NASA’s had some trials and triumphs lately developing its next generation spaceflight system. In the last month, an Orion spacecraft cracked during pressure testing and a Space Launch System (SLS) rocket flew supersonically—at least, a model managed a simulated supersonic flight in a wind tunnel. Overall, it’s progress all around.
First, Orion. Among the battery of prelaunch tests needed to verify a spacecraft as flight-worthy is a pressure test. Like vehicles before it, the Orion capsule is pressurized beyond sea level to mimic the pressure differential between the inside of a spacecraft and the vacuum of space. During the week of November 5, NASA pushed its first flight-model beyond its designed limits, a regular part of pressure testing. Orion was pressurized to 1.5 times its expected flight levels, about 25 pounds per square inch (psi). But the capsule’s bulkhead gave way when the interior reached just 21.6 psi.
Test engineers found three cracks in three adjacent radial ribs on the capsule’s aft bulkhead. But it is a flaw that can be repaired; the whole bulkhead does not need to be entirely re-engineered from scratch to make the Orion spacecraft flight-worthy. Figuring out why the bulkhead failed is a another matter altogether. Determining the fault will require engineers to cut open the cracks and review them using an electron microscope. This will reveal the type of crack, whether it was caused due to stress or fatigue, and help engineers properly repair the problem. Preliminary speculation suggests that one of the beams affixed to the bulkhead may have been stiffer than the others, thus making them more susceptible to the test’s stresses.
It’s not clear yet whether repairing this problem will push back Orion’s debut flight. Currently scheduled for late 2014, Exploration Flight Test 1 (EFT-1) is a proof-of-hardware mission that will test the vehicle’s vital systems, like its heat shield and avionics. The unmanned Orion will orbit to Earth before reentering the atmosphere at almost 20,000 miles per hour, which is roughly 80 percent of the lunar return velocity. This flight will likely be the only time Orion flies on a Delta IV Heavy launch vehicle. After this mission, the spacecraft will move to bigger things, namely the SLS.
Though there’s no flight hardware to show for SLS just yet, the rocket is making steady progress. Managed by NASA’s Marshall Spaceflight Centre in Huntsville, Ala., it was members of the Aeroelasticity Branch at the agency’s Langley Research Center in Hampton, Va., that oversaw the latest round of tests.
Last week, engineers put a ten-foot-long buffet model of the SLS in Langley’s Transonic Dynamics Tunnel (TDT). Langley research engineer Dave Piatak called the test one of the more critical milestones in the vehicle’s development. Buffet testing helps engineers design a rocket that will give astronauts and touchy payloads a smooth ride from the launch pad to orbit.
Buffet testing sees wind tunnel models put through their paces at transonic and low supersonic speeds. This is the area roughly between Mach 0.8 and Mach 1.2, where shock waves bounce around and rattle a vehicle. Measuring the aerodynamics at these speeds is essential to understanding the structural interaction to the flow field around the vehicle and determining loads. The data then helps engineers determine the rocket’s structural margins and, of course, helps ensure the vehicle is safe before it’s loaded and launched.
These tests at Langley returned a substantial amount of data. A total of 360 miniature unsteady pressure transducers on the model’s surface are like tiny microphones scanning the model thirteen thousand times per second. This data shows engineers the unsteady flow that exposes the vehicle to rapidly changing forces during its flight through the atmosphere.
Of course, this model isn’t quite like the real thing. Unlike the rigid SLS buffet wind-tunnel model, the real launch vehicle is quite flexible and will react differently to transonic buffeting. The rocket will bend and shake in response to forces during flight, and engineers use tests like this to make sure the bending loads and vibrations during a launch are within the rocket’s safe limits. Buffet forces measured during wind-tunnel testing are applied to a computational structural model of the launch vehicle to determine what kind of bending forces and accelerations the vehicle can handle. Once the data is retrieved and analyzed, NASA’s engineers can refine SLS’s design before the full-size rocket is built for flight tests.
SLS’s first mission, and the first time we’ll see an Orion/SLS combination launch, is currently scheduled for sometime in 2017. Exploration Mission-1 (EM-1) will send an unmanned Orion spacecraft to lunar orbit to check out the vehicle’s systems, heralding a new era of spaceflight.