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Diagnosing my Satellite?


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I'd say it has gone all wibbly-wobbly due to the lack of struts. 

You may not have struts yet if early in the game so you'll need to move the first stage up a bit to try to improve stability. 

Edited by Foxster
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2 minutes ago, Foxster said:

I'd say it has gone all wibbly-wobbly due to the lack of struts. 

You may not have struts yet if early in the game so you'll need to move the first stage up a bit to try to improve stability. 

Its strutted near the bottom you just cant see it in the pic. Its very stable in flight so thats not the problem.

The problem is that when I turn east for my gravity turn it starts to kinda fall north and rotate around the inner circle. Its hard to explain.

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6 minutes ago, Solis said:

Its strutted near the bottom you just cant see it in the pic. Its very stable in flight so thats not the problem.

The problem is that when I turn east for my gravity turn it starts to kinda fall north and rotate around the inner circle. Its hard to explain.

Do you have it rotated in the VAB so it's already facing out over the ocean?

Your triple booster setup is going to be heavier on the one side than the other, which will most likely be OK so long as you keep it either in front or behind, and don't let it get off to one side.

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2 minutes ago, Rocket In My Pocket said:

Do you have it rotated in the VAB so it's already facing out over the ocean?

Your triple booster setup is going to be heavier on the one side than the other, which will most likely be OK so long as you keep it either in front or behind, and don't let it get off to one side.

Ah, okay, that makes sense. Should I add another booster so that its equal on all sides, then?

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Just now, Solis said:

Ah, okay, that makes sense. Should I add another booster so that its equal on all sides, then?

That would probably make it easier to pilot. 4 boosters/ 4 fins is far more user friendly than 3/3.

Triangle formations are certainly doable though, but you want to carry the heavier side either on your rockets "belly" or it's "back" it's easier to compensate for that way.

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I've just built a close replica of your craft and it flies to orbit OK. 

Thing is that the cost is different to your craft, so something must be a bit different. Are those all Swivel engines?

Maybe you could post the craft file in Dropbox or similar?

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If you are not using a swivel engine for that middle engine, it might cause problems.

Fins should be useless.

The 3-way symmetry shouldn't be an issue unless you eyeballed the placement instead of using the in-game symmetry.

It's probably unstable on the launch pad since the clamps are below the CoM.

Your boosters are placed a little weird in comparison to the side decouplers. Try to offset the parts until the decouplers are roughly in the middle of the decouplers and/or add struts if you can.

Did I mention those fins are probably useless, heck, a problem even? The fins are potentially an issue. Fins on rockets make them not want to turn, plus the lift they provide is going to be unequal as the trajectory changes during the gravity turn. Also, if you are trying to steer with them... ugh. Use a swivel and avoid fins.

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@Ohm is Futile:

All of the engines have swivel.

I thought that you were supposed to put the clamps at the bottom-most part of the rocket?

I think you mean to have the decouplers in the middle of the boosters? I will fix that. Also I do have struts at the bottom. You can very slightly see them if you zoom in.

Those are winglets with control surfaces, will those still cause a problem?

Edited by Solis
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8 minutes ago, Solis said:

@Ohm is Futile:

All of the engines have swivel. Good, might actually be too much. Does your rocket go nuts if you use precision controls? (caps lock during flight) Either way, it's generally a good idea to have gimbals on your central engine.

I thought that you were supposed to put the clamps at the bottom-most part of the rocket? That will likely cause your rocket to wobble on the launch pad, they should be near or above the CoM so that the rocket kind of hangs from them.

I think you mean to have the decouplers in the middle of the boosters? I do have struts at the bottom. You can very slightly see them if you zoom in. Oh yeah, I see them now. Well, I mean, as long as the boosters don't wobble, you're golden.

Those are winglets with control surfaces, will those still cause a problem? Yes, they would. Usually better leaving anything with lift for planes. If you have stability issues with rockets, it's better to try to remove drag from the top and to check for any structural weaknesses or unbalanced engines, that type of thing. As I said, wings/winglets will generally cause all sorts of weirdness on rockets as they create drag at the bottom, which makes the rocket want to hold prograde(and not move from there, regardless of input). Also, they can do weird things once the rocket turns compared to going straight up, due to unbalanced lift. Control surface or not, it's not ideal.

