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Post by Topaz on Feb 26, 2005 9:50:21 GMT
Perhaps a little math can shed some light upon this. I ran these numbers for myself initially, but I think they have direct bearing on your discussion.
Let's start with some basic assumptions:
Cylinder dimensions WxH: 90' x 270' (length three times diameter)
Let's assume that about 75% of the vehicle length is used up in deceleration. Between the length used for deceleration and the depth of the crater, this would put the end of the cylinder at about ground level, as is described in the book. In that 75%, I'm including both the 'collapsing nose' Lancer suggests and simple penetration into the ground such that, from the moment of first contact, the crew cabin moves 202 feet before coming to a rest. Some of that is length of cylinder pushed into the ground and some of it is physical shortening of the 'crush zones.' (The no-crush-zone case is much worse, so this is conservative.)
That covers the characteristics of the vehicle and the landing. Regardless of the landing velocity, we have to decelerate from that value to zero in 202 feet.
So let's see what levels of deceleration result from varying landing velocities.
Unbraked ballistic entry:
We don't know exactly what velocity the Martian cylinders would have at atmospheric entry, but the Apollo vehicles returning from the Moon hit atmosphere at about 25,000 mph, so we'll use that value.
A lot of that speed is going to be lost during atmospheric entry for a craft like the cylinders, having a fairly low density compared to something solid like a meteorite. Since I don't know the mass and aerodynamic characteristics of the cylinders, I'm going to make a really wild assumption and say that, by the time the cylinder reaches the ground, fully two-thirds of the initial velocity is scrubbed off due to aerodynamic drag, even on a fairly good aerodynamic shape like a cylinder entering end-on. Pointing the front end just reduces the drag and increases the impact speed.
That leaves 25,000 x 0.333 = 8325 mph at impact (Yikes!)
Okay, here we go. I'm picking up this equation from one of my old engineering texts. It goes like this:
G = V^2/(29.914*d')
where:
G = force of deceleration in "G's"
V = velocity at impact in mph
d' = distance used for deceleration in feet.
The constant includes the value of G on Earth (32.174 ft./sec^2), adjusted for the units we're using here.
Entering in our values gives:
G = 8325^2/(29.914*202) = 11,469 G's.
Remember, this includes mechanical crush-zones in the forward end of the craft. I think that kind of impact precludes any kind of landing that doesn't use retro rockets or some other in-flight braking system. Even if roughly 88% of the velocity was lost to drag, the resulting deceleration is still the better part of two-thousand G's. Anything biological in the cylinder would be turned to jelly by that kind of landing, no matter how you packed or cushioned it.
Although I think it unlikely, one could make the argument that the cylinders would slow down more during descent. In fact, one could suggest that they'd get all the way down to their aerodynamic terminal velocity. We don't know the mass characteristics of the cylinders, although we can approximate the aerodynamic characteristics of them as, well, as a cylinder. Unfortunately, the lack of mass values makes calculating a terminal velocity in Earth's atmosphere impossible, since it's the speed at which the drag of the vehicle equals its weight. However, even side-on, the value is likely to be somewhere in the high hundreds of miles per hour. End-on, as needed for our 202' deceleration distance, the terminal velocity is almost certainly supersonic, and maybe highly so. There were British WWII bombs (GrandSlam and Tallboy) that achieved supersonic speeds when dropped from only 30,000-40,000 feet. Would a low supersonic speed be slow enough to allow the Martians to survive? That answer comes up at the end of the 'braked' case, which we'll tackle now:
Braked Landing, but impact still digs the crater:
This one assumes some kind of retro-rocket system (or some other in-flight braking system) is significantly slowing the vehicle prior to impact. The tough part of this one is figuring how much velocity is still required to shove 202 feet of 90 foot wide cylinder into a relatively unknown soil. This one's gonna be even more rough than the first case. I'm no soil mechanics expert, so I'm going to have to guess.
Let's try two different cases:
V = 300mph
V = 200mph
My own 'gut feeling' is that anything less than these wouldn't shove a 90' wide cylinder far enough into packed sandy soil like that of the Common, even with a pointed end, and still create the kind of crater and mounds of dirt that are described in the book. (Crushing the house is no indicator - you could drop the cylinder on it from twenty feet and get largely the same result.) Slower than that and you probably would not only not get the crater, but also the cylinder would likely just hit and tumble. I'm going to maintain the same total deceleration distance becaues making it shorter increases the G-loads. We're still using the full-on crush-zone system here.
So...
