Can someone tell me why the LV-N is prefered for orbital engines?

[stildawn]

Hi All

By looking at the stats I cant see why this engine is preferred, so obviously I'm missing something, so why is it in detail good for orbital engines? Compared to the others which seem to me have better twr and t/fuel etc.

[Master Tao]

Its specific impulse (Isp). Isp is basically a measure of efficiency; higher numbers means more maneuvers/change in velocity. The LV-N has the highest Isp of all the stock rocket engines.

[StrandedonEarth]

Isp is the measure of how efficiently a rocket engine uses its fuel. It'll get you more dV per unit of fuel than any other engine except ion.

[Pursuedtank]

It is prefered due to the effeciency that it has compared to the other engines in the game, except the ion engine. This means that you have more delta-V to work with. Thrust to weight doesn't matter to much while in orbit.

[Superfluous J]

800 I_sp. Other engines are in the 200-400 range.

I_sp - without getting so technical that I'm giving false information - is essentially how efficient your engine is. So with the LV-Ns you get more bang (literally) for each liter of fuel you burn.

Do this to test. Make a ship with tier-0 parts. Command module, one fuel tank, one engine. Launch that into orbit (with a lifter or hack it with Hyperedit) so it's orbiting with a full tank. Aim prograde and fire the engines until it's out of fuel.

Make another ship, replacing that starting engine with the LV-N. Get that into a similar orbit, aim prograde, and fire.

The first thing you'll notice is that it doesn't accelerate as quickly.

The second thing you'll notice is that it burns a LOT longer.

The third thing you'll notice is that your resultant orbit is higher (or escapes Kerbin more quickly) than the first orbit.

[edit]Holy cow I just got triple-ninja'd[/edit]

Edited July 15, 2014 by 5thHorseman

[stildawn]

Ah I see, so it will have longer burn times but give more dV than the others. Would I just use lots of them to decrease burn time and keep efficiency? So mass doesn't matter in orbit?

[GluttonyReaper]

Ah I see, so it will have longer burn times but give more dV than the others. Would I just use lots of them to decrease burn time and keep efficiency? So mass doesn't matter in orbit?

You can, but bear in mind nuclear engines are rather heavy. Adding extra engines can significantly reduce dV, depending on the craft, as mass does affect dV.

[maltesh]

It's not that bad of a landing engine, either.

This would be one of my Cheddar-Class landers, variants of which have been a staple of my game since 0.18. Two LV-Ns provide 0.79 gees of acceleration and its twin FLT-400 tanks give it 2.35 km/s of delta-V. Much more than you need for Mun-class worlds (which is what it was designed for), Probably would be cutting things close on a Moho or a Vall.

Swap the LV-Ns for a single LV-909, and the delta-V drops to 1.65 km/s.

Rockomax 47-8s, Delta-V is 1.54 km/s.

LV-T30, 1.44 km/s.

Use a Poodle instead, 1.34 km/s.

Yes, those options would tend to be a slightly less massive, but the Cheddar class isn't intended to be used in one-off missions, it's intended to deliver itself to the intended destination, and remain there so that the next mission (or any colonists) can use it over and over again.

[stildawn]

Ok, so what would be the optimal number for a interplanetary engine system (something that the actual craft docks into and out of at different planets orbits)

[GoSlash27]

I use 1, with a couple 48-7S available to give additional punch when I need to make more rapid DV changes. I would personally never use more than one LV-N on an interplanetary driver.

Best,

-Slashy

[Master Tao]

I tend to use 4 LV-Ns for large ships going interplanetary. Even then, I often have to either start from a very high orbit or make multiple 4 minute burns to go interplanetary. It all comes down to a matter of preference, balancing thrust with efficiency.

[Superfluous J]

I've used up to 6 on a large vessel.

The answer really is "as many as you need to get the thrust you want without sacrificing too much efficiency."

It helps to have KER or something like that installed, so you can see in the VAB the affect of putting more of each type of engine on your ship.

[UmbralRaptor]

Since others have covered why the LV-N is good, I'll just want to clear something else up. The VAB fuel consumption figures make assumptions so iffy that they will mislead you in the vast majority of cases.

[stildawn]

Hmm ok, will experiment a bit lol. Thanks for all the replies though.

[KerikBalm]

The fuel consumption appears higher because of the wya KSP handles ISP, and the very poor atmospheric ISP of the LV-N.

Its ISP is only 220 at 1 atmosphere, and the game lists the fuel consumption needed to generate 60 kN of thrust at 220 ISP. Use it in space, and you get 800 ISP, and your fuel consumption to generate 60 kN of thrust is much much lower.

However, it weight 2.25 tons, while a LV-909 for reference weighs half a ton. So for the same weight, you could send up a LV-909 and 1.75 tons of full fuel tanks.

Thus for small dV needs, the LV-N is not so attractive - but for craft that you will repeatedly re-use (by refueling), or craft with large payloads and fuel tanks (that can get away with poor acceleration), the LV-N will "catch up" and surpass all other engines (except ions)

[Red Iron Crown]

I think LV-Ns may become a bit less desirable with 0.24. I expect them to be very expensive compared to chemical engines.

[NecroBones]

I think LV-Ns may become a bit less desirable with 0.24. I expect them to be very expensive compared to chemical engines.

This is probably true. But then again, for a lot of the designs that people make in which they use several LVNs, the incentive may be to use only one or two, which is more efficient than spamming them all over the back of your ship anyway.

