Transfering to Mun: First to LKO or Directly to Mun?

[bewing]

Well, if we're going to necro this thread anyway, @arkie87 is right and all the gravity turn boosters are wrong. Every bit of the supposed gravity turn dV advantage gets burned up in Kerbin's atmosphere from excess drag (from the much longer airpath), and a necessarily inefficient (< 100% thrust) use of the engines during ascent to LKO.

[Plusck]

  On 5/11/2016 at 9:22 AM, bewing said:

Well, if we're going to necro this thread anyway, @arkie87 is right and all the gravity turn boosters are wrong. Every bit of the supposed gravity turn dV advantage gets burned up in Kerbin's atmosphere from excess drag (from the much longer airpath), and a necessarily inefficient (< 100% thrust) use of the engines during ascent to LKO.

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Oh no, not this again.

It has been conclusively proved, with maths and by experiment, that the difference between vertical ascent and an orbital approach only just evens out with a TWR approaching 10 and a target altitude of the Mun or higher.

The main reason it evens out is because you cannot use a TWR of 10 going sideways through the atmosphere or you'll burn up. If Kerbin's atmosphere were thinner, you'd need an even higher TWR to break even with an orbital approach. With an atmosphere, you break even with a TWR of around 10 and a target orbit at or around the Mun's altitude. With no atmosphere, this obviously tends towards zero difference between the two methods as TWR rises to infinity and as required burn out velocity (V_bo) rises to infinity. With a lower TWR, however, it rapidly becomes impossible to break even, and the orbital approach will always be better for any target altitude.

To quote Iain M. Banks in The Algebraist:

• Seer Taak: "How do you navigate this thing?"
• Dweller 1: "Point."
• Seer Taak: "Point?"
• Dweller 1: "Get to the general volume and then point in the right direction."
  • Dweller 2: "Secret is plenty of power."
• Dweller 1: "Delicate finessing of delta-V is sign you haven't really got enough power."
• [...]
• Dweller 1: "Though sometimes you have to sort of allow a bit for deflection."
  • Dweller 2: "That's a technical term."

This is objectively true: with enough power, pointing and pushing is simpler and costless. Allowing for a bit of deflection sometimes.

The problem is that getting that power is expensive: show me a rocket with a high enough TWR to make this method worthwhile, and I or any number of people here can show you a cheaper rocket that can do the same thing using the orbital method, drag losses notwithstanding.

[mrclucks]

Gravity turn where you just keep burning to transfer to the mun is more efficient.  Once you get high enough, the oberth effect starts weighing more than the atmospheric drag(somewhere between 60km and 45km if I were a betting man)

[awfulhumanbeing]

I always do LKO first. It's an optimal position for waiting for phase angles (no hurry), and if you miss your transfer burn, you can do 1 more orbit and try it again. No need to say, multiple transfer passes are impossible without circularizing.