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Phantom Oxidizer


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During flight testing of my latest SSTO, I'm noticing a rather bizarre behavior... 

I have a Tri-Quad Rapier configuration; so 12 engines.  During ascent, all 12 run out of oxidizer and flameout at the same time exactly as I would expect; but then something bizarre happens.  I throttle up and one of the twelve engines runs for like 15-30 seconds and then flames out.  Is there a hidden store of oxygen in the closed intakes?  ...perhaps only enough to power one engine?  My expectation would be that all twelve would suck every bit of oxidizer and/or air out of the system simultaneously? 

I have checked every oxidizer tank and they are all completely dry.  I also cycled the RAPIERS to air mode and back, just to see if they were auto-closing the intakes and trapping air or something.  After toggling the RAPIER mode, the single engine still finds a way to run.  I can't seem to locate the mystery source of this extra air?

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I'm slightly confused. Are you in open-cycle mode (running off liquid fuel + intake air) or closed-cycle mode (running off LF+O)?

If you're in open-cycle mode, there's a simple reason why you could have 1 run after the rest flame out: your aircraft has enough intake air coming in to power one, but not twelve, RAPIER engines.

If you're in closed-cycle mode, I will have to admit confusion there. I could see why you might have a very brief bit of power on one engine if the game says "okay, not enough LF+O left for all twelve, shut-down", but there's still a tiny amount left in the tank, enough for a few physics ticks of one engine, but that should be less than a second.

 

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What is happening is at the height you are at, the atmosphere is a lot thinner, and the intakes can't collect enough to run all the engines. But all of them combined can collect enough air for one engine. The title was a bit confusing, it should have said intake air, not oxidiser.

Hoep this helps! :)

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11 hours ago, XLjedi said:

Is there a hidden store of oxygen in the closed intakes?

I'm sure there used to be a bug where the intakes would carry oxygen as a resource - but not enough to run an engine on for anything more than a split second.

As @MiffedStarfish says, it's just likely that 12 intakes scraped enough oxygen out of the upper atmosphere to run one engine.

You can use this to your advantage by grouping your engines and shutting those groups down sequentially (thus diverting more oxygen to fewer engines) and squeezing out the last bits of velocity from the air-breathing engines. 

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I seem to have solved the problem; but it was very odd indeed.

Here's a pic of the one engine restarting after all 12 were operating AND flamed out due to lack of OXIDIZER?  ...and hence my referring to the situation as Phantom Oxidizer.  On this mission we were carrying a 4400 payload of oxidizer so the useable oxidizer resource for the RAPIERS should have reported empty at 4400, but as you can see there was like 100-150 in the system somewhere?  Remember all 12 engines flamed out due to lack of oxidizer, then I cycle the throttle and this one pops back on?

Kodiak_Phantom_Oxidizer.png

It appears to have been a fuel flow issue and I solved the problem by adding the two fuel lines shown below.  This is the first time I've ever had to actually incorporate functional fuel lines in an SSTO design.  

Kodiak_Fuel_Lines.png

So today I figured I'd just remove the fuel lines and see if I could duplicate the previous behavior that I saw on at least 3 previous flights and maybe double-check the oxidizer levels in all the tanks again (I swear I looked at all of them and they were all zero)  ...and now, naturally, I can't duplicate the behavior.  In the end, the problem seems to have gone away.  Although I can't duplicate the behavior, I left the fuel lines in the design cuz I just thought they looked cool and might prevent the one engine from firing by itself again for some reason.

 

 

 

 

 

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3 hours ago, MiffedStarfish said:

What is happening is at the height you are at, the atmosphere is a lot thinner, and the intakes can't collect enough to run all the engines. But all of them combined can collect enough air for one engine. The title was a bit confusing, it should have said intake air, not oxidiser.

Hoep this helps! :)

Nah, title was correct... issue was in closed cycle mode.  The flamout and transition from open to close cycle at about 28km behaved exactly as expected; all 12 kicked over to closed cycle without issue.  My problem was at 72km and the flameout associated with depleted oxidizer.  I posted some clarifying pics above.

Edited by XLjedi
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Heh. You've got way too many air intakes on that thing. But the ship looks basically stock.

My bet is that it's one of your mods causing the issue -- but if you can post a version of your craft that I can load, I could give it a whirl or two and see if I can reproduce your issue or not.

 

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On ‎7‎/‎2‎/‎2017 at 8:46 PM, bewing said:

Heh. You've got way too many air intakes on that thing. But the ship looks basically stock.

