Olex Transfer Calculator

[hhatch]

I'm trying to figure out how to use the Olex transfer calculator to get to Duna and I have questions on the instructions.

 

  

The instructions say to have a  0 degree inclination in a parking orbit. I know how to match inclinations for things like a rendezvous but I've never set an inclination to a specific value so how do I do that ?

 

Then I'm supposed to warp to the phase angle. I have KER so I know what the phase angle is as the ship moves. I've been fast forwarding in the tracking station to do that. I sort of guess how long to warp so it's not too precise.  Negative and positive phase angles are mentioned. I think a positive angle is just for bodies outside of my orbit and negative is for those that are inside my orbit. Is this correct ?

 

The next instruction is to position myself at the right ejection angle. What is that ? Does it refer to the position of the node relative to Duna ? Do I just eyeball it ?  What is Kerbin prograde ?

 

The only mod I have now is KER and I want to limit the number of mods and calculators I use, but other than KER and the Olex calculator are there any other mods I might need like Kerbal  Alarm Clock ?

 

 

 

Edited January 7, 2018 by hhatch

[Jenteb07]

To answer the first question:

You can set it to 0 degrees inclination by setting mun as target and reducing your relative inclination to 0.
Or, as you mention you use KER, you can see what the inclination of your orbit is and how far it is till the an or dn, where you can perform a correction burn.

 

[Geonovast]

1 hour ago, Jenteb07 said:

You can set it to 0 degrees inclination by setting mun as target and reducing your relative inclination to 0.

I'm fairly certain this only works while in orbit of Kerbin.

[Jenteb07]

42 minutes ago, Geonovast said:

I'm fairly certain this only works while in orbit of Kerbin.

Well, he is going to Duna and did not specify the origin  (later he says Kerbin)

[Geonovast]

Just now, Jenteb07 said:

Well, he is going to Duna and did not specify the origin  (later he says Kerbin)

Eesh, I read it as he was already in orbit of Duna.  Whoops.

[OhioBob]

If you just launch eastward from KSC you'll be close enough to zero inclination.  I doubt you need to fine tune it anymore than that.  This is because KSC is only a tiny fraction of a degree off the equator.

A positive phase angle is when the target planet lies ahead of Kerbin in its orbit, and a negative phase angle is when it trails Kerbin.  Positive phase angles are used when the target planet has an orbit larger than Kerbin's, and negative angles are used when the target has an orbit smaller than Kerbin's.  In the first case Kerbin is moving faster and is catching up with the target, and in the second case the target is moving faster and is catching up with Kerbin.    A planet that has a larger orbit is called a superior planet (e.g. Duna), and one having a smaller orbit is called an inferior planet (e.g. Eve).

Prograde is the direction that Kerbin is moving in its orbit, and retrograde is the opposite direction.  To visualize it just imagine a line drawn tangent to Kerbin's orbit.  The line will point in the two directions, with the forward direction being prograde and the trailing direction retrograde.  The ejection angle is the angle between the position of the maneuver node and either the prograde or retrograde direction.  The prograde direction is used when going to a superior planet, and the retrograde direction is used when going to an inferior planet.

Kerbal Alarm Clock is a very useful mod, I highly recommend it.  If you want a way to more easily and accurately set up your maneuver nodes, then I also recommend either Precise Node or Precise Manuever.  Another mod that really like is Transfer Window Planner, which does everything the Olex Transfer Calculator does and more, but it does it from inside the game.  Those four mods - KER, Kerbal Alarm Clock, Precise Node, and Transfer Window Planner - I consider to be my most essential mods.
 

Edited January 8, 2018 by OhioBob

[hhatch]

8 minutes ago, OhioBob said:

If you just launch eastward from KSC you'll be close enough to zero inclination.  I doubt you need to fine tune it anymore than that.  This is because KSC is only a tiny fraction of a degree off the equator.

A positive phase angle is when the target planet lies ahead of Kerbin in its orbit, and a negative phase angle is when it trails Kerbin.  Positive phase angles are used when the target planet has an orbit larger than Kerbin's, and negative angles are used when the target has an orbit smaller than Kerbin's.  In the first case Kerbin is moving faster and is catching up with the target, and in the second case the target is moving faster and is catching up with Kerbin.    A planet that has a larger orbit is called a superior planet (e.g Duna), and one having a smaller orbit is called an inferior planet (e.g. Eve).

Prograde is the direction that Kerbin is moving in its orbit, and retrograde is the opposite direction.  To visualize it just imagine a line drawn tangent to Kerbin's orbit.  The line will point in the two directions, with the forward direction being prograde and the trailing direction retrograde.  The ejection angle is the angle between the position of the maneuver node and either the prograde or retrograde direction.  The prograde direction is used when going to a superior planet, and the retrograde direction is used when going to a superior planet.

