h a l f b a k e r yYeah, I wish it made more sense too.
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The situation: Man has started to venture out into the solar system and has begun to exploit the natural resources found out there.
The problem: A need to mass-produce cheap spacecraft to be operated/owned by private individuals/prospectors.
The solution: A resinous mixture of carbon-fibre,
Kevlar, and ceramics is created and pressurised in tanks with air. The mixture is taken up the gravity-well (using some kind of space-elevator type affair) and delivered to an orbiting space platform. The mixture is then extruded from the arm of an orbiting space station and made to envelope a pre-built skeleton of metal. The skeleton is designed to provide structural rigidity and has multiple mounting points for equipment later on.
After pumping a glob into space, the compressed air in the mixture is able to expand, creating bubbles within. The bubbles grow and form compartments of varying sizes within the glob.
Assuming that the air will expand proportionally with temperature, keeping the skin cool will form a tough ‘crust’ of many small bubbles. The insulating property of the material will ensure that the central bubbles are warmer, and therefore grow larger. The ‘baking’ of the glob will be done in shadow, so as to keep the mixture relatively cool. At times, allowing sunlight to warm the mixture may be allowed in order to assist formation of particular shapes/features etc.
Differing materials can be used for the composition of the goo, to provide additional properties like resistance to radiation, heat etc.
After the glob has hardened, it resembles a roundish, bulbous ‘wotsit’ and contains multiple bubbles in which living quarters, control-rooms, fuel and storage compartments can be fashioned. The metal skeleton provides mounting points for engines, mining equipment, and anything else that might require an extra element of structural integrity. It also contains a ‘cofg shifter’ which allows for improved manoeuvrability (idea to be posted soon).
Outfitting the hardened glob entails cutting out a door, and ‘knocking through’ adjoining bubbles to provide access between compartments. Drilling the initial hole into the craft and mounting the door should be done carefully in order not to de-pressurise the ship. Equipment is then mounted within the craft (either by attaching it to the skeleton, or latching onto the walls of one of the internal bubbles) and connecting it all up.
The structure should be incredibly strong (a series of connecting spheres), and contains enough redundancy to withstand a number of damaging micro-impacts. For additional toughness, treatments of other materials might provide a tougher skin, or a reflective layer etc. The advantage of using this method is that the hard work of putting together a series of plates, ensuring they are airtight and then pressurising the whole thing is already done.
N.B. I had posted this idea some time ago, but deleted it, apologies to any of you with unwanted deja-vu effects.
Wotsits
http://walkers.corp....asp?snacktypeid=40
I guess they're the same as cheesy-puffs [zen_tom, Oct 04 2004, last modified Oct 05 2004]
Proposed Applications of Amorphous Metal Foam
http://www.liquidme...dsp.news.04x104.asp
.. last paragraph esp. relevant .. [bpilot, Oct 04 2004, last modified Oct 05 2004]
related...
http://www.halfbake...om/idea/Space_20Jam
[RayfordSteele, Oct 04 2004, last modified Oct 05 2004]
C-Of-G Shifting to assist manoeuvrability/balance
COM Shifting
[zen_tom, Oct 04 2004, last modified Jan 26 2009]
But here's a simpler, cleaner, sexier way to do it...
Low_20budget_20spacecraft
OK, really it's just a gratuitous cross-link [normzone, Nov 18 2007]
Reaction Wheel
http://en.wikipedia...wiki/Reaction_wheel
Real "COG shifter" in satelites. [Spacecoyote, Jan 26 2009]
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//the hard work of putting together a series of plates, ensuring they are airtight and then pressurising the whole thing is already done.
