fuel velocity vs exhaust velocity difference?

liddojoe

hi all,

i've been confused about what the difference is between fuel velocity and exhaust velocity. apparently, fuel velocity is equal to rocket velocity minus exhaust velocity. attached are screenshots of the textbook reading that confused me. if anyone can explain this section to me, i would greatly appreciate it!

thanks!
joe

Attached Thumbnails

     

tiny-tim

hi joe! welcome to pf!
fuel velocity, v_fuel, is the velocity of the fuel (relative to earth)

exhaust velocity, v_ex, is the velocity of the fuel relative to the rocket

v_ex is constant (because the rocket engine has constant power)