Nuclear/Ammonia+Alcohol

Nuclear/Ammonia+Alcohol propellant. Nuclear thermal engines use the heat of a nuclear reactor to heat a propellant. Although early Russian designs used ammonia or an ammonia/alcohol mixture as propellant, the ideal working fluid for space applications is the liquid form of the lightest element, hydrogen. Although successfully ground-tested in both Russia, they have never been flown due primarily to environmental and safety concerns.