Toxn

5 minutes ago, Sturgeon said:

The Su-25 is also substantially smaller, and cheaper to operate.

The load carrying fraction also appears to be about the same, although comparing empty weight to MTOW is not the most useful metric ever devised.

So... what I'm getting from this is that someone needs to shoop the SU-25 and the A-10 into the matrix "red pill, blue pill" meme asap.

14 hours ago, Domus Acipenseris said:

Is it around the level of the A-1?  The AX program originated when the A-1 was in use but the end of the line for the A-1 was in sight.  The SBD can dive for accurate delivery.  Can the A-10 do that?

 

The link is broken but I saw a claim that the expected life of the A-10 was 2 missions on the Central Front.

That makes sense. I've also heard that the lifespans of things like Harriers were expected to be measured in minutes once they got into the Soviet air defence envelope.

14 hours ago, Sturgeon said:

I would like to introduce the idea of the A-10 Threshold:

This is a concept to determine what technological threshold the A-10 Thunderbolt II occupies. We have determined that it's clearly worse than an A-4, arguably worse than an F-100D, and probably worse than an A-26 Invader. 

Discussions are ongoing as to whether it's inferior to the SBD Dauntless.

Where does the Frogfoot sit in all of this?

55 minutes ago, Lord_James said:

Would decoys or active countermeasures (like chaff) be counted as stealth, since they make it more difficult to gain a target? Or is stealth, in this context, not being seen in the first place? 

I think certain forms of ECM count. But my understanding is that stealth is about minimizing signature rather than hiding it.

On 11/20/2021 at 8:09 AM, Toxn said:

I'll have to check, although there aren't great sources online as far as I know.

Having had a look at the performance of the rounds, it looks like the propellant loads would be quite different for each round. The 60mm needs around 1.9dm3 of case volume for the propellant, the 76mm around 4.3dm3 and the 105mm around 7.1dm3.

This means less than you'd think in terms of the space that the rounds take up, though, since volume scales cubically. As a toy example: a conservatively bottle-necked case for the 60mm (1/3 larger base than tube diameter) would have to be something like at least ~450mm long, an equivalent case for the 76mm would be ~630mm long, and the 105mm would be ~600mm long.

The ease of handling of a round is more or less a function of the case length and weight (with weight being one of those factors that becomes exponentially more of a hassle as it increases). So I'd say that, within a certain floor of effectiveness and ceiling of round mass and size, your operational requirements will probably determine where on the graph of size/weight and effectiveness you land. For instance: the 25% jump in penetration performance from the 60mm to the 76mm at the cost of a 40% increase in case length is a decent trade-off if stowed ammunition and rate of fire is worth trading. As is the 30% jump from the 76mm to the 105mm (at the cost of something like a 70% increase in weight), if you need the extra effectiveness against more modern armour.

Again; I'm getting the impression that small-bore APFSDS slingers show up more in second and third-tier armies because those are the forces where the floor of effectiveness is lowest. Zapping T-55s and T-62s instead of T-64s and T-72s just gives you a lot more leeway when it comes to choosing what other trade-offs are worth it. 

9 hours ago, Gauntlet said:

Not completely sure, but I believe that the air intake of the Naval Strike Missle also contains some features in order to reduce the RCS.

Also i quite liked the book, Skunk Works.

From the few pictures available of the inlet, this does indeed seem like the case.

10 hours ago, Lord_James said:

Do those guns have the same propellant load as the normal L7? I would think squeezing the round down would necessitate lengthening the cartridge, which isn’t preferable to reducing the size of your tank, or necking it down significantly. 

I'll have to check, although there aren't great sources online as far as I know.

10 hours ago, eggs benedict said:

One thing that urgently needs to be mentionned is that just like aerodynamics, or Armor vs shell behaviour, RCS is never ever to be evaluated by eye. There are so many details going up about how EM waves scanner or get absorbed that not even a general guess can be made when it comes to radar stealth vehicles.

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.

8 hours ago, W. Murderface said:

 

Now I'm not an expert in any of this, but this guy seems to know what he's on about. 

Thanks, that's a decent find.

