Trouble with tutorial about getting to the moon

[jonpfl]

All,

I keep running out of fuel, I read from some older threads I am supposed to get a delta V of 800 but I cannot get better than 2000.

Any suggestions?

upload your pics

Thx
jonpfl

[Pand5461]

You have wrong phase angle, that is, the angle between directions to the maneuver node and to the Mun from the center of Kerbin.

First, you need to make a prograde node, i.e. move only prograde gizmo. Add deltaV until the apoapsis after maneuver is the same as Mun height. Then just move node around the trajectory until projected orbit gets into Mun's SOI.

Good luck.

[jonpfl]

1) Still confused, not sure what you mean by "wrong phase angle"?  
2) What is a maneuver node vs a prograde node?
3) What does "add deltaV until the apo after man is same as Mun height" mean?  
4) If I try and move node around the trajectory, it just rotates around the planet at stays at 2k.

Thx
jonpfl

EDIT : I figured it out, I was trying to go from in front of the planet, figured out I am supposed to do from the backside.

 

 

Edited May 20, 2017 by jonpfl

[FloppyRocket]

"Phase angle" means you have to add delta-v at the correct place in your orbit so your ship and the Mun arrive at the same point at the same time.

A "maneuver node" is the place in the orbit where you're going to make a maneuver. Prograde is the direction your ship is heading at that moment.

You can add delta-v at the maneuver note using the "prograde" tool, just until the predicted apoapsis is the same as the Mun's orbit.

As you drag the maneuver node around, the ship will be aiming in different directions (because you're changing the phase), but the amount of delta-v doesn't change automatically. You have to change the length of the prograde tool yourself.

Edited May 23, 2017 by FloppyRocket
(remove a typo - totally Jeb's fault)

[jonpfl]

All,

What is the orange vs green path?

How do I get my burn time down?  I can only get it down to around 50 seconds.

Thx
jonpfl

[Ten Key]

12 minutes ago, jonpfl said:

What is the orange vs green path?

The blue line is your current orbit. The orange line is your projected path after you execute your planned maneuver. 

The green path is your projected orbit after your Mun encounter. Presumably you'd want a second maneuver in the Mun's sphere of influence to enter orbit, but if you only execute that first burn and don't touch your craft again, the green path is the orbit it will end up on.

15 minutes ago, jonpfl said:

How do I get my burn time down?  I can only get it down to around 50 seconds.

You are burning way too hard as you leave Kerbin orbit. Not only does this waste fuel on the way out, it results in an excessively long braking burn once you get to the Mun. The burn to go from Kerbin to the Mun should only be about 800-900 m/s. Try reducing the burn to about 850 m/s, then move the node along your current orbit until you get an intercept. Try to avoid going past the Mun-- see how your projected apoapsis is beyond the Mun's orbit? 

Also, note how heavily inclined your resulting orbit (the green path) is in your screenshot. See how the Mun amplifies your slight 1.6 degree Kerbin orbit into something around 70-80 degrees? This can be useful for entering polar orbits around the Mun, or even for adjusting the inclination of existing satellites in Kerbin orbit. 

[Capt. Hunt]

The orange path is the trajectory immediately resulting from the maneuver you've set up, that's the path your ship is going to follow after the maneuver.  The green line is the trajectory after you pass the mun (purple line) and return to kerbin's SoI.  You can reduce your burn time either by using a more powerful engine or creating a maneuver that uses less delta-V.

This maneuver you have pictured is still a very high delta-v maneuver because of the extreme vector change.  You might want to try a different part of the orbit to see if you can get it lower.

[jonpfl]

2 minutes ago, Ten Key said:

The blue line is your current orbit. The orange line is your projected path after you execute your planned maneuver. 

The green path is your projected orbit after your Mun encounter. Presumably you'd want a second maneuver in the Mun's sphere of influence to enter orbit, but if you only execute that first burn and don't touch your craft again, the green path is the orbit it will end up on.

You are burning way too hard as you leave Kerbin orbit. Not only does this waste fuel on the way out, it results in an excessively long braking burn once you get to the Mun. The burn to go from Kerbin to the Mun should only be about 800-900 m/s. Try reducing the burn to about 850 m/s, then move the node along your current orbit until you get an intercept. Try to avoid going past the Mun-- see how your projected apoapsis is beyond the Mun's orbit? 

Also, note how heavily inclined your resulting orbit (the green path) is in your screenshot. See how the Mun amplifies your slight 1.6 degree Kerbin orbit into something around 70-80 degrees? This can be useful for entering polar orbits around the Mun, or even for adjusting the inclination of existing satellites in Kerbin orbit. 

1) So essentially I am trying to get a green path as that means I will be in orbit around the Mun?  
2) Is SOI the same as orbit?
3) I keep seeing posts saying that getting to the Mun should be 800 to 900 m/s but I have NO idea how to control that.  When I doing a maneuver around Kerbin, I am just playing with the symbols until I am getting a path that gets to the moon.  Whatever I am doing, should I be doing the opposite and that should get me to that number?