My bet is that it's one of your mods causing the issue -- but if you can post a version of your craft that I can load, I could give it a whirl or two and see if I can reproduce your issue or not.

 

It's among "My Designs" below...  good point on the overdone air intakes.  I shoulda caught that and scaled em back.  I posted an update to the design with divertless intakes and scrapped a few of em.  Air intake is at 22 now instead of like 32 and it seems to manage ascent just as well.  Looks nicer too!

Thanks for the suggestion.

Edited by XLjedi
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You've only got 12 engines, so anything over 12 intakes does you no good. And the shock cones are worth at least 3 engines each. Heading to KerbalX now.

 

Edited by bewing
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20 hours ago, bewing said:

I can't reproduce it. I took the 2 little fuel ducts off, fly almost to orbit, Ox runs out, and all the rapiers shut down.

 

I can't either...  seems to have gone away.  I did see it on at least 3 missions though, across more than one day.  It was on the third or fourth occurrence that I finally snapped the screenshot.  Probably just a mod-induced gremlin; wish I still had the .craft file before I added the fuel lines. 

The flight test crew was a little iffy too.  At one point, they mentioned seeing a baby kraken playing with one of the air intakes...

Iffy_Crew.png

When you say "anything over 12 intakes", I assume you meant "anything over 12 resource air"?  I've cut back on the intakes in the final design, that was helpful feedback thanks!  ...and shock cones = 3 engines, check! 

Also, I have not been to other atmosphere planets yet, would your rule-of-thumb for resource air apply to those planets as well? 

I very much appreciate you taking the time to flight-test and provide feedback!

 

Edited by XLjedi
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By the way, @XLjedi:  I see that you appear to have a solution to your problem, this time 'round, but just one side note about How Things Work, since that may be useful in understanding potential future difficulties:

Oxygen != oxidizer.  They're completely different things and aren't interchangeable at all.

It's easy to make this mistake, because IRL, oxygen is an oxidizer (thus the name "oxidizer"), and a material that will burn in oxygen will generally burn with an oxidizer, too.  But in KSP, they're completely different things that simply happen to have similar-sounding names.

Specifically:

  • Jet engines (or Rapiers in "jet" mode) will run only with atmospheric oxygen from an intake.  They will not and cannot use oxidizer, ever, under any circumstances, even a little bit.
  • Rocket engines (including Rapiers in "closed-cycle" mode) will run only with oxidizer.  They will not and cannot use atmospheric oxygen from air intakes, ever, under any circumstances, even a little bit.

It's a useful thing to know when you're debugging a situation like this.  For example, if you're dealing with Rapiers and trying to figure out "why does / doesn't it burn", just look at which mode it's in.  If it's in atmospheric mode, you know it's purely an issue involving air intakes, and there's no point in looking for oxidizer anywhere, because oxidizer is irrelevant in that scenario.  If it's in closed-cycle mode, you know it's purely an issue involving oxidizer, and therefore the air intakes have nothing to do with it.

Just something to be aware of.  :wink:

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@Snark  Pro tip, thanks! 

LOL, on the one hand, you think I may need schooling on Oxygen vs. Oxidizer.  I am fairly dense; so not a bad assumption there... 

On the other hand, you give me credit for understanding your esoteric reference to a C# operand for inequality.  I'll take that as a complement, thank you.

Edited by XLjedi
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6 hours ago, XLjedi said:

@Snark  Pro tip, thanks! 

LOL, on the one hand, you think I may need schooling on Oxygen vs. Oxidizer.  I am fairly dense; so not a bad assumption there... 

On the other hand, you give me credit for understanding your esoteric reference to a C# operand for inequality.  I'll take that as a complement, thank you.

Maybe not that much of both cases.

Unfortunately the way you put things give space for some confusion, and clearing it may be helpful in the future. Like in next time you ask a question or when someone look at this thread because a possible related doubt.

OTOH, maybe Snark is so used to C# that "!=" come more easily than "not equal" to him. (and also he stated it in the sequence exactly after the "!=" was used.

 

Back to the topic: jet engines, after a change along the versions, now flameout at a given point in the atmosphere where the "oxygen concentration" is not high enough. (I put it between quotation marks because I really don't know how exactly the game handle it: pressure, altitude, combination of both?). So at the altitude of your screenshot the engine should be in rocket mode and should require Oxidizer to work. If it worked without oxidizer its clearly a bug. Unfortunately non-reproductible bug are hard to smash.

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