Geopolitical and US political considerations in 2014 led to an effort by ULA to consider the possible replacement of the Russian-supplied RD-180 engine used on the first stage booster of the Atlas V. Formal study contracts were issued in June 2014 to a number of US rocket engine suppliers [REF].
The Aerojet AR1 rocket engine under development as of 2017, is a backup plan to the successor rocket Vulcan, to re-engine the Atlas V [REF].