"The most obvious difference between Constellation and the shuttle is the abort/escape design," says Bryan O'Connor, chief of NASA's Office of Safety and Mission Assurance. "We did not require crew escape for the shuttle past the fourth flight. The Constellation abort system, like Apollo, Gemini, and Mercury, will be designed to save the crew from any number of catastrophic system failures."
Lesser known rockets called ullage settling motors are being tested; they'll fire for a few seconds at stage separation to nudge the top half of Ares I forward from the booster. This will cause fuel in the second, liquid-fuel stage to slosh rearward in the tanks, helping to ensure second-stage ignition. And Pratt &Whitney Rocketdyne's cryogenic engine for NASA's new lunar lander, based on the company's RL10 lunar landing engine from the Apollo days, is a critical human-rating element of Constellation. Last January the new engine completed a third round of hot-fire tests that showed it can be throttled from 100 percent down to 10 percent, and should allow for a feather-soft touchdown on the lunar surface, with humans aboard, when that day comes.
Michael Milstein is a frequent contributor to Air & Space/Smithsonian.