A free program for designing your own rocket is called Open Rocket.
Here is a good link that talks about the program:
software source:
http://openrocket.sourceforge.net/download.html
Supporting documentation:
http://openrocket.sourceforge.net/techdoc.pdf
http://openrocket.sourceforge.net/thesis.pdf
Rocket plans that can be viewed with Open Rocket of RocSim:
Rocket plans database:
http://www.rocketreviews.com/openrocket-design-library.html
RocSim plans database: