Ares 1-X test flight an engineer's delight
BY STEPHEN CLARK
Posted: October 26, 2009
Engineers have meticulously tailored Tuesday's Ares 1-X test launch as a learning exercise, using more than 700 high-fidelity sensors to collect gargantuan amounts of data during the booster's six-minute flight.
"We can be very successful in this flight even if some things don't go exactly as planned," said Bob Ess, Ares 1-X mission manager. "It's really hard not to get really good data, no matter what happens."
The Ares 1 critical design review, currently scheduled for the middle of 2011, will be heavily based on what engineers learn from Tuesday's launch.
"What that helps us with is then to make our computer models that we use to design the Ares 1 much more refined and take out the uncertainties that we have," said Jeff Hanley, manager of NASA's Constellation program.
"As we build those models, we keep a lot of conservatism in the models because we're uncertain about exactly what environments we're going to see. So this vehicle lets us interrogate that," Hanley said.
Officials are particularly interested in how the 327-foot-tall rocket performs in the moments after liftoff, as it flies through the period of maximum dynamic pressure, and during the stage separation and recovery sequence.
"Will it really work as planned in one big vehicle, from the guidance, navigation and control, thrust vector control, mass properties, the winds and the weather? That's the part we really want to see if it's all going to work out next Tuesday," Ess said.
The rocket is built as an aerodynamic model of the Ares 1, but only the first stage is active. The solid-fueled four-segment booster, borrowed from the space shuttle program, is topped with a dummy fifth segment that would also be filled with propellant on operational Ares 1 missions.
The upper stage, planned to be fueled by hydrogen on the Ares 1, is loaded with steel plates to simulate the mass of a full-up rocket.
"How do you characterize in a computer, with the uncertanties, what the sound levels are going to be and what the pressure levels are going to be? What's the affect of the upper level winds as it flies through the atmosphere on the loads that the rocket sees, in other words, the bending of the rocket? That's what Ares 1-X is going to inform us most about," Hanley said.
"On a shuttle flight, you have the main engines start at about six seconds. You have full up thrust at three seconds. You have three seconds of steam that's coming out. In this case, you're not going to have any of that," said Ed Mango, Ares 1-X launch director.
"You won't have very much at all until T-zero when it will be just like the SRB ignition we have on shuttle," said Ed Mango, Ares 1-X launch director.
Liftoff will occur less than a quarter-second after the ignition command and four explosive hold-down bolts fire to release the rocket from the mobile launch platform.
"Once we ignite the booster, that's when the game starts," Ess said.
It will take less than a half-second for the powerful motor to build up to 3 million pounds of thrust, equivalent to more than 23 times the power output of the Hoover Dam.
Computer models show the blast from ignition will bounce off the launch tower and amplify the acoustic environment in the first few seconds of the flight. In addition to the rocket's instrumentation, the launch pad and mobile platform are rigged with a network of 49 sensors to measure how the ground systems hold up.
"During liftoff, just like the shuttle, we have this big explosion, for lack of a better word," Ess said. "There's about five or six seconds when we're kind of in the influence of the pad."
Ess said he is confident the rocket can handle the acoustic loads at liftoff, even though they could be more intense than the environment produced by shuttle launches.
"It really comes down to a vibration more than anything," Ess said. "So that vibration is included in our models when we determine structural integrity and we make sure we can handle the liftoff environment."
Unlike the space shuttle, which naturally flies laterally away from the pad during liftoff, Ares 1-X will go almost straight up. Engineers have designed a slight steering maneuver in the moments after liftoff to guide the mammoth vehicle away from the launch pad's tower.
Officials aren't worried about the rocket smashing into the pad, but NASA wants to minimize potential damage to the complex from the plume of hot gas spewing from the motor as it clears the tower.
There will be about 15 feet of clearance between the rocket and pad structure, according to the best predictions, but there is an uncertainty of up to 9 feet.
"Because the drift of Ares 1-X potentially could be much different from shuttle, we actually had to do a lot of beef-up to some of the (pad) structure because if it drifts a certain way we could get some plume impingement and see higher loads," said Bill Stover, deputy ground systems project manager for Ares 1-X.
The roll control system, or RoCS, will be activated six seconds into the flight, just as the rocket flies beyond the top level of the pad's service tower and begins to turn on an easterly trajectory.
The RoCS thrusters, harvested from the military's Peacekeeper missile, can produce up to 2,250 pounds of force in vacuum and will immediately begin to roll the rocket 90 degrees.
The roll program will demonstrate the rocket's controllability and align antennas with ground stations, according to NASA officials. A black barber-pole Z-stripe was added to the first stage's outer shell to allow observers to optically track the roll.
"From a camera standpoint, you can get roll rates from that, you can get pitch rates from that. In a developmental flight test, you want to have as many cues as you can," Mango said.
