RAPID CITY, S.D. — An airman at Ellsworth Air Force Base has been sentenced to 30 years in a military prison for the beating death of his infant son two years ago.

Twenty-four-year-old Airman 1st Class David MacDonald pleaded guilty to murder shortly after his court-martial began Wednesday at the air base. As part of his sentence handed down Thursday, he will be dishonorably discharged from the Air Force.

MacDonald admitted striking his 3-month-old son, Elliot, after he got frustrated trying to calm the infant at their on-base residence on Nov. 4, 2016. He had been watching the infant on his own while his wife was out working.

An autopsy found the baby died of a head injury.