"SpaceShipTwo could be single stage to SUBorbit"
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From: Fevric J. Glandules on 22 May 2010 16:16 Pat Flannery wrote: > On 5/21/2010 2:50 PM, Fevric J. Glandules 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? > > 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. Oh, I understand that bit. But I don't understand why controlling bouyancy is such a problem. All you need, or perhaps that is "all" you need, is to regulate the volume displaced by the lifting gas cells. This could be done using *pressurised* ballonets. What am I missing? > 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 Thanks, that's some interesting stuff. If I've got this right, in a blimp the system is essentially stable and self-compensatory. As the blimp gains altitude, the lifting gas expands, due to reduced pressure; but the weight of displaced air remains the same, as it follows the same equation, and hence the blimp continues to have neutral bouyancy [1]. The whole system operates at essentially ambient pressure, with a bit of over-pressuring to keep the envelope taught [2]. My proposal involves running the system at higher pressures; have pressurised ballonets [3] that reduce the volume of lifting gas as required. That would involve a non-stretchy gas cell. Or use another method: have stretchy gas cells contained within a net. By mechanical means, compress the gas cells to reduce the volume of air that they displace. This might be expensive in terms of energy, but it would only be for fine-tuning: the main lifting cells would operate on the blimp / ballonet principle. What am I missing? What is so difficult that it requires condensing exhaust gases, etc., etc.? [1] Ignoring temperature changes, and assuming the ideal gas law, PV=nRT, says he, to demonstrate that he has actually studied physics to at least 'O' level, which you'd probably call "high school level". [2] Saggy blimps can't be good for business, after all. [3] inside gas cells inside a rigid airship.
From: Pat Flannery on 22 May 2010 23:06
On 5/22/2010 12:16 PM, Fevric J. Glandules wrote: > Pat Flannery wrote: > >> On 5/21/2010 2:50 PM, Fevric J. Glandules 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? >> >> 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. > > Oh, I understand that bit. But I don't understand why controlling > bouyancy is such a problem. All you need, or perhaps that is > "all" you need, is to regulate the volume displaced by the > lifting gas cells. This could be done using *pressurised* > ballonets. > > What am I missing? The weight of ballonets capable of doing that, which would cut badly into the lifting capacity of the airship. I once thought about using another concept; instead of pressurizing the ballonet, use a compressor to take some of the helium and store it in high pressure tanks till it's needed to generate lift again. But again, weight is the problem with that concept, although maybe you could make some sort of carbon fiber composite tanks that wouldn't weigh too much. An idea that has repeatedly come up in the history of both airships and balloons, is heating the lifting gas to decrease its density and increase its lift and controlling your altitude by varying the gas temperature; its a simple and elegant concept, but how exactly to efficiently heat the gas is the problem. Obviously, helium would be a far safer choice to do this with than hydrogen. A idea that has been used with success is to combine a hot air balloon with a gas filled one and use the hot air one to control the buoyancy. This idea goes clean back to 1785, although the first test of it proved fatal: http://www.spartacus.schoolnet.co.uk/AVrozier.htm http://xplanes.tumblr.com/post/221178130/aeronauts-end-of-part-1-jean-francois-pilatre > Thanks, that's some interesting stuff. > > If I've got this right, in a blimp the system is essentially stable > and self-compensatory. As the blimp gains altitude, the lifting gas > expands, due to reduced pressure; but the weight of displaced air > remains the same, as it follows the same equation, and hence the > blimp continues to have neutral bouyancy [1]. The whole system > operates at essentially ambient pressure, with a bit of over-pressuring > to keep the envelope taught [2]. That's the idea, by giving the helium room to expand inside the envelope as the blimp rises by the use of the ballonets, its buoyancy remains stable > My proposal involves running the system at higher pressures; have > pressurised ballonets [3] that reduce the volume of lifting gas > as required. That would involve a non-stretchy gas cell. There's something like that in use already called a superpressure balloon; in its case the balloon is filled completely with helium on the ground, and the gas can't expand as it ascends due to the strength of the envelope. These are used for high altitude meteorology in regards to winds, as they will ascend to a particular flight level and stay there: http://www.space-travel.com/reports/Successful_Flight_Of_NASA_Prototype_Super_Pressure_Balloon_In_Antarctica_999.html I don't know if anyone has used this concept in relation to a manned balloon though. > > Or use another method: have stretchy gas cells contained within a > net. By mechanical means, compress the gas cells to reduce the > volume of air that they displace. This might be expensive in > terms of energy, but it would only be for fine-tuning: the main > lifting cells would operate on the blimp / ballonet principle. > > What am I missing? What is so difficult that it requires > condensing exhaust gases, etc., etc.? It's a matter of weight again... though now we have a lot more sophisticated materials than they did in the age of the great dirigibles, so maybe something along the lines you are suggesting could be done. Considering that it was both successful and trouble free*, its odd the Navy didn't follow up the metal-skinned ZMC-2 with more work on that concept: http://nasgi.org/zmc2.htm The odd thing about the water recovery gear is that they were actually coming out ahead on the deal in regards to ballast water; not only was there water in the exhaust from the burning of the gasoline, but the water in the air the engine was ingesting would condense also, so you ended up with more weight in water than the weight of the gasoline you were burning. You can see the water condensers on this photo of the USS Macon: http://www.history.navy.mil/photos/images/g440000/g441983.jpg They are the lines of rectangular things above the engines. > [1] Ignoring temperature changes, and assuming the ideal gas law, > PV=nRT, says he, to demonstrate that he has actually studied physics > to at least 'O' level, which you'd probably call "high school level". > [2] Saggy blimps can't be good for business, after all. > [3] inside gas cells inside a rigid airship. They have actually gotten hot-air blimps to work, although obviously they can't fly any too fast or the bow will cave in due to the headwind: http://www.personalblimp.com/ ....not for use in high winds. ;-) * Which by Navy standards meant it didn't crash, unlike all the other rigid airships we had other than the one we got from the Germans. The one the British built for us came apart in mid-air before it could even be delivered. Pat |