"SpaceShipTwo could be single stage to SUBorbit"
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From: Fevric J. Glandules on 21 May 2010 18:50 Pat Flannery wrote: > On 5/21/2010 12:41 PM, Fevric J. Glandules wrote: >> Greg D. Moore (Strider) wrote: >> >> [burnt fuel / weight problem with airships] >> >>> I thought they also collected atmospheric water (either rain or condensate) >>> for ballast also. >> >> I must confess that I don't understand the problem. It occurred to me >> that you can simply pump air into and out of a balloon *inside* a lifting >> gas cell to alter its bouyancy. A quick google revealed that this is >> the way blimps operate. What am I missing? > > On a blimp the "ballonet" doesn't alter the ship's buoyancy much, it > keeps the outer envelope stretched taut as the gas swells or contracts > either due to its changing temperature, or climbing and descending > making the exterior air pressure change around the airship, meaning the > lifting gas will take up more or less volume. > The Wikipedia article seems to place too much emphasis on the > pressurization of the gas bag, which is pretty slight...I would imagine > well under a pound per square inch. > In a rigid airship the gas cells need not be taut as the interior > framework maintains the airship's shape, so the gasbags can swell or > contract as it changes altitude. ....and I still don't understand the problem. The gas cells can be slightly over-pressurised, and made of material that doesn't stretch. Inside the gas cells are stretchy balloons, into and out of which you pump air as needed. What am I missing?
From: jimp on 21 May 2010 19:36 In sci.physics Fevric J. Glandules <fjg(a)invalid.invalid> wrote: > Pat Flannery wrote: > >> On 5/21/2010 12:41 PM, Fevric J. Glandules wrote: >>> Greg D. Moore (Strider) wrote: >>> >>> [burnt fuel / weight problem with airships] >>> >>>> I thought they also collected atmospheric water (either rain or condensate) >>>> for ballast also. >>> >>> I must confess that I don't understand the problem. It occurred to me >>> that you can simply pump air into and out of a balloon *inside* a lifting >>> gas cell to alter its bouyancy. A quick google revealed that this is >>> the way blimps operate. What am I missing? >> >> On a blimp the "ballonet" doesn't alter the ship's buoyancy much, it >> keeps the outer envelope stretched taut as the gas swells or contracts >> either due to its changing temperature, or climbing and descending >> making the exterior air pressure change around the airship, meaning the >> lifting gas will take up more or less volume. >> The Wikipedia article seems to place too much emphasis on the >> pressurization of the gas bag, which is pretty slight...I would imagine >> well under a pound per square inch. >> In a rigid airship the gas cells need not be taut as the interior >> framework maintains the airship's shape, so the gasbags can swell or >> contract as it changes altitude. > > ...and I still don't understand the problem. The gas cells can be > slightly over-pressurised, and made of material that doesn't stretch. > Inside the gas cells are stretchy balloons, into and out of which you > pump air as needed. > > What am I missing? A clue as to the state of materials technology when airships were in fashion? -- Jim Pennino Remove .spam.sux to reply.
From: Fevric J. Glandules on 21 May 2010 20:14 jimp(a)specsol.spam.sux.com wrote: > In sci.physics Fevric J. Glandules <fjg(a)invalid.invalid> wrote: >> >> ...and I still don't understand the problem. The gas cells can be >> slightly over-pressurised, and made of material that doesn't stretch. >> Inside the gas cells are stretchy balloons, into and out of which you >> pump air as needed. >> >> What am I missing? > > A clue as to the state of materials technology when airships were in > fashion? I'm talking about *now*.
From: jimp on 21 May 2010 20:25 Fevric J. Glandules <fjg(a)invalid.invalid> wrote: > jimp(a)specsol.spam.sux.com wrote: > >> In sci.physics Fevric J. Glandules <fjg(a)invalid.invalid> wrote: >>> >>> ...and I still don't understand the problem. The gas cells can be >>> slightly over-pressurised, and made of material that doesn't stretch. >>> Inside the gas cells are stretchy balloons, into and out of which you >>> pump air as needed. >>> >>> What am I missing? >> >> A clue as to the state of materials technology when airships were in >> fashion? > > I'm talking about *now*. A clue as to the demand for airships *now*? -- Jim Pennino Remove .spam.sux to reply.
From: Pat Flannery on 21 May 2010 23:34
On 5/21/2010 2:50 PM, Fevric J. Glandules wrote: > Pat Flannery wrote: > >> On 5/21/2010 12:41 PM, Fevric J. Glandules wrote: >>> Greg D. Moore (Strider) wrote: >>> >>> [burnt fuel / weight problem with airships] >>> >>>> I thought they also collected atmospheric water (either rain or condensate) >>>> for ballast also. >>> >>> I must confess that I don't understand the problem. It occurred to me >>> that you can simply pump air into and out of a balloon *inside* a lifting >>> gas cell to alter its bouyancy. A quick google revealed that this is >>> the way blimps operate. What am I missing? >> >> On a blimp the "ballonet" doesn't alter the ship's buoyancy much, it >> keeps the outer envelope stretched taut as the gas swells or contracts >> either due to its changing temperature, or climbing and descending >> making the exterior air pressure change around the airship, meaning the >> lifting gas will take up more or less volume. >> The Wikipedia article seems to place too much emphasis on the >> pressurization of the gas bag, which is pretty slight...I would imagine >> well under a pound per square inch. >> In a rigid airship the gas cells need not be taut as the interior >> framework maintains the airship's shape, so the gasbags can swell or >> contract as it changes altitude. > > ...and I still don't understand the problem. The gas cells can be > slightly over-pressurised, and made of material that doesn't stretch. > Inside the gas cells are stretchy balloons, into and out of which you > pump air as needed. > > What am I missing? That the total ability of the airship to lift its own weight and its payload is a fixed amount, and the fuel has weight, so as the fuel is burnt its weight is lost and the airship becomes lighter and tries to rise to a higher altitude. The ballonets inside the envelope aren't stretchy, they are made of gas-proof material and would look like plastic bags, not rubber balloons. At sea level they are almost fully inflated, and as the blimp climbs and the helium expands, they become progressively less full. The crew of the blimp doesn't control this like blowing ballast on a submarine; a pair of intake ducts are mounted behind the blimp's propellers and the backwash of the props goes up the ducts to and ballonets and pressurizes them as needed to keep the envelope firm. Here's a good photo of the intake ducts: http://realneo.us/system/files/Goodyear-Blimp-P1300304.jpg At some point, at fairly high altitude, the ballonets would be completely empty, and then you would have to vent helium or risk having the envelope rupture if you climbed further. There is an interactive cutaway of a blimp here that lets you click on the parts and will show the interior ballonets: http://science.howstuffworks.com/blimp1.htm Their description of them isn't quite accurate, as they seem to suggest that if you let air out of them the airship will ascend; in fact, all that would happen is that wrinkles would start to form in its envelope as it would no longer have its interior volume totally filled with either helium or air. You can make the blimp pivot nose-up or nose-down though by venting air from either the forward or stern ballonet respectively, as the other one will swell up more to still keep the envelope firm. They also look too large in comparison to the size of the envelope in that drawing; an airship with that large of ballonets would be able to climb to around 20,000 feet before the ballonets were fully emptied, but would lose a lot of lift by devoting so much of its internal volume to the ballonets rather than helium. Pat |