William Elliot wrote on Sat, 18 Aug 2018 22:38:40
-0700:

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?


If it wasn't insulating something I suspect they wouldn't call it
'insulation'.


Why does a boom need to be prevented?


Because booms lead to little bits of rocket scattered about the
landscape.


The article mentions how the flame gets sucked inside.
All of this defies my physics intuition.


But then, I've come to the conclusion long ago that your 'physics
intuition' sort of sucks.


Would you explain how this works?


What's the velocity of the flame front when compared to the outflow
velocity before ignition?


--
"Some people get lost in thought because it's such unfamiliar
territory."
--G. Behn