It uses less dV to circuluarize at higher orbits beyond a certain point. Why?

[Cunjo Carl]

@Nich  Absolutely! And I have additional reading if desired . Gravitationally the cheapest launch to orbit the Mun is a huge blast of 3000ish m/s right from the surface of Kerbin, then burning to enter the orbit of the mun just off its surface. Along these lines, I've heard that in real life it's actually easier to enter orbit around the Moon than it is to enter geostationary orbit around the Earth... Let's see Scott Manley says it in this video or one near it. I'll try to find the timestamp later: www.youtube.com/watch?v=_uMTMGQDUoc .  I'll be honest that I haven't run the numbers for this in the Kerbal universe, but I do something similar in the Realtime Career Mode Speedrunto save deltaV and time on a trip around the Kerbin system.

   That said, there's a few reasons no one normally does this, the first being that there's this darn thing called the atmosphere in the way! If you try to go 3000m/s in the middle atmosphere, it burns you to a crisp. Some people do this anyways, and I think it's fun watching: Fastest time to space and return Challenge. Also, it's hard to aim properly at your target when you're at full burn in atmosphere, it's much easier to do efficient aiming from orbit. Finally, drag slows you down, but for most ships (surprisingly) it's not as big a deal as the gravity! So despite the burning, tough aiming and atmospheric drag, high thrust from within the atmosphere is the most deltaV efficient way to reach a target in space.

Just to be clear, it's more deltaV efficient, but it's definitely not the cheapest! The cheapest launchers have very few engines and very slow lifts. With fewer engines, you haul up more cargo and less... well, engine! Here's a bunch of very slow lifters dedicated to saving your money, which is often the more important goal:1.1 Economy challenge

Typically no one would do this for the mun, but for a target like Jool, it's difficult but doable to save a tiny bit of deltaV by performing your interplanetary burn at around 30km altitude on Kerbin, just above the super burny bit of the atmosphere. By my experience it saves only a few percent, and it's a lot of extra hassle! But still, it does what you're considering it should do, which is pretty cool!

[Callmedave]

Stand up and throw a rock, how far does it go? a few meters?

Now stand up a ladder and throw it it goes further yeah?

now stand on the ladder but throw it as far as it went the first time, you wont need to throw it as hard to reach the same distance.

That's the simplest way i can explain it. - Hope it helps.

[The_Rocketeer]

Not bad @Callmedave, but if I can borrow your analogy, the difference between burning at apopasis (more circular) and burning at periapsis (more elliptical) is the same as the difference between how many meters up you can throw a rock vs how many meters across you can throw the same rock.

[Waz]

On 5/27/2016 at 7:36 PM, qromodynmc said:

Did you calculated difference between needed deltav for 70km and 80km?

You're misreading the 80Mm. Don't worry, I did too at first.

[Kerbart]

Apparently the most expensive Kerbin (circular) orbit (in terms of DV, coming from a 70km orbit) is 9840 km  (1231 m/s). Once you go beyond that, DV after circularizing goes down. I'm sure someone can conjure up the math to show why it's 9840. Not me; it's late and I want to go to sleep. On a side note, 2462 m/s DV must be all you guaranteed need to go from a 70km orbit to any circular orbit around Kerbin, and back to 70km.