G = 300^2/(29.914*202) = 14.894 G's
G = 200^2/(29.914*202) = 6.620 G's
Okay, that's a little more like it! Taking the human body as a reference, I believe we can take approximately 25 G's for 'extended' periods without sustaining damage to the blood vessels in our eyes, the retina, brain tissue, etc. We can take much more in nearly instantaneous loadings. Would that apply here? How long would the deceleration in our example take?
t = v/G
where:
t = time to decelerate, in seconds
v = landing velocity in feet per second
G = landing deceleration, multiple of 32.174 ft/sec^2
which gives,
t = (300*1.467)/(14.894*32.174) = 0.92 seconds
t = (200*1.467)/(6.620*32.174) = 1.4 seconds
Neither one of those is an instantaneous G-load (where they're talking about durations of mili- or micro-seconds), so we have to figure out the Martian's G-tolerance to sustained loads. If we assume that their G-tolerance is similar to ours, but proportional to the ratio of the gravity fields on Earth and Mars, we might have a shot.
The gravity on Mars is about 33% of that on Earth, or Earth's is about 300% of Mars', if you want to do it the other way.
So if the Martian's G-tolerance is proportional to that ratio, we get:
25G * 0.33 = 8.25 G (Earth gravities) maximum G-tolerance.
So the Martians should be able to take about eight and a quarter G's, if they're relatively as robust as we are, proportional to their native environment. From that, it looks like they could survive a 200mph impact, assuming they use 202 feet to slow down. A 300mph impact would probably do them serious injury, so that speed is too high, as is any reasonable terminal velocity in the unbraked case.
I think this pretty clearly shows that they're going to need some kind of in-flight braking system, either by retro-rocket or some kind of drag-brake system that somehow isn't visible after landing. I'm sure some of my assumptions are eminently challengeable, but I think I'm in the ballpark for the most part.
This still begs the question as to whether you'd want to land at about 80% of your maximum G-tolerance. It's an engineering/medical/strategy decision, and we don't have enough information from Wells to see into the Martian psyche that far.
One thing to consider is that the launch acceleration would likely be in the same range as we're talking about here for landing.
Personally I still like what Bayne and I come up with - an even softer landing and the pit is 'dug' by a missile launched from the cylinder. If you have a retro system that can slow you to 200mph, why not get down to less than that?
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Post by Topaz on Feb 26, 2005 10:37:45 GMT
I did post those links in the begining of that "Great Gun" thread I started a while back, what kept ya? Ah yes, so you did. I hadn't come along yet - I joined the forum in January - and I have to admit that I didn't... read... the first few posts in that thread very... thoroughly. Ooops. ;D
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Post by Lensman on Feb 27, 2005 2:43:48 GMT
If you have a retro system that can slow you to 200mph, why not get down to less than that? Exactly the point I made above. Clearly they did *not* have a retro system to slow their landing. And if they had, the crash landing would not have been as violent as the landing of the 5th cylinder is described. And a capsule wall nearly two feet thick does *not* suggest something with a "crush zone" to me. In From the Earth to the Moon, Verne has his astronauts survive launch from the "space gun" with an elaborate series of shock absorbing devices inside the capsule. Apply the math, tho, and it doesn't work; the shock absorbers are completely inadequate, and the passengers would be smashed to jelly. What I proposed above, using a water capsule to cushion the impact, is a more realistic version of Verne's idea. Since the capsule is rather long, the water-filled "landing chamber" could itself be suspended in some sort of shock absorbing mechanism, if necessary running the length of the capsule, giving the passengers that distance to stop. This would have the same advantage of a "crush zone" in the capsule wall, without the structural weakness necessitated by that. Fragile mechanical components could be cushioned in a similar fashion. Regarding Lanceradvance's claim that atmospheric braking cannot slow the capsule significantly: Not so. That's exactly how the (pre-Space Shuttle) American space capsules lost most of their velocity on re-entry. The idea is not to come straight in, but to strike at a shallow angle, and "bounce" off the atmosphere several times until enough velocity is lost to safely descend. Think of it as skipping a stone across a pond. After most of the velocity was lost, American space capsules deployed parachutes for the final deceleration. Obviously the Martians don't do that, and hit at what is still a high velocity. I'm not sure what velocity would be required to "splash" soil out of a crater, as Wells describes, but I'm guessing several hundred miles per hour.
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Post by lanceradvanced on Feb 27, 2005 3:28:45 GMT
Exactly the point I made above. Clearly they did *not* have a retro system to slow their landing. And if they had, the crash landing would not have been as violent as the landing of the 5th cylinder is described. Unless they wanted it to be, the foxhole effect having been discussed at length, and witch seems to suit their purposes perfectly... 2 feet of -screw- shows, that doesn't mean the hull is two feet thick... =========== |////////////////| } = 2 feet... =========== And there's as much evidence for that as there is for a crush zone or retros, or explosive braking module or any other form of crash protection, not to mention the techinical difficulties in the design of the chamber which would be as great or greater as any of the more conventional systems others have proposed. Problem is that the reentry speeds weren't coming in at interplanetary speeds, and they -did- use retros to drop themselves out of orbit in a controled fashion, they wern't plummeting in from mars in the totally uncontrolled manner you argue they did, relying -solely- aerobraking, and all of those capsules were far smaller than the cylinder. Simple summation, I don't think the case for coming in totally uncontrolled is very good, being more likely to produce flattened landscape and splatterd martians. and that the best case is that they came in in a controled fashion, and hit in a manner calculated to fortify their bases in the course of landing, I perfer mecanical means , over Topaz and Bayne's "digger charge" but we both agree that the decent was under control.
