Keep Coding

I have an app that performs some complex numerical analysis and presents information about this in 3d. there are additional 2d screens that show information about the result of this analysis.
the way the app is currently implemented is through a single activity that has several fragments that it rotates through. each fragment is full-screen, and never really meant to be used in the modal way that the guides talk about.
is this an acceptable/standard way to handle the situation, or should i have separate activities for these separate screens?

I believe in terms of efficiency having a single activity to your application is the most efficient way to go.
Big warnings here:
Having said that, having only 1 activity is extremely easy to overlook one tiny thing and the app go "down" since your activity will live longer.eg .memory leak, passing an activity argument to a widget so on so on. Free memory, recycle destroy objects you don't need anymore
http://android-developers.blogspot.com/2011/03/memory-analysis-for-android.html
http://android-developers.blogspot.co.uk/2009/01/avoiding-memory-leaks.html
The other warning is that your code will be really messy if you don't structure it really careful. Things can go really bad there.
I've been down that road, came across the challenges and i can say for sure that one activity is at least performance-wise really great.