Multiple in-line LV-Ns on Interplanetary Transfer Vehicle

[Jouni]

I wish there was an engine in the 5-600s range for Isp with better TWR than the LV-N. Not necessarily realistic, but would make for more variety in IP ships. LV-N is the best choice in almost all IP scenarios, it would be good to have more variety of efficient choices.

You can simulate that with a cluster of different engines. A Poodle or an LV-T30 with 3-4 LV-Ns is roughly 500 s, while an LV-909 with two LV-Ns is a bit over 600 s.

[Red Iron Crown]

That's a good idea Jouni, I hadn't thought of that. My clusters are always homogenous, I should try mixing them up a bit. Still be nice to have a single engine that could do it, it would fill a gap in the Isp vs TWR curve.

[NecroBones]

You can simulate that with a cluster of different engines. A Poodle or an LV-T30 with 3-4 LV-Ns is roughly 500 s, while an LV-909 with two LV-Ns is a bit over 600 s.

Great suggestion! I think a lot of us can easily get stuck "inside the box" and will tend to use homogenous configurations.

[thomasjkenney]

I've used 4 LV-N's radially, in tug-style ships, but that's usually by launching the ship to Kerbin orbit first, then launching several fuel tanks to dock behind. The LV-N's are attached with short modular girder and radial attachment point, with some struts.

Ship close-up:

In the Jool system:

[magnemoe]

Mounting more LV-Ns is much better compromise in trading off efficiency for TWR. For large transfers you still end up with lighter ship than if you mount a powerful engine and fuel to get the same dv.

Of course if you're impatient you need a lot of TWR. But that's not a compromise. That's brute force.

Say you have a 40 ton mothership,landers and the fuel needed to move around in the target system and return.

4 LV-N and a 1440 liter tank will give you 0.35 TWR and 2300 m/s, drive module weight 30 ton,

Replace the the LV-N stack with a skipper. you will get an TRW of 0.67 but will need three times as much fuel to get the 2250 m/s this drive module weight 48 ton, but you will need one or two LV-N anyway on the mothership.

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Now let us think big, we use an 80 ton mothership.

for the LV-N version I double the fuel size and the number of engines, I still have 0.35 TWR and 2300 m/s, drive moduele is 60 ton.

Now for the skipper version I would now have to use a 106 ton drive module, however the skipper TWR is now down to 0.35 or the same as the LV-N version.

And you would still need extra engines for mothership.

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I like to put LV-N on 45 liter tanks radially on the main fuel tank with a pipe from the main fuel tank, stable and short, it also let up put a docking port in bottom here you can put a drop tank if needed.

It let me put 2-8 engines after need.

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Another idea is the LV-N tug, its a heavy tug with 4-6 LV-N, 1440 liter fuel tank a reaction wheel, some propellant. docking port on front and back.

You use this to push the interplanetary ship to the edge of Kerbin SOI, undock tug and continue with mothership, tug will return to Kerbin orbit where you can refuel it and use it again.

Perfect for Eve and Duna, but it also helps going to other places.

[magnemoe]

You can simulate that with a cluster of different engines. A Poodle or an LV-T30 with 3-4 LV-Ns is roughly 500 s, while an LV-909 with two LV-Ns is a bit over 600 s.

I use this all the time on heavy landers like miners, has the benefit that you can select high trust or high TWR, use Poodle for takeoff then fully loaded and turn it off for circulate and orbital operations.

[quantumpion]

Use the thrust plate adaptor + interstate fairing adaptor from the procedural fairings mod. It's the only part mod I use and find it makes things a lot simpler.

[-root]

Mounting more LV-Ns is much better compromise in trading off efficiency for TWR.

That depends on how you play and what you're aiming to achieve.

Packing more LV-Ns onto a craft will only get you so far with TWR.

But that's not a compromise. That's brute force.

It is a compromise. a burn over a shorter period of time gives a more accurate manuever. Burning over 15+ mins, whilst more efficient, can leave you outside of the planned parameters of your mission.

[Moss]

you LOSE all that efficiency by strapping on a bunch of empty mass - which is exactly what a bunch of LV-Ns are.

Not strictly true. Multiple LV-Ns gives you more thrust overall, which is useful for moving massive payloads around. Say you take a fuel tank and a LV-N. The payload weighs a tonne. You have a delta-v of, let's say, 3km/s. Say you want to move a payload that weighs two tonnes. You add another LV-N and fuel tank. You'll end up with the same overall delta-v. Well, I'm pretty sure.

[NecroBones]

Not strictly true. Multiple LV-Ns gives you more thrust overall, which is useful for moving massive payloads around. Say you take a fuel tank and a LV-N. The payload weighs a tonne. You have a delta-v of, let's say, 3km/s. Say you want to move a payload that weighs two tonnes. You add another LV-N and fuel tank. You'll end up with the same overall delta-v. Well, I'm pretty sure.

Yes and no. Any non-fuel mass in the ship will reduce the dV available to you, and that includes the mass of the engines. If you add more fuel, generally speaking your dV will go up, but it will do so with diminishing returns. There is a theoretical limit that you will reach, due to all of that extra fuel mass that you have to accelerate.

From a purely dV specific standpoint, the most efficient option is to keep your fuel-to-non-fuel mass ratio as high as you can (within practical limitations), and only use a single LVN (since additional engines are non-fuel mass). However, as noted in a previous message, longer burn times make for less precise maneuvers (and thus potentially less efficient maneuvers, due to corrections afterward, and travel-distance through orbital trajectories during execution, time spent burning off of the prograde/retrograde marker, etc).

I'm using the word "fuel" here as a generic term. Strictly speaking, it's "reaction mass" in terms of the physics.

So yes, there's always a compromise in there somewhere. Some dV efficiency can be worth sacrificing in order to have greater thrust to mass ratio, shorter burns, etc, depending on what your mission goals are.