Using of delete obsolete in Oracle 11gR2 RMAN on Linux
Tell RMAN what you want/need to keep and let it do the worrying. 

CONFIGURE RETENTION POLICY TO REDUNDANCY 2 ; 


That will keep two copies of your database and consider anything before that as obsolete. 

I am a little concerned that you say you're only taking "full backups".  

Are you backing up the ControlFiles?  The ArchiveLogs?
Without the former, you may have problems restoring a database after changing its structure (removing or even adding DataFiles) and without the latter, you lose any ability to perform a Point-in-Time Recovery.