Did they nerf the LV-N rockets?

[sedativechunk]

I just now unlocked the nuclear engine in KSP post 1.0. I am building a ship for Duna atm and flight engineer is telling me I have better delta V with LVT-45 engines instead of the LV-Ns, I also remembed to remove the oxidizer from the tanks for the LV-Ns. ...?

Did they severely nerf these engines now? Why are regular engines better now??? Is there even a point to the nuclear engines anymore? This doesn't make any sense!

[Xannari Ferrows]

The advantages of the Nuclear engine only get better as you go bigger. If your ship isn't too heavy, it's better to use a light engine with decent thrust rather than ISP. It starts getting better when you want lot's of Delta-V for a large craft, because the mass of the nuclear engine has a gradually smaller and smaller influence on the full to dry mass ratio, and then the ISP starts to really take over for it's greater multiplying factor.

Edited July 3, 2015 by Xannari Ferrows

[AlexinTokyo]

Also, are you looking at the atmospheric I_sp and ÃŽâ€v values?

The LV-N has very poor atmospheric qualities, experiencing its full potenial only in space, whereas LVT-45s have (relatively) good performance in atmosphere, because they're designed as lifting engines.

[sedativechunk]

AlexinTokyo, I am looking at the values for vacuum, NOT atmosphere, I've been using flight engineer for a very long time. I think Ferrows comment is interesting. Something is definitely different, though. These things use to ALWAYS have better numbers. I am putting them on a smaller-sized vessel, though. Maybe it is the larger vessels where it will make a difference. I will have to see.

[Xannari Ferrows]

These things use to ALWAYS have better numbers. I am putting them on a smaller-sized vessel, though. Maybe it is the larger vessels where it will make a difference. I will have to see.

The plane parts are suited for the LV-N well, because they carry pure liquid fuel. The LV-Ns used to get better numbers because they used to use fuel and oxidizer, which is denser, and because fuel consumption is not directly proportional to volume, the Delta-V is greater because you're using the same amount of fuel mass per second, but packed into a smaller space. The newer LV-N uses just liquid fuel, and as such must use more units per second because of it's density being constant, and the overall fuel mass being less.

EDIT: Let's just use some numbers.

Old LV-N:

1 Rockomax X200 has 1600 total units of fuel, weighing a total 8 tons. That's all we need to know for a calculation.

So, [800 • 9.81] • Ln[11.25/3.25] = About 9745 m/s.

New LV-N:

Rockomax X200 has 720 units of liquid fuel, adding up to 3.6 tons.

Math math math, and we get about 5037 m/s.

Edited July 3, 2015 by Xannari Ferrows

[Randazzo]

I also remembered to remove the oxidizer from the tanks for the LV-Ns. ...?

Use the LF only tanks. Lighter, better, more than double the LF they used to hold.

If you don't mind mods to improve your LF tank selection, there's Stock Fuel Switch and Interstellar Fuel Switch.

[Arkalius]

If you use LV-Ns with LF/LOX tanks and pull the LOX out, it's not going to be very useful. Your fuel mass fraction has gone down, plus the LV-Ns are quite heavy (3t each) which also reduces mass fraction. So, unless you have a lot of fuel tanks on there, your mass fraction is likely to go down enough to make the gain in Isp insufficient to actually add dV.

You're going to want to use LF-only tanks with LV-N's, and best to do only with larger craft, due to the mass of the engines.

[Sharpy]

...also, LV-N is heavy. LVT-45 is 1.5t while LV-N is 3t. That means more dry mass fraction, and that in order means using LV-N for short range flights makes little sense - you will get more dV out of a classic engine in a light, short-range craft simply thanks to a lower dry mass fraction. It's good for interplanetary flights, for tugs that fly a lot so and fro between refuellings, it makes sense in SSTO because it uses the same LF as the jets (although bridging the 1100-2300m/s gap of orbital speed before sinking back into the atmosphere is more than a little tricky), but if you're making a transfer stage to bring a small lander to Mun, you're really better off with "classic" engines.

And yeah, using them with standard fuel tanks after removing all oxidizer really kills all benefits - the dry mass fraction gets so high the ISp ceases to matter.

