LV 909 or LV-N?

[THobson]

I have just spent 100's if not 1000's of hours on 0.9 and have now upgraded to 1.02 and started all over again. In my 0.9 activities I thoughtlessly used the nuclear engine for all inter-planetary travel because of its high ISP. All my rockets were big and industrial but now I have to pay for things I am needing to be a bit more careful.

I see that the LV-909 is 0.5 t with an ISP of 345 in vacuum and costs 390. The LV-N is 3.0 t with an ISP of 800 in vacuum and costs 10,000. Both deliver the same thrust. For relatively light rockets the LV-909 gives a much better delta-v but I expect for rockets so heavy that the weight of the engine is relatively insignificant the LV-N will do better in terms of delta-v. Is there a formula that relates mass of rocket, ISP of engine and delay-v so that I can work out the crossover point (i.e. the mass of the rocket) where the LV-N provides better delta-v?

Many thanks for all your help so far.

Edited June 10, 2015 by THobson

[cephalo]

Someone posted an 'optimal rockets' series of graphs on the main forum that may have you covered.

[Starhawk]

Try Meithan's Engine Charts.

It's an extemely useful tool.

Happy landings!

[Red Iron Crown]

If instead of Meithan's tool you'd rather a simple rule of thumb: If you are planning to use more than 5t of fuel for the LV-909 you are likely better off using the LV-N with Mk2 LF tanks, or Mk3 LF tanks if you need a lot of fuel. Avoid the 1.25m LF tank or any LFO tanks with the LV-N if possible.

[Snark]

The rocket formula is:

dV = Ve * ln(Mwet/Mdry)

...where Ve is the rocket's exhaust velocity (in terms of KSP numbers, it's the published Isp in seconds, times Kerbin's 9.81 m/s2 gravity).

Let's assume that your total mass consists of the following:

Me = mass of engine

Md = dry mass of everything except your engine

Mf = = mass of fuel.

So let's say we want to figure out the mass cross-over point where LV-909 and LV-N have equal performance. We'll assume that Md and Mf are the same in both cases; the only thing that's different between the two rockets will by Isp and Me.

So we start out with this:

Isp1 * ln((Me1+Md+Mf)/(Me1+Md)) = Isp2 * ln((Me2+Md+Mf)/(Me2+Md))

There are two variables to play with: the dry mass of the rocket excluding engine, and the fuel mass. Where the cross-over point happens depends on these two things.

Plugging in a few numbers:

If Mf is __ tons => crossover Md = __ tons

1 => 0.96

2 => 0.63

4 => 0.23

Basically, the more tons of fuel you have, the less dry mass you can have at crossover.

The "point of no return" beyond which the LV-909 can't win: in the theoretical extreme, where the only dry mass of the rocket is the engine itself (which clearly can't happen, because there's no such thing as a massless fuel tank), then the max fuel amount is 6.17 tons.

Being a little more realistic, and assuming that your fuel tank has a 1/8 mass ratio (like most of the game's LFO tanks), we get a point-of-no-return of 3.08 tons of fuel.

Summarizing the above:

If you're comparing LV-909 against LV-N, and you have a rocket whose mass is entirely fuel tank + engine, then the LV-N will always win (in terms of dV) if you have more than 3.08 tons of fuel. If you have any mass at all besides tank + engine (which of course you do), then this lowers the cut-off point. e.g. if you have 0.3 tons of non-engine, non-fuel, non-tank mass, the cut-off fuel mass drops from 3.08 tons to 2.21 tons.

Of course, that doesn't tell the whole story. That's assuming that all you care about is the dV of that particular rocket stage. However, you have to get that rocket stage launched into orbit in the first place, and the LV-N solution masses more than the LV-909 solution, which means you'll need a bigger rocket to launch it in the first place (or, put another way, if you have the same booster stage in both cases, the LV-N will get less dV out of that.)

[THobson]

That is really helps fuel. Thank you for taking the time to explain that and for pointing me to other resources. Much appreciated.

Edited June 10, 2015 by THobson

[Sharpy]

1. Replace your fuel tanks with Liquid Fuel only ones (there's a bunch in the airplane branches of the research tree). Or at least remember to remove all the oxidizer.

2. Your efficiency will suffer - a bit. But the nuclear engine is still the best for heavyweight interplanetary, just not -as- awesome as before.

3. If you used it for atmospheric launch, sorry. You may look into ramjet first stage.

[THobson]

Thanks. I now see why the LV-N seems less useful that in 0.9:

Changes

1.0

LV-N “Nerv†Engine now runs solely on Liquid Fuel.

No longer has a gimbal.

Cost increased to 10000.

Mass increased from 2.25 to 3.

Isp curve extended past 1 atmosphere; Isp drops to 0.001 at 2 atm.

I will check it out with no oxidiser.

