TWR and Patched conics.

[cicatrix]

Please, can someone explain to me how to interpret the TWR readout from KER or MJ?

What I mean is that depending on the chosen celestial body, the TWR of my engines changes. If I design a rocket in VAB and put some LV-909 into the craft the TWR says it's 0.35 at LKO (in vacuum). Now, when I change the reference body to Mun or Minmus, for example, it shows something like 10-20. That's understandable also. What I do not understand is how my engines magically increase their TWR at the SOI change. 1 second before the SOI change their TWR was 0.3 and 1 second after it miraculously became 10. Should I believe these figures and shrug it off to game engine faults or these figures are pure gibberish and I shouldn't even look at them?

What I also noticed is that real acceleration dynamics does not change all that much when I change SOI.

My feeling is that I should not base my burn duration estimations on these readouts or am I wrong about that?

Edited May 8, 2015 by cicatrix

[Ljas]

"Weight" implies mass multiplied by local gravitational force. Your TWR is a percentage your ship's max acceleration compared to the local free fall acceleration. If it's less than one, gravity overpowers your thrust and the net acceleration vector will point towards the planet.

[cicatrix]

"Weight" implies mass multiplied by local gravitational force. Your TWR is a percentage your ship's max acceleration compared to the local free fall acceleration. If it's less than one, gravity overpowers your thrust and the net acceleration vector will point towards the planet.

So, it is meaningful only on surface/suborbital flights.

What I mean is I roughly estimate burn duration basing on TWR readouts, i.e. with high TWR I should start the burn closer to maneuver node and when it it's not very high I should start the burn earlier. Apparently I do it wrong because when in orbit the TWR readout means nothing.

[Deadpangod3]

So, it is meaningful only on surface/suborbital flights.

What I mean is I roughly estimate burn duration basing on TWR readouts, i.e. with high TWR I should start the burn closer to maneuver node and when it it's not very high I should start the burn earlier. Apparently I do it wrong because when in orbit the TWR readout means nothing.

it does still matter a bit, because if you have a low TWR it will take a while to complete a burn, while with a high TWR it may not take very long.

[Red Iron Crown]

Use the "max acceleration" figure instead, that doesn't change with celestial bodies.

Moving this over to Gameplay Questions.

[cicatrix]

it does still matter a bit, because if you have a low TWR it will take a while to complete a burn, while with a high TWR it may not take very long.

If that's the case, then imagine I placed a maneuver node right at the point of Kerbin-Minmus SOI change. When my craft is still in Kerbin SOI its TWR is about 0.3, but once it enters the Minmus SOI its TWR becomes 30+.

Will the thrust increase dramatically once the SOI has changed?

[Red Iron Crown]

If that's the case, then imagine I placed a maneuver node right at the point of Kerbin-Minmus SOI change. When my craft is still in Kerbin SOI its TWR is about 0.3, but once it enters the Minmus SOI its TWR becomes 30+.

Will the thrust increase dramatically once the SOI has changed?

No. Thrust and actual acceleration remain the same. TWR is max acceleration vs local body's gravity acceleration, it changes with what SoI you're in but does not actually change vessel performance in any way.

[arq]

If I recall correctly, KER has two possible TWR readouts - one for 'current' and one for 'surface.' 'Current' (or maybe I'm mislabeling it) is based on the gravitational force in your current SOI at your current altitude, while 'surface' is based on the gravitational force in your current SOI at sea level (0km). If you go into options, I believe you can change which one you display (or both). The 'current' TWR should change more gradually between SOI, but honestly isn't very useful. Surface TWR lets you know what kind of launch/landing profile to use. I usually display the surface TWR and acceleration (m/s^2), since acceleration is much more interesting in orbit than TWR.

[cicatrix]

I see now, what I was doing wrong. Thank you for your explanations, rep given.