 10 2014


back: main index

business  computer  culture  fashion  food  halfbakery  home  other  product  public  science  sport  vehicle