Help with maneuver nodes

[KrisKelvin]

Ok, so I have tried out maneuver nodes for the first time to get into an orbit. Can you take a look at this and let me know of any tips, advice, criticisms etc? Annoyingly I ran of fuel before I completed the maneuver. (maybe that's what the red part of the burn indicator means?)

https://drive.google.com/open?id=1aARGGL2w-gVXIWb6eWfB3iprkcMxsW_9

(I find the controls a bit hard - dragging the prograde marker out in front. It's very laggy! I pull a bit too far and the orbit shoots way out the other side and I have to delete the node and start over)

Thanks

[AHHans]

1 hour ago, KrisKelvin said:

Annoyingly I ran of fuel before I completed the maneuver. (maybe that's what the red part of the burn indicator means?)

Yup. That's what the red part on the indicator and the red color of the burn time wants to tell you. One word of warning: that calculation is notoriously wrong while your craft is still in atmosphere - even if the maneuver node is in vacuum - and underestimates your dV and TWR (thus overestimates burn time). So don't panic if your maneuver node readout says that you don't have enough fuel while the dV calculation in the staging display says that you have enough.

1 hour ago, KrisKelvin said:

(I find the controls a bit hard - dragging the prograde marker out in front. It's very laggy! I pull a bit too far and the orbit shoots way out the other side and I have to delete the node and start over)

You don't have to delete the node, you can:

• pull the prograde marker in the other direction
• pull the retrograde marker
• mouse over the pro- or retrograde marker and use the mouse-wheel to adjust the burn
• switch the maneuver-node display (in the lower left corner, where it says how many m/s you burn in which direction) to maneuver-node-manipulation and klick on the retrograde marker
• manually enter a m/s value in the maneuver-node display when it is in list mode

As with many things in KSP (and RL): practice makes perfect.

[N_Danger]

36 minutes ago, AHHans said:

 

• switch the maneuver-node display (in the lower left corner, where it says how many m/s you burn in which direction) to maneuver-node-manipulation and klick on the retrograde marker

 When you are in this mode there is a scale to the right that you can set the magnitude of the clicking. 1 meter per, 2 per, 10 per etc..

 

[Vanamonde]

Looks fine to me.

[KrisKelvin]

22 hours ago, AHHans said:

 

• switch the maneuver-node display (in the lower left corner, where it says how many m/s you burn in which direction) to maneuver-node-manipulation and klick on the retrograde marker
• manually enter a m/s value in the maneuver-node display when it is in list mode

 

Thanks for this. Great tips for adjusting finely. Not sure I understand your last two bullets though. WOuld you be able to make screenshots? I can see velocities that can be set, but I am not sure what they mean. Is this just another way of setting apo height?

[AHHans]

Does this video from when the maneuver-node display was introduced answer your questions?

 

[KrisKelvin]

23 hours ago, AHHans said:

Does this video from when the maneuver-node display was introduced answer your questions?

 

err.. thanks. Kind of. But I still can't seem to make a node that gives me the orbital parameters I want. Endless fiddling about and I still can't get the periapsis to be the same as the apoapsis. I placed the node on the apoapsis and then tried to bring the periapsis up to match it. But the damned apoapsis changes too! So frustrating.

And why on earth are the settings in the maneuver node box in m/s instead of metres? Surely it would be better to be able to set the orbital parameters you are wanting? What use is setting a speed and then having to try and get to a height by trial and error?

[AHHans]

1 hour ago, KrisKelvin said:

And why on earth are the settings in the maneuver node box in m/s instead of metres?

The maneuver nodes are a tool to plan maneuvers, it show you how your trajectory will change if you perform the specified maneuver(s). It is not an autopilot that will tell you which maneuver(s) you need to do to achieve a certain outcome (e.g. to reach a certain orbit). And an orbital maneuver means running your engines to change your velocity at a certain point in time by a certain amount in a certain direction. So you get to specify that: the point in time (i.e. the position on your current orbit) and the dV in three perpendicular directions, pro-retrograde, radial, and normal.(*)

1 hour ago, KrisKelvin said:

Endless fiddling about and I still can't get the periapsis to be the same as the apoapsis. I placed the node on the apoapsis and then tried to bring the periapsis up to match it. But the damned apoapsis changes too! So frustrating.

How precise are you expecting it to be??? It shouldn't be hard to get into an orbit that is way more circular than what you manage with manual flying.

