On Sat, 18 Aug 2018, Jeff Findley wrote:
says...

A description of the Parker Solar Probe take off is included in
http://www.planetary.org/blogs/jason...e-preview.html

In the web page it metions that upon ignition, the rocket burps out a
bunch of hydrogen. What does that do? What's the purpose of it?

I'm pretty sure it has to do with the engine start sequence so that
you don't get a "hard start" (i.e. "boom"). That's why every Delta
IV has insulation that catches on fire. And why every launch causes
the uninformed to say "the rocket caught on fire!".

Yes, it's part of the engine start sequence.
The exterior insulation is burned? Does the insulation
actually insulate anything or is it just there to be burned?

Why does a boom need to be prevented?
The article mentions how the flame gets sucked inside.
All of this defies my physics intuition.
Would you explain how this works?