Bug re: Jet engine CoM

[The_Rocketeer]

First apologies if this has already been reported, I've made a few web searches and found nothing.

KSP Version: v1.0.2.842 Mac OSX, also Windows 64-bit

What Happens: Jet engines moves CoM in opposite direction to part placement (i.e. forward if engine is placed at the back)

Mods / Add-Ons: All Stock

Steps to Replicate:

1) Go to SPH, new craft

2) Place FL-T100 fuel tank as root part (any tank will do, but small tank makes issue clear). Turn on Centre of Mass indicator.

3) Place J-33 "Wheesley" Basic Jet OR J-X4 "Whiplash" Turbo Ramjet Engine on either attachment node.

4) Notice CoM moves forward beyond fuel tank to impossible position.

Result: Balancing jet VTOLs is nearly impossible (reason I noticed it). This just doesn't seem to be working as intended.

Can upload pics if required.

Edited May 29, 2015 by Claw

[Claw]

Thanks for submitting a proper bug report.

Unfortunately, this one is done by design. I believe it was implemented to simulate the weight of an actual engine, and to help with balancing CoM for "regular" style airplanes (same with the fuel flow rules change for jet engines). Unfortunately it has some (definitely) unwanted side effects.

Cheers,

~Claw

[The_Rocketeer]

Claw, thanks for your response, I had suspected that something like that was the case, but have you actually tried putting the parts together in the way I described? This puts the CoM outside the craft! Madness!

[Claw]

Yep, I understand.

Personally, I did want to see it moved forward some, but not quite as much as it is.

[Starwhip]

Claw, thanks for your response, I had suspected that something like that was the case, but have you actually tried putting the parts together in the way I described? This puts the CoM outside the craft! Madness!

Yeah, I've noticed this as well, thanks for bringing it up.

You could pull some REALLY crazy stunts with an out-of-bounds COM...

[steve_v]

I kinda like it TBH, real jets are nowhere near as short as the ones we have, and e.g. mounting a jet on a cubic strut in a cargo bay shouldn't really work - where would the compressor stage go?

Making jet VTOLs in KSP is insanely easy as it is, IRL it requires very fancy engines.

Honestly I'd prefer jet engines were actually shaped like jet engines... but I'll take a compromise.

Doing it this way does leave the door open for exploits though. Better would be to, say make the precooler into a compressor (comprising much of the mass), and jet engines won't function without one attached in-line.

Edited May 30, 2015 by steve_v

[The_Rocketeer]

steve_v, I think a little imagination can go a long way here. For example, why shouldn't the compressor be perpendicular to the thrust nozzle, as in vectored-thrust engines? If there's to be a mass location that indicates the position of 'engine' parts/mass, I'd be much happier if it was associated with the intakes rather than the thruster. KSPs 'jet engine' parts are really just the holes for heat and noise to come out (and gimble controls) after all. I've always assumed it was down to imagination where the gubbins were supposed to be.

Anyway, what really distresses me about this is my awesome AV-8K Harrier design doesn't fly anymore with the engine mass moved so far forward, it's just impossible to make it look right and still function (tho at some future date I might have to go crazy with some I-beams and weigh the tail down...).

[steve_v]

For example, why shouldn't the compressor be perpendicular to the thrust nozzle, as in vectored-thrust engines? If there's to be a mass location that indicates the position of 'engine' parts/mass, I'd be much happier if it was associated with the intakes rather than the thruster. KSPs 'jet engine' parts are really just the holes for heat and noise to come out (and gimble controls) after all.

A) Because IRL, that nerfs performance pretty hard and requires a specialised engine nozzle... In this case the entire engine, bar the vectoring nozzle, is still horizontal ('cos turbine shafts don't like corners )

This is exactly what I'm getting at, though I'd really prefer an intermediate part as many of the intakes are far too short to contain a compressor/turbine assembly.

For making realistic VTOLS I tend to stick to specialised mod parts that work/look like the real thing, and for inline jets I put something like a precooler or intake body in front of it... for the sake of my imagination.

IMO, the whole "slap a jet nozzle on, and have the airflow somehow magically routed through fuel tanks, cargo bays, etc." really isn't right. Moving the "engine" centre of mass outside the part is a dirty hack that doesn't fix it at all.

If I had my way, an engine assembly must be: Intake -> precooler/compressor/turbine (most of the mass here) -> nozzle (vectoring or otherwise), all in line with no other cruft (bar an 'air passage' hollow tube type part, maybe) in between. Mayhap that's too realistic for some.

Edited May 30, 2015 by steve_v

[The_Rocketeer]

A) Because IRL, that nerfs performance pretty hard and requires a specialised engine nozzle... In this case the entire engine, bar the vectoring nozzle, is still horizontal ('cos turbine shafts don't like corners )

Sure it does, but since KSP (stock) doesn't have those things I'm quite happy making do with 'engines' pointing out in funky directions. KSP's engineering problems aren't full-real (thank God!), so as long as there's a simple set of physical rules/laws that aren't too tough to get to grips with (i.e. engine is good if fuel and air exist). When I make crafts that look like real life I flatter myself that I'm getting closer to understanding the engineering process that developed the real thing (NASA-STS was a hell of a learning curve), but if I can cut a corner to two and still have something that performs adequately and looks cool, that's all I'm really after.

That being said, I do mind when KSP's 'laws of physics' get drastically altered at the drop of a hat. Ok, so it wasn't realistic before, but now it's not just unrealistic, it's not even familiar anymore.

[Temeter]

A) Because IRL, that nerfs performance pretty hard and requires a specialised engine nozzle... In this case the entire engine, bar the vectoring nozzle, is still horizontal ('cos turbine shafts don't like corners )

This is exactly what I'm getting at, though I'd really prefer an intermediate part as many of the intakes are far too short to contain a compressor/turbine assembly.

For making realistic VTOLS I tend to stick to specialised mod parts that work/look like the real thing, and for inline jets I put something like a precooler or intake body in front of it... for the sake of my imagination.

IMO, the whole "slap a jet nozzle on, and have the airflow somehow magically routed through fuel tanks, cargo bays, etc." really isn't right. Moving the "engine" centre of mass outside the part is a dirty hack that doesn't fix it at all.

If I had my way, an engine assembly must be: Intake -> precooler/compressor/turbine (most of the mass here) -> nozzle (vectoring or otherwise), all in line with no other cruft (bar an 'air passage' hollow tube type part, maybe) in between. Mayhap that's too realistic for some.

Realism isn't the issure here, but the way craft are put together in KSP. Your version would be even less realistic since they would completely ignore how planes are designed (e.g. not as long tubes consisting of single purpose parts). The current way, as well as the engines weight offset, is pretty much an approximation how weight would be distributed in reality.

Edited May 30, 2015 by Temeter

[sal_vager]

Just gotta be a bit creative...

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[Captain Sierra]

Just gotta be a bit creative...

http://imgur.com/a/iUMn3

now thats just borderlining on a ridiculous solution to a frustratingly unnecessary change.

Gonna toss this one into Legacy.cfg for removal/readjustment. I do not approve Squad.