Nuclear engines not working in 1.0!!!!!

[Bzz]

Hi. I have noticed that nuclear engines "ISP" isnt working. They are not most effective engines.

"kerbal engineer redux" mod showed me, that nuclear engines give way less delta-v than LV-909. I though this is mod error, so i tested it...

I flew 2 similar space ships into orbit (140 km around kerbin) [ Mk1 comand pod + FL-T200 Fuel Tank + Engine ] with another rocket. Saved game (F5), then flew "prograde" with rocket from same point of space, same speed. (I quickloaded before i flew another one).

And results are :

Starting speed (orbit) : 2182 m/s

Final speed with LV-909 - 3927 m/s

Final speed with Nuclear - 2911 m/s

This means that LV-909 is way better that nuclear. Is it just for me, or this is global bug? I though nuclear must be most efficient engines... with 800 ISP... and its not...

[Red Iron Crown]

That is as intended. Nuclear engines won't outperform the LV909 until you are using much more propellant than a single small tank.

Review the rocket equation:

dV = Isp * G₀ * ln(WetMass/DryMass)

For the rocket described, the additional dry mass of the heavier LV-N outweighs its higher Isp. Try it with a much larger tank and you will see the efficiency benefit of the higher Isp.

Oh, and welcome to the forums!

[Gaarst]

Also since 1.0 nuclear engines no longer use LF+O but only liquid fuel; then the oxidizer in your tank is just dead weight.

[WafflesToo]

Fuel fraction is a factor in Delta-V. At very small fuel fractions the tinier, high TWR/low Isp engines will out-perform the lower TWR/high Isp engines.

Just so you can do the math to see it for yourself:

dV = LN (m0 / (m0-mf)) * 9.8 * Isp

[Temeter]

Nuclear engines are heavier than the remainder of your ship together, a 3 ton engine compared to the pod+fuel tank being 1.4/1.9 ton combined. Their low thrust, high weight and insane efficiency makes them prominent interplanetary long range engines. You need to put them on larger tanks, though, and shouldn't go for a thrust to weight ratio higher than 0.5 (usually 0.3 is a good starting point).

I recommend using the kerbal engineer mod, it will show you delta-v and T/W ratio during construction and flight, aside from adding some nice optional hud elements.

http://forum.kerbalspaceprogram.com/threads/18230-0-23-Kerbal-Engineer-Redux-v0-6-2-3

Edited May 1, 2015 by Temeter

[Bzz]

Thaks for answering! I tested by adding more "dead mass" without fuel and... looks like Nuclear engine becomes better than LV-909 only at about 0.03 TWR...

Thats not good enougt! So basicly i should always use LV-909 ? Btw, have you got any exaples, where nuclear is better than LV-909?

[Red Iron Crown]

When the amount of fuel mass is sufficiently high the LV-N will produce more delta-V than the LV-909. I don't know how to put it any more clearly than that.

Nukes are almost never advantageous for going to orbit, the Mun, or Minmus from Kerbin. They really start to come into their own on interplanetary missions.

[Temeter]

Thaks for answering! I tested by adding more "dead mass" without fuel and... looks like Nuclear engine becomes better than LV-909 only at about 0.03 TWR...

Thats not good enougt! So basicly i should always use LV-909 ? Btw, have you got any exaples, where nuclear is better than LV-909?

You misunderstand. The 909 is only better with little fuel tanks, it results in lighter ships, since the LVN is 9 times as heavy. Take a 18 ton tank and then compare both engines performance.

Never go for fuel tanks lighter than the engines they are attached too, that will always result it terrible efficiency.

Edited May 1, 2015 by Temeter

[Bzz]

You misunderstand. The 909 is only better with little fuel tanks, it results in lighter ships, since the LVN is 9 times as heavy. Take a 18 ton tank and then compare both engines performance.

Never go for fuel tanks lighter than the engines they are attached too, that will always result it terrible efficiency.

Now i get it! Thanks a lot!

[m00nchile]

I've done a bit of testing with a common design I used for interplanetary transfers, an MK1-2 command pod, 3 half rockomax tanks and 4 engines, the results were as follows (using KER):

4x LV-909s: 0.40 TWR, 5.2k dV, 61 tons

4x LV-Ns (no oxidizer of course): 0.55 TWR, 5.2k dV, 44 tons

So, you get the same amount of delta V with a better TWR and less weight. Sounds good to me. When I added enough LF to approach the weight of the LV-909 setup, I had about 6.5k dV. So yes, when it comes to bigger ships with low TWR, LV-Ns are still king.