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Post by I own a cylinder on Mar 2, 2005 3:15:12 GMT
The whole novel contradicts itself about the landing of the cylinders. This implies that wells did not have the scientific facts of the landings in mind but simply a vehicle to get the martians on earth. For example. More than once in the novel does wells state that the cyliders land heavily, while at others implying they land lightly.
'Many People in Barkshire, Middlesex and surrey' saw the fall of the meteor. He also says that miss elphinstone saw the landing of the seventh cylinder on Primrose hill from miles away. (The flash.) This suggests that the the cylinder lands with a heavy impact. Yet, it is silent enough to slip past the Narrators house without knowing despite the fact Horsell common is only about 2 miles from Woking and considering how large the cylinder is, (30 yards is still big) it would be impossible for the Narrator not to feel some effect of the cylinders landing on Horsell Common. It is also a soft enough landing that the Narrator and Curate can survive impact when they are trapped in the house.
The logical conclusion i came too, is similar to what is shown in the 1953 film. THe Cylinder exicutes a controlled landing. heavy enough to make a hole but not to heavy to destory the ship completely. i.e. sliding along the ground, digging in as it goes.
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Post by Lensman on Mar 3, 2005 0:40:46 GMT
Reality Check - Kinetic energy increases to the -square- of the velocity. double the speed, and you need 4x the amount of stopping distance. If I have the math right you'd need 62,500x the stoping distance... or about 27 miles of airbag... Sorry, your reality check bounced. You're misapplying the meaning of kinetic energy. If what you say were true, a car going at 60 mph would need 4x the stopping distance of a car going 30 mph. Not so; it needs only twice the distance. Rule of thumb for a car braking is that you need one car length of stopping distance for every 10 mph. That's a linear equation, not an exponential one. The important limit here is not the amount of kinetic energy a body can withstand without damage; it's the speed of accelleration it can withstand, and under what circumstances.
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Post by Lensman on Mar 3, 2005 0:46:18 GMT
we are assuming they just hit the ground full on .. now although there is no mention of it ...could they have skidded along the ground before coming to rest .. ie landed with a shallow dive,.hitting the ground at a shallow angle and then ploughing into the ground to create the crater The "splash" pattern of a crater is only made when the impact is all in one spot. Skidding along the surface wouldn't make a true crater. Oddly enuff, it doesn't matter what angle a meteorite hits at, it always creates a round crater. (Again, I'm referring to hitting, not skidding.)
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Post by lanceradvanced on Mar 3, 2005 1:55:31 GMT
Sorry, your reality check bounced. You're misapplying the meaning of kinetic energy. If what you say were true, a car going at 60 mph would need 4x the stopping distance of a car going 30 mph. Not so; it needs only twice the distance. And probably twice the force applied to the brakes as well... go look in any physics textbook, the formula is KE = 1/2 MV^2. The point about KE hold, because it shows how much energy the cylinders had to dissapate, the faster they come in, the more they have to loose,untill you get an amount of energy that cannot reasonably disapated in "the sudden stop at the end" I've allready done the math to point out that a surviable speed, is one that is aproximating that of a fall from mebbe a -mile- up, not the 100+ miles of reentry. Interplanetary speeds are just to much to be dissapated on an -uncontrolled- by solely atmospeheric means, the leftover velocity simply is too high to decelrate safely, and isn't needed to create the crater, a 2-300 mph impact can do that in and of itself... In short, retros are reasonable, and as well supported in the book as any liquid landing capsule, or other exotic shock absorber.