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Post by Topaz on Feb 27, 2005 8:18:40 GMT
Exactly the point I made above. Clearly they did *not* have a retro system to slow their landing. And if they had, the crash landing would not have been as violent as the landing of the 5th cylinder is described. Well, Lensman, I've got to disagree with you there. I'd say exactly the opposite. Clearly the Martians must have had some kind of in-flight braking system, else they would not have had any kind of survivable landing. What I proposed above, using a water capsule to cushion the impact, is a more realistic version of Verne's idea. Since the capsule is rather long, the water-filled "landing chamber" could itself be suspended in some sort of shock absorbing mechanism, if necessary running the length of the capsule, giving the passengers that distance to stop. This would have the same advantage of a "crush zone" in the capsule wall, without the structural weakness necessitated by that. Crush zone, shock absorbers, the system you use really doesn't matter. As I've shown earlier, the equation doesn't care what you use to create the deceleration distance, it's that deceleration distance itself that matters, along with the change in velocity required. There's no way to change the physics. Floating the Martians in water in no way reduces the G forces they encounter. The water and the Martians still undergo the same amount of deceleration, together. Even flowing water into their lungs does nothing to protect other tissues such as the brain, which would be crushed inside the cranial cavity by an unbraked landing. It's like putting an egg inside a tin can, then placing the can inside a tank of water. The whole system is decelerated equally, so the egg simply crushes against the inside of the can, rather than inside the tank itself. If the can moves down the length of the tank during the process, the deceleration of the can/egg system can be reduced somewhat, but I find it unlikely that the cylinder is actually a long tube filled with water, containing a relatively small crew/cargo cabin that slides the length of that tube. Easier and lighter to do almost anything else, especially since that system alone would still not be enough to make for a survivable landing. Regarding Lanceradvance's claim that atmospheric braking cannot slow the capsule significantly: Not so. That's exactly how the (pre-Space Shuttle) American space capsules lost most of their velocity on re-entry. The idea is not to come straight in, but to strike at a shallow angle, and "bounce" off the atmosphere several times until enough velocity is lost to safely descend. Think of it as skipping a stone across a pond. Actually, only orbital systems have ever used that method (and I believe only the Soviets/Russians actually did it, although I might be wrong about that). The Apollo capsules - which are the only ones to enter from an interplanetary trajectory like the Martian cylinders - didn't dare skip on the atmosphere, as they came in far above orbital velocity and would continue away from the Earth if they did. They couldn't scrub off enough speed to get down to orbital velocity in one 'skip', so they'd keep right on going off into space after that first one. As for how much they'd slow down once into the atmosphere, the capsules to which you refer had much higher drag coefficients because of their blunt, short, shapes than the relatively long, pointed Martian cylinders most people visualize. They would slow down much more in the atmosphere than a Martian cylinder having equal density. The Apollo capsules had a terminal velocity of around 300mph without parachutes. Even with a deceleration distance of 75% of the length our assumed Martian cylinder, I've demonstrated that a landing at that speed is not survivable for the Martians, whether you use crush zones, shock absorbers or any other device. Deceleration distance is deceleration distance, period. And the Martian cylinders would have a terminal velocity - assuming equal density to the Apollo capsules - somewhere well above the speed of sound, not 300mph. The Martians would be crushed on impact - from the inside if they were floating in water. It's not that we have no evidence for a retro-rocket system, it's that the physics of the situation requires such a system, or something that provides an equal effect prior to impact. Without one, the Martians die or are severely injured on impact. The equations don't care about anything but velocity change and distance. Wells may not describe a braking system in the few scraps of information we get about the cylinder, but I don't need to have him do so. The physics require it, whether he mentions it or not. I'm not sure what velocity would be required to "splash" soil out of a crater, as Wells describes, but I'm guessing several hundred miles per hour. Which is unsurvivable. Something else splashes the ground. If the cylinder strikes with enough force to do it, the Martians die. Mathematics. Just for the sake of knowing, let's say the cylinders do slow aerodynamically to, say, 600 mph. How long would the deceleration distance have to be to keep the load under the 8.25G maximum-tolerance figure I used earlier? The formula is rearranged to: d' = V^2/(29.914*G) (same variables as my last post) so... d' = (600^2)/(29.914*8.25) = 1,458 feet. So, even if the crew cabin were able to move the entire length of the cylinder (using water, shock absorbers, or a combination), the cylinder would have to be over a quarter-mile long to keep the landing within the sustained G-tolerance we calculated for the Martians earlier. Getting the value down to the same 6.620G that we arrived at last time means the cylinder would have to be just over 1,817 feet long. I don't believe that it's realistic to posit a cylinder that's a quarter-mile long, just to accomodate a crew/cargo cabin that's a tiny fraction of that size. And a water column 90' wide and 1,458' long would weigh almost exactly 290,000 tons. How could you hold that together during launch, let alone landing? How could you launch it in the first place?
