Jet engines near centre of mass produce little or no thrust?

[fiesty]

More than once, I've tried to make a space plane or atmospheric plane that has an engine at or near the centre of mass, and often when I do this, I get virtually zero thrust.  Has anyone else experienced this, or am I just going loopy?  Here's a screenshot of one aircraft with this weird zero thrust problem.

[sal_vager]

Hmmm it looks like you're blocking the thrust with the elevator.

[Foxster]

Yup, that's your problem. 

I've built several craft with similar central engine arrangements and had no thrust problems. Just flip your elevators to the other side of the fuselages. 

[AeroGav]

Think there's  a collider that operates out to a certain distance behind the nozzle, if it sees something in the way, it determines the thrust to be blocked and you get no power.    Because the elevator is dead central the collider hits it, even though it's not occluding a large % of the nozzle area. 

I've built "engine tunnels" to recess the engine with wing segments before, and other times RCS build aid has had me moving wing mounted engines up the pylon until the rim of the nozzle was clipping into the wing, in order to get centre of thrust in line with centre of lift.  Thrust did not occlude.    Around the rim of the nozzle, you can get away with quite a lot it seems.

[renhanxue]

Yep, it's thrust occlusion. A very confusing phenomenon, because there is nothing in the game that will explain to you in any way what's going on - IIRC even things like KER show that thrust levels are normal, you just notice that you're going really really slowly. The stuff that's occluding the nozzle doesn't heated up by the exhaust, either, and just to make it even more inconsistent I'm pretty sure it only happens with airbreathing engines, not with rocket motors.

I spent long while scratching my head just last week trying to work out why a variant of this design that had wing panels extending much further inboards aft of the engine flew like a brick even when empty:

Some of my ramjets were doing basically nothing except produce noise and fancy lightning effects. Spent a good ten minutes looking for misattached nodes causing drag, until finally the other shoe dropped.

(docked the mk2 mun/minmus commuter successfully and brought it back down - it was bad, and mistakes were made, leading to it ending up in a retrograde orbit, so I decided I didn't want it anymore)

Edited February 1, 2017 by renhanxue

[FancyMouse]

3 hours ago, renhanxue said:

because there is nothing in the game that will explain to you in any way what's going on

F3 will show that some part is heated by exhaust of some engine, which means not only heating, but losing thrust as well if the part being heated and the engine are both on the same ship.

[foamyesque]

5 hours ago, renhanxue said:

The stuff that's occluding the nozzle doesn't heated up by the exhaust, either, and just to make it even more inconsistent I'm pretty sure it only happens with airbreathing engines, not with rocket motors.

 

It 100% happens with rocket motors, and they will in fact heat up whatever's behind them, too. If you don't believe me, put a Hammer on top of another Hammer and fire them both at once to see what happens

[Whisky Tango Foxtrot]

I can confirm not only that occlusion also affects rockets but that it severely screwed up some of my attempts to land base modules on surfaces.

[fourfa]

I think we've established the problem, LOL.  What next?  You might be surprised how little you need to raise the elevators up before the game decides it's not occluding the thrust anymore.  Try a few small increments up and see what happens.

[renhanxue]

I was wondering how I could get my impression of how thrust occlusion actually worked so wrong, so of course I had to test it, and the results explain perfectly why I came to the wrong conclusions.

Everything in the game (including right clicking on the engines) will tell you this thing is producing around 230 kN of thrust, while in reality it is producing zero thrust (the speed of 0.3 m/s is only caused by the runway not being completely flat):

However, the heating caused at this range is almost completely negligible and won't show up on the thermal overlay. I have the KER thermal window open here, and there you can notice something is going on - after a while.

I think the rocket part was just because the thrust "column" is so narrow that I just happened to miss the wings with the rocket engines.

Edited February 3, 2017 by renhanxue

[Foxster]

You can press F3 and the log will tell you if an engine exhaust is hitting another part so leading to zero thrust. 

[renhanxue]

2 hours ago, Foxster said:

You can press F3 and the log will tell you if an engine exhaust is hitting another part so leading to zero thrust. 

I don't know what the log may or may not have done in the past but I can tell you that right now it most definitely does no such thing. It won't even say that if you attach something directly to the node on the bottom of the engine. It'll tell you that whatever you put there exploded from overheating sure enough, but nothing about thrust occlusion.

Edited February 2, 2017 by renhanxue

[Jarin]

40 minutes ago, renhanxue said:

I don't know what the log may or may not have done in the past but I can tell you that right now it most definitely does no such thing. It won't even say that if you attach something directly to the node on the bottom of the engine. It'll tell you that whatever you put there exploded from overheating sure enough, but nothing about thrust occlusion.

Yeah, the log only seems to note when there's a sharp increase in heating from engine exhaust, such as might cause damage if left alone.

[Foxster]

3 hours ago, renhanxue said:

I don't know what the log may or may not have done in the past but I can tell you that right now it most definitely does no such thing. It won't even say that if you attach something directly to the node on the bottom of the engine. It'll tell you that whatever you put there exploded from overheating sure enough, but nothing about thrust occlusion.

