3006. The Moment of Inertia of Fluids – Part 2


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Richard Boynton: 3006. The Moment of Inertia of Fluids – Part 2. 2000.



As much as 80% of the mass of a booster rocket or 40% of the mass of a satellite or aircraft can consist of fuel and other liquids. Engineers spend countless hours calculating the mass properties of the solid elements in a flight vehicle with an accuracy of 1 or 2 percent, but the contribution due to the fuel is often based on assumptions that are in error by as much as 50%. Last year one of the authors of this paper (Richard Boynton) published a paper entitled ?The Moment of Inertia of Fluids? (SAWE number 2459). In this paper he summarized a series of measurements which were made on the fluid within different shaped tanks to determine the relationship between total fluid mass and moment of inertia. Some mass properties engineers assume that a tank rotates independently of the fluid contained within it, so that the mass of the fluid has a small effect on the MOI. Others assume that the fluid acts like a solid. As paper number 2459 and this paper show, both assumptions are incorrect.
There were a number of issues which Mr. Boynton was unable to resolve when he wrote last year?s paper. This second paper gives the answer to several of them. In particular, this paper:
Summarizes experiments on rectangular tanks (the previous paper focused on cylindrical tanks;
Confirms the assumption that the roll MOI of fluid in cylindrical tanks is a greater percentage of the solid equivalent MOI for smaller diameter tanks;
Gives additional data on the effect of fluid viscosity on MOI;
Evaluates the effect of baffles within the tank;
Answers the question whether the effective MOI is a function of the rate of angular acceleration of the tank. This is particularly important for satellites that turn very slowly.
The experiments we have conducted recently indicate that the relationship between tank geometry and fluid moment of inertia is more complex than we originally surmised. For example, it appears that the rate of oscillation and the size of the tank have a significant effect on the results of our experiments. The conclusions of the previous paper – that moment of inertia increased with aspect ratio and viscosity – are still valid, but additional variables must also be taken into account.


SKU: Paper3006 Category: