Rocket flipping backwards @ 14-22km.

[qoonpooka]

I have this space station structural component I'm trying to launch. It's a ton of girders, girder-to-small adapters, and clamp-o-trons. Eventually it will hold the fuel tanks and any large craft that dock at Minmus for fuel.

 

But first I need to get it into the sky. The vessel is very stable at launch and at low speeds. As it exceeds 300m/sec, however, I run into problems. It begins to gravity turn too steeply and any deviation from prograde results in a loss of control and tumble.  The payload is only 6 tons, IIRC.  I suspect the fuel needed to lift it to orbit is simply too far back but I can't figure out how to generate anything like enough drag in the back. I'm already running 8 AV-R8 winglets.  When I ran with full-on wings as stabilizers, the elevons didn't have any control authority.

 

Image included, happy to clarify.

[Pappystein]

@qoonpooka

1st) there is not a lot of information provided here to solve your issue.

a) What mods beyond engineer

b) what speed and altitude did you start your gravity turn

c) what AOA were you at when the rocket flipped out

d) Picture of the Entire rocket?

e) Does this happen before or after you eject the SRBs

f) main engine stats (Type/Number)

 

2nd) SUGGESTION;   Spend time doing a SHALLOW Gravity turn.  Sure not as "efficient" but it eliminates a lot of the issues you are reporting.

3rd) fact, I am not a fan of the AV-R8 winglet.  It does not seem to control as well as the "Standard Canard" I never use them because they do not have enough control authority

4th) Suggested design change.   Launch a small probe based tug that can assemble the side arms in flight.  SURE it is more parts but it will solve the issues caused by the massive frontal area with minuscule sized control surfaces.   Better your payload be TALL and THIN than Short and FAT on a Tall and Thin Rocket

 

Hope this helps!

PS your rocket is atttempting to simulate a Delta III.   Delta IIIs had a lot of issues due to the huge faring small rocket....

Edited September 28, 2016 by Pappystein

[Starhawk]

It's like trying to throw a dart backwards.

You have the light draggy thing at the front and the heavy thing at the back.

I wouldn't be surprised if you had better luck trying to launch it without any fairing.  It might actually have less drag.

I'm not sure why your elevons didn't have any control authority.  It's probably best to use bigger ones and/or more of them.

An overpowered engine with a good gimbal range might help.

If all else fails try just launching it more straight up and more slowly and don't pitch over significantly until you're well above the thick stuff.  It may cost a bit more dV to get to orbit that way, but it's much easier to control an unwieldy craft on such a trajectory.

Happy landings!

[MaxL_1023]

Hey Qoonpooka,

Your rocket has too much drag on the front end. That huge fairing has a large cross section. When you tilt a little away from prograde, the drag force increases a lot (due to a higher AoA) and since that drag is applied forward of your center of mass, your rocket will flip. That little deviation causes a torque on your rocket which adds to the initial disturbance - the definition of an unstable system. 

You want to have the most drag on the back of your rocket. In that case, the drag force causes a torque which OPPOSES the initial disturbance, stabilizing your rocket. You are on the right track with the fins on the SRBs - I have a couple ideas to help you get that to orbit:

1. Put the SRBs on longer decouplers or on the end of a structural part. This will move your fins farther out and increase the drag cross-section of your rocket's cheeks end. 

2. Use liquid fueled boosters - the control authority from engine gimbals might counteract the fairing drag by brute force.

3. Use vector engines (insane gimballing)

4. Use a 3.75m rocket (think shorter but stubbier) so your fairing doesn't stick out as much. 

If all else fails, I doubt you actually need a fairing for structural parts. They would be fine exposed to the elements, even though it breaks immersion somewhat. You should see my rockets when I am using a modset which puts both interstages and fairings late in the tech tree. I basically brute-force them to orbit. 

Edited September 28, 2016 by MaxL_1023

[Superfluous J]

In addition to the above, you may have some luck disabling the top orange fuel tank. That way the fuel stays in it and the bottom tank empties first, causing the center of mass to shift FORWARD instead of BACKWARD. When the engine cuts out, re-enable the tank and by the time the COM shifts too far back, you'll hopefully be high enough up that aerodynamics doesn't matter so much anymore.

