Rösing was born on 28 September 1905 in
Wilhelmshaven, at the time an
exclave of the
Province of Hanover, a
province of the
Kingdom of Prussia. He was the son of
Vizeadmiral Bernhard Rösing (1869–1947) and his wife Elfriede, née Wünsche (1882–1961). He had three brothers, Wilhelm, Kurt-Wolf and Bernhard, all killed during World War II, and a sister named Elfriede. He joined the
Reichsmarine on 31 March 1924. During this training period he advanced in rank to
Fähnrich zur See (officer cadet) on 1 April 1926. Following additional, land based trainings he was stationed on for 13 months (9 January 1928 – 10 March 1929). While stationed on
Nymphe he was commissioned as officer and attained the rank
Leutnant zur See (Second Lieutenant) on 1 October 1928. He then participated in the training course (11 March 1929 – 27 March 1929) for naval
anti-aircraft artillery at
Wilhelmshaven before again boarding
Nymphe for three weeks (28 March 1929 – 16 April 1929). Rösing was then stationed on (17 April 1929 – 31 October 1929) before he was appointed company officer with the 2nd department of the standing ship division of the Baltic Sea on 1 November 1929. During this assignment he was promoted to
Oberleutnant zur See (First Lieutenant) on 1 April 1930. Germany was forbidden to operate submarines under the
Treaty of Versailles and Rösing was one of a small number of chosen personnel who were detached to foreign navies to gain submarine experience. On 6 February 1931 he was officially assigned to the special task force of the naval command and put on an exchange program with the
Swedish Navy. After this assignment on 10 July 1931 he was transferred back to the 2nd department of the standing ship division (11 July 1931 – 21 September 1931). During this posting he also spent one month on board the
sailing yacht Asta (1 August 1931 – 30 August 1931). His next assignment (22 September 1931 – 22 December 1931) placed him on the Admiral's staff at the Baltic Naval Station, while in parallel he attended a technical officers training course at the Naval Academy at Mürwik. Rösing was then transferred to the 1st Schnellboot-Demi-Flotilla (23 December 1931 – 1 October 1933) where he commanded his first boat,
Schnellboot S-15. During this command he attended two more training courses, anti-gas defence (4 February 1932 – 17 February 1932) and a
Spanish language interpreter training vacation (5 May 1932 – 15 June 1932). He passed his Spanish interpreter test on 26 October 1932. From 4 November 1932 the end of his assignment with the 1st Schnellboot-Demi-Flotilla on 1 October 1933 he commanded
S-3. After spending two years at the
Unterseebootsabwehrschule, Rösing, recently promoted
Kapitänleutnant, was given his first submarine command when he commissioned the new . After two years in this small Type IIB coastal submarine, he took command of , a larger Type VIIA ocean-going boat. In 1937 Rösing was given command of another Type IIB, , and assigned to the
Torpedoerprobungskommando, responsible for the testing of new torpedo types. After a year in this post he was appointed commander of
5th U-boat Flotilla - Flotille Emsmann - at Kiel. ==World War II==