// |
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really? ever made a baked alaska. trying to make foamy stuff air tight aint always that easy. |
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<ping ping ping beepppp beepppp > |
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"Captain, we are picking up an anomoly in the Terran Sector...there seems to be an armada of..huge...croissant forms.....Sound the alert!! The Halfbakers have broken out!! "" |
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[etherman] No, I must admit, baked alaska isn't part of my repertoire - however, if you've ever seen builders foam (which is more sticky and expands due to the internal pressure of compressed gas, rather than gasses heated from the outside as in baking) you'll notice that the outside skin has a smooth consistency and is likely more airtight than most confections. |
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Keeping the outside cool during the formation process ought to allow for this type of smooth surface to develop. Indeed, any pops should self-heal (the sticky substance no-longer under pressure would fall back and help provide a tougher surface for the next bubble underneath) |
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but surely it would be very hard to avoid weak spots in the structure. As you said its a blob, how do you make it uniform? how do you shape it around your struture? An d wouldn't all these tanks of compressed carbon and kevlar be pretty heavy to get into space? |
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Firstly, It doesn't need to be uniform - as long as the attached enginery can be adjusted to cater for a non-uniform structure (see the Centre-Of-Gravity-Shifter to be posted soon). |
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Getting the blob to engulf the structure could be done by winding it, candyfloss-style, or could be made to extrude from within the structure itself. |
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And yes, the materials would be heavy, though possibly not as heavy as the traditional, bolted-plate method. |
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Yadda yadda yadda falls apart when the engine pushes it yadda yadda yadda. |
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I liked this originally. Thought it was really cool, and it garnered some interesting annos. But I hate idea deletion. So a bone from me unless there is a great reason why this got deleted. |
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See link for some current research. Skip
to last three paragraphs for the foamy
part. |
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It feels a little strange to be making the same annos with the same people - and [bungston], the reason the idea got deleted was that, being new on hb, after posting one too many physics-theses, and getting batted down once too often, I rather impetuously deleted my account. Yes I know, it was rash, and no, I'm not about to do that again. There are other people who have taken a lot more stick than I ever did and remained cool-headed enough not to press the [destroy] button – and also perhaps a few who haven’t. What’s important I guess is that I've come to accept the bakery for what it is, and not what I wanted it to be. So Bone-on if you must. In the meantime, let me answer some more questions. |
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[bpilot] - thanks, those last few paragraphs do rather validate what's being proposed here, whether that makes this baked or not I shall leave up to you. (Dammnit, it's either a shitty idea, or it's bakable, or people wanna bone because I got click happy once before - I'm stuck between a rock and a number of hard places here!) |
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[Contracts] your debates are most welcome, the more vigorous the better, so thankyou. |
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[pheonix] there are two reasons why the engine wont fall off/the thing fall apart - 1) the skeletal sub-structure that it's bolted onto, and 2) The fact that we really don't need a great deal of force here - we're not proposing planetary landing. |
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I was reading one of the Conan stories, and thinking about how Howard committed suicide in his early 30s, but I was enjoying his creation 70 years later. A strange thing about the HB is that when the creator goes, all his or her ideas go too. |
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But you brought it back, so I will cut you some slack and give back the bun this originally got from me. |
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Another benefit to this model - easy to fix micrometeroite damage. In fact, if some interior areas are not hardened, viscous goo from these will ooze into and self seal some damage. |
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It's going to be very tricky to figure out the dynamics of the shape, for powering puroposes. Each ship would have its own flight characteristics. |
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Shitty or bakeable aren't necessarily polar opposites, nor are they mutually exclusive. A lot of lousy ideas are baked, and a lot of silly ones have offshoot applications that take off. |
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[Rayford] Yes you're right, however a Toyota has different driving dynamics to a Ford without issue. |
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Each ship would indeed have different flight characteristics, however, by utilising a Centre-Of-Gravity adjusting frame (see link) much of the problems associated with this should be ironed out (as well as assisting in manoeuvrability) One thing that would be required after the craft is built is some analysis of its mass distribution and dimensions, to be fed into the control-computer, once installed. |
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In a vacuum, the shape is irrelevant. When the engine is on, it will propel the spacecraft in a straight line, even if the engine itself is not on the centreline of the craft. Weird but true. With no atmosphere to provide resistance, there can be no moment, so no rotation of the ship (unless pairs of thrusters are specifically employed to do so). |
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[ds] I'm ready to accept that's true, but am not 100% convinced. In an extreme example, say a long beam of certain mass, with an engine positioned at one end providing thrust perpendicular to the length of the beam, would the beam be propelled forwards, or would the centre of mass of the beam, being distant from the source of propulsion, cause a spinning/curved motion (assuming the engine was left switched on)? |
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In another example, again using a beam, but this time travelling at some speed, in a direction perpendicular to the length of the beam. If one end of the beam hits an object of mass large enough to effect the beam, wouldn't the beam after impact, go into a spin? |
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//In a vacuum, the shape is irrelevant. When the engine is on, it will propel the spacecraft in a straight line, even if the engine itself is not on the centreline of the craft. Weird but true.// |
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Wierd and false I'm afraid.
Rotation does not require friction. |
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The engine would have to point directly away from the center of mass to avoid changing the angular rotation of the craft.
