Proton First Stage is a medium-sized first stage , unlocked after the completion of Mission 11. It is used to make the Russian Proton rocket. It costs 15,000,000$ in Career Mode . The first stage as part of the Proton rocket. Retrieved from "https://spaceagency.fandom.com/wiki/Proton_First_Stage?oldid=64583" Categories: Parts Medium Stages Content Community content is available under CC-BY-SA unless otherwise noted.