Solar hot air balloon?

I admit it, I'm a total newbie, having only had one ride in a hot air balloon. I'm a paraglider pilot. And I suspect this has been discussed and discarded many times as yet another crackpot idea. I looked for FAQ's and didn't find anything, so I'm posting my questions to the newsgroup without reading it for awhile. My apologies.

I have to wonder, with the improvement in materials and techniques, could it be possible to build and fly a totally passive solar hot air balloon?

I suspect that historically the objections have been that such a balloon is: 1. Big 2. Slow to heat up.

But perhaps the materials today would allow you to build a quasi-insulating two-layer bag, where the flimsy outer bag can be rotated to present either the half that is reflecting or the half that is transparent so the absorbtive surface of the inner structural bag can heat up in the sunlight, which weighs less than 200 lbs? 100 lbs? Perhaps there's no gondola, and the single pilot just wears a harness.

But the other problem is that by the time the sun has been up long enough to heat it up enough to take off, the ground has been heating also and the air is becoming active. I'm guessing that the thermals and rotors aren't that much of a problem when you're at a high altitude, but landing is dangerous in gusty mid-day winds.

So, how about if the *standard* landing was not to ride the balloon to the ground, but to cut away and parachute to the ground while still at a very comfortable altitude? Maybe you trigger an elastic motor or something that wraps the envelope up into a small enough package that it's descending at a rate a little greater than your minimum sink rate under canopy. So you can fly over to it, latch on, and guide it to someplace where it can crash without causing danger to persons or property on the ground. Then you release it at maybe 100 feet and finish setting up your own landing.

So then the only problem is when you're close to the ground, mostly at takeoff. Takeoff is going to be prolonged due to the large size and hence slow ascent rate. Perhaps one should launch at the edge of a cliff so that one would very quickly have sufficient altitude to safely deploy a parachute. I can find places where the wind is blowing off of an open launch area. Or maybe you'd launch when the wind is blowing up a cliff, so that you'd be carried rapidly up above it. You could wait until a thermal was coming through, and cut loose from the ground in the middle of the thermal, so that you wouldn't just be blown back into the rotor area behind the hill.

This does seem like a lot of trouble, but remember, the flight might be six or more hours long.

When you consider the set of all aircraft for which the number of landings do not have to be equal to the number of takeoffs, the possiblities are enormous.

The balloon that I fly is 90,000 cu ft of air. I have been told that one burner is the equivalent of 10,000,000 BTUs/hr of heat flow (4920 hp). How much would a solar array weigh if it were big enough to put out that amount of energy? Note that the balloon is about 600 lbs plus four FAA Standard Bodies (150 lb each!) is a gross weight of 1200lb. Please post the results of your research.

Back in about 1978, I remember a Aerostar (Raven) built using 1/2 upper clear fabric. two small engines with props were mounted, and could be used to turn the clear section in the direction of the sun.

As I recall, it flew pretty well, except that it still used some fuel to get airborne, and flying in the middle of the day to catch the best sun was hell on the thermals!

It did, however, attract a lot of interest at the balloon races it flew in. Anybody remember who that was? Was it Schoo?

Reed,

Solar balloons have indeed been constructed, and flown successfully, but I must admit some of your ideas show a VERY active imagination!

Randy Danneman Fantasy Flights Silver Spring, Maryland http://www.airtravel.com/fantasy

Tracy Barnes built and flew a tetrahedron solar balloon. Not real practical, butproves the concept. As mentioned, there was a partially clear balloon with small fans to position the solar collector. Also was a balloon with black curtains, and I believe a black AX-4 inside a clear AX-7 (not sure if this ever flew, there were problems with keeping them in proper relative position.

It's also been conjectured that solar powered flight may have been achieved by ancient Egyptions. Wasn't it Archimedes who theorized about a huge copper sphere with a vacuum? Huh! Hanger flying before there were hangers! <G>

Try a light weight black plastic bag on a sunny day. Then think big.

You could check my math here: the solar constant is 2 cal/min/cm2 which converts to 441 BTU/hr/sq.ft. The projected area of a 90,000 cu ft sphere is 2428 sq. ft., so your balloon would collect up to 1.072E6 BTU/hr, a factor of 10 less than your burner puts out. How much of the time is your burner on? If a double envelope were used, how much would that reduce heat loss, especially if it were reflective on the side away from the sun? How much lighter could a balloon be made, if you were wearing a parachute, and could accept a 1/100 chance of it failing? Bill Arras made a 54,000 cu. ft. balloon which weighed 75 lbs.

On Sat, 17 Oct 1998 15:36:37 -0400, 'William W. Plummer'

There have been several attempts at Solar balloons. One of the best was a balloon that one that half of the balloon was clear and the other half fabric was black on the inside and silver on the outside. Fans were attached at the equator to turn the clear side toward the sun to climb and toward the silver side to cool off. Also the balloon had a normal burner for Inflation and quick manurvering. The ballon was developed by an Iranian Expat Fredric Essoo, (Spelling may not be correct) and built by Aerostar, then called Raven. if you can find old (Seventys) issues of Ballooning Journal you will find an article on the system. A friend of mine took a ride in the thing. The main problem I heard about was the longevity of the clear fabric. But from the article I read, a three hour flight used less than 10 gals of propane.

You might contact the National Balloon Museum in Indianola they may have other material on the system

Phil Booth