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Terminal Velocity Achievement with details
By mechalynx
Details on how to get the 'Terminal Velocity' achievement (fall 30k feet) including screenshots and alternate methods.
Things that can will go wrong
Update 2017: This is very easy to get with portal funnel turned on from the options. If you want to do it without that option turned on, read on.

If you've tried this already, you'll know that the problem with getting this isn't air pressure or Gs, it's that it's very hard to keep falling long enough.

You're falling but slight misalignments have to be corrected - you overcorrect and hit the floor, fall's over. Typically, you'd try doing this achievement by making a portal on the ceiling and one right under your feet (usually on test chamber 15). Below are details on how to improve your chances of getting the proper angle and getting the achievement, including possible alternatives to the usual method.

As far as how long you'll have to fall, I timed it (with a wristwatch) at 5 minutes 29,5 seconds. However, I started the timer a moment before making the floor portal (with the aformentioned method), took a few screenshots while falling (slight delay) and checked the achievement screen twice since, because I was taking screeshots while falling, I thought I might have missed the achievement popup (add a couple seconds because of that). So, it should take about 5:20 to 5:25 if you time it accurately (I'm assuming).

By the way, in the options there is a 'portal funnel' option, which makes it easier to aim into floor portals (not when shooting, but when trying to move into them). I think it can make this achievement quite a bit easier to get (I haven't tried using it; enough science for today, I wanna play the game!), but this guide assumes it is turned off and I got it without it, so you don't need it but you can use it if you want to - it'll take off the edge ;) (shut up, I love my puns :P)

{addendum: plottingCreeper has tested portal funnel and it appears it works fine in the main game but has no effect in challenge maps (which makes sense). So, while it does help, if you want to use it you should attempt this on a main story map.

{addendum: MahMoot says the portal funnel works in the advanced chambers now. I tried it and it works on both advanced and challenge maps.
Ceiling to floor method
This obviously means using a portal on the ceiling and on the floor to fall vertically until you get the achievement. You can try doing this but unless you get near-perfect alignment, you're going to have to correct your trajectory, which quickly becomes difficult (the furthest I managed was ~45 seconds and you need to fall for ~5,5 minutes). If you're good at correcting, go ahead. For the rest of us, here's how to get better alignment so you have to correct less or (hopefully) none at all.

You'll need to have the crosshair turned on for this. Pick a spot in the room with enough space from the walls so your portal doesn't get pushed (i.e. your control over the final position of the portal is absolute) - also, make sure you avoid the extended platform from the wall (pretty obvious but meh :P). Look straight up as far as possible and align your crosshair with the tiles. Shoot a portal and try to fit it into the grid like the above screenshot shows (notice the left/right and bottom edges of the portal). After you shoot the first portal on the ceiling don't move - seriously, take your hands off the keys if you have twitchy fingers. This is to make sure the portal's orientation is exact and to make it easier to reproduce in the other portal's orientation.

Now look straight down and align your crosshair like you did with the ceiling's tiles. As far as I can tell, the textures for the ceiling and floor don't align exactly, so don't try to visually project one portal's position onto the other's. Simply make sure you're looking straight down and your crosshair is aligned vertically and horizontally with the tiling (as much as you can tell - perfection is practically impossible here).

Ideally you should get something like this: the portal's top and left/right edges are (almost) exactly right. I got lucky on my first try using this grid alignment approach, but it might take you more - don't sweat it, just try again and you should get it within 3-5 attempts. To make sure, just check it once you start falling (and if you see any mistakes in my assumptions mention it in the comments so I can correct this).

It is possible to get alignment so good that you won't have to correct at all during the fall - that's what happened in the fall you see in the screenshot, which gave me the achievement too. The next section has some details I noticed while trying to get better alignment.
Trajectory biasing
Any non-perfect trajectory (which is practically all of them in our case) while falling, which is due to the relative position and alignment of the portals (unless you moved before placing the second portal), will have some bias to move you towards a direction (on the horizontal plane) and thus ensures you will hit the floor and get angry.

The practical effect of this bias, besides making correcting necessary in the first place, is that movement/corrections in the opposite direction of the bias will be less severe but also more necessary.

When I used the method from the previous section but without aligning to the tiles (just straight up -> portal -> straight down -> portal and weeeeeeeeee...) I noticed that there always was a forward bias (i.e. I had a tendency to slightly move forwards while falling from my initial position and orientation) and that it wasn't exactly in the same direction always (there was a miniscule variation to the left or right, which I had to correct, with disastrous results because even the least of movement was more than the bias, which was by itself guaranteed to make me hit the floor if I didn't correct at all). However, correcting backwards, opposite of the most severe bias, was the safest and it brought me closer to the center than any other movement (movement meaning very slight taps of the 'wasd' keys).

