Toxn

True, and as mentioned above getting stealth designs right is as much a matter of computing power and special software (and obsessive attention to detail) as anything else.
 
But there are some general principles that can be gleaned by eye. You don't need complex simulations to see that a B-52, for instance, is going to have a massive radar signature while a B2 is not. Similarly, debunking the idea that the Ho. 229 was a magical stealth plane doesn't require one to make a mock-up and put it into a radar chamber. A look at the intakes shows that any stealthy features were more or less a happy accident rather than the result of intent.

No complaints on my side either.
 
The one massive issue I had was that theatres were only showing it in 3D, and of course the shitty post-processing that they used to convert it made the beautiful cinematography muddy.
 
The only substantive complaint about the movie itself was that Paul didn't do the "give water to the dead" thing after his first fight. A small nitpick, but you're trying to establish that this character is still a kid out of his depth at this point. Instead he just ganks a man and moves on with his day while his would-be love interest looks worryingly aroused by the whole thing.
 
Edit: oh, and they did my man Gurney dirty in this one. Poor lad got better-looking (he's described in the book as looking more or less like a walking knuckle, with a livid scar running across most of his face) but lost his wits and the ability to sing. 

No complaints on my side either.
 
The one massive issue I had was that theatres were only showing it in 3D, and of course the shitty post-processing that they used to convert it made the beautiful cinematography muddy.
 
The only substantive complaint about the movie itself was that Paul didn't do the "give water to the dead" thing after his first fight. A small nitpick, but you're trying to establish that this character is still a kid out of his depth at this point. Instead he just ganks a man and moves on with his day while his would-be love interest looks worryingly aroused by the whole thing.
 
Edit: oh, and they did my man Gurney dirty in this one. Poor lad got better-looking (he's described in the book as looking more or less like a walking knuckle, with a livid scar running across most of his face) but lost his wits and the ability to sing. 

1. Introduction
 
Stealth is one of those buzz-words that everyone knows. Stealth makes aircraft invisible to radar, allowing a stealth plane to sneak up and punch other aircraft or SAM sites with impunity. Stealth is widely acknowledged as one of the most fundamental technologies that all new combat aircraft need to have. Stealth is also, like SH's old friend NERA, mostly completely misunderstood. This thread will attempt to change that, at least a little.
 
HUGE DISCLAIMER: I know, at best, the basics of what is essentially one of the darkest arts in an already black magic-heavy field (radio and radar engineering). I'll be relying heavily on others to correct my obvious mistakes, but this is and will be the lies-to-children version of the field, as told by another child. Still, given the state of knowledge out there, it's probably better than nothing.
 
2. The most basic basics
 
Okay, you ask, what is stealth then if we're all misunderstanding it? Here I think that the best analogy is that stealth is like camouflage, but for aliens. Camouflage famously entails the 5 (sometimes expanded to 7 with speed and spacing) S's: Shape, Shine, Shadow, Silhouette and Sound. Each operates on somewhat different principles, and can be more or less important in different scenarios, but are united in terms of how human senses work. We are pattern-finding creatures with passive senses, so anything that breaks up visual or auditory patterns, blends one into the background, or limits the amount of noise or reflected light one gives off will make you harder for another human to spot.
 
Radar, however, is generally not passive. Instead, a radar set sends out a beam of electromagnetic energy and looks for an echo. There's a huge amount of complexity in how this can be done (what frequency to use, how to generate and send the beam, how to track the returns and so on), but that's radar at it's most basic. So, like camouflage, ways to avoid radar will be united in trying to trick or defeat this basic mechanism. These principles are, roughly: Absorption, Redirection, Scattering and Emissions. Finally, and just to complete a fun acronym, there's also the side issue of radio-related Shenanigans. Taken together, these measures can significantly reduce how easy an aircraft is for a radar to "see" from certain angles.
 
Absorption is simple in concept: if something eats up the radar waves before it can get reflected, then the receiver doesn't get to pick up a signal and the plane doesn't get found. There's a whole realm of sneaky material science that goes into this, but from my understanding the two most common techniques currently being used are non-metallic structural components (which can be more or less transparent to radar) and foams or paints with nanomaterials in them (the famous grey stealth paint job generally being a weather coating instead of the magic material itself). These can be used in all sorts of clever ways: for instance, by making the forward edge of your wing out of radar transparent composite and then packing the area behind it with cones made out of radar-absorbing foam. Absorption can't make a non-stealthy design stealthy, however. It's more of a "cut 10% off our already-low radar return" sort of strategy.
 
Redirection is one of the single biggest reasons why stealth aircraft have their characteristic look. Generally the principle here is to make as many surfaces on the aircraft as parallel as possible, in order to direct the majority of your return to one or two places rather than scattering it all over the sky. Since the most common place you don't want returns to come back to is directly to the front, this also means that swept wings and tails are a must. It's also why flat bottoms are preferred: if someone is looking at your aircraft from below, then a flat bottom is the one shape guaranteed not to provide a good return until you are right above them.
 
