NASA team obtains the 'unobtainium' for next space observatory

Sep 28, 2010 by Lori J. Keesey
Doug McGuffey is pictured here standing next to the Integrated Science Instrument Module (ISIM) Flight Structure. Credit: NASA/Chris Gunn

Imagine building a car chassis without a blueprint or even a list of recommended construction materials. In a sense, that's precisely what a team of engineers at the NASA Goddard Space Flight Center in Greenbelt, Md., did when they designed a one-of-a-kind structure that is one of 9 key new technology systems of the Integrated Science Instrument Module (ISIM). Just as a chassis supports the engine and other components in a car, the ISIM will hold four highly sensitive instruments, electronics, and other shared instrument systems flying on the James Webb Space Telescope, NASA's next flagship observatory.

From scratch — without past experience to help guide them — the engineers designed the ISIM made of a never-before-manufactured composite material and proved through testing that it could withstand the super-cold temperatures it would encounter when the observatory reached its orbit 1.5-million kilometers (930,000 miles) from Earth. In fact, the ISIM structure survived temperatures that plunged as low as 27 Kelvin (-411 degrees Fahrenheit), colder than the surface of Pluto.

"It is the first large, bonded composite spacecraft structure to be exposed to such a severe environment," said Jim Pontius, ISIM lead mechanical engineer.

The 26-day test was specifically carried out to test whether the car-sized structure contracted and distorted as predicted when it cooled from room temperature to the frigid — very important since the science instruments must maintain a specific location on the structure to receive light gathered by the telescope's 6.5-meter (21.3-feet) primary mirror. If the structure shrunk or distorted in an unpredictable way due to the cold, the instruments no longer would be in position to gather data about everything from the first luminous glows following the big bang to the formation of star systems capable of supporting life.

"The tolerances are much looser on the Hubble Space Telescope," said Ray Ohl, a Goddard optical engineer who leads ISIM's optical integration and test. "The optical requirements for Webb are even more difficult to meet than those on Hubble."

Despite repeated cycles of testing, the truss-like assembly designed by Goddard engineers did not crack. The structure shrunk as predicted by only 170 microns — the width of a needle —when it reached 27 Kelvin (-411 degrees Fahrenheit), far exceeding the design requirement of about 500 microns. "We certainly wouldn’t have been able to realign the instruments on orbit if the structure moved too much," said ISIM Structure Project Manager Eric Johnson. "That's why we needed to make sure we had designed the right structure."

Obtaining the Unobtainium

Achieving the milestone was just one of many firsts for the Goddard team. Almost on every level, "we pushed the technology envelope, from the type of material we would use to build ISIM to how we would test it once it was assembled," Pontius added. "The technology challenges are what attracted the people to the program."

One of the first challenges the team tackled after NASA had named Goddard as the lead center to design and develop ISIM was identifying a structural material that would assure the instruments' precise cryogenic alignment and stability, yet survive the extreme gravitational forces experienced during launch.

An exhaustive search in the technical literature for a possible candidate material yielded nothing, leaving the team with only one alternative — developing its own as-yet-to-be manufactured material, which team members jokingly referred to as "unobtainium." Through mathematical modeling, the team discovered that by combining two composite materials, it could create a carbon fiber/cyanate-ester resin system that would be ideal for fabricating the structure's square tubes that measure 75-mm (3-inch) in diameter.

How then would engineers attach these tubes? Again through mathematical modeling, the team found it could bond the pieces together using a combination of nickel-alloy fittings, clips, and specially shaped composite plates joined with a novel adhesive process, smoothly distributing launch loads while holding the instruments in precise locations — a difficult engineering challenge because different materials react differently to changes in temperature.

"We engineered from the small pieces to the big pieces testing along the way to see if the failure theories were correct. We were looking to see where the design could go wrong," Pontius explained. "By incorporating the lessons learned into the final flight structure, we met the requirements and test validated our building-block approach."

