RS-68

The Pratt & Whitney Rocketdyne RS-68 (Rocket System 68) is a liquid-fuel rocketengine that burns liquid hydrogen (LH2) with liquid oxygen (LOX). It is the largest hydrogen-fueled engine in the world. Development of the engine started in the 1990s with the goal of producing a simpler, less-costly, heavy-lift engine for the Delta IVlaunch system. The engine has three versions: the original RS-68, the improved RS-68A, and the RS-68B for NASA.

Design and development

The RS-68 was developed at Rocketdyne Propulsion and Power, located in Canoga Park, Los Angeles, California, to power the Delta IV Evolved Expendable Launch Vehicle (EELV). The combustion chamber burns liquid hydrogen and liquid oxygen at at 102% with a 1:6 engine mixture ratio.

At a maximum 102% thrust, the engine produces in a vacuum and at sea level. The engine's mass is at. With this thrust, the engine has a thrust-to-weight ratio of 51.2, and a specific impulse of 410 s (4 kN·s/kg) in a vacuum and 365 s (3.58 kN·s/kg) at sea level. The RS-68 is gimbaled hydraulically and is capable of throttling between 58% and 101% thrust.

A leading goal of the RS-68 program was to produce a simple engine that would be cost-effective when jettisoned after a single launch. To achieve this, the RS-68 has 80% fewer parts than the multi-launch Space Shuttle Main Engine (SSME). Simplicity came at the cost of lower thrust-efficiency versus the SSME: the RS-68's thrust-to-weight ratio is significantly lower and the RS-68's specific impulse is 10% lower. The benefit of the RS-68 is its reduced construction cost: To build an RS-68 for the BoeingDelta IV program costs about $14 million, compared to $50 million for the SSME. While the SSME's higher costs were designed to be spread across multiple launches, the larger, less-costly, more powerful (50% more thrust) RS-68 was a more cost-effective engine for an expendable launch vehicle.