Estimating characteristics from photos and drawings

[totoro]

we have no data on which we can base any sort of sensible answer. we can't know which one is overall stealthier nor can we tell for which threats they are designed. We can only assume and guesstimate.

 

[quellish]

we have no data on which we can base any sort of sensible answer. we can't know which one is overall stealthier nor can we tell for which threats they are designed. We can only assume and guesstimate.

Sure we can. An object's radar signature is largely driven by shaping, and the shaping on the vehicle is in the public domain. Coatings have limitations, and it's relatively easy to estimate best and worst cases for the contributions of coatings to the signature. From the shaping of the aircraft you can get a very good idea of what threats the vehicle was designed to counter and model the signature of the aircraft from different aspects.

You can "assume and guesstimate", or you can model and measure.

 

[2IDSGT]

we have no data on which we can base any sort of sensible answer. we can't know which one is overall stealthier nor can we tell for which threats they are designed. We can only assume and guesstimate.

Sure we can. An object's radar signature is largely driven by shaping, and the shaping on the vehicle is in the public domain. Coatings have limitations, and it's relatively easy to estimate best and worst cases for the contributions of coatings to the signature. From the shaping of the aircraft you can get a very good idea of what threats the vehicle was designed to counter and model the signature of the aircraft from different aspects.

You can "assume and guesstimate", or you can model and measure.

"Public domain," exactly. Sorry, but the *models* used by various and sundry self-appointed internet experts are still so-much eyeballing based on photos, marketing materials, and fan-art.

 

[sferrin]

we have no data on which we can base any sort of sensible answer. we can't know which one is overall stealthier nor can we tell for which threats they are designed. We can only assume and guesstimate.

Sure we can. An object's radar signature is largely driven by shaping, and the shaping on the vehicle is in the public domain. Coatings have limitations, and it's relatively easy to estimate best and worst cases for the contributions of coatings to the signature. From the shaping of the aircraft you can get a very good idea of what threats the vehicle was designed to counter and model the signature of the aircraft from different aspects.

You can "assume and guesstimate", or you can model and measure.

"Public domain," exactly. Sorry, but the *models* used by various and sundry self-appointed internet experts are still so-much eyeballing based on photos, marketing materials, and fan-art.

Not to mention the fact that some of the finer detail you're just not going to see in your average online picture.

 

[quellish]

"Public domain," exactly. Sorry, but the *models* used by various and sundry self-appointed internet experts are still so-much eyeballing based on photos, marketing materials, and fan-art.

Those would be the "assume and guesstimate" crowd. Those who, yes, eyeball a photo and declare something stealthy or not.

There is a reason it's called "computational electromagnetics". Computers are pretty good at this math stuff.

 

[2IDSGT]

"Public domain," exactly. Sorry, but the *models* used by various and sundry self-appointed internet experts are still so-much eyeballing based on photos, marketing materials, and fan-art.

Those would be the "assume and guesstimate" crowd. Those who, yes, eyeball a photo and declare something stealthy or not.

There is a reason it's called "computational electromagnetics". Computers are pretty good at this math stuff.

Computer models are only as accurate as the information they're given, which is still based on the crap I've already mentioned. Garbage in, garbage out.

 

[chuck4]

If you can't quantify what consititute "crap", and what what constitute useable data; or if you can't quantify any upper limit on how much impact so called "finer detail" are likely to have, then face it, you don't know enough to know how much information can be geathered reliably from available photos.

The knowing, self-satsified snicker about "crap in, crap out" is itself crap.

 

[2IDSGT]

If you can't quantify what consititute "crap"...

I already did. You might have noticed had you read to whole conversation.

 

[Sundog]

Computer models are only as accurate as the information they're given, which is still based on the crap I've already mentioned. Garbage in, garbage out.

How bad are the computer EM models we have? I had no idea they were so poorly designed. You would think with all of the money we spend we could come up with a pretty good system? Of course, garbage in, garbage out is true. Conversely, put "not garbage" in and see what you get out? The programs I've written weren't too bad, but that was way back when and that was just CFD.