[qoonpooka]

11 hours ago, Pappystein said:

@qoonpooka

1st) there is not a lot of information provided here to solve your issue.

a) What mods beyond engineer

b) what speed and altitude did you start your gravity turn

c) what AOA were you at when the rocket flipped out

d) Picture of the Entire rocket?

e) Does this happen before or after you eject the SRBs

f) main engine stats (Type/Number)

 

2nd) SUGGESTION;   Spend time doing a SHALLOW Gravity turn.  Sure not as "efficient" but it eliminates a lot of the issues you are reporting.

3rd) fact, I am not a fan of the AV-R8 winglet.  It does not seem to control as well as the "Standard Canard" I never use them because they do not have enough control authority

4th) Suggested design change.   Launch a small probe based tug that can assemble the side arms in flight.  SURE it is more parts but it will solve the issues caused by the massive frontal area with minuscule sized control surfaces.   Better your payload be TALL and THIN than Short and FAT on a Tall and Thin Rocket

 

Hope this helps!

PS your rocket is atttempting to simulate a Delta III.   Delta IIIs had a lot of issues due to the huge faring small rocket....

Thank you for taking the time! Here's the answer to your question:

1a) Just Precise Node. What you see is what you get.

1b) Several. As you can see the TWR is quite low, so at 1km I'm only doing 80m/sec or so.  When I start the turn with 5-deg there, I lose control at 15,000m pretty consistently, and prograde is around 30 degrees inclined from horizon.  When I go for a shallower turn, as you suggest in #2, I can get up to 22,000m, but again 30 degrees inclined.  What happens in all scenarios is that this is the point where I don't feel my vertical velocity is sufficient to not crash and I have to hesitate away from prograde or it will naturally lawn dart.

1c) Loss of control happens at probably about 2 degrees AoA, full-on flip out is hard to distinguish from this but somewhere in the 20-30 range. I can keep it to slow, unstoppable turn until about then.

1d) Rocket does not fit on screen, attaching another image here for the bottom half. Both photos are at maximum zoom-out.

1e) A good time after SRB ejection. The whole thing is very stable (and yet still fairly responsive to control) through the liftoff. It's only when it wants to gravity turn way too fast that we run into problems.  Once you've started the gravity turn it will readily yaw itself at an ever-increasing rate.  I've tried to slow it as much as I can by hanging out in the westward edge of the prograde marker. (As long as I'm in the circle I seem to be okay.)

1f) In photo, aside from the four thumpers, 1 skipper. All engines fire at launch, the thumpers are there to give me velocity, without the skipper their TWR is 1.01.  When they cut loose, the skipper is only .84. It's enough to continue ascending, however.  TWR is around 1.2 when loss of control happens.  The skipper is the most powerful engine I have.  Kickbacks won't fit because the launch pad can't handle the weight. I've tried using two kickbacks and two Thumper/Hammer stacks but the whole rocket becomes very wiggly all of a sudden, below the 2nd orange tank.

 

2) I've done a straight ascent to 10,000 meters and then initiated a gravity turn, control was maintained but the gravity turn still eventually ran away from me, loss of control at 22,000m instead of 15,000m.  I suspect I don't have a fine enough control to initiate a gravity turn gentle enough for this thing.  I can try going even higher to reduce the aerodynamic forces and essentially go for a dragless horizontal flight.  Gonna need more dV, I suspect, not sure how to get it.

3) I usually use the Delta-Deluxe and have no problems at all.  The AV-R8 is beefier.  I don't see the in 'standard canard' in my aeroparts.  Is it the 'tail fin' or have I not unlocked the node in question.

4) I'm worried about that many clamp-o-trons. This is a station that will be moving from LKO to Minmus to satisfy two contracts (at a steep loss, but I wanted the fueling station anyway).  So finding a way to put this payload in orbit is important to me.  This structure will be under thrust stresses.

Replying to other folks trying to help:

@MaxL_1023 
1) Rocket is stable while SRBs are attached, fins are on the lower orange tank.

2) An asparagus staging option is interesting, but it'll make that fairing stick out roflsauce far.

3) Skipper does gimbal, or are you referring to some other engine? I probably don't have it unlocked. The Skipper is as far as I've managed this game.

4) Don't have 3.75m parts.

The fairing here isn't to protect the payload so much as it is to reduce drag. There's 20 or so girder segments on that thing, then PV panels and lights and struts and ports. As bad as this fairing is, I can't imagine the drag on the truss isn't going to be so very much worse.

@5thHorseman That's an interesting idea! I'm going to try it and see how it goes.