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Post by Lensman on Mar 3, 2005 4:06:55 GMT
It doesn't matter what you use to decelerate the body in question. Airbag, shock absorber, controlled release of gas... It's all the same. C'mon, Topaz, I think you know that's not so. If you were in a car crash, would you rather your body be stopped by an air bag, or a bunch of needle-sharp steel spikes? There's no magic to an airbag or shock absorber. They just provide a longer deceleration distance than you'd otherwise have with a rigid system. Increase the distance over which you apply the slowing force and you lower the G-loads, but in any case the distance can't exceed the length of the vehicle. Mostly correct, but you forgot to add in the distance the vehicle travels into the ground while making the crater. You or Lanceradvanced suggested a "crush zone" to allow some travel distance during the crash. I don't think that would be necessary; the cylinder being driven into the ground for most of its length provides the same effect without the Martians having to weaken their vehicle. Floating in water only spreads the load around, it doesn't reduce it. What's happening is that the water presses against you instead of the seat or bulkhead. Your body doesn't distort, which reduces injury in that sense, and the load against your flesh is spread out, rather than being concentrated into small points like the edge of the steering wheel in the auto example. Correct. I'm so glad we agree on at least one thing! A form-fitting couch does the same. Sorry, but no. As you just said, the water prevents your body from distorting. A form-fitting couch only reduces this--it doesn't eliminate it. The water provides protection all around, and automatically distributes the pressure evenly across your body. Designing a couch to do as good a job would be difficult or impossible. Some parts of your body can withstand accelleration better than others, and certain parts weigh more than others, which means you need different depths of padding and different stiffnesses on different parts of the couch. Water immersion avoids all these difficulties. I'm not sure about the side of your body away from the impact. Does water immersion convert kinetic energy to hydraulic energy? If so, then you've just doubled (at least, probably more) the amount of accelleration you can withstand, as hydraulic pressure will be applied evenly all around your body, including the "upper" side, whereas with an accelleration couch it's applied only to your lower side. But your brain is still sitting against a flat, brick wall - the inside of your head. Whatever load the system endures, your brain will endure against the inside of your cranium. Yes. But since you keep hammering away at that one point, I'm guessing you're missing the point that concussions typically happen because the skull was *suddenly* hit with a powerful impact. With the air-bag system--or its equivalent--we're eliminating that impact, and spreading the force over distance and time. The limit becomes not what the brain can withstand in a momentary impact, but rather what it can withstand in the way of gradually applied acceleration against the skull. Clearly you want to stop short of massive bruising or bursting of blood vessels beyond minimal capillary damage. Okay, but now you've shortened the stopping distance to a tiny fraction of what was previously discussed. The G loadings increase on the inverse of that change in distance. Instead of 8.25 G's, we're back to hundreds of G's. I don't know what acceleration air bags produce. The distance traveled may be--probably is--greater than I indicated in my earlier back-of-the-envelope calculation, because of the telescoping of the front of the car, but even if you double the stopping distance to 5 feet, there's still enuff room for the deceleration capsule to stop even without adding in the distance the cylinder travels thru the earth before stopping. Clearly, air-bags provide very good cushioning. Add in water immersion, and it's even even better. When running, the human foot withstands a momentary G force over 1000 G's. With proper cushioning, the human (or Martian) body may be able to withstand a lot more accelleration than we're used to thinking about. <snip>the real problem for survivability is the final terminal velocity at impact. You'd like that to be in the "several hundred mph" range to produce the desired 'splashing' effect around the crater. That speed isn't survivable unless the cylinder is absurdly long. I've demonstrated that it is survivable, just as car crashes cushioned by air bags are survivable. My proposed system is based on real life experience with air bags. Your attempt to disprove what I've said is based only only math using a lot of assumptions and guesses, plus a misapplication of the term "kinetic energy". [Continued next post-- I received a "post too long" error!]
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Post by Lensman on Mar 3, 2005 4:07:32 GMT
[Continuing...] Let's say the foam layer is twenty feet thick and only covers the side of the hull (for simplicity in the calcs). Let's also use the cylinder dimensions you propose: 90' x 180', and make it a simple cylindrical shape (again to ease my fingers on the calculator!). The volume of the foam layer surrounding the hull is 1,244,069 cubic feet. You'd want a foam that has some structural rigidity so it doesn't get blown off by the supersonic airflow during braking, so let's use something like the 2lb/cu. ft. blue styrene foam that homebuilders use as the core of composite airplane wings. The stuff is very strong and amazingly light for the strength. Well, let's be generous and cut the density in half, positing a superior Martian foam. At 1 lb/cu. ft, in the volume calculated above, your foam aerobrake weighs almost 1.25 million pounds. Then you have the questions as to how the foam shell is deployed, how it survives launch if it's already on the outside of the vehicle... Only 20' of foam wouldn't add that much aerobraking. If there is a need for a significant increase in the cylinder's cross-section, I certainly wouldn't stop at a piddly 20 feet. I'd at least double the cylinder's diameter-- maybe even a lot more than that. Presumably the reason for the cylinder's shape is so it can be launched like an artillery shell from the space cannon. If the foam is present upon launching, there's much less reason for a cylindrical shape; a sphere would be stronger. You could coat it with foam first and protect it with discarding sabot, but this has several disadvantages, both in requiring a larger space cannon and reducing the effective strength of the cylinder during launch. I'd emit the foam from a number of tubes running to the outside of the cylinder. I don't think the Martians want to do a space walk! The sensible thing to do would be to give the foam very large air pockets, increasing the cross-section significantly with a relatively small increase in weight. The average density would be far less than the 1 lb. per cubic ft. you propose. As you said, this is essentially a parachute. Parachutes don't weigh that