The major enemy of this approach is the dreaded corner reflector, which is where any right-angled surface will reflect a return straight back to it's source. This is why stealth aircraft all have angled fuselages, hard chines and cranked tailplanes, and also why even things like landing gear hatches and bomb bay doors end up with saw-tooth profiles (note: not 90-degree saw teeth if you can help it, because corner reflector). The other major enemy of this approach is aerodynamics, which inherently prefers rounded frontal profiles that are great at reflecting returns back along an entire wing or fuselage segment. So stealth aircraft also tend to have aerodynamic features (sharp-nosed, flat-bottomed airfoil profiles, for instance) that make them a bastard to fly.
 
Scattering: if you're doomed to reflect something in an unwanted direction, then it helps to make the surface convex in order to disperse the return. This is seen in the shallow, curved fuselage profiles of stealthy aircraft which, along with their beaky fronts and hard chines, gives them a sort of alien bird quality. It's also really useful when designing air intakes for the engines you've sensibly buried inside the fuselage (seriously, the front of a jet engine is like a disco ball for creating noticeable radar returns): an S-shaped intake reflects about half as much energy as a straight intake with a similar profile.
 
Emissions are more or less self-explanatory: if you're trying to hide in the dark, then don't bring a flashlight with you. This means no big radio sources or old-school radar sets that a receiver can easily pick up on. I've heard that modern AESA radars are harder to spot for {electronic black magic} reasons, but the principle still stands.
 
Shenanigans are what you resort to in the corner cases where one or the other approaches described above are not possible. These usually make use of unintuitive electromagnetic wave-specific physics like half-wave resonance. The intake screens on the F117, for instance, seem to be sized so that the radar wave "sees" it as a solid surface and bounces off while still allowing at least a trickle of air in to feed the engines. These tricks tend to be fiddly, however, and can go very wrong when faced with radar systems that use frequencies much higher or lower than the ones that they were designed to counter.
 
3. Artists are dumb and wrong
 
So, having learned the barest minimum about how stealth works, let's point and laugh at the mistakes of artists who ape the form of stealth without understanding the content. Note: it's now almost impossible to grab high-resolution images off of websites, so you'll just have to google these things if you want to see them in any sort of level of detail.
 
Example 1: the F-19 from model kits in the late 80s
 
A fictional stealth plane from the time where people could be forgiven for not knowing a damn thing about stealth. The top-mounted air intake and engines are a good idea, but the rounded wings/fuselage profile and anhedral wingtips look like a great way to get returns from every direction. 5/10 for effort at a time when nobody knew what stealth really was.
 
Example 2: F/A-37 Talon from the movie Stealth
 
Considering that the damn movie is called "Stealth", the Talon is a remarkable example of a bunch of artists googling stealth aircraft and then adding enough greebles so that the result is neither stealthy nor much of an aircraft. It has intakes everywhere, a bunch of curves but few parallel lines, a swing-wing setup that I can only imagine puts a bunch of nooks and crannies into the airframe that reflect well, random greebles off at right angles and on and on. 0/10, the aircraft plays Incubus when it's angry and is therefore canonically a moody teenager.
 
Example 3: XA-20 Razorback from Tom Clancy's giant, throbbing brain
 
It's an F-20 that's inexplicably been converted into a CAS aircraft (presumably because, in the dark future of 2020, transaircraft rights are now government policy). That idiocy aside, it's more or less fine. Turns out that when you crib directly off of someone else's work you won't fuck things up too badly.

1. Introduction
 
Stealth is one of those buzz-words that everyone knows. Stealth makes aircraft invisible to radar, allowing a stealth plane to sneak up and punch other aircraft or SAM sites with impunity. Stealth is widely acknowledged as one of the most fundamental technologies that all new combat aircraft need to have. Stealth is also, like SH's old friend NERA, mostly completely misunderstood. This thread will attempt to change that, at least a little.
 
HUGE DISCLAIMER: I know, at best, the basics of what is essentially one of the darkest arts in an already black magic-heavy field (radio and radar engineering). I'll be relying heavily on others to correct my obvious mistakes, but this is and will be the lies-to-children version of the field, as told by another child. Still, given the state of knowledge out there, it's probably better than nothing.
 
2. The most basic basics
 
Okay, you ask, what is stealth then if we're all misunderstanding it? Here I think that the best analogy is that stealth is like camouflage, but for aliens. Camouflage famously entails the 5 (sometimes expanded to 7 with speed and spacing) S's: Shape, Shine, Shadow, Silhouette and Sound. Each operates on somewhat different principles, and can be more or less important in different scenarios, but are united in terms of how human senses work. We are pattern-finding creatures with passive senses, so anything that breaks up visual or auditory patterns, blends one into the background, or limits the amount of noise or reflected light one gives off will make you harder for another human to spot.
 
Radar, however, is generally not passive. Instead, a radar set sends out a beam of electromagnetic energy and looks for an echo. There's a huge amount of complexity in how this can be done (what frequency to use, how to generate and send the beam, how to track the returns and so on), but that's radar at it's most basic. So, like camouflage, ways to avoid radar will be united in trying to trick or defeat this basic mechanism. These principles are, roughly: Absorption, Redirection, Scattering and Emissions. Finally, and just to complete a fun acronym, there's also the side issue of radio-related Shenanigans. Taken together, these measures can significantly reduce how easy an aircraft is for a radar to "see" from certain angles.
 