Anyhow, one issue I usually have is that I don't put the maneuver node exactly at the AP. If the predicted AP moves away from your current AP while the PE is still well below your current AP, then the maneuver node is not exactly at the same time as your current AP. Move the time of the node a bit to try to get it better. (I usually dial the sensitivity of the "button-interface" in the maneuver-node display all the way down, and then use the two arrows in the center of the tool to adjust the time of the node to get the predicted AP back to the current AP.

(*) Note: the maneuver nodes assume you do all your velocity change in a single event with infinite TWR. In reality you need some time for the acceleration and long burns with low TWR will differ significantly from the predictions of a maneuver node.

[JoeSchmuckatelli]

On 5/31/2020 at 2:15 PM, AHHans said:

Does this video from when the maneuver-node display was introduced answer your questions?

 

...I just learn that now?

 

NOW?!?

 

SMH.

 

Tango yankee

 

( @KrisKelvin -- I'll try to not hijack this one!)

[king of nowhere]

On 5/30/2020 at 11:25 AM, KrisKelvin said:

(I find the controls a bit hard - dragging the prograde marker out in front. It's very laggy! I pull a bit too far and the orbit shoots way out the other side and I have to delete the node and start over)

Thanks

It's not laggy, it's sluggish. the rocket is big and the control wheel aren't very powerful, so the rocket will respond slowly to your attempts to turn it. and once it responds, when it gains enough momentum, it will alos take time to slow it down.

You can fix this by adding reaction wheels, they aren't too heavy and they make a rocket much more manueverable.

On 5/31/2020 at 11:41 AM, KrisKelvin said:

Thanks for this. Great tips for adjusting finely. Not sure I understand your last two bullets though. WOuld you be able to make screenshots? I can see velocities that can be set, but I am not sure what they mean. Is this just another way of setting apo height?

those velocities are accelerations that you plan to make.

From the way you talk, I assume you don't know much about orbital mechanics, so I'll make a flash course. when you are in orbit, you stay in that orbit indefinitely if nothing disturbs you. when you give yourself a push, you change your orbit. So, you need a rocket burn of a certain delta-V to change your orbit to a different one. But there are rules.

The first, most important rule is that, the moment your rocket stops firing, your current position will be part of the new orbit. because once you stop your rocket, you are in an orbit, and you are going to go round and round without changes. so, in order to change something about your orbit, you much give yourself a push on the opposite side of your orbit, in a place where the old and new orbit intersect. You want to change your apoapsis? you must burn your engine at periapsis, and viceversa. You want to move to an orbit with a different inclination? you must do so in the point where your orbit intersects the new inclination you want to achieve.

Second, know your direction. you have prograde and retrograde, the greenish-yellow ones. those are the most important, they increase or decrease your orbital speed. burning in those directions you can move your orbit lower or higher. i don't remember the names of the other directions, but the purple ones change your orbital inclination, so that an orbit going around the equator may pass over the poles instead. those are very expensive manuevers, it's better to not make them if possible - barring small corrections when you need to set up an orbital rendez-vous. the blue ones change an orbit's eccentricity, they turn a circular orbit elliptical, or an elliptical orbit into a different ellipse. they are seldom used, because they are of limited scope; but sometimes you get a mission to bring a satellyte into a specific orbit, and you will need those to move your periapsis around, because that's one thing they can do: move your periapsis and apoapsis from one part of your orbit to another part.

I can't tell what's wrong on your attempts to circularize the orbit, but the only way you can achieve that is by going on periapsis and burn retrograde until you lower your apoapsis to your current level, or go on apoapsis and burn prograde until you raise the periapsis to the current level. You burn in any other place of the orbit, or with any other direction, you are going to change both apoapsis and periapsis in ways that can only be predicted by an elaborator. And after you set up the nodes, you have to actually make the burns. and of course, you are exactly at the apoapsis/periapsis only for one second, so your burn is never going to be perfect. but getting a circular orbit, barring a few kilometers of difference, is very easy. be sure to burn very slowly when you are close to the end of the manuever

[KrisKelvin]

thanks again everyone. I need to practice this a LOT more, I think. Useful tips.

[glibbo]

Hi, just seen your video.

I'm no expert and you asked for some help, hope this may do ....

in your video you are burning your first stage engine well beyond the 70 -80 - altitude required for orbit and wasting fuel ...

second, even if you don't want to use it for maneuvers, mechjeb  will provide a lot of information for you to see what is happening...

not much I know but hope it helps.