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Post by Lensman on Feb 27, 2005 8:44:39 GMT
2 feet of -screw- shows, that doesn't mean the hull is two feet thick... I can't think of any rational reason for the screw to be significantly longer than the thickness of the hull. Can you? And there's as much evidence for that as there is for a crush zone or retros, or explosive braking module or any other form of crash protection, not to mention the techinical difficulties in the design of the chamber which would be as great or greater as any of the more conventional systems others have proposed. But my system doesn't require any change or external addition to the system of the Jules Verne type giant "bullet" shot from the space gun, which clearly is what Wells was using. And, as I said, Verne put in internal shock absorbers to protect the passengers. I'm merely modifying that internal system from what Verne described to something that would actually work. More importantly, my proposal for atmospheric braking plus an improved internal shock absorber does not in any way contradict the text of what Wells wrote, nor (so far as I can determine) even anything suggested by the text. Problem is that the reentry speeds weren't coming in at interplanetary speeds, and they -did- use retros to drop themselves out of orbit in a controled fashion, they wern't plummeting in from mars in the totally uncontrolled manner you argue they did, relying -solely- aerobraking, and all of those capsules were far smaller than the cylinder. Apollo 13 came in with no retro fire, only course corrections after the accident. Of course we don't know the Martians' velocity relative to earth, but I would suspect it was rather less than twice Apollo 13's velocity. In other words, if Apollo 13 could use atmospheric braking coming directly from the moon, without using orbital insertion first, then I suggest the Martians coming from Mars could do the same thing. As far as the size of the cylinder vs. the capsule, that's not an important factor. What's important is the density (mass in relation to size) and cross section (amount of wind resistance). A larger object has more wind resistance, hence more atmospheric braking. I'll admit that in light of what we know about manned flights to the moon, it's hard to justify being shot from a giant cannon on Mars and hitting the Earth's atmosphere accurately enuff to bounce several times and still hit an area the size of southern England. You're right that it's more believable if they can make mid-course corrections. I would prefer to say "Well, the Martians are said to be more intelligent than we are, perhaps they figured out how to make the shots so accurate they didn't need mid-course corrections." However, even if we are forced to give them some sort of "thrusters"-- i.e. very weak rockets -- that can be something very simple, involving nothing but compressed air jets. That could be a very simple system, with little added mass. A retro rocket system would add much complexity and considerable mass, especially when you consider the mass of the fuel necessary. If retro rockets are used in the manner you suggest, then the Martians get the worst of both -- the weight, fuel requirement and complexity of a large rocket system added to the launch package-- without the benefit of a controlled, soft landing, which a full retro rocket package should allow. The text does not support this, nor do I think it reasonable to believe the Martians -- who are supposed to be smarter than us -- would take such an overly complex approach. If the Martians used a giant cannon to fire a giant bullet-shaped capsule, there must have been a rational reason for them to use that system rather than a multi-stage rocket. It seems reasonable to me that the reason was simplicity (less chance for failure) and reduced weight. If you add a powerful rocket system and the weight of a large amount of fuel back in, you've negated most of the advantage of using the cannon.
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Post by Lensman on Feb 27, 2005 9:06:25 GMT
Oh, I missed this earlier: The earth -splashed- but it didn't -explode- the way bolide impacts do, yes it blew a good sized pit, but if it had come in uncontroled there should have been rocks raining down all over Woking after the impact. It's the bolide which explodes, not the earth. Bolides contain a large amount of ice/snow, in addition to rocky chunks. The ice/snow when superheated by passage thru the atmosphere can explode, sending the rocky chunks flying for a long distance. Clearly, this has nothing to do with the cylinder landing. Non-bolide meteors which crash don't explode.
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Post by Lensman on Feb 27, 2005 9:15:18 GMT
If the martians were coming in "uncontroled" they could have skipped the figting machines completly, a cylinder sized mass slamming in from martian orbit would have the effect of a medium sized nuke, in and of itself. It's clear from the text that the Martians couldn't control their precise landing point-- which is another indication they didn't use retro rockets-- they landed all over the place. Therefore they wouldn't have been able to, say, "nuke" just the population centers. Dropping large rocks in random locations would not have served their purpose. They wanted to take over England, not destroy it.
