Heavy-duty heat shield prepares for launch to Mars
BY STEPHEN CLARK
Posted: May 18, 2011
A massive carbon-covered heat shield, one of the largest ever built for space travel, is now at the Kennedy Space Center being readied for launch on NASA's next mission to Mars.
The centerpiece of the mission, a six-wheeled rover named Curiosity, will drive around Mars for at least two years studying the planet's archive of organic compounds, climate and geology to determine whether it was ever capable of supporting microbial life.
The mission's 15-foot-wide heat shield and aerodynamic shell touched down at KSC's space shuttle runway inside a U.S. Air Force C-17 cargo plane May 12, then workers trucked the hardware to a nearby clean room overnight, according to George Diller, a NASA spokesperson.
Manufactured by Lockheed Martin Corp. in Denver, it's the largest heat shield ever designed for flight through the Martian atmosphere. It's also wider than the blunt side of the Apollo command module, making the MSL heat shield the largest to ever fly on a capsule-shaped craft in space.
"It's almost 15 feet across," said Bill Willcockson, a Lockheed Martin entry systems engineer. "That's enormous."
Only winged spaceships like the space shuttle, the Soviet Buran, and the Air Force's X-37B space plane have used a larger thermal protection system. Lockheed Martin's Orion capsule, the prototype for NASA's next government-owned exploration spacecraft, will fly a wider heat shield to protect against scorching temperatures returning into Earth's atmosphere.
Covered with a one-inch coating of special carbon ablator, the aeroshell will take the brunt of the loads as the rover descends to the Martian surface. Temperatures will soar to 3,300 degrees Fahrenheit as friction builds on the saucer-shaped craft, according to Lockheed Martin engineers.
The wind-facing heat shield will absorb most of the blazing temperatures, leaving the underlying aluminum honeycomb and graphite epoxy structure at a relatively comfortable 400 degrees Fahrenheit.
"The heating rate is pretty high," Willcockson said. "That's actually not that far off from what Apollo saw when it was coming back from the moon, as far as the heat flux. That's because this vehicle is very heavy and it has what we call a high ballistic coefficient."
The ballistic coefficient, a function of mass, diameter and drag, is a measure of how well an object overcomes air resistance in flight. The Mars entry vehicle's high ballistic coefficient number means it loses velocity slowly when encountering the planet's thin atmosphere.
But the Curiosity rover must touch down on Mars gently, so the mission uses a three-step entry, descent and landing plan with a heat shield, parachutes and a rocket-powered "sky crane" system to place the rover wheels-down on the surface.
The mission's biscuit-colored heat shield will discharge most of the probe's approach velocity, slowing the craft to about 1,000 mph for the deployment of a supersonic parachute.
Not only is MSL's entry vehicle larger than any earlier Mars lander, the capsule will steer the rover toward its landing site for a precision touchdown. The rover's landing ellipse is less than 15 miles wide, smaller than the margin of error for previous missions.
"In order to support the science of this mission, we wanted to be able to go to smaller places which were still totally safe," said Pete Theisinger, the MSL program manager at NASA's Jet Propulsion Laboratory. "That drives us to smaller (landing) ellipses. The way to do that is with guided entry."
The shape of the aeroshell's curve is asymmetric, giving the craft the ability to generate modest lift and glide through the Martian atmosphere. The vehicle will control its trajectory through a series of roll maneuvers.
By comparison, the Spirit and Opportunity rovers fell to Mars seven years ago on a ballistic trajectory, meaning there was no way to control the flight path after reaching the upper layers of the atmosphere.
"It's a challenging entry," Willcockson said. "It's a lot more challenging environment than what Spirit and Opportunity were designed for, for example. That's why we went with the PICA."
The extra rigor of MSL's atmospheric entry means the heat shield will be subjected to more heat, stress and a turbulent flow of plasma across its flush surface.
It turns out the heat shield material used on preceding Mars missions wasn't tough enough for the job required by MSL. Called Super Lightweight Ablator 561V, the material was never subjected to the temperatures and shears the Curiosity rover's entry capsule must survive.
"This environment is much more severe than any other Mars mission has ever been," Willcockson said. "It literally was a struggle to just come up with a test that would at least envelope this condition. The test they came up with was a combination of high shear and high heating. The heritage Mars heat shield material, SLA 561, had never had to undergo that."
The heat shield will be subjected to about 105,000 pounds of compressive force across its surface, causing the structure to deflect with bending and shear loads more severe than other missions experienced, according to Lockheed Martin.
Officials decided to switch to the stronger PICA heat shield after engineers thought they had already settled on a final design.
"It was very late in the game," Hund said. "We had a very high intensity development program that was mostly accomplished within about a year."
Willcockson said it was "later in the program than we would have liked, but we found out it wasn't passing that test. It wound up being a real scramble."
Despite the stringent design requirements for a costly mission like MSL, plus the enormous size of the entry vehicle, Hund said the most challenging part of Lockheed Martin's work on the program was the late switch to PICA.
Lockheed Martin's planetary sciences division flew a PICA heat shield on NASA's Stardust mission that returned cometary dust particles to Earth in 2006. But Stardust's capsule was just 32 inches in diameter, more than five times smaller than the Mars vehicle.
