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(Going higher gets one going too fast while the atmosphere is still too thick which causes problems with trying to steer. In KSP I find a thrust to weight ratio of about 2 (on the pad) to be ideal, using a less powerful engine makes for a more expensive rocket. I have played with the numbers in Kerbal Space Program, though, and I've seen going up almost straight costing about 10% more velocity (and remember that the rocket equation is exponential-that's a lot more than 10% more fuel.) I suspect it's worse in the real world as KSP rocket engines are cheap and thus KSP rockets tend to be a lot more powerful than real world engines and thus suffer less gravity loss. I do not know how big an effect it has on Earth. With a properly designed track you can boost to your desired velocity without even lighting your rocket.) It's riding on rails as it builds velocity, thus you are not expending any energy countering gravity. (The reason I say it can be avoided on airless bodies is that if you build a big enough base you can launch your spacecraft with a maglev train.
![kerbal space program escape trajectory kerbal space program escape trajectory](https://i.imgur.com/oqbExta.jpg)
This ends up being a path that goes almost horizontal while still in the outer edges of the atmosphere. In practice you pick a path that minimizes the sum of gravity loss and drag loss. 0 - Release - KSP 1 The Transfer Window Planner is a plugin for Kerbal Space Program that brings some functionality from some great web apps in-game It can be expressed as the sum of you parking orbit velocity and v applied during the escape burn It can be expressed as the sum of you parking orbit velocity and v applied during the escape burn. Doing this too soon on a body with an atmosphere will cause too much loss due to atmospheric drag, though. Thus, for the purposes of minimizing gravity loss you want to get as close to horizontal as you can (consistent with not falling down) as soon as you can. (If your orbit is elliptical you keep trading back and forth between speed and height but it averages out the same.) This does not magically switch on as you reach orbital velocity, any horizontal velocity produces some effect. Now, consider what orbit is: For a circular orbit you are moving fast enough that your path climbs as fast as gravity pulls you down. While there is no way to get around this on a body with an atmosphere it's energy expended for no progress, you want to minimize it. That kind of dV takes quite a bit of mass, and launching such an upper stage to an escape trajectory would be very expensive early in the game. So stopping would require an additional 3000+ m/s of dV. As you climb into the sky on your tail of fire some of your energy is going to simply holding your rocket in the sky. On an escape trajectory anywhere near Kerbin, your rocket will still have an upward velocity on the order of 3000 m/s. Yet another question where the answer becomes obvious if you play Kerbal Space Program.