Elegant Solutions in Missiles

[Jonfliesgoats]

This uses a quote from Wikipedia; not from any AFTTP, etc.  

Pilots learn that two aircraft on a collision course have no apparent motion in the cockpit.  So planes that appear to be moving aren't going to hit you without a course change, while stuff that will hit you will appear fixed or nearly fixed, blending in with cultural lighting, stars, etc.  This is why deliberate visual scanning still remains important.

The same principal is used to automatically compute appropriate lead in some Air to Air missiles.  Rather than use an eleaborate lead computing algorithm that depends on a processor and rapid communication to relevant servos, some missiles make course adjustments by change in relative bearing to target rather than relative bearing itself.

Pretty neat, eh?  This is why you should hug an engineer!  (Engineers hate hugs, though.)

"The Sidewinder is not guided on the actual position recorded by the detector, but on the change in position since the last sighting. So if the target remained at 5 degrees left between two rotations of the mirror, the electronics would not output any signal to the control system. Consider a missile fired at right angles to its target; if the missile is flying at the same speed as the target, it should "lead" it by 45 degrees, flying to an impact point far in front of where the target was when it was fired. If the missile is traveling four times the speed of the target, it should follow an angle about 11 degrees in front. In either case, the missile should keep that angle all the way to interception, which means that the angle that the target makes against the detector is constant. It was this constant angle that the Sidewinder attempted to maintain. This "proportional pursuit" system is very easy to implement, yet it offers high-performance lead calculation almost for free and can respond to changes in the target's flight path,^[7] which is much more efficient and makes the missile "lead" the target."

Edited December 13, 2016 by Jonfliesgoats

[magnemoe]

It makes sense, wonder if mechjeb does it then it docks? it has two modes, first is to aim at an spot forward of docking port, distance depend on size of ships, then it moves forward while doing fine adjustments. 

[Jonfliesgoats]

You know, this is why I am always impressed with folks that pull stuff off like this.  There are some pretty neat solutions in IRCM and IRCCM too.

[p1t1o]

You can actually use this phenomenon in everday life - for example, if you are approaching an intersection (in a car) and see another approaching from another direction, you can tell who will reach it first. Or you can judge if you need to speed up or slow down in order for a merge to go smoothly.

I learned this concept from military flight sims and immediately started using it when driving

 

Fun Fact(s): The AIM-9 is called "Sidewinder" because in its early iterations it would produce a "snaking" path through the sky caused by the way rotating seeker head produced its tracking inputs against a maneuvering target.

The seeker was very simple back then, not a camera by any means, but a rotating mirror which reflected incident IR onto several strategically-placed sensor "strips". Whenever a strip was illuminated by IR, the seeker would know the current angle of the mirror and which strip was being illuminated, and a bearing of the incident IR could be calculated from that.

**edit: ooh I remebered a detail - the strips are arranged in a spiral, in this way the seeker can get the angle off-axis and the axial bearing just by knowing which strip is illuminated and the position of the mirror. As described above however, it doesn't have to compute the actual figures.

Todays variants have more sophisticated imaging IR seekers (essentially an IR camera, made possible by advances in high-temperature IR CCDs and compact cooling systems) and more sophisticated guidance principles based on image recognition. They can reportedly, for example, target cockpits instead of just whatever part is hottest and can reject flares more easily - in fact the newest generation of "staring array" seekers are all-but-immune to this form of countermeasure.

Edited December 13, 2016 by p1t1o

[Jonfliesgoats]

The evolution of self-guidance systems has really been fascinating,  It's really interesting to learn how different engineering teams tackled similar challenges, especially before solid state electronics became viable.

There are really useful tactical applications of this wonky knowledge too.  In real life a lot of this knowledge regarding modern systems can't be discussed here.  Legacy systems, some of whose details make it into popular games and books are fair game.  

Optimizing warheads for specific applications is another fascinating design challenge too.

