Security of key schedule that only XORs a key with constants Suppose that:


$MK \in \{0, 1\}^{n}$ and the main key of a block cipher.
$RK_{r} \in \{0, 1\}^{n}$ and is the $r$th round key.
$RC_{r} \in \{0, 1\}^{n}$ and is the $r$th round constant.
$RK_{r} = MK \oplus RC_{r}$


What's the security of this key schedule. I'm imagining that it is not very strong.
This key schedule is Totally Linear. If two Master Keys $MK$1 and $MK$2 have a difference of d, all the round keys will have difference of d with probability of 1. This makes the cipher vulnerable to Related key Attack. But its hard to workout and comment without the knowledge of the cipher description that how badly such key-schedule effects the security of cipher 

The PRINCE – A Low-latency Block Cipher for Pervasive Computing Applications have very simple key-schedule, and its designers say 


  for our cipher it holds that decryption for one key corresponds to
  encryption with a related  key.  This  property  we  refer  to  as
  α-reflection


For more information about key-schedule requirements see "What are the requirements of a key schedule?"