Kerrison Predictor

[Hanno Spoelstra]

Originally Posted by Rob Fast

http://www.ebay.ca/itm/150713504340?...84.m1423.l2649

For future reference:

WWII AntiAircraft Battery Kerrison Predictor sighting scope gunsight 1943 AA RAF
Starting bid: GBP 30.00
Item number: 150713504340
Item location: Rouen, France
Ships to: Worldwide

SCARCE Anti-Aircraft Battery Kerrison Predictor sighting scope


1943 dated. close to mint condition, complete, found in France, old WWII flying instrument stock, so certainly see some service here in Normandy in 1944 ! AA interest, very special item, please read full story bottom !


The Kerrison Predictor was one of the first fully automated anti-aircraft fire-control systems. The predictor could aim a gun at an aircraft based on simple inputs like the observed speed and the angle to the target. Such devices had been used on ships for gunnery control for some time, and versions such as the Vickers Predictor were available for larger anti-aircraft guns intended to be used against high-altitude bombers, but the Kerrison's electromechanical analog computer was the first to be fast enough to be used in the demanding high-speed low-altitude role, which involved very short engagement times and high angular rates.



By the late 1930s, both Vickers and Sperry had developed predictors for use against high altitude bombers. However, low-flying aircraft presented a very different problem, with very short engagement times and high angular rates of motion, but at the same time less need for ballistic accuracy. Machine guns had been the preferred weapon of choice against these targets, aimed by eye and swung by hand, but these no longer had the performance needed to deal with the larger and faster aircraft of the 1930s.

The British Army's new Bofors 40 mm guns were intended as their standard low-altitude anti-aircraft weapons. However, existing gunnery control systems were inadequate for the purpose; the range was too far to "guess" the lead, but at the same time close enough that the angle could change faster than the gunners could turn the traversal handles. Trying to operate a calculating gunsight at the same time was an added burden on the gunner. Making matters worse was that these ranges were exactly where the Luftwaffe's dive bombers were attacking from, which were quickly proving to be a decisive weapon in the Blitzkrieg.

The Kerrison Predictor was a relatively simple device compared to high-altitude predictors, and was designed to meet these particular requirements. It was designed by Major A.V. Kerrison at the Admiralty Research Laboratory, Teddington, in the late 1930s. After the war, Kerrison went on to become Director of Aeronautical and Engineering Research at the British Admiralty.

The Predictor solved the problem by doing all of the calculations mechanically through a complex system of gears. Inputs to its calculations included wind speed, gravity, ballistics of the gun and the rounds it fired, angle to the target in azimuth and altitude, and a user-input estimated target speed. Some of these inputs were fed in via dials, which turned gearing inside the Predictor to calculate the range (from the change in angle and estimated speed) and direction of motion. The "output" of the device drove hydraulic servo-motors attached to the traversal and elevation gears of the otherwise unmodified Bofors gun, allowing it to follow the predictor's indications automatically without manual intervention. The gunners simply kept the gun loaded, while the three aimers simply had to point the Predictor, mounted on a large tripod, at the target. The Kerrison predictor did not calculate fuse settings, as the shells fired by the 40 mm Bofors gun, with which it was designed to work, were contact fused.

The Predictor proved to be able to hit practically anything that flew in a straight line, and it was particularly effective against dive bombers. However, it was also very complex, including over 1,000 precision parts and weighing over 500 lb (230 kg), even though much of it was made of aluminum to reduce weight. With the demands of the RAF for almost all light metals and machinists, the Predictor was far too difficult for the Army to produce in any quantity.

While the Predictor proved to be an excellent addition to the Bofors, it was not without its faults. The main problem was that the system required a fairly large electrical generator in order to drive the gun, increasing the logistics load in supplying the generators with fuel. Setting the system up was also a fairly complex task, and not something that could be done "on the fly". In the end they were used almost entirely for static emplacements, field units continuing to rely on their original iron sights or the simple Stiffkey-Stick sights that were introduced in late 1943.

There were two of these sighting scopes fitted to the side of a Kerrison Predictor, one for Bearing and on for Elevation. Two operators would sight and track an aircraft through the scopes, and the Predictor would calculate an amount of Lead required based on the rate of change of the scopes, along with a value for Altitude (from a Barr and Stroud rangefinder), an estimated speed, and time of flight of the shell . Angle values for the scopes would be transmitted to the Gun positions, where the angles would be read off a scale in front of the "Layer for Elevation" and the "Layer for Bearing". This way 4 Guns could be brought to bear on the same target aircraft without the crews on the guns "Aiming" at the target. They just had to concentrate on the scale. These were used with 40mm Bofors, 3.7in AA Guns, and were even used to control AA Searchlight Batteries.