Thx
jonpfl

[bewing]

1) not in orbit -- just touching the SOI.

2) not exactly. Each celestial body has a region around it where its gravity is stronger than every other body's gravity. That is the SOI. When you enter the SOI you will start to fall toward that body.

Look at the Mun's orbit from above. Pick a spot 90 degrees ahead of the Mun -- you have to lead your target (the Mun by 90 degrees, Minmus by 60 degrees). Pick the spot on your current orbit in LKO that is on the exact opposite side of Kerbin from the spot you picked on the Mun's orbit. Set your maneuver node right there. Grab the prograde on the maneuver node (and only the prograde) and pull it until your Ap is at about 10Mm. That should get you a Mun encounter for 870 m/s of dV or so.

Edited May 23, 2017 by bewing

[HvP]

@jonpfl Your orbit lines change color every time they cross into another planet or moon's sphere of influence. The blue line is the orbit you are on before your burn. The orange line is the orbit you will be on after you do your maneuver but BEFORE you cross into the gravity of the Mun. At the point that the Mun's gravity becomes stronger than Kerbin's gravity you see a little circle on your path and the line changes color. If you hover over the little circle it will tell you if you are entering or exiting the Mun at that point. This means you are crossing the sphere of influence, or SOI.

If you zoom in towards the Mun a little you will probably see that the orbit line is now purple. It's purple because you will be in the sphere of influence of another gravitational body. When your path leaves the Mun's gravity then the line changes color again - to green in this case. It does this so that you will know which line is further in the future. Some orbits might take you around the Mun and then line up almost exactly with the path you are on. It's important to know that after you do your maneuver you travel along the orange line first, then the purple line, then the green line. If you make any more maneuvers at a point later on then all of the lines after that maneuver will change position to show you your new path.

The reason you are getting such a high number for your maneuver is because you are probably starting the maneuver in the wrong place. Try this. Angle the map view so that you are looking almost straight down over the north pole. Hover over the Mun and look at what it's orbital altitude is. Now create a maneuver node anywhere along your orbit and increase your prograde direction until the apoapsis of your new orbit is around the same height as the Mun's orbit. This should give you a maneuver requiring an engine burn of only about 900 delta-v. Now click and hold on the center of your maneuver node so that you can now drag it forward along the line of your current orbit. If you're doing it right you should be able to drag the point at which your burn starts to a later time in your current orbit.

What happens as you move the maneuver node around? The high point of the predicted orbit also travels around touching the inside circle of the Mun's orbit. And there should be a point where the tip of that orange line just barely meets the Mun in its orbit. That way you will get an encounter that uses much less fuel. Remember, it takes time to cross the distance between Kerbin and the Mun, so when you get to the Mun it won't be in the same place as it is right now. So you have to make your maneuver line cross the Mun's orbit way out in front of where the Mun is right now.

Edited May 23, 2017 by HvP

[Ten Key]

2 minutes ago, jonpfl said:

So essentially I am trying to get a green path as that means I will be in orbit around the Mun?  

No, the green path is irrelevant. It's only there because you haven't told the game you intend to do another burn to enter Munar orbit.

There are actually three different projected paths in your screenshot. The orange one is first, and is within Kerbin's sphere of influence. The second is hidden behind the Mun in your screenshot-- it is purple, and represents your projected path inside the Mun's SOI. The green path is third, and represents your orbit once you have crossed back into Kerbin's SOI. Each color change corresponds to a sphere of influence change. If you intend to enter Munar orbit, then you don't want to leave the Mun's SOI, so presumably you'll perform a maneuver somewhere on the purple path to make sure the green one never happens.

 

7 minutes ago, jonpfl said:

Is SOI the same as orbit?

No. A sphere of influence represents a spherical chunk of space where the gravity of a particular object is so dominant that, for mathematical purposes, the gravity of other objects can be ignored. When you are within Kerbin's SOI, the game only calculates Kerbin's gravitational influence on your ship. The Mun's sphere of influence is a sphere that extends. . .2,200 km I think, from the center of the Mun. As soon as you cross into this region, the game switches to using the Mun's gravity for its orbital calculations. When you exit the Mun's SOI, it starts using Kerbin's again.

Kerbin's SOI is (again from memory) about 84,000 km. If you exceed this altitude about Kerbin, you leave Kerbin's SOI and enter the sun's. 

 

14 minutes ago, jonpfl said:

I keep seeing posts saying that getting to the Mun should be 800 to 900 m/s but I have NO idea how to control that.  When I doing a maneuver around Kerbin, I am just playing with the symbols until I am getting a path that gets to the moon.  Whatever I am doing, should I be doing the opposite and that should get me to that number?

