ISP Abruptly change in Vaccum?

[Stilgard]

Sorry I was looking in the forum, but I couldn't see any question about it.

The other day I was landing on Minmus when I suddenly saw my ÃŽâ€v dropping in Mech Jeb. To realise that the ISP of my LV-N was at 220s. I was slowing down my descent by small burst and all was fine until that happen. Changing focus and coming back to my vessel fixed it. But it did it again later on. And now at 100m AGL, it’s not a good idea to change focus. So I landed it like that, I had plenty more fuel on board so it was not so much of an issue. And again changing focus for another vessel and coming back fixed it. My LV-N was back to his 800s.

I already had the same issue back in 0.23 (if my memory is correct) with an LV-909. I was landing on the Mun when only one of my three reactors changed ISP. Making this side of my Lander burning fuel more quickly that the 2 other ones.

Sorry I don’t have pictures or screenshot, but you get the idea.

I know I get it, ISP change in function of the atmospheric pressure. But here in both case it was in a Vacuum all the time.

Did somebody already had similar issue? What did you done to fix it?

[NASAHireMe]

That is very strange and sounds like a bug to me. I don't think I have the answer, but I can try to narrow it down:

1. How did you view the ISPs for either the LV-N or LV-909? By right-clicking on them and viewing the readouts? Because then, such a dramatic drop, resolved by simply focusing on another vehicle, is almost certainly a bug.

2. Fuel lines?

3. Did you update MechJeb to be compliant with 0.90 (or whatever version you're playing)?

4. Do you have a .craft file?

[Stilgard]

I just wanted to see if others had the same issue, because I cannot reproduce this thing. Or at least I don’t see want can trigger it. And it will not happen when I focus the view on the vessel. So I can easily done multiple small burst to control the descent and all goes well. But on this specific attempt it changed, I can’t tell if it was in the middle of a burst, or in between two bursts.

Yes I view the ISP by right clicking on the engine.

It’s a really simple design, no fancy things here. Reactor, one Fuel tank, 2x hitchhiker (it was for a station in orbit contract of 9 kerbonauts), little can for the pilot, batteries, solar panels and mech jeb. Not even any landing gear. But landing on the greater flat on the engine is not so much of a big deal on Minmus, so I give it a try to collect ground samples, EVA and crew report. (and I landed it without any other problem that the one described here).

Yes MechJeb is up-to-date for 0.90 and the career was started in 0.90

No Fuel line.

I can, later today, attach the craft file, but again it didn’t seem to be specifically attached to this craft. (or at least it’s not my impression)

Thanks NASA Hire me, yes I wish NASA can hire me too

Edited January 20, 2015 by Stilgard