Imagine speccing that "wind tunnel".
We need you to build a vacuum chamber for us.
Big enough to put a satellite in.
And then, and you're going to love this, we need you to turn it into a wind tunnel.
We need you to make it a really powerful wind tunnel too. One to simulate 7.8km/s."
"Back up. A wind tunnel in a vacuum?"
Yeah. Well, that's okay because we both know you engineers can't make a pure vacuum. We need space levels of vacuum. And we need it to move really fast.
"Hey! We can make a vacuum! But you want your vacuum to move really fast? There's nothing there to move."
We know. That's why you'll need a really big fan. That nothing has to do 7.8km/s. Then we're going to compress it, ionise it, and see if it goes forwards.
"You want to compress high speed nothing, fiddle with it, and see if it will give you forward thrust into a 7.8km/s headwind of nothing?"
It's at this point half the engineers quit to go become landscape gardeners and the other half get a slightly crazed look in their eyes and start muttering into their beards.