A couple of gameplay questions

[Kitspace]

Hello!

Quite recently I have encountered a new problem somehow. No idea on why this was not an issue for me before so maybe I am doing something simple terribly wrong.

My rockets tend to flip over uncontrollably after gaining some positive angle of attack at the very begging of the gravity turn. The only reason I can see is that the rocket is going to fast at the moment of initial pitch over when the prograde vector did not move from its vertical position yet. But also I have no idea of what is the best time and speed for that and what amount of input should be applied.

I am using mechjeb guidance and FAR.

Is there a way to limit the thing to terminal velocity and get correct atmospheric predictions with that combination? Maybe there should be a special version of mechjeb for that, what do you think of it?

Thankyou!

[Pecan]

MechJeb usually doesn't play nicely with FAR. The improved aerodynamics of FAR mean you must start your gravity turn from much lower and turn much more gradually than MJ does. I don't use FAR myself but IIRC the recommended gravity turn starts at 2km or so, never pitching-over so fast that your heading indicator on the navball moves outside the circle of the prograde marker.

An expert should be along soon ...

[Claw]

Yes, if you are using FAR the gravity turns need to be very gentle. MJ tends to be pretty abrupt with it's default turn. Being that agressive will result in tumbling or the rocket breaking up.

As Pecan said, do the turn a bit more gradual. You can also start it sooner with the improved FAR aerodynamics.

[Hodo]

The best profile I have found with MJ and FAR, is to start your gravity turn at 1km altitude, and I think set it for 13-15deg.

I use MJ with FAR and it works fine with rockets, just don't use the stock launch to 10+km and turn 90deg and burn to orbit. That is not a gravity turn, that is a gravity right angle.

[Jimbimbibble]

You may also want to reduce your TWR for Ferram. Shoot for an initial TWR of 1.2-1.5 and start your turn when you get to between 50m/s and 100m/s respectively. Pitch over about 5 degrees and you know the rest.

[Kitspace]

Sorry, what do you mean under setting the MechJeb for a certain amount of degrees to pitch over? Is it possible to adjust that? Is it possible to make it more gentle and make it wait for the prograde vector to follow the nose direction not to apply more input and increase the angle of attack further?

I usually just set the ascent turn shape to ninety or even a hundred percent and the final flight path angle to a few degrees below prograde to make it start slower with less control inputs and burn longer at the end to realistically achieve a stable orbit in a single burn without circularization.

I have never done the stock shot strait up and then turning for fourty degrees at a time even when I just first came across the game and used no mods.

So, you suggest starting to pitch over at airspeeds of about fifty to a hundred metres per second, did I get it right?

The problem is that most of my rockets usually tend to accelerate very rapidly and get supersonic at around seven kilometres or so. I have to throttle down very aggressively and leave myself with not that much control torque from thrust gimballing, at least until I progress to much larger rockets and payloads through my space program.

I really need a good way to forcibly limit rockets to a reasonable terminal velocity.

Also, I suppose those rockets which have their center of lift higher than their center of mass and center of thrust are aerodynamically unstable and will tend to rapidly increase any angle of attack value other than zero. That is just the way physics work here. That could be the main explanation of my problem. So, most things capable of ballistic flying ranging from toy rockets to missiles have aerodynamic fins to counter that. But they are an extremely rare occasion on space launch vehicles. How do they counter this problem and achieve an acceptable level stability and controllability, while still in the dense atmosphere just as the matter of general safety? If the real rockets behave the same way they should without fins and the same way they do in the game, why can not a strong wind gust or a momentary unexpected wobble of some sort in the controls make the rocket flip upside down and destroy an expensive payload or even lead to casulties?

Thank you!

[Wallygator]

Deleted...

Edited June 7, 2014 by Wallygator
Attempt to be helpful gone wrong.

[Hodo]

Sorry, what do you mean under setting the MechJeb for a certain amount of degrees to pitch over? Is it possible to adjust that? Is it possible to make it more gentle and make it wait for the prograde vector to follow the nose direction not to apply more input and increase the angle of attack further?

I usually just set the ascent turn shape to ninety or even a hundred percent and the final flight path angle to a few degrees below prograde to make it start slower with less control inputs and burn longer at the end to realistically achieve a stable orbit in a single burn without circularization.

