How James Webb’s Deployments MUST Work

by Taath · April 1, 2022

On Christmas Day, the James Webb Space Telescope left the planet. After a flawless launch on the Ariane five, Webb separated from its upper stage and, thanks to a built in camera, we even got to see Webb make its critical solar panel deployment. As Christmasse go, that was one for the books! But if there was one deployment that absolutely had to happen on time, it was that solar array. Without it, there is no power and without power, there is no mission. So it was really great to see it opening up while it was still on camera. About twelve hours later, the critical Midcourse Correction burn 1a completed. The reason this burn is needed at that time is because if Webb is moving too fast, then it would have to turn around to fire its thrusters in order to slow down. But turning around would expose the telescope directly to the sun and overheat, and that would effectively kill the mission before it ever began. So the Ariane five deliberately under burned its second stage to give Webb a slightly lower than required initial velocity. And that allowed the flight dynamics team to assess Webb’s exact heading and velocity and determine the exact duration and direction of Webb’s first midcourse correction burn 1a. And this would be the first of three burns to bring Webb into its desired orbit around L2. And the idea here is to make these burns in stages in order to ease up to the correct velocity. But of the three burns, this first one was the most time critical. Because of Webb were moving too slow for too long, Then a longer burn would be needed to bring it back on course and speed, and that means less fuel is available for the rest of the mission, and that would have been bad. So the first burn took place twelve hours from Webb’s release from the rocket while it was still close to Earth. The burn started at 7:50 pm Eastern Standard Time on Christmas night and lasted 65 minutes. This and the solar panel deployments were the only two activities that were time critical. The exact time and duration of the rest of the activities, including its next two burns, do have some flexibility. So everything we’re going to talk about today is going to be taken with a certain nominal timeline grain of salt. Shortly after 10:00 a.m. Eastern on December 26, Webb deployed its gimbal antenna assembly. And this houses Webb’s high gain antenna, and it’ll be used to send at least 28.6 gigabytes worth of science data back to Earth two times each day. On Monday, December 27, Webb executed its second midcourse correction burn called MCC 1b, and the second burn is used to further adjust Webb’s velocity to keep it on course to L2. Now why execute a second burn, It just had one the other day. Well, that’s because if everything went correctly with the first burn, Webb should still be moving just a tad slow again. That’s deliberate. Now, if Webb overshoots L2 by even the slightest amount, it cannot be turned around to fire its thrusters back toward Earth. So this is a weird case where we actually want Webb to play the role of Sisyphus and keep rolling that rock ever so slightly uphill. So engineers will analyze the results of the first burn and adjust the second burn’s duration to keep Webb on course, but never quite fast enough to overshoot. However, now comes the hard part – the deployment of its sunshield – followed by the less complicated, though equally critical, deployment of its mirrors and optical system,. And then the fine alignment of its 18 hexagonal mirrors into a single perfect mirror. In total, Webb must execute more than 50 major deployments using 178 release mechanisms and avoid 344 single points of failure. So, yeah, launch is behind us and believe me, that retired a lot of risk right there, but the hardest parts lay ahead. So we’re going to look at all of the remaining deployments in detail, and along the way, we’ll understand just why it has to work the way it does. But first, I’d like to thank Magellan TV for sponsoring today’s video. The James Webb Space Telescope will be able to probe the alien atmospheres of exoplanets near our sun. “Planet Hunting with the James Webb Space Telescope” is just one of the more than 3000 documentaries available exclusively on Magellan TV. Documentaries about nature, history, science and technology are added each week and are presented without commercial interruption. And for the remainder of this holiday season, my viewers can take advantage of a special buy one get one free offer for an annual membership gift card. Simply click on the link in the description of this video. Even before the Hubble Space Telescope launched, astronomers knew that a next-generation space telescope would be needed to peer even farther back in time than.Hubble ever could. Because of the universe’s expansion, light from those first stars is shifted all the way into the infrared, As a visible light telescope, Hubble could never see this light from the very first galaxies to form in the universe. So the Next Generation Space Telescope, as it was called back then, would need to be large enough to collect enough photons, but also cold enough that it could sense the faint infrared heat coming from the early universe. And that meant it would have to fold up to fit inside its launch vehicle and then unfold itself once in space, deploy a large sunshield and move to the Sun Earth L2 point to block the heat from the Sun, Earth and Moon. It’s a gravitational stable point where the Sun and Earth’s gravity combine to keep an object at this location orbiting the Sun at the same rate as Earth. The practical upshot is that Webb remains at the same general distance from Earth throughout its mission, far enough away to avoid the heat from the Earth and Moon, but close enough to maintain a continuous high data rate communication. However, one does not simply put a spacecraft at the exact L2 point. And that’s because this position is stable in the way a marble sitting on top of a round bowl is