Absorption is simple in concept: if something eats up the radar waves before it can get reflected, then the receiver doesn't get to pick up a signal and the plane doesn't get found. There's a whole realm of sneaky material science that goes into this, but from my understanding the two most common techniques currently being used are non-metallic structural components (which can be more or less transparent to radar) and foams or paints with nanomaterials in them (the famous grey stealth paint job generally being a weather coating instead of the magic material itself). These can be used in all sorts of clever ways: for instance, by making the forward edge of your wing out of radar transparent composite and then packing the area behind it with cones made out of radar-absorbing foam. Absorption can't make a non-stealthy design stealthy, however. It's more of a "cut 10% off our already-low radar return" sort of strategy.
 
Redirection is one of the single biggest reasons why stealth aircraft have their characteristic look. Generally the principle here is to make as many surfaces on the aircraft as parallel as possible, in order to direct the majority of your return to one or two places rather than scattering it all over the sky. Since the most common place you don't want returns to come back to is directly to the front, this also means that swept wings and tails are a must. It's also why flat bottoms are preferred: if someone is looking at your aircraft from below, then a flat bottom is the one shape guaranteed not to provide a good return until you are right above them.
 
The major enemy of this approach is the dreaded corner reflector, which is where any right-angled surface will reflect a return straight back to it's source. This is why stealth aircraft all have angled fuselages, hard chines and cranked tailplanes, and also why even things like landing gear hatches and bomb bay doors end up with saw-tooth profiles (note: not 90-degree saw teeth if you can help it, because corner reflector). The other major enemy of this approach is aerodynamics, which inherently prefers rounded frontal profiles that are great at reflecting returns back along an entire wing or fuselage segment. So stealth aircraft also tend to have aerodynamic features (sharp-nosed, flat-bottomed airfoil profiles, for instance) that make them a bastard to fly.
 
Scattering: if you're doomed to reflect something in an unwanted direction, then it helps to make the surface convex in order to disperse the return. This is seen in the shallow, curved fuselage profiles of stealthy aircraft which, along with their beaky fronts and hard chines, gives them a sort of alien bird quality. It's also really useful when designing air intakes for the engines you've sensibly buried inside the fuselage (seriously, the front of a jet engine is like a disco ball for creating noticeable radar returns): an S-shaped intake reflects about half as much energy as a straight intake with a similar profile.
 
Emissions are more or less self-explanatory: if you're trying to hide in the dark, then don't bring a flashlight with you. This means no big radio sources or old-school radar sets that a receiver can easily pick up on. I've heard that modern AESA radars are harder to spot for {electronic black magic} reasons, but the principle still stands.
 
Shenanigans are what you resort to in the corner cases where one or the other approaches described above are not possible. These usually make use of unintuitive electromagnetic wave-specific physics like half-wave resonance. The intake screens on the F117, for instance, seem to be sized so that the radar wave "sees" it as a solid surface and bounces off while still allowing at least a trickle of air in to feed the engines. These tricks tend to be fiddly, however, and can go very wrong when faced with radar systems that use frequencies much higher or lower than the ones that they were designed to counter.
 
3. Artists are dumb and wrong
 
So, having learned the barest minimum about how stealth works, let's point and laugh at the mistakes of artists who ape the form of stealth without understanding the content. Note: it's now almost impossible to grab high-resolution images off of websites, so you'll just have to google these things if you want to see them in any sort of level of detail.
 
Example 1: the F-19 from model kits in the late 80s
 
A fictional stealth plane from the time where people could be forgiven for not knowing a damn thing about stealth. The top-mounted air intake and engines are a good idea, but the rounded wings/fuselage profile and anhedral wingtips look like a great way to get returns from every direction. 5/10 for effort at a time when nobody knew what stealth really was.
 
Example 2: F/A-37 Talon from the movie Stealth
 
Considering that the damn movie is called "Stealth", the Talon is a remarkable example of a bunch of artists googling stealth aircraft and then adding enough greebles so that the result is neither stealthy nor much of an aircraft. It has intakes everywhere, a bunch of curves but few parallel lines, a swing-wing setup that I can only imagine puts a bunch of nooks and crannies into the airframe that reflect well, random greebles off at right angles and on and on. 0/10, the aircraft plays Incubus when it's angry and is therefore canonically a moody teenager.
 
Example 3: XA-20 Razorback from Tom Clancy's giant, throbbing brain
 
It's an F-20 that's inexplicably been converted into a CAS aircraft (presumably because, in the dark future of 2020, transaircraft rights are now government policy). That idiocy aside, it's more or less fine. Turns out that when you crib directly off of someone else's work you won't fuck things up too badly.

