LV-N "Nerv" underpowered?

[Pixel of Life]

I built a Mun lander a few days ago and noticed that 909's seem to be giving me a lot more delta-v than the Nerv nuclear engines - 3999 m/s on the 909's compared to only 2176 m/s on the LV-N (from Mechjeb Delta-V Stats). So I checked the values with an online dV calculator (http://www.strout.net/info/science/delta-v/) using the masses and ISPs listed below:

Mun lander, 4 x LV-N "Nerv" engines:

Full mass: 45400 kg

Dry mass: 35400 kg

Engine ISP: 800 s

Delta-V: 5900 m/s

Mun lander, 4 x LV-909 "Terrier" engines:

Full mass: 35400 kg

Dry mass: 11400 kg

Engine ISP: 345 s

Delta-V: 3833 m/s

Now, as you can see, the LV-N is clearly WAY underpowered in terms of efficiency since it achieves less than half of the delta-V it should. Or is it just Mechjeb being dumb and unable to calculate dV properly?

Discuss.

[Temeter]

You're probably carrying oxidizer? New nerva is liquid fuel only.

[WWEdeadman]

Yeah, you're probably carrying around oxidizer you don't need. THe LV-N only uses Liquid Fuel now, like a real live NERVA would.

[Pixel of Life]

You're probably carrying oxidizer? New nerva is liquid fuel only.

Yeah, you're probably carrying around oxidizer you don't need. THe LV-N only uses Liquid Fuel now, like a real live NERVA would.

Removing oxidizer improved the situation a bit, however, it still only has 3306 m/s of delta-V compared to the 5900 m/s it should have. There is definitely something weird going on.

E: Can someone else test this too and post results?

- - - Updated - - -

Derp, I think I *might* be stupid. I ran the calculations again with no oxidizer mass included and it said the dV is a bit over 3200. I guess that's because the empty weight with 909's is only about half of what it is with the LV-N's. So apparently it isn't the go-to engine for almost everything anymore. In fact, it seems to be completely useless for anything under 60 or 70 tons. I tried the same thing on my asteroid tug (~55 tons) and I get nearly 30% more dV with the LV-909's.

Edited April 30, 2015 by CaptainKorhonen

[KerikBalm]

We need more LF only tanks now... currently, they are only available for spaceplane parts...

As much as its cool to cruise around in a spaceplane looking thing.... I'd prefer if the spaceplane parts were left in LKO, and I instead released pure spacecraft from my cargo spaceplanes.

The other nerf to the LV-N is that its TWR is only 75% of what it used to be... so the dry mass is even higher