As for atmospheric use... at 10,000m it has already some 700 ISp and most of its vacuum thrust.

[FancyMouse]

But also remember if you use the same tank for your comparison, you don't have the same fuel mass between LV-N and others to begin with (because you consciously removed oxidizer for nukes), makes the comparison unfair or even invalid.

[Edax]

The advantage of the LV-N with spaceplanes with a LF only design, is that when you reenter Kerbal, you don't have to deal with dead-weight oxidizer, all of the remaining fuel can be used to safely put you back at KSC. Another thing to negate the bad TWR is to have a small spaceplane that is capable of refueling in orbit, that keeps the weight low, and the DeltaV and fuel efficiency high, and you only ever have to deal with one fuel gauge.

[Sharpy]

when you reenter Kerbal,

Kerbin

[Red Iron Crown]

But also remember if you use the same tank for your comparison, you don't have the same fuel mass between LV-N and others to begin with (because you consciously removed oxidizer for nukes), makes the comparison unfair or even invalid.

This. Almost always when I see this sort of thread the OP is simply swapping engines on a given design and maybe tweaking oxidizer out. Such comparisons give the LV-N less than half the propellant as the chemical engines, and the vessel will be much lighter.

LV-Ns are still great if you use appropriate tanks, though they have been nerfed slightly compared to 0.90 and before due to their greater mass.

[John FX]

You could do worse than looking here when designing a craft

http://forum.kerbalspaceprogram.com/threads/121565-Optimal-engine-charts-for-1-0-2

[ajburges]

This. Almost always when I see this sort of thread the OP is simply swapping engines on a given design and maybe tweaking oxidizer out. Such comparisons give the LV-N less than half the propellant as the chemical engines, and the vessel will be much lighter.

LV-Ns are still great if you use appropriate tanks, though they have been nerfed slightly compared to 0.90 and before due to their greater mass.

Exactly. To compare dV per fuel mass, you need 2.22 times the tank capacity! At that point, you are eating a huge dry mass penalty.

A X200-8 has the equivalent fuel mass to 800 LF units. Ergo an orange tank is analogous to 16 Mk1 LF fuselage modules.

Since the needed Mk1 tank buff. My go to nuke solution is a NEVA, 2 Mk1 tanks, and a cheap nosecone. This configuration is atheistically pleasing, easy to install, and has acceptable performance (800 fuel unit, a fairly minimal dry ratio, and good unit TWR). 800 units a nuke should be plenty for any single burn, so increased range can just come from backup tanks and fuel transfer.

For mass considerations, nukes beak even at about 1000 units of fuel a nuke. For fuel economy considerations, I don't know the nuke break even point. I only know that it has a lower break even point. Maybe I'll extrapolate the formulas and report back.

Follow up:

I derive fuel mass as a function of dV to be:

m=f/(g(f/w)(1/(e^(dV/g*Isp)-1)+1))

m: fuel mass required for delta-v

f: engine thrust

g: force of gravity (9.81 m/s^2)

(f/w): start TWR

dV: delta-v

Isp: engine Isp

Assuming tank mass is 1/8 fuel mass. The payload mass is not present because of the use of desired TWR.

Simply plug in appropriate values for engine thrust, Isp, and desired start TWR and it becomes clear that nukes are practically always the most fuel efficient liquid fuel engine. Their mass never outweighs their Isp in useful scenarios. Wish I would have followed my intuition there.

Edit:

There seems to be an error in my equation. It is logarithmic not exponential. I will try to find the error. Any thoughts would be appreciated.

Follow-up 2:

An earlier step is verified to have the expected exponential behavior. Apparently my error lies in substituting TWR equation solved for engine mass.

m[f]=(m[p]+m[e])/(1/(e^(dV/g*Isp)-1)-m[t])

m: mass of fuel (f), payload (p), engines (e), and tanks (t)

g: force of gravity (9.81 m/s^2)

dV: delta-v

Isp: engine Isp

Assume tank mass is 1/8 fuel mass. Just pick a payload and near equivalent thrust engine arrays and you see that nukes are still more fuel efficient for almost all positive dV choices.

Edited July 4, 2015 by ajburges