[p1t1o]

Friend of mine did a degree in "missile dynamics". How do these people even find out about these sorts of courses and why did I end up doing boring chemistry?

 

[Nuke]

computing lead is easy when position and velocity are known. for a game, its very easy to look at the object classes and get the needed data. in the real world position and velocity are kinda known. not with any certainty or precision. also in the time it takes to do the computation that position is going to change enough to throw off your guidance solution. so if you can streamline the whole system to avoid reliance on software, you are going to want to do that. instant > in a few cycles.

Edited December 13, 2016 by Nuke

[Jonfliesgoats]

2 hours ago, p1t1o said:

Friend of mine did a degree in "missile dynamics". How do these people even find out about these sorts of courses and why did I end up doing boring chemistry?

 

As I move into earlyish middle age, I find sticking my nose where it doesn't belong usually pays big dividends.  If you chase strange leads down, you wind up studying and working in some really unique niches.

Also, chemistry is far from boring.  If you want to do something "expeditionary"with chemistry, I am sure there are ways.

[Jonfliesgoats]

The original Strela (SA-7) would roll its entire airframe in flight and maneuver only about a single axis.  That was a pretty simple solution to the problem of packing enough guidance and attitude control into a man-portable SAM.  The sights of the first Strelas were aligned with the launch tubes.  This meant an operator had to superelevate their launcher (or engage a target more than thirtyish degrees above the horizon) or else the missile would splat into the ground in front of you before it's boost motor could ignite.  So it took a little bit of training to use this system well.  There were/are issues regarding the thermal salt batteries used for seekers, too.

The Blowpipe took a lot of training.

Later versions solved this problem by tilting the sight so the launch tube was automatically super-elevated in any engagement.

On a related note,

In the West we tend to think our effort to push Stingers and Blowpipe missiles into Afghanistan during the Soviet War in the 80s was the first time MANPADS had a strategic impact.  This is an artifact of Western transparency and pop culture.  In the final days of the Vietnam conflict HUMINT and other sources reported strange new weapons being smuggled into South Vietnam.  During the final drive by the North Vietnamese into the South, SA-7s made an appearance and were responsible for shooting down a South Vietnamese AC-119.  There were other engagements with these weapons in the quiet wars in Laos and Cambodia.  My point is that new weapons technologies cross borders and are used much quickly than we generally believe.  There's a really good archive preserved at the University of Texas at Dallas for interested parties.

Here is an article regarding some field engineering on these systems:

http://armamentresearch.com/improvised-manpads-batteries-employed-in-syria/

[Jonfliesgoats]

Here's that archive:

http://www.utdallas.edu/library/specialcollections/hac/cataam/

 

[magnemoe]

28 minutes ago, Jonfliesgoats said:

The original Strela (SA-7) would roll its entire airframe in flight and maneuver only about a single axis.  That was a pretty simple solution to the problem of packing enough guidance and attitude control into a man-portable SAM.  The sights of the first Strelas were aligned with the launch tubes.  This meant an operator had to superelevate their launcher (or engage a target more than thirtyish degrees above the horizon) or else the missile would splat into the ground in front of you before it's boost motor could ignite.  So it took a little bit of training to use this system well.  There were/are issues regarding the thermal salt batteries used for seekers, too.

The Blowpipe took a lot of training.

Later versions solved this problem by tilting the sight so the launch tube was automatically super-elevated in any engagement.

On a related note,

In the West we tend to think our effort to push Stingers and Blowpipe missiles into Afghanistan during the Soviet War in the 80s was the first time MANPADS had a strategic impact.  This is an artifact of Western transparency and pop culture.  In the final days of the Vietnam conflict HUMINT and other sources reported strange new weapons being smuggled into South Vietnam.  During the final drive by the North Vietnamese into the South, SA-7s made an appearance and were responsible for shooting down a South Vietnamese AC-119.  There were other engagements with these weapons in the quiet wars in Laos and Cambodia.  My point is that new weapons technologies cross borders and are used much quickly than we generally believe.  There's a really good archive preserved at the University of Texas at Dallas for interested parties.

