Is the nuclear engine still efficient?

[CobraA1]

I've had an odd thing whenever I replace a standard engine with a nuclear one on some of my spacecraft - the delta V seems to go down, according to MechJeb. Even if I take out the oxidizer. Does this sound right? If so, what is the current role of the nuclear engine?

[Palaceviking]

Sounds weird?!?  If it's a super light craft then the sheer weight of the thing could be the problem. 

Oh and use liquid fuel only tanks to save carrying dead dryweight around with you.

[steuben]

Is it calculating atmospheric dv or vacuum?

[Red Iron Crown]

34 minutes ago, CobraA1 said:

I've had an odd thing whenever I replace a standard engine with a nuclear one on some of my spacecraft - the delta V seems to go down, according to MechJeb. Even if I take out the oxidizer. Does this sound right? If so, what is the current role of the nuclear engine?

It does sound right, you're using less than half the propellant mass in the nuclear version compared to the nuclear one (look at the total vessel mass between the two versions). Try replacing the LF+O tanks with LF-only ones and you'll see the nuclear engine comes out on top once you have more than about 4 tons of propellant per engine.

[magnemoe]

15 minutes ago, Red Iron Crown said:

It does sound right, you're using less than half the propellant mass in the nuclear version compared to the nuclear one (look at the total vessel mass between the two versions). Try replacing the LF+O tanks with LF-only ones and you'll see the nuclear engine comes out on top once you have more than about 4 tons of propellant per engine.

Yes, use liquid fuel only tanks, for larger ships the MK3 tanks are very nice here. The only reason why you should bring oxidizer is to fuel other crafts. 
This can be an nice feature however. 

Else it works as before, exception is that isp now affect trust not fuel use so you can not use it on kerbin or Laythe for an powered landing. 
The engine are heavy so don't use it for light stuff like one man craft or probes. 
For heavy ships you can cluster them, the slanted nose cone is perfect to put them radial on an fuel tank, makes landing easier and you got the bottom free for an docking port. 
 

[GoSlash27]

1 hour ago, CobraA1 said:

I've had an odd thing whenever I replace a standard engine with a nuclear one on some of my spacecraft - the delta V seems to go down, according to MechJeb. Even if I take out the oxidizer. Does this sound right? If so, what is the current role of the nuclear engine?

CobraA1,

 A design for less than 2km/ sec DV is actually lighter if you use the LV-909 instead of the LV-N, so I'm not surprised. And as RIC pointed out, oxidizer is dead weight when using LV-Ns.

Best,
-Slashy

[Red Iron Crown]

4 minutes ago, GoSlash27 said:

And as RIC pointed out, oxidizer is dead weight when using LV-Ns

I didn't point that out as such as the OP already mentions tweaking out the oxidizer. I pointed out that using the same tank arrangement for both designs is misleading, as there is over twice the propellant mass in the chemical version.

[GoSlash27]

1 minute ago, Red Iron Crown said:

I didn't point that out as such as the OP already mentions tweaking out the oxidizer.

I stand corrected

-Slashy

[CobraA1]

Thanks for everything :).