Moho periapsis for return trip to Kerbin?

[MZ_per_X1]

I'm trying to do a Moho mission. I transferred from Kerbin directly when Kerbin was at the Ascending node (=Moho apoapsis). I just eyeballed it. I didn't worry about the inclination because I had an encounter on the next solar orbit with a little tweaking. I have the Moho capture lined up for 2100 m/s that will put me in a low orbit (35km circular). Obviously I will be arriving on an inclined orbit around Moho. 

I guess the same way I should be able to get back to Kerbin when Moho is at its apoapsis? (Plus minus a few orbits.) I have just under 3000m/s left. I probably don't have enough to correct the inclination for the return trip and have to aerobrake at Kerbin. I also wonder if I will be coming in slower this way instead of leaving Moho at its periapsis. 

How can I see the periapsis of a planet? Shouldn't the map show you such information? 

[Vanamonde]

Well, there's the wiki. 

https://wiki.kerbalspaceprogram.com/wiki/Moho

[MZ_per_X1]

1 hour ago, Vanamonde said:

Well, there's the wiki. 

https://wiki.kerbalspaceprogram.com/wiki/Moho

I am asking about the Moho apoapsis and periapsis in game. The wiki makes us no wiser apart from eyeballing. I'd like to be able to show in game the precise location so I could experiment with the rest of my questions. 

[Spricigo]

6 hours ago, MZ_per_X1 said:

I guess the same way I should be able to get back to Kerbin when Moho is at its apoapsis?

You should be able to get back to Kerbin the same way, when Moho is at the Ascending Node. No idea why you brought up the apoapsis.

Also keep in mind, an encounter on the next solar orbit with a little tweaking is less likely to happen because the Kerbin's orbit is longer than Moho's

[Hotel26]

Try: go to Map View and click Moho and set it as the target of your vehicle.  I believe then that Moho's AP and PE markers should be shown in the yellow target orbit.  (You've probably found that yourself, by now.)

On 10/4/2020 at 7:30 AM, Spricigo said:

No idea why you brought up the apoapsis.

Haven't confirmed, but it's entirely possible that Moho's AP and PE just happen to coincide with its AN and PN.  So @MZ_per_X1's departure point is "onto something".  (I found this note actually searching for anyone who had already written about the following.)  That would make this little KSP Kerbolar "graded puzzle" in Orbital Mechanics quite a pretty one!

Instead of performing the expensive inclination alignment burn at high speed in the Kerbol (or Kerbin) orbit, wait for Kerbin to reach its equivalent point in its orbit to Moho's DN/PE, burn for a PE matching Moho's AP, arrive at Moho's AP and then burn retrograde until one more orbit will be synchronized with arrival at Moho's AP, albeit with 7 deg inclination relative to Moho when entering its SOI; much cheaper to fix in Moho orbit.  As a bonus, you get to intercept Moho at its slowest-moving point.  This is longer (but far easier and safer) than the fastest transfer,

Edited October 19, 2020 by Hotel26

[MZ_per_X1]

Moho obit periapsis and apoapsis are almost where the AN/DN nodes are in respect of Kerbin's orbit. (That is not necessarily the inclination of your ship unless you nail it or use mods.)

 

That is why I was thinking that catching Moho at Periapsis results in higher capture cost as Moho is faster at periapsis. So you would try to catch it at its apoapsis when Moho is slower. 

I wonder if the return would be the opposite, I guess when Moho is faster and gives you a bit of a boost. OR if you leave Moho at the apoapsis, your orbit will be higher and therefore closer to Kerbin?

I haven't figured it out because I lost my save and I haven't bothered to try this in Sandbox.

 

[Hotel26]

On 10/19/2020 at 9:17 PM, MZ_per_X1 said:

So you would try to catch it at its apoapsis when Moho is slower. 

This is correct.  Your dV expenditure is about bridging the difference in orbital speeds.

Therefore, leaving Moho's at periapsis is a mistake because your orbital speed there (relative to Kerbol) is higher which makes that lower at the other side of your transfer ellipse.  That would give you a bigger deficit to make up reaching Kerbin.  Orbital speed at Moho's AP is closer to Kerbin's orbital speed.  That is the difference you wish to attempt to bridge.

(By the way, I think I was incorrect earlier in thinking you could get Ap and Pe markers for the target in Map View.)

[Spricigo]

On 10/19/2020 at 10:17 AM, MZ_per_X1 said:

That is why I was thinking that catching Moho at Periapsis results in higher capture cost as Moho is faster at periapsis. So you would try to catch it at its apoapsis when Moho is slower. 

As Moho's velocity will vary with orbital height so will craft's velocity. Be careful to not follow a hunch to something the actual math and experimentation may tell is not the best option.

 

[Hotel26]

On 10/22/2020 at 11:14 AM, Spricigo said:

Moho's velocity will vary with orbital height so will craft's velocity. Be careful to not follow a hunch to something the actual math and experimentation may tell is not the best option.

These are words to the wise!!

You are right.   I was wrong. 

All we can say from the reasoning I was using above is that a craft with kerbin apoapsis, arriving at either moho periapsis or apoapsis will have a considerably higher speed than moho itself at those points.   Which one has the least difference just depends. 

And as I now read in Laie's(?) Wiki  tutorial,  it is the PE that is better. 

[Spricigo]

48 minutes ago, Hotel26 said:

You are right.

That is the good thing about vaguely expressing doubt, hard to be wrong about not being sure. 

 

[Hotel26]

1 hour ago, Spricigo said:

That is the good thing about vaguely expressing doubt, hard to be wrong about not being sure. 

Thank you for that.

It is only my personal belief, and possibly a minority one, that, being unafraid of being wrong, in combination with assiduous open-mindedness and the willingness to easily give up a wrong position in favor of a better one, holds some considerable utility.

Direct speech is good.  (Opinion.)

[Hotel26]

With the aid of Fredfry Kerman and my new Mongoose HETV high-energy test vehicle:

 

I have been able to determine the orbital speed at Apoapsis and Periapsis of a) Moho and b) an HETV with apoapsis at 13.318 Gm (Kerbin altitude):

	alt: 3.949 Gm	alt: 6.054 Gm
Moho	18,278.8 m/s	12,185.9 m/s
HETV	20,614.6 m/s	15,917.9 m/s
dV	2,335.8 m/s	3,732.0 m/s

There we have it.  Thanks for signalling an alert, @Spricigo.

Edited October 25, 2020 by Hotel26