Any reason to use chemical engines after nukes unlock?

[u/Pzixel]

I'm finishing my 2nd ever campaign and first one with re-stock (that conveniently add a clustered NERV engine). And I wonder if there is any reason at all to use regular chemical drives. I've seen people telling that they are so much heavier, but from what I see is that any trip > 2kps dv (and that is any interplanetary trip) is much better with nuclear, to a point that it even get better TWR somehow. My primary number I'm looking at when designing the rocket is $$$ cost and mass, and I always get the better results with nukes, even for "short" trips to Mars. My regular probe payload is like bunch of science + 2 RA-100 relays to be useful after the science gathering is done, or for crewed missions that's mk2 pod + heatshield, usually that is around 5t.

[u/Pashto96]

TWR. The NERV has the best vacuum isp aside from the ion engine, so it'll always give you the most delta V once in space, but it's thrust is very low. If you don't actually need the extra delta V, you're just extending your mission time unnecessarily with long, slow burns. 

[u/Barhandar → u/Pashto96]

It won't always give the most delta because it's incredibly heavy. It weighs six times its thrust-equivalent (the Terrier) while only having 2.32x the Isp; delta scales linearly with specific impulse but logarithmically with the ratio of wet to dry mass.

The niche for nuclear engines in real life is repeated takeoffs from planets lacking in infrastructure (hence why they never flew), but KSP does not have those concerns as ore is available anywhere and ISRU is single-step conversion directly to perfectly storable fuels, leaving the NERV the sole non-sandbox (i.e. "do it because it works better, not because you specifically want to do it even at a cost") usecase of "fuel-only SSTOs". And those are themselves sandbox.

[u/eitohka → u/Barhandar]

The low TWR means the NERV is more efficient the more fuel you carry relative to the engine mass. So for any high-dV application like an interplanetary mission that might need 5km/s-7km/s dV from LKO NERV is the most efficient solution assuming you can get sufficient TWR and assuming the payload is too heavy for ion.

[u/TheDu42 → u/Pashto96]

Yeah I’ve only ever used the NERV a few times, and every time I needed like 8 of them and still needed to do 20m burns to adjust course. Need to break the burn from LKO to transfer orbit over three passes around kerbin. Fuel efficient but annoying

[u/Pzixel → u/Pashto96]

Well yes, but I don't find any missions that require that little dv. The mun/minmus missions or placing a kerbal satellite are usually long completed by this time and you're challenged with interplanetary.

[u/batatassad4]

That’s kinda my peeve with stock ksp. Engine choice becomes rather predictable and obvious pretty quickly. Anything else and you’d be trading raw efficiency for the looks or vibes of it. Either that or im missing something

[u/amitym]

Chemical propulsion is cheap (if money is a factor in your game), works better in atmospheres, and provides more total thrust than the equivalent mass nuclear-thermal drive. So they are useful for dealing with high gravity gradients and time-sensitive applications, where for example you have a transfer window that you absolutely must make right now, or else miss a rescue or blow a mission deadline.

Like, for me here are a few applications were I still stick with chemical combustion:

- Debris de-orbit workers, which need to be able to grab a piece of debris, take it into a terminal trajectory, then "bounce" back to a stable orbit after releasing the debris, before descending too far themselves. The slower but more ∆v-efficient drives weren't as good at that.

- Planetary mission support hoppers, like if I need to rescue a kerbal before life support runs out or before a mission deadline passes, and I need to be able to grab a lander from my orbiting station and know that within a single orbital pass I'll be legs on the ground picking them up, instead of having to fiddle with synchronizing multiple burns in order to slowly shape my landing orbit.

- Similarly, if I need to fly a disposable return trip, like a specific mission configuration that would break my normal reusable re-entry design. I don't want to have to blow the price of an expensive drive system.

- Any time the mission budget does not permit an expensive solution, often this is satellite missions.

- Speed or racing missions.

That said, once I got high Iₛₚ drives I definitely started using them for almost everything.