1. Introduction
 
Stealth is one of those buzz-words that everyone knows. Stealth makes aircraft invisible to radar, allowing a stealth plane to sneak up and punch other aircraft or SAM sites with impunity. Stealth is widely acknowledged as one of the most fundamental technologies that all new combat aircraft need to have. Stealth is also, like SH's old friend NERA, mostly completely misunderstood. This thread will attempt to change that, at least a little.
 
HUGE DISCLAIMER: I know, at best, the basics of what is essentially one of the darkest arts in an already black magic-heavy field (radio and radar engineering). I'll be relying heavily on others to correct my obvious mistakes, but this is and will be the lies-to-children version of the field, as told by another child. Still, given the state of knowledge out there, it's probably better than nothing.
 
2. The most basic basics
 
Okay, you ask, what is stealth then if we're all misunderstanding it? Here I think that the best analogy is that stealth is like camouflage, but for aliens. Camouflage famously entails the 5 (sometimes expanded to 7 with speed and spacing) S's: Shape, Shine, Shadow, Silhouette and Sound. Each operates on somewhat different principles, and can be more or less important in different scenarios, but are united in terms of how human senses work. We are pattern-finding creatures with passive senses, so anything that breaks up visual or auditory patterns, blends one into the background, or limits the amount of noise or reflected light one gives off will make you harder for another human to spot.
 
Radar, however, is generally not passive. Instead, a radar set sends out a beam of electromagnetic energy and looks for an echo. There's a huge amount of complexity in how this can be done (what frequency to use, how to generate and send the beam, how to track the returns and so on), but that's radar at it's most basic. So, like camouflage, ways to avoid radar will be united in trying to trick or defeat this basic mechanism. These principles are, roughly: Absorption, Redirection, Scattering and Emissions. Finally, and just to complete a fun acronym, there's also the side issue of radio-related Shenanigans. Taken together, these measures can significantly reduce how easy an aircraft is for a radar to "see" from certain angles.
 
Absorption is simple in concept: if something eats up the radar waves before it can get reflected, then the receiver doesn't get to pick up a signal and the plane doesn't get found. There's a whole realm of sneaky material science that goes into this, but from my understanding the two most common techniques currently being used are non-metallic structural components (which can be more or less transparent to radar) and foams or paints with nanomaterials in them (the famous grey stealth paint job generally being a weather coating instead of the magic material itself). These can be used in all sorts of clever ways: for instance, by making the forward edge of your wing out of radar transparent composite and then packing the area behind it with cones made out of radar-absorbing foam. Absorption can't make a non-stealthy design stealthy, however. It's more of a "cut 10% off our already-low radar return" sort of strategy.
 
Redirection is one of the single biggest reasons why stealth aircraft have their characteristic look. Generally the principle here is to make as many surfaces on the aircraft as parallel as possible, in order to direct the majority of your return to one or two places rather than scattering it all over the sky. Since the most common place you don't want returns to come back to is directly to the front, this also means that swept wings and tails are a must. It's also why flat bottoms are preferred: if someone is looking at your aircraft from below, then a flat bottom is the one shape guaranteed not to provide a good return until you are right above them.
 
The major enemy of this approach is the dreaded corner reflector, which is where any right-angled surface will reflect a return straight back to it's source. This is why stealth aircraft all have angled fuselages, hard chines and cranked tailplanes, and also why even things like landing gear hatches and bomb bay doors end up with saw-tooth profiles (note: not 90-degree saw teeth if you can help it, because corner reflector). The other major enemy of this approach is aerodynamics, which inherently prefers rounded frontal profiles that are great at reflecting returns back along an entire wing or fuselage segment. So stealth aircraft also tend to have aerodynamic features (sharp-nosed, flat-bottomed airfoil profiles, for instance) that make them a bastard to fly.
 
Scattering: if you're doomed to reflect something in an unwanted direction, then it helps to make the surface convex in order to disperse the return. This is seen in the shallow, curved fuselage profiles of stealthy aircraft which, along with their beaky fronts and hard chines, gives them a sort of alien bird quality. It's also really useful when designing air intakes for the engines you've sensibly buried inside the fuselage (seriously, the front of a jet engine is like a disco ball for creating noticeable radar returns): an S-shaped intake reflects about half as much energy as a straight intake with a similar profile.
 
Emissions are more or less self-explanatory: if you're trying to hide in the dark, then don't bring a flashlight with you. This means no big radio sources or old-school radar sets that a receiver can easily pick up on. I've heard that modern AESA radars are harder to spot for {electronic black magic} reasons, but the principle still stands.
 
Shenanigans are what you resort to in the corner cases where one or the other approaches described above are not possible. These usually make use of unintuitive electromagnetic wave-specific physics like half-wave resonance. The intake screens on the F117, for instance, seem to be sized so that the radar wave "sees" it as a solid surface and bounces off while still allowing at least a trickle of air in to feed the engines. These tricks tend to be fiddly, however, and can go very wrong when faced with radar systems that use frequencies much higher or lower than the ones that they were designed to counter.
 
3. Artists are dumb and wrong
 
So, having learned the barest minimum about how stealth works, let's point and laugh at the mistakes of artists who ape the form of stealth without understanding the content. Note: it's now almost impossible to grab high-resolution images off of websites, so you'll just have to google these things if you want to see them in any sort of level of detail.
 