If you don't get perfect alignment (or at least negligible mis-alignment within the necessary time frame - my successful fall had a slight backwards bias) you can try correcting a bit, but remember to favour the slightest of slight taps on the movment keys and remember that movement opposite of the portal's bias will be safer and will take longer before you need to correct again. If you notice a slight bias but are already 2-3 mintues in, I'd advise against correcting. Here's how close my fall was in the last minute (I started out practically in the center, check the previous screenie to see):
I've tried this:

but I didn't leave it long enough to see if I'd get the achievement. However, it seems that it requires no correcting if it starts ok and it's much much easier to execute. Just make sure you fall gently into the edge of the floor portal and you should be fine (unlike objects, the game seems to align the player to the center of the portal as they exit regardless of the 'portal funnel' option, which I have turned off). I just don't know if, with this arrangement, it counts as falling at all, since the portal on the wall is more or less level with the ground - you are falling, but I have no idea how the game counts fall distance when exiting portals (it might be expecting you to reach a proper orientation and in this arrangement, you're always changing orientation as you move through the portals). If you try this and it gives you the achievement, please mention it and, if possible, time it for reference ;)

{addendum to this part: in the challenges, falling vertically does not add steps to your count but doing this does increase your step count, so it's possible it doesn't count as falling}

{update: plottingCreeper has tested the above for at least 15 minutes and hasn't got the achievement - didn't fall out, but no achievement. I'd recommend avoiding this approach unless you are curious, as I am, to find out if it is possible and how long it takes. If you know of other means to find out if the game actually counts this as falling, please mention it in the comments}

You can also try doing the celing to floor method in the corridor that precedes the room in the screenshots (right at the entrance of the test chamber) if you'd like a shorter fall distance. The effect is that it looks like it's going a lot faster (though it should be exactly the same) and, theoretically, alignment errors will become amplified faster (because it's the portals' relative positions that cause bias and you pass through them more frequently will less height), so it might not be a good idea unless you can get good enough alignment that you don't have to correct (of course, that's just my theory). The time to fall should be practically the same (depending on whether and how it counts the distance you fall while entering and exiting portals).

There is a third alternative, in test chamber 18:

{images pending}

The screenshots are from the challenge version (not that it has any differences from the original, but I'm not certain, so I'm mentioning that just in case).

The screenies aren't from the same pair of portals since it's practically impossible to make them properly and get out of the hole. Regardless, this fall is longer than the one that is usually used and might be preferable to those who have already finished the game as it is easily accessible through the bonus maps (it shouldn't prevent getting the achievement, because it's on a challenge version of the map, but I haven't tested it). Again, the fall time should be the same, but some people might prefer it as it can be easier to correct, depending on your preference.
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mechalynx  [author] Mar 21 @ 12:44pm 
ty for the award <3
Sinister_P1g Feb 24 @ 8:16am 
I did it on test chamber 16 and somehow after a couple attempts managed to get a pixel perfect portal alignment
neptunestar999 Dec 13, 2020 @ 10:56am 
i did test chamber 15, and i went into the right corner and turned down my sensitivity to 0.01, so i could get the portal right in the corner. I found it took around 6:03, but thanks for the guide.
GwenTotalDramaIslandGaming Nov 25, 2019 @ 9:24am 
OMG thank you so very much this worked much better than the other guides i tried!
Hellbyte Sep 3, 2018 @ 9:40am 
@mechalynx can you firgure out a way for long jump????
Amigo Sando Jul 10, 2018 @ 9:04am 
there's a MUCH easier method using exactly the same location in these screenshots but not having to do any "aligning" effort of your own ... just fire the portals in the corner of the square space and the edge will literally align it for you and do the same on the same corner on the other end of the vertical plane
I'm honestly kinda surprised no one said anything about this ridiculously easy method
mechalynx  [author] Jul 6, 2017 @ 4:25am 
@MahMoot np and thanks for the update on the portal funnel! For the record it's totally doable without it - I had never tried the funnel and to be honest, it's super easy with it :P but ultimately this is a matter of personal preference. Updated the guide.
MahMoot Jul 6, 2017 @ 2:10am 
Jesuuuuus, I forgot to turn on the portal funnel............. That's why I just kept hitting to side of the portals! Thanks my friend, that alone helped a lot! :)
By the way, the portal funnel works with the advanced chambers!
EphemeralPants Feb 15, 2014 @ 11:51am 
@MechaLynx Well, I can't change your way of life!
mechalynx  [author] Feb 15, 2014 @ 11:41am 
If it works for you better, then it's better. I simply prefer doing more work beforehand so that I need to do less while falling (as mentioned above, I didn't have to correct at all during my fall). If you've tried it, mention the details and I'll add it as an alternative to the guide.