much. Are you sure you wouldn't rather use a retro-rocket? Of course I'd rather use a retro-rocket! The question isn't what system we'd *prefer*. The questions are: Presuming the Martians used the system Wells described, how did they survive? And what advantages are there to the system? Adding in a series of complex systems in a Rube Goldberg approach, making it absurdly overcomplicated for no good reason--not taking advantage of a retro-rocket for a controlled landing--is not the correct approach. As I said, your proposal fails the test of Occam's Razor. Airbags were added to put some additional distance into the equation as I've discussed above. They're a little bit of added insurance, not a panacea. I completely disagree. The main advantage of the air bag is that it prevents the body from slamming into the front of the car. It converts an impact-- an instantaneous deceleration-- into a strong deceleration spread over a brief time. Which is exactly what is needed for the Martians to survive their cylinder crashing down at a speed of hundreds or possibly even thousands of miles per hour. Are there any of my assumptions that you feel are incorrect? I'd be happy to rework the math if we can find even more accurate starting points. I'm not trying to bag on your ideas, Lensman. My entire goal here is to come up with a system that isn't precluded by the text and yet still works in the 'real world.' It just so happens that I went to school for aerospace engineering and while I ended up in graphic arts (a very long story!) I've still got a lot of my old engineering reference material up on bookshelves. It doesn't mean my ideas are 'better' than yours, it just means that we can 'test' our designs against reality. I sincerely appreciate you doing the math. That was never my strong suit; I flunked out of college trig twice. I looked at my copy of "Asimov on Physics" but can't see how to convert the formulas he uses to figure distance travelled given a fixed acceleration. If I could figure out what the G forces were assuming a 40 MPH impact with a stopping distance of 5 feet, I'd use that number. I think there's a fundamental, irreconcilable difference in our approaches. Your approach seems to be: Figure out what will work best from currently known engineering applications, and then see how the novel's text can be re-interpreted to allow it. My approach is to assume the novel is a true account, and try to theorize a more realistic system within those confines, and without adding more complexity than is necessary.
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Post by lanceradvanced on Mar 3, 2005 4:53:08 GMT
[Continuing...] Adding in a series of complex systems in a Rube Goldberg approach, making it absurdly overcomplicated for no good reason
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Post by Lensman on Mar 3, 2005 5:00:05 GMT
The [retro rocket] system would be at the front of the cylinder, and thus underground from the point of view of human observers. Let me get this straight: You're seriously proposing putting a liquid rocket at the front of something that's intended to crash to earth? Quick, what's this a design for? (Hint: BOOM!) The problem with using Apollo as an example is that it did use a braking system in the terminal phase: Parachutes. The unbraked impact speed of the Apollo system would've been just as fatal to the astronauts as an unbraked cylinder impact would've been to the Martians. As I've said a number of times, it's clear the cylinder hit at a high speed, and therefore the survival of the Martians must have depended on a system inside the cylinder. If you don't like my survival capsule/air bag tube design, I'm quite willing to discuss an alternative proposal for a survival system inside the cylinder, so long as the proposed system isn't so useful or advanced (like retro rockets, or anti-gravity) that the Martians would have used it in preference to a meteoric landing. 'Wind resistance' (drag) is determined by the drag coefficient of the object shape and the exposed area of the object, and the velocity through the air. Scaling factors mean that the weight/surface area ratio increases with a gain in size, since area (and drag) increases as the square of the scale factor, but volume (and therefore weight) increases as the cube of the scale factor. A big cylinder has a higher terminal velocity than a little one, given equal densities. Rats! You're right, I forgot the cube-square law. But that doesn't mean the cross-section vs. density is a "minor factor" at all. It does mean the overall mass is also a factor. The gun only provides enough velocity for a very high sub-orbital trajectory, almost straight up to the limits of the gravity well. The rocket system provides the remaining 'kick' to boost the vehicle into trans-Earth flight. In transit, the same system can provide course adjustments. After entry, the same system provides the retro-thrust to soften the landing. So you're just using the space cannon as the booster stage. Well that's not a bad idea, considering that IIRC the Apollo Saturn V system used over 1/2 its fuel just lifting itself off the ground. But you haven't answered my point that if they had a retro rocket system, they would have used it to make a controlled landing, and would have picked their landing places. There is no way to reconcile that with the text. The landing of the 5th cylinder clearly describes a meteoric landing. In fact, to interpret it any other way would require that for some reason the Martians deliberately faked a meteoric landing. Nor have either you or Lanceradvanced addressed my point about the first and second cylinders not landing near each other. My point was only dismissed, not actually responded to, earlier in this debate. Clearly the Martians, from their actions, were very concerned for their safety upon the initial landing. If they could have landed the first two cylinders near enough for each other for mutual support, they would have done so. They didn't because they couldn't. They couldn't because they couldn't control their landing places. Someone also said during this debate that the subsequent cylinders landed at the "front lines". That's having the tail wag the dog. The Martians moved the front to where the new cylinders were in order to protect them, not vice versa. A reading of the text is entirely consistant with the idea tha the cylinders landed at random locations. Again, there's no indication of controlled landings as you're proposing. A retro rocket system would require large rocket nozzles. They would have been seen if they were on the back of the cylinders. If they were on the front, there would have been the problem of lack of stramlining, likely causing the cylinder to tumble as it came in, plus unless the cylinder really had a soft landing, there'd be the danger of explosion. I think we all know just how susceptible liquid-fuel rockets are to exploding upon impact. This is not a good design-- to put it mildly!-- and the fact that none of the cylinders exploded upon landing is a strong indication they didn't make a crash landing with a rocket on the front of the cylinder.