 

[quellish]

Computer models are only as accurate as the information they're given, which is still based on the crap I've already mentioned. Garbage in, garbage out.

How bad are the computer EM models we have? I had no idea they were so poorly designed. You would think with all of the money we spend we could come up with a pretty good system? Of course, garbage in, garbage out is true. Conversely, put "not garbage" in and see what you get out? The programs I've written weren't too bad, but that was way back when and that was just CFD.

They're very good, and have been for a long time. Today we have both hardware and software that is much, much faster than just a few years ago.
Just like with CFD, there are some methods that lend themselves especially well to modern hardware. Today both with CFD and EM you can do much more complex simulations in far shorter times than back in the 90s. Modeling how surface and travelling waves interact with complex geometry is now not just possible, but very much practical as a design tool (even with MATLAB).

 

[latenlazy]

I don't doubt that someone somewhere has done a relatively accurate RCS model of the J-20 somewhere, but it's somewhat moot to the discussion if we don't know what the conclusions are.

 

[2IDSGT]

Computer models are only as accurate as the information they're given, which is still based on the crap I've already mentioned. Garbage in, garbage out.

How bad are the computer EM models we have? I had no idea they were so poorly designed.

How many times does it have to be explained? It's not the computer models that are in question; it's the information on which they are based that is suspect.

 

[LowObservable]

The information at a basic level is as bad or good as a photograph. Deriving an accurate 3D model from photos is not easy but is not the stuff of miracles and guesswork either.

Yes, there is RAM and RAS, but barring miracles there is only so much that can be done with these without driving weight through the ceiling.

Result - It is no more fundamentally impossible to use photos for a first-cut estimate of RCS, particularly in the regions where specular scattering dominates, than it is to estimate weight and performance.

Some people, however, have a big problem with accepting this.

 

[2IDSGT]

Result - It is no more fundamentally impossible to use photos for a first-cut estimate of RCS, particularly in the regions where specular scattering dominates, than it is to estimate weight and performance.

Like estimates of the Mig-25's performance that were based on photographs? Or the Mig-29 panic in so many 1980's books? Then again, I suppose people of your job description might lose credibility were it understood that speculations based on photographs ARE NOT authoritative or reliable. :

 

[LowObservable]

Exactly, Sarge! Not as authoritative or reliable as one would wish, but not half-bad if done properly.

Like working out that the MiG-29 was range-deprived because of losing 1500 pounds of gas to the auxiliary inlets and having limited external tank options... Or that the MiG-25 went like a rocket and burned fuel the same way... or that the Tu-22M did not have the legs for a strategic bomber (and nailing the fuel capacity within 5 per cent)... or Braybrook's right-sizing of the MiG-21 in FRI in 1964...

Just a few things done open-source, bang-on-accurate and usually from grotty photos.

The classic mis-estimates, on the other hand, were usually the result of people trying to use pseudo-intel to promote the answer that they'd already thought of.

 

[2IDSGT]

Like working out that the MiG-29 was range-deprived because of losing 1500 pounds of gas to the auxiliary inlets and having limited external tank options...

No one actually knew how limited the bag options were back then. As for the auxiliary inlets, I don't recall anyone speculating how much fuel was lost to the things; it was more along the lines of: "Oh woe is us! The Soviets have better fighters that can use alfalfa fields as runways!"

 

[sferrin]

Exactly, Sarge! Not as authoritative or reliable as one would wish, but not half-bad if done properly.

That's quite a stretch. They really screwed the pooch on the Mig-25. Same with the "Su-19" which they thought was a land-based Tomcat analog.

 

[LowObservable]

Who be dat "they", Sferrin?

If you can cite someone serious who thought Fencer was a Tomcat analog, fire away. And, yes, people in the US did misoverestimate the MiG-25, but they did it in order to sell a bigger and hotter F-X (F-15). Anyone who could look at that canopy and think "WVR dogfighter" quite obviously needed his head examined, once a team of proctologists had retrieved it.

2IDSGT - I'm sure the people you were talking to did not think of or know about these things, but that doesn't mean that there were not more perceptive folks somewhere else.

