Preheat oven to 350°F.

In a large bowl, cream butter and sugar until light and fluffy. Beat in the eggs, orange peel, and vanilla.

In a separate bowl, combine the flour, baking powder, cream of tartar, baking soda, and salt; add to creamed mixture alternately with sour cream. Fold in rhubarb and walnuts.

Transfer to two loaf pans coated with cooking spray and bake for 40-45 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes before removing from pans to wire racks.