Edit: this one of theirs might be even better as an intro:

And this one explains the issue of detection range not scaling linearly with RCS, as well as some of the other, more fiddly aspects of reducing cross-section:

Something that I should also emphasise about stealth is that small fuck-ups have big consequences. A completely un-stealthy aircraft has an average RCS of something like a few square metres. Meanwhile, a well-shaped stealth aircraft supposedly has an average RCS of around 0.1 square metres. The area of a landing gear well for such an aircraft might be something like 0.5x1m, if cunningly made. This means that, if you fuck up the stealth shaping of a wheel well (by, for instance, making it out of something radar-transparent or forgetting to shape the doors correctly) you can increase the cross-section of the entire aircraft by as much as five times! This sort of issue is all over the place, and is one of the reasons why simple, clean external lines are preferred - the more structures you have hanging off the fuselage, the more chances there are for something fucky to happen and for an accidental radar reflector to be formed. And, of course, the smaller you try to make your cross-section, the bigger the relative effect of any fuck-ups.

Getting a very small RCS across even a very tightly specified arc is accordingly a process of almost obsessive attention to detail. And getting a small RCS across a large arc is the sort of thing that you need very specific and potent computer-aided design tools to accomplish.

On 11/15/2021 at 1:19 AM, Sturgeon said:

I was not sure why so many people thought the soundtrack was mediocre, it was one of my favorite parts of the movie.

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.

8 hours ago, Lawrenicium said:

Wasn't this the same guy who loves m113 so much because its light? He always complains about how heavy bradley or abrams is and proceeds to tell how some vietnam era patton is superior to abrams.

 

And now Im seeing him design a tank even bigger than abrams??! What even is this guy thinking. He is in conflict with himself!!!!

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

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.

16 hours ago, Lord_James said:

 

Since you like weird ideas @Toxn, I'll @ you specifically  

 

So, with more modern APFSDS having smaller fins that the bourrelet fins of old, couldn't the bore size be reduced for many AP slingers? Reducing the bore diameter (down to roughly 90-95% of the fin diameter, but not to where the fins interact with the tube) might reduce the weight of the barrel, assuming it stays the same thickness, and would definitely reduce the mass of the carrying sabot, meaning more energy towards the penetrator. This might also mean savings to the weight of propellant needed to get a round up to a particular energy, or increase the energy of an already powerful round without increasing the propellant or chamber size. It might be possible to get a 105mm (or less) gun with a thicker barrel and larger chamber that your normal L7A or M68, but slinging full size 120mm (or larger, if the fins fit) sabots at targets. This gun might be somewhat lighter than the 120's, but I don't think by too much. 

 

I'm going to assume I'm not the first person to come up with this, and that there must be problems associated with doing something like this. I'm going to guess there are barrel droop and vibration problems, thought these might be less so than a normally designed gun because I specifically stated that the barrel stays the same thickness as the original gun (read: 120mm L/44) to cope with the (assumed) higher pressures of the larger parent gun. There's also the fact that the HE shells you will want to sling at infantry are also less effective, unless you make the HE abnormally long, which might also have hidden consequences like strange shrapnel dispersion or tumbling or something else you don't want. I'm disregarding HEAT due to it falling out of favor based on greater ERA usage across many combat vehicles, but it's problems would be similar to the HE shell. 

 

Any thoughts from anybody? I'm always open to being corrected and learning real world experience. 

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. 

13 hours ago, Lord_James said:

Is it more efficient mass wise to have a gun with a longer barrel and lower pressures, or a shorter barrel and higher pressures? The 75mm PaK 42 seems to be about 1 ton by itself, while the similar 77mm HV is 1.5 tons (don’t have a source for the QF 17 lb without carriage). 

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.

Mobility gimmicks:

12 hours ago, Lord_James said:

Can I use this mobility gimmick 

We already had a competition for that 

2 hours ago, Lord_James said:

Are we going to do a competition with wacky mobility gimmicks? 

That sounds like a fun idea.

It can go into the pile.

On 9/14/2021 at 8:28 AM, Toxn said:

Someone finally built a pedrail. I've always wondered how these things worked in the real world...

More pedrail:

Someone has a few points and a generalized hatred of EE and Pierre Sprey.

https://www.youtube.com/c/LazerPig/videos

I wonder who this guy is?

Edit: a bit of digging shows that he's a dude who goes by ricewynd on (ugh) reddit and has been banging around their history/tank forums there for a while.

6 hours ago, Lord_James said:

If I may request, can the competition wait until after holiday season? 

Normally we leave at least a few months between contests to allow everyone to cool off.

So yeah, in the new year at the earliest 

Tank cock-ups: an ancient British tradition.

1 hour ago, Sturgeon said:

 

It's like nobody's ever even watched the Sacred Reels!

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