I have never done the stock shot strait up and then turning for fourty degrees at a time even when I just first came across the game and used no mods.

In Mechjeb there is a tab, "acsent guidence" in that you will have a sub tab at the very bottom which lets you set the actual flight path. Set it between 13 and 15 deg, based on your TWR and desired altitude.

You can also launch the rocket using a real gravity turn, which is having your TWR around 1.2 and taking off, once you hit 100m/s tip the craft 5deg in the direction you want it to go, and let it gradually tip on its own. By the time you hit orbital altitudes it should be pretty much level with the horizon.

[Kitspace]

I certainly know what an engine gimballing is. If you read carefully you can see this term in my posts, and all my actual questions are about a quite different matter, aerodynamic forces applied to the launch vehicle during the ascent to be exact.

I know about the flight path editor window. It lets me adjust the altitude of the initial pitchover maneuver and the actual path shape. What should I set to fifteen degrees there? It has no means of adjusting angles other than the final attitude the maneuver should end on in orbit.

Does not the mechjeb perform the actual gravity turn? It does not seem to apply any control inputs other than heading corrections after performing the initial pitchover maneuver? If not, how to make it perform one? How to perform a dogleg properly? How to launch something into the plane of an existing target, but with a different orbit inclination?

[Hodo]

Ok now you are getting into advanced questions for MJ. I can't help you much there, I usually do those things manually. But I know you can set MJ to do most of those things.

As for the settings in the flight path editor, the curve that you see that is the flight path indication line, should have a reading of some type of deg... I am not at home or on KSP right now so I cant tell you off the top of my head.

[Link]

Ok now you are getting into advanced questions for MJ. I can't help you much there, I usually do those things manually. But I know you can set MJ to do most of those things.

As for the settings in the flight path editor, the curve that you see that is the flight path indication line, should have a reading of some type of deg... I am not at home or on KSP right now so I cant tell you off the top of my head.

The value being spoken of is in form of a percentage, not degree. It's the last value on the 'edit ascent path' window.

[Hodo]

The value being spoken of is in form of a percentage, not degree. It's the last value on the 'edit ascent path' window.

Thank you, I haven't messed with my MJ flight path editor in weeks, because I fly space planes most of the time, and in RSS my rockets are pretty much self orbiting.

[Claw]

I know about the flight path editor window. It lets me adjust the altitude of the initial pitchover maneuver and the actual path shape. What should I set to fifteen degrees there? It has no means of adjusting angles other than the final attitude the maneuver should end on in orbit.

Yes, you are in the right spot. If you move the percent slider up and down you can watch how it flatens the shape, or makes it curve more upward. For FAR, you want a more gentle curve and not one where it bends in half and flies out.

There is also options for what altitude to start and complete the turn. For FAR, you'll want to move the start altitude down. The end altitude will depend on the abilities of your rocket, but has much less of an impact on your ship's stability except to control how quickly MJ completes the pitch down maneuver during flyout.

Does not the mechjeb perform the actual gravity turn? It does not seem to apply any control inputs other than heading corrections after performing the initial pitchover maneuver? If not, how to make it perform one? How to perform a dogleg properly? How to launch something into the plane of an existing target, but with a different orbit inclination?

Yes, MJ performs the turn but it does it however you specify in that trajectory window mentioned above. You don't specify pitch in degrees, but you specify how sharp the turn is (with percentage) and how long it will take over that curve (with the altitudes). The more distance between the start and end altitudes, the less quickly the craft will pitch down (overall rate) throughout the whole turn. The percentage will also control how quickly the craft pitches, but in a slightly different way. Essentially controlling how sharp the initial turn is, and the shape of the trajectory.

Make sure that you actually engage MJ's autopilot thoug, or it won't do anything. After the initial pitch over, it is providing inputs. But it doesn't take much for it to follow the curve after that. You can also play with the "corrective steering" option. That can help compensate for under-powered or under-controlled craft.

[Pecan]

...how quickly MJ completes the pitch down maneuver during flyout...

...pitch down (overall rate) throughout the whole turn...

