A question about TWR.

[LostElement]

I know what it means Thrust to Weight Ratio. It needs to be at least one for it to lift off from the Gravity of Kerbin. Does ASL gravity mean the TWR needed to lift from a planet/moon? Thanks/

[Taki117]

I know what it means Thrust to Weight Ratio. It needs to be at least one for it to lift off from the Gravity of Kerbin. Does ASL gravity mean the TWR needed to lift from a planet/moon? Thanks/

Your question is a tad confusing, but if you are talking about how your TWR is affected by gravity then you use Surface Gravity to determine your TWR, so if you are using MechJeb or KER to tell you your TWR it is using Surface Gravity (Hence why it doesn't change based on altitude, but on vessel mass)

[LostElement]

From the ground. I know Kerbin requires a TWR of 1 or more to lift off from it. When I look at the info panel of Mun, It says ASL .166. Would that mean I would need a TWR of at least .166 to liftoff from the surface of Mun?

[Dave Kerbin]

"ASL Gravity" ?

If you point out where you got this term we might be able to help more. ASL is altitude above sea level which has nothing to do with thrust. As for TWR it's a calculation based on the engine's thrust (kN), the rocket's total mass (t) and the planet or moon's gravity (m/s).

You start out with the engines thrust or total thrust if you have several engines. If you are using a LV-T45 engine you'll have 200 kN of thrust under ideal conditions (this information is shown on the tooltip). You then divide that by the total mass of the rocket. If I attach the engine to one of those tall skinny T800 fuel tanks and then put one of the 3 man Mk1-2 pods on top the total weight of my rocket would be 10 tons, so 200 / 10 = 20. Then divide by the gravity of the planet or moon you are on. You can find the gravity by looking on the Wiki or using the GravMax instrument. Kerbin has a surface gravity of 9.81 m/s. 20 / 9.81 = 2.04. So my TWR is 2.04 which mean I will be able to lift off on Kerbin (if it was less then 1 then I would be putting out less force then the gravity pulling me down and gravity would win).

[LostElement]

To simplify it, If your TWR was 1.5, You could overcome gravity from the surface Kerbin, which Scott Manley has stated, your TWR needs to be 1 or more to lift of from Kerbin. It says that the Mun has an ASL Gravity of .116. That should mean in order to overcome its gravity and lift off from the surface I would need a TWR if .116 or higher.

Edited March 18, 2014 by LostElement

[cantab]

ASL probably means At Sea Level in this context. Although for worlds without oceans, IIRC "sea level" or datum is the lowest point, if not lower, because KSP doesn't like negative altitudes.

And yes, it means that a Kerbin TWR of 0.166:1 would be a Mun TWR of 1:1.

[Poynting]

TWR = Thrust force/Mass*g

Where g is the current gravitational field strength in m/s^2

As long as this value is greater than 1 in any given situation the rocket can lift the ship.

MechJeb and Kerbal Engineer only ever use kerbin surface gravity and are to be used as a tool for launch from the pad. (An ASL TWR of 1 will give a ship in space an acceleration of 1g )

[Red Iron Crown]

MechJeb and Kerbal Engineer only ever use kerbin surface gravity and are to be used as a tool for launch from the pad. (An ASL TWR of 1 will give a ship in space an acceleration of 1g )

Kerbal Engineer actually lets you select which body to calculate TWR for, which is really useful in the design phase. MechJeb uses the body in whose SOI the craft is currently, in the VAB or SPH it's always Kerbin.

[Padishar]

Kerbal Engineer actually lets you select which body to calculate TWR for, which is really useful in the design phase. MechJeb uses the body in whose SOI the craft is currently, in the VAB or SPH it's always Kerbin.

