# R4M vs. 2.75" FFAR



## Zipper730 (Feb 20, 2017)

I'm curious how the R4M's accuracy and range compare to the 2.75 FFAR


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## fubar57 (Feb 20, 2017)

The R4M was WW2 and the 2.75FFAR was Post-War. The 3.5" FFAR was WW2 and comparable


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## Shortround6 (Feb 20, 2017)

Zipper730 said:


> I'm curious how the R4M's accuracy and range compare to the 2.75 FFAR


 Basically the R4M was toy compared to the 2.75 FFAR. It weighed about 46% of what the early 2.75 FFAR did. 
The 1950s equivalent to the R4M was the Soviet 57mm rocket system (launch tube was 57mm, the rockets were 55mm) 

There are several threads on the R4M.


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## Zipper730 (Feb 20, 2017)

Shortround,

It's smaller, but I get the vibe that it was much more accurate: The 2.75 FFAR seemed abysmal; the Soviet 57mm seemed closer to the FFAR. Why did we fuck up the FFAR so much?


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## Shortround6 (Feb 21, 2017)

And where does this "vibe" come from?

Germans with war time production can make a much more accurate rocket than the Americans, French and Russians can in peacetime with several years minimum for research into rocket propellant, nozzles and fin shape/manufacture? 

If you have data on accuracy of the rockets please make sure it is at the same distance or uses the same criteria, like all rockets with target area XXX or 75% within the target area or????


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## fubar57 (Feb 21, 2017)

Zipper730 said:


> Shortround,
> 
> It's smaller, but I get the vibe that it was much more accurate: The 2.75 FFAR seemed abysmal; the Soviet 57mm seemed closer to the FFAR. Why did we **** up the FFAR so much?



Language!!!!!!

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## Zipper730 (Feb 21, 2017)

Shortround6



> And where does this "vibe" come from?


The Germans seemed to not have too many complaints about the R4M, and I've seen footage of them firing and they didn't look as bad as the FFAR.

And I did a little research and found the following

R4M: Ballistics were close enough to their 30mm cannon that they could use the gun-sight for gauging ballistic drop
R4M: Dispersal was approximately 30 meters wide (98.43) by 15 meters high (43.21 feet)
2.75" FFAR: Dispersal was about the same as a football field with 24 rockets launched. Presuming without end-zones is 100 yards (91.44m), and 120 with (109.73m)
The R4M and 2.75" FFAR had substantially different ranges, which read as follows

R4M: 600-1000m normal: (656.17 to 1093.6 yards; 1968.5 to 3280.84 feet); 1500m maximum (1640.42 yards; 4921.26 feet)
Mk.4 FFAR: 3400m normal range (3718.3 yards; 11,154.86 feet); maximum range: 6000 m (6561.7 yards; 19685.04 feet)

fubar57



> Language!!!!!!


I didn't know there were any rules on that here, but I'll reign it in.


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## Shortround6 (Feb 21, 2017)

Dispersal figures are _useless_ without the range they were obtained/achieved. 
Wiki claims the figure for the R4M was at 1000 meters but gives no source. 
The Dispersion figure for the US rocket may have been obtained at a much longer range and since dispersion, *at best, *follows a geometric pattern (double the distance doubles the size of the pattern) a longer range of engagement would naturally have a a much larger dispersion pattern. In real life dispersion patterns actually follow a sort of curve. doubling the distance gives slightly more than double the dispersion. Including all projectiles fired can sometimes give a larger than meaningful figure as often 2-3 projectiles out of 10 will be responsible for doubling the group/dispersion figure. 

I would also like to know how the Germans got that 15 X 30 meter figure when fired from launchers about 7 meters apart. The rockets veered left and right more than up and down? The launchers were aimed slightly outward? The last is a real good trick with fin stabilized rockets. They will align themselves with the flight path of the launching aircraft as they are launched and as they accelerate up to speed.


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## tyrodtom (Feb 23, 2017)

Somewhere in the 2.75 FFAR's developement they changed the nozzle design.
The early 2.75 evidently depended only on the pop out fins for spin stability, meaning when first launched they took some time to spin up for good stability.

Later when they were used from slower helicopters, they knew spin stability provided by the fins wouldn't be enough, so they canted the exhaust tubes, to start the missile spinning even before it exited the launcher.

I've seen many 2.75 launches with my own eyes, not U-Tube. They varied greatly, because bad storage/handling could cause so many problems with the motors. A uneven burn inside with the solid propellant could cause guidance problems no fins could correct. Then loose, vibrating fins would cause their own problems too.