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Post by Lensman on Mar 3, 2005 5:10:01 GMT
...now are we to assume the cylinders are piling in from the atmosphere .....like a bullet ie stright in .... then only the nose cap/cone would need to be heat shielded BUT this would exclude the use of RETROS , because the exhaust ports would allow heat in, UNLESS the cylinder came in arse end first ..ie the screw bit ..now this is 2 ft thick ..big and blunt, then the martians would have to reorient for the landing ..which leads to my statement above about being able to control the landing Thank you! Yes, so far as I know all the American and Russian spaceships used retro fire only outside the atmosphere. There are several very good reasons for this-- all involving danger to the spacecraft. Suggesting they fired retros all the way down to the ground... Uh-uh.
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Post by Topaz on Mar 3, 2005 5:12:50 GMT
I looked at my copy of "Asimov on Physics" but can't see how to convert the formulas he uses to figure distance travelled given a fixed acceleration. If I could figure out what the G forces were assuming a 40 MPH impact with a stopping distance of 5 feet, I'd use that number. The formula I'm using is: G = Speed 2/(29.914*distance). Speed is in MPH and distance is in feet. Note the square on the speed variable - makes a big difference! I think there's a fundamental, irreconcilable difference in our approaches. Your approach seems to be: Figure out what will work best from currently known engineering applications, and then see how the novel's text can be re-interpreted to allow it. My approach is to assume the novel is a true account, and try to theorize a more realistic system within those confines, and without adding more complexity than is necessary. That's fine. I disagree with some of your conclusions and methods, but you've certainly got some creative and interesting ideas. In the end, I don't think either of us is all about being 'right.' I've enjoyed the discussion!
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Post by lanceradvanced on Mar 3, 2005 5:46:50 GMT
But there's no point in having a "stronger" vehicle... the cylinders are one use only, and then they get scrapped... Crush zones are a definite assist to the survival of the contents, probably weigh less than a cylinder designed to penetrate 200 feet of earth untouched.
If you want a real life example, back in the 30's through the 40's cars were made -very- solidly, with curved steel body pannels and a solid frame, if there was an accident (especially in racing) the -car- usually survived mebbe with a few scratches... -the passenger- was another story, because of the strong construction of the car, there was nothing slowing the passenger down, he got tossed -immediatly- forward, when the car hit... the car and the passenger declerated at -vastly- diffrent rates. Then Crush Zones and seatbelts came in (and eventually airbags) they not only slow the acceleration down, they also keep everything together as a unit.
You can't, without knowing how long it took to declerate...
but the basic equation will be something like...
