🚀 go-pugleaf

I have a question about gas dynamics that people in that area may find
simple.  I hope.  I need an answer for a "back-of-the-envelope" calculation,
which I must try to solve as quickly as possible, so while I always 
appreciate being referred to textbooks, I will not be able to use them in
this case.  I therefore ask for answers of another sort.

I have a nozzle in which gases escape at known pressure and velocity.
It's the jet of a spacecraft, so the gases escape into a vacuum.  I
want a first approximation of the dynamic pressure of the escaped gases
at a known distance downstream of the nozzle, along the primary line of
action of the jet.  In other words, given p_0 and v_0, what's p_x if
x is known and along a straight line and the medium is a vacuum?  Assume
x is short enough that each molecule of gas is still moving in a straight
line, and that the profile of the escaping gases is conical.

Thanks in advance,

RG

Who became an engineer thinking "free-body diagrams" might be interesting.