Ubuntu Servers :: Su As Backup User Inside Of Bash Script - Lock And Flush Tables On A Mysql Db

I am attempting to write a backup script that will do the following:

1) lock and flush tables on a mysql db
2) dump the db to a file
3) unlock the tables
4) rsync the file to offsite storage

It all seems to be going well. However, obviously I don't want to setup ssh to the storage server on another network as the root user without a password. so I am attempting to su as the backup user inside of the script but when I try to run the script everything happens as it should until I try to so.. then it jumps out of the script .. akss me to login as the backup user.. proceeds to rsync to the offsite storage it does all this and then resumes execiting the script. it is not going to be setup as a cron job. it will be executed manually. assuming that is the case, how can I get the script to run without prompting for a password?

Here is what I've come up with so far... assuming that the script is run as root and the identity of the backup user will need to be assumed inside the script without perstering the user to enter the backup user's password.

[Code]....