Re: tank slosh baffle comment

On Tue, 4 May 2004 [email protected] wrote:
>      From my experience with ballast tanks on a ship, where sloshing produces 
> undersirable rolling movement of the entire ship: Seems like four vertical 
> partition baffles would work well. Looking down from the top of the tank, they
> would be arranged in the shape of the pound sign,#.

The potential problem with compartmentalizing the tanks is that you can
then get sloshing within the compartments.  (Well, that plus the fact that
partitions are heavy.)

And non-circular shapes complicate the analysis, because higher-order
slosh modes become significant:  in a cylinder, the mass moving around in
the second slosh mode is only 3% of that in the first mode, so everything
but the first mode (simple side-to-side sloshing) can be ignored... but in
a quarter-cylinder, second-mode slosh mass is 43% of the first-mode mass. 
Similarly, multiple cylinders with open connecting plumbing can have quite
complex slosh behavior, as MSFC found out to its dismay on the Saturn I.

> They would prevent the bulk 
> of the propellant from moving to one side of the tank and thus allowing only 
> a short Center of Gravity shift.   Rings would only slow the shifting motion
> of the propellant, not preventing it as much as vertical baffles...

Control problems from the oscillating mass are usually more important than
the actual movement of the center of mass, given that rocket tanks are
typically tall cylinders, in which only a fraction of the liquid mass is
involved in sloshing unless the tank is nearly empty.  So damping of the
motion -- as supplied by rings -- is usually the key requirement.  (The
wiping action as the propellant surface sweeps up and down the wall does
not contribute much damping with a smooth wall.)

>      Obviously, the partitions would make the tank stronger and it would 
> become possible to design the tank to do double duty, 1] a storage tank and
> 2] a monocoque framework for the rocket.

Any pressurized tank, *especially* one pressurized to the point needed for
pressure-fed engines, typically makes excellent structure with little or
no added stiffening.  (Witness the classical Atlas, whose tanks are just
sheet-metal balloons, with essentially no strength of their own.)

                                                          Henry Spencer
                                                       [email protected]