easily replaced, making such raids futile. Therefore, this mission was abandoned. In addition, previous campaigns had made use of medium bombers flying well behind the frontline to block the arrival of reinforcements. This mission, however, was rarely attempted during Citadel. The Luftwaffe command understood that their support would be crucial for the success of Operation Citadel, but problems with supply shortfalls hampered their preparations. Partisan activity, particularly behind Army Group Center, slowed the rate of re-supply and cut short the Luftwaffe's ability to build up essential stockpiles of petrol, oil, lubricants, engines, munitions, and, unlike Red Army units there were no reserves of aircraft that could be used to replace damaged aircraft over the course of the operation. Fuel was the most significant limiting factor. To help build up supplies for the support of Citadel, the Luftwaffe greatly curtailed its operations during the last week of June. Despite this conservation of resources, the Luftwaffe did not have the resources to sustain an intensive air effort for more than a few days after the operation began. For Citadel, the Luftwaffe confined its operations to the direct support of the forces on the ground. In this mission the Luftwaffe continued to make use of the Junkers Ju 87 "Stuka" dive-bombers. A new development to this aircraft was the "Bordkanone" 3,7 cm calibre cannon, one of which could be slung under each wing of the Stuka in a gun pod. Half of the Stuka groups assigned to support Citadel were equipped with these Kanonenvogel (literally "cannon-bird") tankbuster aircraft. The air groups were also strengthened by the recent arrival of the Henschel Hs 129, with its 30 mm MK 103 cannon, and the F-subtype ground attack ("jabo") version of the Focke-Wulf Fw 190. In the months preceding the battle, Luftflotte 6 supporting Army Group Center noted a marked increase in the strength of the opposing VVS formations. The VVS formations encountered displayed better training, and were flying improved equipment with greater aggressiveness and skill than the Luftwaffe had seen earlier. The introduction of the Yakovlev Yak-9 and Lavochkin La-5 fighters gave the Soviet pilots near parity with the Luftwaffe in terms of equipment. Furthermore, large numbers of ground-attack aircraft, such as the Ilyushin Il-2 "Shturmovik" and the Pe-2, had become available as well. The Soviet Air Force also fielded large numbers of aircraft supplied via lend-lease. Huge stockpiles of supplies and ample reserves of