Invented by NASA's Ames Research Center, PICA is only produced in small blocks, so designers drew up a new heat shield using more than 100 tiles of PICA to cover the composite structure underneath.
"There was quite a development effort in a short period of time, and that was very challenging for us," Hund said. "Our heritage was in the Stardust-sized capsule. Getting it to an MSL-sized heat shield with the tiled approach and such was very challenging."
The Mars Science Laboratory's attention-grabbing size and hefty payload necessitated the costliest and most robust spacecraft, heat shield and rover to ever fly to the Red Planet.
It also drives the extraordinarily high temperatures the capsule will encounter when it falls into the Martian atmosphere.
"It's based on two things," Willcockson said. "I think the first driver is the mass. It's double the mass per unit area of Spirit and Opportunity. But also, in order to slow something like this down in the thin Martian atmosphere, we need to use a lot of lift to bleed off the energy."
The MSL Entry Descent and Landing Instrumentation, or MEDLI, will stream information back to Earth as the probe enters the atmosphere.
"Although this is the seventh time we've entered the Martian atmosphere, we really don't have good detailed data on the actual profiles, pressure and temperature profiles, as we fly into the atmosphere," Theisinger said. "They provided us with an instrument, MEDLI, which looks at the entry conditions through the heat shield."
Managed by NASA's Langley Research Center, the MEDLI sensors will measure temperatures, pressures and how much PICA material has burned off and ablated throughout the entry sequence. It's the most instrumentation ever placed on a robotic mission's heat shield.
Another first for the mission's atmospheric entry is the use of roll maneuvers to bleed off the probe's blazing speed and set the stage for deployment of a parachute in the lower atmosphere.
Without the roll reversals, the vehicle would plow into the atmosphere at even higher speeds, subjecting the heat shield to dangerous temperatures and making a precision landing impossible.
The entry capsule will fall into the atmosphere at a carefully planned angle of attack based on the vehicle's center of gravity, which is unbalanced after the probe jettisons a cruise stage on the final approach to Mars.
"It's all built in as to where they place that center of gravity inside the vehicle, which will give it its angle of attack," Willcockson said. "That's passive, if you will. The active control is the rate damping and the roll control, which is doing these bank reversals in the atmosphere to slow it down."
According to Theisinger, the probe's guidance system will initially only adjust the dive angle of the atmospheric entry, correcting errors in the downrange piece of its trajectory. The capsule's computers will also compute sideways steering commands in the later phases of the entry.
Once the spacecraft reaches the lower atmosphere, pyrotechnics will jettison tungsten ballast slugs to re-establish its center of gravity along its axis of symmetry.
"After we get through the hypersonic entry, we're now unbalanced," Theisinger explained. "But we want to be balanced for parachute deploy. We don't want the parachute to come out with an angle of attack. We throw away more mass in order to rebalance the vehicle."
The parachute will slow the craft to subsonic speeds, then a rocket-powered descent stage will decelerate the landing package to nearly a hover. A state-of-the-art "sky crane" will lower the nearly 2,000-pound Curiosity rover on a bridle to the surface directly on its wheels.
All three components will be unpacked and prepared for the Curiosity rover's arrival at KSC some time in late June. Liftoff is scheduled for Nov. 25 on an Atlas 5 rocket.
"From a hardware and equipment standpoint, we're in very good shape. The test program is going very well," Theisinger told Spaceflight Now. "I think there's no reason to believe we won't be ready the first day of the launch period."
Theisinger said the project is in good shape with funding through launch.
"We are OK with respect to funding that NASA has committed to us for the pre-launch phase," Theisinger said. "We have some studies that NASA has asked us to do with respect to post-launch operations budgets."
Engineers plan to mate the heat shield and backshell for precise mass and balance measurements in the next few weeks, then take the components apart again to prepare for the arrival of the Curiosity rover and descent stage in late June.
Once Curiosity ships to Florida, technicians will unpack the rover and make sure it weathered the cross-country trip. Then engineers will start final systems testing to simulate the launch, cruise, descent and driving phases of the mission.
Later in the summer, NASA will start assembling the medley of parts into their launch configuration. The rover and descent stage will be loaded into the aeroshell and backshell, then the solar-powered cruise stage will be connected to keep the spacecraft alive on the journey from Earth to Mars.
"This is kind of like a Russian doll," Theisinger said, noting the spacecraft will handed over for launch operations by late October, when it will be bolted to a rocket adapter ring and encapsulated inside the Atlas booster's payload fairing.
Once the probe is at the launch pad, engineers will attach the rover's nuclear power generator about one week before blastoff.
Liftoff is possible between Nov. 25 and Dec. 18 when the alignment between Earth and Mars is positioned for a trip between the two planets. The mission's start is more than two years late due to rising costs exacerbated by a heap of technical problems.
When it was clear MSL would not meet its original October 2009 launch date, NASA management ordered a two-year delay to the next flight opportunity this year.
"This is a very exciting mission," Theisinger said. "We've been in the throes of putting it together. We've talked about the engineering challenges and the programmatic challenges of doing that. But we're now getting to the payoff here. It is an extremely exciting mission with a lot of really good science capability."