“The rocket engine nozzles are going into a flow field that's supersonic, so you're going to set up shock fields, pressures behind the shock that the engine has to start against,” says Brown. “Those don't look like they're going to be insurmountable, but it's going to be a highly dynamic event.”
Charles Campbell, an expert in computational fluid dynamics at Johnson Space Center, is developing a sounding-rocket flight test for NASA's Space Technology Mission Directorate (STMD) to gauge just how difficult that ignition will be. At Mars multiple engines will be required, says Brown, and the flow fields of supersonic retropropulsion are likely to require some thermal protection on the body of the spacecraft, which will add more mass. There is also the question of using the system to land precisely and, in the case of the crew-habitat vehicle, ideally within walking distance of the pre-positioned cargo carrier.
The Curiosity EDL system achieved unprecedented precision in landing by jettisoning ballast as it entered the atmosphere to create enough lift to “fly” the entry vehicle toward its target, and then dropping more ballast to stabilize itself under the parachute. That technique got it down in an ellipse measuring 20 X 7 km (12 X 4 mi.), and it used all of the atmosphere to achieve it.
For human-sized landers, says Brown, “the most efficient trajectory is one that waits until almost the last minute, fires a very high thrust, and then you touch down. But you . . . have very little ability to throttle the engines to provide precision landing. And we want to start working the precision landing problem as soon as we enter the atmosphere.”
Engineers have some tricks up their sleeves as they work the precision-landing problem for human landings, according to Jim Masciarelli, a guidance, navigation and control expert at Ball Aerospace. Most of the generic hardware and software necessary for the needed level of landing accuracy is in the works and “almost ready to go,” he says. Hazard avoidance with flash lidar, radar and other sensors probably will be needed for the final few hundred meters of descent, which presents a challenge as well. Under current estimates there will only be 90 sec. from entry to landing, which will make the 7 min. of terror look like a data-processing luxury.
“You have gobs and gobs of data from these sensors to process,” Masciarelli says. “You probably have redundant sensors for reliability in case something fails, so how you process all that data is probably the biggest challenge, and get it in a package that is radiation tolerant, that can survive the trip to Mars, and is a small, lightweight package as well.”
Work on hazard avoidance under NASA's Science Mission Directorate has been halted because of budget constraints, says Doug McCuistion, until recently the head of NASA's Mars Exploration Program. Other EDL technology for human landings on Mars is just getting underway, says Mike Gazarik, the STMD associate administrator.
Campbell's supersonic retropropulsion concept will be briefed at agency headquarters this week, he says. Last year the agency ran a subscale inflatable-decelerator flight test called the Inflatable Reentry Vehicle Experiment (IRVE-3) on a sounding rocket from Wallops Flight Facility in Virginia (AW&ST July 30, 2012, p. 16).
“We're pushing on all the tools here from an entry, descent and landing viewpoint for that future mission,” Gazarik says.