Everyone knows the cliché about needing rocket scientist to figure out a complex problem. But the biggest ongoing challenge for launch firms isn't building rockets per se, but making sure the engines to get to space do what they're supposed to without failing. 

Building a powerful, light and reliable engine continues to be a major challenge, humbling everyone from newcomers such as SpaceX (News - Alert) to established firms like Pratt Whitney Rocketdyne. Leaning on Russian know-how has had risk as well.

SpaceX's engine failure a few weeks ago on its CRS-1 commercial cargo mission should be viewed as a glass half-full. The Falcon 9 rocket was indeed able to lose one of its nine first stage Merlin 1C engines and still able to deliver its primary payload into the proper orbit – just as CEO/founder Elon Musk had designed. However, with only four flights of the Falcon 9 and an issue on the first Falcon 9 flight with an "oxidizer-rich shutdown" of engines, NASA and SpaceX both want to understand what happened on the October 7, 2012 launch.

Traditional space firms that might have sought some mileage out of SpaceX's engine problem had issues of their own less than a week before. On October 4, a United Launch Alliance (ULA) Delta IV launch of a GPS satellite encountered problems with the rocket's second stage. The Pratt Whitney Rocketdyne RL 10 engine delivered lower thrust as it boosted the GPS satellite into the appropriate orbit. 

The Delta IV stage compensated for the reduced thrust levels to make sure the primary payload made it to where it needed to go – just like the SpaceX Falcon 9 first stage – but the problem is being investigated by both ULA and the Air Force.

The RL 10 has been flying since 1962 with the newest version in production since 2002 – a decade's worth of testing and flight use logged. A different version of the engine is used by ULA's Atlas V upper stage. 

Since the Atlas V is the primary workhorse of the U.S. military to put satellites and spaceplanes into orbit, everyone wants to make sure there's not a common problem in the RL 10 that could affect future launches. 

Orbital Science Corporation's Antares two-stage vehicle scheduled for a first test flight late this year uses dual Aerojet AJ26 engines that are essentially remanufactured Russian NK-33 engines developed for the Soviet moon rocket back in the 1960s. Recent test stand firings of NK-33 hardware have mostly gone well, but a summer 2011 breakdown when testing engines for Orbital resulted in a pad fire, adding a couple of months of delay.

In spite of the many challenges of building a rocket engine, U.S. commercial companies are investing their own money into building better engines. SpaceX plans to start flying its uprated Merlin 1D engine next year on Falcon 9 and Musk has said the company plans to build bigger liquid hydrogen/liquid oxygen that would have several times the power of Merlin. 

Blue Origin, funded by Amazon billionaire Jeff Bezos, recently completed testing of parts for its 100,000 pound BE-3 liquid oxygen/liquid hydrogen rocket engine. The BE-3 will go into Blue Origin's reusable rocket to lift people and satellites into space.