In MS Project can a summary task be a fixed duration and auto scheduled?

Question

I have a number of summary tasks that I want to allocate a fixed amount of time to while still allowing them to be auto scheduled based on other dependencies.

Currently there are two modes:
1) If I set the summary task to auto scheduled, it will expand and contract to it's sub tasks.
2) If I set it to manually scheduled it will stay to a set duration but it won't move with it's dependencies as they expand and contract.

Question
Both of these modes make sense to me but I would like a hybrid where I can define a fixed duration but then allow it to move with it's dependencies. Is this possible?

More Background
One use case is I know how long a phase is but I want to be able to enter individual tasks without worrying about them perfectly aligning to the phase duration. Similar to scrum.

Currently I add a general task at the top of the list that sets it to the duration but this seems like a hack.

Answer 1

Create a meta-task that describes the minimum duration of the phase

I would create this as the first sub-task of the summary.

This expands the summary task to the planned duration.

When you change your sub-tasks the summary will expand further.

Also, by using a constraint on your summary task, you can ensure you don't go over your planned deadline:

Answer 2

The duration of a summary task will expand and contract according to the duration of its subordinate tasks. If they grow or shrink then so will the summary task. A summary task is not a task in itself, it is just a "rolled up" timeline of all its subordinate tasks so fixing its duration has no practical meaning.

It is the underlying tasks that you need to make fixed duration- that will freeze the duration of both them and the summary task.

This is irrespective of manual or automatic scheduling which only describes when tasks will start according to dependencies and other factors.