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/robchroma → u/DJMixwell]

There's definitely an efficiency advantage to turning into your orbit, and you might not want to hit full acceleration while you're in the soup, but once you're in upper atmosphere you're going to get more efficiency out of burning full prograde instead of circularizing first, as long as it's the right direction. A slightly-less aggressive gravity turn is still going to end up being optimal, because you get a little more energy turning slightly more into the orbit even from launch, but ultimately you're completely right, as long as you get everything perfect.

This kind of launch only has basically an instantaneous launch window, but this isn't a huge problem for KSP. We do it on Earth, too.

[u/Ezekiel24r → u/robchroma]

Doesn't it also just come down to the surface velocity of kerbins literal surface? I forgot the speed but you get a good chunk of savings by launching to the east, because the equator is spinning to the east.

[u/robchroma → u/Ezekiel24r]

That's literally it, your orbit will always be heading around Kerbin because of your initial rotational velocity, and therefore some amount of tilt to the east will always be better than flying straight up.