Komet weapons: SG500 Jägerfaust

Short history

The 'Sondergerät 500 Jägerfaust' (also often referred to as Jagdfaust, which could be correct too) was a radical weapon, firing heavy 50 mm projectiles vertically into the lower sides of bombers. The system was triggered by an optical system, and the pilot's only task was passing underneath the bomber. The Jägerfaust was tested on the Fw 190, destined for installation in the Me 163B and the Me 262B. The installation in the Me 262B apparently did not work successfully, and it was not used operationally. Trials with the Komet showed very promising results (see the 'White 05' page for some details), and it is reported that some six operational aircraft were modified.

One bomber was shot down with it by Fritz Kelb. This happened on 10 April 1945, the last day that Komets operated from Brandis (p56-59 of Flugplatz Brandis 1935-1945). The victim has been reported as a Lancaster in Ransom's book, and as a B-17 in others, but only recently new evidence came to light via Stephen M. Fochuk from Canada. He reports:

On the 10th of April 1945 a rather large formation of No.6 (RCAF) Group Lancasters and Halifaxes carried out a day light raid on the railway centre and marshalling yards at Leipzig. Other than moderate flak being reported the only enemy opposition was in the form of 1 to 3 Me 163s. Several air gunners fired at the rocket interceptor(s) but no hits were confirmed. According to one witness a Me 163 destroyed a Halifax, which "blew up right in front of me".

This description agrees with the destructive power of a Jägerfaust armed Komet, and it is a almost sure match. So it was a Halifax instead of a Lancaster or a B-17.

Technical description

Little was known about the weapon itself, and its installation. For example, Mano Ziegler's book shows a Komet on the cover with four launching tubes in the wings; their location however interferes with the C-Stoff tanks. The pictures of the wreck of 'White 05' page don't reveal much either. But it appears logical that the launching tubes were fitted in the gun bay in the wing root. According to Ziegler's description, five tubes were fitted on either side, each projecting some 2-3 centimeters from the wing's upper surface. Possibly they pointed up at an angle and also fanwise to achieve some spread, but this could also apply only to the Fw 190 installation. Ziegler reported that a single hit was sufficient to bring a bomber down.

Big was the surprise when an ammunition collector (who wishes to remain anonymous) contacted me, and told about the Jägerfaust he had in his possession, obtained from a German source. He even kindly supplied Autocad drawings, shown on these pages. He also reported the following details:

The Jägerfaust is a recoilless weapon. It ejects its barrel downwards to compensate for the upwards firing shell. The system consists of a thin-walled launching tube (dimensions and shape are estimated, no clear information on this part is available), a riffled barrel, a projectile and a charge (all shown in the drawing). The charge is electrically ignited, causing the projectile to move upwards, and the barrel downwards after breaking two pins that held it in position. The riffling of the barrel makes the shell start to spin which results in greater stability in flight.

The installation in the Komet consisted of 10 tubes, but exact data is lacking. The installation shown in the drawing is an approximation

A normal projectile has both blast and shrapnel effect. A standard 50 mm projectile has 8-10 mm wall thickness for shrapnel effect, and the rest is explosive. The weight ratio of steel to explosive is around 80:20. The Jägerfaust uses an 'Minengranate' or 'M-Granate' which has considerably thinner walls (2-3 mm), which changes the weight ratio of steel to explosive into 60:40. The shrapnel effect is very limited, but the blast effect is enormous. This is particularly effective on sheet-metal aircraft structures, where the skins would be blown off. 'M-Granate' were also available for nearly all other German calibers between 20 and 88 mm. The concept was taken over by other countries after the war.

Grenade weight is 1 kilogram, consisting of 610 grams steel casing and 390 grams explosive. The charge is 513 grams, resulting in a muzzle velocity of 400 m/s

The Jägerfaust was manufactured by 'Hasag' in Leipzig

Jeffrey F. Bell (associate professor of planetary science, university of Hawaii) sent additional observations and insights:

The SG500 shells shown here are not very much like the Minengranaten designed for normal German aircraft and flak guns. The 'bottle' shape with sharp shoulder and distinct neck below the fuze is aerodynamically inefficient and would cause a lot of drag at high velocities. This shape was probably intended to allow inexpensive manufacture by stamping or forging, instead of machining, and is good enough for a low-velocity short-range weapon.

