Getting to Moho orbit and dV requirements

[Some Kerbal]

I seem to be having a problem getting an orbit around Mojo. I launched a satellite a bit ago game time and got an intercept on the burn from Kerbin. Set up a mid course correction to fine tune the right pe approach. Waited the 80ish game days for the maneuver node to happen while doing other things. Correction time comes and I adjust the PE at Mojo to a bit under 30k. Then I set up for the next maneuver to bring the AP into view. I look at the dv required to make the circularization burn and I see its going to cost over 6000dv. 

I don't understand why it is costing me so much. The dv charts show 2400 to go from a flyby to an orbit.

I used Transfer Window Planner mod to set up the transfer time and the mod detailed that it would be just under 2k for insertion. TWP has another window coming up and again shows ~3kdv ejection, ~2k insertion. 

I can write off the satellite (will still get the flyby science and stuff) and launch another but would really like to know what I did wrong. Any advice would be appreciated. 

 

[Aegolius13]

Moho is the toughest place in the Kerbol system to get to.  Problems are very common - I think it took my fourth mission before I was even able to get into orbit.   

The reason, in a nutshell, is that since it's so close to Kerbol, it's zipping around really vast.  So if you get an intercept where you hit Moho's SOI, but you're not going very close to the same direction as Moho, that makes for a very large velocity differential - i.e., a large delta-v burn to capture.  Adding to the complication, Moho is at a fairly inclined angle, and somewhere along the way, you'll likely have to pay a lot to match its plane.  

How to fix it?  The basic idea is that you have to get your transfer burn exact.  You want your periapsis around Kerbol to pretty much exactly match Moho's orbit, and your want your orbit and Moho's to be tangent.  On other planets, you can get a way with an orbit that overshoots the target and intersects it at an oblique angle.  Not so on Moho.  

It's also wise to just build in a high delta-v tolerance, and expect some degree of imperfection versus what the transfer window planner tells you.  Nukes and ions are both valid options, depending on the size of your rocket.  

Also, there is a cheaper and, in my mind, easier way to do the whole thing.  It involves launching at Kerbin's ascending/descending node with respect to Moho, rather than at a launch window based on phase angle.  I found it a little tough to conceptualize, but once I got it, it made Moho missions much easier. Some more info here:

 

 

 

 

[Blaarkies]

Using gravity assists from Eve saves a lot of dv...but it does require some fiddling about. Launch for a flyby of eve, then when still near Kerbin use a maneuver node to see the results depending on which side of Eve you arrive on, how close you approach Eve and so forth. A Pe at Moho and Ap at Kerbin is much harder to fix than the same orbit with an Ap at Eve. Don't bother with gravity assist at Moho, it is barely a planet. Just remember Sir Oberth

Edited February 9, 2017 by Blaarkies

[Ohm is Futile]

2 hours ago, Aegolius13 said:

It's also wise to just build in a high delta-v tolerance, and expect some degree of imperfection versus what the transfer window planner tells you.  Nukes and ions are both valid options, depending on the size of your rocket.  

As mentioned, Moho goes around Kerbol very fast. For this reason, the transfer window is key. The link provided shows that.

I am going to somewhat disagree on using nukes or ions for this though, unless I'm crazy from my own experience at shooting for Dres with low TWR, you're going to have to perform an ejection burn of roughly 2,800 m/s of dV. With low TWR, this can take 20 minutes of burning. Patience is not the issue here, but making your burn precise due to it being spread over a long period of time is going to be difficult and result in a lot of wasted dV in the end.

Anyways, hopefully, shedding some light on this may help my own case.

[Aegolius13]

51 minutes ago, Ohm is Futile said:

I am going to somewhat disagree on using nukes or ions for this though, unless I'm crazy from my own experience at shooting for Dres with low TWR, you're going to have to perform an ejection burn of roughly 2,800 m/s of dV. With low TWR, this can take 20 minutes of burning. Patience is not the issue here, but making your burn precise due to it being spread over a long period of time is going to be difficult and result in a lot of wasted dV in the end.

Yeah, TWR can potentially be a limiting factor.  But you can mitigate this somewhat by starting your burn before you get to Moho periapsis (potentially before you even get to Moho SOI).  In particular, if you use the node method, you can once you reach your periapsis around Kerbal, you do a retrograde burn for whatever amount is necessary to create an encounter on your next orbit.  This is often about 1000-1500 m/s, and TWR is basically irrelevant in solar orbit.  But this essentially pre-pays the amount of capture burn you'll have to do around Moho itself.  I've ended up with a capture as low as ~500 m/s.  