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Post by Lensman on Feb 27, 2005 9:22:35 GMT
From what I remember in the book, even when discussing their remains, or the bodies that the dogs are getting to, what the dogs pull off is generally labeld as "gristle" and the birds were able to tear the martian in the hood of the machine to "shreads" I'm inclined to belive this describes a more cartiliginous skeleton at the least, rather than a bony one. It's certainly reasonable to believe the Martians had a cartilaginous skeleton, rather than a bony one, what with Mars' lower gravity.
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Post by Topaz on Feb 27, 2005 9:53:39 GMT
More importantly, my proposal for atmospheric braking plus an improved internal shock absorber does not in any way contradict the text of what Wells wrote, nor (so far as I can determine) even anything suggested by the text. Strictly speaking, neither does a retro-rocket system. The system would be at the front of the cylinder, and thus underground from the point of view of human observers. Apollo 13 came in with no retro fire, only course corrections after the accident. Of course we don't know the Martians' velocity relative to earth, but I would suspect it was rather less than twice Apollo 13's velocity. In other words, if Apollo 13 could use atmospheric braking coming directly from the moon, without using orbital insertion first, then I suggest the Martians coming from Mars could do the same thing. None of the Apollo capsules used retro-fire prior to atmospheric entry, as it wasn't part of the system design, so you're right about that. 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 far as the size of the cylinder vs. the capsule, that's not an important factor. What's important is the density (mass in relation to size) and cross section (amount of wind resistance). A larger object has more wind resistance, hence more atmospheric braking. Not true, except for very minor factors involving the viscosity of the air and the length of the vehicle. '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. I would prefer to say "Well, the Martians are said to be more intelligent than we are, perhaps they figured out how to make the shots so accurate they didn't need mid-course corrections." Theoretically possible, I suppose, but some of the factors (atmospheric conditions, for one) are not controllable during launch and so would have to be predicted in advance. I find it a stretch to imagine that they could predict every wind shear and breeze with enough accuracy to land precisely where they wanted to after an interplanetary flight. It's so much simpler to include a propulsion system, which would be a small fraction of the vehicle weight. If retro rockets are used in the manner you suggest, then the Martians get the worst of both -- the weight, fuel requirement and complexity of a large rocket system added to the launch package-- without the benefit of a controlled, soft landing, which a full retro rocket package should allow.... If you add a powerful rocket system and the weight of a large amount of fuel back in, you've negated most of the advantage of using the cannon. IMHO, a combined gun/rocket system is the best of all possible worlds - presupposing a requirement for a gun - particularly in the following type of system: 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. Doing so provides the following benefits: - By restricting the performance of the gun to sub-orbital speeds, this system is vastly simplified. You're leaving the planet, so why develop the system to the limit?
- Limiting the gun to sub-orbital speeds provides an abort mode, allowing a malfunctioning cylinder to return.
- Using a rocket for trans-Earth injection, course adjustment, and retrofire means the system is of minimum size and weight, as opposed to separate systems.
- Being able to do course corrections means you don't have to try to be perfect on launch, which is almost impossible anyway. Even an 'adaptive' gun would have to rely upon absolutely accurate predictions of gun performance and atmospheric conditions.
- Any weight penalty involved in providing a retro-rocket system for the cylinder is going to be more than offset by the weight reduction in cylinder structure resulting from obtaining a 'soft' landing.
As an aside, the rocket-boosted gun concept is exactly that used by the proposed 'space booster' product of the SHARP tests Lancer originally linked. I suspect that this was a result of trading off gun complexity against reduced payload for the round, as I briefly discuss above.
