tons of bombs. In addition to interdicting the road and rail system of North Vietnam, Linebacker also systematically attacked its air defense system. The VPAF, with approximately 200 interceptors, strongly contested these attacks throughout the campaign. Navy pilots, employing a mutually supporting "loose deuce" tactical formation and many with TOPGUN training, enjoyed a kill ratio of 6:1 in their favor in May and June, such that after that the VPAF rarely engaged them thereafter. In contrast, the USAF experienced a 1:1 kill ratio through the first two months of the campaign, as seven of its eventual 24 Linebacker air-to-air losses occurred without any corresponding VPAF loss in a twelve-day period between 24 June and 5 July. USAF pilots were hampered by use of the outdated "fluid four" tactical formations (a four-plane, two element formation in which only the leader did the shooting and in which the outside wingmen were vulnerable) dictated by service doctrine. Also contributing to the parity was a lack of air combat training against dissimilar aircraft, a deficient early warning system, and ECM pod formations that mandated strict adherence to formation flying. During August the introduction of real-time early warning systems, increased aircrew combat experience and degraded VPAF ground control interception capabilities reversed the trend to a more favorable 4:1 kill ratio. Linebacker saw several other "firsts". On the opening day of the operation, USN Lieutenant Duke Cunningham and his radar intercept officer, Lieutenant (j.g.) William P. Driscoll became the first U.S. air aces of the Vietnam War when they shot down their fifth MiG. On 28 August, the USAF gained its first ace when Captain Richard S. Ritchie downed his fifth enemy aircraft. Twelve days later, Captain Charles B. DeBellevue (who had been Ritchie's backseater during four of his five victories) downed two more MiGs, bringing his total to six. On 13 October another weapons officer, Captain Jeffrey S. Feinstein, was credited with his fifth MiG, making him the final Air Force ace. Operation Lion's Den Although Linebacker was largely carried out by air, naval forces were also deployed to provide counter-battery fire against enemy targets along the coast and other important logistical areas and in support of ground troops. One such operation was Operation Lion's Den, or "The Battle of Haiphong Harbor". On 27 August 1972, Vice Admiral James L. Holloway III took with him his ship, the heavy cruiser , the guided missile cruiser