Can someone explain the use of the nuclear rocket to me?

[Clockwork13]

It is really high up in the tech tree and it's really expensive but it can barely launch my ship off the ground.

Edited February 22, 2014 by Clockwork13

[Tex_NL]

It was never designed to lift your rocket from the ground.

It's heavy, has low thrust but is extremely efficient. It is the ideal engine to do long interplanetary burns.

[Clockwork13]

It was never designed to lift your rocket from the ground.

Oh that might explain why I was having trouble trying to lift my ship off the ground with RAPIERS...

[Captain Sierra]

Oh that might explain why I was having trouble trying to lift my ship off the ground with RAPIERS...

RAPIERS are hybrid jet-rocket engines. They won't lift your craft for different reasons.

Jets have spool-up time. By default, they start as jets (assuming you have air intakes) and take time to produce power. They are also high altitude/speed engines and only give you a measly 90kN of thrust on the pad, but up to 190 at full speed in the upper atmosphere. They are primarily spaceplane engines and use as rocket first stages is something that's very hard to make work. If you're only building rockets, the RAPIER has a very small niche role there and I'd advise avoiding it until you enter the world of SSTO spaceplanes. They aren't great for efficiency and in rocket mode, the aerospike is pretty much a better choice.

The nuclear engine is designed to work in space. It has terrible thrust but it's insane efficiency means you can go the same places with less, or more places with the same, depending on how you let it change your design philosophy. Expect long burns, but less fuel needed.

[SilentSkills]

Oh that might explain why I was having trouble trying to lift my ship off the ground with RAPIERS...

Rapiers aren't actually bad, but their thrust and efficiency depend on how fast your rocket/spaceplane is going, which is zero at launch, and increases incredibly slowly if you are launching vertically (assuming you are using it as a jet engine). If you use the rapier as a regular rocket engine it will behave as such, just not as good as some other options.

[mdatspace]

The Nuclear rocket(or NTR)is used for interplanetary travel. It has a very high Isp, meaning it is very efficient. It is heavy and does not have much thrust, so it is a poor choice for a booster stage. Do not bring too many along because of their weight.

The NTR can also be used as a lander engine in places where there is low gravity and tenuous atmospheres, but you need to place landing legs on structural parts such as girders or beams because it is quite long.

[Jean Deaux]

Yep, don't even need to turn them on really until you get to space, preferably out of the gravitational pull of whatever planet/moon you may be around. Once you get to space, they are pretty much a must have if you want to get any distant travel in.

[Streetwind]

You need to understand that things higher up in the technology tree are not necessarily upgrades. Not in KSP.

The game only got a tech tree in 0.22, quite a short while ago. All the time before it, there was only sandbox mode. And if you have a sandbox game and you make parts for it, there is no sense in making parts that are clearly better or worse than other parts. Instead, you make sidegrades. Parts that perform a different function, that have their own niche somewhere.

For nuclear rocket engines, it's amazing fuel efficiency in vacuum. For R.A.P.I.E.R.s, it's being an all-in-one spaceplane solution. Neither of the two is suited for launching rockets. You know what's suited for launching rockets? The LV-30T, for instance. You start the game with it unlocked, but as it happens, it has among the best stats in the game for lifting something off the pad. That's not a mistake; that's simply its niche. Exactly what it was designed to do.

[capi3101]

To be clear - you can use a chemical engine (LV-909 / Poodle) to go to other worlds. It's just that in space the LV-N Atomic Rocket Motor has an Isp of 800; over twice that of the next highest pair of engines (the aforementioned LV-909 or Poodle), which means you need about half the fuel you would need otherwise to get where you want to go. I've done interplanetary travel on chemical engines before for challenges.

It is definitely NOT a launch stage engine; you want it for your payload.

[MrAnonymous]

Nuclear rockets are best used in space where their efficiency can be used to its potential. Don't try to takeoff with it.

[Dispatcher]

You've read the preceding posts; pretty good information there. Each engine has its uses (and is completely useless in some cases). Check out the graphs linked in my sig line; they compare engine performance tests. Easier to read than the chart they are based on.

Edit: the blue is the maximum mass, the orange is the maximum altitude reached when half the maximum starting mass is non-fuel.

