Captain Knapp was the co-pilot of a B-26C with the 8th Bomber Squadron, 3rd Bomber Wing based at Kunsan Air Force Base (K-8), Korea. On 7 June 1951, while on a night intruder mission to attack a highway between Pyongyang and Sariwon, contact was lost with the aircraft. He was listed as Missing in Action and was presumed dead on 31 January 1954. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

For his leadership and valor, Captain Knapp was awarded the Distinguished Flying Cross with Oak Leaf Cluster, the Air Medal with 7 Oak Leaf Clusters, the Purple Heart, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.