Example 1: the F-19 from model kits in the late 80s
 
A fictional stealth plane from the time where people could be forgiven for not knowing a damn thing about stealth. The top-mounted air intake and engines are a good idea, but the rounded wings/fuselage profile and anhedral wingtips look like a great way to get returns from every direction. 5/10 for effort at a time when nobody knew what stealth really was.
 
Example 2: F/A-37 Talon from the movie Stealth
 
Considering that the damn movie is called "Stealth", the Talon is a remarkable example of a bunch of artists googling stealth aircraft and then adding enough greebles so that the result is neither stealthy nor much of an aircraft. It has intakes everywhere, a bunch of curves but few parallel lines, a swing-wing setup that I can only imagine puts a bunch of nooks and crannies into the airframe that reflect well, random greebles off at right angles and on and on. 0/10, the aircraft plays Incubus when it's angry and is therefore canonically a moody teenager.
 
Example 3: XA-20 Razorback from Tom Clancy's giant, throbbing brain
 
It's an F-20 that's inexplicably been converted into a CAS aircraft (presumably because, in the dark future of 2020, transaircraft rights are now government policy). That idiocy aside, it's more or less fine. Turns out that when you crib directly off of someone else's work you won't fuck things up too badly.

He's also a nationalistic furry. Consistency is not this dude's strong suit.

So part of how APFSDS works, at least as I understand it, is that you get to use a larger swept volume of bore to propel your projectile (subject to the usual diminishing returns as you take the concept out to its extremes). So having a bigger bore is positively an advantage from an internal ballistics perspective (rather than a barrel mechanics perspective), and results in more efficient use of propellant. There's also the additional side benefit, as you noted, that a bigger bore means a bigger HE round and better HEAT-FS performance.
 
The testers here are things like the hot 76mm used on the Rooikat or the 60mm used on Chilean Shermans. From what I can find, the round is a comparable size to the 105mm gun, and has comparable APFSDS performance (if a little poorer overall). The gun is actually heavier than the 105mm L7, however, even though it's a nearly a metre shorter. It also seemingly has a longer recoil stroke. From what I can find about the 60mm HVMS, it's significantly lighter than either the 105mm or 76mm, but has an intermediate length. It's also significantly less powerful, clocking in at something like 240mm of RHA penetration at 2000m vs 270 (76mm) or 300 (105mm).
 
So if the trade-off you're willing to make is in the realm of overall dimensions, then going for a smaller bore might be worth it up to a point. It also provides some interesting options, such as the three-round burst for the 60mm gun (before the Chileans removed it). If, however, you have turret space to spare, then a bigger gun seems to provide few downsides. Once again, subject to the spectre of other trade-offs as you reach the outer ends of the performance envelope. 

Just to look at what different bore configurations can do for you, I used an internal ballistics spreadsheet and mocked up a bunch of guns of different calibres which run at the same pressure (650 MPa) and have the same tube lengths:
 
57mm L66.7: 2.1MJ 
76mm L/50: 3.74MJ
90mm L/42: 5.24MJ 
105mm L36: 7.13MJ
120mm: L31.7: 9.31MJ
 
Using this approach, I can get the performance of the 76mm gun by using a 105mm gun of the same length which runs at a substantially lower pressure (341 MPa, which is achievable using WW1-era metallurgy). The kicker, of course, is that the 76mm projectile and sabot are somewhat lighter than an equivalent configuration for the 105mm: a hypothetical 15:1 soviet-style APFSDS for the 76mm weighs in at 1.38kg for the penetrator, and ~1kg for an aluminium spindle sabot (note: both calculated using another spreadsheet and so are for illustrative purposes only). The equivalent 105mm sabot is more like 2kg. So to get true equivalence in performance, your hypothetical 105mm goes back up to 522 MPa (ie: roughly equivalent to the pressures found on early 125mm guns like 2A26) and a muzzle energy of  5.73MJ.
 
All of which means, again, that the 76mm probably provides a saving in terms of the overall dimensions of the gun and the overall volume of the cartridge. But it also requires a disproportionately higher pressure to achieve the same results. All of which means that, as you hit the limits of your materials and gun penetration, a bigger bore is going to loom larger and larger as a way to get more out of your platform.
 
This, I think, also neatly explains why small-bore APFSDS slingers tend to go into service in less developed countries: a modern, high-pressure 60 or 76mm is perfectly capable of dealing with the T-62s and the like that your opponents will be fielding, so the advantages of a smaller bore are more apparent.

That's a really interesting question! Which I'm deeply unqualified to answer 
 
Anyway, the equation for hoop stress is Pd/2t under the thin-wall assumption, which seems to indicate a linear relationship between pressure and wall thickness. The relationship between pressure and velocity is a bit more complicated, but running the numbers on test barrels (a 75mm L/40 gun running at 375MPa, and a 75mm L/30 running at 500 MPa) seems to show a similar relationship in terms of the length needed.
 
The terms thus appear to cancel out, at least for the calibre range you were referring to: a high-pressure gun, having the same performance as a low-pressure one, will weigh about the same.
 