stable. The slightest push would send the marble rolling off. In fact, holding station exactly at L2 means frequent burns over the lifetime of its mission, and more burns means less science. However, it turns out to be a lot more fuel efficient to enter into a halo orbit around L2. Now, even then, Webb’s orbit will be ever so slightly leaning back towards Earth so that occasional station keeping burns will be used to nudge Webb ever so gently back into its orbit. And that’s why those midcourse correction burns that Webb is doing right now are so critical. The more fuel saved early on means more fuel for the life of the mission,. But there is no mission if the telescope can’t get cold enough. Previous infrared missions like Spitzer and Herschel used a combination of rigid shields and cryogenic cooling systems to keep their telescopes cool, but their rigid design ultimately limited how large those telescopes could be. But with a 6.5 meter telescope to protect, Webb’s shield needs to be much larger. How large? Well, in a perfect world, the shield would be just exactly large enough to keep the telescope cold while it pointed in any direction it needed to as long as it was away from the shield. But the telescope needs to be kept at around 40 Kelvin. That’s -233 degrees Celsius, or -388 degrees Fahrenheit. Motors on the telescope side would have to be kept warm in order to point the telescope, and that would have added more complexity and more mass to the spacecraft. So that idea was dropped early on. Instead, the telescope is fixed in position and the shield was made a little bit larger to about the size of a tennis court when fully deployed. It’s also shaped like a large bent diamond, and the shape allows the spacecraft to roll, pitch, and yaw to give Webb a larger field of regard than it otherwise would have. And this design still allows Webb to view the entire sky over the course of a six month period, albeit with some restrictions on exactly when and for how long a given target is available. So here’s how the nominal deployment works. The shield is folded up into the forward and aft Unitized Pallet Structures, or UPS’s. These are stowed parallel with the telescope, so they’re largely shielded by the bottom of the spacecraft prior to deployment. The spacecraft is maneuvered to let sunlight reach the forward UPS, and heaters are activated to warm up key deployment components. Release devices are activated and the forward UPS is then lowered and locked into place. Next, the spacecraft is repositioned to warm up and then lower the aft UPS. Both deployments should nominally occur on the same day, but they’re not automatic. They’ll only occur when ground control tells them to. About four days after launch., the telescope’s Deployable Tower Assembly or DTA is raised off the spacecraft in order to give the necessary clearance for the sun shield to deploy. And this separation also provides a little extra thermal isolation from the sunshield. And this step is necessary because the whole thing has to be made as compact as possible to fit into the Ariane five’s launch fairing. Fortunately, this is a relatively straightforward step. It starts with activating several release mechanisms, turning on some deployment heaters, spinning up the software electronics and deployment motor, and then extending the tower up about two meters. You know, come to think of it, it doesn’t sound very straightforward at all. Next is the deployment of Webb’s Aft Momentum Flap. To understand its purpose, we need to consider that when deployed, the sunshield essentially becomes a large solar sail. Because of the shield’s angled design, solar radiation pressure will gradually cause the observatory to pivot. So the aft momentum flap provides a continuous yet gentle offset to some of that solar radiation pressure. Now, like most of Webb’s deployments, the aft trim tab is “one and done”; meaning, once it’s in place, there’s no further adjustments to be made. But it also means that once it’s in place, it can’t be further controlled, so it pretty much has to work the first time. Now it’s time for the main event, the sunshield deployment. This is the most complex Rube Goldberg series of deployments ever attempted in space. To understand why, let’s take a look at the shield’s design. The shield is made of five ultra thin layers of Kapton This is a polyimide film which is highly resistant to heat and can tolerate temperatures ranging from -269 to +400 degrees Celsius. However, Kapton is transparent, which is why each layer is coated with a very thin film of aluminum. Not only does this make the layers opaque, but it also makes them electrically conductive, and that allows the shield to be grounded to the rest of the telescope. So, any static charges that build up are conducted away and dispersed into space. The first layer is 0.05 millimeter thick. That’s the thickest of the five layers! The remaining layers have decreasing surface areas and are only 0.025 millimeter thick. The aluminum coating is even thinner, at just 100 nanometers in thickness. Light from the Sun, Earth and Moon primarily hit the first layer and occasionally will hit part of the second. That’s why these two layers are also coated with a 50 nanometer film of silicon to reflect more of the Sun’s heat back into space. And the silicon coating is what gives the first two layers their pink color. So why five layers and why not just one thick layer? Well, as each layer is heated, it radiates in all directions. That means some of that heat gets radiated back up to the spacecraft, which is why another layer is there to reflect that heat as well. The layers use the vacuum of space in between them as an insulator, and the heat energy is bounced between layers and away from the telescope. But in order for the sunshield to work, the layers must be pulled taut and separated from each other very precisely. This is why the shield alone uses 140 release mechanisms, approximately 70 hinge assemblies, eight deployment