In this thermal imagery captured shortly after stage separation, the top of the SpaceX Falcon 9 first stage appears as a dim dot with a fading plume within the brighter upper-stage plume. In the inset, the restarted first-stage engines power the first stage as it performs a propulsive descent to Earth.
NASA/Scifli Team/Applied Physics Laboratory Images
An innovative partnership between NASA and SpaceX is giving the U.S. space agency an early look at what it would take to land multi-ton habitats and supply caches on Mars for human explorers, while providing sophisticated infrared (IR) imagery to help the spacecraft company develop a reusable launch vehicle.
After multiple attempts, airborne NASA and U.S. Navy IR tracking cameras have captured a SpaceX Falcon 9 in flight as its first stage falls back toward Earth shortly after second-stage ignition and then reignites to lower the stage toward a propulsive “zero-velocity, zero-altitude” touchdown on the sea surface (see images).
Engineers at NASA and SpaceX are now correlating that data with company telemetry from the Sept. 21 Falcon 9 launch of a Dragon cargo carrier to the International Space Station to learn exactly what the vehicle was doing in terms of engine-firing and maneuvering when it generated the signatures collected by the aircraft.
The deal is a “win-win” for both parties, who obtain valuable data that would otherwise be unavailable to them, says Robert Braun of the Georgia Institute of Technology, principal investigator on the Propulsive Descent Technologies (PDT) Project. For SpaceX, NASA is providing detailed information on temperatures and aerodynamic loading on the Falcon 9 vehicle as it rides an exhaust plume of hot gas toward its launch site. And NASA engineers get a chance to collect data on supersonic retropropulsion that may one day lower payloads the size of two-story buildings to the surface of Mars.
“This is the kind of thing that NASA couldn’t have done five years ago,” says Braun, who was chief technologist for the agency in 2010-11.
He learned that the hard way. After returning to Georgia Tech, Braun—a specialist in entry, descent and landing (EDL)—worked with engineers from the university and various NASA centers to develop a proposal for a $50 million sounding-rocket program to flight-test supersonic retropropulsion (AW&ST May 20, 2013, p. 30).
NASA’s Space Technology Mission Directorate (STMD) rejected the plan “because of its cost,” Braun says. But the agency still needs a way to land payloads weighing more than 20 tons to support a human expedition to Mars, leading Braun and his colleagues to find common cause with SpaceX.
“If you look at the requirements for returning a first stage here on the Earth propulsively, and then you look at the requirements for landing heavy payloads on Mars, there’s a region where the two overlap—are right on top of each other,” Braun says. “If you start with a launch vehicle, and you want to bring it down in a controlled manner, you’re going to end up operating that propulsion system in the supersonic regime at the right altitudes to give you Mars-relevant conditions.”
Basically, the PDT Project struck a deal with SpaceX to use airborne -infrared-imaging techniques developed to study the space shuttle in flight after the Columbia accident as a data-gathering technique for the supersonic retro-propulsion SpaceX will need for its reusable launch vehicle development. Collecting the data is easier said than done, according to Tom Horvath of NASA’s Langley Research Center, the PDT imagery lead.
After unsuccessful attempts to image the rocket on the third SpaceX Commercial Resupply Services (CRS-3) mission that ultimately flew on April 18, and the July 14 launch following delays of six Orbcomm low-Earth-orbit communications satellites, the project struck pay dirt with the CRS-4 flight last month. Launched at night, the flight was captured by mid-wave IR sensors on NASA’s WB-57 aircraft based at Ellington Field in Houston, and on a Navy NP-3D Orion operating from Jacksonville, Florida.
“The sensors are actually following a pre-determined trajectory,” says Horvath of the passive tracking conducted by both aircraft. “It really boiled down to our ability to accurately point these systems and have them looking at a particular point in the sky at a very particular instant.”
Working off GPS position data for the aircraft and preloaded trajectories from SpaceX, the WB-57 at 50,000 ft. and the P-3 at 27,000 ft. were able to follow the Falcon 9 from the time it emerged from the clouds that had threatened the launch until its upper stage separated and the single Merlin engine on that stage ignited.
The cameras then followed the Falcon 9 first stage as it coasted to its apogee while the upper stage powered its way up toward orbit, picking up the IR signatures of the attitude-control jets positioning the stage with its engines facing away from the coast. The “boost-back burn” of three Merlins to move the stage closer to the coast registered clearly, followed by more attitude thrusters as they turned the engines into the direction the stage was traveling. Finally, the cameras caught reignition of three Merlin engines for the supersonic retropropulsion portion of the flight.
That was the main event for the NASA-backed team, which observed not only the changing temperatures from the maneuvering vehicle to spot any instabilities in the propulsion system, but also the effect of the engine-firings on the vehicle loading from the flow field surrounding the stage, as well as how the changing flow field affects vehicle drag. That aerodynamic data will help future developers design a Mars-landing trajectory for heavy payloads, Braun says.
“On the aero side, what you have to realize is of course you’re trying to decelerate, and drag is how we decelerate,” he says. “When the vehicle’s flow field changes, with the propulsion going from off to on, . . . you’re blowing off, or you’re losing most of your aerodynamic drag. That’s OK, because if you have enough propellant, you just do your descent propulsively. But if you’d like to manage how much propellant you have to bring with you all the way to the Mars surface, you’d like to minimize that mass . . . and so understanding how much drag will still be present when the propulsion system is on is also an important effect.”
For NASA, the period of the flight most relevant for future operations over Mars came when the first stage was traveling at about Mach 2, 100,000-150,000 ft. above the surface. The two midwave IR sensors—mounted in a nose pod on the WB-57 and internally on the P-3—were about 60 nm from the rocket when it reignited its engines for supersonic retropropulsion. That produced raw images in which the stage appeared 1 pixel wide and 10 pixels long, but subsequent enhancing by specialists at the Johns Hopkins University Applied Physics Laboratory improved the resolution dramatically.
The final, single-engine touchdown was out of the cameras’ fields of view because of clouds, but the project plans to image at least one more Falcon 9 launch and may be able to capture the entire first-stage descent trajectory, if weather permits. Charles Campbell, an expert in computational fluid dynamics at Johnson Space Center and NASA’s project manager on the work with SpaceX, says the agency is spending about $10 million on the effort, which produces far better data for much less funding than the once-proposed sounding-rocket flight test. That is in keeping with a push at NASA to stretch its funding with outside partnerships (AW&ST Sept. 1, p. 18).
“Through our partnership with SpaceX, we’re gaining access to extraordinary real-world test data about advanced rocket-stage design and retropropulsion,” says Michael Gazarik, associate administrator for space technology. “By working with SpaceX and imaging their great technology, we’re saving the taxpayer millions of dollars we’d otherwise have to spend to develop test rockets and flights in-house.”
SpaceX founder Elon Musk has based his “disruptive” business approach in part on a stated goal of colonizing the red planet (AW&ST Aug. 15, p. 24). The NASA technology work is right in line with those plans.
“SpaceX was excited to support NASA’s efforts to capture infrared imagery of the Falcon 9 first-stage reentry maneuvers following the CRS-4 flight,” the company states. “In addition to informing our first-stage recovery efforts, the data captured on the stage’s supersonic retropropulsion may provide key insights toward understanding the propulsive descent technologies necessary to one day land people on Mars.”
Quelle: Aviation Week