KENNEDY SPACE CENTER, Fla.--A preliminary look at data from NASA's Ares I-X test flight Wednesday shows the towering rocket performed as well or better than computer modeling predicted during the climb out of the dense lower atmosphere, a senior NASA manager said Friday.
One of three huge parachutes failed to inflate during the spent booster's descent to the Atlantic Ocean and a second chute only inflated halfway, resulting in a hard splash down that caused the rocket's case to buckle.
But Mission Manager Bob Ess said the parachute system, flying for the first time, was designed for NASA's planned Ares 1 rocket, which is 15 percent lighter than the test version, and that engineers will have plenty of time to correct whatever went wrong.
"No one is concerned about it," Ess said. "In fact, the parachute guys were ecstatic, was their words, (about) the information they got from this flight. They really wanted to test this out."
The Ares I-X rocket was designed to match the characteristics of NASA's planned shuttle replacement, the more powerful Ares I. The test version featured a four-segment shuttle booster, a dummy fifth segment housing guidance and control equipment and an unpowered mockup of the rocket's upper stage and crew capsule.
The 327-foot-tall test rocket was launched Wednesday from shuttle complex 39B at the Kennedy Space Center. The major goals of the unmanned six-minute flight were to collect engineering data on how the tall, slender rocket flew through the lower atmosphere, how the structure responded to aerodynamic and acoustic forces and how the new parachute system, scaled for the planned Ares I, performed.
During the initial seconds of flight, the rocket's nozzle moved 1 degree as planned to help the booster "walk off" the pad, preventing its hot exhaust plume from hitting the upper sections of the shuttle service gantry. As expected, the plume caused minor damage to the lower sections of the gantry, but Ess said that would not be a problem for the new service tower that will be used for Ares rockets.
Six seconds after liftoff, the vehicle rose above the gantry and the rocket's roll control system fired to rotate the booster about its vertical axis, ensuring that antennas relaying telemetry would remain in line of sight with ground stations throughout the flight.
The 5-degree-per-second roll maneuver went off with out a hitch and Ess said the rocket remained "very, very stable" after that. The booster experienced a maximum aerodynamic pressure of about 900 pounds per square foot just after it accelerated through the sound barrier.
"The booster guys have been looking at all the data," Ess said. "Everything looked great. This was the oldest booster we've ever flown, it looked just like a shuttle flight. All the parameters were right in the middle, there were no dispersions, all the pressures, the oscillations were what we typically see for a shuttle flight."
One major concern early in the development of the Ares system was a phenomenon known as thrust oscillation, which can cause unwanted vibrations toward the end of powered flight as the booster exhausts its load of solid propellant. Data from a few test firings early in the shuttle program indicated potentially excessive vibrations, but data from recent shuttle flights and ground firings, including one using a five-segment Ares I booster, show only minor disturbances.
Based on a quick look at the Ares I-X data, "the oscillations look very small, similar to what we see for shuttle data or that one test firing we did," Ess said. "So we didn't see anything unusual. ... It was very benign, there was very little there. Again, we'll get the data back from the recorder, which is high-speed data, and we'll verify that. But at this point, there's nothing to indicate thrust oscillation was even a factor."
The first stage lofted Ares I-X to an altitude of about 24 miles and a velocity of 4.6 times the speed of sound in two minutes of powered flight. Explosive charges then fired, separating the spent first stage from the dummy second stage. An instant later, small upward-facing rockets fired to pull the first stage away.
"The rates of the vehicle at separation were supposed to be within 2 degrees per second in pitch and yaw and 3 degrees per second in roll," Ess said. "And indications we had are those were all within less than half a degree per second. So the flight control (system) kept the vehicle very, very stable, it wasn't rotating at all during the separation, so it was very solid."
In what appeared to be a surprise at the time, the upper stage went into a slow, flat spin instead of continuing upward on a nose-forward trajectory. Ess said Friday engineers, in fact, expected that behavior and had seen it in scores of computer simulations.
"Two days ago at the (post-flight) press conference I used the phrase "a little different," Ess said, describing the upper stage tumble. "We went back and looked at all the (computer simulations) we ran and we found thousands of them that matched what we saw. So my comments were incorrect when I said 'a little different.'"
The upper stage was loaded with 30,000 pounds of ballast near the bottom to simulate a full load of liquid oxygen rocket propellant and another 30,000 pounds higher up to simulate liquid hydrogen fuel.
"So the center of gravity is very far aft in this thing once it's by itself and the center of pressure is more towards the middle, so it's inherently unstable," Ess said. "With about 90 or so pounds per square foot of dynamic pressure (at that altitude) and an unstable vehicle, it's no wonder the simulations showed just what we saw, that when you separated there's nothing to control it.
"As a reminder, for Ares I there's an attitude control system on it. So as soon as you separate, there are attitude control motors that will keep the upper stage where it needs to be and then the J2 engine will kick off as well and we'll have active control. So that is something that's very, very different between I-X and Ares I."
After separation, small rockets at the base of the first stage fired to put the booster in a flat spin of its own to prevent a nose-down descent that might interfere with parachute deployment.
Ess said a pilot chute deployed as planned, pulling out a larger drogue parachute to slow and stabilize the rocket. The three main parachutes then were pulled out, each one initially inflating to about 50 percent of their full 150-foot width as planned to ease the shock on the system.
"The parachutes came out in the reefed position and they all inflated, all three of them," Ess said. "So that was good. Soon thereafter, one of the parachutes failed. It's tough to tell exactly from the video, we're hoping to look at the parachutes sometime over the weekend, or Monday or Tuesday. It appears the suspension lines themselves may have failed as opposed to the parachute material.
"So we had two parachutes then in the reefed position, one was just kind of trailing, it wasn't open. Then as we went and disreefed to the fully open position, a second chute appeared to have some damage. It was still pretty much intact, but part of it, it seemed like the riser lines or suspension lines seemed to get fouled or cut, and so part of the parachute was not inflated.
"So we had one good one, one completely failed one and then one was probably about halfway (inflated)," Ess said. "So that caused the booster to hit the water at a higher speed than expected, it was a pretty hard impact. We think because this one parachute was partially inflated, we got a little more horizontal velocity than nominal. ... What that caused was a hard impact and the vehicle slapped down pretty hard in the water. So that caused some damage on the booster."
The impact apparently damaged the nozzle steering actuators and caused one of the booster's fuel segments to buckle.
Ess said the damage was of no immediate consequence because NASA had no plans to reuse any of the hardware. As for the test, he said the parachutes were subjected to a heavier load than what is expected for the actual Ares I.
"I-X empty is about 15 percent heavier than Ares I empty," he said. "That's because of the fifth segment simulator, our dummy simulator, is full of electronics and avionics. We also added a bunch of ballast in the back end of it to make it more stable. So this was an overtest of the Ares I chutes. Again, there's 15 percent more weight it had to carry than it would for Ares I. And so, therefore, there's a whole lot more load on the chutes."