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Why is this craft veering off the runway?


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https://www.dropbox.com/s/8n196bb00qs8vzf/ksr ssto.craft?dl=0

craft in question^^

I've been playing ksp on and off for years now, just got back into it. I have another craft similar to the one posted above, with the rear landing gear placed on the wings, and it goes in a fairly straight line down the runway EVEN WITHOUT the steerable front landing gear this SSTO has. Are the wings bending and causing the rear landing gear to steer the ship off course?

Sidenote, I like to see the game have a large landing gear variant that has the capability to steer

Edited by fireblade274
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I can't load your craft, because it has a modded part and I'm running stock. I think it's a mechjeb part?

Anyway, this is a very common problem. Common enough that it actually has an entry in the FAQ on this forum. Unfortunately, there are many possible specific causes for runway veering and each has a somewhat different fix.

As far as I'm concerned, the most likely source is that there is too much drag against the ground on the front gear, and not enough on the rear to make the SSTO directionally stable on the ground. The easiest way to fix/test for this is to open the front gear in the SPH, switch friction from automatic to Manual, and reduce the friction to somewhere between .4 and .6 -- if that doesn't fix it, try reducing it incrementally all the way down to zero.

 

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As Bewing noted, wheel drag is a very common problem. But this statement also caught my eye:

On 9/26/2019 at 7:23 PM, fireblade274 said:

…Are the wings bending and causing the rear landing gear to steer the ship off course?

…Do you happen to have the rear gear attached to the wings? KSP has a bug where mirror symmetry gives parts slightly unsymmetric stiffness, which can manifest as veering to the side on takeoff or landing, and the way to combat it is by attaching your gear as rigidly as possible to the center fuselage. Some even cheat wing gear by attaching to the fuselage and shifting them outward with the offset tool.

Otherwise, avoid wing gear like the plague. Any additional joints between the center and the gear make the problem worse.

If this is your problem, you’ll also see behavior like unwanted roll occurring during hard pitch-ups. You may need to find ways to add stiffness to your design.

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3 hours ago, bewing said:

The easiest way to fix/test for this is to open the front gear in the SPH, switch friction from automatic to Manual, and reduce the friction to somewhere between .4 and .6 -- if that doesn't fix it, try reducing it incrementally all the way down to zero.

 

This.  I generally set my rear wheels to 90-100% friction and the front at 30 or less.  Also, if you have steerable gear, disable the steering on the rear wheels.  

Edited by Klapaucius
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very interesting, yes i do have the gear on the wings. the ship seems to take off OKAY now with traction control turned down, it just precariously auto corrects down the runway now back and forth. My last craft also had the gear on the wings, but worked just fine. Last craft was similar in construction to this one but is over 4 years old in the making, made it around ksp 1.0-1.2. 

 

side note; I would have bet money back around 1.1 that by the time 1.7 came out there'd be no bugs like this

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40 minutes ago, fireblade274 said:

very interesting, yes i do have the gear on the wings. the ship seems to take off OKAY now with traction control turned down, it just precariously auto corrects down the runway now back and forth. My last craft also had the gear on the wings, but worked just fine. Last craft was similar in construction to this one but is over 4 years old in the making, made it around ksp 1.0-1.2. 

 

side note; I would have bet money back around 1.1 that by the time 1.7 came out there'd be no bugs like this

Friction and traction control are two different things. Make sure you are looking at friction. And this particular thing isn't a bug -- this is all engineering stuff that needs to be dealt with IRL too. If you want bugs, you should try playing some bethesda games.

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