Half a croissant, on a plate, with a sign in front of it saying '50c'
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Solar Ramjet

Thermal Propulsion
(+4, -4)
  [vote for,

This can be made from a plastic kitchen funnel. Make one side black with marker or paint and put it in the sun with the dark side up. The air in the cone of the funnel will heat up and expand. New, unexpanded air will be sucked in through the tube. After being left out in the hot sun for about 20 minutes, air will be screeching through it and about 15 lbs of thrust will have developed. With two of these, this is enough to propel a skateboard + rider at up to 27 mph.
jmvw, May 14 2012


       //New, unexpanded air will be sucked in through the tube.//   

       Why? If the air in the funnel is expanding as it would be expected to, it will also expand back down the tube, preventing any air from moving in.
MechE, May 14 2012

       It's much easier for the air to go the other way. Air, being lazy, will not want to go back into the tube.
jmvw, May 14 2012

       If the air's momentum isn't enough to keep it flowing, maybe it needs a gas-turbine-like arrangement, with a small fan blowing air into the black-painted expansion region, and a large fan tapping some of the thrust to turn the smaller one.   

       Would heat be transferred into the air quickly enough for this to work?   

       //about 15 lbs of thrust will have developed//   

       Where did you get this figure? Did you calculate it or measure it? Either way, could you expand on this a bit?
Wrongfellow, May 14 2012

       This is an excellent idea, and I do not think we should let the fact that it won't work, has no grounding in reality, and cites spurious and irrelevant numbers detract from that by one iota.
MaxwellBuchanan, May 14 2012

       [jmvw], I have two questions:   

       A) If this could be constructed using only a plastic funnel and some black paint, why didn't you make one and find out for yourself that it won't work instead of throwing it up here for us to ridicule?   

       3) If something so simple could actually provide motive power, don't you think it would be in common use by now?
Alterother, May 14 2012

       1. I may or not may have built this myself. But if I did (and I am not saying that I did), I might not be able to share this information with you as my life could be in danger because of the obvious economic importance in skateboard propulsion. I have to protect my work and I may or not be in the process of perfecting this invention (by selecting the best type of marker and so on).   

       2 //Where did you get this figure?//   

       Thank you! I think sitting at a desk, eating bread and drinking beer really helped me get where I am today.   

       2. //could you expand on this a bit?//   

       The air expands on the outlet side of the funnel.
jmvw, May 14 2012

       //      I might not be able to share this information with you as my life could be in danger because of the obvious economic importance in skateboard propulsion. //   

       Okay, you're forgiven.
Alterother, May 14 2012

       Do you think an old fashioned metal funnel might work better?
pocmloc, May 14 2012

       About the same, I would think.
jmvw, May 14 2012

       I don't think so; beyond the obvious weight issues, there is also thermal retention to consider. There would be an advantage in conductivity, but I don't see it outweighing the benefits of a lightweight plastic construction.   

       Of course, a carbon-fiber funnel would be ideal.
Alterother, May 14 2012

       Bun for the exactitude of the estimated 27 mph skateboarder and because it inspired my time machine idea.
doctorremulac3, May 14 2012

       Thank you for keeping this going. I appreciate the comments, I really do. And I agree, carbon fiber is a neat material for certain specialized applications.   

       But now I have no idea what got into me to write this nonsense. It's nice weather outside and getting later and I haven't gotten much work done yet today. It's really time to stop playing and instead I need to contort myself back into the sullenness that puts bread on the table.   

       Dr. Remulac was right with his mock idea. My idea is worthless.   

       So I apologize for wasting everybody's time in such a trivial way. I'll come back if I think of a more effective way to waste your time.
jmvw, May 14 2012

       The real problem with this idea is that you didn't couch it in copious technical-sounding verbiage, nor did you back up your point with several links of questionable relevance and scientific value. Observe:   

       skateboard propulsion by means of solar-powered simulated "seebeck effect"   

       massive energy can be generated at nearly no cost except for paint and kitchen utensils by applying free ubiquitous sunlight and converting it to heat energy   

       Through means of the well-known Seebeck Effect large quantities of power can be generated by heating the junction of two dissimilar metal wires. As the point where the wires connect has increased heat applied electricity (which is just a form of power useful to do work) is produced. A similar process to the SE can be achieved by replacing the wire with two sides of a common kitchen funnel and applying a coating of black paint in opposition to the white outside of the funnel. Thus creating a dielectric that can be operated with no electrical charge, but providing the basis for the SE to apply to the ambient air instead of charge carriers in the funnel (instead of the wire).   