Making Cold, Colder

The test inside Goddard's Space Environment Simulator — a three-story thermal-vacuum chamber that simulates the temperature and vacuum conditions found in space — presented its own set of technological hurdles. "We weren't sure we could get the simulator cold enough," said Paul Cleveland, a technical consultant at Goddard involved in the project. For most spacecraft, the simulator's ability to cool down to 100 Kelvin (-279.7 degrees Fahrenheit) is cold enough. Not so for the Webb telescope, which will endure a constant temperature of 39 Kelvin (-389.5 degrees Fahrenheit) when it reaches its deep-space orbit.

The group engineered a giant tuna fish can-like shroud, cooled by helium gas, and inserted it inside the 27-foot diameter chamber. "When you get down to these temperatures, the physics change," Cleveland said. Anything, including wires or small gaps in the chamber, can create an intractable heat source. "It's a totally different arena," he added. "One watt can raise the temperature by 20 degrees Kelvin. We had to meticulously close the gaps."

With the gaps closed and the ISIM safely lowered into the helium shroud, technicians began sucking air from the chamber to create a vacuum. They activated the simulator's nitrogen panels to cool the chamber to 100 Kelvin (-279.7 degrees Fahrenheit) and began injecting helium gas inside the shroud to chill the ISIM to the correct temperature.

To measure ISIM's reaction as it cooled to the sub-freezing temperatures, the team used a technique called photogrammetry, the science of making precise measurements by means of photography. However, using the technique wasn't so cut-and-dried when carried out in a frosty, airless environment, Ohl said. To protect two commercial-grade cameras from extreme frostbite, team members placed the equipment inside specially designed protective canisters and attached the camera assemblies to the ends of a motorized boom.

As the boom made nearly 360-degree sweeps inside the helium shroud, the cameras snapped photos through a gold-coated glass window of reflective, hockey puck-shaped targets bolted onto ISIM's composite tubes. From the photos, the team could precisely determine whether the targets moved, and if so, by how much.

"It passed with flying colors," Pontius said, referring to the negligible shrinkage. "This test was a huge success for us."

With the critical milestone test behind them, team members say their work likely will serve NASA in the future. Many future science missions will also operate in deep space, and therefore would have to be tested under extreme cryogenic conditions. In the meantime, though, the facility will be used to test other Webb telescope systems, including the backplane, the structure to which the Webb telescope’s 18 primary mirror segments are bolted when the observatory is assembled. "We need to characterize its bending at cryogenic temperatures," Ohl said.

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User comments : 18

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Quantum_Conundrum
3 / 5 (2) Sep 28, 2010
Once again, with all the costs of R&D, why not mass produce them? Why not make 5 or 10 of these?
otto1932
5 / 5 (2) Sep 28, 2010
Once again, with all the costs of R&D, why not mass produce them? Why not make 5 or 10 of these?
Maybe, because, we only need one? -is my guess
RocketScientist5KN
5 / 5 (2) Sep 28, 2010
They'll typically make one or two spares, but they only have the funds to launch one.
Quantum_Conundrum
4 / 5 (6) Sep 28, 2010
Maybe, because, we only need one? -is my guess


There are so many stars in our own galaxy that estimates vary by as much as 200 billion, or 50%, because just counting and cataloging the stars is that hard and time consuming.

One telescope is a joke by comparison to the task of studying just what is in our own galaxy.

Then when you look at just the Hubble Deep Field, you see there are far more galaxies in the universe than there are stars in the milkyway.

How the heck is one telescope supposed to study all this?
thales
5 / 5 (2) Sep 28, 2010
How the heck is one telescope supposed to study all this?


Amen, brother. To be fair, there are at least thousands of telescopes in use; they're just on Earth. With adaptive optics, they can be surprisingly powerful. Having said that, it seems like pulling teeth to get decent gear in the skies. I personally can hardly wait for a super massive moon-based telescope.
GSwift7
2.5 / 5 (8) Sep 28, 2010
"Why not make 5 or 10 of these?"

The age old question: If you could do it again, would you do it the same way? The answer is always no on a project like this one. They also don't really know if it will work. Why build two if you don't know whether the first one will work? If you want to build another one later, you still have the plans, right?
DKoncewicz
5 / 5 (4) Sep 28, 2010
I'm sure once we actually have significantly cheaper ways to send things into orbit we'll build more, but as was mentioned earlier getting them into space and actually maintaining them up there is quite expensive.
daywalk3r
3.4 / 5 (18) Sep 28, 2010
Why not make 5 or 10 of these?
Exactly as GSwift7 allready mentioned, projects like this usually are their own prototypes, so there is allways a big question mark hanging over their functionality/reliability in real-world use.