Target the Mun. Create a maneuver node. Pull on the green prograde marker () until the dV indicator reads 850 m/s. Then drag the node around your orbit until you get an encounter. Ideally, you want your projected Munar periapsis to be below 50 km. . .15-20 km is good. If the green path goes away entirely, that means you're on a collision course and your Munar orbiter is about to become an impactor.  Perform the burn. Next, create another burn at your Munar periapsis, and this time use the retrograde marker (). Pull on the retrograde marker until you have an orbit. 

When you're ready to leave Munar orbit, use the prograde marker  to set up your exit burn. Your exit trajectory should be a path roughly parallel to the Mun's orbit, but moving in the opposite direction. 

Because of your initial inclination of 1.6 degrees, you may very well end up in a near polar orbit around the Mun. If this happens, there are only two points in the Mun's orbit where you can return to Kerbin. Your polar orbit must be nearly parallel to the Mun's at the time of your departure burn. 

[Starman4308]

One small additional thing is that, if you want to tweak your entry into the Mun, it's often best to perform your correction maybe halfway to 2/3 of the way there. The farther away you make the correction burn, the easier it is to change direction due to having slowed down so much, but the maneuver has less time to adjust your eventual position.

Also, as mentioned, burn prograde about 90 degrees behind the Mun, and you'll meet up with it at apoapsis.

[jonpfl]

Wow, great stuff guys!!!

I know my questions seem really basic but it is nice to ask them and not get snarky replies!

When I was messing around with it last night, I could get my ship to go around the moon but it would leave orbit (it was curving around the moon).  I assume that had to do with me going too fast and I had to slow down.  Should I be doing a man node at some point around the earth and slowing down?  

Thx
jonpfl

Edited May 23, 2017 by jonpfl

[Starman4308]

45 minutes ago, jonpfl said:

When I was messing around with it last night, I could get my ship to go around the moon but it would leave orbit (it was curving around the moon).  I assume that had to do with me going too fast and I had to slow down.  Should I be doing a man node at some point around the earth and slowing down?  

Thx
jonpfl

With how KSP handles physics, it's physically impossible to get into an elliptical Mun orbit from outside the Mun's SOI. You need to make another maneuver node at Mun periapsis to achieve a stable orbit, burning retrograde. You have some velocity relative to the Mun coming into its SOI, plus some from falling inwards, and you need to remove some of that to stay inside its SOI.

Also, I suspect the sarcasm is stayed by being polite about asking questions, the questions literally being rocket science, and the experienced players being those who can do rocket science. This forum is usually pretty good about staying polite.

EDIT: The reason for this is conservation of orbital energy. When you enter a sphere of influence, your orbital energy will be constant unless something happens (such as an engine burn or aerobrake). When entering an SOI, you by definition have enough orbital energy to escape again, because you have A: sufficient gravitational potential energy to be at the outer edge of the SOI, and B, a finite amount of kinetic energy from having some velocity relative to the central body (you did enter the SOI in a finite amount of time, yes?).

Because of that, to insert into lunar orbit, you need to shave off some of that orbital energy, for which the best option is almost always a retrograde burn at periapsis, as that benefits from the Oberth effect.

Edited May 23, 2017 by Starman4308

[Loren Pechtel]

On 5/20/2017 at 11:44 AM, jonpfl said:

EDIT : I figured it out, I was trying to go from in front of the planet, figured out I am supposed to do from the backside.

 

 

Welcome to the wonderful world of orbital mechanics where many things do not work as you would expect.  In space you do not burn towards your target unless the velocity involved is quite small compared to your orbital velocity and your target is nearby.

While the orbital mechanics in the game are simplified for the sake of getting the answers fast enough what you see in the game is pretty much what NASA sees.  Burning at the Moon would be just as disastrous for NASA as you found when you burned at the Mun.

[jonpfl]

Ok,

Trying this tutorial again, any suggestion on trying to get both Ap and Peri around 200k?  Whenever I try to move one, the other moves and vice versa.

Thx
jonpfl

[bewing]

To change your Pe/Ap, you need to burn prograde or retrograde. When you burn prograde or retrograde, you affect the opposite side of your orbit. If you burn somewhere between the Ap and Pe on your orbit, you will affect both of them, and the percentage will be based on how far from each of them you are. If you burn right at your Ap, you will only change your Pe -- and vice versa.

It is also possible to change both your Pe and Ap at the same time by burning Radial In or Radial Out. But generally that is inefficient, so it is not recommended unless you are just making a tiny change.

 

Edited May 26, 2017 by bewing

[jonpfl]

4 minutes ago, bewing said:

To change your Pe/Ap, you need to burn prograde or retrograde. When you burn prograde or retrograde, you affect the opposite side of your orbit. If you burn somewhere between the Ap and Pe on your orbit, you will affect both of them, and the percentage will be based on how far from each of them you are. If you burn right at your Ap, you will only change your Pe -- and vice versa.

It is also possible to change both your Pe and Ap at the same time by burning Radial In or Radial Out. But generally that is inefficient, so it is not recommended unless you are just making a tiny change.

 

Great stuff, thanks for explaining!!