Best design of a spreadsheet for layering of four different projects into one projection timeline

I am in the process of building an Excel spreadsheet to project cash flow for four distinct projects that will make up one cash flow projection. The product to be delivered for each project is the same, but the number of units of product delivered to each project each week and the total number of units delivered to each project will differ.

Each project has a different start-date (as yet unknown) and the units of product need to be ordered with a six-week lead time. Each project has a different duration. When projects occur simulaneously we will need to smooth production as best as possible to keep deliveries to each project steady.

Has anyone designed a similar spreadsheet who might have some helpful hints on how to optimally design this, or a good formula or two that you found particularly helpful? Do I build four different projections and then aggregate them or is there a better way to do this, perhaps by arranging everything in a lookup-table format? 

I'm just hoping to get some ideas from anyone who might have taken on this sort of project in the past, to see ways in which I might approach it.