The Henschel Hs 129, often referred to by its nickname, the Panzerknacker, (tank cracker), was a World War II ground attack aircraft fielded by the Luftwaffe. Although likely to be a good anti-tank weapon, it never really hand a chance to prove itself in any real way, the plane was produced in only small numbers and deployed during a time when the Luftwaffe was unable to protect them from attack.
By the middle of the 1930's the idea of using aircraft against ground targets had been "well understood" to be of little use other than hurting enemy morale. Experiences during World War I had demonstrated that attacking the combatants was generally much more dangerous to the aircraft than the troops on the ground, a problem that was only becoming more acute with the introduction of newer weapons. For much of the 1920s and 30s the use of aircraft was seen primarily in the strategic and interdiction roles, where their targets were less likely to be able to fight back with any level of coordination. For high-value point targets, the dive bomber was the preferred solution.
Condor Legion experience during the Spanish Civil War turned this idea on its head. Although armed with generally unsuitable aircraft such as the Henschel Hs 123 and cannon-armed versions of the Heinkel He 112, their powerful armament and fearless pilots proved that the aircraft was a very effective weapon even without bombs. This led to some support within the Luftwaffe for the creation of an aircraft dedicated to this role, and eventually a contract was tendered for a new "attack aircraft".
Since the main source of damage would be from rifle and machine gun fire from the ground, the plane had to be heavily armored around the cockpit and engines. They also required the same protection in the windscreen, which required 75 mm thick armored glass. Since the aircraft was expected to be attacking its targets directly in low level strafing runs, the cockpit that had to be located as close as possible to the nose in order to see the ground. One last requirement, a non-technical one, ended up dooming the designs; the RLM demanded that the aircraft be powered by "unimportant" engines of low power that were not being used in other designs.
Four companies were asked to respond, and only two of the resulting three entries were considered worthy of consideration; Focke-Wulf's conversion of their earlier Fw 189 reconnaissance plane, and Henschel's all-new Hs 129.
Design and Prototypes
The Hs 129 was designed around a single large "bathtub" of steel sheeting that made up the entire nose area of the plane, completely enclosing the pilot up to head level. Even the canopy was steel, with only tiny windows on the side to see out of and two angled blocks of glass for the windscreen. In order to improve the armor's ability to stop bullets, the fuselage sides were angled in forming a triangular shape, resulting in almost no room to move at shoulder level. There was so little room in the cockpit that the instrument panel ended up under the nose below the windscreen where it was almost invisible, some of the engine instruments were moved outside onto the engine nacelles, and the gunsight was mounted outside on the nose.
In the end the plane came in 12% overweight and the engines 8% underpowered, and it flew like a pig. The controls proved to be almost inoperable as speed increased, and in testing one plane flew into the ground from a short dive because the stick forces were too high for the pilot to pull out. The Fw design proved to be no better. Both planes were underpowered with their Argus Ar 410 engines, and very difficult to fly.
The RLM nevertheless felt they should continue with the basic concept. In the end the only real deciding factor between the two was that the Henschel was smaller and cheaper. The Focke-Wulf was put on low priority as a backup, and testing continued with the Hs 129A-0. A series of improvements resulted in the Hs 129A-1 series, armed with two 20 mm MG 151/20's and two 7.92 mm MG 17's, along with the ability to carry four 50 kg bombs under the fuselage midline.
Even before the A-1's were delivered the plane was redesigned with the Gnome-Rhone 14M radial engine, which were captured in some number when France fell. This engine supplied 700 hp (522 kW) for takeoff compared to the Argus at 465 hp (347 kW). The A-1 planes were converted into Hs 129B-0's for testing (although some claim that some A's were sold to Romania) and the pilots were reportedly much happier. Their main complaint was the view from the canopy, so a single larger windscreen and a new canopy with much better vision were added, resulting in the production model Hs 129B-1.
B-1's started rolling off the lines in December 1941, but they were delivered at a trickle. In preparation for the new plane, I./Sch.G 1 had been formed up in January with 109's and Hs 123's, and they were delivered B-0's and every B-1 that was completed. Still, it wasn't until April that 12 B-1's were delivered and its 4th staffeln was ready for action. They moved to the eastern front in the middle of May, and in June they received a new weapon, the 30 mm MK 101 cannon with armor-piercing ammo in a midline pod.
By May of 1942 after only 50 of the planes had been delivered, they started to deliver the new Hs 129B-2 model side-by-side with the B-1. The only difference between the two were changes to the fuel system – a host of other minor changes could be found almost at random on either model. As time went on these changes were accumulated into the B-2 production line until you could finally tell them apart at a glance, the main differences being the removal of the mast for the radio antenna, the addition of a direction-finding radio antenna loop, and shorter exhaust stacks on the engines.
In the field the differences seemed to be more pronounced. The R-kits were renumbered and some were dropped, and in general the B-2 planes received the upgraded cannon pack using a MK 103 instead of the earlier MK 101. These guns both fired the same ammunition, but the 103 did so about almost twice the rate.
Even by late 1942 complaints started about the MK 103 against newer versions of the Soviet T-34 tanks. One obvious solution would be to use the larger 37 mm gun, adapted from an anti-tank gun that had recently been abandoned by the army. These guns had already been converted into pod-mounted weapons for the Ju 87 and found to be a fearsome weapon. When mounted on the Hs 129 the empty area behind the cockpit could be used for ammunition storage, which would address the only problem with the Ju 87's mounting, limited ammunition.
But for some reason the Luftwaffe decided to skip over this gun for the Hs 129, and install a gigantic 75 mm gun from the Panzer IV. A huge hydraulic system was used to damp the recoil of the gun, and an auto-loader system with twelve rounds was fitted in the large empty space behind the cockpit. The resulting system was able to knock out any tank in the world, but the weight slowed the already poor performance of the plane to barely flyable in this new Hs 129B-3 version.
B-3's only started arriving in June 1944, and only 25 were delivered by the time the lines were shut down in September. A small number were also converted from older B-2 models. In the field they proved deadly weapons, but with only 25 of them they had no effect on the war effort.
In order to address the poor performance of the aircraft, plans had been underway for some time to fit the plane with newer versions of the Italian Isotta-Fraschini Delta engine that delivered 850 hp (634 kW). However the engine ran into a number of delays, and was still not ready for production when the plant was overrun by the Allies in 1943.