Abstract: A protective door assembly is disclosed which will cover missile altitude control system engines during storage and launch while still being easily jettisoned when the ACS engine is fired. The titanium door attaches to the missile skin by a hook on it's lower edge and by a tapered boss which attaches to the skin and fits into a tapered hole near the door upper edge. Behind the door, the ACS engine nozzle is surrounded by a seal housing which prevents engine gases from escaping into the body of the missile and causing a pressure buildup when the engine is fired. At pressures between 35 and 60 psi, the door retainer screw fails in tension and the door is blown off.