Coming Out of the Fog: Getting Clarity on NGSS Unit Structure

I have had the opportunity to meet several members of the NGSS writing team at the NSTA conference.   So, you know me, I started asking them with questions.  

1. How were the topic pages organized (this has preoccupied me for months)?  The most logical response has been that through the process of development, the team joined the PEs that would hang together under a theme.  
2. Were specific culminating events/performances in  mind when they organized the PEs?  No, they were specifically told not to do that.  The committee did not want to appear to dictate curriculum.
3. I asked about my proposed structure of using the topic pages as the basis for units.  The major concerns were student endurance and force fitting.  Looking at some of the unit lengths, I can see the endurance issue.  The grade 3 "Forces and Interactions" topic pages is going to be a long unit.   
4. This particular page  brings me to something that I have been feeling lately as I watch the direction of some of the unit outlines my team is developing.  So, here is the revised plan.  If it fits and is logical, write it as one unit (see "Bee and Engineer").  If it feels forced, make it one storyline but with "sub-units" with specific performance assessments.  

My current curriculum has an example of this idea.  Last year, we re-developed a unit that had some serious implementation issues.  It covered Newton's Laws and for unknown reasons also included light (reflection, refraction).  The masterful writing team created a unit scenario focused on the Orion Deep Space System.  Students built straw rockets to demonstrate their understanding of Newtons Laws.  Part 2 of the unit, challenged students to create a "visual docking system" for the crew module (aka Periscope).   See image below.  The astronaut sits in the position shown but has to see out the window.  Build a device that will allow the astronaut to see out the window in order to dock if the ISS.