Here the obvious problem is that cannons are not thin-walled, which means that you need to do integration and constant-finding to get a good answer. Now, without wanting to touch that particular mess at all, my gut feeling is that the higher the pressure the thicker the walls need to be relative to lower-pressure tubes. Which probably leads, in turn, to lower-pressure guns being lighter than higher-pressure ones for the same level of performance. The functional constraint then becomes about length and tube stiffness, which starts to become a significant factor as your low-pressure potato cannon scales up in velocity.
 
Extrapolating, then: a low-pressure gun is great if you can get away with it, being lighter than a high-pressure one of the same performance (which is probably why things like the low-pressure 90mm guns are built the way they are). In fact, as a designer you should probably look for the lowest-possible pressure to run a gun at so long as it fits within the maximum dimensions that you're allowed.
 
Unfortunately, if you're looking at slinging an energetic (read: AP or APFSDS) shell, then this also implies that you're doomed to chase after higher and higher pressures (and thus relatively heavier guns) simply to keep within reasonable size and stiffness constraints.

That's a really interesting question! Which I'm deeply unqualified to answer 
 
Anyway, the equation for hoop stress is Pd/2t under the thin-wall assumption, which seems to indicate a linear relationship between pressure and wall thickness. The relationship between pressure and velocity is a bit more complicated, but running the numbers on test barrels (a 75mm L/40 gun running at 375MPa, and a 75mm L/30 running at 500 MPa) seems to show a similar relationship in terms of the length needed.
 
The terms thus appear to cancel out, at least for the calibre range you were referring to: a high-pressure gun, having the same performance as a low-pressure one, will weigh about the same.
 
Here the obvious problem is that cannons are not thin-walled, which means that you need to do integration and constant-finding to get a good answer. Now, without wanting to touch that particular mess at all, my gut feeling is that the higher the pressure the thicker the walls need to be relative to lower-pressure tubes. Which probably leads, in turn, to lower-pressure guns being lighter than higher-pressure ones for the same level of performance. The functional constraint then becomes about length and tube stiffness, which starts to become a significant factor as your low-pressure potato cannon scales up in velocity.
 
Extrapolating, then: a low-pressure gun is great if you can get away with it, being lighter than a high-pressure one of the same performance (which is probably why things like the low-pressure 90mm guns are built the way they are). In fact, as a designer you should probably look for the lowest-possible pressure to run a gun at so long as it fits within the maximum dimensions that you're allowed.
 
Unfortunately, if you're looking at slinging an energetic (read: AP or APFSDS) shell, then this also implies that you're doomed to chase after higher and higher pressures (and thus relatively heavier guns) simply to keep within reasonable size and stiffness constraints.

Nice to see some attempts to push back all those Wehraboos shit on the net.
 
 
 

Mobility gimmicks:
 

Mobility gimmicks:
 

It is said that the man with no name wanders the desert wastes to this day, searching for his fistful of dollars.

Main Battle Tank, 2247, project names "Derebus" and "Derebus-M"
 
 



Manufacturer: Manufactuer: Paramount-Allen-Fullerton (Para-allful) Conglomerated
 
Table of basic statistics:
 
Note: all statistics provided are for Derebus unless otherwise noted.
 
Parameter
Value
Mass, combat
Armour mass: (1-2" (25-50mm) RHA base plus ERA, composites, side skirts and engine bay liner): 20.6t (18.5mt)
 
43.1t (39.1mt) modelled, 43.4t (39.4mt) calculated
Length, combat (transport)
246" (6.25m) hull, 379" (9.63m) total 
Width, combat (transport)
150" (3.8m) with skirt
Height, combat (transport)
95" (2.41m) to top of commander's hatch, 109" (2.77m) total
Ground Pressure, zero penetration
Ground pressure (calculated MMP): 29.4 PSI (203 KPa). 
 
Nominal ground pressure (based on calculated weight): 10.3 PSI (70.77 KPa)
Estimated Speed
37 mph (60km/h)
Estimated range
490 mi at 30 mph
Crew, number (roles)
4 (commander, gunner, loader, driver)
Main armament, caliber (ammo count ready/stowed)
5-inch 55 calibre (127mm L/55) high/low pressure gun, (19 charges, 9 active projectiles, 10 inert projectiles in turret/ 16 charges, 8 active projectiles, 8 inert projectiles in hull)
Secondary armament, caliber (ammo count ready/stowed)
3 x .30 cal MG (600 rnd belted each ready/ 1200 rnd belted each stowed)
 
Vehicle designer’s notes:
 
The Derebus family of vehicles (provisionally named Derebus and Derebus-M) are intended to fulfil a procurement strategy emphasizing mobility (tactical, operational and strategic), reliability and superb value for money, achieved using a lightweight vehicle design, proven automotive components and a high/low capability mix. Derebus sports a state-of-the-art fire control system allied to a powerful 5" gun, while Derebus-M provides supporting firepower and a larger ammunition load thanks to it's 4" gun. Both vehicles provide superb protection across their frontal arcs, with the Derebus making use of cutting-edge composites to save weight. Both Derebus and Derebus M are immediately available to fulfil all of your defense needs.
 
