Oh! Oh! Talking about pressure;
While considering the meaning of life and other important things, such as how to get the lint out of ones navel, a thought arose. If thrust outside of ground effect is less than thrust in ground effect then thrust in the vacuum of space must be even less.
A quick search for the answer uncovered the following statement. ~
" a space shuttle main engine can produce 1.67 million N of force during liftoff and 2.1 million N of thrust in a vacuum. " 
Oh sh:t. Back to the navel.