Kerbal Alarm Clock is a very useful mode, I highly recommend it.  If you want a way to more easily and accurately set up your maneuver nodes, then I also recommend either Precise Node or Precise Manuever.  Another mode that really like is Transfer Window Planner, which does everything the Olex Transfer Calculator does and more, but it does it from inside the game.  Those four mods - KER, Kerbal Alarm Clock, Precise Node, and Transfer Window Planner - I consider to be my most essential modes.
 

Thanks for this info. I think it should help.

[OhioBob]

Just now, hhatch said:

Thanks for this info. I think it should help.

Note that I just corrected something in my post.  The retrograde direction is used when going to a inferior planet (I mistakenly wrote superior).

To reach a superior planet you want your spacecraft to be going faster than Kerbin's orbital speed after escaping Kerbin's sphere-of-influence, so you launch in the prograde direction so you're adding to the speed that Kerbin is already traveling.  And to reach an inferior planet you want your spacecraft to be going slower than Kerbin, so you launch in the retrograde direction.  The ejection angle places the maneuver node at the correct spot so that after the spacecraft leaves Kerbin's SOI, it is moving in either the prograde or retrograde direction.  Maneuver node placement must take into account how much the spacecraft will swing around the planet while escaping.
 

[Streetwind]

19 hours ago, Geonovast said:

I'm fairly certain this only works while in orbit of Kerbin.

Fun fact: Ike's inclination relative to Duna is only 0.2°. So it just so happens that even at Duna you can get "close enough for practical purposes" by using the same cheap trick.

It also works at Jool, with your choice of Laythe, Vall, or Tylo.

Around Moho, Eve, Dres, or Eeloo, you are definitely out of luck though.

Edited January 8, 2018 by Streetwind

[Spricigo]

9 hours ago, Streetwind said:

Around Moho, Eve, Dres, or Eeloo, you are definitely out of luck though.

One may use the KerbNet to get the location relative to the surface. It's quite troublesome and hardly necessary for game purposes but possible.

Also, a contract to put a satellite in equatorial orbit can be handy to setup a nice reference.

[Zhetaan]

@hhatch:

Pretty much everything @OhioBob said, though I will add one other thing:  ejection angle.

I know that OhioBob defined it, but I'll flesh it out a bit more because it's surprisingly important.

The idea is this:  let's assume that you're going to the Mun from an equatorial orbit around Kerbin, so there are no inclination changes, corrections, or anything you need to worry about other than burning at the right time.  If you burn prograde for a Hohmann transfer, then the point where you burn is the periapsis and the Mun's orbit is at the apoapsis.  If you additionally burned at the right time, then the Mun will be at that apoapsis when you get there yourself.  This is all elementary and I've no doubt you know it already given that you're working on interplanetary transfers.  A return from an orbit in empty space is similar; point retrograde at the apoapsis and burn.

Now consider a return from the Mun:  you can't just point to the Mun's retrograde because you're not in empty space; the Mun's in the way and its gravity will warp your path.  You also can't wait until you're on the Mun's prograde side and burn to raise the apoapsis on the far side to escape; this is closer to what you want, but again the Mun's gravity will warp your path.  As you circle round the Mun, the gravity will sling your vessel to a slightly higher apoapsis than the Mun's orbit.  Because the Mun puts some orbital energy into raising your Kerbin apoapsis on escape, it takes that energy away from lowering the periapsis, so you end up with a trajectory that usually isn't enough to reenter at Kerbin without correcting, which wastes fuel.

Instead, you need to account for the warp, burn a little early, and as a result use that warp to put your vessel where you want it to go without wasting fuel.  The Mun still warps your path, but the idea is that by changing your burn time, when the Mun slings you out, it raises your exit path so that you leave parallel to the Mun's retrograde, which gets you where you want to go.  The difference between a burn that accounts for the warp (and burns a little early) and a burn that does not (it burns to raise apoapsis to escape at the Mun's prograde point) forms an angle, and that is the ejection angle.

When going on interplanetary journeys, ejection angle is even more important, because the burns are more complicated.  Unlike returning from the Mun, where you're leaving from a point (the Mun) to enter a gravitational field you want to reach the bottom of (Kerbin's), interplanetary journeys involve leaving from one point (Kerbin) and crossing a gravitational field (the sun's) in order to hit another point (Duna in this case).  Failing to account for the ejection angle puts you on the wrong path through the sun's gravitational field, which further warps your trajectory and may cost you the encounter.  It will certainly cost you fuel.

In summary, failing to pay attention to ejection angle can ruin an otherwise good encounter.

The simple, rough way to check ejection angle in KSP is to watch your post-escape trajectory and compare it to Kerbin's orbit line.  So long as the two are parallel at your point of escape (they don't need to be in the same location; they only need to point in the same direction), then you have it right.

[hhatch]

1 hour ago, Zhetaan said:

@hhatch:

Pretty much everything @OhioBob said, though I will add one other thing:  ejection angle.

