Protected by the shell of its huge launch-rocket during blastoff, NASA’s Orion Multi-Purpose Crew Vehicle (MPCV) must get back to earth on its own at mission’s end.
The flight plan for this next-generation craft includes a dramatic ocean splashdown reminiscent of the Apollo program that predated the Space Shuttle’s smooth runway landings.
To keep capsule and crew safe under the huge reentry and splashdown loads — temperatures exceeding 4800° F and speeds up to 25,000 miles per hour — a 16.4-foot diameter ablative thermal protection system is secured to the MPCV’s base with a carbon graphite and titanium carrier structure.
As the ablative thermal protection system of this heat shield reaches extremely high temperatures, portions of it fall away from the vehicle to remove excessive thermal energy.
The remaining carrier structure has to survive the brunt of the impact when it hits the water to help keep the astronaut module intact.
With the first unmanned launch-and-return test of Orion scheduled for the fall of 2014, NASA engineers and contractors were highly motivated to get to a final design for the MPCV that achieved ideal weight and performance targets.
In late summer of 2012, NASA’s chief engineer for the Orion project, Julie Kramer, contacted the space agency’s Engineering and Safety Center (NESC) and requested some novel ideas for how to reduce the spacecraft’s mass.
NESC’s mission is to perform value-added independent testing, analysis, and assessments of NASA’s high-risk projects to ensure safety and mission success.
In the zero-failure tolerance environment of spaceflight, a second look from NESC boosts confidence that results are optimum and the final concept has widespread team support.
How to Take a Load Off
At some 3,000 pounds, the “baseline” composite-and-titanium design for the wagon-wheel shaped carrier structure that supports the MPCV’s thermal protection system (See image 3, left-side bottom figure ) was one of the largest components of the crew module, and thus became a prime target for the weight-reduction exercise.
“The goal of the Orion program is to go well beyond Earth orbit, around the moon and eventually to an asteroid or Mars,” says Mike Kirsch, project manager and principal engineer of the NESC’s Orion Heat Shield Carrier Structure Assessment Team. “Mass obviously becomes of paramount importance on such long trips.
“Wouldn’t you rather carry extra water, food, oxygen and propellant? By being efficient in our structural design, we can bring along more of those consumables. And when it comes to splashdown, a lighter vehicle comes in with less energy upon impact.
Kirsch’s team, which included technical lead Jim Jeans, president of Structural Design & Analysis, Inc. (a longtime contractor for NASA as well as private companies) knew what engineering software they’d apply to the heat shield design assessment program: Collier Research Corporation’s HyperSizer.
They’d used the tool extensively on the Composite Crew Module (CCM), an earlier NESC project designed to demonstrate that composites could indeed be used as a primary structure of the crew module.
Jeans is currently employing HyperSizer on other aspects of the Orion vehicle, and on the James Webb Space Telescope.
The first-ever software commercialized out of NASA, HyperSizer provides stress analysis and sizing optimization, reducing the weight of aircraft, wind turbine blades and other structures in addition to space vehicles.
Whether designed with composite or metallic materials, a typical HyperSizer optimization produces weight savings of between 25 and 40 percent.
“There is no way we could have optimized the heat shield alternatives we came up without HyperSizer,” says Jeans. “The sheer capability of the product enabled us to get through complex analyses of huge models in record time.”
Alternate Structural Concepts Developed with HyperSizer
The baseline design for the heat shield consisted of a solid laminate carbon-graphite skin secured to the capsule by a carrier structure with a spoke-like pattern of titanium I-beams in the aforementioned wagon-wheel shape.
The concept is similar to the aeroshell that protected the Rover for Mars entry. Carbon graphite designs are tailorable, in that modifications can continue to be made en route to final manufacturing.
However, in this case, the result was a design that weighed more than it needed to. With an initial goal of cutting out 800 lb., the NESC team considered both material and structural modifications to the baseline.
Splashdown is the toughest challenge
“We needed to come up with a lighter structure that could still withstand the aerodynamic pressure of the Earth’s atmosphere re-entry and support the thermal protection system so the ablative material in the heat shield could do its job,” says Kirsch. “Reentry is a pretty severe load case. But even more important is when the crew module actually hits the water. That water landing is the event that drives the design of the heat shield carrier structure. Using parachutes we try to take as much energy out of it before that impact, which is a tricky, dynamic situation based on wind and wave conditions. Ideally you want the capsule to knife in, not belly-flop. The design must be robust to the wide range of possible wind and wave conditions.”
Image 4. As the crew module impacts the water a wave of high stress moves over the heat shield. The landing simulation is first performed in LS-DYNA, then HyperSizer imports the internal loads at each dynamic millisecond time step.