Sometimes you'd see shots that would make you think, "was he aiming for that ? " because it was that good, and then other times you'd question just what were they aiming at.

Later, in the late 70's or early 80's the changed the 2.75 nozzle again, gave it even more rapid spin-up in the launching tube to give even better accuracy, changed the fin design too.

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## Zipper730 (Mar 1, 2017)

Shortround6 said:


> Dispersal figures are _useless_ without the range they were obtained/achieved.
> 
> The Dispersion figure for the US rocket may have been obtained at a much longer range and since dispersion, *at best, *follows a geometric pattern (double the distance doubles the size of the pattern) a longer range of engagement would naturally have a a much larger dispersion pattern.


So that means even if the dispersal followed the pattern accuracy would be 180-300 meters?



tyrodtom said:


> Somewhere in the 2.75 FFAR's developement they changed the nozzle design.
> The early 2.75 evidently depended only on the pop out fins for spin stability, meaning when first launched they took some time to spin up for good stability.


I was told they miscalculated the spin-stability and used a smaller number off fins: How did they miscalculate the spin and why did they reduce the number of fins?



> Sometimes you'd see shots that would make you think, "was he aiming for that ? " because it was that good, and then other times you'd question just what were they aiming at.


I usually saw them corkscrew and fish-tall everywhere[/QUOTE]


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## Shortround6 (Mar 1, 2017)

Finding actual firing tests is difficult on the web but one should view statements like "blanket a football field" with suspicion as it sounds like a press release for a non-technical audience. 

A test I have found from 1961 (US Army) while evaluating the 2.75 rocket for helicopter use _after _modifying the rocket nozzles for spin says.

"Impact patterns showed excellent area coverage, with the initial pair of rockets impacting approximately 10 - 20 meters apart and succeeding pairs impacting at approximately 15 meters greater range with the same lateral spacing. Employing a ripple of 46 rockets, a beaten zone of approximately 40 x 400 meters resulted at an opening range of 1500 meters from 150 to 200 feet absolute altitude and 70 knots indicated airspeed (IAS)."

This is obviously for air to ground use and it was noted earlier in the report that the _unmodified _rockets when fired at zero air speed had a dispersion of 40-50 mils while the modified rockets had a dispersion of 10-12 mils in a Navy test.
The Army test does note the difficulty in getting accurate results when firing from a hover as opposed to having forward velocity on the helicopter but this is a result of the stability of the helicopter (firing platform) and not anything to do with the rockets. 

Saying the US made a mistake in reducing the number of fins _might _be correct and might not be as it takes no account of the other differences in the two rockets like the much more powerful motor in the 2.75 and the higher flight speed. It takes no account of differences in burn time of the motors. And so on.


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## tyrodtom (Mar 1, 2017)

They shot them from several different launchers. I've seen everything from a 3 tube launcher, to 7 tube, to a 19 tube launcher, and more, on each side of the aircraft.
When I was in the USAF in the late 60's I only saw them used by watching gun camera films, and most times the USAF used the 19 tube launcher. Fired in ripple fire. The rockets left the tube fractions of a second between launches, the rockets being launched through the exhaust of the others had problems. If they were launched in single pairs, they guided better. But very few tactical situations is going to allow 19 passes.

In the Army a few years later I saw many 2.75 rockets fired with my own eyes. The Army helicopters tended to use the rockets more sparingly, sometimes they'd shoot the whole load, but more often in singlets, ( 1 from each side ). How accurate each individual rocket was depended on sometimes of how many were fired with it.

Improper storage and handling of solid fuel rockets motors can have a lot of influence on how accurate they are. Drop a 2.75 rocket motor when it's cold and you might have just guaranteed that it will explode on launch, or at least have a very weird flight on launch. You may have cracked the propellant inside, which will expose much more propellant to burning on ignition. A uneven burn will produce a flight that can't be controlled by the fins. If the weather seal on the nozzle isn't good then the propellant will dry out and crack.

The RM4 that the Luftwaffe used were probably used about as soon as they were delivered to the airfield, the 2.75s the Americans and allies used later might have been in storage for decades.

Zipper730, if you're determined that the 2.75 rocket was a dud then there's probably nothing me or anyone else can say that will change your mind.
The 2.75 FFAR was like any other weapon, how effective it was depended on how it was maintained and how it was used.