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Post by Lensman on Feb 27, 2005 11:16:45 GMT
Floating the Martians in water in no way reduces the G forces they encounter. The water and the Martians still undergo the same amount of deceleration, together. Correct, it does nothing to reduce the G forces. What it does is stop the body from being squashed like a bug by the force of impact. Immersion in water (or any fluid) would spread out the force of impact over the entire body, similar to the way an air bag works, only better. Even flowing water into their lungs does nothing to protect other tissues such as the brain, which would be crushed inside the cranial cavity by an unbraked landing. It's like putting an egg inside a tin can, then placing the can inside a tank of water. The whole system is decelerated equally, so the egg simply crushes against the inside of the can, rather than inside the tank itself. Yes, I'll admit I also realized this was a weakess in my proposal. I know you can get a concussion from your brain hitting the inside of your skull. So what we do is, put the Martians inside the deceleration capsule, and put the entire capsule inside a cylinder running for much of the length of the cylinder, and put something like an airbag in front of the capsule to cushion it. (Technically, I wouldn't have them use an actual airbag, but rather allow compressed gasses to escape at an exactly controlled speed, but the air bag analogy is immediately understandable.) Of course, the same thing is used in reverse for launching. I find it unlikely that the cylinder is actually a long tube filled with water, containing a relatively small crew/cargo cabin that slides the length of that tube. Great hawk! I never suggested filling the entire cylinder with water. Like I said, you use a small capsule to contain the crew. And another relatively small capsule to contain fragile components. Only the *capules* contain water. If you rig up the capsules to contain your water supply for the voyage, you don't add much mass. You do lose some space with the internal deceleration tubes the capsules slide up and down in. But with the entire cylinder 90 feet wide and presumably at least 180 feet long, you don't lose that large a percentage. Actually, only orbital systems have ever used that method (and I believe only the Soviets/Russians actually did it, although I might be wrong about that). The Apollo capsules - which are the only ones to enter from an interplanetary trajectory like the Martian cylinders - didn't dare skip on the atmosphere, as they came in far above orbital velocity and would continue away from the Earth if they did. They couldn't scrub off enough speed to get down to orbital velocity in one 'skip', so they'd keep right on going off into space after that first one. This was, indeed, a concern for Apollo 13. Too steep and they'd burn up; too shallow and they'd skip completely out of the atmosphere and possibly be stranded in orbit. The rock-skipping-on-a-pond analogy is, like most analogies, an inexact one. You want to go deep enuff into the atmosphere to have effective braking, but not deep enuff to burn up. And you don't want to "skip" so far up that you leave the atmosphere entirely, until you're below orbital speed. Part of the way you control this with the "attitude" of the spacecraft: The angle of attack. Presumably the Apollo command modules controlled this angle with the attitude jets; I propose the Martians control this with gyroscopes inside the capsule. As for how much they'd slow down once into the atmosphere, the capsules to which you refer had much higher drag coefficients because of their blunt, short, shapes than the relatively long, pointed Martian cylinders most people visualize. They would slow down much more in the atmosphere than a Martian cylinder having equal density. So increase the drag by increasing the cross-section of the cylinder. Add a huge layer of some sort of foam to the outside. The foam burns off during descent, so it's gone by the time it crashes to earth. We can either assume that's a layer separate from the heat shield, or we can assume the heat shield wasn't homogenous, but instead it was light and foamy on the outside, gradually getting denser toward the center until it was the hard flaky material seen after it landed. The Apollo capsules had a terminal velocity of around 300mph without parachutes. Even with a deceleration distance of 75% of the length our assumed Martian cylinder, I've demonstrated that a landing at that speed is not survivable for the Martians, whether you use crush zones, shock absorbers or any other device. Deceleration distance is deceleration distance, period. Your math included a lot of assumptions. Let me throw a real-world example at you: An air bag allows someone to walk away uninjured from a crash against a solid brick wall at, say, 40 MPH with a stopping distance of about 2 1/2 feet (for the skull; less for the body) If we assume the cylinder hits at 1000 MPH, an air bag would need 1000/40*2.5= 62.5 feet to bring the body safely to a rest. Since the suvival capsules should be able to travel up to 180 feet or more, I think that's plenty of room to absorb the impact, even with the Martians' more fragile bodies. Water immersion should allow us to reduce the safe stopping distance even further. Actually I would hope the cylinder would hit at subsonic velocity. As a matter of fact, they may have had a lighter density than the Apollo capsules, but I don't want to reduce the velocity *too* much or they won't create the true crater that Wells describes.
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Post by lanceradvanced on Feb 27, 2005 16:11:01 GMT
An air bag allows someone to walk away uninjured from a crash against a solid brick wall at, say, 40 MPH with a stopping distance of about 2 1/2 feet (for the skull; less for the body) If we assume the cylinder hits at 1000 MPH, an air bag would need 1000/40*2.5= 62.5 feet to bring the body safely to a rest. 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... I think the english would have noticed that...
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Post by TOMAHAWK on Feb 27, 2005 18:39:29 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 ...would seem to pan out when the narrator/curate was trapped...
