How do you balance mass of a rover to land it safely?

[Kulebron]

1. How do you fine-tune the CoM of a vehicle?

I have a rover with center of mass a bit off the center, and when tried to land it, it pitched when thrust was applied, so it started slightly rotating (which was slow, but too much to maintain control) and crashed into the surface. I had to test it about twenty times to finally eliminate any rotating moment. If there were a tool for that, it would be nice.

2. How can I balance weight dynamically? If I put a kerbanaut in the rover before landing, it will be very disbalanced and surely will start rotating with thrust.

All that comes into mind is just ditching some monopropellant and move all of it to the other side.

Edited September 16, 2013 by Kulebron

[Diazo]

In the VAB, you can enable Center of Thrust and Center of Mass indicators.

Then I shift parts around to get them to line up as close as possible.

You can never get them to line up perfectly however, so I stick Reaction Wheels at each corner. (I prefer the square sky-crane with an engine at each corner design.)

Leaving SAS enabled as I descend, the Reaction Wheels compensate for the difference between the CoM and CoT and keep me on course without tipping.

Note that the CoM and CoT do not have to be on top of each other directly, it is the Thrust arrow of the CoT that needs to line up in the 2D plane with the CoM.

D.

[Kulebron]

Well, I did use these indicators, yet if it's slightly off-center (which you can't clearly see), it starts rotating with thrust. I tried installing four engines on long trusses, which helped, but there were other issues (the 1.5MN engines were too weak, the 20MN were too strong, so it either had too low dV or too overpowered, hard to control).

Which RW part do you use?

[Diazo]

I used the Reaction Wheel part that fit.

After the SAS update in the recent patch, all Reaction Wheels offer the same torque for the same ElecCharge cost, so I match the Diameter of my fuel tank/engine combo I have on my sky crane corners.

On the engines thing, keep in mind you can mount the radial mount engines to tweak thrust levels.

On one of my smaller rovers I actually had to get rid of the big inline engines and use 12 of the really small radial engines (LV-1R with only 1.5kN thrust) to get my TWR down low enough to control my descent.

D.

(At work, will try to post pics when I get home to my KSP computer.)

[Kulebron]

By the way, I put a big reaction wheel in a main stage over mainsail engine, and it collapsed. Are they so soft?

[Skorpychan]

By the way, I put a big reaction wheel in a main stage over mainsail engine, and it collapsed. Are they so soft?

Compared to tank -> decoupler connections, yes they are. What I tend to do is stick the smaller wheels on the upper stage, and use enough to turn the bits that are designed for space operation without too much difficulty.

The initial lift stages can be steered with winglets anyway, and the main lifting stage steers with thrust vectoring, since it only really needs to keep pointing prograde and thrust that way.

[Diazo]

It's more the mainsail is just that powerful.

If you are going with mainsails, struts here, struts there and struts everywhere.

Having said that, what are you lifting? It does not sound like you have that big of a rover so you might be able to downsize to a skipper?

Drag is a thing, so streaking off the launchpad with a TWR of 5 is losing a lot of fuel to fighting drag.

D.

[Kulebron]

I hadn't problems with Mainsails, maybe because I dispense with a 1440 tons in main stage and 2*1440 & Skippers on the sides, and this is the most common thing I use, with some adjustments to get proper dV & TWR. I only need a pair of struts on each side stage to keep them straight, and in those upper stages that have a narrow engine between wide tanks. Apart from that Mainsail is just enough, and TWR is about 1.7-2.

This way I have few stages and a lot of TWR to circularise easier. I tried to do in Apollo style, with a Poodle on 3rd stage to circularize and translunar-inject, but the whole ship became bigger and more expensive, and indeed oscillated.

That experiment with reaction wheel was just out of curiosity.

Thanks for help!

Edited September 16, 2013 by Kulebron