       There are many processes for converting trapped solar heat energy to kinetic energy, and such a process could be applied to the air that the dome of the funnel surrounds. Air in general will undergo expansion when subjected to rapid heating, leading to movement and the transfer of energy from the heat form to the kinetic form. As photons strike the coated interior funnel surface they bounce off but leave a heat signature that is depleted rapidly by cooler air. But then the cooler air in the funnel expands through the kinetic transfer process forcing the now cooler air out through the neck of the funnel, completing the open Newtonian system. The ejection of the cooler air and the partial vacuum created in the space the air previously filled causes the propulsion of the skateboard. Based on the composition of sea level air being 78.084% N2 and 20.947% O2, with trace gasses including Ar (0.934%) and CO2 (0.033%) and Ne (in quantities of 18.2ppm), the rate of propulsion would be enough to accelerate a 60kg rider and skateboard total to a speed of up to ~72.5 kilofurlongs/fortnight.   

       While looking at the links to various kitten videos provided, think of the kitten as the funnel and the fur as the paint, and the piano music as the air rushing in the funnel and out through the neck.
ytk, May 14 2012

       //It's really time to stop playing and instead I need to contort myself back into the sullenness that puts bread on the table. Dr. Remulac was right with his mock idea. My idea is worthless.//   

       Crazy entertaining ideas are worth a lot more than sullen table breading. Sometimes being the bread-winner should just consist of getting a couple of buns from the Halfbakery. We all need a break now and then. [+]
doctorremulac3, May 14 2012

       //in the hot sun for   

       As a UK citizen I have to ask what is this "sun" you speak of? We have no knowledge of this mysterious, possibly legendary, phenomena.
not_morrison_rm, May 14 2012

       //Thatcherism came in and flares went out of fashion   

       which was the greater tragedy?   

       jmvw - you might as well run this idea past S&P, they signed off on the collateralized debt, so they probably sign this one off too..
not_morrison_rm, May 15 2012

       //flares went out of fashion//

They did? Oh dear. Time to go shopping again.
DrBob, May 15 2012

       [jmvw]--never apologize! Take your lumps with pride, use incoming ridicule for targeting data, and turn all derision into motivation! Stay proud, my friend.
Alterother, May 15 2012

       /wasting everybody's time/   

       I was thinking about this.   

       1: Only thin part of funnel is black.   

       2: Thin part is not only black but also freaking long. Maybe a metal tube attached to funnel. Maybe it has fins too - stylish, but also to increase surface area and captured heat. Or maybe it is flat with just a slot for air to pass.   

       3: /it will also expand back down the tube- MechE/ Inside funnel, juncture of cone and freaking long thin part has a propellor which spins only one way. It acts as a valve. Air cannot exit the long part back into the cone. It must shoot out the back.   

       4: A series of these could be placed atop a skateboard. The passenger (for purposes of the video) would be a gnome, bobblehead or comparably low mass test subject.
bungston, May 16 2012

       A small amount of the airflow should be diverted through a cutout, which would cause a sound to be generated.   

       Thus your over-speeding skateboard would wail its way, Stuka-like, through traffic... Safety feature, don't you think?
lurch, May 16 2012

       Only for a couple of years. Later in the war, ground- pounders learned that the Stuka siren meant 'he's diving in a straight line, if we shoot at him there's nothing he can do about it'. Considering how irritating it is when skateboarders are weaving through traffic, the same phenomenon would probably take hold.
Alterother, May 16 2012

       //As a UK citizen I have to ask what is this "sun" you speak of?//   

       As a Seattleite, I call him 'BOB', as in Big Orange Ball. He goes into semi-hibernation from mid-September to mid-April, occasionally poking his head out between the nice cushy clouds. He's shy, though, and any mention of BOB quickly results in his return to hiding.   

       When he does finally wake up in late Spring, he hangs out for a while and seems to be quite friendly.
Freefall, May 17 2012


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