(D/T)/G=X where T=(Sum of the Series of the Square of the Time) or... 1.5*(T)/9.8 = X (done in tidy meters/sec)
(man,I hate doing physics problems in english units)
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Post by Lensman on Mar 3, 2005 6:07:00 GMT
The Airbag needed to slow down from 1,000 mph would only need to be a measly, 1562.5 ft long, or a bit less than a third of a mile long... Nowhere near as big, but still noticible. You're assuming, what was it, 8.25 G's? I didn't find much in a brief Internet search of G forces, but a couple I did find suggested an upper limit of 30 G's for rocket sleds and air bags. If Martians can withstand 40% of that, it's 12 G's just with an air bag/acceleration couch. As I said, I dunno how much better the water immersion is. Maybe only a bit better; maybe more than twice as good. And I think you've left out the distance the cylinder travels between the time its nose first strikes dirt, and when it finally stops moving. That's about the full length of the cylinder, from the descriptions. (The first cylinder is described as "almost entirely buried in sand", and of course the bottom of the pit it is in is below the level of the surrounding land. So all together, the stopping distance may be slightly longer than the length of the cylinder.) Anyway, it sounds like at a barely-subsonic speed -- around 650 mph -- it would be survivable. ====================== The following doesn't support my proposal-- in fact, I think it throws into doubt the entire concept of atmospheric braking for something as massive as the cylinder-- but here is some "hard" data which has a bearing on this discussion: ~~~~~~~~~~~~~~~~~~~~ Meteorite Impact VelocitiesThe average velocity of meteoroids entering our atmosphere is 10-70 km/second. The smaller ones that survive the trip to the Earth's surface are quickly slowed by atmospheric friction to speeds of a few hundred kilometers per hour, and so hit the Earth with no more speed than if they had been dropped from a tall building. For meteorites larger than a few hundred tons (which fortunately are quite rare), atmospheric friction has little effect on the velocity and they hit the Earth with the enormous speeds characteristic of their entry into our atmosphere. Thus, for example, it is estimated that the meteorite that produced the Barringer Crater was still travelling at 11 km/second when it struck what is now the Arizona desert 49,000 years ago. Such objects do enormous damage, because the kinetic energy carried by the meteorite is the product of the mass and the square of the velocity. ~~~~~~~~~~~~~~~~~~~~ --from csep10.phys.utk.edu/astr161/lect/meteors/impacts.html
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Post by lanceradvanced on Mar 3, 2005 6:39:59 GMT
Why not, if the fuel is totally expended a few hundred feet to a mile up, why not, the empty tanks would make a good crumple zone, not that it -has- to be a liquid fuel rocket, a solid fuel system, might make more sense, and it could allways be jettisoned befor impact...
And yes our ships fired retros outside of the atmosphere -because they were in orbit- and the retros were use for the sole purpose of slowing the ship down -enough to cause it to fall naturally out of orbit- landing a ship "on it's jets" so to speak is something we haven't tried yet, (except for a couple of test flights in the DCX) but would be perfectly possible if you had a reliable enough engine, in that case you -wouldn't- be hitting the atmosphere fast enough to create friction, having slowed most of the way down before you got there, and would be coming in at an almost leisurly speed (thoug upper atmospehric temps would probably make you want to get through it in a hurry).
It clearly describes the powerfull forces of the landing, a flash, a bang, the earth moving, but just because the landing involved powerfull forces doesn't mean that that landing was uncontroled.
They're not "faking" a metoritic landing, they're just makeing a -very limited- one, involving moving just enough earth to bury the cylinder, and that kind of landing requires -more- control, than pure frictional aerobraking can accomplish
You're making an all or nothing agument, either the martians had -total- control of their landings and could have come down softly, without the impact crater, chooshing exactally where and when to land or they came in -utterly- uncontroled, landing completly uncontolled...
There's -plenty- of room between those two extremes, as for what the martians would have "perfered" that's a very -hard- call, I still think they chose their landing method as opposed to being -forced- to it.
If the martians were coming in completly uncontoled, why did none of them hit a rock outcropping and get smashed? or land in a lake or pond? Hitting Horsell Commons, A Golf Course?, Bushey Park and Regent's Park, and gardens in Wimbelton? pretty good luck for "random" shots, the 5th one is the only one that seemed to hit anything but open ground, even in the middle of london..
As an aside, I don't really consider any proposal that requires the launching cannon back on mars to be bigger/heavier/longer etc, to be -all- that much of a disadvantage, the advantage of "leaving your engine at home" would tend to outweigh just about any other consideration. By using a launching cannon the martians bascally -halved- the needed weight of their ships, since they only needed to consider what they needed to land, and not what they needed to -take off- as well in the design of their vessels.
If we dismissed it, nobody would have responded to it, as it is, it I think the distances between the first two shots were logistically trivial, they -did- land close enough to "support" each other, close enough that they were able to cross (in your argument not mine) in an "incomplete" FM and come back and wipe out the troops on the common before waltzing over to the third shot to show off for the narrator and walking back again by the time the narrator and artilleryman went up to the window to watch them.
But once the fighting started the Martians got there -before- the cylinders did..
