William Elliot schrieb:
Planetary Society Blog

Parker Solar Probe preview: 10 hot facts about NASA's cool mission
to the Sun

http://www.planetary.org/blogs/jason...e-preview.html

This weekend, NASA’s Parker Solar Probe spacecraft leaves Earth on
a mission to touch the Sun.

What's the reason for the hydrogen burp upon take off?

Some heat will be transferred to the hydrogen storage aboard the
rocket. Without countermeasures, this would lead to an increase
of pressure above the liquid hydrogen, eventually resulting in
the tank's structural failure.

Bleeding off some the gaseous hydrogen will lead to evaporation
of some liquid hydrogen, thus cooling the tank and keeping the
pressure low.