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Getting (short) flameouts and don't know why


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So I'm flying a Rapier plane to space, everything going as usual, when suddenly the vessel is in a flat spin. I presume, but do not know for certain, that one of my engines cut out for a moment. By the time I'm paying attention, it's fine with all engines running. But sadden flat spin, what could that be besides a flameout, however brief?

This doesn't happen on every flight, and not always in a similar situation. But it has happened often enough that I wonder what is going on.

As I possible explanation, I can offer part clipping:skylon.jpg

Both the shock cone and the engine is angled relative to the part they're attached to, and offset into the parent so as to not show a gap. Is such a setup known to be problematic? Or could it be that I'm having another problem, one I'm not aware of?

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6 hours ago, Laie said:

But sadden flat spin, what could that be besides a flameout, however brief?

The 3 degree gimbal on the Rapiers is tiny and not very effective.

Pretty much all airplanes (and definitely yours) are somewhat unstable at speed. There is always much more drag at the front of the craft than at the back. If your craft goes even the smallest amount off prograde, the tendency is always for the craft to flip around backwards. So you need extensive control surfaces and gimbal to prevent the natural instability of the craft from actually swapping ends. So I think that's what is happening. Your "yaw" control surface (the tail) plus the engine gimbal is simply not enough to keep your plane pointing mostly forwards.

 

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Ah, no. It is aerodynamically stable, even very much so.

Whatever sends it spinning must be a momentary event, as I can easily bring it back under control - often quickly enough to still make orbit. This makes me wonder why I'm losing control in the first place.

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At what altitude and what speed does this happen? Did you SAS enabled? In what mode?

It could be that you run out of intake air (which is what @Fierce Wolf meant). In that case typically one engine fails first, and then the other engine gobbles up the supply meant for both and keeps running a little longer.

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2 hours ago, AHHans said:

It could be that you run out of intake air

Exactly. Once I built a VTOL with 8 engines pointing down that had a severe intake problem. The solution was to dive deep into the root parts system that KSP uses, and place a combo of intake+jet engine x 8, instead of intake x 8 and then jet engine x 8, because at low speeds (going up the runway) the entire supply tends to go to the 1st engine in the list, while depleting the last one.

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20 hours ago, Fierce Wolf said:

Fly your ship and have an eye in each engine's AIR supply. See if the supply gets low or uneven.

Did that -- it's totally inconspicuous whenever I look, even right after the event.

9 hours ago, AHHans said:

At what altitude and what speed does this happen? Did you SAS enabled? In what mode?

I'm inclined to say "any time, at any speed" but think it's more likely to happen either when I'm going supersonic (say 300-360 m/s, when going is tough and the rams haven't kicked in yet), or much later when im going >1000m/s. Hmm.  That's the two times when I'm closest to level flight (I never really stop climbing, but at these times I'm barely going up). Maybe that's no coincidence.

Control is by kRPC autopilot.

I know how flameouts usually work, and how they get worse once the intakes no longer point prograde. Engines may quickly go on- and offline as you spin, usually not helping matters. This is not what I see here: in this particular case, it is more as if the vessel received one slap across the face -- it happens in the blink of an eye; by the time I notice, it's already recovering(1). I'm calling flameout because I can't think of anything else that would create sudden strong torque on the yaw axis. But really I don't know what's going on here. I came here in the hope that it might be a reasonably well-known quirk.

There's two shock cones for two rapiers, it shouldn't be lacking air under any circumstances one would call "ordinary". It even has air right after the slap, when it's pointed far from prograde.

--

(1) well, "recovering": the tailfin gives it a clear sense of prograde in the yaw axis, but due to the low wings it turns belly-up more often than not. As the wings are also angled, this leads to a quick loss of altitude, not to mention that the event as such already costs some airspeed. Usually this costs too much fuel to still make space afterwards. And besides, I don't fancy comical interludes on my ascents.

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You offer very little to go on. There's is no craft file to try/test, and the one screenshot shows only a glimpse of a wing section, no yaw control, no effective roll control, and a very flip-happy nose that cannot be corrected by anything at the back.