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Post by Lensman on Mar 3, 2005 6:41:13 GMT
You could put the heat sheild between the retros, and the cylinder body, as was done in mercury space capusles... When John Glen came after his orbital flight, they were worried that his heatsheild might have been jarred lose, and they had him come in, withoug jettisoning the retropack, so that the retrounit, would keep the heat shield in place. en.wikipedia.org/wiki/Image:Mercury_Capsule2.jpgIn addition, there are some proposed aerobraking designs, where the exhaust from the retro's themselves is used as the "heat shield" as long as the rocket exhaust escaping faster than the ship is traveling, it would provide a buffer between the ship and the hot atmospheric gas. Ack! I had completely forgotten about the incident with John Glenn having the retro pack still in place outside the heat shield. And that's fascinating about using the exhaust as a "heat shield." My arguments are burning up at meteoric speed! However, I still maintain that if the Martians used retro rockets, they used them very stupidly. You're suggesting they used them to reduce their incoming speed by something like 99%, but not that extra bit that would have allowed a soft landing. Or that they actually did have a soft landing, but used a missile to fake the meteor crater. And as to "why," you say "digging a foxhole." But "foxholes" made by artillery shells do not show the "splash" appearance of a meteor crater, and Wells very clearly describes the splash effect. And again, there's no good reason for them to have partially buried themselves. It didn't provide them protection; it just made them helpless for many hours. In fact, burial would have slowed the cooling of the cylinder, so this would have caused them to be helpless even longer than just laying out in the open. Nor do I find the argument persuasive that they woundn't have desired controlled landings in a remote area, "because that would have put them farther from London." So what? With the speed their tripods moved, it would have made little difference.
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Post by Lensman on Mar 3, 2005 7:28:48 GMT
The 'table-like' section does suggest that the surface was flat, or nearly so, while I personally see the entire segment with the emerging Martian working better with the opening more perpendicular to the ground. A table tilted on its side is still flat and "table-like". The fact that things kept falling out of the end of the cylinder-- first the screw plug, then the Martian-- indicate to me it was closer to horizontal than vertical. This would be consistant with a relatively shallow entry angle, as is the description of the meteoric entry as "a line of flame high in the atmosphere" (from near the beginning of Book one, Chapter 2). All this is consistant with the shallow entry angle required for atmospheric braking.
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Post by Topaz on Mar 3, 2005 8:05:50 GMT
C'mon, Topaz, I think you know that's not so. If you were in a car crash, would you rather your body be stopped by an air bag, or a bunch of needle-sharp steel spikes? Of course, but if the spikes stop you in the same distance as the air bag, the G-loading would be the same. The equation doesn't care, or there would be a term in there to account for the media used to stop the moving body. In terms of G-loads, it's absolutely true. With these speeds, we're already into G levels that make even a soft airbag lethal via internal organ damage. Spikes aren't required. Mostly correct, but you forgot to add in the distance the vehicle travels into the ground while making the crater. You or Lanceradvanced suggested a "crush zone" to allow some travel distance during the crash. I don't think that would be necessary; the cylinder being driven into the ground for most of its length provides the same effect without the Martians having to weaken their vehicle. Absolutely true, and having a movable capsule inside the cylinder effectively 'telescopes' the distance. However, the sum of the deceleration distances still equals the total distance required. So, if the total distance to keep the G's to an acceptable level is 1,400 feet and the capsule inside the cylinder is slowed over 100 feet inside the cylinder, you'd still have 1,300 feet of penetration into the ground to keep the G loads down to an acceptable level. I wonder if the Martians brought really tall ladders? ;D Sorry, but no. As you just said, the water prevents your body from distorting. A form-fitting couch only reduces this--it doesn't eliminate it. The water provides protection all around, and automatically distributes the pressure evenly across your body. Designing a couch to do as good a job would be difficult or impossible. Some parts of your body can withstand accelleration better than others, and certain parts weigh more than others, which means you need different depths of padding and different stiffnesses on different parts of the couch. Water immersion avoids all these difficulties. It's only a matter of degree, and a relatively small degree at that. Well-fitting ouches get almost all of the effectiveness of the water-tank method at a fraction of the weight. Both methods support the body smoothly and without the distortion that would cause injury to bones and ligaments. Water spreads the load hydraulically, whereas the couch allows the body to distort a little bit to fill in the tiny gaps between the body and the couch surface. This is why real-world spacecraft use couches instead of water tanks. And no matter what system you use, none of them can protect the body beyond the individual G-tolerance of the internal organs. It's not much use keeping your ribs or legs from breaking if your brain is turned to mush! I'm not sure about the side of your body away from the impact. Does water immersion convert kinetic energy to hydraulic