Vehicle feature list:

Mobility:
 
1.     Link to Appendix 1
2.     Engine- V-12 Diesel (Kharkiv V-2-55 derivative), 2441 ci (40l) displacement, 600HP (448kW), liquid-cooled. 
 
Note: alternate engine and transmission arrangements are provided for in Appendix 3
 
3.     Transmission - hydraulic torque converter feeding into Merritt-Brown-style double differential system, 7 forward/1 reverse gears.
4.     Fuel - diesel, ~2400lb total (639lb/290kg in tanks flanking the driver, 1764lb/800kg in rear sponson tanks, estimated range of 490mi at 30mph.
5.     Engine, transmission and cooling are arranged in removable aluminium tub housed in engine bay. The tub is removable by sliding out the rear of the bay.
6.     Suspension - torsion bar, variable travel (presently 11.8" (30cm)), 20" (0.5m) ground clearance, geared torsion bar suspension, each axle pair in detachable units shrouded by aluminium housings. Wheels are 23.6" (0.6m) in diameter, with a track width of 27.6" (0.7m) and a pitch of 7.7" (0.195m).
 
Survivability:
 
1.     Link to Appendix 1
2.     Link to Appendix 2
3.     Non-specified survivability features and other neat tricks - highly sloped turret and hull front (75 degrees), charges and active ammunition (HEAT-FS and HE) arranged in sealed tubes leading to a blast chimney that outlets to blow-off panels in the turret roof, turret sides, hull roof and hull sides.
 
A.    Weapons:
 
1.     Link to Appendix 1
 
2.     Main Weapon-
 
a.      Type: smoothbore, vertically trainable +15/-10 degrees
 
b.      Caliber: 5"/127mm
 
c.      ammunition types and performance:
 
Note: the armour used for the target has the same hardness (360BRN) as the armour used in the vehicle. The target was at 0 degree obliquity for calculation purposes.
HEAT-FS (low-pressure setting): 46lb (20.85kg), penetration of around 21" (535mm), 3074fps (937m/s). HE (low-pressure setting): 46lb (23.2kg), 201oz (5.7kg) fill, estimated blast penetration of ~55mm RHA, 2910fps (887m/s) APFSDS (high-pressure setting): 15:1 LD, 550BHN monosteel body, tungsten insert, 115mm cap, ring sabot, 1800m/s, 15.7/13.8" (400/350mm) penetration at 100/2000y (lower estimate, 17.1" (435mm) at 2000y upper estimate). d.     Ammo stowage arrangement - 19 charges, 9 active projectiles, 10 inert projectiles in turret; 16 charges, 8 active projectiles, 8 inert projectiles in hull.
 
e.      FCS:
Duel axis stabilized main gun Semi-autoloader: the loader places the charge and warhead on trays in the bustle. these are then fed into the gun using an automatic mechanism (horizontal rammer, pivoting loading tray and rigid chain actuator to ram the warhead and charge home). The gun automatically returns to the loading position after each shot. A short spring at the end of the actuator helps to smooth out the loading impulse.  
f.      Neat features: 
Gun has a high-pressure and low-pressure recoil option, selectable on the slide – this doesn’t affect the recoil mechanism, it just changes where the trip key is to unlock the breech (warning: don’t fire high-pressure ammo with the low-pressure setting selected!) Gun uses a separate 6.9x27" (175x685mm) charge: 44lb/20kg mass, semi-combustible case built along the lines of the 4Zh-40 charge used with the historical 125mm 2A26 gun), matching the length of the HEAT-FS round. The charge gives space to produce a more powerful round to match higher future barrel higher pressures (when using a secondary charge with the APFSDS projectile itself). 74000 PSI (510 MPa) gives a potential power of 15MJ. 94000PSI (650 MPa) gives a potential power of 19MJ. Final penetration potential of the gun with early monobloc DU projectiles is something in the region of 22" (550mm) at 2000y (putting it on par with Mango and Vant). Being able to store and handle a longer projectile (ie: above 27") would probably allow something a bit better than Snivets.  Low-pressure charges are shortened (17.1" / 435mm) and come with an ejection spring to work with the same storage tubes as the high-pressure charges. 3.     Secondary weapons - 3 x .30 cal MG, 1 coaxial, 2 in mountings attached to the commander and loader's turret hatch
 
4.     Link to Appendix 3. 
 
B.    Optics:
 
1.     Primary gunsight: single axis stabilized gunner’s sight
 
2.     Secondary gunsight: vertical coincidence rangefinder (stadiametric, 39.4" (1m) base), doubles as a redundant back-up sight.
 
