Problem No acceleration with full power applied

[Marijn]

I am practicing reentries with the XR5. Most of the times, I end up with too little energy somewhere in front of the runway. I'd like to practice the landing anyway, so if this happens, I'll cheat by appling some throttle to make it to the runway.

But I am always stalling and crashing anyway. Even with 100% power applied. That cannot be right.

I noticed that the Eng ISP displayed by BurnTimeCalc MDF is falling. All the way to zero. It seems to fall with altitude. But that means that the engines will become less and less effective while losing altitide. Below 500 meters, even 100% power is not enough to maintain 70m/s of airspeed, so I will stall and crash.

I think this must be a serious bug. It's completely ruining my Orbiter experience so I hope someone can explain what's going on or how to get rid of this.

---------- Post added at 12:34 PM ---------- Previous post was at 11:45 AM ----------

Here's a clip. The Eng ISP is already way too low when the video starts. I am descending below 300 meters and try to stay airborne after that by pitching up and applying full power. But then the Eng ISP is so low there is no chance to avoid disaster anymore.

Edit: Removed the video from YT. It can be reproduced by setting 'CONFIG_OVERRIDE_MainFuelISP 1' (or CONFIG_OVERRIDE_MainFuelISP=1) in the default Ready for Takeoff to ISS scenario. 100% power does not make the vessel move.

Any suggestions on what might be going on here?

 

[Abloheet]

You can try unchecking the "Realistic flight model" check box under the Parameters tab in the Orbiter Launchpad. Unchecking that option disables the effect of external atmospheric pressure on real thrust generated. This is recommended when flying in the atmosphere of planets like Venus and the gas giants.

Anyway, I didnt expect that this will effect the engine performance of the XR5 in the Earth's atmosphere to such a degree. Atleast it should be flyable.

 

[Marijn]

Nah. I never ran into this situation before and I've been doing poweless landings many times before. It must be a bug coming from somewhere I am not yet aware of.

Eng ISP at the default ISP=2 setting should be 20.779 km/s at at KSC ground level. Not anything less.

 

[dgatsoulis]

The ISP of the XR5 is pressure dependent, meaning that it drops as the external static pressure rises. The drop shown in the video is a bit dramatic, but changing the ISP only changes how long you can burn your engines, not how "hard" your engines burn.
That value (engine acceleration), is given by dividing the thrust by the total ship weight. And as you can see, it is constant. (Well.. near constant because your weight changes as you burn fuel)
Thrust 8.525e6 N / 356.052e3 kg = 23.94 m/s² acceleration.

The main problem that I found in the video you posted, is that you were already too low and too slow to avoid a stall.

I don't have the XR5 installed right now, but IIRC you can change the engines in the cfg file to not be pressure dependent. (So you can lift off Venus or something like that).

Look for a EnableATMThrustReduction in your Vanguard's prefs cfg file, change it and see if it works better.

Hope this helps

 

[turtle91]

The only thing what comes in my mind is the ISP setting in XR5-config and
EnableATMThrustReduction=1 in XR5-config (XR5VanguardPrefs.cfg).

I would check this settings, and also check that you don't have any cheat-values enabled within this config.

But the question is, how did you took-off ?

EDIT: forget my post Dimitri was faster...

 

[Marijn]

Found it! It's a bug for sure. A very nasty one.

My scenario contained a line I forgot about:
CONFIG_OVERRIDE_MainFuelISP 1

If you put that line into the 'Ready for Takeoff to ISS' scenario, applying 100% power does not even move the vessel an inch. BurnTime indicates Eng ISP = 35.436 m/s

If I set MainFuelISP=1 in the XR5VanguardPrefs.cfg file, the ISP indicated by BurnTime is 16.739 km/s

To me that seems a bug in the XR5 and possible in all XR vessels. It can be avoided by not using the CONFIG_OVERRIDE's.

---------- Post added at 01:28 PM ---------- Previous post was at 01:24 PM ----------

But the question is, how did you took-off ?

I was coming back from the moon in this scenario. But you're right, you would not be able to move in this conditions when sitting on the runway.

 

[Marijn]

Can someone please confirm whether this is a bug and not something related to my system?

Reproduction is easy: Just launch the default 'Configuration File Override Example 2' scenario from the XR5 add-on and try to take-off. In my case, the vessel won't be able to move when 100% power is applied.

If it's a bug I'd like to report it. But this feature is documented quite well, so it seems unlikely to me that the config_override feature is not working at all.

 

[turtle91]

I can confirm this.
When oaded the scenario, I am niot bale to move the XR5 using full-thrust.
BTCMFD showed something about 45 m/s.

So...yes....seems to be a bug.
Btw. I never used this overrides, if needed I create a custom XR/1/2/5 - config...

 

[Marijn]

So...yes....seems to be a bug.
Btw. I never used this overrides, if needed I create a custom XR/1/2/5 - config...

Thanks for checking. Creating .xrcfg files is indeed another method to achieve the same. But the override method is more elegant. I think it was working earlier and broke at a later stage.