Moho- Apoapsis or Periapsis?

[RocketPilot573]

Hello, I'm planing a mission to Moho, but there's a problem: When transferring to Moho, which is more fuel efficient, encountering it at it's Apoapsis or Periapsis??? Based on research I have done, there seems to be conflicting answers, some say Apoapsis, while others say Periapsis. Also, how much Delta V will an optimal round trip to there and back cost (excluding the Delta V to land and return to orbit)?

Thanks in advance.

Edited August 12, 2013 by RocketPilot573

[Mr Shifty]

For a Hohmann transfer, here are the considerations:

1) For a periapsis rendezvous, your transfer ellipse will be smaller, and so it will take more delta-V to lower your orbit from Kerbin's heliocentric altitude to Moho's.

2) For an apoapsis rendezvous, you're meeting Moho when its speed is slowest, but your orbital speed is highest, so it will take more delta-V to get captured.

3) There could be a significant inclination difference between your orbit and Moho's.

In reality Hohmann transfers to Moho are very difficult because of its high eccentricity and inclination relative to Kerbin. You're better off using a Lambert calculator to plot your intercept trajectory. The best one out there is here:

http://alexmoon.github.io/ksp/

Using that, you can see that there's a launch window on year 1, day 67, that can get you from 100 km LKO to 100 km LMO for under 5000 m/s.

[painking]

Encounters at Apoapsis. Moho is terribly close to the sun, and hitting it at periapsis means you're going at it as fast as anyone ever needs to be in this game, resulting in a massive deceleration burn. My first trip to Moho resulted in an 8K dV burn to get into any kind of orbit around it, let alone something at a reasonably low altitude and inclination.

[RocketPilot573]

For a Hohmann transfer, here are the considerations:

1) For a periapsis rendezvous, your transfer ellipse will be smaller, and so it will take more delta-V to lower your orbit from Kerbin's heliocentric altitude to Moho's.

2) For an apoapsis rendezvous, you're meeting Moho when its speed is slowest, but your orbital speed is highest, so it will take more delta-V to get captured.

3) There could be a significant inclination difference between your orbit and Moho's.

In reality Hohmann transfers to Moho are very difficult because of its high eccentricity and inclination relative to Kerbin. You're better off using a Lambert calculator to plot your intercept trajectory. The best one out there is here:

http://alexmoon.github.io/ksp/

Using that, you can see that there's a launch window on year 1, day 67, that can get you from 100 km LKO to 100 km LMO for under 5000 m/s.

Thanks! That's a really good calculator, how have I not come across it before!? This has brought down my initial estimation of Delta V from an impossible 20,000 m/s to only 7,500! Looks like a transfer window will open up in about 60 days for me.

Edited August 12, 2013 by RocketPilot573

[maccollo]

Encounters at Apoapsis. Moho is terribly close to the sun, and hitting it at periapsis means you're going at it as fast as anyone ever needs to be in this game, resulting in a massive deceleration burn. My first trip to Moho resulted in an 8K dV burn to get into any kind of orbit around it, let alone something at a reasonably low altitude and inclination.

Completely wrong. You want to burn as close to the sun as possible to exploit it's oberth effect. You will get a substantial delta-v penalty for encountering Moho at it's apoptosis. You can think of it like this: Ideally, when would you want to burn from Moho to Kirbin? The answer is when Moho is at it's periapsis. This means that you also want to encounter it at the periapsis, because the lawls of physics work the same in reverse.

The velocity of Moho at apoapsis is be 12 186, yours will be about 15 550 if you're coming from Kirbin. That's a difference of almost 3400. At Moho's periapsis it's velocity will be 18 279, and yours will 19 850. That's only a difference of roughly 1500, less than half compared to an apoapsis encounter.

Edited August 12, 2013 by maccollo

[DMagic]

You want to burn as close to the sun as possible to exploit it's oberth effect.

I agree that aiming for Moho's periapsis is best. But more important, I think, is making sure that you actually get a good encounter.

These huge insertion burns, 5000 or 8000m/s are not due to differences in encountering Moho at its periapsis vs. at its apoapsis. They are due to getting non-ideal intercepts. The ideal intercept is one where you encounter Moho at your periapsis (or perihelion to be precise, the closest approach to the sun).

Having been to Moho more than any other planet I have found that the easiest way (though not the most fuel efficient) is to treat a Moho encounter like docking. Bring your periapsis down until it just meets Moho's periapsis (it's best to do this at the ascending node between Moho and Kerbin, which should get you close to Moho's periapsis). Then use smaller retrograde burns at the point where you cross Moho's orbit to setup a near ideal intercept. This might take several orbits around the sun, but I've always found this easier than trying to get the perfect intercept from Kerbin.

You lose some efficiency due to making burns far away from Moho, but it's not very much. Doing this I can almost always get from LKO to a 100km, polar, Moho orbit in under 5000 m/s, and I never have to worry about finding the perfect time to leave Kerbin or getting to Moho and finding that I need to a make a 5k m/s burn.

[krenshala]

Assuming Kerbin's orbit defines the ecliptic, you may want to shoot for the ascending or descending node as well. That should minimize the deltaV needed for capture. Though, due to my newness at the game still, I could be missing something important here in my assumption.

[SRV Ron]

Try to get a capture of Moho at a retrograde so that the planet's gravity can bleed off your encounter speed as much as possible as it slingshots you to a path against its orbital direction. It may take several attempts to get it right to cut down on the fuel needed for orbit.

[RocketPilot573]

Assuming Kerbin's orbit defines the ecliptic, you may want to shoot for the ascending or descending node as well. That should minimize the deltaV needed for capture. Though, due to my newness at the game still, I could be missing something important here in my assumption.

As it turns out, Moho's Descending/Ascending Nodes is in the same place as it's Ap and Pe. So that makes it even easier. I still can't believe I thought is costed at least 20,000 m/s of Delta V to get to that planet and back!