3.     Miscellaneous optics:
Commander and loader's rotating hatches, including vertically trainable (+/- 15 degrees) periscope in front of hatch, degree markings on hatch ring to allow rough direction of gunner to target. Driver's periscope, vertically trainable +/- 15 degrees  
C.    FCS:
 
1.     List of component systems, their purpose and the basic system architecture:
Simple electronic gun-follows sight fire control system (encoder connected to sight mirror feeds elevation data into a transistor-based PID controller, which tries to match position on a similar encoder connected to the gun. When gun position and sight position align, the firing mechanism is electronically triggered). LRF mounted above barrel, solid-state components, maximum operating range of 5km in clear conditions, average estimation error of 1%. Uses flashlamp-pumped ruby laser, optical sensor, quartz timing circuit and the sequential event time sampling approach (with post-sampling amplification) to allow time-of-flight rangefinding using a lower timebase and bandwidth compatible with current electronics.  
2.     Link to Appendix 3.
 
Fightability:
 
1.     List vehicle features which improve its fightability and useability:
 
Engine bay approach simplifies engine and transmission replacement via rear bay doors. Generous rear hull roof hatches simplify servicing and maintenance. Bolt-on suspension units simplify field replacement and repair. Commander and loader's hatch design improves buttoned-up visibility   
Additonal Features:
 
See Appendix 3  
Free expression zone: 
 
"...bellicis"

judge's opinions:
For the sake of convenience, these are posted in order of posting in the submission thread.

XG-48E3 Comanche Battle-cruiser
 

Main Battle Tank, 2247, project names "Derebus" and "Derebus-M"
 

Brownsville Armour Engineering Systems FV601 “Cossack”
 

VK-55.01 - Versuchsträger NK
 
Persson Engineering Solutions and Brewing, Main Battle Tank, MBT-01, "Gigan"
 

East Oil Company MBT-1 Monolith
 
 

So, as results are published, i wanted to make notes about each submission i reviewed.
 
   First is @Sturgeon
Overal impression of "Comanche".
   Generic and somewhat mediocre design, but mostly gets job done. One of more realistical submissions of this competition.
 
 
 
   Next is @Toxn
Overal impression of "Derebus" and "Derebus-M":
   More interesting submission and, as soviets would call it, a "duplex" of tanks was suggested. Sadly didn't reached all required perfomance, "unreliable" perfomance of armor. Derebus-M looks like a vehicle for more mass use and/or tight budget. Derebus is a bit too specialised.
 
 
 
   @Fareastmenace's Cossack. Would like to note a very good submission itself that showed plenty of details of proposed design.
Overal impressions:
   Very good vehicle overal with very few bad or strange decisions. 2 configs were noted and i like both.
 
 
 
@Sten
My overal impressions about Gigan:
   We are getting into memezone. Huge, heavy, pancakes, overarmored.
 
 
 
   @Dominus Dolorem's Memelith
Overal impressions:
   Insane, huge, overarmed, meme on tracks.
 

FROM THE FILES OF LFS ORDNANCE DEPT.
 - Lone Free State proprietary information -
9.18.2247 
To: LFS central command
CC: relevant industrial concerns
 
@Sturgeon
@Toxn
@Fareastmenace
@delete013
@Sten
@Dominus Dolorem
SUBJ: RE: candidate heavy armored truck designs

Kind sirs,
we apologize for the delay in responding to the technical request in your last communication on this topic. As you are no doubt aware, the LFS Ordnance dept. was set up only recently, and administrative affairs have delayed the trials and testing of the proposed vehicle designs.

With the above said, enclosed are our recommendations for the selection of a new heavy armored truck for the newly formed 1st Heavy Ranger Brigade.

FIRST PLACE: 
Brownsville Armour Engineering Systems FV601 “Cossack”

LFS Ordnance was very impressed with this design, featuring a very good blend of features both for the current threat environment and for future threat environments.
Congratulations, @Fareastmenace!

 
SECOND PLACE:
Persson Engineering Solutions and Brewing, Main Battle Tank, MBT-01, "Gigan"


Another very impressive beast, again featuring a design focused not only on the current threat environment but on the future as well.
@Sten, very well done.

THIRD PLACE:
East Oil Company MBT-1 Monolith


An extremely large, extremely powerful beast, which while perhaps somewhat poorly tailored to the requirements of the LFS nevertheless would offer substantial performance in service.
@Dominus Dolorem, good show.

Detailed opinions on all designs to follow shortly.
 

Long arrow adequate arrow...

Why do you think I keep responding to this inanity? I've been trying to praise what I see as good in you, in the idealistic hope that you'll become a better poster.
 
Slavic pessimism would have saved me some trouble on this front...

Because I'm not Slavic. My psycho-social gestalt is firmly on the praise good/romantic idealist end of the spectrum.
 
My patience is also a finite resource, especially when getting lip from someone I'm in the process of trying to gently correct instead of chastise.

Cool your jets with the conspiratorial stuff.
 
LoooSeR is one of our most experienced and valuable posters, and provides something that every project needs in his role as a judge: terminal slavic pessimism. And he does so without fear, favour or bias.
This is a man who could look upon the face of God himself and then provide detailed commentary on elements in need of improvement. And we're damn thankful for it.

Because I'm not Slavic. My psycho-social gestalt is firmly on the praise good/romantic idealist end of the spectrum.
 
My patience is also a finite resource, especially when getting lip from someone I'm in the process of trying to gently correct instead of chastise.