Here is an article regarding some field engineering on these systems:

http://armamentresearch.com/improvised-manpads-batteries-employed-in-syria/

Manpads was important in the 67 and 73 wars between Arabs and Israel. 
An weakness was that they went after the back of the engines who was places at the back plane so they detonated in the rear and into the exhaust an quick fix was to add an pipe behind the engine, this set of the missile well behind the control surfaces and the pipe was easy to replace. 
This is also an issue with later manpads used in Iraq and other places, large passenger or transport planes have huge engines with serious back pressure and has been pretty resistant against manpads, 
Modern ones is probably almost as smart as the modern sidewinders, they probably also require an unlock code to make combat ready. 

[p1t1o]

13 hours ago, Jonfliesgoats said:

As I move into earlyish middle age, I find sticking my nose where it doesn't belong usually pays big dividends.  If you chase strange leads down, you wind up studying and working in some really unique niches.

Also, chemistry is far from boring.  If you want to do something "expeditionary"with chemistry, I am sure there are ways.

I know, I know,  Im a chemist at heart really, by this point. From it you can study a great many things from explosives to rocketry to the greater workings of the universe. In reality though I have worked on scented candles, perfumes, household cleaners and now concrete! Though I have dabbled in several interesting areas including wound care, pharmacology, electronics, research etc.

Chose chemistry precisely because it opened up a lot of avenues, but probably would have done a different degree had I known then what I know now, I was pretty clueless straight out of uni.

[Green Baron]

 

Just an annotation @Jonfliesgoats: constant bearing is as you say the principle to eyeball a "close encounter", it has it's roots in maritime navigation and boat "captains" (savvy :-)) of recreational vessels as well as the professionals do this muliple times a day. In 2D and with the speeds of boats and in dense traffic it's very easy to watch another traffic over a few minutes and tell whether it crosses your bow or your stern, depending on how bearing changes. It is an approved method and besides others used to judge which rule of collision avoidance applies (see "colreg").

 

Having flown propeller planes myself years ago i can however say that in 3D and with the speeds of aircrafts, even small ones, reaction times are very tight and the silhouettes of the "opponent" very small. I can't remember that it was officially tought and wouldn't rely on the method when underway in VFR, because the application of constant bearing needs time to judge whether the bearing is ... well ... constant.

Fly safe :-)

 

Edit: just for clarification: constant bearing & decreasing range in combination are the interesting part. A vessel with constant bearing but increasing range is uninteresting.

 

Edited December 14, 2016 by Green Baron
Which witch

[Jonfliesgoats]

Great input, Green Baron!  Aviation pulled heavily from maritime and naval techniques and procedures.  This is why we have the conventions regarding navigation lights, passing, right of way, etc.

With regard to constant bearing, depending on where/what you learn, we don't really teach constant bearing as a means of judging WHETHER a collision is a risk.  Aircraft nearby can and do maneuver onto collision courses and away from them.  We do, however, teach that a thorough and deliberate visual scan is important because the greatest hazards don't always jump out at the pilot with relative motion.   

Also, with regard to your experience flying props, it is valuable experience and you have as much unique insight into observations made as anyone.  Humility is admirable, and qualifying your experience is good for information.  This said, the chest thumping that remains rampant in aviation is something that stops people from sharing ideas and harms us.  Your input is as valuable as anyone else's.  Even in a video game forum, we are better off with your viewpoints shared than otherwise.

 

[benzman]

When I was boating we were taught that an approaching boat was on a collision course if it was not moving against the background.  This works if the boat is against a land horizon but not so well with a sea horizon.  Then you fall back on the constant bearing idea.  Aircraft are so much quicker that boating rules do not apply so much.

