Center of Pressure Determination

[dclarion]

I'm still fooling around with the demo, which is purported to accurately reflect the operation of the full version; I'm hoping to have the funds to consider buying the full version next week. Anyway, in building launch vehicles, I was turning on the center-of-gravity and "center-of-lift" (which seems to be misnamed; center of lift applies to winged vehicles, while center of pressure applies to vehicles with and without wings) indicators, but not seeing the center-of-pressure marker. After stumbling upon shift + right-click to raise the launch vehicle, I found the CP marker. It was approximately half the launch vehicle's length behind the first stage engine bell.

I designed small rockets ("model" rockets) for many years, and the rule of thumb I learned was that the CP of a symmetrical launch vehicle roughly corresponded to the CG of its silhouette. Considering that I rarely had CP-related stability problems, I took this rule to be a good one. So why is it that I'm seeing a calculated CP well aft of where I'm thinking it should be (which would go a long way to explaining why my vehicles are pretty much unmaneuverable right off the pad)? Do I have it wrong, after all?

Edited June 30, 2015 by dclarion

[Juicy]

If you mean the "purple ball", it's the Center of Thrust, not Center of Pressure. It's more or less the place where the expanding gasses from the explosion of the fuel enter the exhaust. So it is exclusively the force generated by the thruster(s) and does not include any other forces applying to the vessel, like drag, gravity, etcetera. The builder only shows the static forces acting upon the ship: Center of Mass, Center of Lift and Center of (maximum) Thrust. Center of Pressure is a derivate of CoL and CoT, but would also require a Center of Drag, which varies depending on direction&heading, speed, and air pressure (altitude), and thus is not shown during building.

[ajburges]

Isn't the demo still on something like 0.16?

I wouldn't say it accurately reflects the full game anymore. The atmo overhaul had very far reaching consequences.

Still, CoL and CoP are different. Stock game does not show CoP so most use CoL as an approximation.

[selfish_meme]

Demo was updated to 1.0

[A_name]

Like the other said, there is no center of pressure indicator. Just add winglets on the bottom of your rocket and you'll be fine (unless your payload is something ridiculously un-aerodynamic and you don't have a fairing on it, in which case just add moar winglets).

[RainDreamer]

I first read this as "Center of Pleasure" and was like, what.

And if in doubt, add more booster. Get out of atmosphere fast and you won't have to deal with the aerodynamic stuff any more! Even if you are tumbling all the way out.

[Warzouz]

Adding more boosters/engines is a very good way to flip even faster and/or beeing very unefficient. The only case you want to add engines is if your rocket doesn't lift off (or very slowly). TWR requirement is lower than in 0.9. Further more the fastest you go, the harder the rocket is to fly (except over 30km where it doesn't matter)

[Chaos_Klaus]

I dont really understand how the center of lift marker works in the VAB. It's always been a mystery to me and I've been playing since before 0.24.

What I do know is that the VAB presumes that you ar moving upwards and the hangar presumes that you are moving fowards (out the door).

Aerodynamic forces can be seperated into lift and drag. There is no center of drag marker in stock KSP. Only center of lift (blue) and center of thrust (red ... or is it purple?)

What you can do, is grab the root part with the rotation gizmo and turn it slightly out of the presumed airstream. That way the center of lift should tell you If your rocket will have lifting forces that make it go straight again. You can do this the easy way by adding fins to the bottom of your rocket. However, KSP also simulates body lift since version 1.0. Although: The model for the wings is differnt from the model for the body. It's a little inconsistent if you really get to the details.

If you like tinkering with real aerodynamics, take a look at the mod Ferram Aerospace Research (FAR). It introduces a way more accurate and realistic aerodynamics model, including supersonic design ... area rule ... stuff like that. It uses a voxel based technique to analyze the actual shape of your rocket. Stock KSP only approximates that shape in a very rough way.

[dclarion]

I'm speaking of the blue ball. (Take that and run with it, guys, as far as it will take you ) What I think I'd like to do, once I buy the full game (hopefully next week) is go with the RO mod. I don't mind difficult, it gives me more to learn.

- - - Updated - - -

I've noticed the part about faster being more difficult to fly. As well, I believe that aerodynamic drag varies with the square of the velocity, so you really don't want to be trying to go like a bat out of Hell in the lower atmosphere?

Edited June 30, 2015 by dclarion
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[Chaos_Klaus]

As well, I believe that aerodynamic drag varies with the square of the velocity

In general yes, but the drag coefficient also changes with mach number (which is speed aswell ...). When you pass 270m/s you get to transonic speeds, where part of the airflow around you vehicle goes supersonic. That creates a lot of drag and might cause a lot of instability. Once you pass Mach 1 (ca. 340m/s) drag decreases again.

RO ... well, I have an RO install aswell. It doesn't feel very polished. It has too many engine options. Each engine part can be clicked and you can choose like eight different setups/versions of this engine that all have different stats. I think it is overcomplicated. Someone went to town on the engine stats, but the models all look ugly.

But don't believe some guy on the internet (me). One advice: Copy your KSP folder and do a seperate install for RO/RSS.