The team developed a series of analytical models to predict how the heat shield carrier structure as a whole — particularly the internal support webs — would react under a wide range of splashdown scenarios (see Image 4).
Landing simulations were run in LS-DYNA transient non-linear Finite Element Analysis (FEA) solver.
The dynamic landing simulations were loaded into HyperSizer, which then controlled relevant parameters (such as material thickness and location of stiffeners) within each model to optimize, and then compare, different design solutions.
Changing the Game with HyperSizer
“Because it can concurrently evaluate so many different combinations of the variables that influence design, HyperSizer very rapidly identified those configurations that had the lowest mass,” says Kirsch. “We could look at different solutions, materials, layouts—in this case orthogrid patterns—height, density. It’s a very effective way to down-select to the most efficient solution quickly.”
Sharing interim results among designers and analysts was enabled by the software’s ability to display summary images showing critical load case, margins of safety or failure modes.
“This is so powerful from a presentation standpoint, as it enabled the designers and the experts to readily visualize together what was going on,” says Jeans. “And a game changer for me is the ability to do trades between different construction methods, with apples-to-apples comparisons. We could investigate many different configurations and be confident that we were making the right choices.”
“We were a brand new team with very little exposure to this software design environment,” says Kirsch. “But HyperSizer enabled us to rapidly study about 40 different variations — looking at steel, aluminum, stainless, titanium, carbon graphite, honeycomb systems, T-stiffened, I-stiffened and so on. Within ten weeks we’d identified a half dozen candidates with minimum mass configurations that significantly exceeded our original goal of an 800-pound reduction.”
James Ainsworth, a structural engineer for Collier Research, was the HyperSizer technical advisor to the NESC group. “This was a complex problem that we had not previously analyzed in HyperSizer,” he says. “The landing simulations evaluated are similar to a car crash where a vehicle is slamming into something at a high velocity and the entire event takes place over a few milliseconds. Our software allowed the team to evaluate the stresses and strains at every single time step, and use that data for detailed sizing and final analysis in HyperSizer. That was one of the biggest technical challenges we achieved on this project, to set up a process that could import the loads from every time step and size the model to find the optimum lightweight, robust structure.”
“To put it in perspective,” says James, “each one of those landing events was about 12 gigabytes of data and we crunched through some 50 landings.”
New Nonlinear Analysis Capabilities in V7
The kind of transient, dynamic analysis that the NESC performed on heat shield candidates is a new feature in the latest release of HyperSizer, Version 7.
“We included new analytical methods to account for the load redistribution that happens when you have nonlinear material and geometric responses, such as material plasticity and plastic bending. HyperSizer also handles the dynamic landing events by processing thousands of time steps for each landing simulation,” says Ainsworth. “A benefit of working closely with a customer like NASA is that we could readily accommodate their analysis needs by enhancing our software in real-time to keep pace. As a result, in addition to the full range of finite element analysis (FEA) solvers, we can now support, and automatically optimize, a structural design based on LS-DYNA technology [which is widely used in industry for this kind of dynamic analysis].”
Their in-depth evaluations of a variety of engineering concepts led the NESC team to consider alternative designs that incorporated load sharing with the crew module backbone, replaced the exiting wagon-wheel stringer with an H-beam configuration, or switched the composite carbon graphite skin to a titanium orthogrid skin. The titanium orthogrid version (see Image 5) emerged as their final proposal.
NESC built a test structure to verify that they understood the physics of the dynamic impact on this final alternative. The real-world tests (see Image 6) provided sufficient data to validate the ability of HyperSizer to model those physics with its enhanced analytical tools.
As NASA’s Orion team continued their own work on reducing the weight of the baseline design, NESC’s insights from the HyperSizer analyses informed discussions that led to a reduction of the final weight of the baseline design by 23 percent, eliminating hundreds of pounds of unnecessary weight.
“Our work provided some independent perspective on where the baseline design was in the spectrum of alternatives, and highlighted additional efficiencies that were possible,” says Kirsch.
The lighter, stronger MPCV that resulted from the NASA/NESC partnership is scheduled for launch in 2016 with the redesigned version of the heat shield. (The first un-crewed mission, Exploration Flight Test- 1, is already planned for later this year using the original version.)
Optimization from Early Concepts to Final, Certified Design
As the Orion team finalized their own design for the crew module, the NESC and Collier Research continued to work on maturing the alternate carrier structure they developed for NASA.
“We developed a production-ready, light-weight design that has been optimized to withstand the harsh thermal environment of reentry and the violent water landing,” says Ainsworth. “Our success on this project proves that HyperSizer software goes beyond just a weight-saving optimization tool. We have shown that it can be used throughout the design process, from the early stages of preliminary design optimization to the writing of the final margins of safety.”
In other words, from lift off to splashdown.