 

[chuck4]

To be fair, reasonable WVR combat capability and poor rear vision are not muturally exclusive. Later models of Mig-21 had rear vision no better than Mig-25 and were still formidable dog fighters for the 1970s.

I think Mig-25 was something of a special case because both its missions and its construction was unusual, so assuming its mission and its construction were conventional led to an incorrect picture of its performance potential.

In the particular case here, the RCS estimates for T-50 and J-20 don't make too many assumptions about mission and construction because they rely on shape only. So the Mig-25 analogy was inappropriate.

 

[2IDSGT]

2IDSGT - I'm sure the people you were talking to did not think of or know about these things, but that doesn't mean that there were not more perceptive folks somewhere else.

Tell ya what. If I ever get my damned thesis finished, I'll make a historiographical study of your work and that of your peers to make it official; but if vague memory serves, the general whine was (then as it is now) that the other guy's stuff is always somehow cheaper AND better than ours. Silly in retrospect, but I suppose that's always sold more magazines.

 

[LowObservable]

If I ever get my damned thesis finished, I make a historiographical study of your work and that of your peers to make it official.

If you do that with some degree of integrity I will read it with interest.

 

[2IDSGT]

In the particular case here, the RCS estimates for T-50 and J-20 don't make too many assumptions about mission and construction because they rely on shape only.

Yeah... when APA manages to steal some actual blueprints and material samples, let me know. Otherwise, I don't really care what the fanboys feed into their computers; it'll still be "garbage in, garbage out."

 

[LowObservable]

So you don't agree with Denys Overholser that the four most important aspects of stealth are "shape, shape, shape and materials"?

 

[overscan (PaulMM)]

Who be dat "they", Sferrin?

If you can cite someone serious who thought Fencer was a Tomcat analog, fire away. And, yes, people in the US did misoverestimate the MiG-25, but they did it in order to sell a bigger and hotter F-X (F-15). Anyone who could look at that canopy and think "WVR dogfighter" quite obviously needed his head examined, once a team of proctologists had retrieved it.

2IDSGT - I'm sure the people you were talking to did not think of or know about these things, but that doesn't mean that there were not more perceptive folks somewhere else.

As I recall from released intelligence reports it was the USAF who over-egged the MiG-25 threat, the CIA/DIA and RAF were much more conservative. This same pattern was observed in other "scares" like the "missile gap" and the "bomber gap" and probably owed more to USAF procurement plans than sober fact analysis.

 

[2IDSGT]

So you don't agree with Denys Overholser that the four most important aspects of stealth are "shape, shape, shape and materials"?

You of all people should know that RCS shaping is about a lot more than the broad strokes, which is all that can be learned from photos... sometimes not even that. Remember what the first picture of the F-117 led to? What a goofy looking plane.

 

[quellish]

You of all people should know that RCS shaping is about a lot more than the broad strokes, which is all that can be learned from photos... sometimes not even that. Remember what the first picture of the F-117 led to? What a goofy looking plane.

What aspects of the vehicle's shape are you arguing are missing from photographs?

 

[2IDSGT]

As I recall from released intelligence reports it was the USAF who over-egged the MiG-25 threat, the CIA/DIA and RAF were much more conservative. This same pattern was observed in other "scares" like the "missile gap" and the "bomber gap" and probably owed more to USAF procurement plans than sober fact analysis.

I can tell you that the "missile gap" was just a stick for JFK to beat Nixon over the head with. Ike knew better and told Kennedy as much in a pre-election briefing, but that didn't stop his campaign from continuing to use the phrase. After the election, McNamara actually told reporters in a press briefing that the gap didn't exist, necessitating some quick damage control on Camelot's part. Ike had only wanted a total missile force of ~400 (including SLBMs); JFK's whining about a gap meant building a total force of ~1700 to save face.

 

[LowObservable]

My first interpretation of what you might call Overholser's Law is that the main use of RAM/RAS is to make the real signature conform to the specular ideal defined by the shape.

This means dealing with the scattering sources represented by edges, apertures, discontinuities, creeping waves and the rest of the complex stuff.

My second interpretation (a major point that he was making, I think) is that fixing a bad shape with RAM/RAS is unlikely to work.

