The US Air Force has had to overhaul its program plans due to a number of external pressures. It will likely need to keep more old aircraft and buy fewer new fighters than planned. It also faces higher costs to support an expanded Army and Marine Corps. The Air Force Chief of Staff says the service needs an extra $20 billion annually to maintain readiness and will have to completely rebalance its financial and structural plans. Its top acquisition priorities are replacing the KC-135 tankers, acquiring a new combat search and rescue helicopter, developing new space systems, and procuring F-35 fighters.