The challenges CAE solved while NASA designed the $10 billion James Webb Space Telescope.
Siemens has sponsored this post.
For many on terra firma, the most amazing things to come from the James Webb Space Telescope (JWST) have been the beautiful images of outer space. By processing the satellite’s data, NASA has produced high-quality pictures of everything from the Eagle Nebula’s Pillars of Creation to distant galaxies from the early universe.
But for Paul Bagdanove, JWST Mechanical Systems Engineer and Lead Stress Analyst on the project, safely getting the equipment into space, unfolding it, aligning it and getting everything to steady state temperatures is about the coolest thing imaginable.
“What I’m amazed at are the foldable, align-able mirrors and the fact that they can correct wave fronts, right there on the go,” says Bagdanove. “You have this backplane that they are mounted to, and the thing fold up. Then you have to unfold it and latch lock the side wings in place. And then the mirrors themselves have actuators on them that push and pull these rods and distort the focal plane of each of the eighteen individual segments to bring the entire telescope into one uniform focal plane to get these beautiful images. That’s so amazing to me.”
Though this technology is used on the ground, admits Bagdanove, he notes what is new is the fact that such a large version has never been launched, deployed, aligned and calibrated in space. To make that possible, his engineering team had to worry about a lot more than the damage and stress that a launch could bring—more on that later. The most interesting stresses JWST experiences, he says, are due to extreme temperature gradients. So, let’s dig into these challenges.
JWST Is More Than Pretty Pictures, It’s an Engineering Wonder
There is no question that the JWST is an interesting tool of science. Bagdanove is personally interested in how the satellite will be used in research of early star formations and the search for life on far-off exo-planets.
However, he, like many other engineers, realizes that these experiments are only possible because someone had to figure out how to get the equipment in orbit without error. Unlike Hubble, which orbits near Earth, the JWST is floating at a LaGrange point 1.6 million kilometers (1 million miles) away. Bagdanove notes this engineering challenge is what captured his imagination and really makes the JWST a marvel.
“There is no way you can send a servicing mission out there just like we could for Hubble,” argues Bagdanove. “It would take too long, it’s too far and honestly, we could have made things modular but everything is just too sensitive to have an astronaut monkeying around on the structure while it’s in a deployed state.”
He continues to say that if one were to run maintenance on the JWST they would have to fold it up first, which is a capability that isn’t engineered into the satellite. So, to do so would require special tooling and people on the ground. No matter how you cut it, JWST isn’t serviceable. As a result, it needed to work once deployed in the field.
To achieve this, the JWST engineers needed to integrate redundancy, assurances and tolerancing to make sure that everything launched, moved into position and worked at steady-state temperatures once in space. This didn’t just need redundant circuits; it needed many simulations.
Simulation’s Design Role for the JWST
To ensure that the equipment could survive launch, or any other transportation, the engineers needed to assess how the parts would react to the vibrations it would experience within a given vehicle. These simulations are standard for companies big and small and in many fields—especially if they feature sensitive equipment, heavy payloads that can affect vehicle control/fuel consumptions or if the vehicles used are associated with bumpy rides.
“We get a vibration requirement; it’s called a sine-vibe or sinusoidal vibration,” says Bagdanove. “It’s not a waveform we get from the rocket folks, they give us an environment that they measure as a modal frequency response that we can input into a sine-vibe environment … This environment must be met precisely, within some bounds, such that the rocket can control the payload, without damaging it, as it launches into space.”
Each rocket and vehicle will have its own vibration environment, and each needs to be assessed using simulation. To do that, NASA engineers use Simcenter Femap software from Siemens to pre- and post-process the model. Because Simcenter Femap is solver-agnostic, the team was then able to easily use other commercially available solvers for the project without issue.
Within the simulation, the engineers include a conservative factor so that when the equipment is in the real-world vehicle it will experience a vibration environment that is well within acceptable margins and safety limits.
How Simulation Helped JWST Survive the Harsh Reality of Space
Though vibrational simulations can be seen in many industries, what isn’t common are the cold temperatures and gradients the JWST would experience in space. Companies big and small focusing on manufacturing and aerospace will need to solve similar temperature gradient issues, but Bagdanove explains that what sets this apart is that much of the satellite’s equipment operates near absolute zero. Given that the overall telescope portion operates at 30 to 40 Kelvin and the sunshield could run as hot as room temperature, this is not an everyday engineering problem.
“It’s an overstressing condition,” said Bagdanove. “Coefficient of thermal expansion (CTE) is a real thing. Laypeople may not understand CTE, but it’s a major factor in the overstressing of any part going from 293 Kelvin all the way down to 40 Kelvin.”
How can these stresses affect Webb? At these various temperatures, materials will expand and contract at different proportions. If those proportions are not accounted for and there is no strain relief, it could distort the telescope. As a result, elements of Webb could push and pull on each other until at best a wavefront distortion causes blurry images and readings, or at worst something hits something else hard enough that it pops off the vehicle.
To account for this, engineers and material scientists took a two-pronged approach. The first was the selection of materials that would optimize their structure and reduce strain at the given operational temperatures. Once the material scientists settled on the right materials, those properties were passed onto the second task, where design engineers produced geometries and evaluated them using simulation.
“You have to mount everything kinematically,” says Bagdanove. “And you have to try to pick materials that can bond and bolt together with a close enough CTE such that when you go to temperature you don’t have that huge strain.”
For those in the aerospace industry, this solution might sound familiar to the design of fighter jets. These vehicles experience extreme heat in the engine and cold atmospheric temperatures at most other locations. As a result, when the jet sits on the runway it might not look like all the parts fit together. However, they are toleranced, mounted and assembled in a way that while flying the parts flex and stress into alignment.
To design Webb in the same way as those jets, engineers need to simulate each part and how it interacts with the overall assembly at operational and non-operational temperatures. This way they can predict that the parts will expand and flex so that up in space nothing is damaged. Bagdanove notes that his team used Femap extensively to set up these models.
“It’s something that we have always used at NASA,” said Bagdanove. “I’ve been there for 22 years and used Femap. I’ve used other pre- post-processors, but most of us, a good 80 percent, use Femap and understand it really well. You learn where all the buttons are, where all the inputs are and how to make a model, so we’ve stuck with it … It is a robust program, and the software itself has many features and they’ve kept up with us and helped if we had some issues. They would write programs for us, little sub-routines and add-ons. We have good customer service.”
NASA isn’t the only organization that uses Simcenter Femap. It is popular with startups, medium and large organizations in various industries due to its flexibility and software compatibilities. To produce the simulations mentioned here, or to make some for your own applications, try Simcenter Femap with Nastran with this free trial from Siemens Digital Industries Software.