And a valuable lesson that everyone in this business learns at one time or another is that one photo is not much use unless it's a direct overhead with something to give you scale.

PaulMM - An interesting aspect to the MiG-25 story was range, because both the Sinai overflight tracks and 1970s radar tracks over the North Sea suggested range that was out of line with what was known about the jet. But I don't know if anyone was aware of the 5300 lit centerline tank on the recce-strike versions (which IIRC explained Sinai) and the North Sea tracks were Tu-123s.

 

[sferrin]

And, yes, people in the US did misoverestimate the MiG-25, but they did it in order to sell a bigger and hotter F-X (F-15).

A bit disengenuous wouldn't you say? Open any book on the development of the F-15 and you can read that the *perceived* capability of the Mig-25 heavily influenced the characteristics of the F-15. Here: http://www.afhso.af.mil/shared/media/document/AFD-120516-036.pdf

A few highlights (heh, there's Sprey whining about heavyweight fighters 40 years ago):


Good enough for ya? It's pretty obvious they (the USAF/DOD/industry) were pretty concerned about the Foxbat. Not only the F-15 but the Tomcat was heavily influenced by the perceived capabilites of the Mig-25 to the extent that one of the first tests of the F-14/Phoenix system was to shoot down a Bomarc to demonstrate it could deal with the Mig-25.

As for the Su-19/24 thing that was from an old book back in the day called, "The Soviet War Machine".

In it they said the nearest equivalent was the F-14 Tomcat. Anyway, in looking for info from the time on the "Su-19" to see if there was anything more to that I stumbled upon this:

http://www.flightglobal.com/FlightPDFArchive/1978/1978 - 1406.PDF
(You'll appreciate the irony.)

 

[Sundog]

So you don't agree with Denys Overholser that the four most important aspects of stealth are "shape, shape, shape and materials"?

You of all people should know that RCS shaping is about a lot more than the broad strokes, which is all that can be learned from photos... sometimes not even that. Remember what the first picture of the F-117 led to? What a goofy looking plane.

I remember when I saw that and I LMAO. You shouldn't confuse poor graphic artists with knowledgeable engineers.

 

[LowObservable]

Sferrin - What you perceive as "whining" reads more accurately as the conceptual roots of the F-16.

Moreover, note what the fighter generals were using the Foxbat to push for - Mach 2.7, STOL and supersonic on the deck. Thank goodness some sanity prevailed and the F-15's seldom-if-ever--used Mach 2.5 dash was the only legacy.

As for Su-19s... back in the day you worked with what you had, so I fail to see what is ironic about it. And offhand I can't recall who did the Soviet War Machine stuff on the Su-19. I think it was Gunston.

 

[quellish]

My first interpretation of what you might call Overholser's Law is that the main use of RAM/RAS is to make the real signature conform to the specular ideal defined by the shape.

This means dealing with the scattering sources represented by edges, apertures, discontinuities, creeping waves and the rest of the complex stuff.

Shaping reduces the specular backscatter return, and to a (much) lesser extent the nonspecular as well.
Materials are typically used to address the nonspecular return by absorbing and/or channeling that energy, though there are cases where materials are used to address specular returns such as structures that have multiple bounce potential (like cavities, control surfaces, etc.) or where shaping is not practical.

The outer mold line of an aircraft is the most important factor in the RCS of an aircraft. For the most part, that tells you just how bad of an antenna you can make. If your antenna is bad enough, then you start worrying about other sources of scattering like cavity returns, apertures, etc. In the case of the F-117 and Bird of Prey, a great deal of attention was paid to even very very small sources of scattering because the RCS of the planform was so low.

On the other hand, if your planform's RCS is large enough to drown out these sources to begin with, you don't worry as much about those things. Case in point, the J-20 or the F-15 Silent Eagle.

My second interpretation (a major point that he was making, I think) is that fixing a bad shape with RAM/RAS is unlikely to work.

That is certainly true. RAM can make a good shape *better* in some bands, but it can't magically make a bad shape good.

