Dragless parts in 1.0.2 � ?

[Teilnehmer]

KSP Wiki says:

Physicsless parts are parts where the physics don't work in the usual way. No drag is experienced by this part and the mass of the part is added to the parent's part.

List of physicsless parts:

 2HOT Thermometer

 Double-C Seismic Accelerometer

 GRAVMAX Negative Gravioli Detector

 PresMat Barometer

. . .

I made two identical rockets.

Then I put 4 “dragless†sensors onto the front side of the left one. Each sensor weighs 5 kg, so the four weigh 20 kg.

As for the right rocket, I added a 50-kg battery between the guidance unit and the nose cone, so it was 30 kg heavier than the left one.

Just after the launch the lighter rocket with the sensors charged into the lead as expected.

But after a while it started to slow down and tilt.

When the fuel ended, the rocket with the sensors was 1 km behind and 10° from the vertical direction.

What can this behavior be ascribed to if not the drag force applied to the sensor parts?

[Rocketfeline]

Try putting sensors on each fin.

[xrayfishx]

Are you sure the wiki is up to date? As far as I know the majority of it has always been outdated.

[Prepper-Jack]

What does the ascent look like in aerodynamic overlay mode? (F12 I believe)

[pincushionman]

Instead of adding different parts to another rocket, use the same rocket, say with an otherwise-unused monopropellant tank, and add the "physicsless" parts to that part (preferably at the CoM of that part), and adjust the monoprop levels to match masses. That way you'll eliminate all the differences except the sensors' presence.

I suspect they do in fact contribute to drag.

[Greenspan]

Try putting sensors on each fin.

This, the sensors slightly shift the CoM out of alignment with the CoT causing a torque making the rocket turn.

Or, use VAB to spread out the sensors evenly around the rocket.

[Teilnehmer]

What does the ascent look like in aerodynamic overlay mode? (F12 I believe)

No red arrows were visible at the sensors.

- - - Updated - - -

Instead of adding different parts to another rocket, use the same rocket, say with an otherwise-unused monopropellant tank, and add the "physicsless" parts to that part (preferably at the CoM of that part), and adjust the monoprop levels to match masses. That way you'll eliminate all the differences except the sensors' presence.

I tried the rockets of equal mass (10 sensors 5 kg each vs one 50 kg battery). The results are even clearer. The rocket with sensors dropped behind just after the launch.

[cephalo]

I think you'll have to repeat this test a good number of times. Since I'm terrible at launching, I usually have to revert many, many launches, and I have seen that even the same rocket can behave very differently depending on the exact starting conditions. The starting of the physics engine results in weird random stresses right off the bat that can cause rockets to lean a little etc.

[Prepper-Jack]

No red arrows were visible at the sensors.

That would lead me to believe they are indeed dragless. The turn is most likely coming from mass imbalance, as suggested above.

Good to know - I've been shoving them in the service bay. Having them on the side of the rocket is much more convenient.

[NathanKell]

Wiki is wrong. All parts (unless they have dragModelType = none and do not have a lifting module) produce drag, no matter their physics significance.

[moronwrocket]

This is because the CoM is shifted ahead in your rocket on the right.

It is the "my rocket keeps flipping" problem writ small. That battery pulls the CoM slightly ahead, which makes the fins work "better" and keep the rocket straighter. If you do like @pincushionman says (attach the sensors to a monoprop tank, adjust monoprop to match weights) you should now see essentially similar behavior.

This could also explain why the sensor rocket was slightly behind - the slight tilt causes the fins' drag to increase, losing just a tiny bit of speed.

Edited May 29, 2015 by moronwrocket

[NikkyD]

I usually put the mechjeb module on the side of the mk1 cockpit that has no door and this can "pull" my rocket into the direction of the MJ when ascending... everything has drag now! So you need those service bays

[Starhawk]

Wiki is wrong. All parts (unless they have dragModelType = none and do not have a lifting module) produce drag, no matter their physics significance.

It takes time for volunteers to update the Wiki when an update drops.

This one contains vast amounts of new content, so updating everything will take time and willing people.

I haven't heard whether the PhysicsSignificance flag does anything in the current version.

Happy landings!

Edited May 29, 2015 by Starhawk

[Eric S]

No red arrows were visible at the sensors.

The sensors won't have their own red arrows, they'll change the red arrows of the part that they're attached to if my understanding of the one time I saw a dev comment on this correctly. The statement was made that attaching a physicsless part to another part would alter the mass and the aerodynamics of the other part.

If you think back to the heat shield issue in 1.0 where heat shields were marked as physicsless, if they didn't alter the aerodynamics of the pod they were attached to, there wouldn't have been a problem with the heatshields causing the pod not to orient itself correctly, since the pod without a heatshield would correctly orient itself. Just changing the mass of the pod wouldn't have caused it to become unstable.

[Mister Kerman]

So there's no more physicsless parts now basically?

If so, what's the relation of physicsless parts and their parent part in terms of drag and mass placement?

[rotorwashed]

I thought the physicsless parts added their mass to the parent body?

[Mister Kerman]

We need a smarty pants to clear this up. The thread jumped around a little and I want a simple definitive analysis of this stuff cuz I'm a dumb dumb.

[Volix]

This was mentioned in the squadcast today. I didn't watch it, but here's the relevant piece from the summary by ObsessedWithKSP:

PSA: right now, physicsless parts aren't truely physicsless anymore. They transfer drag and mass (i.e. all physical properties) to their parent.

The full summary is here. Since it adds to the parent part it would just make the drag arrow of the parent part a little longer, probably why it wasn't noticed in testing.

[Mister Kerman]

That's simple enough. So we can glue all the crazy physicsless stuff we like off of a part as long as we consider the added drag and mass to the parent part. I was avoiding placing things asymmetrically up until now.

Edited May 30, 2015 by Mister Kerman

[Volix]

So we can glue all the crazy physicsless stuff we like off of a part as long as we consider the added drag and mass to the parent part. I was avoiding placing things asymmetrically up until now.

As I understand it yes, anything "physicsless" will not matter if it's asymmetric so long as it's placed on a central part. If you had radially attached boosters and covered only one with physicsless parts (for some reason) that booster would have more mass/drag than the others, and your craft would want to turn over.

[Mister Kerman]

Kk cool. Thanks for clearing that up Volix!

[szpachla]

F 12

Press F 12

On your keyboard while in the game.

Wheels (landing gear) are also drag less.

[Foxster]

Ah but what if you add a physicless part to another physicless part? Now that gets interesting