I think we need to be careful with terminology here, to avoid later confusion over controls, especially with aeroplanes. Yes, the pitch of the heading is being reduced but, in a normal equatorial launch, that's done with the "yaw-right" D key, not the "pitch-down" W key. The term I try to use for the gravity turn is 'reducing pitch' as it also covers all those situations where the vehicle has rolled during the ascent. Similarly, I use 'reduce/increase/adjust' pitch when discussing landing, regardless of which controls are actually required.

Yes, I realise this sounds a bit like nit-picking, but it's not meant to be - I'm trying to avoid situations where people automatically hit W/A to adjust pitch. Apologies if it's just rubbish.

[Laie]

I know about the flight path editor window. It lets me adjust the altitude of the initial pitchover maneuver and the actual path shape. What should I set to fifteen degrees there? It has no means of adjusting angles other than the final attitude the maneuver should end on in orbit.

There's at four figures in the MechJeb ascent planner:

1. turn start altitude
   
2. turn end altitude
   
3. final angle
   
4. "turn shape", a percentage.
   

You need to set 1) to a pretty low altitude, or 4) to a pretty high figure, or a combinatrion of both.

[Claw]

I think we need to be careful with terminology here, to avoid later confusion over controls, especially with aeroplanes. Yes, the pitch of the heading is being reduced but, in a normal equatorial launch, that's done with the "yaw-right" D key, not the "pitch-down" W key. The term I try to use for the gravity turn is 'reducing pitch' as it also covers all those situations where the vehicle has rolled during the ascent. Similarly, I use 'reduce/increase/adjust' pitch when discussing landing, regardless of which controls are actually required.

Yes, I realise this sounds a bit like nit-picking, but it's not meant to be - I'm trying to avoid situations where people automatically hit W/A to adjust pitch. Apologies if it's just rubbish.

Yes, I'm aware of the difference in pitch/yaw. It's going to depend on the orientation of probe cores and yadda yadda. I was trying not to get caught up in every detail, but you are correct. If I would have said yaw, it would be the same problem.

Since we're talking about rockets, there's usually less of an obvious difference in pitch/yaw (besides the buttons you push).

Anyway, nitpicky but accurate.

Pecan: I read this post again and I was worried I came off a bit snotty. I don't mean to be. You are correct to point it out. I was just trying to simplify and be consistent with the OP.

Edited June 7, 2014 by Claw

[Tortoise]

A picture would help.

But I use FAR and I gently gravity turn at 10km.

[Kitspace]

As you started talking of what should be considered pitch and yaw in our case here.

Is it possible to make mechjeb perform a roll programme after liftoff as the real rockets do and pitchover at the axis which is structurally the pitch axis of the vessel?

Is there a reliable way of getting the current terminal velocity with all that mods?

How to keep my center of lift reasonable? Things like fairings affect it very much. How do I get it back down?

After all this I want to perform a real downrange gravity assisted ascent instead of all those kerbal things called gravity turns for some reason.

How do I calculate it?

How do I need to adjust my rocket stability and thrust performance?

How do I make it take me where I need my rocket to be not where it would end up on its own?

Can I present all that as a bunch of mechjeb settings somehow to make its ascent path the same or very close to what would happen anyway?

I do certainly not want it to affect my efficient and safe ascent path and destroy my rocket again somehow but I really love the other features of the ascent guidance such as firmly keeping me on the desired heading or steering a dogleg into the target plane.

How to achieve that if that is possible at all? Or is it possible to limit guidance to horisontal control only?

[Claw]

As you started talking of what should be considered pitch and yaw in our case here.

Is it possible to make mechjeb perform a roll programme after liftoff as the real rockets do and pitchover at the axis which is structurally the pitch axis of the vessel?

Is there a reliable way of getting the current terminal velocity with all that mods?

How to keep my center of lift reasonable? Things like fairings affect it very much. How do I get it back down?

After all this I want to perform a real downrange gravity assisted ascent instead of all those kerbal things called gravity turns for some reason.

How do I calculate it?

How do I need to adjust my rocket stability and thrust performance?

How do I make it take me where I need my rocket to be not where it would end up on its own?

Can I present all that as a bunch of mechjeb settings somehow to make its ascent path the same or very close to what would happen anyway?

I do certainly not want it to affect my efficient and safe ascent path and destroy my rocket again somehow but I really love the other features of the ascent guidance such as firmly keeping me on the desired heading or steering a dogleg into the target plane.

