Project not available in sales orders

Project not available in sales orders

Hi,
Could you tell me why projects are not available in Sales Orders?
As a work around I start by using estimates, but the project do not follow in the Sales orders, wich is a problem to follow all invoices on a project.
If I use estimates, I cannot partially bill my estimate. When I try, I have a message saying the estimate has been invoiced even if I deleted line items in the invoice.
What can I do?
Thanks
Marie-Andrée