23-77 Twitch engine issue

[Finndibaenn]

Hi,

I've been working on putting together rovers for a mining base on Minmus, they are made like this

As shown by KER, this is supposed to give a TWR of 1.18 when the fuel tanks empty (on Kerbin), but when testing, the rover never lifts on Kerbin. I've also tested with 8 Twitch instead of 4 as shown in the image, the result was the same.

This is my first try with Twitch engines, I was previously using Thuds when needing radial attachment, and it was working fine.

To compare , i've tried replacing the 8 Twitch , (which should give 8*13.7 = 109 Thrust), with 2 Thuds , throttled down to 33.5 % and it lifts up just fine when the tanks empty (TWR tops up at 1.18 similar to above).

Am I doing something wrong ? or is this known ?

Edited July 20, 2015 by Finndibaenn

[Reddeyfish]

your 64 kN of thrust in the shown TWR readout is for in-space ISP, not atmosphere ISP. That's probably the source of your problems.

Also, check to make sure that center ore tank as well as the monoprop tanks are empty or at least taken into account for your TWR.

Edited July 20, 2015 by Reddeyfish

[ajburges]

your 64 kN of thrust in the shown TWR readout is for in-space ISP, not atmosphere ISP. That's probably the source of your problems.

Also, check to make sure that center ore tank as well as the monoprop tanks are empty or at least taken into account for your TWR.

KER does account for resource mass.

Click on the atmospheric button, and you should see the TWR drop. Once in atmospheric mode, there are also sliders to play with altitude and speed.

[Finndibaenn]

Yes you're right, KER only shows TWR based on vacuum, but Twitch Thrust doesn't change much : 16 in Vacuum and 13.7 in Atmos.

However, I'm not sure what was going on, i've tried again the twitch version after fiddling with the thud one, and it now works as expected, I guess i had done something wrong, but no idea what

[Padishar]

I make the max Kerbin sea-level TWR to be 1.013 at burn-out so you are unlikely to notice it lift off before it runs out of fuel. When you had 8 engines attached, I suspect that the extra engines had their thrust blocked by other parts of the vessel...

Edit: also, if you angle your engines then make sure you turn on the option in the settings to account for thrust vectors...

Edited July 20, 2015 by Padishar

[Finndibaenn]

When you had 8 engines attached, I suspect that the extra engines had their thrust blocked by other parts of the vessel...

That may very well be the issue, I know I tried at some point with the exhausts pointing on the wheels.

Edit: also, if you angle your engines then make sure you turn on the option in the settings to account for thrust vectors...

I'm not aware of that, guess i'll have to experiment with it

Thanks