 

[sferrin]

Sferrin - What you perceive as "whining" reads more accurately as the conceptual roots of the F-16.

Which was completely outclassed by even the Mig-23 in BVR, couldn't self-designate, had limited night attack capability. . . It didn't get useful until it got BVR with AIM-120 and LANTIRN. In other words the aircraft Sprey dreamed of was ineffective even then.

As for Su-19s... back in the day you worked with what you had, so I fail to see what is ironic about it. And offhand I can't recall who did the Soviet War Machine stuff on the Su-19. I think it was Gunston.

Ironic in that I was looking for info supporting the notion that the Su-19 was a fighter and I stumble upon an article written by you, during that timeframe, outlining what it *actually* was.

 

[latenlazy]

On the other hand, if your planform's RCS is large enough to drown out these sources to begin with, you don't worry as much about those things. Case in point, the J-20 or the F-15 Silent Eagle.

How do you know that the J-20's return is big enough to drown out smaller sources?

 

[LowObservable]

Me, I never said Fencer looked like an F-14, so you are thinking of some other guy.


As for the F-16A being ineffective, I suggest that there is an air force not one million miles from Tel Aviv that would be impressed as heck with that penetrating insight.


On paper, the MiG-23 should prevail against an F-16A in BVR. However (exactly as BVR critics such as Sprey knew) the drawbacks of an early semi-active weapon, including poor Pk at range and the need for constant illumination from launch to impact, would make its use indecisive and force a merge. At that point the -23 would be highly disadvantaged due to the performance trades that were made in order to provide BVR, not to mention other features such as lousy cockpit design, poor visibility and difficult handling (also anathema to the Fighter Mafia).


Fortunately, nobody would be dumb enough today to design a fighter so completely dependent on a single design attribute...


As for "couldn't self-designate", first, not many things could in those days and second, ask the Pakistanis.

 

[sferrin]

Me, I never said Fencer looked like an F-14, so you are thinking of some other guy.

I wasn't saying you said that. That other book did. What I was saying is that I was looking for information from that time period that supported the "they thought the Fencer was a fighter" notion and in doing so found *your* piece that indicated it was already known that the Fencer was a STRIKE aircraft. It just struck me as amusing.

As for the F-16A being ineffective, I suggest that there is an air force not one million miles from Tel Aviv that would be impressed as heck with that penetrating insight.

Ask yourself why they planned on adding BVR ASAP if it wasn't a shortcoming.

Fortunately, nobody would be dumb enough today to design a fighter so completely dependent on a single design attribute...
As for "couldn't self-designate", first, not many things could in those days and second, ask the Pakistanis.

My point was that a non-BVR, light fighter ONLY force (as Sprey wanted) would not have been a viable option, which was recognized even then.[/quote][/quote]

 

[chuck4]

To say Su-19 (Su-24) could have been filled function similar to F-14 wasn't so far out there as the flippant might suggest. The Su-24 was perceived to be similar to F-111, and a derivatives of the F-111, the F-111B, was to have filled the stand off interception role later filled by F-14.

Also, Su-24's closest European equivalent, the Tornado, also spawned an interceptor derivative in the Tornado ADF. So what is so remarkably outlandish about Su-24 airframe being used to fill a similar role?

If there was one thing which many early estimates of Su-24 severely erred on, it was fuel capacity and range. Early suggestions that Su-24 had combat radius well over 1000 miles and could dominate the NorthSea and strike most of Britain from Warsaw pact bases was far off the mark.

 

[RyanC]

Moreover, note what the fighter generals were using the Foxbat to push for - Mach 2.7, STOL and supersonic on the deck. Thank goodness some sanity prevailed and the F-15's seldom-if-ever--used Mach 2.5 dash was the only legacy.

I think they were pushing for M2.7 cruise, but my recollection may be wrong.

 

[quellish]

How do you know that the J-20's return is big enough to drown out smaller sources?

A guy named Maxwell gave me the solution. The method only took a moment.

 

[latenlazy]

How do you know that the J-20's return is big enough to drown out smaller sources?

A guy named Maxwell gave me the solution. The method only took a moment.

If you did estimates, why not publish them?