#101
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article ,
Pat Flannery wrote: The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. ...Considering that I've seen propane cylinders for refueling lighters that have very thin aluminum walls, is it even necessary to chill it? It's desirable for several reasons. For the same pressure, wall thickness rises as the tank gets bigger, so big tanks won't be as thin as those little ones. The vapor pressure of pure propane(*) is about 8.5atm at room temperature, and a rocket tank pressurized solely to support structural loads probably needs less than 1atm overpressure, so we're talking an order-of-magnitude difference in wall thickness. (* Note also that commercial "propane" is a mix of light hydrocarbons, and often has considerable butane in it to lower vapor pressure. In fact, my recollection is that the fluid for those lighters is mostly butane. ) Then too, room-temperature tanks would be bigger, because propane at LOX temperatures is about 50% denser than at room temperature. Finally, chilled liquids make pump design easier -- might not even need a boost pump for the propane -- because they're much less prone to cavitate. (In fact, it might be worth chilling the LOX below its boiling point too; Rockwell's X-33 design did that.) -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#102
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The 100/10/1 Rule.
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#103
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The 100/10/1 Rule.
On Mar 11, 10:51 pm, Pat Flannery wrote:
My sister was once bitten by a SSTO vehicle. ;-) Is that curable? Or does she turn ballistic from time to time? |
#104
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The 100/10/1 Rule.
On Mon, 12 Mar 2007 06:34:56 -0600, Jorge R. Frank wrote
(in article ): I understood what you were getting at, but can understand why Herb didn't. I'm not disputing anything you wrote, just disputing Paul Dietz's assertion that you'd explained the issue completely. And bearing in mind that my orbital mechanics education is getting VERY rusty with disuse over these last 10 - 15 years . . . :-) -- You can run on for a long time, Sooner or later, God'll cut you down. ~Johnny Cash |
#105
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The 100/10/1 Rule.
On Friday 09 March 2007 22:48, kT wrote:
Pat Flannery wrote: Even then though you are going to have to do something to store it, as it will boil off fairly quickly once the sun starts warming the exterior of the LOX tank. I'll shade it then, and use it up as fast as possible. Convert LOX and H2 into water. You could save it easier and longer. 'Course, you'd need lots of electricity to get O from the water. And it'd be a shame to waste all the energy from making water. Maybe use the electricity with hydrogen to raise or change your orbit ... -paul- -- Paul E. Black ) |
#106
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote: In article , Pat Flannery wrote: The oxidizer is LOX -- cheap and dense. The fuel is probably propane -- slightly better performance than kerosene, less tendency to leave oily residues and otherwise misbehave, and it's still liquid and quite dense at LOX temperatures. ...Considering that I've seen propane cylinders for refueling lighters that have very thin aluminum walls, is it even necessary to chill it? It's desirable for several reasons. For the same pressure, wall thickness rises as the tank gets bigger, so big tanks won't be as thin as those little ones. The vapor pressure of pure propane(*) is about 8.5atm at room temperature, and a rocket tank pressurized solely to support structural loads probably needs less than 1atm overpressure, so we're talking an order-of-magnitude difference in wall thickness. (* Note also that commercial "propane" is a mix of light hydrocarbons, and often has considerable butane in it to lower vapor pressure. In fact, my recollection is that the fluid for those lighters is mostly butane. ) Then too, room-temperature tanks would be bigger, because propane at LOX temperatures is about 50% denser than at room temperature. Finally, chilled liquids make pump design easier -- might not even need a boost pump for the propane -- because they're much less prone to cavitate. (In fact, it might be worth chilling the LOX below its boiling point too; Rockwell's X-33 design did that.) I'm not sure if I understand what you are saying here. Are you proposing to make a rocket where structural strengh is provided by pressure in the tanks and where this pressure comes from LOX chilled below its boiling point? If not, maybe it would be a good idea to point out that we aren't talking about baloon tanks anymore or that some other gas is inserted to provide pressure or what ever. Alain Fournier |
#107
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fun with expendable SSTOs (was The 100/10/1 Rule.)
