Orbital Transfer Vehicle, Ion engine powered or Nuclear powered?

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3600 Isp is not "way too low". It's 4.5 times that of a LV-N! That is a big deal, especially when it has a TWR of 9.84. For comparison: the LV-N has 2.72, the Poodle has 8.97, the LV-909 has 10.19, and the PB-ION thruster has... wait for it... a whole 0.2. In other words: the DS01 has nearly ten times the Isp of other engines with comparable TWR. That is really, really, really good.

You may however be getting deceivingly low dV ratings from it anyway. The reason for that is that it runs on hydrogen, which has an abysmal density. Even the biggest tank you can mount has hardly any significant mass in it. And because the rocket equation looks at mass, and not at the number of arbitrary units of fuel that the fuel tank uses to count its contents, that means you don't get much dV from just one tank. If you want to go places with hydrogen, you need to build big. Really big. Bundle up those 2.5 meter tanks. The fun fact is that this won't actually make your vessel heavier than if you used a single small liquid fuel tank. It will just be much larger. In fact, because the Isp is so much higher than that of a LV-N, and the thruster itself is so much lighter too, the ship will weigh a whole lot less despite being much larger.

But then, yeah, the power generation. The plasma thrusters buy their impressive TWR by paying in electric charge. Let's calculate an example:

Mounting three DS01 MPDTs and one MX-1 reactor weighs 6.87 tons, plus weight for the radiators, which I don't know off the top of my head. Let's guess blindly and say 7.5 tons for the whole assembly. That gives you 84 kN of thrust, for a "propulsion section only" TWR of 1.14. Compare that to the LV-N: 2.72 for the propulsion section (in this case only itself). For the weight of the plasma thruster/reactor assembly, you could mount three and have a little weight left over.

However, now comes the Isp difference. You say your LV-N fueled craft weighs 21 tons wet and 6 tons dry, and is meant to carry 10 tons of payload. Let's quickly check the rocket equation: 800 * 9.82 * ln(31/16) = ~ 5196. That doesn't really match up, to get 4500 dV you'd need about 17.5t dry mass + payload, not 16. Or the wet mass is lower, but that's ultimately arbitrary, so let's just go with this. Assuming one LV-N in those 7.5t dry mass gives a total vessel TWR of 0.197; if it's two LV-Ns, then that's 0.394.

Replacing the LV-N's with the MPDT/reactor assembly, assuming one LV-N we remove 2.25t and add 7.5t. Together with the 10t payload that makes 22.75t dry mass. How much fuel mass does this need to get 4500 dV? 4500 = 3600 * 9.82 * ln ((22.75+x)/22.75), solve for x. The result is roughly about 3.1t, for a wet mass of 15.85t, or 25.85t with payload. Note how this is less than the 31 that the nuclear rocket tug features. In fact, since the MPDT assembly gives 84 kN thrust instead of the 60 of one LV-N, the total vessel TWR with payload is now 0.331 - which is almost 70% higher than the LV-N! The reason the MPDT craft weighs less despite the huge reactor is all down to the fact that the Isp is so high that it requires barely any fuel mass.

What if the original tug had two LV-N's? Then the new dry mass is 10.5t plus payload, and total vessel mass with payload and enough fuel for 4500 dV would be 23.3t, which gives a total vessel TWR of 0.367. This is slightly less than the twin LV-N config, showing that higher-thrust, lower Isp engines still have a good reason to be used in certain cases. But the MPDTs are certainly competitive still, and if there was a higher dV requirement (such as longer distance or higher payload) they would once again come out on top. You could double the dV of this ship and the vessel TWR with 10t would still be over 0.3 - not something you can easily match with LV-Ns.

Now, if you paid attention, you'll have noticed that the MX-1 reactor is actually slightly short of being able to supply three DS01 MPDTs. They would drink 2250 Ec/s, and the reactor outputs 2000 Ec/s. The remaining 250 could be supplemented by a MX-4 reactor, or solar power and/or by utilizing buffer storage. Near Future actually adds parts predestined for this, namely the capacitors. They store 8 times as much Ec per weight as normal batteries do, but cannot be drained directly; they must dump their contents into regular batteries for it to be used. So what you do is have enough regular battery space to receive one capacitor worth of power, and then just stack a large number of capacitors and dump them one by one when necessary during burns. Then afterwards the capacitors can be recharged. This will of course add weight, but that should be under 2 tons so the craft will still easily remain over 0.3 TWR.

All in all, this shows that electric propulsion using the DS01 MPDT can be competitive to LV-Ns even in a medium dV light cargo tug application, but technically plays its real strength only when more dV (or bigger payloads) are required.

I didn't know spoiler is back =S

I had a similar design before, but everything goes way too big for my rocket..

and sadly I don't have a rocket with big enough fairing to fit that in yet =S..

FAR hates huge rockets xD

Maybe I can make it REALLY long...

Perhaps I should do it =D

Or constructed in orbit instead launch them all at once, thanks!..

Dock it to a station or something, or fly disposable tankers up to it to restock it with fuel. Mechjeb can handle rendezvous nicely, but docking is best left to manual controls.

Since other accessories does not weight much, it does not make a huge difference =S... either launch intact or launch fuel compartment only...