MechJeb and Atomic Engines

[Detroit12]

Hello everyone. Just registered but I've been a silent reader for a long time. Been loving KSP for months now, but I'm relatively new to MechJeb and I only recently unlocked the LV-N Atomic Rocket Motor. My question is:

Does MechJeb have an issue with calculating the Delta-V of this engine? I mean could just be missing something painfully obvious, I'm no pro at the game yet by any means. But whenever I attach the LV-N, it never gets the most Vac. Delta-V of the various engines I try on the ship. Since it's heavier, I even add enough dry-mass when trying on the other engines to bump the TWR to the same as it was with the LV-N attached. Still no luck. The LV-909 "Terrier" for example...added dry-mass when trying it on so that both it and the LV-N had the same TWR and the same amount of fuel at their disposal: even with the LV-N's much higher 800 Vac. ISP it gets less Delta-V.

Is there something specific I'm missing that is the key to using these Atomic Motors? (I apologize in advance if it's a stupid question)

Edited June 20, 2015 by Detroit12

[Red Iron Crown]

Congratulations on delurking, and welcome aboard!

LV-Ns have changed to use liquid fuel only rather than liquid fuel + oxidizer, so if your tanks have a bunch of oxidizer then the LV-N could very well deliver less delta-V as the oxidizer is dead weight. Try either tweaking the oxidizer out of the tanks or switching to liquid fuel only tanks like the Mk2 or Mk3 fuselages.

[Detroit12]

Congratulations on delurking, and welcome aboard!

LV-Ns have changed to use liquid fuel only rather than liquid fuel + oxidizer, so if your tanks have a bunch of oxidizer then the LV-N could very well deliver less delta-V as the oxidizer is dead weight. Try either tweaking the oxidizer out of the tanks or switching to liquid fuel only tanks like the Mk2 or Mk3 fuselages.

Thank you for the welcoming

I emptied the oxidizer, which brought up the TWR, but didn't really seem to affect anything because when I matched it's new TWR with the Terrier engine on, the Terrier was still higher . Each went up, but they went up proportionately so the Terrier still ended up being higher even though it has less than half Vac. ISP.

Also something of note- it seems that when matching the TWR of other engines, the LV-N has a much higher Vac. Delta-V, it's just the Terrier engine that it can't surpass. Exact same TWR, exact same accessible amount of fuel, yet there's is literally no situation where I can get the Vac. Delta-V higher for the LV-N than the Terrier if their TWR's are the same and have the same amount of fuel.

[Red Iron Crown]

Thank you for the welcoming

I emptied the oxidizer, which brought up the TWR, but didn't really seem to affect anything because when I matched it's new TWR with the Terrier engine on, the Terrier was still higher . Each went up, but they went up proportionately so the Terrier still ended up being higher even though it has less than half Vac. ISP.

Also something of note- it seems that when matching the TWR of other engines, the LV-N has a much higher Vac. Delta-V, it's just the Terrier engine that it can't surpass. Exact same TWR, exact same accessible amount of fuel, yet there's is literally no situation where I can get the Vac. Delta-V higher for the LV-N than the Terrier if their TWR's are the same and have the same amount of fuel.

If you are just tweaking out the oxidizer of a given design, the LV-N version has less than half the available fuel. The Terrier uses both liquid fuel and oxidizer, so it has the mass of both to use for propellant. The LV-N has only the liquid fuel, the same amount that the Terrier had but no oxidizer. Since the mix is 9:11 LF:O, that means the LV-N is delivering close to the same dV for about 41% the propellant mass. Have a look at how the total vessel mass changes between the two designs.

For a proper comparison, you should add enough tank space to give equivalent propellant mass to each engine (i.e. mass of LF+O for the Terrier equal to the mass of LF only for the LV-N). The LV-N will also do better with dedicated LF-only tanks rather than tweaked LF-O tanks, they have less dry mass per unit of LF.

Finally, there actually are situations in which the Terrier delivers more delta-V than the LV-N. If the total propellant mass is small enough (less than about 5 tons of propellant mass per engine), the lower mass of the Terrier outweighs the higher Isp of the LV-N.

[Randazzo]

Red Iron Ninja beat me to it, but here's some illustrations anyway:

The procedural tanks aren't the best example since their weights aren't identical, but as mentioned above, just removing the oxidizer from tanks gives a different mass of fuel.

[MaverickSawyer]

At that size, there's no real reason to choose one over the other. The difference comes into play with larger ships and longer missions, which is where the LV-N begins to beat the LV-909 hands down.

[Detroit12]

Thank you, that makes sense. I knew I was thinking about it wrong!

What I did was look at ExtraPlanetary LaunchPads' VAB screen to see how much actual fuel was there, in units (adding the weight of the fuel and oxidizer for the Terrier). Once I had 5t of fuel+oxidizer / fuel according to EPL's stats for the Terrier and LV-N, respectively, then I matched the TWR's (which basically consisted of adding a little to the Terrier one to account for it being lighter), and sure enough...the LV-N had much more Vac. Delta-V. Guess I should've looked at the weight of tons of fuel in EPL right away, and added the extra weight from the oxidizer for the Terrier one. Thanks again.

[Detroit12]

Red Iron Ninja beat me to it, but here's some illustrations anyway:

http://i.imgur.com/Fvxr12R.pnghttp://i.imgur.com/Hl0AfGH.png

The procedural tanks aren't the best example since their weights aren't identical, but as mentioned above, just removing the oxidizer from tanks gives a different mass of fuel.

At that size, there's no real reason to choose one over the other. The difference comes into play with larger ships and longer missions, which is where the LV-N begins to beat the LV-909 hands down.

Thank you both as well. Yeah on top of the mistake I was making explained above, I was also apparently incorrectly assuming that just doing a small/quicker comparison would scale to a larger one. I'll definitely go ahead with something bigger now that I can understand it a bit better.

[Skorpychan]

That's because the LV-N is ridiculously heavy, and the low thrust leads to inefficient burns.

They don't really work out more efficient now unless you're going for 4k+ DeltaV, and using spaceplane tanks.