The spanner is placed across the aperture where your scope mounts, to prevent light damaging the internals of the unit. The internal mechanism is powered electrically, but the calculating is wholly worked by gyros and gears!

Memories of Frank Yates CHAPTER 4 (http://www.bbc.co.uk/ww2peopleswar/s...a7120298.shtml)

« Now the predictor; The sergeant instructor told us that a large number of them had been landed in Norway in the disastrous invasion, and had been thrown into the sea, still in their wooden packing cases, in the equally disastrous evacuation. A predictor was useful only in a static situation with a power supply laid on. They were far too heavy to be carted about in action, as we were about to find out!
A heavy duty steel tripod stand, fitted with plate sized feet was put in position, the top having three mounting “pinkles”, each of which could be raised and lowered, by turning a screw operated handwheel, allowing the predictor to be accurately levelled.
Our friend, the instructor, before giving us the pleasure of lifting the thing, told us about the inventor, Colonel Kerrison and his team who had demonstrated their prototype to the powers that be, by hitting the towed drogue target, with astonishing accuracy, and then bringing down the house by deliberately severing the tow cable. Be that as it may, during the next two or three years, I saw many predictor hits, but nothing approaching that level of skill. I suppose that the 3 scientists responsible for the design and building of the instrument had the edge, both in intelligence and know how
A large, dark green, steel box, about 30 inches square, and 24 inches high, standing on four tubular, spring loaded feet, protecting it from shock, was equipped with 4 hefty rings, through which long steel porter bars were threaded. Then four gunners, facing each other, gripped the ends of the bars, and at the command “Lift” raised the bars up to chest height in the weight lifters’ “Snatch” position. Then No 1, by cajoling, with verbal instructions and tactical pushing and shoving, manoeuvred the base of the box over the tripod and, not too soon for the lifters, had it lowered, securely, onto its mounting points. Once in position it was a fixture, weighing about a quarter of a ton.
High up, on opposite sides, were two, right angled, telescopes, the rubber cupped eye pieces at a convenient height for most people. These telescopes could be elevated and depressed, by means of a six inch diameter hand wheel, mounted low down below the left hand telescope. Similarly, another wheel on the other side caused the box to traverse. Once the two layers could see the target in their ’scopes, they could, by inserting their fingers in convenient finger holes in the hand wheels, keep the target smoothly and instinctively in their crosshairs.
The predictor would then accurately follow the flight of the target, and thus constantly measure its angle of velocity, both in azimuth and elevation. When an estimated range was set into the machine, by means of a calibrated hand wheel, operated by No.1, a mechanical device multiplied the angles by the time of flight of the shell, thus giving the “lead” needed for the hopeful meeting of shell and target.
A mains junction box, containing a 50 volt transformer and rectifier was connected, with hefty rubber covered cables, both to the gun and the predictor. I well remember the 4 plugs, being 10in. long, and 2in in diameter, “D” shaped in section, and having 15 beautiful copper rings. When inserted into the sockets, they were turned through 90 degrees and locked. The gun, itself, was fitted with two electro-hydraulic motors, one to traverse it, and the other taking care of the vertical movement.
This then was the sequence of events;
1. The No 1 grasped a clutch handle, conveniently positioned near his right hand, and bodily turned the box, using an open sight on one of the telescopes, instructing the No 3 to elevate until both layers yelled “ON.ON”, where upon the No 1 released the clutch and the predictor followed the target. He then “wound in” the estimated range. With a competent crew all this took about two seconds. »

[Neil Ashley]

Presumably these Predictors can only be used with Bofors mounted on the Power Mounts?

Have many of these survived as most I have observed appear to be manual operation only?

Presumably if the battery wanted to engage more than one target multiple Predictors would be required. I am assuming more than one gun can be operated from a single Predictor.

[Damien Allan]

Hi Neil,

in answer to your questions, the Kerrison could only be used with the powered mounts. The power units were referred to as hydromotors and they bolted onto the rectangular plates in front of the traverse and elevation geartrain. There was also a lot of other gear that had to be incorporated into the gun mount. In Australia, all powered mounts were converted back to hand cranked by removing the hydromotors. These mounts often retain some of the other parts like the electrical junction box near the equilibrators,the traverse indication gear under the platfrom and the drive input shafts that connect the hydromotors to the gun mount gearboxes.