This is a part of the career mode that I'm really looking forward to.... some trade-offs. Will the LVN save me money over a conventional rocket and a little more fuel? Each situation may be a little different. Hopefully the prices will be well balanced.

[Red Iron Crown]

This is a part of the career mode that I'm really looking forward to.... some trade-offs. Will the LVN save me money over a conventional rocket and a little more fuel? Each situation may be a little different. Hopefully the prices will be well balanced.

That's my thought on it almost exactly. The way we play is going to change quite a bit.

I'm imagining reusable modular nuclear tugs will be the way to go. That way you can amortize the cost of the nukes over many missions, the only additional cost is enough fuel to return the tug to Kerbin. Though whether that is viable may vary by destination.

[NecroBones]

That's my thought on it almost exactly. The way we play is going to change quite a bit.

I'm imagining reusable modular nuclear tugs will be the way to go. That way you can amortize the cost of the nukes over many missions, the only additional cost is enough fuel to return the tug to Kerbin. Though whether that is viable may vary by destination.

You know, I wasn't even thinking that far ahead. That's a great point-- reusing tugs and other equipment. As it is now, when I return from an interplanetary mission, I usually ditch the whole ship in the ocean. Now, the incentive is going to be to put things in parking orbits and re-use them. Awesome.

[Red Iron Crown]

You know, I wasn't even thinking that far ahead. That's a great point-- reusing tugs and other equipment. As it is now, when I return from an interplanetary mission, I usually ditch the whole ship in the ocean. Now, the incentive is going to be to put things in parking orbits and re-use them. Awesome.

There was a good thread about reusable transfer stages a little while ago, here. Pecan and LethalDose arrived at similar elegant solutions independently, worth a look.

Edited July 15, 2014 by Red Iron Crown

[Pecan]

Yeah, our direction of travel is the same but LethalDose is working with FAR, so he has a harder time getting such awkward-shaped ships into orbit in the first place. There are a lot of different ways and reasons to play KSP, of course, but my preferred approach is to develop reusable infrastructure. Launching single-missions - for me - is harder, since you have to launch or space-assemble so much every time, but at the same time you have less to show for it afterwards, just one mission achieved, no resuable vehicle parts.

The structure of my tutorial takes single-missions everywhere for satellite placement (it's not worth bringing the transfer vehicle back, although 0.24 may change that) but only as far as Mun/Minmus manned. After that I concentrate on reusability, space-stations and tractors. Incidentally, since I'm still in the fan club, I point to that same thread in the final chapter with the quote "these [design criteria] lead to a 'mini-station' or 'mothership' design which Red Iron Crown, particularly, illustrates well in the thread mentioned above."

The tutorial's for beginners/intermediate though, of course. As I mentioned above I try to build an infrastructure using SSTOs ('plane and rocket) for crew and fuel transfer to/from Kerbin station, tractors between Kerbin and 'wherever destination' station and dedicated landers around each system, possibly moon. A lander for Eve would probably be larger than Gilly (!!!) so Gilly gets its own lander (but not station) whereas Ike can make do with Duna's lander. Jool, of course, is a whole set of different issues all in one package, and not one for which I've got a satisfactory set of vehicles. That'll be target 1 for 0.24 sandbox then :-)

Anyway; all that together means all I have to launch, once the infrastructure's in place, is fuel and additional/rotation crews. The only thing that is consumed (barring accidents) is the fuel and, in the tutorial, the fuel's launch-vehicle.

[RoboRay]

Ok, so what would be the optimal number for a interplanetary engine system (something that the actual craft docks into and out of at different planets orbits)

The optimal number of engines to maximize fuel efficiency is always "one".

Two is often worth doing, as you're doubling your TWR for a modest increase in mass. Four can be reasonable on extremely massive ships, for another doubling of TWR. Beyond that, especially if you're having to add support structures to mount the engines, you begin to approach the point where adding engines offers little value. It can even become counterproductive, if taken to extremes.

Edited July 15, 2014 by RoboRay

[Claw]

The optimal number of engines to maximize fuel efficiency is always "one".

Well, generally but not always true.

One example: Harnessing Oberth Effect.

If the TWR is too low, the ship will have a harder time taking advantage of Oberth Effect. The amount of Oberth assistance missed out on will, of course, depend on the specifics of the ship and burn. However, it can be significant.

[Red Iron Crown]

Chasing ideal fuel efficiency is a bit painful anyway. Almost no one wants hour long burns, I'd rather sacrifice some dV on the altar of playability.

[NecroBones]

The optimal number of engines to maximize fuel efficiency is always "one".

Well, generally but not always true.

A good way to reword the above, would be "The optimal number of engines to maximize fuel efficiency is always "one", from a pure delta-V perspective."

Chasing ideal fuel efficiency is a bit painful anyway. Almost no one wants hour long burns, I'd rather sacrifice some dV on the altar of playability.

Very, very true.

[RoboRay]

Well, generally but not always true.

One example: Harnessing Oberth Effect.

If the TWR is too low, the ship will have a harder time taking advantage of Oberth Effect. The amount of Oberth assistance missed out on will, of course, depend on the specifics of the ship and burn. However, it can be significant.

Not really. It just depends on how many periapsis kicks you're willing to make. And, by extension, how patient you are. And, how well you planned your transfer window.