This is not necessarily that difficult to arrange; the engine attachment point could permit some angle adjustment. |
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excuse me if someone has mentioned this but the expansion of the gas would be dependent on the pressure of the surrounding air so you would need an enviroment with air pressure. if you built one in space it would be easy to release pressure and expand the foam though. |
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[gpm] Yes, the idea is to build these in space, where the pressurised air-mixture mixed in with the resinous material is free to expand in a vacuum. |
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[Loris], friction is not required, but a moment is. To produce a moment, two forces are required, offset from one another. This is why, on spacecraft, thrusters are fired in pairs. |
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For instance, consider a large square object. To make it spin clockwise, one thruster must be fired from its top left corner, while another fires from the bottom right. Omitting either of these will lead to sideways motion and _no rotation_. |
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[zen_tom], my every instinct says it will rotate in both cases, but that isn't the case. In the example of the moving beam striking an object, a couple is still required to initiate rotation, and we haven't got one. |
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[ds] I'm less convinced about this now - Hitting a snooker ball off centre causes it to spin and there are no coupled forces there - unless you count the inertia of the ball as supplying the countering force. What we need is proof goddamnit! - anyone able to find proof positive one way or the other? |
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Ok, here's my latest rotational proof. Balance a beam between two walls. Gravity applies a force based on the mass of the beam in a downward direction. Likewise, the walls apply an equal and opposite force at the two points of contact. The result - the system remains in equilibrium and the beam remains stationary. |
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Knock down one of the walls. Now the gravitational acceleration is countered by only one side of the beam. And the beam pivots around the point of contact with the wall until it comes to rest against the ground. |
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Now, since gravitational force is equivalent to acceleration, consider the following system in a weightless environment: |
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A beam is constantly accelerated by two rockets. Each rocket is located at opposite ends of the beam, but are facing in the same direction. The beam is accelerated forwards. As long as the rockets apply a constant force of 1g, this system is equivalent to the balanced beam described above. Now cut the power to one of the rockets. The beam will rotate in the same manner as the wall example, due to the force being applied off centre. |
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[zen_tom] For a snooker ball, the countering force is provided by the friction between ball and table. |
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I agree, this sounded weird when I first was told it (by a mechanical engineering lecturer). It still sounds weird, and I live in fear of being proved wrong. Feel free to do so... |
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(I'm working in a design engineering department at the moment, and the question is thoroughly confusing us all). I'm with [zen_tom], we need some sort of proof... |
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[zen_tom], I'm not sure that the two systems are analogous to one another. In the stationary frame of reference, the beam has weight acting downwards from its centre of mass. When supported at two points, then, the system is in equilibrium. Taking a support away will produce a couple as the weight and the reaction are offset from one another, and rotation results. In the moving frame of reference, the rockets' thrust results in an acceleration. There is obviously a net force to provide this. The reactions on either end of the beam are the same on earth or in space. The net force is thus provided by the absence of weight. Switching off one of the rockets does not then produce a couple, and hence no rotation results. |
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<EDIT>
Having involved half of Rolls-Royce and a couple of Oxford physicist undergrads, I think we have an answer: |
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If the line of thrust of the engine does not pass through the centre of mass, then particles in the rocket exhaust will have angular momentum about the centre of mass of the rocket. Hence, by conservation of momentum, the rocket will rotate. |
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Bummer, I hate being proved wrong... |
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OK, different tack. Just to see where this goes. How would a rope (or a set of nunchuks) behave? If you pulled on a length of rope from the side, would it rigidly float sideways towards you, or bend towards you? (I've never tried this, but it should be reasonable to assume that in space, or on an ice-rink (or any close to frictionless surface) rope behaves in a similar way to down here on earth. |
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A more interesting example is a chain, you pull on one link, the link itself is relatively light so moves towards you, but it is linked to the next part of the chain. Your link stops, and now you are exerting your force, via the link, to the next link, and so-on down the chain. Each link provides a tiny extra piece of mass, and has a pair of forces acting upon it, the force of you pulling, and the force of the stationary rest-of-the-chain whose inertia provides a coupling force. |
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I think the idea I'm trying to get across (and this is where I wish I had done better at school and remembered my physics/maths etc) is that there must be a difference between point masses and distributed ones. |
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I don't think it's too far to go to consider a rigid body as a collection of particles, each of which operates within the network of it's friends, and that a force, once applied to one set of particles, is transmitted through their various bonding mechanisms to all other neighboring particles, etc. In this view-point, the rigid-body becomes very much like the chain, and as such, should become 'spinnable'. |
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Aha - sorry, my last post took so long to write, I didn't realise you'd posted. |
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I'm not sure I fully understand the angular momentum conservation wordage (again, should have paid more attention at school!) but am quite happy to bow to superior knowledge. |
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And don't think of it as being proved wrong, you've had me scratching my head trying to prove something that I now realise is something I've up till now *believed* in, without really knowing why. |
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"[pheonix] there are two reasons why the engine wont fall off/the thing fall apart - 1) the skeletal sub-structure that it's bolted onto, and 2) The fact that we really don't need a great deal of force here - we're not proposing planetary landing."