The shells lack the usual driving band of copper or sintered-iron that seals the bore and transmits the rifling spin to the projectile. Instead, their bases are slightly flared and pre-engraved at the factory to mesh with the lands (ridges) and grooves of the barrels. Again ultra-simple and cheap to make.

The idea of a gun that throws away a complete barrel with each shot seems crazy at first. Most recoilless guns use A) propellant gas or B) a separate 'counter-projectile' to balance the recoil. But the Germans had stopped building type A guns in 1944 due to their high consumption of propellant and type B would have greatly increased the weight. I expect that a metallurgical analysis would show that both barrels and projectiles are made of soft steel without any of the hardening alloy elements Ni, Cr, Mo, V etc. which were running short in 1945. Also, probably the usual dimensional tolerances were relaxed, since accuracy is not a requirement. So both barrels and shells could be machined rapidly with little tool wear. The substitution of a simple 20mm fuze for the complex AZ 39 safety fuze is another economy measure. The SG 500 barrels were probably shipped already loaded; in this mode the fuze is protected from accidental shocks. So, SG 500 is really a very economical weapon, very well adapted to mass production in an overstrained war economy (unlike most German weapons).

It would be nice to know more about the triggering mechanism for SG500 and similar weapons. Most descriptions imply that a photocell was used to detect the fighter's entry into the shadow of the bomber. But since the fighter is some distance below the bomber, the random orientation of the sun will introduce a timing error. Did Me 163 pilots dial in last-minute corrections on some kind of fire-control computer? From my own experience making astronomical observations at 14,000 feet, NO! So I think the SG 500 trigger system must have had a simple telescope which looked straight up and focused a small patch of sky onto the photocell. The trigger circut would respond to the change in voltage when the darker or brighter underside of a bomber enters the field of view. With either system, an approach from the rear means that there is a high chance of a firing against the target's horizontal tail, which is smaller and harder to hit than the wing. So preferably, SG500 attacks would be made from the FRONT of the bomber formation.

The SG500 trigger system could not have worked in the IR, but must have been in the visible spectrum. If (as I am almost certain) the trigger device looked straight up at the sky and NOT at the sun, a detector most sensitive in the blue or ultra-violet would be best. There were already in 1945 both vacuum tubes and primitive solid-state detectors sensitive in this range, and the technology was well known and commercialized ('talking' movie projectors read the sound track on the film this way). The system as I envision it is a small camera lens or telescope that focuses a patch of the blue sky onto the detector. When the sky is replaced by the shadowed underside of the target aircraft, the output signal will make a sudden jump. The detector output is amplified and fed to a circut that detects any abrupt change in the voltage and then generates the firing signal. US bombers in 1945 were bare unpainted aluminum, British bombers had the underside painted flat black, so I would make the system sensitive to both positive and negative voltage jumps. A possible Allied counter-measure would be to paint all bomber undersides in the RAF color called 'PR Blue' which attempted to closely match the color and brightness of the sky as seen from ~10 kilometers altitude. This color was actually used only on photo-recon aircraft which flew alone without escorts.

Andreas kindly sent the following information:

As far as I know the only suppliers for advanced light sensitive switches and detector tubes on ZnSe and CdTe (short band gap semiconductors) base were the AEG Telefunken labs and its subsidiaries. Maybe the answer lies here, somewhere in the archives. AEG did some groundbreaking work concerning sensors in the near and far IR, around 3.5 micrometer and up to 10 micrometer which would suit the IR spectrum from a normal, 40 degrees hot object or from the motors. The research work was started early in the 1930ies, not when the war started. These sensors were so highly developed they could be and were used on specially equipped night fighter tanks in the Ardennes forests during the Battle of the Bulge. AEG sold a very similar infrared system of the second generation of IR sensors to the Bundeswehr later in the beginning of the 60ies.

The setup for a trigger circuit without the need to generate a picture for the pilot (which is completely unnecessary in this case) would be quite simple. Two ZnSe cells that are compared by a circuit, some kind of integrator so the system does not react to minor differences in brightness and a relay to trigger the Jägerfaust. Not to forget some kind of safety so the Jägerfaust can be switched of on the ground or during flight. If those sensors were housed in a pipe with the same angle as the Jägerfaust or with a little lead angle (travel time for the round) the pipes would blind out any sideward alternations of the light and give a directional sense to the trigger. Even while flying a slight curve under the enemy aircraft, it still would be shot down since the sensors always look in the same target area as the Jägerfaust does.

