My technique (fully upgraded) was to go upwards on full rocket power at about 40° until the rocket ran out of fuel, and then go into a glide. I used an angle so that speed and height would converge at about zero at the same time. 6000ft was easy. I was still 60ft up by the end, and the fade-to-white...
Now that is a fun game! Doesn't take years to complete, has objectives and isn't just like clicking everywhere until something happens like the other game!
Completed it after 26 days.
Well it's kind of easy actually, all you need is to keep constant and keep at high altitude with a low level of resistance. I achieved the 6'000 feet achievement easily, could do much farther. (on my first try I thought I was supposed to go 6'000 feet flying upward, but after failing hard I just realized I was supposed to go forward lol).
i also disccovered an easy way to get the altidue and speed achievement at the same time.just go straight up as high as u can(also use the rockets on this up goin) then just go straight down and you can reach amazing speeds
if u do the code right the safe will open by itself.. and if u have got the wrong code then better reload the game in a new tab/window and press continue to re-enter the code. If it says for example 12L at the start you will place the cursor on the left side of the wheel and press until you reach the number 12 (Doesn't matter how many times you press it, it matters what number is pointing at). Hope I've helped.