Edited June 10, 2015 by THobson
Updated information

[THobson]

Update

Prompted by Sharpy's comment I have just been playing around with a spreadsheet and in checking my numbers against MechJeb I have come to the view that a straight comparison is slightly complicated by the fact that the LV-N does not consume oxidiser. So in the formulae provided by Snark the

- mass of fuel for the LV-909 is the mass of liquid fuel + the mass of oxidiser

- mass of fuel for the LV-N is only the mass of the liquid fuel

- and the dry mass for the LV-N should include any oxidiser in the tank (hence the suggestion to dump to first)

Apologies if this well known. It wasn't to me. Once again thanks for all the help

[Red Iron Crown]

Don't use LFO tanks for LV-Ns, or if you must just tweak the oxidizer out. There are a few LF only tanks in among the plane parts that are much better for LV-N use.

[THobson]

But they don't seem the correct shape for a rocket.

[ajburges]

But they don't seem the correct shape for a rocket.

Just need to change your expectations.

For non atmospheric craft mass and structural integrity against both negative hull pressure and heading force are the sole considerations. Look at the lunar landers and most long range probes. Rockets get their shape from atmospheric considerations.

Realistic non-landing craft would be huge spheres to hold fluids, engines, trusses to hold stuff together, and optional habitation modules.

Realistic tugs only need the aerodynamic features for launch. Otherwise, they should resemble the previously described composition. Given unlimited operation time, the cost optimal approach would be to enclose a vacuum operation craft in a huge faring. Or assemble in orbit.

Mk2 vacuum craft look rather stylish and Mk3 craft look very similar to rockets. You could also use those tanks as catamarans for the rest of the ship. Though since most ship volume is tanks and payload storage, I don't know what you'd attach it to.

[THobson]

I suppose I am okay having a strange shaped ship, but more fundamentally it would mean me spending science to unlock a whole load of spaceplane stuff that I have no interest in.

[Snark]

I suppose I am okay having a strange shaped ship, but more fundamentally it would mean me spending science to unlock a whole load of spaceplane stuff that I have no interest in.

Amen to that. The lack of rocket-friendly LF tanks is a real annoyance; they're all so clearly designed for spaceplanes (both in terms of shape and in tech tree placement). Really wish they'd add a couple of rocket LF tanks (such as a LF equivalent of the FL-T800), for example on the science node that gives you nuke engines.

[Wanderfound]

But they don't seem the correct shape for a rocket.

Looks fine to me...

[Gwtheyrn]

The LV-N engines were nerfed in another way too. In 1.02, they produce significant amounts of heat. Due to the low thrust output, a heavy ship is going to need a lot of burn time to perform your delta-V maneuvers, but after about 7-8 minutes or so, you're looking at an overheat. This means you have to strap more of them on to reduce the burn time. However, once you get up to 3 of them, the Poodle becomes the more efficient choice, even with the added mass of the oxidizer.

[Red Iron Crown]

The LV-N engines were nerfed in another way too. In 1.02, they produce significant amounts of heat. Due to the low thrust output, a heavy ship is going to need a lot of burn time to perform your delta-V maneuvers, but after about 7-8 minutes or so, you're looking at an overheat. This means you have to strap more of them on to reduce the burn time. However, once you get up to 3 of them, the Poodle becomes the more efficient choice, even with the added mass of the oxidizer.

It's true that managing heat from LV-Ns has required some rethinking in how to build vessels with them, but it's not unmanageable. Take this simple nuclear tug that uses 4 LV-Ns:

It can burn its entire fuel load (10+ minutes) at full throttle in a single burn without overheating. Simply connecting LV-Ns directly to largish tanks is enough to keep things from exploding in most cases (though they get very warm). If I needed more fuel connected to the docking port for longer burns it might be trouble, I would likely need some radiators wings mounted to the tanks to radiate heat away. Or possibly redesign to attach those Mk2 tanks to a larger Mk3 tank that can absorb the heat.

Or another example, this spaceplane that has LV-Ns for orbital maneuvering:

This plane can burn its entire remaining fuel load at full throttle (coincidentally also 10+ minutes) immediately after reaching orbit, without waiting for the heat accumulated during ascent to dissipate. Note that the LV-Ns are attached to different fuel tanks (it would overheat if they were both mounted to the central bicoupler). In this case, the tail fins and wing parts attached to the side bicouplers help with radiation, and the bicouplers themselves are attached to another thermally massive part, the large Mk2 LF tanks.

Neither of these designs has a single part added just to manage heat; they just use the parts needed anyway arranged in a way that keeps temps under control. The real trick for me was not using low thermal mass parts attached directly to the LV-N for clustering them. Having a part with high thermal mass connected directly to the LV-N is the key, and if you can connect that part to another thermally massive part it's better still.