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Space Forum / Space Flight / September 2005RXF1: Polyethylene Spaceship Material?
Here's a claim by NASA: http://science.nasa.gov/headlines/y2005/25aug_plasticspaceships.htm They say they've developed a modified version of polyethylene which has 3 times the tensile strength of aluminum and yet is 2.6 times lighter. The article also says it has superior radiation shielding properties compared to the heavier metal alloys. Can anyone speculate on whether this material is a suitable or superior candidate for building spaceships out of? If this material were compatible with conventional plastic molding processes, then it sounds like it would be much cheaper to mass-produce parts with it. Actually, why stop at spaceships -- would this thing be good for automobiles, airplanes, boats? Can anyone give some informed critical analysis here? What are the weaknesses or liabilities with RXF1? (Other than the fact that someone could dissolve your vehicle with paint thinner -- hopefully in outer space at least, there's not a whole lot of hydrocarbon solvents floating around.) Is durability an issue? Maybe if this stuff is cheap enough to work with, then could it make the disposable rocket approach more affordable? Comments, please? > http://science.nasa.gov/headlines/y2005/25aug_plasticspaceships.htm Do you actually read and comprehend these articles? --Damon > They say they've developed a modified version of polyethylene which has > 3 times the tensile strength of aluminum and yet is 2.6 times lighter. That's not unusual. Spectra and Dyneema have been on the market for years and they're very strong forms of polyethylene. Both have a tensile strength about 5-6 times stronger than high grade aluminum alloys while being 2.6 times lighter. (Ultra-high molecular weight polyethylene is about as dense as water; aluminum is about 2.7x as dense as water.) > Can anyone speculate on whether this material is a suitable or superior > candidate for building spaceships out of? Depends. Do you like spaceship components that: 1) Are flammable; 2) Lose strength above 100C and melt at 150C; 3) Embrittle at -150C; and 4) Are prone to stretching when put under continuous stress (like pressure hulls and pressurized fuel tanks)? > Actually, why stop at spaceships -- would this thing be good for > automobiles, airplanes, boats? Same issues as spacecraft, but more oxygen and chances to burn. High-strength polyethylene fibers certainly have their uses and you can find safe applications for them in almost any vehicle, but you need to be pay attention to their shortcomings. I wouldn't trust polyethylene as pressure hull material, but it'd be acceptable in temperature-controlled exterior structural applications. Mike Miller, Materials Engineer Hi Mike, I'd like to ask if the hull temperature of a launchcraft would come close to 100C during ascent. I can understand that nobody wants a polyethylene heat shield for re-entry, but if it's a disposable rocket then perhaps having your polyethylene boosters/stages burn up on re-entry ensures a more thorough/carefree disposal. The embrittlement from cold sounds like more of a problem to me. Regarding flammability, I'd imagine that a thick enough coating could be put on to shield from stray sparks, but otherwise your combustible rocket fuel might be the main failure mode to worry about, flammability-wise. (ie. if you can tolerate sitting on top of a large bucket of flammable propellant, then perhaps you can tolerate the fact that the bucket is itself flammable, although it would be protectively coated.) Regarding stretching/deformation under prolonged stress, I'd like to know how long is long. Isn't a typical ascent to orbit measured in minutes, and not tens of minutes? I'm imagining that a disposable plastic rocket hull could stand up to the 8 minutes of accelerative stress. And actually, if your plastic rocket hull is molded onto a skeletal framework made of high-strength alloy or composite, then why should the hull even be experiencing a lot of stress once you've cleared the atmosphere? Wouldn't the main accelerative stress be between the engine and the skeletal superstructure beyond that point? When you talk about pressure stress due to being a pressure hull, then why couldn't that stress be sufficiently mitigated through bulkhead compartmentalization? And if we're talking about an unmanned rocket taking a spaceprobe to mars, then it might not need a pressure hull at all. As far as exploiting the radiation-shielding abilities, I'd think that once you're out in orbital space then the hull isn't going to experience so much mechanical stress from that point onwards, so it should last awhile in space. If your paint was reflective, can you could avoid excessive radiative heating? > I'd like to ask if the hull temperature of a launchcraft would come > close to 100C during ascent. Leading edges can get fairly hot. The shuttle external tank's foam gets scorched. > Regarding flammability, I'd imagine that a thick enough coating could > be put on to shield from stray sparks Coating = weight penalty. The shuttle's external tank was only painted for the first two launches. Removing the paint saved 500kg, an extra half ton then available for payload. If fire's enough of a risk that you need to add fire protection to the material, it might be worthwhile just to use a strong material with integral fire resistance (like a metal or aramid fiber) rather than slather on a parasitic coating. > Regarding stretching/deformation under prolonged stress, I'd like to > know how long is long. Depends on the load and temperatures, and the problem depends on how sensitive the structural tolerances