Launch @ <200m/s?

[Anizer]

I'm seeing this everywhere; many people say that when you're leaving the Kerbin atmosphere, keep your velocity around 200m/s or less. Why? I've been keeping my throttle pretty high the entire time and I get into orbit perfectly fine. Has someone calculated that 200m/s is most efficient? (in terms of fuel and time)

[blizzy78]

I think the keyword here is terminal velocity.

[Mr Shifty]

The most efficient atmospheric flight speed is terminal velocity due to how KSP handles atmospheric drag:

http://forum.kerbalspaceprogram.com/showthread.php/6664-Mini-challenge-max-altitude-with-this-supplied-spacecraft?p=100912&viewfull=1#post100912

Terminal velocity at 7000 meters, which is about where most people start their gravity turn, is around 200 m/s, so the advice is to keep speed below 200 m/s before gravity turn. There's a table of terminal velocity vs. various altitudes in the wiki here:

http://wiki.kerbalspaceprogram.com/wiki/Kerbin#Atmosphere

[maccollo]

You should try to keep your speed below terminal velocity for any given altitude. The ksp wiki has a list of these. http://wiki.kerbalspaceprogram.com/wiki/Kerbin

If you go faster the atmosphere will slow you down a lot faster. So you will waste a lot of fuel. If you go much slower than terminal speed you will waste fuel fighting gravity instead.

Basically, when you launch you should try to reach terminal speed as fast as possible, then slowly accelerate as you climb until you start your gravity turn at around 10 000 meters. At that point you can usually go full throttle and just focus on your assent profile.

[Scarecrow88]

You'll get in to orbit no probs using full thrust, but it's not the most fuel efficient way of doing it if you're exceeding 200m/s early in the flight. Your using a load of fuel to overcome a shed load of drag created by the thickest part of the atmosphere at low altitudes.