Following the
German invasion of Norway in April 1940 Kjelstrup participated in the defence of Norway as a soldier. He was hit by a German bullet, but saved his life because he was carrying a pair of wire cutters that the bullet hit before entering his body. The bullet stayed in his body for the duration of his life. When
the battles were over he travelled to Sweden, and after a long journey, via Moscow, Odessa, Istanbul, Suez, Durban, Cape Town, Trinidad, Halifax and Liverpool he ended up in London, where he became a member of the
Norwegian Independent Company 1 (). He was recruited a member of the
Grouse team which was parachuted onto the
Hardangervidda plateau on 18 October 1942, along with
Jens-Anton Poulsson (leader of the group),
Knut Haugland and
Claus Helberg. The team landed at Fjarefit in Songadalen, and relocated to
Møsvatn where they prepared to receive British troops, the
Operation Freshman. He later returned to the United Kingdom via Sweden. As a sergeant in the Linge company he joined various military courses, receiving training in explosives and sabotage. In 1944 he participated in
Operation Sunshine led by major
Leif Tronstad. He lived his later life in
Bærum, and died on 5 May 1995, just days before the 50-year anniversary of the
liberation of Norway. ==Honors==