Sudden low altitude airbreathing thrust loss?

[KerikBalm]

So I was trying to make a more efficient, aerodynamic SSTO. (with a more reasonable m3 cargo bay, instead of the massive 3.75m fairing designs like I have as my avatar)

Previously, I got to orbit with this and plenty of payload:

(that was in 1.04, still worked in 1.05), but that is 10 engines just to get a payload in the long mk3 bay to orbit (plus lots o space dV with the nuke and extra fuel).

I reduced that to 6 airbreathing engines, an everything was going fine:

But then suddenly, the thrust dropped to 50% on all engines:

Reverting and relaunching, and trying different ascent profiles always had the same result, throttling up or down did nothing to change it... for some reason, a certain time after launch, the engines justhave their prop requirements reduced to exactly 50%, and they stay there regardless of throttle/airspeed/altitude.

So a 6 rapier design becomes a 3 effective rapier design, and thus it effectively becomes a 40 ton/rapier design, which cannot go supersonic.

What is going on? is this a bug? has anyone else encountered it?

 

[Snark]

Hmm, that's really strange.  The only clue I can see (great screenshots, btw! nice capture of detail) is the "Prop. Requirement Met: 50.00%", indicating that the engines are getting starved of propellants somehow.

Since the engine is in AirBreathing mode, it needs two propellants (intake air and LF), but I don't see why you should be having a problem with either one all of a sudden-- you've got an LF tank right in front of each engine (they have fuel in them, right?), and you shouldn't be running out of air at such a low elevation.  Can you see what your intake air volume is?  Anything interesting happening there?

Is there any other equipment running on the ship?  You're not running an ISRU or something, are you?  (I'm guessing not, just grasping at straws here.  There's a bug, or at least there has been one, don't know if they fixed it, that running an ISRU at the same time as running multiple engines causes weird engine smothering problems.)

[cephalo]

I bet you have some action keys set to close the intakes in an unexpected way. Depending on your test environment, sometimes the toggles on the rapier mode switch and the intake closure get out of sync if you've made a lot of changes in the SPH.

[cantab]

Prop. Requirement Met refers to air. You don't have enough air intakes. Try flying faster or adding more intakes.

[KerikBalm]

Yes, I saw the prop requirement met thing before making the post, and obviously from the staging display, there is plenty of LF available.

I did not make any action groups to close intakes... they are all open, I suppose I should run the test again looking at the airflow.

Its 1 shock intake per rapier, when in other designs, 2 rapiers per shock cone is enough. If I was running out of intake air, I would expect to see it drop from 100% to lower levels in a smooth fashion, and the prop req met to vary based on the throttle setting. As it is the prop requirement is 100% as my craft continues accelerating in a very shallow manner, until it suddenly drops to exacty 50%, an stays there as speed an altitude change.

I wonder if this is some bug like a repeated kraken strike that will go away when I restart the game. Oterwise, I will try to remove the intakes and engines an then place them again.

No ISRU... the payload is just 2x "low gravity airless body landers" (ie, a modified version of my Mun lander with science stuff added as it was unlocked- sufficient for landing on any airless body except Tylo)

[AeroGav]

Try mousing over the intakes , make sure they all say "status - nominal" rather than "occluded" which means the game thinks a cargo bay or something is blocking the inlet.     It's wierd though, how it doesn't appear to strike until the second climb segment,  and thrust is not restored even when falling below the airspeed / altitude / pitch angle where the power rollback first occured.

That's a lot of engines btw !  I've flown an SSTO with gross weight over 30 tons on one rapier, 2 nervs and no oxidiser, but i did have about three times as much wing as you had,  and the ascent took a long time.    

[KerikBalm]

Well, I've narrowed down the problem, but it makes no sense to me... It is not a matter of intake air, and it is a very sudden loss, its always happened between 1:19 and 1:23

Fullpower, doing fine:

Less than a second later

So I took off the engines and intakes, and added them back, I also gave it some wingstrakes to improve its wing area while maintaining aesthetics (unrelated to the problem):

and by 1:23... same problem

 

The funny thing was... my payload is tilted in the bay, and twice when I launched, the control point was my payload, not the probe core on the main craft. Once it automatically loaded a pilot in the payload (leasie), but earlier it hadn't - I suspect it was related to the choice of control point. I figured there may be something funky going on with the payload (although it should be very standard).

I removed the payload (and added another set of wingstrakes), and then everything was fine....