Four test inputs are programmed into the Ares 1-X computers to gauge the rocket's reaction to manually induced loads.
"By knowing how much input we put to the nozzle and measuring what we get from the vehicle, we can understand a little better about the vehicle itself and how it flies," Ess said.
The first two inputs, occurring less than a minute after liftoff, will move the first stage motor nozzle by about one-tenth of a degree.
The commands "basically move it one way, move it to center, move it the other way and move it back. That sinusoidal input should make the vehicle move a little bit and we'll pick that up on our sensors," Ess said.
A third test maneuver will deflect the nozzle more than one-third of a degree about 75 seconds into the launch. The final input will pulse the nozzle about 1 degree to check the booster's response in the yaw direction.
Ess said the extra nozzle movements will be minimized as the rocket passes through the phase of the flight known as maximum dynamic pressure, or Max Q.
That's because engineers want to measure how the rocket naturally reacts as it traverses the portion of the flight with the most forces acting upon it.
Models predict Ares 1-X will experience maximum loading of about 865 pounds per square foot, appreciably higher than the forces seen during shuttle launches.
"In this high (loads) region with a long vehicle, if we get more bending than we thought we would, then it could affect our thrust vector control and how it affects the flight. It could make things worse instead of better," Ess said. "Or our aerodynamics could be off enough that we get some strange dynamics. All of those are part of the test."
Officials in charge of Ares 1-X don't believe the vibrations will cause any problems during the launch, but engineers are curious to learn how the loads manifest themselves throughout the rocket's upper assembly, including the crew module.
Ares 1-X will fly a different trajectory than the Ares 1 rocket to match the maximum dynamic pressure level the operational launcher will experience. This means the test rocket will separate from the upper stage simulator lower in the atmosphere than the real Ares 1.
"Because we're flying a four-segment booster and we're trying to match the trajectory of a five-segment booster, we had to do a couple of different things. By matching the dynamic pressure profile, pretty soon you run out of oomph and you can't quite go as high as Ares 1 will be with that extra segment," Ess said.
In the final seconds before burnout, pressure sensors will look for signs of a rocket characteristic known as thrust oscillation.
Thrust oscillation is a phenomenon in solid rocket motors that can trigger intense and damaging vibrations. Engineers have instrumented solid rocket boosters during the last few shuttle launches and have not detected any prominent loads caused by thrust oscillation, which is also called resonant burning.
"The probability is we will see nothing of note on this flight," said Joe Oliva, Ares 1-X project manager at ATK, the prime contractor for the rocket's first stage.
"I don't see thrust oscillation being a huge driver to us at this point in the program. It's really what we call vibro-acoustics that drives us," Hanley said.
During staging, pyrotechnics will fire at the base of the frustrum, the conical connecting point between the first and second stages.
Eight deceleration motors will fire simultaneously to separate the first stage from the simulated portion of the rocket. A few seconds later, four more motors will ignite to put the first stage in a yawing tumble similar to what solid rocket boosters experience after being jettisoned during shuttle launches.
"We need that to happen so the parachutes will properly deploy," said Jon Cowart, Ares 1-X deputy mission manager. "If we don't get it spinning enough, there's always a chance they might get fouled on the rocket."
The top part of Ares 1-X, comprising the inactive components of the launch, will continue on an uncontrolled trajectory and crash into the Atlantic Ocean. It will not be recovered.
Data from instruments on the forward assembly will be stored in a recorder located inside the first stage, which will be plucked from the ocean. The information will also be beamed live back to tracking stations near Cape Canaveral and in Jupiter, Fla.
The first stage will coast to an altitude of 153,000 feet before beginning its parachuted descent to splashdown less than 150 miles due east of Cape Canaveral.
Ares 1-X will demonstrate the three main parachutes to be employed by the reusable Ares 1 first stage. Covering more than two acres, the 150-foot-wide chutes are made of a kevlar material that is stronger and lighter than the nylon parachutes currently used.
High speed cameras inside the booster will record the parachute deployment. A small Cessna Skymaster airplane near the recovery zone will also observe the splashdown sequence.
Both of NASA's booster recovery ships, the Liberty Star and Freedom Star, will be on station near the expected landing site to retrieve the rocket and tow it back to port.
"It lets us get real test data on the first stage recovery system. It lets us test the guidance system. We have the real Ares 1 guidance algorithms in the flight computer that will be guiding the rocket. So that is some of the engineering value that we'll get (from Ares 1-X)," Hanley said.
Officials say they expect to learn many lessons from Tuesday's launch, especially if things don't go exactly as planned.
"Any human being around here knows you learn a lot more from things that don't go right than you do from things that do," Cowart said.
"The only failure on this flight is the failure for us to learn from it," Ess said. "And no matter what happens, we will learn a lot from it."