motors, bearings, springs, gears, about 400 pulleys and 90 cables, all of which must work. Six days after launch, protective covers are released and partially rolled back. These covers protected the shield membranes throughout the ground and launch activities. Before they can roll up all the way though, a set of covers over the core region of the ahield are released. Now comes the most complex part of the entire deployment. All 107 of the sunshield’s release mechanisms must fire at exactly the right moment in exactly the right order to release the sunshield for deployment. Here we go. The port-side membrane covers fully roll up and the port mid boom begins to deploy. And this boom is driven by electric motors to control the speed of the deployment and not damage the sunshield. After the port boom extends, pulleys reel in the cables to fully extend their layers. Next, the starboard membrane covers are rolled up and the starboard amid boom deploys, mirroring the port boom deployment sequence. You may notice that the shield layers have corrugated patterns embedded in them, and these are rip stops to prevent micrometeoroid impacts from destroying the rest of the shield. Yet another reason to have multiple layers! And then more release mechanisms fire to allow the five layers to move into the relative separations. And this process will take about two days as motors and pulleys work to precisely align and tension the sunshield. By the eighth day, the shield will be fully deployed. The side that is closest to the Sun will be hot enough to boil water, while the dark side is cold enough to freeze air. That’s a 600 degree differential, but it will take several months for the observatory to fully cool down to those cryogenic temperatures. In the meantime, there’s still more work to be done, including what is arguably the most critical deployment since the solar panel. Ten days after launch, the secondary mirror deploys. And this is the mirror that will reflect light from the telescope’s primary into its instrument bay. All of these other deployments have had some degree of fault tolerance. For example, the sunshield is designed to make Webb slightly colder than it actually has to be. So if it doesn’t deploy perfectly, the telescope can still do science. If one or both of the primary mirror wings don’t open. Webb could still make use of the light reflecting off of the twelve fixed mirrors. But if the secondary mirror doesn’t deploy, there is no telescope. And that’s bad. Fortunately, this is a much less complex deployment than the sunshield. So once the deployment arms are latched, some further adjustments will be made to the secondary mirror at a later time. Behind the primary is the Integrated Science Instrument Module. This module is where Webb’s science instruments and supporting cryogenic systems are housed, and it also contains the supporting electronics for those systems. Those instruments and electronics generate heat, which must be disposed of in order to keep the instruments as cold as possible. So on day eleven, the Aft Deployed Instrument Radiator is released, and this allows heat from the instruments to start flowing away. And this deployment is relatively straightforward, but it’s also critical for keeping the instruments from being blinded by their own radiation. Starting around day twelve, Webb begins its deployment of its primary mirror wings. Each wing holds three of Webb’s 18 hexagonal mirrors. The port wing is deployed first, and it’s driven by a motor to control its motion. When fully deployed, the wing is latched into place. This is followed by the starboard wing the following day. At long last, the largest telescope ever sent into space has finally unfolded. It still has a couple of weeks to go before it reaches L2, and it will use much of their time aligning its mirror segments to form a single image. Each mirror segment is moved by six actuators that adjust the segments’ position by just a few nanometers at a time. That’s about 1 ten-thoudandth the width of a human hair! Webb does this by looking at a star and gradually combine the 18 images into a single image. This is a process called “phasing”, and it can take anywhere from several days to a month to complete. Now, the details of Webb’s optical system are so fascinating that I really ought to save them for another video. Besides, were about to reach L2! Twenty-nine days after launch, Webb executes its final midcourse correction maneuver. This final burn corrects any residual trajectory errors and nudges Webb into its halo orbit around L2. This final maneuver completes Webb’s 29 day deployment and brings it into its orbit. For the next five months, the telescope will continue to cool to its operating temperature. Then the science instruments will have their cryogenic systems brought up to cool them down even further. The optical systems will continue to be aligned, instruments will be calibrated, and Webb will be commissioned as a working observatory. Obviously, there’s a chance that something could go wrong somewhere in one of these deployments. That’s why controllers have been practicing Webb’s deployment over and over again for several years now. In that time, they’ve imagined every possible anomaly that Webb might experience, and they’ve developed contingency operations for each of them, including, but not limited to commanding Webb to shake itself back and forth to loosen a stop deployment. Now the great thing about this is that we’re going to continue to follow Webb and see how it’s going throughout its deployment and to help me do that, I’ve got these wonderful people here who are supporting me on Patreon to help keep Launch Pad Astronomy going, and I’d like to welcome Nikson and Christopher Peck as my newest supporters. And if you’d like to stay informed on how Webb is doing and indeed everything else going on in the universe, well, please make sure you subscribe and ring that notification bell so that you don’t miss out on any new videos. Until next time, stay curious, my friend.