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Post by Topaz on Feb 27, 2005 19:06:54 GMT
Yes, I'll admit I also realized this was a weakess in my proposal. I know you can get a concussion from your brain hitting the inside of your skull. So what we do is, put the Martians inside the deceleration capsule, and put the entire capsule inside a cylinder running for much of the length of the cylinder, and put something like an airbag in front of the capsule to cushion it. (Technically, I wouldn't have them use an actual airbag, but rather allow compressed gasses to escape at an exactly controlled speed, but the air bag analogy is immediately understandable.) Of course, the same thing is used in reverse for launching. I think you're missing a fundamental point. 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. They're all a means of producing a stopping distance, but in any case the stopping distance cannot exceed the length of the cylinder. In the example I used in my last post, I calculated the stopping distance for an 8.25G landing from 600mph. That's about a quarter-mile, no matter what system you use, so you'd have the crew cabin sliding from top to bottom of a quarter-mile-long cylinder if they hit that fast and you set that G-limit. No mechanical or hydraulic system is going to shorten that distance without also increasing the G-loads. An airbag works by increasing the distance of slowing your head before it is stopped by the steering wheel. Once your car hits an object, your head moves through a certain distance to the steering wheel. Without an airbag, your head isn't slowed until it meets the wheel, then the actual stopping distance is only that amount of distance provided by the deflection of the wheel (and your head!) All the G-loads are incurred in a fraction of an inch, and so they're huge. With an airbag, the stopping distance is now increased to the depth of the airbag because the bag is slowing your head over the entire distance. Because of that increased distance, the G-loads are much lower. 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. The reason floating in water was never adopted is that it provides little more protection than some kind of fitted couch, which would be much lighter. 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. A form-fitting couch does the same. 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. The weight factor and almost equal usefullness of a fitted couch are why water-tank systems have never been used for real-world spacecraft. Couches and such are better than laying on nails, I'll grant you, but they only allow you to reach the G-tolerance of your internal organs; no more. Great hawk! I never suggested filling the entire cylinder with water. Like I said, you use a small capsule to contain the crew. And another relatively small capsule to contain fragile components. Only the *capules* contain water. If you rig up the capsules to contain your water supply for the voyage, you don't add much mass. You do lose some space with the internal deceleration tubes the capsules slide up and down in. But with the entire cylinder 90 feet wide and presumably at least 180 feet long, you don't lose that large a percentage. 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. This was, indeed, a concern for Apollo 13. Too steep and they'd burn up; too shallow and they'd skip completely out of the atmosphere and possibly be stranded in orbit. The rock-skipping-on-a-pond analogy is, like most analogies, an inexact one. You want to go deep enuff into the atmosphere to have effective braking, but not deep enuff to burn up. And you don't want to "skip" so far up that you leave the atmosphere entirely, until you're below orbital speed. Part of the way you control this with the "attitude" of the spacecraft: The angle of attack. Presumably the Apollo command modules controlled this angle with the attitude jets; I propose the Martians control this with gyroscopes inside the capsule. Yes, the Apollo capsules used a reaction control system (RCS) of small rocket motors to control the attitude of the spacecraft. I suppose aerocapture like you're describing is possible - it's being proposed for some unmanned spacecraft going to Mars - but 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. So increase the drag by increasing the cross-section of the cylinder. Add a huge layer of some sort of foam to the outside. The foam burns off during descent, so it's gone by the time it crashes to earth. We can either assume that's a layer separate from the heat shield, or we can assume the heat shield wasn't homogenous, but instead it was light and foamy on the outside, gradually getting denser toward the center until it was the hard flaky material seen after it landed. Okay, that'd slow down the cylinder, certainly. A drag brake. A big, soft, cylindrical parachute, not to put too fine a point on it, since they perform the same function. 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... Are you sure you wouldn't rather use a retro-rocket? Your math included a lot of assumptions. Let me throw a real-world example at you: An air bag allows someone to walk away uninjured from a crash against a solid brick wall at, say, 40 MPH with a stopping distance of about 2 1/2 feet (for the skull; less for the body) If we assume the cylinder hits at 1000 MPH, an air bag would need 1000/40*2.5= 62.5 feet to bring the body safely to a rest. Since the suvival capsules should be able to travel up to 180 feet or more, I think that's plenty of room to absorb the impact, even with the Martians' more fragile bodies. Water immersion should allow us to reduce the safe stopping distance even further. Yes, my math certainly does include a lot of assumptions, but that doesn't mean they're incorrect. There is danger in assumptions, as I'd like to point out in your real-world example. The deceleration distance in your case is not just the 2.5 feet provided by the airbag. The nose of the car is crushing down and providing additional deceleration distance and is, in fact, the larger part of the stopping distance in an auto wreck at those kinds of speeds. 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. Lancer has already pointed out the problem with using straight ratios in this case. 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. ;D
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Post by Topaz on Feb 27, 2005 19:15:50 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 ...would seem to pan out when the narrator/curate was trapped... I think we've gotten locked into that assumption simply because Wells never mentions any kind of 'skid marks' or other manifestations of what you describe, although he describes the original landing site in some detail. The '53 movie uses that method, however, probably for the very reasons we're talking about here. It'd work, although the predictability is dependant upon knowing the topography of your landing site exactly.
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Post by TOMAHAWK on Feb 27, 2005 20:51:54 GMT
Well we know they landed on Hosell Common , which I presume is wide / open and from what I gather from the book didn't land in a town/city ..although Wells is vague as to the locations of the other cylinders , I presume that the Martians could control were they landed, so that would mean that they could control to an extent THE landing. (how would they cope if they landed on the icecap /or North sea)
...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
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Post by lanceradvanced on Feb 27, 2005 23:15:50 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... I think the english would have noticed that... Well, it took me about 10 minutes after I left the house and got down the road, but I made a boo-boo in my math and slipped an extra 0 in my numbers, figuring the airbag length needed for decelerating from 10,000 mph, not 1,000 mph.... 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. And the minimum speed that they'd be coming in at would be 11,310 mph, (the escape veloicty of mars) wich would give a travel time of about a year and 3 months, not counting any time lost to accleration en-route, due to solar or terrestrial gravity.