 

[Jonfliesgoats]

Another piece of information related to this has to do with TCAS/ACAS.  in a previous life, we mounted some extra devices on a plane that was going to be shipped overseas.  The TCAS on the plane got some weird interference that caused azimuth information to go wonky while range to targets displayed remained normal.  Eventually a new location for the TCAS antenna had to be found and the system recertified.  What was eye-opening about this was the number of blind spots that exist in most TCAS systems.  Even after recertification a target would disappear off TCAS entirely when we flew into a certain position and reappears when we gently turned a few degrees another way.  Also degradation of antennae can exist without causing any fault messaging in a lot of the systems, as we found out by pulling components and testing them.  So a lot of what we read about regarding the necessity for visual vigilance still remains.  

There are ICAO-defined standards that these systems have to meet to make certification, but even a system that passes certification and meets design requirements will have a lot of inherent weaknesses.  The simpler a particular system is designed, the fewer hidey-holes unknown weaknessses have to exist.  Weaknesses that are known and appreciated can be handled with different procedures, complimentary equipment etc.  

In trying to hang a new lump of stuff off a production airplane, the team I was on wound up chasing these technical rabbits throughout an airplane's avionics and electrical system.  KISS is really important.

 

 

Edited December 14, 2016 by Jonfliesgoats

[Green Baron]

Thank you :-) The forum can be fun.

These were only reminiscenses, my license expired looong ago.

As you mention TCAS: this a very sad example of how it works.

 

[Jonfliesgoats]

10 hours ago, p1t1o said:

I know, I know,  Im a chemist at heart really, by this point. From it you can study a great many things from explosives to rocketry to the greater workings of the universe. In reality though I have worked on scented candles, perfumes, household cleaners and now concrete! Though I have dabbled in several interesting areas including wound care, pharmacology, electronics, research etc.

Chose chemistry precisely because it opened up a lot of avenues, but probably would have done a different degree had I known then what I know now, I was pretty clueless straight out of uni.

Concrete is awesome!  Roman architecture wouldnt be as cool without it!  Scented candles are part of modern, human mating rites.  Go chemistry!

[todofwar]

On ‎12‎/‎13‎/‎2016 at 0:15 PM, p1t1o said:

Friend of mine did a degree in "missile dynamics". How do these people even find out about these sorts of courses and why did I end up doing boring chemistry?

 

I know the feeling, if I had taken physics or bothered to learn about aerospace engineering in high school I probably would have never stuck with chemistry.

[Jonfliesgoats]

Chemistry is awesome!  Our understanding of itty-bitty physics grew from and involves chemistry.  Chemistry drives at the nature of what and who we are.  

Edited December 14, 2016 by Jonfliesgoats

[FleshJeb]

13 hours ago, p1t1o said:

In reality though I have worked on scented candles, perfumes, household cleaners and now concrete! 

Can you make scented concrete a thing?

[DBowman]

20 hours ago, p1t1o said:

and now concrete!

Mars ISRU concrete would be apropos for Science and Spaceflight - sub zero temperature and the water will sublimate right out of it ...

[mrfox]

On 12/13/2016 at 7:44 AM, p1t1o said:

The seeker was very simple back then, not a camera by any means, but a rotating mirror which reflected incident IR onto several strategically-placed sensor "strips". Whenever a strip was illuminated by IR, the seeker would know the current angle of the mirror and which strip was being illuminated, and a bearing of the incident IR could be calculated from that.

**edit: ooh I remebered a detail - the strips are arranged in a spiral, in this way the seeker can get the angle off-axis and the axial bearing just by knowing which strip is illuminated and the position of the mirror. As described above however, it doesn't have to compute the actual figures.

 

The classic sidewinder (not the X) is also one of my favorite mechanical systems - there were so many elegant mechanical solutions in that design. 