From the screenshot I can't even tell if this plane is capable of take off, let alone fly stable. The glimpse of wing visible looks like regular wing section (not 'fat' enough to be Big-S wings), and since it hides almost entirely behind the short engine stack, it must also be pretty narrow, which alludes to insufficient lifting surface. Lift deficiency seems to follow also from the wing apparently having no angle of incidence at all, which would make it very draggy and inefficient.

Your word that it flies, is stable, and can make orbit is all we got. The one visual you give us doesn't support this at all - this looks like a very unstable plane.

Can you at least add a top-down screenshot, so we can see what the main lifting surface is like and where it is located (allowing more accurate guesstimate of CoM/CoL locations, which would make it possible to identify problematic instability)? We also have no idea of weight distribution: the fuel sections could be anything from full to empty to anything in between, and the contents of the cargo bay section is unknown.

I tried building it looking as closely as possible to the picture, and in test get the expected difficult control, unstable flight, and too little lift to fly efficiently. But much of it is forcibly purely guessed.

 

In any case: part clipping in and of itself is never a cause for flame outs: the way KSP calculates intake air completely ignores any parts in front or 'around' intakes: with the exception of fairings or bays (shielding code automagically kills the basic function of a multitude of parts - worth a rant all of its own but not relevant to this craft), all other parts are basically 'transparent' to airflow; for all intents and purposes, they may as well not be there at all. The angling you use, the function of which eludes me, also appears not to be enough to cause intake insufficiency. The precoolers offer a good bit of static intake air which should work regardless of the velocity vector.

Barring any additional data, and if you continue to suspect intake air disruptions and wish to observe it, I would recommend opening both the rapier PAWs and pinning them, and then either recording the flight so you go back to see the actual moment of disruption, or keeping a finger on Esc to stop the simulation as soon as you notice something's up, to give you time to read both the PAWs for the tell-tale 'Prop Requirement Met' drop below 100%, which by default will always happen asymmetrically.

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13 hours ago, swjr-swis said:

You offer very little to go on. There's is no craft file to try/test, and the one screenshot shows only

That was on purpose: I'm quite certain that the issue is some kind of gameplay quirk, and that a generic discussion of spaceplane design is not going to help.

If the vessel was inherently unstable, I'd feel that at all times. But I'm experiencing singular events that have perhaps a 60% chance of happening on any given flight. Sometimes it doesn't happen at all.

13 hours ago, swjr-swis said:

In any case: part clipping in and of itself is never a cause for flame outs: the way KSP calculates intake air completely ignores any parts in front or 'around' intakes: with the exception of fairings or bays

Dang. Thanks for making that clear.

13 hours ago, swjr-swis said:

Can you at least add a top-down screenshot, so we can see what the main lifting surface is like and where it is located

By all means:
skylon.jpg

skylon-top.jpg

BEWARE: it's a biplane. There's two layers of wing, but non-obvious, because clipped. Mass is 54t at take-off.

--> craft file <-- Requires Making History and Community Resource Pack (because Life Support: Food, Water etc). Any missing part modules can probably be ignored, except perhaps Modular Fuel Tanks. I think MFT stores it's values in such a way that the vessel still works without the plugin, but am not certain. And if the game can gracefully ignore unknown resources, even CRP isn't necessary.

13 hours ago, swjr-swis said:

The angling you use, the function of which eludes me, also appears not to be enough to cause intake insufficiency.

Well, in part the angling helps with pitch torque: the engines are mounted below the CoM. Not sure if it's 100% necessary... mostly I'm trying to allude to this:

_75169153_img_0029.jpg&f=1&nofb=1

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CG shift aft in flight, set fuel priorities.

A canard design without a horizontal stab with fuel draining evenly and large parts at the back is destined to spin out, and your canards being standard size may not have the moment to recover.

Basically what I'm saying is that your plane may not need more air, unless you mean at 16+km in which case clip another shock come in there, but it could be that your plane lacking passive stability in the rear is causing the nose to rise slowly, increasing your alpha, and decreasing the air you get to a point you lose controllability and stability.

Edited by TheTripleAce3
Last paragraph.
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