Morning. As we near the end of #JWST’s deployments (& how mad is that?! 😱), the big focus (😉) will naturally be on the primary mirror wings swinging into place 🔭
But don’t forget the aft deployable radiator, key to the instruments keeping their cool as they do science 😎
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But don’t forget the aft deployable radiator, key to the instruments keeping their cool as they do science 😎
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Unfortunately I don’t have time today for a megathread on the physics of the radiator, as I’m going to be driving my son back to university in Groningen.
But there’s one equation to keep in mind with the ADR: sigma.A.T^4
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But there’s one equation to keep in mind with the ADR: sigma.A.T^4
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That’s the amount of power radiated by a black body in Watts & can be used to determine how big that radiator & others on the roof of the Integrated Science Instrument Module (ISIM) need to be to shed the heat generated by the instruments.
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sigma is the Stefan-Boltzmann constant & is roughly:
σ = 5.67 × 10^−8 W⋅m^−2⋅K^−4
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σ = 5.67 × 10^−8 W⋅m^−2⋅K^−4
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Then you have A, the area of the radiator in square metres & T, the temperature in Kelvin.
The key thing to realise is that the power radiated depends on T to the *fourth* power.
That means as T goes up, the radiated power goes up very quickly.
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The key thing to realise is that the power radiated depends on T to the *fourth* power.
That means as T goes up, the radiated power goes up very quickly.
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Conversely, at low temperatures like the very chilly 40K or -233°C that JWST will get down to on the cold side behind the sunshield, you are going to get very little power radiated at all.
Unless you compensate by increasing A, the area.
Which is why the ADR is huge.
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Unless you compensate by increasing A, the area.
Which is why the ADR is huge.
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As an example, at 40K, one square metre of perfectly black radiator (& the JWST ones are very black, around 98% so) will emit just 145 milliwatts.
Not a lot. A (living, warm 🙂) human being emits about 1,000 x that much just standing still.
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Not a lot. A (living, warm 🙂) human being emits about 1,000 x that much just standing still.
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So to shed the tiny amounts of power generated by operating the science instruments & parasitic heat creeping through from the hot side, #JWST needs a whole bunch of big black radiators on the cold side.
Some are bolted to the roof of the ISIM, but the ADR folds out.
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Some are bolted to the roof of the ISIM, but the ADR folds out.
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To get a sense of scale, here’s a CAD drawing of the fixed radiators for NIRSpec & NIRCam, two of the four science instruments on #JWST from this paper by Franck et al. at Ball Aerospace.
ttu-ir.tdl.org/bitstream/hand…
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ttu-ir.tdl.org/bitstream/hand…
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And here’s a diagram from the same paper showing where they’re located on the roof of the ISIM & how the ADR will add more surface area when deployed, all angled to beam the heat out in the deep cold of space, away from #JWST.
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Relative to some other deployments on #JWST, this should be a fairly easy one. But again, an important one, as are they all.
Anyway, a little further insight into the crazy physics of this machine & just how much clever work has gone into solving its challenges 👍
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Anyway, a little further insight into the crazy physics of this machine & just how much clever work has gone into solving its challenges 👍
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Coda: to clarify the nomenclature, the official name of the big black flap is the ADIR, for Aft Deployable ISIM Radiator, where ISIM in turn is the Integrated Science Instrument Module.
I’ll let you be the judge of whether nested acronyms are ever justifiable 😬🤪
I’ll let you be the judge of whether nested acronyms are ever justifiable 😬🤪
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