are. A fuel tank may swell "slightly" when it is pressurized to keep its shape - that may be enough to ruin a fit, or it may be allowed for in the design. > When you talk about pressure stress due to being a pressure hull, then > why couldn't that stress be sufficiently mitigated through bulkhead > compartmentalization? The bulkheads and pressure walls of the pressure vessel carry substantial loads. For example, a cylindrical space station module 4m diameter with a 1mm shell and 1 atmosphere of internal pressure carries a continuous load of 30,000psi in the skin ("hoop stress.") That stress will be there so long as the air presses again the skin of the station. Obviously, you can vary hull thickness to alter the stress, but the loads in a weight-conscious pressure hull are going to be substantial and constant. > As far as exploiting the radiation-shielding abilities, I'd think that > once you're out in orbital space then the hull isn't going to > experience so much mechanical stress from that point onwards, so it > should last awhile in space. Yes, but the only way a polyethylene hull is going to add much radiation protection is if you're just piling on extra mass simply for radiation shielding. The polyethylene used in that fashion can't be used for much else while water, food, wastes and fuel used for radiation shielding do have alternate uses. > If your paint was reflective, can you > could avoid excessive radiative heating? As I understand, the shuttle usually orients its black underside toward the sun to minimize heating. A black surface not only captures heat better, but it also radiates heat better. Anyway...The cost of materials is rarely a driving factor in spacecraft costs. The fact that you're building an almost one-of-a-kind vehicle with no room for failure will make the spacecraft expensive whether it's made of cast iron or gold. This RFX1 is nice stuff, but there are better spacecraft materials out there, ones with higher strength and better tolerances to environmental extremes. Mike Miller Am 4 Sep 2005 13:02:12 -0700 schrieb "manofsan@yahoo.com": >Hi Mike, I'm not Mike, but I'll try to answer just one of your questions... >I'd like to ask if the hull temperature of a launchcraft would come >close to 100C during ascent. I can understand that nobody wants a >polyethylene heat shield for re-entry, but if it's a disposable rocket >then perhaps having your polyethylene boosters/stages burn up on >re-entry ensures a more thorough/carefree disposal. >The embrittlement from cold sounds like more of a problem to me. As a matter of fact, especially nose cones/payload shrouds are often insulated with a thin layer of cork, that makes a very good isolator, because its surface is charred away and acts just like an ablative heat shield. cu, ZiLi aka HKZL (Heinrich Zinndorf-Linker)  Signature"Abusus non tollit usum" - Latin: Abuse is no argument against proper use. mailto: heinrich@zili.de http://zili.de [...] > As a matter of fact, especially nose cones/payload shrouds are often > insulated with a thin layer of cork, that makes a very good isolator, > because its surface is charred away and acts just like an ablative > heat shield. Didn't some of the early Russian capsules use oak heatshields, because it turned out the have just the right combination of insulation and ablatative properties? (Why did they stop, BTW?) - -- +- David Given --McQ-+ "If you're up against someone more intelligent | dg@cowlark.com | than you are, do something insane and let him think | (dg@tao-group.com) | himself to death." --- Pyanfar Chanur +- www.cowlark.com --+ > Didn't some of the early Russian capsules use oak heatshields, because it > turned out the have just the right combination of insulation and ablatative > properties? I've heard that, at various points, Russian, American, and Chinese capsules used oak heat shields. I'm not sure which of the claims to believe. > (Why did they stop, BTW?) Because you can make lighter ablative heat shields from composite materials. Mike Miller, Materials Engineer >> Didn't some of the early Russian capsules use oak heatshields, because it >> turned out the have just the right combination of insulation and ablatative [quoted text clipped - 3 lines] > capsules used oak heat shields. I'm not sure which of the claims to > believe. The early *planned* Chinese manned capsules involved oaken heatshields, at least according to astronautix, though I believe these never flew. Signature-Andrew Gray andrew.gray@dunelm.org.uk Lighter is always a plus but use cost per payload pound delivered However if this is a foamed material these numbers are misleading. Modulus and fatigue also have to be critically evaluated in a design. The article mentions why light molecules are helpful with cosmic rays and adds that this is flammable material. This does allow grinding up structure and using it as fuel once a section has served it's purpose. I would expect the first partial solutions for shielding crew will be layered. This material may well be one of those layers. Water too is useful and ideas dealing with greater biological tolerance are also in the mix. As with the shuttle tiles it is hoped an adequate solution will be developed. Experience and experiments will later render that solution obsolete with even newer ideas (but it does require the doing). ---- On a side note, steel seems to work out as the disposable rocket material of choice. This on a cost/strength basis. Large pressure vessel fabrication facilities are fairly common and the material well understood. Some plans even consider soft landing a pressure-fed lower stage and floating it back. SignatureAnvil* |
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