How to achieve that if that is possible at all? Or is it possible to limit guidance to horisontal control only?

(I suppose this thread should be moved to the add-on forum...)

Anyway. Yes, MJ will automatically manage throttles for you with the "Limit to terminal velocity" option in the ascent guidance. Alternatively, you can set up a custom window to display the current terminal velocity. HOWEVER, these options do not work with FAR installed, since FAR zeros out the drag. I'm not sure if FAR has a way to display terminal velocity, but I would guess so. For MJ, you can also use options such as "limit acceleration to..." or "limit throttle to...". If you limit acceleration to 2Gs, that will approximate flight at terminal velocity for the initial portion of the flight. Once you have pitched over a good ways and are in thinner atmosphere, you can remove that limit.

Unfortunately, turning on MJ is just about an all or nothing proposition. If you want it to control the heading, it will also want to control the ascent angle. It will also max the throttle unless you give it some limit.

I recommend that you play with the various MJ ascent settings to try and understand them. For ecample, you can open the ascent path editor and move the slider left/right while the rocket is in flight. This will cause the rocket to pitch up/down during flight and you can watch what it does.

MJ also has a lot of settings that are not on the default ascent guidance page. You can dig around in the window editor (another MJ tab) to find them all.

Edit: I almost forgot. You can move the center of lift around by placing fins and control surfaces near at the bottom of your craft. That will pull the CoL down and also make your rocket a bit more stable.

[Pecan]

MJ is what MJ is, if you want to be able to do all sorts of other 'autopilot' things you might want to look at KOS, Telemachus and/or one of the scripting-language add-ins (I still vote for Lua!).

@Claw - no worries, I know you're not the 'snotty' type and I didn't take it even as disagreement particularly - just "yes, terminology is a problem, I'm using it like this ..."

[cantab]

As you started talking of what should be considered pitch and yaw in our case here.

Is it possible to make mechjeb perform a roll programme after liftoff as the real rockets do and pitchover at the axis which is structurally the pitch axis of the vessel?

I can't answer this, but I do know it's possible to pick up the whole ship in the VAB and turn it 90 degrees, so that pitch down will push the nose east right from the launchpad.

[Kitspace]

I need that most not because of the equatorial orbits, but because of those that are not equatorial or polar...

For example, Soyuz launch vehicles can not perform programmed rolls while airborne and their launch tables are capable of turning to the correct azimuth with the rocket installed, but most launch vehicles still perform a roll programme a few seconds after liftoff.

And I also really need some way of firmly holding the rocket on the desired heading as if I do not use mechjeb or stock stabilizer the rocket tends to float away all the time.

For me calculating what would the natural path be and then make the mechjeb follow the very same trajectory still remains the best idea. What do you think of this?

[Claw]

If you want to launch into a particular inclination, MJ does that. There's no need to roll the craft to get a pure pitch or pure yaw input unless your rocket is only 2x symmetric around the z axis. If you are just trying to launch at something that isn't directly east or north, put in the desired angle into the "inclination" window in the ascent guidance module. Bt the way, inclination is measured from the equator. 0 is due East.

The shuttle performed a roll so it could pitch over onto it's back. I don't know if there is a way to do that with MJ's ascent guidance, but the SmartASS module may be able to help with this.

Edited June 8, 2014 by Claw

[Kitspace]

Most real rockets, sertainly not only the space shuttle launch vehicle perform a roll programme even if they have other sorts of radial symmetry, just because many things from their guidance systems to their engine gimballing abilities make it probably the only possible way to pitch over onto the side which is on its structural pitch side.

Imagine that you have a couple of gimballing engines to pitch you up and down, and a couple of gimballing engines to yaw left and right. Combination of both groups of those can be used to roll. Neither of these can gimbal on two axis. Such system is used on lots and lots of different rockets in the real world. Do you agree that it is easier for this system to roll and pitchover on your back then to point your nose in a direction that would require a very weird combination of inputs on all three axis all the time?

[Claw]

It's not that I disagree, it's that KSP doesn't limit the gimbal angles on engines. And if you are using 4x symmetry, it won't matter for the control surfaces. MJ (or a human pilot) will have just as much authority either way.

Are we discussing reality here or how MJ controls a KSP rocket?