Like for example, an unnoticed bug in the control software could cause the whole thing to drift into atmosphere, and we would get to see not ONE, no no.. but even TEN fireballs, at the same time! Excellent ;)

The other "issue" is with the pace of advancement of technology nowadays. In about 4-5 years we could build a telescope with twice the performance, making the old model/design automaticaly obsolete. Give another 4-5 years, and it's double that again.

So unless we have a more practical way of getting things into orbit, it would be a considerable waste of resources to build and deploy multiples, unless there is some specific crucial need for it..
GSwift7
1.5 / 5 (8) Sep 28, 2010
One small point about the Webb telescope daywalk3r: It won't be orbiting the Earth, so an error in the software is unlikely to cause it to drift into the atmosphere. Webb is set to occupy one of the La Grange points and will actually be orbiting the Sun, far away from the Earth. Other than that, good post. With the acceleration of technology, by the time a big project like this is completed, it's already obsolete. By starting from scratch a year later, you actually gain a whole generation of advancement, as you correctly state.
GSwift7
1.5 / 5 (8) Sep 28, 2010
by the way, this was a really good article. I really liked the detail and the way it was written. Good job on this one, Physorg.
daywalk3r
3.3 / 5 (16) Sep 29, 2010
One small point about the Webb telescope daywalk3r:..
Yes, ofcourse, that one specific outcome most probably does not apply to the Webb telescope, but the cause of the failure certainly does.

The point of the example was rather to show one of many flavours of flaws that could be expected from the bulk of similar projects/prototypes. You can't just easily send someone up to have it fixed - the more when it is not even close to Earth.
nevdka
not rated yet Sep 29, 2010
What concerned me most about this article is when they mentioned the diameter of the square tubes.
CaptBarbados
5 / 5 (1) Sep 29, 2010
Great article. The lab sounds like it's gaining huge experience.

This is pathfinding R&D!
LivaN
5 / 5 (2) Sep 29, 2010
Once again, with all the costs of R&D, why not mass produce them? Why not make 5 or 10 of these?


There is a big difference in being able to build something, and then making it mass producible.

I assure you, were it at all possible to mass produce this telescope, and maintain each one at a reasonable cost, it would be done. Remember a lot of the technology was specifically developed for this project, which means it's no doubt highly advanced. How much more difficult do you think it would have been to develope this if it had a prerequisite of being mass producable?
Quantum_Conundrum
1 / 5 (1) Sep 29, 2010


There is a big difference in being able to build something, and then making it mass producible.

I assure you, were it at all possible to mass produce this telescope, and maintain each one at a reasonable cost, it would be done. Remember a lot of the technology was specifically developed for this project, which means it's no doubt highly advanced. How much more difficult do you think it would have been to develope this if it had a prerequisite of being mass producable?


If you made 5 of them then at least you've distributed the R&D costs across 5, reducing the average R&D cost by 80% per observatory.
Adriab
not rated yet Sep 29, 2010


There is a big difference in being able to build something, and then making it mass producible.

I assure you, were it at all possible to mass produce this telescope, and maintain each one at a reasonable cost, it would be done. Remember a lot of the technology was specifically developed for this project, which means it's no doubt highly advanced. How much more difficult do you think it would have been to develope this if it had a prerequisite of being mass producable?


If you made 5 of them then at least you've distributed the R&D costs across 5, reducing the average R&D cost by 80% per observatory.


The cost of mass-production doesn't make sense for 5 or 10, it just wouldn't be worth it. Building the assembly line would be more difficult and expensive than building a few of the units themselves.
lengould100
not rated yet Sep 29, 2010
I suppose they've allowed for the measuring camera's parts (lenses, detector) to shrink in the cold while taking those precision measurements? Sure hope so.
Quantum_Conundrum
1 / 5 (1) Sep 29, 2010
lengould100:

They are probably designing it to be as close as possible to accomodate the shrinking and deformation expected, and then have some sort of calibration mechanism.

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