Later versions of the A-7 used different engines; according to Gunston and Gilchrist, this was largely due to production difficulties in keeping up with numerous military and civil demands. These new powerplants included the more powerful Pratt & Whitney TF30-8 and the Allison TF41-A-2 engines, a licensed model of the Rolls-Royce Spey engine. The TF41 corrected issues that had troubled initial A-7 operations, such as severe compressor stalls and low thrust. The Air Force A-7D had self-contained starting using internal batteries and a gas turbine starter. The Navy A-7E used an air turbine starter driven by an external air supply. Air was fed to the engine through ducting from a simple nose inlet, similar to that on the F-8, despite the potential hazard it posed to flight deck personnel. An aerial refueling probe was mounted on the righthand side of the nose. Two cannon were installed on the underside of the nose. For self-defense against aerial threats, the A-7, in addition to the cannon, had a mounting for AIM-9 Sidewinder air-to-air missiles on either side of the fuselage. Later variants had the two cannon replaced with a single M61A1 Vulcan rotary cannon, along with other improvements. To reduce vulnerability to ground fire the flying control hydraulic systems were triplicated, other systems duplicated and much of the fuselage had armor protection. The A-7 was fitted with an AN/APQ-116 radar, later followed by the AN/APQ-126, which was integrated into the ILAAS digital navigation system. The radar also fed an IBM navigation and weapons delivery computer which made possible accurate delivery of bombs from a greater stand-off distance, greatly improving survivability compared with faster aircraft such as the McDonnell Douglas F-4 Phantom II. It was the first U.S. aircraft to have a modern head-up display, (made by Marconi-Elliott), now a standard instrument, which displayed information such as dive angle, airspeed, altitude, drift and aiming reticule. The integrated navigation system also had another innovation – the projected map display system (PMDS) which accurately showed aircraft position on two different map scales. The A-7 had more modern avionics and systems than contemporary aircraft. This included data link capabilities that, among others, provided "hands-off" carrier landing capability when used with its approach power compensator (APC) or auto throttle.