Business end
The Rocketdyne-built RS-68 main engine at the base of the Delta 4 is seen here wrapped by a red cover. The hydrogen-fueled engine is derived from the Space Shuttle Main Engine. The two Alliant solid rocket motors attached to the Delta 4 are larger versions of those flown on Delta 2 and Delta 3. The Delta 4 can fly with zero, two or four strap-on SRMs, depending on the size of the payload. The motors have nozzles that gimbal, providing extra steering for the vehicle during ascent.
You also see in this image two tail services masts that will be used for Delta 4 Heavy. The center section of the masts are doors that open for umbilicals to reach the rocket. The umbilicals are retracted at ignition and the doors close like eye lids.
Photo: Justin Ray