1) Why do so many people misspell my username?
2) How do you bolt <anything> to foam? |
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1) Sorry phoenix, I guess it's just easy to misspell.
2) As per //The mixture is then extruded from the arm of an orbiting space station and made to envelope a pre-built skeleton of metal. The skeleton is designed to provide structural rigidity and has multiple mounting points for equipment later on.// |
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It is not the foam that bears the load of the engine, but the aformentioned 'skeleton'. |
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The object would still rotate if the engine power was applied off-center. Things are weightless in space, but they still have all their mass, hence all their inertia. As you applied force at an angle, you'd be applying a greater force to the mass on its side of the center of mass than you would to the opposite side. That side, needing force to move, would not move as fast, and you'd rotate the object. In EVA activity, a space suit does not need to fire opposing thrusters to rotate, they can spin with only one thruster operating. |
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Its true there is no weight in space, but its hard to remember that all the inertia is still there. If you were in space, you wouldn't be able to easily lift a car. Of course, once the car was lifted, it would stay lifted, but you'd still need to apply the force to move it. |
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Anyway, about the idea, I'd say its a good one. Consider monolithic domes which are made here on earth. They inflate a fabric hemispherical skin, and spray polyurethane foam on the inside of it. When the foam sets, they release the air pressure from the skin, lay some rebar on the inside, in a basket formation, and then spray in shotcrete (sprayable concrete). The dome gets holes cut in it for windows, and all the plumbing and wiring conduits are layed before the final shotcrete layer. The dome end up with a fabric outer layer, a few inches of foam, a rebar skeleton, and a few inches of concrete, and are tremendously strong. They can make HUGE ones, and they will support nearly any weather condition that nature can throw at them. |
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Polyurethane foam will stay airtight, its a closed cell foam that produces a smooth outer layer. Instead of a metal skeleton, why not do what monolithic dome constructors do, use an inflatable skin, and spray the foam in an even layer on the inside of the skin. You could have an inflatable skin presown in the shape of the craft you want, inflate it with a couple PSI, then spray a few feet of foam on the inside of it. A metallic skeleton may not be necessary, but could be optionally added to provide engine mounting points. It would be easy to determine the center of mass of the final object, and then when the engine is attached, slight adjustments could be made so it fires in line. |
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Pumping liquids up a space elevator would definately be easier than moving solid building materials up, so I'm rating this +. |
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I want mine made out of whipped cream. |
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More information regarding the structural stability of the foam. |
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Given that Earth structures made out of a flexible skin with a polyurethane foam lining plus semi-rigid metallic skeleton easily withstand the force put on them by gravity, we can assume that a force equal to an acceleration of about 10 meters per second squared is well within the structural capabilities of polyurethane foam. |
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Based on that assumption, if a spacecraft made out of foam was accelerating at around 9.8 meters per second squared, then the forces put on the foam would not exceed the forces put on earth-bound foam, and things would be fine. |
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Is that enough acceleration for a spacecraft? I think so! The space shuttle accelerates at about 29 meters per second squared during launch, about 3 Gs. |
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At 380km, the orbital height of the International Space Station, the acceleration due to gravity is about 8.6 meters per second squared. If you were going to launch perpendicular to the earth from that height, and your spacecraft could only handle accelerations of 9.8 meters per second squared (it could probably handle way more, but we KNOW it would handle 9.8, since we have structures like this on Earth), then if the engines fired to produce an overall acceleration on the craft of 9.8 m/s^2, then we'd have an overall acceleration of 1.2 m/s^2. If you burned at that acceleration for about 6 hours, you'd be moving close to 60,000 miles per hour, which is the same speed that the Cassini craft was moving as it entered Saturn orbit last month. An acceleration of 9.8m/s^2 would also be comfortable for the astronauts, since it would replicate normal earth gravity. |
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In reality, the burn could be even shorter because as you retreated from earth the acceleration from earth's gravity would lessen, and a greater component of your total 9.8 m/s^2 would be devoted to your forward motion, and your overall acceleration would increase. I'm not going to do that differential equation right now, however. ;) |
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Needless to say, I hope this math helps you understand the relative stresses put on a craft in space. In open space you don't have to apply much acceleration at all to produce motion. With small accelerations, you don't put much force on your structure. Open space craft could be built out of MUCH simpler and weaker materials than craft designed for planetary landing, foam could be ideal! |
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"Given that Earth structures made out of a flexible skin with a polyurethane foam lining plus semi-rigid metallic skeleton easily withstand the force put on them by gravity, we can assume that a force equal to an acceleration of about 10 meters per second squared is well within the structural capabilities of polyurethane foam."