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Post by lanceradvanced on Feb 27, 2005 23:38:40 GMT
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 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. One other -minor- or not so minor detail, you also have to remember that as soon as the cylinders get close enoug to earth, they will start acclelerating, one does not only have to deal with their interplanetary speed, but the added velocity, of their fall, which can hit 300mph+, withing 7 seconds, falling from less than a mile up, and low orbit is 100 times that distance. The other detail to point out, is that despite depictions to the contrary, the cylinders seeme to have hit somewhat near vertically, not at an oblique angle, as evidenced by the narrators acount of how he was almost pushed by the crowd "onto the tablelike end of the cylinder" This of course raises questions of it's own, such as how, if the cylinder top was nearly flat, and the narrator was in danger of beind pitched -onto- it, how the top would have room to fall off inside the pit, unless a much smaller section of the top was unscrewing, rather than the entire 30yrd surface.
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Post by Topaz on Mar 1, 2005 0:02:05 GMT
In 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. Have you heard anything lately about that technology? The last I heard (several years ago) they were doing wind-tunnel testing and had found some vehicle control problems because the rocket plume kept moving around where it formed the primary shock with the atmosphere. The engineers thought they could handle the problem with more research, but I haven't heard anything since. Seemed like a really great solution. Another good one for the cylinder application would be what they were calling a 'truncated aerospike engine.' Basically it was a ring of combustion chambers around the base of the vehicle, with a sloped 'plug' in the center to form one wall of the flared exit nozzle. The shockwave off the back of the vehicle formed the outer wall, and it would adapt automatically to the ambient pressure conditions. Where this variant differed was that the bottom half or so of the spike was cut off and replaced by a heat shield. You'd enter the atmosphere tail-first, and the heat shield would form a strong primary shockwave that would divert the really high-temperature flow away from the annular nozzle of the rocket motors. It's easier to draw than describe, so here's a picture: The left-hand side of the side-view shows the ascent phase, while the right-hand side shows what goes on during re-entry. The blue area is the heat shield at the bottom of the aerospike 'plug'. As you can see in the bottom view, the exit nozzle for all the combustion chambers forms a single ring, all the way around the central 'plug.' You'd get a lot of drag from the blunt end of the aerospike in ascent, so they were talking about dumping the turbopump exaust into that area to 'fill in' the void with high-pressure gasses and so reduce the drag. Basically, you get the best of all worlds - a 'variable geometry' engine bell, plus you keep all the 'hot' parts at one end of the vehicle for all phases of flight. Another nice thing is that the major G loads always are 'down': the engine is pushing 'up' during ascent, and the air is pushing against the bottom of the vehicle to decelerate during re-entry. Having all the loads pointing in the same direction makes for a lighter vehicle. Obviously, this design could also be re-ignited on 'final approach' to slow the vehicle for a softer landing. The screw-off hatch would be at the other end, and this end would wind up buried at the bottom of the crater, nicely out of view of the writing public. There was a lot of talk back in the early '70's about using this system to turn the second stage of the Saturn V stack into a re-usable booster. The idea was that you'd throw away the first stage, then the second stage would take the payload all the way to orbit. After deployment of the payload, the stage would re-enter tail first, then touch down on extendable landing legs under parachutes. This was right before the Shuttle program really got going and that latter system killed any chance of this actually being used. The NASA wanted something with wings. A 'linear' version of the aerospike engine alone (pointed, without the heat shield and 'unwrapped' into a straight line) was planned for the X-33 project a few years ago, but that got canceled, too.
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Post by Topaz on Mar 1, 2005 7:52:53 GMT
The other detail to point out, is that despite depictions to the contrary, the cylinders seeme to have hit somewhat near vertically, not at an oblique angle, as evidenced by the narrators acount of how he was almost pushed by the crowd "onto the tablelike end of the cylinder" This of course raises questions of it's own, such as how, if the cylinder top was nearly flat, and the narrator was in danger of beind pitched -onto- it, how the top would have room to fall off inside the pit, unless a much smaller section of the top was unscrewing, rather than the entire 30yrd surface. You know, that's a really good point. It seems like the various descriptions in the book do conflict a bit. I wonder if Wells really had a very good visualization of the whole situation in his head when he wrote those passages. 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. Strictly speaking, the Narrator says he "narrowly missed being pitched on top of the screw," not the top of the end of the cylinder, but I don't know if that really changes anything.
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