I don't understand it, but I think I may have encountered this problem before, as I recall hearing the sudden change in engine sound associated with this thing before. Maybe the other times I simply had enough engines to get going supersonic with only 50% thrust.

14 hours ago, AeroGav said:

Try mousing over the intakes , make sure they all say "status - nominal" rather than "occluded" which means the game thinks a cargo bay or something is blocking the inlet.     It's wierd though, how it doesn't appear to strike until the second climb segment,  and thrust is not restored even when falling below the airspeed / altitude / pitch angle where the power rollback first occured.

That's a lot of engines btw !  I've flown an SSTO with gross weight over 30 tons on one rapier, 2 nervs and no oxidiser, but i did have about three times as much wing as you had,  and the ascent took a long time.    

Yea, they aren't occluded, and as already mentioned, there's no way that they'd stay at precisely 50% of the engine intake needs, regardless of airspeed, angle of attack, altitude, or throttle setting.

It is a lot of engines, I wonder if I've encountered thing bug before and just trained myself to use moar (airbreathing jet) boosters. I see others use less engines, so what you see here is a 40% reduction in engines relative to the previous version (6 instead of 10).

Also, there are other reasons... my computer is kind of slow, so I prefer faster ascents to save real life time. I don't like part clipping(at most I do the minor part clipping as you see with the wing strakes and the small cones on the back of the rapiers- which isn't actually needed if you sufficiently restrict the gimbal of the rapiers), but I do like certain aesthetics, particularly for this design (vs my monster design for oversize payloads... which I've started to lower the part count - I use more mk2-1.25m adaptors instead of bicouplers, and thus less intakes, >1 shock per rapier was a bit wastefull, and I'm thinking of reducing the engine count, although it may still be needed when the payload size results in a draggy fairing, like when I launched my 3-kerbal eve-lander)

[KerikBalm]

So I replaced the payload with an orange tank(fuel in the tank locke)... everything went fine and had 100 of prop requirements met to >1300 m/s and >23,000 m

Then I went back to the original saved craft with the original payload, and made some changes to make it a little more aerodynamic and heat tolerant:

Probecore inside the bay, battery inside the bay, shock cone gone replaced with a pointy fairing(higher heat tolerance)

-So it is now -1 intake... and I then disabled fuel flow from the lander tanks (2 landers, 2 tanks per lander), then I disabled fueld crossfeed on the docking ports, and enabled staging on the docking ports.

This time, there was no thrust loss, prop requirement met stayed at 100% the whole time... I'm not sure which change actually solved it, but I've determined that the payload is somehow causing it.... :/ it shouldn't behave this way, I'm calling it a bug.

[AaronLS]

Had same issue.  When the cargo bay doors are open, no problem.  As soon as they are closed Rapiers and Turbo Jets all drop to 50%. Disabled crossfeed to block fuel in the cargo bay fixed issue.  Took awhile to figure it out though then googled to see if it was known issue.

Edited January 1, 2016 by AaronLS

[Snark]

From reading the various posts on this thread, it sounds like it's a real issue that bites lots of folks.  Has someone reported a bug?  Best way to make sure it's on Squad's radar. 

 

[Geschosskopf]

2 hours ago, Snark said:

From reading the various posts on this thread, it sounds like it's a real issue that bites lots of folks.  Has someone reported a bug?  Best way to make sure it's on Squad's radar. 

 

If this is caused by cargo bays, there's another bug report on them for killing the ability of nearby wings to generate lift.  Might want to add the above to this one, or at least reference this one in a new bug report.

[Wanderfound]

Just had this happen to me as well. It's craft specific; other ships fly just fine, there's just one design that drops to 50% air as soon as it leaves the runway biome.

[Myvekk]

Just had this occur to me.  I found that as soon as the fuel in the cargo hold payload was used up, (it was drained before the SSTO tanks were used at all), I hit 50% thrust.  Manual transfer of fuel into the payload restored full thrust.  Relaunched after disabling crossfeed on the docking clamp in the cargo hold, and no problem.

[AeroGav]

Me too now, bitten by the cargo bay bug.   Guess airplanes just aren't supposed to have cargo bays?

 

Shame to encounter this 9 years into a grand tour though..

[Geschosskopf]

2 hours ago, AeroGav said:

Me too now, bitten by the cargo bay bug.   Guess airplanes just aren't supposed to have cargo bays?

Well, if it's any consolation, I have tried very hard to replicate the cargo bay occlusion bug in 1.1 and so far it hasn't happened.  So fingers crossed....