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Colt
2003-Feb-16, 07:27 AM
I was wondering how much hydrogen (or helium, if you know) gas it would take to lift 100 pounds? Anyone know how to figure this out? This is discounting the rigging and gas envelope.

How this relates to astronomy: Why haven't we ever tried to lift a launch vehicle to a very high altitude and then released it? With this we could even launch it directly on the equator. I would think that as the LTALV (Lighter Than Air Launch Vehicle) rose higher the chances of an accident with hydrogen would lessen as there would be less oxygen to combust with.

How might the lifting values of hydrogen/helium increase or decrease in a thicker or thinner atmosphere (Venus, Mars)?

Thanks. -Colt

Glom
2003-Feb-16, 10:46 AM
There is a slight problem that Archimedes principle states that upthrust is equal to the weight of the fluid displaced. As you get higher, where the air is less dense, you're displacing less air and so upthrust drops. There's only so high up you can go. How much you'd need would depend on how high you want to go.

kilopi
2003-Feb-16, 10:48 AM
Hydrogen advocates discount the dangers of hydrogen, especially since hydrogen weighs half of what helium weighs--still, it's the difference that makes up the lift. Nitrogen, which makes up most of the atmosphere, weighs seven times as much as hydrogen (oxygen weights eight times as much), so each mole of hydrogen can lift a little more than 12 grams. Helium lifts only 10 grams. Lawn Chair Larry (http://www.darwinawards.com/stupid/stupid1997-11.html) used 45 4' diameter weather balloons filled with helium. Let's see, that's 45 x 4/3 x pi x 23, or 1500 cubic feet. Hmmm...maybe I did something wrong.

John Kierein
2003-Feb-16, 04:58 PM
This is a good idea. So-called "rockoons" (rockets on balloons) have actually been planned and small ones launched.

Glom
2003-Feb-16, 05:18 PM
But what's their payload capacity? With a booster and a few tonnes of payload, I'd imagine it must take a helluva load of helium to do it.

Dickenmeyer
2003-Feb-16, 06:29 PM
According to the book ZEPPELIN: Rigid Airships 1893-1940 by Peter W. Brooks typical gross lift of helium is 1,070 kilograms per 1000 cubic meters. Or to look at it another way, just a hair over one kg/cu. meter. A rough estimate on the calculator at 2.2 pounds per kilogram yields approximately 45.455 cubic meters of helium to lift a gross load of 100 pounds. Since you WILL need lifting capacity for the gas bag, some sort of launch control and monitoring devices and some sort of support structure for the rocket your helium requirements will go up accordingly.

Glom
2003-Feb-16, 06:31 PM
And efficiency drops with altitude as air density, and therefore upthrust drops.

Kizarvexis
2003-Feb-16, 10:50 PM
Two nuts (IMO), a French man and an American woman, plan to use a ballon to go to 130,000 feet and parachute back to earth. At that height, the atmosphere is effectively a vacuum and both the nuts (IMO) expect to break the sound barrier when they hit thicker air during their skydive.

Here is a Popular Science article on them.

Jump! Jump! (http://www.popsci.com/popsci/aviation/article/0,12543,409394,00.html)

At least it deals with a ballon taking a payload to 25 miles or so.

Kizarvexis

_________________

"We don't just borrow words; on occasion, English has pursued other languages down alleyways to beat them unconscious and rifle their pockets for new vocabulary." - James D. Nicoll

<font size=-1>[ This Message was edited by: Kizarvexis on 2003-02-16 17:51 ]</font>

Colt
2003-Feb-16, 11:57 PM
No one has really given me an answer as to why an LTALV would not work. /phpBB/images/smiles/icon_wink.gif The nuts that are trying to get the altitude record for jumping are using a type of baloon that expands as they go higher and this somehow offsets the lift-loss. Here is a thread I started on the article: http://www.badastronomy.com/phpBB/viewtopic.php?topic=3415&forum=4

Does anyone know a metric to standard converter so I can convert this stuff? Thanks. -Colt

roidspop
2003-Feb-17, 03:11 AM
I remember a proposal back in the 70s (IIRC) for an "aerostat" of gigantic proportions which would have been used as a launch facility, among other things. It was supposed to have a transparent outer shell comprising a space frame and membrane covering, the whole thing to be about a mile in diameter and would have been stationed at about 100,000 ft. Lift was to be provided by solar heating of the enclosed air...a temperature difference of only a few degrees would suffice. There would have been accomodations for a large number of people; supplies and launch vehicles were supposed to be delivered by an elevator system. I gather small satellite carriers would have been assembled up there, then dropped for an air-start.

Maybe now that nanotube composites seem to be on the horizon, it might be time to reconsider this thing...Maybe it could make a stratospheric way-station for the space elevator when it's finally built?

Dickenmeyer
2003-Feb-17, 03:42 AM
On 2003-02-16 05:48, kilopi wrote:
Lawn Chair Larry (http://www.darwinawards.com/stupid/stupid1997-11.html) used 45 4' diameter weather balloons filled with helium. Let's see, that's 45 x 4/3 x pi x 23, or 1500 cubic feet. Hmmm...maybe I did something wrong.

The DirecTV guide for the program "What Were You Thinking?" on TLC this evening promised a fellow who took to the air with balloons. I assumed it was going to be Larry but it turns out to have been a fellow named Ian Ashpole (or something like that) who built a stack of 600 helium filled party balloons, rode it up several thousand feet and then cut himself loose and parachuted to the ground. Freaks abound. Colt, I don't see any reason why your balloon wouldn't work, John pointed out that rockets have been successfully launched from high-altitude balloons, I just don't know that it's the most practical or efficient way to do it. I know that several small satellites have been placed into orbit by Pegasus rockets launched from high flying airplanes, perhaps those are favored because of control issues.

Colt
2003-Feb-17, 03:52 AM
Actually, there have been plans for a giant metal sphere-city. It would be so massive that the heated air inside would be enough to keep it aloft.

Is there any law against making your own LTA craft? I don't think you would need a pilots license since you would not be piloting it. -Colt

johnwitts
2003-Feb-17, 04:16 AM
Why not take this one step further. Once you're above 99% of the atmosphere, why not just take the balloon with you into orbit? When it was time to come back, you could gently slow down in the upper atmosphere, for hours or days if necessary, and then gently drift home. It's so simple, it's genius!

Colt
2003-Feb-17, 06:05 AM
Anyone ever read Venus by Ben Bova? Not exactly the same thing but close. In that they somehow use vacuum for lift. I understand that since vacuum weighs nothing it would be lighter than the surrounding gases. -Colt

Charlie in Dayton
2003-Feb-17, 08:10 AM
Does anyone know a metric to standard converter so I can convert this stuff? Thanks. -Colt