why are there people who stick more than one nuke on their ships?

[lammatt]

i always wonder why people do that... the nukes have terribad thrust/mass,

together with the fact that the more number of nuke you stick on the ship, the less dV you are getting from your fuel since you know.... dV is a function of ln [m(wet)/m(dry)]

and you are never going to get much thrust from the nukes regardless the number of those you put on the ship,

but you indeed dont have to care about the thrust in space anyways... you just have to burn longer, dont you?

anyone enlighten me please?

[Awaras]

Because they try to reduce the burn time? But honestly, if you need more than 3-4 nukes you might be better off with one of the other engines...

Edited January 22, 2014 by Awaras

[KerbMav]

Depending on the size of the craft it looks cooler to have more than one nacelle?!

[lammatt]

Because they try to reduce the burn time? But honestly, if you need more than 3-4 nukes, you might be better off with one of the other engines...

that's exactly my point.

even without plugging numbers into the rocket function, you can make a safe guess that sticking more nukes onto a ship doesnt make much sense at all.

the nuke is at specific impulse of 800s while the other stock engines are running at 400, i.e half the effeciency, but then for every extra nuke you add, you add 2.25t to you ship.

making the mass (wet)/ mass(dry) a lot closer to one for smaller ships.

[theend3r]

Some of my crafts have more than 700T in orbit, it would take days just to escape Kerbin's SOI. I usually use 4 + 4 detachable of them. When your craft is heavy, their mass doesn't really matter.

[bsalis]

Escape velocity... any day now...

[lammatt]

Some of my crafts have more than 700T in orbit, it would take days just to escape Kerbin's SOI. I usually use 4 + 4 detachable of them. When your craft is heavy, their mass doesn't really matter.

http://i.imgur.com/7len15h.png

this is over-engineering.

and it's another issue... people just cant build smaller, more efficient craft.

[maccollo]

It's all about bringing down the burn times to reasonable amounts, or to reduce the amount periapsis kicks you need to do to get yourself out of a deep gravity well Kirbin space.

Because they try to reduce the burn time? But honestly, if you need more than 3-4 nukes you might be better off with one of the other engines...

The number of engines has nothing to do with it. What matter is what TWR you need. If it's higher than a certain amount you get no benefit from nuclear engines.

If you want to accelerate a 100 ton payload out of Kirbin space you are still much much better of with 4 nuclear engines than one LV-T30.

[lammatt]

It's all about bringing down the burn times to reasonable amounts, or to reduce the amount periapsis kicks you need to do to get yourself out of a deep gravity well Kirbin space.

The number of engines has nothing to do with it. What matter is what TWR you need. If it's higher than a certain amount you get no benefit from nuclear engines.

If you want to accelerate a 100 ton payload out of Kirbin space you are still much much better of with 4 nuclear engines than one LV-T30.

but then, why would you need to fly a 100t to begin with?

you can basically go to EVE and return with less than 50t if you staring point is Kerbin low orbit

[SargeRho]

Cargo freighters, large, complicated multi-stage duna missions, replicating the Design Refference Architecture 5.0 or Race to Mars...

[maccollo]

but then, why would you need to fly a 100t to begin with?

you can basically go to EVE and return with less than 50t if you staring point is Kerbin low orbit

You're now getting into the territory of "You aren't having fun the correct way", and that's irrelevant. They want to move payload X for reasons. What is the best engine combination to do that? Now I prefer to pick a destination to go there and back, and then do it in the most efficient way possible, but that's not what everybody wants to do.

Sometimes people just want to move 500 tons of fuel to Duna because they want to move 500 tons of fuel to Duna.

Basically it all comes down to this:

You want to move payload X.

You need a certain amount of delta V.

You have a minimum TWR requirement.

What is the best engine combination to do this?

There was a pretty detailed chart made that compared all of these statistics that I can't seem to find, and the nuke came out on top at any TWR below 0.5 or something.

There's also the odd exception where the delta V requirement is so low that you'll get the delta V you need from a smaller engine and the fuel mass of a nuclear engine, but I've only had that situation once.

Edited January 22, 2014 by maccollo

[Madrias]

Simple answer: Because I wanted to.

Yeah, it's not quite as efficient as, say, just one turbojet and just one nuke, but two turbojets ends up being a fartload of mass that ends up as useless weight on my SSTO designs. Two nukes means I've got a long-range SSTO that can cruise up to my space station, grab a Rockomax Big Orange Can, and proceed to fly anywhere in the bloody solar system.

That, and two nuke engines just look cool.

[FlamedSteak]

I've literally done 5 orbital passes getting my escape velocity up. It is.......very........boring.

i don't begrudge anyone their extra nukes.

[Silverchain]

When changing orbit it's usually more efficient to do a single instantaneous burn than it is to do a series of small burns.

Also orbital insertion from high speed.

Basically, the loss of efficiency from having too little thrust can be worse than the loss of efficiency from adding more engines.