energy? If so, then you've just doubled (at least, probably more) the amount of accelleration you can withstand, as hydraulic pressure will be applied evenly all around your body, including the "upper" side, whereas with an accelleration couch it's applied only to your lower side. Yes, the water converts the increased weight into hydraulic pressure. Unfortunately, if the load on the bottom half of your body was effectively 'halved' as you're saying, your body still is pushing down with the entire force and your body would sink, and quickly. Neglecting the G-tolerances of your internal organs, what would eventually happen is that the water pressure would crush your body, but in reality your internal organs would smash inside your body before that actually happened. Yes. But since you keep hammering away at that one point, I'm guessing you're missing the point that concussions typically happen because the skull was *suddenly* hit with a powerful impact. With the air-bag system--or its equivalent--we're eliminating that impact, and spreading the force over distance and time. The limit becomes not what the brain can withstand in a momentary impact, but rather what it can withstand in the way of gradually applied acceleration against the skull. Clearly you want to stop short of massive bruising or bursting of blood vessels beyond minimal capillary damage. Yep, I agree with all of that completely. No argument at all. The trouble we're having here is that I don't think you really realize the magnitude of the G-forces involved in an unbraked impact at several hundred miles per hour, assuming simple drag slowed the cylinder even that much. At those speeds, the distance involved in any reasonable depth of airbag still puts the G levels into the range that the type of damage you describe is exactly what would happen. It would probably be lethal to humans, let alone the more-fragile Martians. I don't know what acceleration air bags produce. The distance traveled may be--probably is--greater than I indicated in my earlier back-of-the-envelope calculation, because of the telescoping of the front of the car, but even if you double the stopping distance to 5 feet, there's still enuff room for the deceleration capsule to stop even without adding in the distance the cylinder travels thru the earth before stopping. Fair enough. Let's run those numbers! Let's say the cylinder is 270' long (A shorter cylinder would increase the G's, plus I like a 3-1 ratio for purposes of shooting out of the gun). Let's further say that the entire length of the cylinder presses into the ground, leaving no appreciable distance above the surface. Let's also use the 600mph impact speed we used before. I think that meets your description of the impact speed, for the 'splashing' effect. Yes? Lastly, let's allow that the Martians (or the capsule they're in) is cushioned by a 5' deep airbag. So, the total deceleration distance works out to: d = Cylinder length + Air cushion depth d = 270 + 5 d = 275' V = 600 mph This gives: G = 600 2/(29.914*275) G = 360,000/8,226.35 G = 43.8 No assumptions on this one. Just pure math on a very simple formula. I did a little more research about human G-tolerance. I found the following very interesting PDF article: csel.eng.ohio-state.edu/voshell/gforce.pdfTurns out that our test case is almost exactly the conditions to which one Col. Stapp subjected himself, using a rocket sled and a hydraulic braking system. He hit (and survived, with injuries) about 46G's. The test ruptured "nearly all" the capillaries in his eyes, which rendered him blind for almost a day before the blood cleared. He also suffered disorientation, which implies that he was very near to injuring his brain in this test. You'll note that these injuries are G-load injuries to soft tissues and that a water-tank system would not improve these results. Stapp was adequately supported by his couch to the point that all his injuries were internal. It should be noted that Col. Stapp also suffered broken ribs and other bones at lower G-levels in these tests, so the testing method would produce 'deformation' injuries if he was inadequately supported. So, yes, a human could survive a landing like this - barely. I'm not sure the guy would be ready for battle right away, though! The questions are: 1) Could a Martian survive this kind of impact at all? 2) Would he be useful for doing any kind work afterwards, if he did? Unless you object, I'd like to use same methodolgy for determining maximum G-tolerance of the Martians as before, but adjusted for this new human-tolerance data. GMAX human = 46 Mars gravity is about 33% of Earth gravity, so... GMAX martian = 46*0.33 GMAX martian = 15.2 So this landing would exceed the Martian's maximum G-tolerance by 288%. They'd either be dead or severely injured after landing, even with the airbag and using the entire length of the cylinder to slow down. Even if we double their G-tolerance, they'd still come away with eye and brain injuries probably severe enough to keep them out of action for days, if not weeks. So, using the same technology and cylinder, how deep would the airbag or other system have to be to keep the Martians safe? I don't like the idea of coming in at maxium G-tolerance - it's too big a risk and doesn't have any kind of safety factor. Usual safety factor in aerospace design is 1.25, so let's have the Martians endure 1/1.25 or 80% of their maximum G-tolerance calculated above. G safe = 15.2 * 0.8 G safe = 12.2 Rearranging the equation... d total = V 2/(29.914)*G) d total = 600 2/(29.914*12.2) d total = 360,000/364.95 d total = 986.4 feet Subtracting the length of the cylinder gives... d airbag = 986.4 - 270 d airbag = 716 feet Which is, of course, longer than the cylinder. So no, we still have not demonstrated that an unbraked landing is survivable by the Martians. The landing speed needs to be much lower if they're going to survive a landing like this.