The self stabilization rollerons on the rear control fins are simply genius - a self contained, self powered, purely mechanical SAS system using a only a few moving parts, physics, and aerodynamics.

https://en.wikipedia.org/wiki/Rolleron

[p1t1o]

13 hours ago, Jonfliesgoats said:

Concrete is awesome!  Roman architecture wouldnt be as cool without it!  Scented candles are part of modern, human mating rites.  Go chemistry!

Ha! Yes, I did learn a lot of cool (heh, for a scientist  ) stuff about concrete when I started.

Fun Fact - concrete continues to harden for a long time, sometimes up to centureies. The Hoover Dam, for example, is still getting stronger.

13 hours ago, todofwar said:

I know the feeling, if I had taken physics or bothered to learn about aerospace engineering in high school I probably would have never stuck with chemistry.

Hindsight is always 20/20, at least we get the cred which comes with studying one of the "hard sciences". Muchos nerd cred

13 hours ago, Jonfliesgoats said:

Chemistry is awesome!  Our understanding of itty-bitty physics grew from and involves chemistry.  Chemistry drives at the nature of what and who we are.  

Yup, essentially an extension of physics into the material universe. We studied everything from quantum superposition to weird symmetry effects. Nature is goddang terrifyingly complex!

11 hours ago, FleshJeb said:

Can you make scented concrete a thing?

I'd have to say quite unlikely. For starters the fragrance would fade quite rapidly, however there are also issues with building materials which release substances and regulatory limits are placed on such.

It would at the very least be an incredibly niche product, this is why we have scented candles!

Fun Fact #2: if you work with perfumes from a scientific perspective, knowing what is in them, you suddenly become far less fond of anything with a fragrance...and start to notice how goddang many products are infused with them! That stuff is everywhere! You know I had to classify a fragrance once as "fatal if inhaled"? And that isnt even particularly unusual (its ok - because they stink so hard, you are only ever exposed to tiny amounts...but still hey?).

***

Fun Fact #3: - there is a specific concrete that is formulated especially for rocket launch pads. I checked, we dont make any

***

3 hours ago, DBowman said:

Mars ISRU concrete would be apropos for Science and Spaceflight - sub zero temperature and the water will sublimate right out of it ...

Fun Fact #4: Concrete/cement cannot set without water. The common misconception is that concrete/cement sets as it dries, actually it is a chemical reaction that is started by the addition of water and consumes a fair amount of it. Note that concrete can set underwater. Martian concrete may need to be substantially different from earth-concrete for this reason.

***

Back on-topic

3 hours ago, mrfox said:

The classic sidewinder (not the X) is also one of my favorite mechanical systems - there were so many elegant mechanical solutions in that design. 

The self stabilization rollerons on the rear control fins are simply genius - a self contained, self powered, purely mechanical SAS system using a only a few moving parts, physics, and aerodynamics.

https://en.wikipedia.org/wiki/Rolleron

I believe that rolleron (I know the one you mean and it *is* elegant!) was the invention that enabled the missile in the first place - the rotating seeker requires the missile to maintain its roll orientation.

Without the rollerons, the seeker technology would not work. And an active roll control system would add much mass and complexity.

[monophonic]

On Thursday, December 15, 2016 at 0:18 PM, p1t1o said:

I believe that rolleron (I know the one you mean and it *is* elegant!) was the invention that enabled the missile in the first place - the rotating seeker requires the missile to maintain its roll orientation.

Without the rollerons, the seeker technology would not work. And an active roll control system would add much mass and complexity.

Not completely so - but the rolling motion must be slow. Just the addition of the rollerons transformed the hopeless AIM-9A to the world beater AIM-9B.

It was actually so good that when soviet engineers got their hands on one, they called it a veritable university course in missile design. The first versions of the K-13 (AA-2 Atoll) were such straight clones that they even had the same part numbers stamped on them! And if took a Sidewinder and an Atoll apart, you could build two working missiles from the parts without regard to which part came from which missile originally.

At least I have been informed thus. I've never actually done that myself, nor been within spitting distance of an actual missile for that matter.