Vertical staff spacing between song parts

[brucep16...@outlook.com]

Greetings,

I'm engraving an instrumental that has three parts; each part is two 
lines long.  How can I arrange them on the page so that there is 
vertical space between the parts.

Here is a tiny example:

   \version "2.18.2" \language "english"

   \score {
         \new Staff
         {
           a1^"part 1"  a a a \break
           a1 a a a \break
           a1^"part 2"  a a a \break
           a1 a a a \break
           a1^"part 3"  a a a
           a1 a a a \break
         }
   }

Is there a way to create vertical space between the parts?

Thanks in advance for your assistance.

BruceP