Of course, you lose out on Moho's Oberth effect by doing any of these.  But (1) Moho is small and does not generate that much Oberth, (2) the ISP savings from nukes or ions will likely make up for it, and (3) in the case of node method, your total delta-v cost is still probably going to be lower than the traditional method.

[George van Doorn]

Moho is the only truly 3D encounter out there. Dres and Eeloo are far off and their inclination difference can be easily adjusted. I'd like to give you the theory behind the actual maneuver so that you understand it for basically all maneuvers.

Let's take the planets out of the equation, let's say you are just orbiting the Sun at Kerbin's altitude and there's a ship where Moho is. What do you have to do to reach it? Well, you'd have to:

a) correct your inclination of 7 degrees

b) decrease your peri so that it intersects that of the target and do that so that you reach the target's orbit when the target is there as well, so as to have a rendez-vous

 

Because orbits are very costly to change in Sun's orbit, it's very efficient to bundle these 2 maneuvers into one. And because you can only change the inclination at the AN/DN, it takes more than a year (even though Moho orbits the Sun every 100ish days) to get a good encounter. Anything but this, meaning either correcting the inclination or reducing your orbit during your downward trip to Moho, will be less efficient.

Now let's bring in the planets back into the equation. Kerbin is for you just a slingshot so that your transfer costs less. If you follow the numbers given by the Transfer Window Planner, you should get similar (but not exactly the same) results. Why similar? Minute deviations on your orbit, the fact that your transfer burn doesn't happen all at once, things like that. Let's say you do not get an encounter and do not even get the closest distance markers, pretty common for Moho. Do not despair! That means you need to perform a correction burn, big or small depending on your deviation. When is it the right time to do this? ASAP! Meaning, place a maneuver node 5-10-30 minutes ahead of your trajectory and fiddle with the numbers. Have a tool like Precise Node or Precise Maneuver to be able to add ever smaller increments.

Why is the right time to do the correction immediately? We have to look at the big picture. Irrespective where you are regarding Kerbin, you are still orbiting the Sun. Now your orbit is eccentric and your starting point is basically your apoapsis AND your AN/DN  (where you presumably started this transfer) is nearby as well. Additionally, you can still benefit from some slingshot effects because you are in Kerbin's sphere of influence. Could you have done this correction more efficiently if you were at Kerbin's periapsis? Yes, but what's done is done now; the difference should still be marginal because it's a correction. It's still the best alternative you got.

 

It's really important to understand this theoretically because then, whatever happens, you will be able to adjust your burn to compensate. Now that you know this, you can start your transfer from Minmus fully refueled, maybe adding some 3k dV to your total. You can go refuel at Gilly and start from there, like here:

 

I'm saying, everything is possible. Just know what your objective is, know the theory and then do what humans do best: adapt!

 

 

You can use these tools for transfer window planning:

https://alexmoon.github.io/ksp/

http://ksp.olex.biz/

Transfer Window Planner mod

even the Kerbal Alarm Clock gives you decent transfer Windows

P.S. : You may look at my profile and say: "ah, a beginner giving me advices", but I have about 2000 hours in this game. I've only been posting on steam so far. And information is not more correct if it comes from a reputable source, it's either correct or not.

 

Edited February 9, 2017 by George van Doorn
clarifications and spelling

[Ohm is Futile]

The capture is something, but I meant that mostly for the ejection burn from Kerbin, actually.

[Aegolius13]

10 hours ago, Ohm is Futile said:

The capture is something, but I meant that mostly for the ejection burn from Kerbin, actually.

That's true, but you can always put a higher-thrust chemical engine at the bottom of your rocket, then stage to a nuke or ion or the Moho stuff.  I pretty much always do this with ion probes so I don't have to run the ions on Kerbin ejection (where solar energy is scarce too).   Since the top stage is so light, you can probably get several thousand delta-v with a Terrier and 800 tank, or the like. 

[Some Kerbal]

After reading the responses I went back and looked at my intercept path. Sure enough I was cutting across the face of Mojo and not approaching on a parallel path. I guess I was not careful enough with my transfer and plotting my intercept. I will take the advice provided when I make my next attempt.

Thanks for the help. So many things to learn