In article m,
wrote: (In fact, it might be worth chilling the LOX below its boiling point too; Rockwell's X-33 design did that.) I'm not sure if I understand what you are saying here. Are you proposing to make a rocket where structural strengh is provided by pressure in the tanks and where this pressure comes from LOX chilled below its boiling point? Not quite. The pressure comes from the tank pressurization, just like it does with any other balloon-tank scheme. The one blemish is that you can't easily pressurize subcooled liquids with their own vapor -- you have to pressurize with something else, unless maybe you can put an insulating barrier between gas and liquid. Pressurizing with something else is often done anyway, for other reasons, but here it's pretty much mandatory, especially for the propane, which is rather drastically subcooled. (For the oxygen the subcooling is not so dramatic, given LOX's narrow temperature range. It's also worth noting that most existing LOX-using rockets effectively run their LOX *slightly* subcooled -- it's boiling at countdown pressure, but then they go to a higher tank pressure for flight, which raises the boiling point. It generally doesn't have time to warm up to the new boiling point before it's burned.) The orthodox approach is to pressurize with helium, which is light and inert but does suffer from being relatively expensive and in limited supply. Some experimenting with alternatives would be in order. You might be able to pressurize the propane with methane, which conveniently is also liquid at about LOX temperatures. (You could almost certainly pressurize propane with hydrogen, but that's a pain to store.) Some of the methane will dissolve, but that's an issue for pressurizing with propellant vapor too, and it can be controlled well enough. Another option, for both propellants, is neon -- still not exactly cheap, but the supply is essentially unlimited (unlike helium, it's made from air, as a byproduct of LOX and LN2 production) and it's inert and has a very low boiling point. If you were careful to minimize turbulence in the gas, you could probably pressurize mildly-subcooled LOX with GOX, if you threw in a bit of neon first: you'd get a layer of cold neon gas on top of the LOX, with warmer GOX above it. An experiment or two would be needed -- neon *is* somewhat soluble in LOX, but then, both Delta II and Soyuz pressurize LOX with GN2, which is very soluble in LOX... Some GOX would get through the neon layer, so you'd have a layer of warm LOX at the top of the liquid, but that again is a common issue -- most propellant-vapor pressurization systems heat the pressurant beyond boiling to reduce its density. Whether a neon buffer layer would suffice to let you pressurize propane with propane is less obvious. Maybe. -- spsystems.net is temporarily off the air; | Henry Spencer mail to henry at zoo.utoronto.ca instead. | |
#108
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fun with expendable SSTOs (was The 100/10/1 Rule.)
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#109
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The 100/10/1 Rule.
Pat Flannery wrote:
Proponent wrote: Is it possible that with the booster engines attached all the way to orbit, the vehicle could fly a somewhat more efficient trajectory, therefore boosting the payload a bit? On the other hand, the booster engines might have to be shut down anyway in order to keep the acceleration from damaging the structure. I assume they carefully worked out to the second when the boosters became a net deficit to the ascent, and jettisoned them at that point. No doubt you are correct that the separation of the booster half-stage was timed to optimize the performance of the actual stage-and-a-half vehicle. And we agree that performance would have suffered substantially under the constraint that the booster half-stage not be dropped. I'm just pointing out that re-optimizing the flight program under the no-staging constraint would probably result in performance slightly better than flying the without staging under original stage- and-a-half program. |
#110
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fun with expendable SSTOs (was The 100/10/1 Rule.)
Henry Spencer wrote: The orthodox approach is to pressurize with helium, which is light and inert but does suffer from being relatively expensive and in limited supply. Some experimenting with alternatives would be in order. Was Atlas helium pressurized prior to launch, or was the LOX boil-off used? I assume the latter, as it was venting pretty much right up till launch. In fact, in this shot it looks like it's venting after lifting off: http://users.skynet.be/space-link/Pictures/Atlas3.jpg Pat |
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