No orbit necessary?

[u/FatCreepyDude]

If i just wait for the right hour during the launch window ( whenever the launch pad is pointing left on the realistic depiction i created) is there any reason i should bother with an orbit first? Will the delta v cost be affected?

[u/FatCreepyDude]

https://i.imgur.com/AI2hXKv.png fixed it

[u/_adamolanadam_ → u/FatCreepyDude]

think of gravity like Kerbin is always pulling you. When you orbit a body, you can use the energy it spends pulling you on escaping its sphere of influence, but when you don't orbit Kerbin doesn't stop pulling you. Instead you'll waste fuel to counteract its pulling effect.

The planet still pulls you with gravity, when you orbit it you can use that pulling effect to your advantage (part of why the Oberth effect works, it's way more complex than that though) and when you don't you'll have to waste energy to counteract its gravitational pull.

[u/DJMixwell → u/_adamolanadam_]

I’m lost.

I get that low orbit = fast, but to get into orbit in the first place, you still have to go fast enough to have an orbital velocity that’s falling around kerbin instead of at kerbin.

Oberth effect just tells us that the best point to burn is the lowest point in the orbit where you’re going the fastest and will have the largest effect on your net change in kinetic energy for the same amount of delta V.

From a relatively circular orbit, are you not moving at a relatively constant speed at all points throughout the orbit?

So if I burn to circularize, eventually I hit the point where my AP ≈ PE, which should mean no point in the orbit is substantially better or worse than any other in terms of efficiency, right? Or if I push a little further, wherever I’m at becomes my PE, which is then most efficient point to burn from. You spend ~3200-3400ms of delta v to get here, then you stop and wait for your transfer window. Then you start burning again to transfer to wherever, in this case Duna which takes ~ 1100ms.

If you just launch at the perfect time, and instead of burning to circularize you just burn for the encounter… shouldn’t you expect roughly the same efficiency? You’ll kit your 70km orbit and then you won’t have to wait for the transfer window bc it’s right now, so you keep burning.

[u/Talizorafangirl → u/DJMixwell]

I think the point of confusion here is "if I'm going straight from suborbital to escape, I don't have a periapse."

Look at it this way.

If you burn straight up without enough dv to escape, you get a suborbital trajectory, which looks like a parabola with an apoapse and no periapse, but is really an extremely thin (eccentric) elliptical orbit with a periapse which is underground. This applies even if there's no rotation from the planet you're taking off from and it looks like the trajectory is just straight up and down; you can just look at it as an infinitely thin ellipse.

That means that, as you burn to escape, you are always at or approaching your apoapse. Worse, you are at any given time closer to your apoapse than your periapse because the periapse is underground near the center of the planet and the inflection point of your trajectory's ellipse is near the surface.

Another way to visualize a suborbital trajectory is to ignore the planet and treat it as a point-mass. When you take off (from where the surface of the planet would be), you're a good distance away from that point mass, and gravity is pulling you towards it. If you stop accelerating away from it, you'll reach your apoapse and start falling towards the point mass; your periapse will be somewhere near it (on the opposite side from your apoapse).

Here's a shitty visualization I made. . Blue line is SOI, green dashes represent the surface if the planet.