Apologies. You are quite correct. 

I wonder when that changed? It most definitely used to do this.  It used to say something like, "Fuel tank damaged by the exhaust from mainsail". 

[AeroGav]

12 hours ago, renhanxue said:

I was wondering how I could get my impression of how thrust occlusion actually worked so wrong, so of course I had to test it, and the results explain perfectly why I came to the wrong conclusions.

Everything in the game (including right clicking on the engines) will tell you this thing is producing around 230 kN of thrust, while in reality it is producing zero thrust (the speed of 0.3 m/s is only caused by the runway not being completely flat):

However, the heating caused at this range is almost completely negligible and won't show up on the thermal overlay. I have the KER thermal window open here, and there you can notice something is going on - after a while.

I think the rocket part was just because the thrust "column" is so narrow that I just happened to miss the wings with the rocket engines.

I did attempt to troll the physics once by putting LVN-nukes inside a mk3 cargo bay, to see if they'd produce thrust when the bay doors are open.    The only way this would work is if you left a massive amount of clear space behind them.  For example, one and a half CRG-100 bay's worth of empty space behind the nukes,  which makes the exploit not worth it.      Funnily enough, Jeb could sit in a command chair right in the path of this exhaust and not be remotely distressed, but i suppose that's Jeb for you.

[foamyesque]

1 hour ago, AeroGav said:

I did attempt to troll the physics once by putting LVN-nukes inside a mk3 cargo bay, to see if they'd produce thrust when the bay doors are open.    The only way this would work is if you left a massive amount of clear space behind them.  For example, one and a half CRG-100 bay's worth of empty space behind the nukes,  which makes the exploit not worth it.      Funnily enough, Jeb could sit in a command chair right in the path of this exhaust and not be remotely distressed, but i suppose that's Jeb for you.

 

You can in fact put them vertically in the cargo bays and thrust out the open doors, though. That works just fine, and gets you the majority of the benefits without costing you anything streamline-wise.

[Reusables]

This is what made me think that spark has little to no thrust; Found out that I got an occlusion there. Although it was nearly on the edge, it gave no thrust at all.

[renhanxue]

14 hours ago, Foxster said:

Apologies. You are quite correct. 

I wonder when that changed? It most definitely used to do this.  It used to say something like, "Fuel tank damaged by the exhaust from mainsail". 

No need to apologize, if anything I should be the one to apologize because I think I was rather unnecessarily rude in my response to your well-intentioned advice. I do have some vague memory of the message you describe as well, but I've been away from the game for a long time and can't remember any details.

As an aside, another thing I discovered is that mounting a mk1 structural fuselage on the bottom of an engine works fine. It actually is hollow as far as the thrust collider is concerned.

Edited February 3, 2017 by renhanxue

[Whisky Tango Foxtrot]

12 hours ago, foamyesque said:

 

You can in fact put them vertically in the cargo bays and thrust out the open doors, though. That works just fine, and gets you the majority of the benefits without costing you anything streamline-wise.

Or you could put a cargo ramp on the back of your bay. That way you can more easily keep your CoT in-line with your CoM. A cargo ramp is actually big enough to fully contain a NERV engine without needing to add any regular cargo bay parts, too.

Edited February 3, 2017 by Whisky Tango Foxtrot

[Foxster]

1 hour ago, Whisky Tango Foxtrot said:

Or you could put a cargo ramp on the back of your bay. That way you can more easily keep your CoT in-line with your CoM. A cargo ramp is actually big enough to fully contain a NERV engine without needing to add any regular cargo bay parts, too.

I was playing with something like that recently with an SSTO. Put the heat sensitive stuff like the cabin back there too with the nukes. Cozy...

Little advantage really as the craft is so fat. Kinda fun tho. 

Edited February 3, 2017 by Foxster

[A_name]

On 2/2/2017 at 0:35 PM, renhanxue said:

(the speed of 0.3 m/s is only caused by the runway not being completely flat)

Fun fact: The runway is actually perfectly flat, unlike Kerbin's surface which is spherical. As a result, the middle of the runway sits closer to the ground than the ends, which in turn results in a gravity differential that causes crafts to start rolling forward upon launch.

[foamyesque]

14 minutes ago, A_name said:

Fun fact: The runway is actually perfectly flat, unlike Kerbin's surface which is spherical. As a result, the middle of the runway sits closer to the ground than the ends, which in turn results in a gravity differential that causes crafts to start rolling forward upon launch.

 

There's a very distinct upslope at the ocean end

[Jarin]

1 hour ago, foamyesque said:

 

There's a very distinct upslope at the ocean end

Same issue, I'm pretty sure. Kerbin is sloping away from the runway, not the other way around.

[foamyesque]

Just now, Jarin said:

Same issue, I'm pretty sure. Kerbin is sloping away from the runway, not the other way around.

 

I'm gonna have to go and actually survey this now aren't I

[Jarin]

Just now, foamyesque said:

 

I'm gonna have to go and actually survey this now aren't I

Increase physics range, make a 3 mile long girder and lay it flat?

Seriously though, if you have a good idea for an accurate survey, I'd be curious to see the results.