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## Shortround6 (Mar 1, 2017)

The test I found
http://www.dtic.mil/dtic/tr/fulltext/u2/273738.pdf

used a C-34 Choctaw as the launch platform and a 24 round launcher (four 6 round boxes combined) on each side. Rockets when ripple fired were launched 100 milliseconds apart. This was more of a feasibility study and/or test to see what was needed to actually field a system for the upcoming UH-1 helicopters. 
Even the early 2.75s used a much more powerful rocket motor than the R4M leaving a much larger disturbance in it's wake and no amount of fins is going to make a rocket fly straight through turbulent air.


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## Zipper730 (Mar 1, 2017)

Shortround6 said:


> Finding actual firing tests is difficult on the web but one should view statements like "blanket a football field" with suspicion as it sounds like a press release for a non-technical audience.


It was the only reference I had, sorry.


> A test I have found from 1961 (US Army) while evaluating the 2.75 rocket for helicopter use _after _modifying the rocket nozzles for spin says.
> 
> "Impact patterns showed excellent area coverage, with the initial pair of rockets impacting approximately 10 - 20 meters apart and succeeding pairs impacting at approximately 15 meters greater range with the same lateral spacing. Employing a ripple of 46 rockets, a beaten zone of approximately 40 x 400 meters resulted at an opening range of 1500 meters from 150 to 200 feet absolute altitude and 70 knots indicated airspeed (IAS)."


40 meters dispersal over 1500 meters is 27 mils right?


> This is obviously for air to ground use and it was noted earlier in the report that the _unmodified _rockets when fired at zero air speed had a dispersion of 40-50 mils


40-50 mils seem to conform to the 100 yard @ 3400m estimate


> the modified rockets had a dispersion of 10-12 mils in a Navy test.


Now, that's an improvement: This would result in the ability to have put the rockets in a ring 24-25 yards wide at a distance of 3400 meters. This is actually inside the R4M's ballistics in effective range.


> Saying the US made a mistake in reducing the number of fins _might _be correct and might not be as it takes no account of the other differences in the two rockets like the much more powerful motor in the 2.75 and the higher flight speed. It takes no account of differences in burn time of the motors. And so on.


Of course: I'm not sure if they had the ability to produce the higher flight speeds in those days.
quote]Even the early 2.75s used a much more powerful rocket motor than the R4M leaving a much larger disturbance in it's wake and no amount of fins is going to make a rocket fly straight through turbulent air.[/QUOTE]So, basically the fact that they were ripple fired reduced the accuracy of each weapon?



tyrodtom said:


> The RM4 that the Luftwaffe used were probably used about as soon as they were delivered to the airfield, the 2.75s the Americans and allies used later might have been in storage for decades.


Was the propellant as stable as the early AIM-9 and AIM-7's[/quote]


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## Shortround6 (Mar 1, 2017)

There are also some significant differences between firing fin stabilized rockets from helicopters and fixed wing aircraft. 

From the helo all you had to stabilize the rocket in the first few feet of travel (as the fins are deploying) is the forward speed of the helo. In the test above best results were at 70kts. The rocket can get rather out of shape trajectory wise before the rocket motor accelerates it up to speed and rocket shape and fins get a better "grip" on the air. Please remember that doubling the speed of the rocket will result in 4 times the force acting on the fins to straighten the rocket out. But that is straightening the rocket out on the path it is traveling, not the path it may have been originally aimed on. 
The much faster fixed wing aircraft give a much faster launch speed to the rocket and a much better initial stabilization. 

The Helo, especially at hover or near it, has a considerable down wash from the rotor which can screw up fin stabilized rockets in at least two ways. One is the down wash physically pushing the rocket downward in relation to the line of sight or line of travel of the helo.
The 2nd is the tendency of fin stabilized projectiles to "weather cock" or turn into the "wind" no matter what direction it is coming from.

Another paper describes using the 2.75 rocket as a "sounding rocket" or test rocket for small payloads and "low" altitudes.
It claims the 2.75 rocket accelerated to about 106fps in it's own launch tube and accelerated at 40-49 Gs until burn out in 1.7 seconds with a peak speed of 2415fps. They were generally firing the rockets up at 70-85 degree angles so speed may differ a bit from horizontal fire but at 40-49 G acceleration I doubt the difference amounts to much. Altitude (range?) at burn out was about 2000ft. after burn out the rocket decelerated at -16Gs to start with. 

Now a 600mph Jet is doing 880fps when it fires it's 2.75in rockets so the first few hundreds of a second as the rocket leaves the launch tube has much much higher forces acting on the rocket and it's fins. This is the major reason for the higher accuracy of the 2.75in rocket when fired from fixed wing aircraft and the problem/s they had firing from helicopters. 

