No, an.
The pronunciation of the letter R is very similar to the word "are" which begins with a vowel sound, and when a word begins wih a vowel sound, you use an.
But RD is an abbreviation of Rainbow Dash, thus negating the use of "an"
Well, didn't get a gift card to the fabric store, but I gots the money. It'll be a while before anything comes of it, but it will happen!