Protected by the shell of its launch-rocket during blastoff, NASA’s Orion Multi-Purpose Crew Vehicle (MPCV) must get back to earth on its own at mission’s end. To keep capsule and crew safe under the huge reentry and splashdown loads—temperatures exceeding 4,800° F and speeds up to 25,000 mph—a 16.4-ft. diameter ablative thermal protection system is secured to the MPCV’s base with a carbon graphite and titanium carrier structure.
As the heat shield reaches extremely high temperatures, portions of it fall away from the vehicle to remove excessive thermal energy. The remaining carrier structure must survive the impact when it hits the water to help keep the astronaut module intact.
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.
How to take a load off
At some 3,000 lb, the “baseline” composite-and-titanium design for the wagon-wheel shaped carrier structure that supports the MPCV’s thermal protection system is one of the largest components of the crew module, and a prime target for weight-reduction.
The design team, which included Mike Kirsch, project manager and principal engineer of the NESC’s Orion Heat Shield Carrier Structure Assessment Team, and technical lead Jim Jeans, president of Structural Design & Analysis, Inc., used the Collier Research Corp.’s HyperSizer.
The first-ever software commercialized out of NASA, HyperSizer analyzes stress, optimizes sizing, and reduces the weight of aircraft, wind turbine blades and other structures. Whether designed with composite or metallic materials, a typical HyperSizer optimization produces weight savings between 25 and 40%.
Alternate structural concepts
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 a wagon-wheel shape. Carbon graphite designs are tailorable, in that modifications can continue to be made en route to final manufacturing. With an initial goal of cutting out 800 lb, the NESC team considered both material and structural modifications to the baseline.
“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,” said 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.”
The team developed a series of analytical models to predict how the heat shield carrier structure as a whole and the internal support webs would react under a range of splashdown scenarios. 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 designs.
“Because it can concurrently evaluate different combinations of the variables that influence design, HyperSizer rapidly identified those configurations that had the lowest mass,” said Kirsch. “We could look at different solutions, materials, layouts—in this case orthogrid patterns—heights and densities.”
The software displays 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,” said 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 different configurations and be confident that we were making the right choices.”
“HyperSizer enabled us to rapidly study about 40 different variations,” said Kirsch, “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-lb reduction.”
James Ainsworth, a structural engineer for Collier Research, said, “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.”
“To put it in perspective,” continued Ainsworth, “each one of those landing events was about 12 GB of data and we crunched through some 50 landings.”
The kind of transient, dynamic analysis that the NESC performed on heat shield candidates is a new feature in HyperSizer, Version 7. “We included new analytical methods 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,” said Ainsworth.
The NESC team considered alternative designs that used load sharing with the crew module backbone, replaced the existing wagon-wheel stringer with an H-beam configuration, or switched the composite carbon graphite skin to a titanium orthogrid skin. The titanium orthogrid version emerged as their final proposal.
NESC’s insights from the HyperSizer analyses informed discussions that led to a reduction of the final weight of the baseline design by 23%, eliminating hundreds of pounds of weight.