Phase and ejection angles

[DaMachinator]

I tried to use the Transfer Window Planner mod to get to Moho. Even when I have them displayed, their meaning is still a mystery to me. 

EDIT: Question answered, thanks @Goody1981! For anyone trying to read this, make sure "sort by date" is selected.

Edited July 2, 2016 by DaMachinator

[Jimbo Jangles]

Phase angle describes the position of the target body in its orbit in relation to the Kerbin (or other origin) in its orbit. In other words, looking down from overhead, is the target in front of or behind Kerbin, and by what angle. 

 

Ejection angle describes where to preform your ejection burn in your current orbit. I believe LWP has a handy overlay showing where in your orbit to preform the burn. 

 

Jimbo

[Goody1981]

You need to go into the tracking centre and zoom out until you see Kerbin and the planet you are wanting to go to. 

Rotate the view until Kerbin is at the 3 o'clock position (don't have to do this but it always helps me). Phase angle is the angle between Kerbin and your target planet. 

E.g. If the phase angle is 90 degrees and Kerbin is at 3-o'clock, the target planet would be at 12 o'clock. 

A negative phase angle means it's that many degrees behind you (bottom of screen if Kerbin is at 3), positive phase angle means it's in front of you (top of screen if Kerbin at 3).

Ejection angle is the place where you need to complete your prograde burn and eject from Kerbin, measured as if the path of Kerbin's orbit prograde is 0/360. 

No pics sorry (on phone) but hope that helps!

 

[DaMachinator]

So ejection angle is where I should put my maneuver node for the transfer?

[Goody1981]

Yes! That's a much better way of describing it haha. I usually don't bother that much getting the ejection angle perfect, instead I look at my path as I leave Kerbin's SOI - it should line up pretty closely with Kerbin's orbit 

[DaMachinator]

It's almost impossible to get perfect anyways. I now have to use an orbit around Kerbin that is asynchronous with the orbit I burn from for these seemingly random special Moho windows that take about 1000 m/s less Delta V. 

[Goody1981]

Yeah I've read (not sure what thread) that Moho is the one planet that phase angles aren't as important, because its orbital period is so short you'll get an encounter after not too long..

My fave way to go to Moho is to launch to Eve and use a reverse gravity assist to drop my Pe down to Moho. Gravity assists are so satisfying

[Dafni]

46 minutes ago, Goody1981 said:

Yeah I've read (not sure what thread) that Moho is the one planet that phase angles aren't as important, because its orbital period is so short you'll get an encounter after not too long..

My fave way to go to Moho is to launch to Eve and use a reverse gravity assist to drop my Pe down to Moho. Gravity assists are so satisfying

Could you please explain me what a reverse gravity assist is and how to do it?

many thanks in advance

Daf

[Goody1981]

1 hour ago, Dafni said:

Could you please explain me what a reverse gravity assist is and how to do it?

many thanks in advance

Daf

Hi Daf  no problem

A gravity assist is just using a planet to bend your trajectory. If you bend it so it's more "in line" with that planet's you will get an energy boost (relative to the sun) and raise your Ap. 

In a reverse gravity assist you bend your path so it is less "in line" with the planet's orbit. This causes you to lose energy (relative to the sun) and lowers your Pe.

I'm assuming you know how to eject retrograde to get an encounter with Eve? (If not, let me know!). Do this.

Then, when you encounter Eve, play with your maneouver node until the path leaving Eve is less "in line" with Eve's orbit than it was approaching Eve. You can do this by passing in front of Eve. The close you are to the planet, the more your path is bent and the greater your gravity assist is.

Does that make sense? It's hard to explain without pics!

[Dafni]

It makes perfect sense to me, thank you. I love me my grav assists... even though a lot of the time the benefits are questionable (time spent fiddling with nodes versus dV safed) I do them a lot. I love deep space probes IRL and to try to copy these things in KSP. At the moment working on an ESA probe that will fly to Mercury in 2018... this maneuver will come in handy.

I just never knew you'd call that particular maneuver a reverse assist, but it makes sense I guess. I just call all of them slingshots in my non-english-thinking head. Trading inclination for dV and the other way around.

Thanks again.

[Goody1981]

4 hours ago, Dafni said:

It makes perfect sense to me, thank you. I love me my grav assists... even though a lot of the time the benefits are questionable (time spent fiddling with nodes versus dV safed) I do them a lot. I love deep space probes IRL and to try to copy these things in KSP. At the moment working on an ESA probe that will fly to Mercury in 2018... this maneuver will come in handy.

I just never knew you'd call that particular maneuver a reverse assist, but it makes sense I guess. I just call all of them slingshots in my non-english-thinking head. Trading inclination for dV and the other way around.

Thanks again.

Haha ah no problem  actually you're probably more right - they all "assist" you where you're going, so they're all assists! 

Sorry for thinking I needed to explain from the beginning - you probably know more about them than me  

You might be interested in this guy's tutorial where he recreates the Juno mission - just found it today and going to try it now!

 

Edited July 1, 2016 by Goody1981

[FullMetalMachinist]

9 hours ago, Goody1981 said:

Yeah I've read (not sure what thread) that Moho is the one planet that phase angles aren't as important, 

This is very true. 

9 hours ago, Goody1981 said:

because its orbital period is so short you'll get an encounter after not too long..

This part, not so much. Well, it is true, but it's not the reason that phase angles and traditional transfer windows aren't so important for Moho. 

The reason you want to ignore phase angles when going to Moho is that it's orbit is both highly elliptical (compared to the other planets) and highly inclined. So depending on where Moho is in it's orbit when the phase angle with Kerbin is correct, you could end up having to do a huge plane change just to get an encounter, and then a huge capture burn when you get there. The problem is so bad that both your ejection burn from Kerbin and the capture burn at Moho can be several km/s of dV more than the theoretical best case. 

See here for more info. 

 

[DaMachinator]

9 hours ago, FullMetalMachinist said:

This part, not so much. Well, it is true, but it's not the reason that phase angles and traditional transfer windows aren't so important for Moho. 

The reason you want to ignore phase angles when going to Moho is that it's orbit is both highly elliptical (compared to the other planets) and highly inclined. So depending on where Moho is in it's orbit when the phase angle with Kerbin is correct, you could end up having to do a huge plane change just to get an encounter, and then a huge capture burn when you get there. The problem is so bad that both your ejection burn from Kerbin and the capture burn at Moho can be several km/s of dV more than the theoretical best case. 

See here for more info. 

With life support, this is a bad idea unless your ship can self-sustain for a long time.

I suspect that the reason some transfer windows are significantly less expensive than others is because they line up with this method. I remember Kerbin being at about that position for the unusually good transfer I found last time i tried a Moho trip.

(This trip failed because phase angles are not my thing.)

Edited July 2, 2016 by DaMachinator