 

Edited by Ohm is Futile
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I go the other direction as Ohm -- I use wings and fins for landing legs on all my landers. So I tend to have lots of aerodynamic surfaces on my rockets. I like rockets that follow prograde. :)

Also having the decouplers high on the boosters can be very helpful. It forces the boosters to fly away from the rocket when you decouple them -- rather than having them crash into your rocket, which (for some crazy reason) can mess up your launch.

 

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How much your payload weights?  And what is the target orbit?  (apoapsis,  periapsis and inclination) 

The rocket seems overengineered to me. Making it smaller may allow for a more stable layout. (if nothing else,  make it cheaper) 

Well,  your rocket spin out of control because it is aerodynamic unstable. It means that there is more drag in front of CoM then behind, the rocket want to fly tail first.  

Notice this unstability only appear when you deviate from prograde,  because that is the moment you expose a larger area (of the rocket's nose) to the airflow.  So if you managed to stay aligned to prograde maybe you could maintain control.  However for this you would need to have the rocket inclined East at the launchpad,  and even then there is no guarantee. 

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@Spricigo

You may be right about the over-engineered part because I basically just put satellite stuff on my Mun orbiter.

It weighs 47.5 tons and my target orbit is 12km AP and 10km PE with 11.8° inclination.

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42 minutes ago, Spricigo said:

How much your payload weights?  And what is the target orbit?  (apoapsis,  periapsis and inclination) 

The rocket seems overengineered to me. Making it smaller may allow for a more stable layout. (if nothing else,  make it cheaper) 

I tend to agree with that, but who knows? It depends on the target orbit for the satellite and a bunch of other factors.

 

43 minutes ago, Spricigo said:

Well,  your rocket spin out of control because it is aerodynamic unstable. It means that there is more drag in front of CoM then behind, the rocket want to fly tail first.  

Notice this unstability only appear when you deviate from prograde,  because that is the moment you expose a larger area (of the rocket's nose) to the airflow.  So if you managed to stay aligned to prograde maybe you could maintain control.  However for this you would need to have the rocket inclined East at the launchpad,  and even then there is no guarantee. 

I strongly doubt that there is more drag at the front of the rocket, unless the service bay doors are open during the lower atmosphere part of the flight. If the doors are closed, there's no way the drag is higher at the top than the bottom with those winglets (not that I think they should be there, but that's another debate). The doors are closed during launch, right?

Not a bad idea to incline the rocket on the launch pad, especially with the clamps. It's possible the OP is simply being too agressive with the gravity turn, though. You should never get far(or leave the edge of the circle even) from the prograde marker even with a stable rocket during the lower part of the ascent (<30km altitude).

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@Ohm is Futile yes, there is amore drag in the top than in the botton, you need to consider it relative to the CoM (which in this rocket is way to the bottom):

ahead of the CoM: 2 service bays, , 1 terrier, 5 fuel tanks

behind CoM: 4 swinvel/reliant, 4 fuel tanks

also the service bays and one of the fuel tank is more distant from the CoM so drag in those parts result in more torque .

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2 minutes ago, Spricigo said:

@Ohm is Futile yes, there is amore drag in the top than in the botton, you need to consider it relative to the CoM (which in this rocket is way to the bottom):

ahead of the CoM: 2 service bays, , 1 terrier, 5 fuel tanks

behind CoM: 4 swinvel/reliant, 4 fuel tanks

also the service bays and one of the fuel tank is more distant from the CoM so drag in those parts result in more torque .

Hmm, food for thought... I mean, you are not talking about the winglets which were there initially (which I believe do more harm than good) which should counteract a significant amount of torque. At this point I'd be curious to recreate the rocket with and without the winglets and actually test the thing, because in my experience this rocket doesn't look too crazy in terms of top drag.

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46 minutes ago, Spricigo said:

@Ohm is Futile

ahead of the CoM: 2 service bays, , 1 terrier,  one FL-T800 fuel tank

behind CoM: 4 swinvel/reliant, two FL-T800 fuel tanks

Small correction here. Also the engines on the sides are made up of 2 FL-T800 fuel tanks a piece. (If that even matters)

41 minutes ago, Ohm is Futile said:

Hmm, food for thought... I mean, you are not talking about the winglets which were there initially (which I believe do more harm than good) which should counteract a significant amount of torque. At this point I'd be curious to recreate the rocket with and without the winglets and actually test the thing, because in my experience this rocket doesn't look too crazy in terms of top drag.

I like the winglets there to help with steering, so that I can have all 4 "reliant" engines.

Also I tested with and without winglets (with center engine "swivel") and the results are quite different at first but end up being the same.. (Note that I followed the prograde marker on the navball all the way up)

I 'spun out' far sooner without winglets at 2,300 meters and with them I was able to get to 10k meters before flipping out.

