How do I keep my rover upright in water?

[Loren Pechtel]

I'm trying to collect all the Kerbin science that I can reasonably get.  (Some biomes simply don't exist and some aren't reasonable to reach.)  I have a rover that does a fine job on land and flying low--it has some baby fuel tanks feeding thuds and a small mining rig to refuel them.  To be able to get back out of the water it has some jet engines mounted high.  I would have thought putting some empty tanks at the four corners (they are partially floating when it's in the water) would be enough to make a passable boat out of it but I has crossed a lake and was approaching land when it flipped.

(The tanks are configured to hold monopropellant but are empty, and since the ISRU is not configured to make monopropellant they'll stay empty.)

[HvP]

26 minutes ago, Loren Pechtel said:

it has some jet engines mounted high

Hello, Loren. It's hard to say for sure without pictures of your craft. If you can take a screenshot with the F1 key and upload the picture to a file sharing site like Imgur.com you can paste the link to that picture into a post here in this thread. Also, how did it flip? Did it go forwards and turn upside down? Were you trying to turn and it flipped sideways? Backwards? Did it begin to dive under the water and then partially submerge? How fast were you going?

My guess is that having engines mounted up high could be causing your problem.

Consider that friction with the water will cause a lot of drag down low, and the force of your engine's thrust will be only pushing on the top half of your craft. This means that the bottom half of your craft is being pushed backwards by the water and the top half is being pushed forwards by the engines causing it to flip.

To counteract this effect you would need a longer ship with the engines in line vertically with the center of mass. Adding parts like nose cones on the front of the craft would help reduce drag when moving through the water.

Edited June 15, 2017 by HvP

[bewing]

The easiest way to have water stability is to have some aerodynamic control surfaces below water level. That gives SAS plenty of control authority to maintain stability.

Edit: and as HVP implies -- moving your engines down a lot closer to water level (center of pressure) can help a lot too.

Edited June 15, 2017 by bewing

[Loren Pechtel]

27 minutes ago, HvP said:

Hello, Loren. It's hard to say for sure without pictures of your craft. If you can take a screenshot with the F1 key and upload the picture to a file sharing site like Imgur.com you can paste the link to that picture into a post here in this thread. Also, how did it flip? Did it go forwards and turn upside down? Were you trying to turn and it flipped sideways? Backwards? Did it begin to dive under the water and then partially submerge? How fast were you going?

My guess is that having engines mounted up high could be causing your problem.

Consider that friction with the water will cause a lot of drag down low, and the force of your engine's thrust will be only pushing on the top half of your craft. This means that the bottom half of your craft is being pushed backwards by the water and the top half is being pushed forwards by the engines causing it to flip.

To counteract this effect you would need a longer ship with the engines in line vertically with the center of mass. Adding parts like nose cones on the front of the craft would help reduce drag when moving through the water.

If it had gone end over end I would agree with you and wouldn't be asking about it.  However, this rolled sideways after crossing maybe 50 meters of water at very low speed.  The engines certainly do cause an issue but to roll it had to force two tanks underwater, I wouldn't think it would do that.

[HvP]

With drills and ISRU it's entirely likely to be a weight distribution issue coupled with a draggy front end and high center of thrust.

In my experience, boats in KSP like to have the mass weighted towards the back so that when thrust is applied the front of the craft isn't forced under. Pointy, low drag parts like nose cones really do make a difference in reducing the tendency of a craft to pivot around its front end as well. But you still need more drag at the back than the front, just like in planes. So underwater rudders, keels, etc... in the back will help.

Without seeing pictures of the craft I'm only shooting in the dark here but I can imagine a scenario as follows: When thrust is applied the front of the craft is being forced down into the water where the blunt end of the fuel tanks is catching a lot of drag from the water. The front is now experiencing high drag while the tail is not submerged as deeply and has less drag and is more free to slide. The nose is now a pivot point and any random yaw motion will cause the tail to swing around. If your center of mass is sufficiently high then the top heavy craft will roll because there is now drag acting sideways against the part of the hull that's underwater but the inertia of the mining equipment above pulls it over.

Edited June 15, 2017 by HvP

[Loren Pechtel]

9 hours ago, HvP said:

With drills and ISRU it's entirely likely to be a weight distribution issue coupled with a draggy front end and high center of thrust.

In my experience, boats in KSP like to have the mass weighted towards the back so that when thrust is applied the front of the craft isn't forced under. Pointy, low drag parts like nose cones really do make a difference in reducing the tendency of a craft to pivot around its front end as well. But you still need more drag at the back than the front, just like in planes. So underwater rudders, keels, etc... in the back will help.

Without seeing pictures of the craft I'm only shooting in the dark here but I can imagine a scenario as follows: When thrust is applied the front of the craft is being forced down into the water where the blunt end of the fuel tanks is catching a lot of drag from the water. The front is now experiencing high drag while the tail is not submerged as deeply and has less drag and is more free to slide. The nose is now a pivot point and any random yaw motion will cause the tail to swing around. If your center of mass is sufficiently high then the top heavy craft will roll because there is now drag acting sideways against the part of the hull that's underwater but the inertia of the mining equipment above pulls it over.

I'll see what I can do with rigging a rudder.  Since this is also a rover I don't have a lot of clearance.

All the mining gear is in the back, although the ore tank is empty.  The craft is as symmetric as I could build it other than the science instruments that are it's reason for existence.