Going waaay too fast on Moho intercepts.

[painking]

I don't know about you guys, but I don't think 8K+ m/s is necessary to get into a Moho orbit, especially not according to the Dv maps I've looked at. Am I doing something wrong with my intercepts? I timed it with protractor, and I successfully intercept at the AN of Moho's orbit. How can I slow down enough to not require ultra massive Dv to get caught by Moho's gravity?

Edited July 8, 2013 by painking

[astropapi1]

Well, you're coming deep into Kerbol's gravity well, and as our buddy Hermann Oberth said, you'll be going fast as hell when you intercept Moho. There's really nothing you can do. :/

[HoY]

I don't know about moho specifically since I've never been there myself, but you could consider using a gravity assist from one of its moons(does it have moons??) to help slow you down. (Or change your trajectory more correctly) thus needing a little less to actually capture

[painking]

I understand going fast, but it really seems excessive.

That Dv map says it takes 2200 m/s to get into Moho's gravity after intercept, but my own maneuver burn would've been 8400 m/s. It just really feels like I did something wrong.

edit: @hoy, Moho doesn't have any moons.

[HoY]

Yeah that's showing as only needing 2200m/s to circularize. Needing nearly 3.5x that sounds suspect..

Are you positive your maneuver burn isn't set to circularize you in the opposite orbit that the one you come in at? Ie, coming in prograde but pulling the retrograde marker so far that it zeros out your orbital speed then circularize a you in the opposite direction?

Edit: probably a no, because based on the numbers on that chart it would still o ly cost around 5500m/s to do what I just said.

Edited July 7, 2013 by HoY

[alexmun]

Moho intercepts are tricky and the standard Hohmann calculation fails pretty badly due to Moho's inclination, eccentricity, and orbital velocity. If you don't hit a good window, your intercept will come in at a large angle which means a lot of delta-v since you're so close to the sun. A good launch window can get you from LKO to 100km Moho orbit in ~5000m/s delta-v. This exact problem is one of the main reasons I made my launch window planner: http://alexmoon.github.io/ksp

[stupid_chris]

Delta V maps can't take account of Moho's rather high inclination or eccentricity, so yeah, it's normal. From LKO, a Moho orbit is 4,5km/s in theroy, but really you should expect something more like 7-8km/s :l

[esinohio]

Shew, I guess I don't feel so bad about slinging past Moho on numerous occasions. I can't count how many times I was looking at that exact DeltaV map above wondering just what I was doing wrong. Although now armed with alexmun's nice window planner I think I'll give Moho some more attention with my minimalist rockets/probes. Love trying to get the fuel planned down to the last drop to keep those costs down.

[Colonel_Panic]

The basic principle is that as your kerbol periapsis gets lower, the faster you slingshot around the sun. Given moho's orbital eccentricity, how close to kerbol you have to come, and therefore how fast you'll be moving on capture, will vary with what point on moho's orbit you intercept. If you can plan your transfer to intercept at moho's apoapsis, you'll need the least delta v to capture.

Due to moho's eccentricity and inclination, it may be worth considering abandoning a traditional injection burn from kerbin, and instead launching when kerbin is at moho's ascending or descending node, thereby eliminating the inclination change burn, and leaving your kerbol periapsis above moho's orbit. Then you can wait and plan the most efficient hohmann transfer to moho, by intercepting at it its kaerbol apoapsis.

[AndreyATGB]

8k is too much, most I've seen is 5400 if you burn too much and lower the Pe way under Moho's own. Basically if you have an intercept on the second intersection between your orbit and Moho's then it'll be a lot more since you're catching up with your lower orbit. Best is to intercept Moho at the first node, either ascending or descending don't remember which, then you'll need around 2900 to circularise.

[Nibb31]

Are you orbiting Kerbol retrograde from Moho when you enter its SOI by any chance?

You want to be going in the same direction as the target you are intercepting.

[NoMrBond]

I had to get my crafts orbit closer to Moho's orbit before pulling off a successful intercept, the dV required was just way to high to get into orbit around Moho otherwise.

[tomf]

Surprisingly getting captured at moho PE requires less DV than at AP. You are moving faster but so is the planet.

Capture DVs higher than predicted are normally caused by intercepting Moho while not traveling parallel to its direction of travel. At the speed the ship and Moho are both travelling around the sun at that distance a difference of a few degrees in direction of travel can work out to km/s of relative velocity.

To avoid all this use the launch planner above and make sure that your intercepts happen when paths are parallel.