Acceleration increases with Frequency 1

[sarclee]

Hello,

I notice the acceleration and grms given by the launcher OEM are increasing with frequency... in my prior experience in the interior design, acceleration is higher at lower frequency...

It is interesting to see the difference here but I am wondering the physical meaning why higher frequency higher acceleration?

Thank you!

 

[rb1957]

vibration means displacement. displacement in a sinusoidal wave means if your amplitude is fixed then higher frequency will mean higher acceleration. Maybe in your previous experience higher frequency meant lower amplitude ?

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[IRstuff]

You need to pay attention to the frequency, and look at the total curve; all vibration must eventually start to decrease in amplitude, since displacement cannot continue to increase with frequency. The below is from the Minotaur user's guide

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[sarclee]

Yes... the amplitude will be decreasing ...

In the interior design, the amplitude started with higher g then decreasing but now for s/c, it is increasing ...

The PSD is measured to represent the actual event, right?

 

[rb1957]

AFAIK, the PSD is showing the distribution of cycles in the spectrum.

in a period of time there will be X cycles. X/1000 will be at the load level of 1000 Hz, X/100 will be at 100 Hz.

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[sarclee]

I am attaching a picture for reference...

 

[rb1957]

what can we say ... vibration in a rocket is different to that in a truck.

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[3DDave]

Yes - this is saying that there are higher high-frequency accelerations than the accelerations at lower frequencies. Considering that low frequency means higher amplitude to gain similar accelerations ask what happens to a rocket or an airplane if it is experiencing high amplitude low frequency - think Tacoma Narrows Bridge.

 

[IRstuff]

You are literally comparing a truck to a rocket; trucks have suspensions that attenuate higher frequencies beyond around 2 Hz. The truck would be otherwise not fit for human ridership, since you could possibly ride such a PSD for hours, while a rocket's PSD lasts about 10 minutes

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[sarclee]

Thanks a lot to rb1957, 3DDave and IRstuff!

Right! Truck is different from Rocket

I like how 3DDave and IRstuff motivates the thinking...

I understand that lower frequency would have more vibration but I have to find out how the event likes to understand the opposite physics in rocket ...

Appreciate it!

 

[rb1957]

light a bejesus big rocket under yourself and hang on.

all this rocket data is coming from the real world.

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[GregLocock]

To be fair most vibration psd limits are real world, predicting psds is rarely achievable in practice. That is, given a road profile, an expensive tire model, and superb vehicle model (my bit), we can generate a psd, and it'll be kinda sorta accurate in general, to within say a factor of 8 on fatigue life. The reasons are partially known, for example a plot of shock absorber force vs velocity in the real world bears scant resemblance to the sort of model we use, which again, we know why, and every hole we dig gets deeper.



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[IRstuff]

understand the opposite physics in rocket

Your misunderstanding is in thinking that the phenomenon are related; they're not. Automobile vibration is from rolling on the ground with a suspension; rocket vibration is from instabilities/inconsistencies in rocket fuel combustion coupled with aerodynamics variations. There should be zero expectation that the PSDs would be at all comparable. For example, a car can't generate much more than a couple of g's in non-braking straight line travel, while rockets routinely exceed 6 g's in liftoff. They have a 200:1 ratio of velocities.

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[WKTaylor]

This 'graphic plot' looks like affects of 'the system' or 'a component' may be experiencing resonance.



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[GregLocock]

IRstuff- vertically (and longitudinally) we see a /lot/ more than 2g on cars. And of course on engines 20g is not unusual.

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[rb1957]

but greg I think that's another difference between rockets and trucks. I think the vibration from a rocket launch is well measured and predictable. A truck can drive on any road with "any" load at "any" speed ... so guesswork and conservatism are needed.

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[3DDave]

The peak loads on road vehicles are pretty high, but it is unlikely to be a continuous 2G input to the body of the vehicle. I know from testing that the PSD that was spec'd for our military aircraft equipment, when run on the shake tables, sounded very much like that of the operating aircraft - it required earplugs to be in the same room when the test was running. Given how loud a rocket launch is it doesn't surprise me that there is a peak at 1kHz.