And you may need multiple nukes to land a massive craft, should you wish to do so. Which I do. Or launch one. If you wanted to launch a five ton payload from Tylo's surface, for instance, then a craft with two nukes and the equivalent of 10 small fuel tanks is a good option.

Edited January 22, 2014 by Silverchain

[qromodynmc]

Well Im building a ship with 18 nukes on back,one orange tank,and 6x360 l fuel tank. Also added a heavy lander to it. Im planning to go eelo with it,if it fails,I'll probably stop using nukes.

[theend3r]

Well Im building a ship with 18 nukes on back,one orange tank,and 6x360 l fuel tank. Also added a heavy lander to it. Im planning to go eelo with it,if it fails,I'll probably stop using nukes.

That's a horrible idea. 8-12 are all you need for anything that still holds together and that fuel isn't enough, especially not for this configuration.

That craft I posted on the first page went to Eeloo and back with ease and fuel to spare, but 4 should be enough.

[magnemoe]

this is over-engineering.

and it's another issue... people just cant build smaller, more efficient craft.

Depend on your objective. I have done a grand tour in a 100 ton from LKO ship, land and take off from all planets and moons, except Jool.

The stock Jool "landing" was more than an magnitude larger.

24 nukes, burn took 24 minutes in game so I first lifted Ap outside Mun orbit and did the escape burn on next pass.

[SofusRud]

Well, you can't do a "puller" design with less than two nukes

[DoomtrainInc]

Scott Manley uses multiple Nuclear rockets in some of his designs. Thus I infer it is reasonable to do so in the right situations. All hail the Manley.

[Danish_Savage]

Because they enjoy playing their game that way. Now there really isn't much more to it. You could just as well ask why some people take two spoonfuls of sugar in their tea.

[Ralathon]

From a pure dV perspective it makes no sense to add more nukes. But if dV is all you care about you should be using a single Ion engine for all your spacecraft.

Thrust is still important, even in space. More thrust means less boredom, more efficient usage of the oberth effect and more accurate launch windows. If you spend 2 weeks raising your Apoapsis to escape you aren't going to be very accurate on that interplanetary transfer.

You just have to recognize when it is better to use LV-N's and when to switch to conventional rockets. My tolerance is usually around 0.2G. Anything below that is too slow for my tastes and gets more powerful engines.

[Kasuha]

Why people put more LV-N engines than one on their ships? Because they can.

KSP is not aimed at optimized designs. KSP is aimed at having fun. Doing experiments and overengineering is part of it.

There's no rule how many engines of each type you're supposed to use so if you believe using more than one LV-N on a ship is a waste, just use one and enjoy feeling superior to those who use more.

[Psycix]

There is a big difference between a low TWR and a practically nonexistent TWR with thousands of DV to go.

Adding more nukes on big designs reduce delta-V by a few percent. However, it slashes burn times from hours to minutes.

Is it worth bringing 5% extra fuel to save 40 minutes of watching a craft perform a burn worth it? Definitely.

[RuBisCO]

All of this could be fixed with a time warp mode between physics time warp and non-physics time warp that allows very low level thrusting and limited spacecraft attitude control. Then we could have realistic ion engines and the like with spiral orbits and not wait days.

[Clockwork_werewolf]

For me personally:

Reason one: Wanting a docking port along the center of mass. If the nuke engine needs to be off center then you need at least two to balance thrust.

Reason two: Nuclear engines fairings making everything explode. I hate finally getting a large ship into orbit only to have the fairing on a nuclear engine hit another part and make the thing explode. To have no fairings they have to not be attached and this means either off center docking or side mounted engines (see reason one for why you need at least 2).

Reason three: Dieing of boredom, 10 minute burns, enough said.

Reason Four: One engine weights 2.25 tons, it has twice the efficiency of anything other than ion engines (and they are useless for anything over 5 tons). For a 50 tons ship if 40 tons is fuel then going from one nuke to two doubles your T/W and halves your burn time (reducing burn inaccuracy and Oberth effect inefficiency) but only takes out 5.6% of your fuel (assuming you replace fuel with the engine weight). Replacing these with any other engine would be the same as losing 20 tons of fuel.

Reason Five: Landing on a moon. On atmosphere so the nuke stays at 800isp the whole landing, but I might need the extra engine to take the weight of science equipment and crew back into orbit (even if you dump the sci stuff you still increase T/W with more nuke engines).

Reason Six: Getting into orbit around an atmosphere-less object. You can't do multiple burns and the Oberth effect means the longer burning the less efficient the burn but swapping to any other engine and you might as well though out half your fuel. See reason four.

Reason Seven: Play time verses non existent money. It takes no more game money (since there is none yet) to send up 100 tons and 4 nukes than it does to send up 100 tons and 1 nuke. But it takes four times longer to make a burn and I don't have unlimited time to play games all day every day. This is sort of reason three but that is about fun and this is about what I want to do with my time. If they introduce money later I may cut down on nuke engine numbers.