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Launch stability issue

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I built a simple probe with a nuclear engine just to test interplanetary navigation. I tried many different vectors, but I never succeded in put this thing in orbit: As soon as I bend the trajectory at about 10,000 m.  the rocket becomes uncontrollable. This very rarely happened to me with other manned/unmanned vehicles, but in this case every configuration I tried failed. These are the pics of the last one, with and without fairing. Anybody can tell me where's the mistake?



(Sorry but for some reason the pics don't appear, I hope the link is visible)

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Without knowing much further details:

* The side things look way too big and fat - are these the first stage? Why is it so big? Its going to massively alter the CofG when fuel depletes and its decoupled etc Personally I'd go for about 1000m/s first stage, you have 2546m/s.  If you wanted ~2500m/s first stage, you could do 4x shorter 'pods'.

* Putting those side things to one side, the rocket looks too thin, I imagine it will wobble a lot when thrust is applied. A temporary fix would be to add some structural elements from the tip of the side things to as high up as possible of the main body, but there's other fundamental issues which will not help. Since you have used 2 diameters of fuselage, you could make the upper bit shorter and thicker with no issues and it will probably help stability too. You can attach a smaller diameter engine to a wider fuel tank no worries.

KSP is very forgiving and normally if a rocket "looks right" it flies okay. And if it looks wrong, it has issues. (Sometimes it looks right but flies horribly!)

The resource survey scanner is a big lump at the nose, but not heavy, its the worst thing to get into orbit for stability but it should be possible to do it nicely, especially if the rocket underneath is big in comparison to it. 

Also one final point on gravity turns not appreciated by many - its not really a 'maneouver' where control is applied to change direction, rather you (very early on) set up a curve instead of a straight line, and as the rocket 'falls sideways' due to that initial small maneouver, it does the curve/turn. In theory its possible to put 1 control input at the start, then do nothing and it will fly itself (if you turned off stability control). Or, make a structure from launch that was angled very slightly eg east. 

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When a rocket goes out of control like that, it's usually an issue with too much drag at the front. Most of the time, however, there is no way to fix it.

So what you do is stop trying to do an "efficient launch". Keep your Ap below 70km, and wait until your rocket is above 35km before you start trying to turn. When you are going straight up like that, you will have much better control. Yes, you will lose a bunch of deltaV, but actually getting your rocket to orbit is the important part anyway.

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40 minutes ago, marcast said:

Anybody can tell me where's the mistake?

It aso seems that you are not doing a gravity turn, thus wasting a lot more fuel than necessary. And quite possible you fell for the the 'big is better' fallacy too.

44 minutes ago, paul_c said:

In theory its possible to put 1 control input at the start, then do nothing and it will fly itself (if you turned off stability control).

also in  practice as demonstrated here:


 just notice the lack of a control point



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As others said, too much drag at the front, too little weight at the back.

  • Add fins at the bottom of the rocket for stability.
  • Set your fuel tanks to burn from the bottom tanks first, which will move your COM to the front (top) as you burn fuel.   That will help keep it stable.
  • Long floppy rockets are hard to keep stable.   Try turning on Rigid Attachment for joints, by bringing up the PAW in the VAB.   Also, adding struts will help limit the floppiness.
  • Finally,  avoid aggressive turns, try to always keep your vessel pointed within 5 degrees of prograde.   This will make a long gradual gravity turn necessary.   You can often keep an otherwise unstable rocket on course by not letting it get more than 5 deg off prograde while in the atmosphere.
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That launch rocket is totally overbuilt for the payload you’re trying to launch- three Mainsails and two Rockomax Jumbo tanks per booster is massive overkill and would be close to reaching orbit with the side boosters still burning if the delta-V is showing vacuum numbers, or could make orbit with fuel to spare if they’re the surface numbers. You’re also potentially going too fast at low altitude, so any attempt to adjust your trajectory causes massive aerodynamic stresses that will make the rocket unstable and prone to flip or break apart.

Add fins on the core stage, make your boosters much smaller (1.25m boosters using Reliant engines- or 1.875m with Bobcats or Kodiaks if you have Making History- would probably be enough, or use Thumper or Kickback SRBs with some fuel tanks on the top and crossfeed enabled on the decouplers) and autostrut all the engines to the root part, the fairing to the heaviest part, the booster tanks to grandparent part and the nosecones on the boosters to the root part to keep it all locked together without being rigid to the point of breaking as rigid attachment tends to do.

You should also set the fuel tanks to drain from bottom to top which helps to keep the centre of mass as far forward as possible, then add a 1.25m reaction wheel to the top of the first stage which should be more than adequate to control the rocket at high altitudes or in space. Rockets tend to flip because there’s too much drag at the front, so fins on the bottom of the booster help to correct that, or because the weight balance is too low, which is largely resolved by draining the tanks from bottom to top and can be improved by using smaller, lighter engines in your core stage and radial boosters rather than big, heavy engines on big, heavy stages.

Edited by jimmymcgoochie
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2 things.

1.  Struts.  I dont know if your rocket is shaking or not before flipping, and if it is, struts will prevent that.

2.  Reaction Wheels.  I always have issues with probes flipping, even if I use SAS to burn prograde, without these.  Heck I've started using them on manned rockets to keep myself stable.

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In addition to what everyone else has said:

Your boosters are going to be flexing, because they have huge weight, and huge thrust, and the steering input from the gimbals, but are only attached by a single decoupler each. When the boosters flex, they no longer point perfectly backwards. That contributes to making your rocket veer off course.

