Why Heavier subsystem; Lower Grms 2

[sarclee]

Hi, just to check the Grms for electronic equipment. Why is it smaller component (30gram) can take up to 15Grms,heavier component (400 gram) can only take 8Grms?

 

[rb1957]

It could be to drive the structure to "sensible" sizing. The smaller payload at 8g is only 0.6N and about 1.2N at 15g ... both pretty "silly" (small) forces.

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[WKTaylor]

sarclee... what-the-heck??? We are not mind readers...

Your question is seriously devoid of any relevant background/orientation information.

What specifications/standards/documents is/are Your question related-to?

Also...

As a crtsy & 4-prcsn, alwys dfine unts... espclly whn usng abbrvtns...

I THINK I know that Your terms/units are... but there are so many themes/variations out-there that we run-off-track and 'assume' [ass-u-me] we know.

Grms =?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[rb1957]

seriously, how do you think of a payload of 1 oz ?

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[sarclee]

Grms is G root-mean-square to determine the vibration energy. I attached one of the subsystem, reaction wheel. It is around 250 gram and 23 grms.

I do notice from another company selling reaction wheel 400 gram but 8 grms and a sensor as light as 30 gram with higher vibration energy. Does it mean for heavier component, we try to keep the vibration energy as low?

 

[rb1957]

ok, now this is something different. I thought you were talking about a design spec.

higher mass and lower vibration loads ... makes intuitive sense. A higher mass at the same vibration load would have larger forces than a smaller mass.

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[IRstuff]

It's not necessarily an apples to apples comparison, even if they ostensibly have the same function. Something with 10x the mass would be applicable for platform with possibly 10x the mass, so environment can't necessarily be the same. The launch environment would potentially get dampened by the extra mass of the host platform and its launch installation hardware, for example. An ESPA ring has a lot of mass by itself, so it can't transmit as much environment to a SmallSat attached to it, while something like a Starlink satellite is much smaller and has a very different, and its equivalent of an ESPA ring is possibly much lighter.

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[sarclee]

Thanks @rb1957 and @IRstuff

I just found another brand of reaction wheel that weight 215g with only 8.9g RMS which is much smaller than the one weight 226g reaction wheel with 23g RMS.

However, I have an optical payload predicted g RMS up to 20g RMS and the optical payload is 15kg. I just concern if the normal optical payload can take up to such high vibration energy. I am still searching similar optical payload specification.

 

[rb1957]

are we confusing required standards with achieved limits ?

it's quite reasonable that different pieces of equipment operate differently.

I'm not clear on "an optical payload predicted g RMS up to 20g RMS". Is this saying ...
1) the optical payload needs to be supported up to 20g RMS (as like a spec maximum). With a mass of 15kg, this is much more than your previous units. or
2) the payload, when excited by the launcher actually achieves 20g RMS.

are you designing the payload support structure, or specifying the momentum wheel to restrain this imbalance ??

it's a long time since I've done spacecraft, so my questions could be meaningless.

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[IRstuff]

I just concern if the normal optical payload can take up to such high vibration energy. I am still searching similar optical payload specification.

I think you are seriously confused. The objective is to find components that can withstand THE environment, but not overly so as those will tend to be more expensive. You seem to be thinking that matching specs is a thing that you should be doing, and it's not.

You need to start over and identify the overarching environmental requirement, which is often only the launch environment and select components that can meet those requirements with margin. The more critical spec for optical payloads is to ensure that the reaction wheels don't generate so much vibration that it degrades the image quality of the sensor. THAT is often a harder nut to even properly identify and model.

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[sarclee]

@rb1957 @IRstuff,

We are first time developing optical payload. Based on the launcher environment, the g RMS is pretty high at the optical payload mounting. So, I take a look in other subsystem to see if it is reasonable.

 

[sarclee]

I have another question.. is the random vibration table given in the launcher vibration environment(for example Figure 6.3.5-1 from SMC-S-016 below)meant for Qualification level?

Typical unit test level and duration in the launcher also similar to Table 6-3-3 below.

 

[rb1957]

it sounds like the launcher is applying a high vibration load to your payload, and yes your structure needs to support it.
yes, it sounds like your tests should follow those requirements. Hard to read your table, too low resolution (blurs under zoom).

are you saying that your payload had 20g response to this spectrum ? ... doesn't sounds good.

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

Sounds like you are still confused; if your test environment is 6.9 grms and your payload response is 20 g, then you have a poor design that is resonating crazily, and needs to be redesigned, FULL STOP.

What you are looking at in the spec sheets for reaction wheels is irrelevant to your payload design; those are the environments those components were designed to withstand and perform to spec afterwards, and had only marginal connection to your design, for which you already have a spec and it looks like you didn't design it to survive the spec.

Now, it's hypothetically possible that you can get away with leaving the design alone and so long as your optical payload can survive the load, that's OK, but the resonance, or excessive response, might likely couple into other components and damage them.

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[rb1957]

Nah, whilst the structure may survive the loads manifested by the vibration, the structure is needlessly heavy ... a critical flaw in spacecraft design.

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[IRstuff]

Possibly, but there are ludicrously small satellites like a 3U CubeSat

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[rb1957]

yeah but his payload is 15kg getting excited up to 20g for an applied acceleration of 0.04g, AIUI.

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[sarclee]

I updated the pictures above.

IRstuff, I agreed with you. The design has more room to improve. We size the weight down 25% of the total weight.

G rms of optic has cut down to 8 g RMS but for acceptance level. However, the peak PSD is still pretty high 5 G2/ Hz... But the peak PSD can be brought down with damper, right? I am using 2.5% damping ratio.

 

[IRstuff]

Dampers can work, but "designing" them in a CAD program isn't necessarily going to be the solution; we would have never imagined in CAD that the most effective solution to a resonance problem was a popsicle stick between two boxes.

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[rb1957]

yeah, you can have vibration isolators between your payload and the rocket ... to reduce the vibration loads on your payload

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