I have seen a couple of guns with the hydromotors connected, but I think they were probably reinstalled by the current owners. I have two spare sets of the hydromotors in the shed myself (anyone interested?), so they are probably a few more out there. There are some good photos of Bofors in North Africa, all fitted with hydromotors.

If the battery wanted to engage more than one target multiple Predictors would be required. The intention was that a battery of guns can be operated from a single Predictor, which would be impressibe to see.

Does anyone out there know where a copy of the Illustrated Parts List for a Kerrison might be? I know they exist, as I missed out on one a few years ago. I recovered a Kerrison in its transit case from a barn in Bathurst NSW 10years ago, but the farmer had used it as a source of bits and pieces over the years. I don't think too much is missing (scopes are still there), but the IPL will tell me for sure. Also keen to locate a Kerrison tripod, cheers D

[Neil Ashley]

Damian

Thank you for the reply.

When you say power units/hydromotors, I assume you are refering to the electric motors rather than a generator?

Do have any info on what generator was used for this purpose?

[Damien Allan]

Neil,

Yes, I am referring to the electric motors rather tha the generator.

I have not seen (to my knowledge) a generator in Australia, but there is a good article on them by Kim Siddorn at http://www.wessex-sec.co.uk/2007%20N...rs/Jul07p2.pdf

There is a good often published picture of a Kerrison aimed Bofor battery in the south of England waiting for V1s. There may be a glimpse of the generator in that - I will have a look and see if there is a better picture reference in case you do not know the picture I mean.

cheers,

Damien

[Neil Ashley]

Quite an old thread but I found this picture today of a UK based Bofors Gun still fitted for powered laying via a Predictor.

Presumably quite rare in this unmodified configeration.

[George Moore]

If anyone is going to, or lives near(ish) to Woolwich, the Firepower Museum at the arsenal has a library.....pre arranged visits are required, contact Paul Evans (paule@firepower.org.uk), or phone 020 8312 7125.

I was researching detail of the 3" 20cwt AA gun used early WW2....in the box file was an IPL for a predictor......the 3" was also coupled to a predictor, some photographs show cables running into the base, and a set of photographs taken of the Egyptian Army show a gun with predictor.

I had a quick look through the IPL, I was more interested (from a modelling point of view) to get the dimension drawings, however there were only two photographs of the overall shape, but pages and pages of bits and pieces.

May be of help, not sure. The photocopying fees are very reasonable, and they always welcome a small donation. Very helpful people too.

Here is one photo of the Egyptian gun with predictor.

George.

[Godwin Hampton]

Originally Posted by Damien Allan

I have seen a couple of guns with the hydromotors connected, but I think they were probably reinstalled by the current owners. I have two spare sets of the hydromotors in the shed myself (anyone interested?), so they are probably a few more out there. There are some good photos of Bofors in North Africa, all fitted with hydromotors.

Damien,

Do you still have the Hydromotors? Where are you located?

Regards

Godwin Hampton

[Damien Allan]

Hi Godwin, and hello all of the Kerrison correspondents.

I do have a spare sets of T&E motors which are available. I am in Sydney - contact 0419 230 043. Also I have spare elevation gearboxes including hand cranks for anyone who needs those.

I have been lucky to acquire a Kerrison in transit case with optics ex-barn in Bathurst. It has a few parts missing, and I will have to remanufacture them most likely. That said, I am looking for the No.3 Predictor Illustrated Parts List (to work out what is missing), and the whole No.3 predictor tripod.

I noted previous references in this thread to getting the M5 manual on line, but I have not been able to find it myself. Anyone have a current link?

Looking forward to hearing from anyone who can assist.
cheers,

Damien

PS. No.3 MkI Predictor for Kerrison is different to the 3in No.1 predictor. The No.1 series were for Vickers made for mainly 3.7in AA. They are heavy and densely packed with gears and cams. Japanese predictors were based on the Vickers commercial models that came out between the wars - would be happy to collect one of those too.

[Godwin Hampton]

Damien,

You are halfway round the world from my neck of the woods! I'm in Malta, Europe - so getting hydromotors over would probably cost an arm and a leg! My Bofors can be seen at http://www.anti-aircraft.co.uk/Malta_Bofors.html

The outriggers are not mine and I am looking for a pair, or at least the jacks. I have 6" tubing to make up the arms, so if you have a pair of jacks....

Could you post some pictures of what you have available? Or write me.

Best Regards

Godwin