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davidpsummers

Problems on lift-off from Eve

Question

I have landed and refuled the ship pictured below on Eve.

I have a problem when I try and lift-off.  The ship pitches over uncontrollably.  If I am careful to get it straight up immediately after lift off, I can delay it.  But it always seems to happen.

Now I've come to realize that the vernor engines are probably useless at these pressures.  But I do have 4 large advanced reaction wheel modules and the torque from the engine gimbal of the S3 KS-25x4 Mammoth engine.

So I'm thinking something else must be happening.

Any help?

 

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Hard to say w/o trying "first hand" a lift off.

There are many Eve peculiar "design" issues I can spot at first glance.

I could guess your ship is not "rigid" enough to keep its attitude; then your fins are not "control surfaces"; on top of that you have a lot of drag coming from all the stuff you can not detach before lift off like chutes, solars, "legs" etc.

Lat but not least, Eve atmo is a b***h and you may want to take off at full thrust, then slow down and patiently ascend above the 12k-25k belt that may fry you rocket if you are too fast.

From what you say I could guess it is a matter of drag in the wrong place and lack of control; but as I wrote it is hard to say w/o a first hand test.

BTW, you have got a very kerbal design.

Could you provide more data about the situation? Could you tell us at least your take off altitude and the altitude where you experience your loss of control?

 

PS: do you stay on a straight ascend route for a while or are you trying an early gravitational turn (that imo could be bad advised with a craft like this)? 

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Couple of things...

As said above, drag is the important thing on Eve. You can get that payload to orbit with a fraction of that craft if you eliminate most of the drag. It is also likely what is making your craft flip because there is so much at the front end. Put absolutely everything not associated with getting to orbit on decouplers and dump before taking off - solar panels, radiators, chutes, air brakes, landing gear, etc. Also get rid of of the docking port on the nose - that alone could be causing the tipping and will be robbing the craft of a LOT of dV

Then start taking stuff away. You don't need reaction wheels, a mammoth engine, RCS, antennas and, in fact, most of the tanks and engines. All that having a lot of craft does on Eve is generate drag, requiring more craft to compensate and you end up with a behemoth to get a tiny payload to space. 

Oh and have some steerable fins at the back end but only as many as you absolutely need. 

 

 

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Well, I can think of two reasons:

  1. wobbly rockets / too much gimbal:
    Do the engine bells swivel from full left to full right, while the rocket swishes like a cow's tail? Then it may help if you tweak down the gimbal range on most or all of the engines.
  2. bad aerodynamics / too much drag in front:
    the way aerodynamics work in this game, you will have a surprising amount of drag from the parachutes. Also solar panels, airbrakes, radiators, but on this particular vessel the parachutes are the worst offenders, and also the most forward. It may be that you simply have not enough tailfins to make up for that. And once you jettison the boosters, you have no tailfins anymore.
    If your problem is aerodynamics, you will have to re-design your vessel.
15 hours ago, davidpsummers said:

If I am careful to get it straight up immediately after lift off, I can delay it.  But it always seems to happen.

This sounds more like aerodynamics.

Perhaps you can try which way the vessel will fall if you simply let it drop in the atmosphere? Either by using Hyperedit, or a combination of Cheat Orbit, (short) de-orbit-burn, and turning off heat effects so you can see aerodynamics play out without the vessel blowing up. If you vessel turns itself tail-first into the airflow, that's your problem right there.

Edited by Laie

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On 11/8/2019 at 4:53 PM, davidpsummers said:

The ship pitches over uncontrollably.

Aerodynamic instability is a strong possibility.  Where's the CoM on this beast as it sits ready for liftoff?  If it's very low, that's likely to be your problem right there.  A really low CoM would cause two problems, both of which would make the thing very unstable.  First, it would have a long draggy fuselage sticking out in front of the CoM; and, second, the fins and gimbaled engines at the back would have a reduced lever arm (due to being too close to the CoM) and would be therefore have much less control authority, i.e. much less effective at stabilizing the craft.

 

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Played around with it a bit.  Aerodynamic problems explain the behavior.  But ithat would mean that, as pressure goes up hull mounted components increase dragfaster than fins?

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Posted (edited)

It appears you have a rhino engine staged right above the top of the orange tanks.  The joint looks very weak there as the slight slope is causing the top half to lean over.  I would suggest adding some struts from the decoupler to the base of the engine to prevent bending back and forth in flight.

Edited by MunsterIII

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It isn't great, but the joint isn't the problem.  I can get it verticle and still have it a few moments later.

I'm currently going on the theory that I need to loose the parachutes and stuff when I land on Eve.  But maybe some struts too.

Still don't understand why modest drag surface components would cause more drag than actualy fins, but based on what I've read above, it seems it can happen and it fits what I see.  So I'll chalk it up to models breaking down at v high pressures.

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