Solutions? Well, the meme goes that there are only two solutions in KSP: moar boosters, and moar struts. I'll leave it to you to figure out the answer ;)

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We've all had this a few times I'm sure, and it's very frustrating when you feel like you've done nothing differently from a successful variant.

I can actually see what looks like a strut from the left booster to the main stack.

That still leaves a few of the issues pointed out above: lack of fins at the base, the timing (and severity) of the turn, and the possibility that the tanks are emptying from the top down.
Definitely adding a few fins should help a lot. Once they are fitted, I would suggest starting the turn very early and then using prograde SAS. If the top tanks are emptying first, invert the priorities. (And yes, the boosters really do look like massive overkill :))

Edited by Neilski
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They look like it but if so, doesn't make sense. There's the HECS core just underneath the resource survey scanner. But there's no additional fuel tanks or engines, so they can't be (functional) other satellites? 

For 100s of my satellites I've used the HECS core. On its 6 sides are: 2x solar panels*, 1x  battery, 1x aerial, leaving 2 sides for eg thermometer, another science instrument. 




* if you fit just one, it will probably end up in shade and the satellite will go dead!

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On 1/10/2021 at 8:57 PM, paul_c said:

They look like it but if so, doesn't make sense.

Huh? This is KSP - making sense is optional :)
Also, HECS2 have great features for the mass - huge battery capacity, decent reaction wheels, great SAS... But I will confess I can't think of a good reason to put two of 'em right under a HECS :confused:

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I'm really getting crazy on this.....:mad:

Rover Test (fairing)

Rover Test (no fairing)

Mun Ship Rover (fairing)

Mun Ship Rover (no fairing)

Same vector, Rover Test is a bit lighter and with an apparently lower drag fairing. Still Mun Ship works perfectly, Rover Test always loses control. I'd say it's something related to the shape of payload, But how is it possible when there's a fairing?

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On 1/10/2021 at 9:09 PM, bewing said:

HECS2 probe cores, it looks like.

Yes HECS2. I used 2 to have more space to set batteries and solar panels. Well, I'm not so experienced......:0.0:

6 minutes ago, paul_c said:

Probably to do with the weight and the centre of mass, ie not the aerodynamics


Centre of mass is quite the same for both of them, just at the decouplers' level

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Fatter fairings at a given angle to prograde produce more drag than thinner ones at the same angle to prograde.

That drag is at the top of your ship and can easily cause a flip.

Fairings don't eliminate all drag. The contents of the fairing are shielded from drag but the fairing itself still is a solid wall for the oncoming windshear to hit.

The more you drift from prograde, the more drag you get but on the side which is closest to prograde.

On a day without wind the wind-speed at the front of the ship is as fast as the ship is travelling.




Edited by Daveroski
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3 hours ago, marcast said:

Still Mun Ship works perfectly, Rover Test always loses control.

Can you open up AeroGui (Alt-F12 -> Physics -> Aerodynamics -> Check Display Aero Data GUI) during ascent and try to get a screenshot right when it starts to tip ?

Note altitude, repeat with vessel that works and get a screenshot at same altitude. If I am right dynamic pressure is really high and quite a lot higher for failing one.

My Guess is that you tip close to MaxQ. You have quite decent TWR, but draggy shape: Dynamic pressure might be just little bit too high for gimbal and reaction wheel to cope with small sideway aero forces due to vibrations and suddenly the vessel starts to tip over.

The normal counter measure is to throttle down around 300m/s to limit MaxQ ("dynamic pressure" in Aero GUI) until air gets thinner and you can throttle back up. You probably want 50-75% throttle.

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12 hours ago, marcast said:

Yes HECS2. I used 2 to have more space to set batteries and solar panels. Well, I'm not so experienced......:0.0:

Nah, we just have some fixation with "efficiency", if using the expensive* probe core as a structural element works for you than it is all fine.


12 hours ago, paul_c said:

Probably to do with the weight and the centre of mass, ie not the aerodynamics

If that is the case it still is an aerodynamic issue. The position of the "draggy parts" relative to the CoM makes the craft flip. 

All the craft the OP presented so far have a combination of undesirable characteristics: Multiple heavy engines at the bottom,   light and draggy fairing at the top, a long 'neck' between the fairing and the center of mass and lack of fins. That is basically the recipe for a flipping rocket.

*Even if that it is a sandbox/science , the extra mass and bulk will take its toll from the craft performance.

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  • 2 weeks later...

For such a small payload, and the fuel/engines at the lower stages, you are using a bazooka on a fly. In other words, the rocket engines you have are overkill for such a small payload. I'm guessing the TWR is really high due to using 3 Mainsail engines. Due to the distance between the center of mass (CoM) and the top of the rocket, you are probably experiencing a lot of torque which will induce flipping and when correcting the flipping motion, the rocket is probably overcorrecting since 3 mainsails are vectoring with steering input. Downsize a bit. You could probably get away with 1 mainsail and 2 reliants if you want to have two FL-T series tanks on the side. Just ensure that your Delta V is around 3500 with a TWR of around 2-3 and I think you will be fine having enough to get to orbit. I look to have my CoM slightly below the center of the rocket for launching. I have had better success using that which yielded less flipping tendencies. If it still wants to flip, add fins just above the mainsail on the bottom.

Hope this helps.

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