Concerning the fire and forget setup of the Jägerfaust it has to be said that it was developed and manufactured by HASAG, which is the company that also developed the first one shot anti tank weapon, the Panzerfaust. My guess is that the RLM approached HASAG / Dr. Langeweiler to develop something similar for anti-aircraft use and they gave HASAG information on the 'Schräge Musik' which was an approach to shoot down bombers with night fighters by flying under them and then shooting them down with guns tilted upwards. Since the Erprobumgskommando was one of the pets of Galland I guess he pushed them in this direction. Because pilot training was deteriorated by petrol shortness and flight hours were limited for new pilots, the Luftwaffe searched for easy ways to trigger and fire weapons than by aiming and using cannons which also made it necessary to travel on a straight axis towards your target and made it quite easy for the target to shoot back.

Another thing is that it was relatively safe to attack Allied bombers from the front, since the speeds of both bomber and attacking aircraft add up and therefore, the time window for aim and counterfire from the bombers gunners is quite limited. 800 km/h for the Komet and 350 km/h for a B-17 add up to 1150 km/h or 350-400 meters per second. Autotriggering makes an awful lot of sense here.

Peter Ripley found very useful information in the book 'German Secret Weapons' by J.B. King and John Batchelor, BPC Publishing (UK), 1974.

Quoting page 20: "Having got all this into the aircraft the only problem remaining was to make sure the pilot pushed the button at the right moment, and that wasn't as easy as it looked. With the Me 163 passing under the bomber at 500 mph and the bomber passing over the attacker at 300 mph, the margin of error was incredibly small, and if the pilot was a tenth of a second wrong in his assessment, he would miss. To solve this problem a photoelectric aiming device called 'Zossen' was developed by a Doktor Orthuber of the AEG company at Neustadt. This contained a light source and a photocell arranged so that the light reflected from the target above would enter the photocell and automatically fire the weapon at the right moment. This was first tested at Werneuchen airfield near Berlin in mid-1944, using an aircraft fitted with an ordinary 20-mm cannon and flying it beneath a cloth target suspended between two balloons. It worked quite well and production began, but very few were ever finished and it is believed that only two were ever installed in aircraft for service."

On page 19 it also comments on the SG113A through SG119 barrel block weapons, mentioning the electric ignition system of SG119 and that... "In order that the discharges should not interfere with each other a timing device was included in the firing circuit which gave a very slight interval between the firing of each barrel, giving an effective rate of fire of 1200 rounds a minute to the assembly."

Peter's comments: This book mentions an active light source. So far it has been proposed that the cell would likelier be sensitive to either a sudden increase or decrease in light received, which would seem more practical. Of course, if an attack was to take place in low light, an active light source might have been essential for the system to continue to work. The 2 kW generator of the Komet may have come in handy. The book mentions the designer, Doktor Orthuber. This 1947 document lists German scientists, including a Dr. Orthuber of Neustadt bei Coburg, whose specialty was listed as 'Infra-red Homing Devices'. That makes the 'Zossen' device almost certainly infra-red. From my calculations, 1200 rpm firing rate (20 rounds per second) combined with a combined fly-past speed of 800 mph, would have resulted in shot density of one for every 17.77 metres of bomber fuselage. If both five-barrel guns fired at 1200 rpm, shot density could be as close as 8.8 metres along the bomber fuselage. Therefore, at full combined fly-past speed of 800 mph, a Komet passing under the wing, or across the fuselage, of a bomber would only get in one single hit. The actual rate of fire would thus seem to have needed to be higher, or the designers perhaps realised that in combat conditions, perhaps having been in glider mode for a while, actual speeds would be lower?



In the Volker Schröder DVD 'Das Kraftei - Raketenjäger Me 163 B Komet', one Komet pilot briefly discusses the Jägerfaust installation. He mentions that the tubes were installed in the root, and that a 'Selenzelle' (selenide cell) was fitted in the wing tip.