The Orange Box

To unlock Terminal Velocity, you must create 2 portals directly above and directly below you. These portals must be directly vertical to each other, or you can fall out of your constant fall. which will prevent you from getting the achievement. To make sure that they are perfectly vertical to each other, aim as high as you can, and fire a portal on the ceiling. Now, look directly down, not turning side to side at all, firing the color of portal that you didn't fire at the ceiling below your feet, as low as the game will let you. Once again, make sure that you do not turn left or right while transitioning from the ceiling portal to the portal that is below you. Once you fire the portal below you, you should be in a constant fall. Leave the game sitting there, and wait for about 2 minutes. The achievement should unlock. Before you leave your fall, I suggest you press pause and look at the achievement menu to make sure that you unlocked the achievement before you leave your constant fall as well.

Just find a room where you can place a portal directly above and below you. Place them, and come back 2-3 minutes later and you should have it so as long as you didn't touch the floor when going through the Portals.

Load Chapter 5, Testchamber 13.
Walk straight and look into the first far right corner.
Aim up. Place a portal.
Aim down. Place the opposite colored portal.
Slowly walk straight into the portal that is on the floor.

Make sure that your joystick doesnt move you around while doing this though or you may not fall straight and you will have to line it up again.

I had no issues and obtained this on my first try.

If doing this as soon as you get to this point in the story. it should take around 6 minutes to fall the full 30,000 feet.

Here is a video of myself on Testchamber 13

Possibly the hardest part of this achievement is trying to put the portals in the right spots. The best way to put them exactly opposite each other is to go to a corner of a room. Shoot a portal directly in the top corner, where the walls and roof join. It won't go exactly in the spot you pointed it at, so don't worry about that. Then do the same thing for the bottom corner. They'll be exactly above and bellow each other, now jump in.

Had fun trying to do this one. My solution was to shoot the ceiling above me then the floor below and shoot the floor again as i fell to make it directly below me. this took a few tries but eventually worked out. i did this in a lower ceiling room, no idea if that makes a difference. think it was about 3 dizzying minutes later and I got it.