Edited by Solis
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Ok, so I made this. It looks pretty much the same, it has a different value by 60 moneys, but as far as I can tell, the thing is otherwise a perfect replica from the OP.

I got this into a 250km by 250km orbit with my eyes closed with 2,800 m/s dV left in the tanks. I ditched the middle stage a fair bit earlier due to the lack of torque because of the reliants, I would probably have significantly more fuel left if I used swivels. As I expected, the winglets prevented me from making a really nice gravity turn, which is why the orbit is so high. It flew a bit weird, but even with sloppy controls, it didn't flip.

At this point, I have to ask exactly how is the OP trying to get this thing into orbit? It seems like it's more of a pilot problem than a design one...

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@Solis Depending on when you start getting plasma shock effects it's possible you may be running into control blackout due to ionization. Do you know if you have "Plasma Blackout" enabled in the difficulty settings? The default is off I believe, so it's unlikely to be your problem, but it's worth mentioning.

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20 minutes ago, Ohm is Futile said:

At this point, I have to ask exactly how is the OP trying to get this thing into orbit? It seems like it's more of a pilot problem than a design one...

I got this "thing" into a lunar orbit and back to Kerbin...(blew up in atmosphere)

Though, for some reason, I can barely get into a Kerbin orbit now.

Perhaps the part of my brain required to perform simple tasks died somewhere between losing my quicksave and blowing up on my return trip.

7 minutes ago, HvP said:

@Solis Depending on when you start getting plasma shock effects it's possible you may be running into control blackout due to ionization. Do you know if you have "Plasma Blackout" enabled in the difficulty settings? The default is off I believe, so it's unlikely to be your problem, but it's worth mentioning.

Ive never heard of that before. Also im on PS4 so I don't think its a feature for me.

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2 hours ago, Solis said:

Small correction here. Also the engines on the sides are made up of 2 FL-T800 fuel tanks a piece. (If that even matters)

Yes, the side boosters also matter, that is why I counted 5 in front and 4 behind.

I'm guessing the CoM is roughly between the middle tank and the bottom tank, turn on the CoM indicator in the VAB and confirm it. In any case that is the point you need to consider when looking if there is more drag in front or behind.

3 hours ago, Ohm is Futile said:

... in my experience this rocket doesn't look too crazy in terms of top drag.

and is not too crazy, It even fly straigth while pointed to progade. But with X AoA it become incontrolable.

I would not be surprised if the adjustment needed turn out to be very small, like moving the booster a bit up/down or setting fuel flow priority.

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11 hours ago, Solis said:

Ive never heard of that before. Also im on PS4 so I don't think its a feature for me.

PS4 version might be different enough. I just don't know at this point.

 

9 hours ago, Spricigo said:

and is not too crazy, It even fly straigth while pointed to progade. But with X AoA it become incontrolable.

I would not be surprised if the adjustment needed turn out to be very small, like moving the booster a bit up/down or setting fuel flow priority.

From my test, this is not what happens, at least not with the original design. It was not perfectly stable, but it did not behave like something that wants to flip either. I was pretty sloppy and ended pointing the rocket well away from prograde in the lower atmosphere. I've wrestled a top-draggy design recently, and believe me, this one may not be perfect, but it actually wobbled back and forth because it wants to fly prograde much harder than it needs for a good gravity turn. Something flip-inclined would try to move away from prograde the moment you push it slightly over the edge. I pushed it well over 20 degrees out of prograde.

So at this point, either my replica was not close enough (but I doubt that, as much as I dislike the winglets, they do provide a lot of drag at the bottom and can help prevent flipping) or the PS4 version is different in how in handles aerodynamics. Failing that, I'm guessing there's something the user may be doing during ascent that somehow breaks this rocket. Without more information, I really couldn't tell.

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@Ohm is Futile well, semmed the type that would flip when of prograde to me but I'll accept your analyses since you actually tested the vessel*. Still possible that the OP's vessel have a slight difference that make it want to flip but unlike with the replica actually resisting deviation from prograde.

Maybe the SAS overcorrection? I'm out of ideas now.

 

 

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If you use aerodynamic control surfaces on a rocket instead of a plane, ALWAYS turn off the roll input.  3-way symmetry is fairly poor for control surfaces anyway, though it's not bad for fixed winglets.  Also, it's worth turning down the control authority even if it makes the initial turn more sluggish.

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