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Post by Lensman on Mar 3, 2005 8:14:46 GMT
~~~~~~~~~~~~~~ Lensman wrote: If what you say were true, a car going at 60 mph would need 4x the stopping distance of a car going 30 mph. Not so; it needs only twice the distance. ~~~~~~~~~~~~~~ And probably twice the force applied to the brakes as well... go look in any physics textbook, the formula is KE = 1/2 MV^2. Let's say a rocket requires A amount of delta-V to reach velocity B. To reach velocity 2B, Does it need 4A amount of delta-V, and hence 4 x the amount of fuel burned? No, it only needs 2A. A decceleration is just a negative acceleration. Objects don't "magically" become more difficult to accelerate just because they're moving faster... unless you're talking about relativistic speeds, which we're not. Yes, the way kinetic energy is measured, a body falling twice as far will hit with 4x the force. But the amount of injury a human body sustains vs. distance of falls during accidents is a linear relationship, not a geometric one. This suggests to me that "kinetic energy" is more a mathematical concept that anything relating to real life. Are you, possibly, trying to apply the formula for changing direction at increasing velocities? What we're talking about is a straight-line acceleration. At least, that's what I'm talking about. I'm not sure what you are; possibly one or the other of us lost the context of what we were saying somewhere along the line.
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Post by Lensman on Mar 3, 2005 8:20:07 GMT
In short, retros are reasonable, and as well supported in the book as any liquid landing capsule, or other exotic shock absorber. We will have to agree to disagree on this. I have given many reasons why retro rockets are not reasonable in the context of Wells' novel, and there's no point in my continuing to repeat those numerous reasons.
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Post by Topaz on Mar 3, 2005 8:25:18 GMT
However, I still maintain that if the Martians used retro rockets, they used them very stupidly. You're suggesting they used them to reduce their incoming speed by something like 99%, but not that extra bit that would have allowed a soft landing. Or that they actually did have a soft landing, but used a missile to fake the meteor crater. And as to "why," you say "digging a foxhole." But "foxholes" made by artillery shells do not show the "splash" appearance of a meteor crater, and Wells very clearly describes the splash effect. And again, there's no good reason for them to have partially buried themselves. It didn't provide them protection; it just made them helpless for many hours. In fact, burial would have slowed the cooling of the cylinder, so this would have caused them to be helpless even longer than just laying out in the open. Actually, what I'm proposing is that they did use retros to effect a soft landing - in a pit blown open by another device fired from the cylinder on final approach. A missile with a large penetrator warhead would produce the crater and the ejecta 'splashing' described in the story, but preserves the safer, easier, soft landing. Lancer disagrees with me on this part, I believe, seeing a braked entry to a low altitude and then a fall to produce the crater by impact alone. That might be possible, but we'd need a soil mechanics expert to figure that one out. If the speed necessary to duplicate the effect falls turns out to be low enough, it might be possible. I, for one, think that forming the initial pit/crater as a 'foxhole' does provide a tactical advantage. Yes, it prolongs cooling, but keeps the cylinder out of direct line-of-sight of outside observers for all landings after the first one. That's important because after the first landing, we'll know they're hostile and we'll likely try to fight back. We have two opportunities to do that: 1) Shelling the cylinder from a distance. Being in a pit protects you from the shrapnel of a near-miss and makes the aiming process more difficult, especially compared to a straight-shot into the side of an exposed cylinder. 2) Small-arms fire once the Martians emerge. If the cylinder was on the surface, the Martians would make a perfect target for any sniper who managed to crawl close to the landing site. Down in the pit, a soldier would have to get right to the edge of the crater to have any shot at all. This leaves the first cylinder to land completely exposed, but we can see how that turned out: "Maybe if we wave a flag, they'll know we're intelligent and want to make peace."
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Post by Lensman on Mar 3, 2005 8:34:38 GMT
But there's no point in having a "stronger" vehicle... the cylinders are one use only, and then they get scrapped... Crush zones are a definite assist to the survival of the contents, probably weigh less than a cylinder designed to penetrate 200 feet of earth untouched. Again this seems to point up our fundamental differences in approach. I assume that the cylinder is something which survived relatively intact despite punching a hole in the ground at meteoric speed, and having a wall thickness of nearly two feet, all as described in the novel. You are assuming that it's built the way we humans build modern vehicles, and then going back and trying to figure out how to twist what Wells wrote to fit your idea. Yes, it would serve my proposal well to suggest the cylinder did not have walls of solid metal nearly two feet thick. If I could believe it was much lighter, I could justify atmospheric braking that much easier. But I can't. That's not what Wells wrote. To paraphrase Sherlock Holmes: Theories should be twisted to fit facts, not vice versa.
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