The Rocketdyne J-2

On May 25th, 1961, John F. Kennedy set a seemingly impossible goal for the United States - to put an American on the Moon by the end of the 1960s. Despite being behind years behind the U.S.S.R. in the Space Race, the 1960's saw one of the largest growths of engineering talent, science and technology since the Industrial Revolution. Due also in part to a massive investment of money from the U.S. - $25.4 billion total in 1973 and up to .75% of U.S. GDP per year - the United States met its goal of landing a person on the Moon on July 20, 1969

After the Mercury and Gemini projects established the feasibility of conducting lunar operations in earth orbit it was necessary to design an engine capable of fitting the mission profile of a trip to the moon: it must be steerable, throttleable, designed to operate outside the atmosphere and, most importantly, be capable of restarting multiple times to achieve Earth orbit and lunar insertion. 

The J-2 rocket engine was produced by North American Aviation's Rocketdyne Division as an answer to this need. The first manned booster engine to use liquid hydrogen propellant, the J-2 engine oxidized hydrogen with oxygen stored in liquid form. It was used for the second and third stages of the Saturn V rocket, with the outer four engines of the second stage and the single engine of the third mounted on gimbals to allow attitude adjustment during flight. 

The J-2 was an impressive machine, weighing in at 3,609 lb, eleven feet tall, and over eight feet in diameter.  The thrust chamber was constructed of longitudinally arranged furnace-brazed stainless steel tubes that provided regenerative heating to the propellent. Not only did this vaporize propellent for combustion in the thrust chamber, but it also prevented the chamber from melting under the heat of its own 5,200º F combustion.


In a gas generator, small amounts of propellent and oxidizer are burned to power two two-stage turbines. The first one fed by the gas generator exhaust is a two-stage turbine powering a seven-stage axial flow pump spinning at an impressive 27,000 RPM requiring 7,800HP, feeding LH2 through the engine bell tubes into the thrust chamber. The second pump pushed LOX with a single-stage centrifugal pump requiring 2,200 HP to operate.

J-2 engine oxidizer and fuel delivery system schematic. 

In case those numbers flew by, let me reiterate. The fuel pumps alone consumed over 10,000HP. The reason the oxidizer pump requires so much less power, even though it is mixed in a 5:1 ratio with hydrogen, is due to the increased density of liquid oxygen (1.141 kg/L for LOX vs. .07099 kg/L for LH2.) To achieve a similar mass flow rate, volumetric flow rate must increase, consequently so does the operating pressure and power requirement. There is more information on mixture ratios' effects here.


While those are impressive numbers, equally impressive are the control systems in place to keep the engine from exploding on startup, to provide restart capability, and allow the engine to be throttled. There are complex valves and control systems, as well as ullage rockets which fire to establish liquid propellant head at the turbopump inlet for restarting in zero-g. The Apollo program required computers that could fit within the rocket, pushing scientists to miniaturize computers and develop manufacturing methods that have aided development of everything from iPads to hybrid cars. 

In a testament to the excellent design of these engines, they were the inspiration behind the Space Shuttle Main Engines. Furthermore when tasked with designing a Moon-launch vehicle for the 21st century, Rocketdyne engineers simply dusted off the old drawings and made a few tweaks to create the J-2X. Even with three decades of material and computer advancement, they couldn't do much better than was done during the Apollo era.  

All these systems, consuming 531 pounds of propellant per second, generated 225,000 pounds of thrust in a consistent repeatable fashion over 200,000 miles from Earth. These ingenious engines sent human kind further than it has ever gone before - achieving a goal that hasn't been duplicated in almost 30 years. 

Despite the budget cuts and social problems that our planet has, perhaps by standing on the shoulders of these engineering giants we'll one day make it back to space.

The first-stage F1 engine is even larger at almost three me's tall!

Sources:

http://www.nasa.gov/centers/marshall/pdf/499245main_J2_Engine_fs.pdf

http://boeing.com/history/bna/j2engine.htm

http://www.rocketdynearchives.com/engines/j2.html

http://en.wikipedia.org/wiki/Apollo_program

http://en.wikipedia.org/wiki/Liquid_hydrogen

http://www.braeunig.us/space/comb-OH.htm