What's up with my TWR?

[Starchaser]

So I'm not sure what's going on here. 

I'm throwing together a ship to go rescue a kerbal in minmus orbit. When I did it around the Mun, I burned about 500 d-v more than I expected, so I added more cushion to this one. At any rate, here it is.

TWR is 1.25. All set to launch.

But wait!

'

 

Surface TWR, 0.78

What gives there?

[Superfluous J]

Engines have two TWRs. Well, a low point and a high point. In atmosphere, they have lower TWR than in space. The Terrier, in particular, has terrible atmo TWR. It makes up for this by being GREAT in space.

In KER, it says you're viewing Atmospheric TWR but considering you're using 2 Terriers, I think it's wrong. I don't know why.

The fix is to not use Terriers until you're about 10km up or so. I'd replace those with BACCs or something like that.

Edited November 23, 2017 by 5thHorseman

[bewing]

Maybe it's calculating "surface" TWR from the top of the highest mountain.

[Starchaser]

21 minutes ago, bewing said:

Maybe it's calculating "surface" TWR from the top of the highest mountain.

Oh no. Those engines are all blasting and full out and the rocket is dedicatedly staying where it is. Except just after this screen grab, it fell over.

26 minutes ago, 5thHorseman said:

The fix is to not use Terriers until you're about 10km up or so. I'd replace those with BACCs or something like that

I had Reliants on those boosters. The numbers were fine, except my TWR was 2.94. Since it won't launch as is, I'll put them back on. I was just trying to understand why it said 1 thing in the VAB and another on the pad. I've seen it before, but it's been an overall frustrating night through 3 different games (Diablo3, Endless Space 2 and KSP) It just broke my frustration threshold for the evening and made me actually ask the question.

[bewing]

3 minutes ago, Starchaser said:

Oh no. Those engines are all blasting and full out and the rocket is dedicatedly staying where it is. Except just after this screen grab, it fell over.

I understand that, but "surface" does not necessarily mean "sea level". And the TWR of those engines will change a LOT between 72m altitude, and 5km altitude.

 

[Superfluous J]

Yeah it's been a while but doesn't KER's Atmospheric thing in the VAB allow you to set a height? Maybe you're not at 0 but are instead at 10km or something weird.

[Aegolius13]

KER is showing the ship's vacuum TWR as 1.25, which sounds legit.  So it seems like the other window (mechjeb?) is just failing to include atmosphere in its readout for whatever reason.

@Starchaser, I suspect that if you clock on the "atmospheric" button on the KER readout in the VAB, it will show you the same TWR you're seeing when you actually launch.  

But in any event, as stated above, the underlying problem is that the Terrier is essentially useless at sea level.  I think you have to hit around 10km altitude before it starts to outperform the surface-oriented engines.  

[HebaruSan]

Click here to see the truth about your lower stages, then click it again to switch back to dealing with the upper stage:

The handling of atmospheric TWR is one of the few remaining issues I have with KER. It needs an option for "use atmospheric values until this total dV value and vacuum beyond," but the structure of the simulator makes that difficult to implement.

[qzgy]

1 hour ago, 5thHorseman said:

Yeah it's been a while but doesn't KER's Atmospheric thing in the VAB allow you to set a height? Maybe you're not at 0 but are instead at 10km or something weird.

Yes, but in this case its completely off.

In anycase thoug, the SLT readout from mechjeb works.

[HebaruSan]

2 hours ago, 5thHorseman said:

In KER, it says you're viewing Atmospheric TWR but considering you're using 2 Terriers, I think it's wrong. I don't know why.

No, KER says he's viewing vacuum TWR. If you click Atmosphere, the button depresses like a toggle and some sliders appear:

11 minutes ago, qzgy said:

Yes, but in this case its completely off.

See above; the OP's screenshot is in vacuum mode.