The US made the choice to use a high speed rocket (compared to the R4M) due to the nature of the target/s and the hoped for fire control system. The rocket system was introduced to destroy Soviet bombers of at least TU-4 (B-29) performance if not higher performing aircraft (US was flying B-47s in squadron service and introducing B-52s) and not B-17/B-24/Lancasters. 
The US was also building/introducing collision course fire control radar aiming systems. The radar and on board fire control computer would generate a firing solution (aiming point) for the rockets at several thousand yds range even from a near 90 degree intercept angle. 
All of this took a number of years to introduce and with the stakes being destroying possibly nuclear armed bombers there was a fair amount of testing done. Turns out the system never worked as well as hoped/needed and even early air to air guided missiles were judged superior. However the idea the wartime German rockets were superior in any way takes an awful lot of faith.


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## Shortround6 (Mar 1, 2017)

I would further note that 40 meters by 400 meters is NOT a measure of dispersion but rather the beaten zone of the rockets. 
I can think of no weapon that has a _dispersion_ pattern 10 times longer in one direction than another. 
The Test Helicopter was flying forward at 70kts (118fps) and firing pairs of rockets at 100 milisecond (0.10 second) intervals and fired 23 pairs of rockets. 2.2 seconds? helo would have covered almost 260ftc(78.6 meters) in that time and that assumes the Helo maintained the exact same height *and *the exact same attitude (angle of fuselage to horizontal) during that time while firing around 750lbs worth of rockets. Any change of attitude (angle of attack) while firing would obviously spreed the rockets vertically which would result in a change of range much larger than the change in vertical error.


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## tyrodtom (Mar 1, 2017)

How many aircraft were brought down with the R4M ?


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## Zipper730 (Mar 7, 2017)

Shortround6 said:


> There are also some significant differences between firing fin stabilized rockets from helicopters and fixed wing aircraft.


Very good points...



> It claims the 2.75 rocket accelerated to about 106fps in it's own launch tube and accelerated at 40-49 Gs until burn out in 1.7 seconds with a peak speed of 2415fps.


I would have thought they'd have much longer burn time...


> They were generally firing the rockets up at 70-85 degree angles so speed may differ a bit from horizontal fire but at 40-49 G acceleration I doubt the difference amounts to much.


less than 1g probably...


> Altitude (range?) at burn out was about 2000ft. after burn out the rocket decelerated at -16Gs to start with.


From what I recall it'd be more, possibly something to do with L/D


> The US made the choice to use a high speed rocket (compared to the R4M) due to the nature of the target/s and the hoped for fire control system.


My critique had little to do with speed... the speed is a good thing. The problem was that pilots had even said that they were amazed they'd hit anything with them.


> The rocket system was introduced to destroy Soviet bombers of at least TU-4 (B-29) performance if not higher performing aircraft (US was flying B-47s in squadron service and introducing B-52s) and not B-17/B-24/Lancasters.


They were expecting to use them on aircraft ranging from propellers to subsonic jets at the very least. The F-102 seemed to be designed initially to be able to shoot down subsonic and supersonic jets (it wasn't very successful in this role as it wasn't as fast as desired, hence the F-106), though its primary armament was 6 x AAM's.


> The US was also building/introducing collision course fire control radar aiming systems. The radar and on board fire control computer would generate a firing solution (aiming point) for the rockets at several thousand yds range even from a near 90 degree intercept angle.


That I know about to some degree, and the problem wasn't the computer but the projectile used. It corkscrewed and fish-tailed all over the place. Had the ballistics been more like that of a cannon-round or projectile it's odds of success would be greater.

I'm curious how they calculated the spin-rate for stabilization (it was off), and the decision to reduce the number of fins.


> All of this took a number of years to introduce and with the stakes being destroying possibly nuclear armed bombers there was a fair amount of testing done.


Probably with the fire-control mostly. I'm not sure how they concluded the rockets were okay...


> Turns out the system never worked as well as hoped/needed and even early air to air guided missiles were judged superior.


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## DerAdlerIstGelandet (Mar 7, 2017)

Why do you quote so many posts at one time, despite so many here asking you not to do that?


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## Zipper730 (Mar 7, 2017)

I quoted one member (I deleted the response to tyro)


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## DerAdlerIstGelandet (Mar 7, 2017)

Ah got you. That was one post. You're good. My apologies.

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