Yes, the KER build engineer displays the TWR using the surface gravity of the reference body selected. In flight, it goes further and actually displays 3 different TWR values:

if (settings.Get<bool>("Vessel: Thrust to Weight (Throttle)")) GUILayout.Label(Tools.FormatNumber(stages[Staging.lastStage].actualThrustToWeight, 3), dataStyle);
if (settings.Get<bool>("Vessel: Thrust to Weight (Current)")) GUILayout.Label(Tools.FormatNumber(stages[Staging.lastStage].thrustToWeight, 3), dataStyle);
if (settings.Get<bool>("Vessel: Thrust to Weight (Surface)", true)) GUILayout.Label(Tools.FormatNumber(stages[Staging.lastStage].thrust / (stages[Staging.lastStage].totalMass * (this.vessel.mainBody.gravParameter / Math.Pow(this.vessel.mainBody.Radius, 2))), 3), dataStyle);

The first two values both try to use the local gravity accounting for the altitude above the surface:

SimManager.Instance.Gravity = this.vessel.mainBody.gravParameter / Math.Pow(this.vessel.mainBody.Radius + this.vessel.mainBody.GetAltitude(this.vessel.CoM), 2);

The last one (surface) is explicitly ignoring this and using the surface gravity of the body (just uses the radius).

So, the "current" value should be higher than the "surface" one once you are above the surface (as the gravity and hence the weight will be less). The throttle one will obviously depend on the current throttle setting but at 100% should be the same as "current". If they aren't then there is a bug (I will check this in my updated version and fix it if wrong).

[Red Iron Crown]

Interesting, Padishar. I've only been using KER in the VAB and relying on MechJeb in flight, maybe I should try the flight engineer mode. I'm not good at reading code, what is the difference between the first two TWR values?

MechJeb's TWR readout does have another thing going for it, it displays TWR for both the start and finish of a stage, is that what KER is calculating?

[Padishar]

No, the actualThrustToWeight one (Throttle) uses the current thrust set by the throttles where the thrustToWeight (Current) uses the max thrust of the engines. The surface one also uses the max thrust but uses the surface gravity.

I'll look at splitting the TWR in the build engineer to show start and end TWR. It shouldn't be too difficult to calculate the TWR at the end of a stage...

Edit: In fact, it may be better to show "initial" and "maximum". The maximum would usually be at the end of the stage, but in some situations it may not be. E.g. as in the OPs example, if you have a liquid engine that burns for 30 seconds and 2 solid boosters that burn for 28 seconds all in the same stage, then the TWR at the end will only include the liquid engine whereas the TWR as the solid boosters burn out will be higher. This may be (very) important when using a mod where excessive g-force is harmful...

Edited March 18, 2014 by Padishar

[cantab]

For the time being, if you haven't got complications with different engines burning out at different times, you can find maximum TWR by using the tweakables to remove all or nearly all the fuel from the stage. Just remember to put it back before launch or you will not go to space today.

[Amaroq]

Interesting, Padishar. I've only been using KER in the VAB and relying on MechJeb in flight, maybe I should try the flight engineer mode.

That's exactly what I use K.E.R. for. Its got plenty of good data displays about your Orbit, your Vessel, and your Rendezvous; I can manually plot and pilot an interplanetary burn using just the K.E.R. output.

Padishar, I rate "TWR (Throttle)" as one of the most important pieces of output during launch, as I'll throttle down to keep my TWR around 1.2 when I'm in the low atmosphere, and throttle up as I clear through my gravity turn .. please don't remove that one in favor of TWR (Max)

[Pecan]

...as I'll throttle down to keep my TWR around 1.2 when I'm in the low atmosphere...

Much too low! The point of a good TWR at launch is to get you out of the thick low atmosphere as soon as possible. Throttle-back if your atmospheric drag is excessive (another fine KER information display) above terminal velocity.

[Padishar]

Padishar, I rate "TWR (Throttle)" as one of the most important pieces of output during launch, as I'll throttle down to keep my TWR around 1.2 when I'm in the low atmosphere, and throttle up as I clear through my gravity turn .. please don't remove that one in favor of TWR (Max)

Sorry, I didn't see this the other day. Don't worry, I have no plans to remove TWR (Throttle) and I doubt Cybutek would want to do that either. The "initial and max" TWR thing is for the build engineer. At present it just give the initial TWR of the stage and the plan is to keep track of the TWR during the simulation so a max value can be displayed as well.