How to calculate the RPMs needed to make a spincast lquid parabolic dish?

  • Context: Undergrad 
  • Thread starter Thread starter CosmicVoyager
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Discussion Overview

The discussion focuses on calculating the revolutions per minute (RPM) required to create a liquid parabola of a specified diameter and depth. It explores theoretical aspects related to the physics of liquid mirrors and the practical considerations of motor performance.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant inquires about the method to calculate the RPM needed for a liquid parabola.
  • Another participant references a Wikipedia article on liquid mirrors and notes the challenge of finding a motor that can maintain a low constant speed without jitter.
  • Some participants suggest that, based on their calculations, a wider radius requires fewer RPMs to achieve the same parabolic profile, which contrasts with their initial expectations.
  • There is a mention that larger mirrors typically rotate at very low speeds, implying a relationship between radius and required angular speed.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between the radius of the liquid parabola and the required RPM, indicating that the discussion remains unresolved regarding the specifics of the calculations and assumptions involved.

Contextual Notes

There are potential limitations in the assumptions made regarding the calculations, particularly concerning the definitions of diameter and depth, as well as the performance characteristics of the motors discussed.

CosmicVoyager
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Greetings,

Anyone know how to calculate the revolutions per minute needed to create a liquid parabola of given diameter and depth?

Thanks
 
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Andy Resnick said:
A good accounting can be found here:
http://en.wikipedia.org/wiki/Liquid_mirror

AFAIK, the main problem is getting a motor that can run at low constant speed without any high-frequency jitter.

If I have done the math correctly, it seems the wider the radius is the fewer RPMs that are needed? Is that correct? That is the opposite for what I was thinking.
 
CosmicVoyager said:
If I have done the math correctly, it seems the wider the radius is the fewer RPMs that are needed? Is that correct? That is the opposite for what I was thinking.

The larger the radius, the slower angular speed needed to generate the same profile... IIRC that's correct. Those large mirrors turn *very* slowly.
 

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