As for the LV-N, those are the only ones I use for orbit to orbit flights.

Edited February 22, 2014 by Dispatcher

[Nuke]

it becomes more efficient than any other engine at around 6000 meters. ive launched small nuke ships with just a few srbs and no other engines.

[Benno]

It's not necessarily the best engine to use for interplanetary travel though. The nuclear engines have great ISP, but they are also extremely heavy. For the same amount of weight, you could have an engine with less ISP but take along more fuel. For a big ship or a long journey, nuclear will almost always be the better option. However, with a small ship or a short interplanetary trip, you might still be better off with a good conventional eninge (say an orbital achievement device, which has around half the ISP of a nuclear engine but has about 1/4 of the mass).

E.g. A small 5.6 ton ship with a nuclear engine can get to Duna orbit and back to Kerbin Orbit with a little fuel to spare (3450 delta-V). A 5.1 ton ship with an orbital achievement engine but carrying more fuel can do the same trip with slightly more to spare (3516 delta-V).

[thereaverofdarkness]

The most impressive thing to note about the efficiency of the LV-N is that doubling your ISP turns your post-orbital-to-interplanetary payload fraction from about 25% to about 65%. That means your rocket can consist primarily of the valuable substance known colloquially as stuff-other-than-fuel.

[Mmmmyum]

I'd like to point out that the LV-N is not really the only engine you should be looking at, going to Eeloo Apollo style is more than possible with chemical rockets

[technion]

I'd like to point out that the LV-N is not really the only engine you should be looking at, going to Eeloo Apollo style is more than possible with chemical rockets

It would be incredibly painful though.

[Mmmmyum]

It would be incredibly painful though.

I did it myself! Single Stage to orbit (excluding boosters), second stage to break into Eeloo orbit, lander lands leaving second stage in orbit, on return the lander ditched it's life-support and empty fueltank (was running Ioncross at that point) and was docked with the second stage facing rearwards before firing into a Kerbin encounter

[SSSPutnik]

If you use chemical rockets for Jool or Eeloo, your payloads going to be tiny.

[Dispatcher]

The most impressive thing to note about the efficiency of the LV-N is that doubling your ISP turns your post-orbital-to-interplanetary payload fraction from about 25% to about 65%. That means your rocket can consist primarily of the valuable substance known colloquially as stuff-other-than-fuel.

Ah! And in my post above, I forgot to write that I am a fan of 3 symmetry, so I not only use the LV-N to get around once I'm "up there", but I am using 3 of them. I usually have fuel to spare once they have fulfilled their purpose. But to each their own; that's half the fun of KSP.

[KerikBalm]

"it becomes more efficient than any other engine at around 6000 meters."

- Well, I don't realy agree.

Its ISP is higher than anything else aside from ions and jet engines at around 2000 meter. It never exceeds that of ions. Jet engines still work much better at 6000 meters. I don't think you were comparing to those, but the ones you would be comparing it to (the other rockets) have lower ISP at 2000 meters.

But efficiency is not neccessarily ISP.

It depends on what you are trying to move. A nuke engine is not more efficient than any other engine at putting a small stayputnick probe into orbit, for example.

Benno is right... I typed out a reply, and then realized he had already made the point mor succintly

So I've deleted a large part of my response

In the most extrema case... take a small probe core (Probodobodyne OKTO2), add an Oscar-B Fuel Tank, and then an LV-N or a Rockomax 48-7S... the 48-7S is going to get more delta V.

Now make the stages equal weight, ie give the 48-7S stage an FL-400 tank instead of an Oscar-B fuel tank (it will actually still be a bit lighter).

Even if you do this with a mk1 command pod and an FL-400 tank instead of an Oscar B for the LV-N, and a LV 909 instead of a 48-7S - the light engine gets to take another FL-400 tank to be "equal weight", and it will get more delta-V - sure, it uses more than 2x as much fuel to thrust a given amount, but in this case it will carry 2x the fuel, and its empty weight will be much less.

Less mass to push offsets less ISP in many cases.

[Pecan]

Remember travert's already worked all this out so we don't have to ... mass-optimal engines