I know that OhioBob defined it, but I'll flesh it out a bit more because it's surprisingly important.

The idea is this:  let's assume that you're going to the Mun from an equatorial orbit around Kerbin, so there are no inclination changes, corrections, or anything you need to worry about other than burning at the right time.  If you burn prograde for a Hohmann transfer, then the point where you burn is the periapsis and the Mun's orbit is at the apoapsis.  If you additionally burned at the right time, then the Mun will be at that apoapsis when you get there yourself.  This is all elementary and I've no doubt you know it already given that you're working on interplanetary transfers.  A return from an orbit in empty space is similar; point retrograde at the apoapsis and burn.

Now consider a return from the Mun:  you can't just point to the Mun's retrograde because you're not in empty space; the Mun's in the way and its gravity will warp your path.  You also can't wait until you're on the Mun's prograde side and burn to raise the apoapsis on the far side to escape; this is closer to what you want, but again the Mun's gravity will warp your path.  As you circle round the Mun, the gravity will sling your vessel to a slightly higher apoapsis than the Mun's orbit.  Because the Mun puts some orbital energy into raising your Kerbin apoapsis on escape, it takes that energy away from lowering the periapsis, so you end up with a trajectory that usually isn't enough to reenter at Kerbin without correcting, which wastes fuel.

Instead, you need to account for the warp, burn a little early, and as a result use that warp to put your vessel where you want it to go without wasting fuel.  The Mun still warps your path, but the idea is that by changing your burn time, when the Mun slings you out, it raises your exit path so that you leave parallel to the Mun's retrograde, which gets you where you want to go.  The difference between a burn that accounts for the warp (and burns a little early) and a burn that does not (it burns to raise apoapsis to escape at the Mun's prograde point) forms an angle, and that is the ejection angle.

When going on interplanetary journeys, ejection angle is even more important, because the burns are more complicated.  Unlike returning from the Mun, where you're leaving from a point (the Mun) to enter a gravitational field you want to reach the bottom of (Kerbin's), interplanetary journeys involve leaving from one point (Kerbin) and crossing a gravitational field (the sun's) in order to hit another point (Duna in this case).  Failing to account for the ejection angle puts you on the wrong path through the sun's gravitational field, which further warps your trajectory and may cost you the encounter.  It will certainly cost you fuel.

In summary, failing to pay attention to ejection angle can ruin an otherwise good encounter.

The simple, rough way to check ejection angle in KSP is to watch your post-escape trajectory and compare it to Kerbin's orbit line.  So long as the two are parallel at your point of escape (they don't need to be in the same location; they only need to point in the same direction), then you have it right.

At the moment I am in orbit at about 106 km. The instructions indicate I should burn until I reach a velocity of 3289 m/s which I assume is escape velocity. Should I just eyeball the escape trajectory until the path looks parallel and stop the burn ?

[Zhetaan]

36 minutes ago, hhatch said:

At the moment I am in orbit at about 106 km. The instructions indicate I should burn until I reach a velocity of 3289 m/s which I assume is escape velocity. Should I just eyeball the escape trajectory until the path looks parallel and stop the burn ?

Escape velocity from that orbit should be about 3160 m/s; the remainder is excess velocity that you need to not only escape Kerbin, but also go to Duna.  The way I do it is to set up a manoeuvre that gets close to Duna--I usually don't want the transfer apoapsis to go higher than Duna's orbit, but since this transfer is less than perfect timing, you'll have to make do--and then take the whole node and slide it back along my pre-burn Kerbin orbit.  As I do, the warp pulls my final trajectory more in line with Kerbin's prograde, which makes the burn more efficient ... which raises the apoapsis.  When it won't go up any higher, then I know I'm close and will check the ejection from Kerbin SOI to compare it to Kerbin's own trajectory.  If it looks good, I call it good, and somewhere, a NASA controller dies on the inside a little at my slap-dash methods.

Keep in mind that there's no way to see both your low Kerbin orbit and your Duna transfer orbit without zooming out and back in, so it can be a little tedious until you get a feeling for the correct angle.

Anyway, when I'm close, I pull slightly retrograde to pull my apoapsis back down to Duna orbit and I begin to slide the node again.  I need to do that because the highest apoapsis I got before was the highest for the original burn, but once I changed the burn, that stopped being true.  Since the new burn is close to the old one, the new angle ought to be close to the old one, too.

I generally don't bother fine-tuning it more than once; the savings in fuel isn't worth the hassle for me after that point.  Also, while the tweaks will change your apoapsis height a bit, don't expect something spectacularly dramatic as when you drag the manoeuvre around low Kerbin orbit for a Mun transfer.  If you began with a fairly close encounter, then you should end with a close encounter, but it will cost a lot less in fuel.

Edited January 10, 2018 by Zhetaan