COLORADO SPRINGS — NASA managers looking for at least two commercial vehicles to take crews to the International Space Station have a choice of techniques for returning astronauts to Earth, from parachute landings on land to a gliding touchdown on a runway.

As they consider system-level proposals for the third phase of the Commercial Crew Program, known as Commercial Crew Integrated Capability (CCiCap), space agency evaluators are pondering the eventual use of propulsive vertical landing proposed by Space Exploration Technologies (SpaceX) and perhaps the secretive Blue Origin LLC.

Also on the table are the Boeing and Sierra Nevada entries presented at the National Space Symposium here this week, and an as-yet-undisclosed entry by ATK/Astrium based on the proposed Liberty Rocket.

Boeing and Sierra Nevada both plan to use the Atlas V to launch their crew vehicles. The similarity stops there, and landing is a big difference. Boeing’s CST-100 capsule will ride parachutes to an airbag-cushioned land landing at one of three sites in the continental U.S., while the Sierra Nevada Dream Chaser can return to a runway landing pretty much anywhere there’s 10,000 ft. of tarmac.

“We’ve got three parachutes; there’s six airbags,” says John Mulholland, vice president and manager of Commercial Programs for Boeing Space Exploration. “So we’ll be dual-fault tolerant with that system. Under a normal landing, it’ll be around four or five gs, so it’s really low loads on landing. But we are able to have a single parachute and a single parachute out.”

Inside the capsule, the crew seats are anchored to a “pallet” that sits on struts designed to give if the landing load exceeds 20 gs, Mulholland says. But normally airbags would use a two-stage design to hold the load well below that. “We’ve got an inner airbag that inflates, and then there’s an outer airbag that inflates, and the outer airbags actually have relief valves in them,” he says. “So right on landing, at a certain pressure it blows those relief valves, so it relieves that pressure, which takes a lot of the load out, and then it comes to rest on the inner airbags.”

Mulholland says his company probably will land first in the desert at White Sands, N.M., and has also picked Edwards AFB, Calif., as a landing site. A third site will be selected later. The capsule also is equipped with flotation devices to turn it upright in case of a water landing after a launch abort or mistargeted re-entry, he says.

Sierra Nevada’s Dream Chaser maintains the outer mold line of NASA’s experimental HL-20 lifting body, outfitted with a hybrid-motor pusher escape system to eject it from its Atlas V in a launcher failure. After that, it would fly a piloted return-to-landing site maneuver and land with about a 2g load on the crew.

If necessary, the Dream Chaser could do a trans-Atlantic abort to a runway landing at the international airport in Shannon, Ireland, says former astronaut Jim Voss, vice president of Sierra Nevada’s Space Exploration Group. “Everything that we’ve done right now shows that we can do nominal, we can do aborts, and we can get back to a runway landing, but we don’t think that’s good enough, so we have a bailout capability.”