Comment by scrooched_moose | Hacker News Reader

Comment by scrooched_moose | Hacker News Reader

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scrooched_moose

To everyone saying "what's the big deal, it sees more vibration on launch":

It sounds to me like this is more of an unintended shock event (see zeeb's response below) that caused an oscillation at the natural frequency ("vibration") but they tortuously avoided using the word "shock".

This would result in a given shock response spectrum [1], which effectively can amplify the input if it hits any part's natural frequency - e.g. a 1G input shock could become a 5G response somewhere in the satellite. I assume this wasn't accounted for in design so they'll determine if this could have damaged any onboard systems.

Launch vibration has a well known Power Spectral Density [2] which effectively says how much energy is being input at every frequency. If they design the system so no parts have a natural frequency at a high-energy peak it's safe.

Shock and vibration are closely related, but very different in practice. It's all about natural frequencies and what frequencies the energy is being input at.

1) https://en.m.wikipedia.org/wiki/Shock_response_spectrum

2) https://vru.vibrationresearch.com/lesson/what-is-the-psd/

RUAG makes most of the "payload adapters" that integrate the satellite onto the top of a rocket. Based on the size of JWST they're probably using a 1666 model payload adapter.

RUAG's clamp-bands and separation system have a very well characterized Shock Response Spectrum [0, pg. 4], which would have been accounted for in the design of JWST. But the spacecraft and satellite usually separate after the initial shock of the explosive bolt and clamp-band de-tensioning, so they may need to investigate any coupling or recontact of the clamp-band.

If we're lucky they had some accelerometers running during integration and can review actual response data.

0) https://www.ruag.com/system/files/media_document/2019-03/PLE...

Right, in design shock and vibration are basically 2 separately requirements, analyzed and tested separately. Sometimes by analysis it is shown that vibration test levels "cover" the shock requirement, but a spacecraft that is the first of its type, and with many mechanisms is likely to undergo discrete shock testing.

Minor addition to your reply, design typically avoids similar natural frequencies to the launcher, but the design needs to be compatible with the loads resulting from vibration as

It sounds like the clamp band was accidentally released just after installation, thankfully in a position that didn't drop the spacecraft somehow. The input shock is more like 1000G for these things, usually the largest source of shock for a spacecraft. Of course, it's designed to be released by the same mechanism, although it would not be designed to be released an unlimited number of times. I would guess they already know the components which are critical for shock, and will inspect and/or function test those before proceeding. Since it was an unplanned event, even documenting that it happened and getting everyone to agree to proceed based on the spacecraft having been designed for that input shock, takes at least a few days.

It does not sound like a larger than design shock was somehow applied and they have to check everything at spacecraft level analysis. That would take at least several weeks.

Is that like the difference between being inside a punching bag when punched and a bell when rung?

Both of those are more shock events - there is a one-time impulse of energy into the system. The main difference here would be damping - the punching bag absorbs and dissipates the energy readily via internal friction of whatever the stuffing material is. A metal bell has orders of magnitude lower internal damping so it sits there and vibrates at its natural frequencies, emitting sound as it pushes against the air which slowly bleeds off energy due air resistance.

Random Vibration is usually characterized by a constant input of energy to the system at a range of frequencies - rocket motors, engine rpm + cylinder firing frequency, road noise, etc.

A somewhat imperfect analogy that I've heard is random vibration is analogous to playing white noise/pink noise/etc through a speaker. Shock is analogous to the horrific crackling and popping you get when you plug a guitar into an amplifier that's already on.

Is the JWST in a state where they can open it up and inspect? Or are they just going to run simulations to see if the shock/vibration would cause any anomalies?

My guess based on the very limited press release and some aerospace experience:

Because it's only a 4 day slip, they'll start with simulations to look for any systems that may have been damaged. If they don't see any issues assembly and launch will continue.

If simulations show that this may have damaged a component, we'll see a longer delay as they begin replacing systems.

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