[qoonpooka]

12 hours ago, 5thHorseman said:

In addition to the above, you may have some luck disabling the top orange fuel tank. That way the fuel stays in it and the bottom tank empties first, causing the center of mass to shift FORWARD instead of BACKWARD. When the engine cuts out, re-enable the tank and by the time the COM shifts too far back, you'll hopefully be high enough up that aerodynamics doesn't matter so much anymore.

This was the solution.  Moving the CoM forward like this did the trick. I wasn't even aware that this was a thing that could be done.

 

Payload is in orbit without any other changes.  Took two tries because the fairing is so huge that it blows up the payload if you eject it while under thrust.  Cutting engines just long enough for the fairing to clear did the trick.

[Jens Lyn IV]

In this case, the fairing absolutely, indisputably increases drag. I built a replica based on your screenshots and tested it.

Frontal area with fairing.

Frontal area without fairing.

Stable at high speed and high angle of attack.

I know you already got this one to orbit, but next time you launch a girder structure, try without a fairing first.

Edited September 28, 2016 by Jens Lyn IV

[qoonpooka]

1 hour ago, Jens Lyn IV said:

In this case, the fairing absolutely, indisputably increases drag. I built a replica based on your screenshots and tested it.

I know you already got this one to orbit, but next time you launch a girder structure, try without a fairing first.

I'm impressed by a number of things (including your ability to rebuild my truss from visual, and even got the LT-400 and LV-909 included!

In re the truss having less drag.... Sigh.  Are the fairings one of those parts that's just there for immersion but actually hampers you?  Do they EVER provide less drag than a spiked ball of dragness?

Edited September 28, 2016 by qoonpooka
cleaning screenshots

[MaxL_1023]

They will help if your payload has a flat front close to the diameter of the fairing (think a cupola or a pod facing the wrong way), especially at supersonic speeds. Your fairing is so much larger than your payload (in cross-sectional area) that it counteracts the advantages of a smooth, aerodynamic shape. 

IRL, most satellites would not be able to withstand the aerodynamic forces and heating during ascent - the fairing protects them. When your rocket gets large enough air drag becomes insignificant, as your thrust is proportional to mass (cube of linear size) while drag is proportional to the area (square of linear size). 

In R0 (full scale systems and real engines), I can put ridiculously unaerodynamic payloads on my rockets and they still fly, especially when I have 20 MN of thrust at liftoff from a few F-1 type engines. What is 100KN of drag in that case?

[Superfluous J]

4 hours ago, qoonpooka said:

This was the solution.  Moving the CoM forward like this did the trick. I wasn't even aware that this was a thing that could be done.

Look out in 1.2 for "Advanced Tweakables" (which is not on by default but is in settings and - in my opinion - should be on by default). It will allow you to - among other handy things - set the fuel-flow priority PER TANK both in the VAB and in flight. Using them, you'd be able to tell KSP to drain that top fuel tank last without all the "disable/enable" shenanigans.

[Plusck]

8 hours ago, qoonpooka said:

Do they EVER provide less drag than a spiked ball of dragness?

Spiked ball of dragness is a pretty good description where they should (I guess) actually help. If you have, say, an ion-powered probe with a bunch of solar panels, instruments and an orbital scanner on it, that'll be a fairly spiky ball of dragness (and heatsplosionness) that would certainly be better off* inside a fairing...

* "better off" meaning able to get to orbit quickly and easily on an aggressive and efficient gravity turn, but I can't claim as fact that it would have less total drag...

[Jarin]

Does the angle of the faring nose contribute to amount of drag? I was under the impression that KSP used the slope of the part model to determine drag amount. (i.e. "make faring pointier")

Edited September 30, 2016 by Jarin

[foamyesque]

My solution would've been an additional stage, which would give you some other advantages. Take that upper orange tank and split it into a -16 + poodle and a -32+8 ahead of the rest of the stack. That should give you a nice, efficient and easy to control upper stage, and keep your weight forward during your launch without needing to do enable/disable.

[Kelderek]

On 9/30/2016 at 0:38 AM, Jarin said:

Does the angle of the faring nose contribute to amount of drag? I was under the impression that KSP used the slope of the part model to determine drag amount. (i.e. "make faring pointier")

I have no idea about the specifics of drag for the fairing, but in my own anecdotal experience, a more rounded fairing makes it easier to steer a rocket in an atmosphere than a pointy one - especially for for wide fairings.  I find that pointy rockets really want to go straight and struggle with turns.  No matter what you do with or without fairings up top, it really helps to have fins at the bottom and enough control authority to stay on course. 

Here are a couple examples: http://imgur.com/a/a9YJT