Henschel Hs 129

The Henschel Hs 129 was a World War II ground-attack aircraft fielded by the German Luftwaffe. Its nickname, the Panzerknacker (tank cracker), is a deliberate pun—in German, it also means "safe cracker". In combat service the Hs 129 lacked a sufficient chance to prove itself; the aircraft was produced in relatively small numbers and deployed during a time when the Luftwaffe was unable to protect them from attack.

Design and development
By the middle of the 1930s, the idea of using aircraft against ground targets had been widely 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 1930s, 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.

The German Condor Legion experience during the Spanish Civil War turned this idea on its head. Though equipped with generally unsuitable designs such as the Henschel Hs 123 and cannon-armed versions of the Heinkel He 112, their armament and 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. Similar protection was also needed in the windscreen, which required 75 mm (2.95 in) thick armored glass. The aircraft was expected to be attacking its targets directly in low-level strafing runs, so the cockpit 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 horsepower that were not being used in other designs, so the plane's production would not interfere with that of other types deemed more essential to the war effort.

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.

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, 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% overweight with the engines 8% underpowered, and understandably, it flew poorly. 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 joystick 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 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 the Gnome-Rhône 14M radial engine, which were captured in some number when France fell and produced under German occupation. This engine supplied 522 kW (700 hp) for takeoff compared to the Argus at 347 kW (465 hp). The Gnome-Rhone 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 were delivered B-0s and every B-1 that was completed. Still, it wasn't until April that 12 B-1s were delivered and the 4th staffel (squadron) became ready for action. They moved to the Eastern Front (to the 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.

Hs 129 B-3


By late 1942 reports were coming in about the ineffectiveness of the MK 103 against newer versions of the Soviet T-34 tanks. One obvious solution would be to use the larger Bordkanone BK 3,7 gun, recently adapted from the ground-based Flak 18. These guns had already been converted into underwing 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: a limited ammunition supply.

Few Hs 129s were actually installed with the BK 3,7 however, and the Rheinmetall firm decided to adapt for the aircraft (as had already been done with the heavy-gunned Ju 88P-1) their semi-automatic loading 7.5 cm Pak 40 anti-tank gun into a lighter-weight, fully automatic aircraft-mountable version, with a completely different and more aerodynamic muzzle brake to produce the Bordkanone BK 7,5 model. A huge hydraulic system was used to dampen the recoil of the gun, and an autoloader system with 12 rounds in the magazine was fitted in the large empty space behind the cockpit. This made for an easier design solution due to the PaK 40 already having a semi-automated firing mechanism. The gun and its recoil mechanism occupied a substantial gun pod under the fuselage and a hole in the rear end of the pod allowed spent cartridges to be ejected. The resulting system was able to knock out any tank in the world, but the added weight further hindered the already poor performance of the airplane. The Hs 129 B-3 version was a very poorly handling aircraft.

B-3s finally started arriving in June 1944, and just 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 aircraft available they had no effect on the war effort.

The only other aircraft to serve in World War II that were factory-equipped with as heavy a calibre of cannon as the B-3 version possessed were the 1,420 examples of the North American B-25G and B-25H Mitchell attack bombers, internally mounting the lower muzzle velocity M4 cannon or lighter weight T13E1 and M5 aircraft versions of the same cannon, all hand-loaded, mounted entirely within the fuselage, and of shorter barrel length than the BK 7,5.

The 1,200 kg (2,645 lb) Bordkanone BK 7,5 cannon installation in the Hs 129B-3 was the heaviest forward-firing "big-gun" installation ever made for a series production military aircraft, until the introduction of the Fairchild Republic A-10 "Warthog", with its General Electric GAU-8 Avenger seven barrel 30mm caliber anti-tank Gatling cannon main armament coming in at a total weight of up to 1,830 kg (4,030 lb) with ammunition included.

Hs 129 C
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 630 kW (850 hp). 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.

Operators

 * Nazi Germany
 * Luftwaffe
 * Kingdom of Hungary
 * Hungarian Air Force
 * Kingdom of Romania
 * Royal Romanian Air Force