Subject: How to build a yoyo with a maximum return height

As a school project, we were asked to design and build a yoyo that would have a 
maximum return height once dropped.  The yoyo will be attached to a 10m string, 
and released from a given height, and once it reaches the bottom it must return 
as high as possible, without any added tugs from the person dropping the yoyo.  
I would just like to know if there are tips of things to look out for when 
building this yoyo.  Also, I have been unable to determine the maximum amount 
of tension that the string can take before breaking.  This information would be 
useful for determining how much mass my yoyo can have before it actually breaks 
the string.  The reason for this complication is that I would probably need to 
take into account the velocity of the yoyo at the bottom of the string, and the 
force that it will apply at the very bottom.  Unfortunately, I am not sure how 
to do this, or what equation to use; any help would be greatly appreciated.

PS. We are not allowed the use of any springs, or other things to store 
potential energy.  We must stick to the basic yoyo and string design.

Re: How to build a yoyo with a maximum return height