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 deflect 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' inboard-facing surfaces, as on some models of Messerschmitt's Bf 110 heavy fighter, and the gunsight was mounted outside on the nose.

Henschel's plane came in 12 percent overweight with the engines 8 percent underpowered, and understandably, it flew poorly. The controls proved to be almost inoperable as speed increased, and in testing, the V2 prototype flew into the ground from a short dive on 5 January 1940 because the stick forces were too high for the pilot to pull out. The Focke-Wulf design proved to be no better. Both planes were underpowered with their air-cooled, inverted-V12 Argus As 410 engines, and very difficult to fly.

The RLM nevertheless felt they should continue with the basic concept. The only real deciding factor between the two designs 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 129 A-0. A series of improvements resulted in the Hs 129 A-1 series, armed with two 20 mm MG 151/20 cannons and two 7.92 mm (.312 in) MG 17 machine guns, along with the ability to carry four 50 kg (110 lb) bombs under the fuselage centreline.

Hs 129 B-1

Even before the A-1s were delivered, the plane was redesigned with Gnome-Rhône 14M radial engines, which were captured in some number when France fell and continued to be produced under German occupation. This engine supplied 522 kW (700 hp) for takeoff, compared to the Argus at 347 kW (465 hp). The Gnome-Rhône radials were also made in versions with opposite rotation for the propeller, and were installed on the Hs 129 with the port engine rotating clockwise and the starboard rotating counterclockwise, as seen from nose-on, thus eliminating engine torque problems. The A-1 planes were converted into Hs 129 B-0s for testing (although it has been claimed that some As 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 129 B-1.

B-1s started rolling off the lines in December 1941, but they were delivered at a trickle. In preparation for the new plane, I./SchlG 1 had been formed up in January with Bf 109 E/Bs (fighter-bomber version of Bf 109 E) and Hs 123s, and they delivered B-0s and every B-1 that was completed. Still, it was not until April that 12 B-1s were delivered and the 4th staffel (squadron) became ready for action. They moved to the Eastern Front (to Crimea) in the middle of May 1942, and in June they received a new weapon, the 30 mm (1.2 in) MK 101 cannon with armor-piercing ammunition in a centerline pod.

Hs 129 B-2

Deliveries of the new Hs 129 B-2 model began in May 1942, side by side with the B-1 (of which just 50 planes had been delivered at that point). 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. These changes accumulated in the B-2 production line until they could eventually be told 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 Rüstsatz field refit kits were renumbered and some were dropped, and in general, the B-2 planes received the upgraded cannon pack using a 30 mm MK 103 cannon instead of the earlier MK 101. These guns both fired the same ammunition, but the 103 did so at almost twice the rate.

By late 1942 reports were coming in about the ineffectiveness of the MK 101 against newer versions of the Soviet T-34 tanks. One obvious solution would be to use the larger 3.7 cm (1.46 in) BK 3,7 (Bordkanone 3,7), recently adapted from the ground-based Flak 18. These guns had already been converted into underwing pod-mounted weapons for the Junkers Ju 87G and found to be an effective weapon, despite the fact that only 12 shells per pod could be accommodated. 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: the limited ammunition supply. The B-2/R3 package introduced the BK 3,7 3.7 cm (1.46 in) automatic cannon, but relatively few aircraft were converted in favour of the B-3, mounting the BK 7,5.

Hs 129 B-3

It was decided that the 7.5 cm (2.95 in) semi-automatic Rheinmetall PaK 40 anti-tank gun, which had already been adapted for use in the Junkers Ju 88P-1, would be further modified for use in the Hs 129. This resulted in the BK 7,5 (Bordkanone 7,5), which, even though it weighed 1,200 kg (2,600 lb), was lighter than the PaK 40. Fully automatic, it featured a new, hydraulic recoil-dampening system and a new, more aerodynamic muzzle brake. An autoloader system, with 12 rounds in a rotary magazine, was fitted in the empty space behind the cockpit, within the rear half of the wing root area. The gun and its recoil mechanism occupied a substantial gun pod under the fuselage, and a circular port at the rear of the pod allowed rearwards ejection of spent cartridges immediately after firing. While this new variant, the Hs 129 B-3, was theoretically capable of destroying any tank in the world, the added weight worsened the aircraft's general performance and it was inferior to previous variants.

The Bordkanone 7,5 was the heaviest and most powerful forward-firing weapon fitted to a production military aircraft during World War II. The only other aircraft to be factory-equipped with similar-calibre guns were the 1,420 examples of the North American B-25G and B-25H Mitchell, which mounted either a 75 mm (2.95 in) M4 cannon, or lightweight T13E1 or M5 versions of the same gun.

From June 1944, only 25 examples of the Hs 129 B-3 arrived at frontline units before the production line was shut down in September (a small number were reportedly also created by converting B-2 aircraft). In the field the B-3 proved effective, but its small numbers had little effect on the war effort.

Hs 129 C

In order to address the poor performance of the aircraft, plans had been under way for some time to fit the plane with newer versions of the Italian Isotta-Fraschini Delta air-cooled inverted V12 inline engine that delivered 630 kW (850 hp) and weighed some 510 kg (1,111 lb) apiece, providing more power than Germany's own Argus As 411 engine of similar configuration and lighter (385 kg) weight. The engine installation ran into a number of delays however, and was still not ready for production when the plant was overrun by the Allies in 1945.

Hs 129 D

Planned version of the Hs 129. Powered by either two 1085 hp Junkers Jumo 211 or two 1539 hp BMW 801 to improve its performance. No prototypes were made.

Sources:
Gunston, Bill - The Encyclodepia of the Worlds Combat aircraft, 1976, Chartwell Books, Inc., New York
Brown, Eric, Captain - Wings of the Luftwaffe, 1979, Airlife Publishing Ltd., Shrewsbury
Gunston, Bill & Wood, Tony - Hitler's Luftwaffe, 1977, Salamander Books Ltd., London
Donald, David - The Complete Encyclopedia Of World Aircraft, 1997, Brown Packaging Books Ltd., London
Wikipedia - Henschel Hs 129