Can an astronaut stand on top of a polyurethane foam dome and jump up and down without falling through? At what thickness? What does that mass? How does that compare to the materials already being used? |
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"Can an astronaut stand on top of a polyurethane foam dome and jump up and down without falling through? At what thickness? What does that mass? How does that compare to the materials already being used?" |
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They sure can. Earth domes easily hold the weight of people on top, as well as huge piles of snow, or large piles of earth if they are partially embedded in a mountain. And that is at less than a foot of thickness. |
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The material that makes up current spacecraft is usually some sort of honeycombed metal sandwiched between composite layers. Its heavy stuff. Foam is already 'honeycombed', just on a smaller scale. |
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The materials used to build the Apollo LM aren't fully relevant. It was honeycombed aluminum too, but it was designed for low weight, not high strength. The pressure inside the vehicle was only 5psi, and 100% oxygen. This is not a 'livable' pressure and mixture, it was just designed to support the astronauts for the time needed to make the landing and return to the command module. |
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The strength of foam is that it can provide structure, sealant, and insulation all at once. An aluminum hull will still require layers of insulation and sealing to be useable at atmospheric pressures. The aluminum may be light weight, but once you put on everything that needs to go on, it will probably be heavier than a foam of similar characteristics. |
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Also remember that current spacecraft designs sacrifice insulation for strength, because they need to handle the 3Gs of launch acceleration. A space-only craft would need more insulation for energy efficiency, but less structural rigidity. Hence, foam might make more sense. |
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The biggest reason foam may be usable is the fact that this whole concept is framed around a world with a space elevator. A space elevator may be incredibly restrictive as to the amount of weight it can move, but there probably won't be a reason that it can't be constantly moving small loads. Instead of fabricating big panels here, and trying to lift them on an elevator that can't carry the weight, moving up very small quantities of reagent liquids over a long period of time would be easier. A minimal amount of in-space preparation would be required to make it a ready-to-use building material. |
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For those of you who are still uncertain about the physical characteristics of polyurethane foam, I recommend buying a can of 'Great Stuff' at your hardware store. Its just a can of polyurethane foam (it comes in a few different types that expand different amounts). Spray some around, see how strong it is and how it develops a smooth skin (lots of closed cells for great airseal). Its handy stuff for insulation, air sealing, adhesive uses, but remember that once it sets, its impervious to solvents, and nothing will clean it off. |
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Its just a fun and easy way to try out foam, and get some ideas about how it might be useful for bigger and better things. |
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If the foaming gas were something like Nitrogen, would that cut down on heat loss? Do we need to worry about radiation shielding, maybe doping with lead? But then you'd have lead-based walls... |
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I had a can of that Great Stuff that had a slow leak. When I came back up to the attic, the can had grown a lobulated excrescence that extended up about 2 feet from the can. |
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reminds me of the idea of making space stations, etc. out of balloons & then spraying them with hot metal coating for strengh/shielding.
will such stuff "foam" in the vaccum of space? |
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Of course it will foam - in fact, the vacuum of space, rather than impede the foaming behaviour, will instead cause it to foam like you've never seen it foam before. |
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I used to put polyurethane resin in a vacuum chamber to degas it somewhat before mixing it with a catalyst, in an effort to produce bubble-free material for underwater electrical connector assemblies. |
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Playing with mixing the catalyst before degassing led to some weird foamy conconctions. You may not have to add anything to the process to make it foam. |
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Likey [+]. If you impart some spin to the glob before inflation, you'll get an ellipsoid, well suited to powered spaceflight. |
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I imagine a sub-scaffold will be needed when casting the glob without a mold, especially with no main environmental director to cause differentiation or design.
Not unlike a simplified elastic organ scaffold such that the foam can blow up and differentiate through to form the necessary compartments. |
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