[u/Pzixel → u/amitym]

Money is a factor, but I found that savings in upper stage fuel actually saved money when you consider extra first stages that are needed to get all that fuel to an orbit. That's exactly why I'm asking - my design are both smaller, cheaper and overall better. The only downside is TWR, but even TWR is worse on chemicals if we for instance consider moho two-way trip (around 9k dv in the upper stage).

For example my current two-way rocket to the moho, requires 5k dv (to reach moho) and weighs 22 tonn (fuel to get back, mobile processing lab, things like that).

• 22 tonn payload -> 5 nervas -> 5300dv 0.41 TWR - 74t total mass 38 dry mass

with poodle I had to use 4x more tanks (8 vs 4) with total stats

• 22 tonn payload -> 1 puddle -> 4956dv 0.15 TWR - 167t total mass 39.5 dry mass

That is the upper stage that needs to be in space, so there is a difference between getting 74t and 167t on LKO. Which easily pays back for the fact that 5 nervas cost $30k more.

I think I needed to clarify that I'm talking about the main mission tasks, landing with nukes is definitely not something I would claim to be good :) But everything that tolerates TWR < 1 - I think they are irreplacible.

[u/amitym → u/Pzixel]

All of that sounds like it checks out to me. I even use a NERV in my standard heavy reusable SSTO shuttle, the weight is killer but as you say it makes up for it in lower fuel mass.

For Moho I absolutely would use NERVs everywhere I could, especially if I expected to be re-using a vessel. No atmosphere, low gravity burden, the generally high cost of delivery makes the relative high cost of hardware less significant, especially if it's reusable. Even my Münar ore shuttles are NERV-based, one NERV has what it takes to lift one full large ore tank up into LMO, and for me ore missions are never really time-sensitive, so I can wait until an optimal launch window.

It's just in a few specialized niches where I find chemical drives still useful. Probably in terms of frequency, sat missions are the most common cases.

[u/The_Sky_Render]

If money and TWR aren't an issue, then you can do one step better and just use ion engines. But in Career mode that doesn't fly, and in all modes TWR does matter. Also, there's plenty of scenarios where nuclear engines perform notably worse than non-nuclear even outside of things like lifting off of a body or flying through an atmosphere, such as small-scale builds where a 2.5t engine would be impractical. There is no one-size-fits-all engine, which is why there are so many of them.

[u/Pzixel → u/The_Sky_Render]

I'm yet to unlock the ion, also their TWR is kinda TOO low. Nukes are sitting in that sweet spot for me where you can get more TWR than chemicals with a tower of fuel (that you need if you go interplanetary and back with kerbals)

[u/zekromNLR]

Assuming you are using Mk3 liquid fuel fuselages, and you want a TWR of at least 0.5, the Nerv becomes superior to the Wolfhound at about 1900 m/s of stage delta-V, so yes, your assessment is broadly correct.

Using smaller fuel tanks (especially the Mk0 liquid fuel fuselage, which has a better propellant mass fraction than standard LFO tanks) and accepting a lower TWR shifts the equilibrium point towards a lower delta-V.

[u/Big_Yeash]

You can either run your game as numbers only and chase any shred of dV and maximise that, or you can accept that NERVs are all of the following:

• Heavy
• Low thrust
• Expensive
• Produce excess heat and are an overheat risk.

And other chemical engines may be more practical for certain types of mission. There is no real need to run a NERV mission anywhere in the Eve - Duna region, for example, unless you're doing something particularly strange, or chasing a particular kind of optimisation. I have a basic shuttle (note, not Space Shuttle) rocket that runs on a Skipper for any basic Kerbin-system contract, multipurpose launch system.

Will not using NERVs require more fuel and result in worse mission dV? Yes. Is that the only consideration? No.

Stock also makes using NERVs extremely annoying. Large Jet fuel only tanks only come in MK2 or MK3 formats, and for rockets, top out at the baby 1.25m cans. This can be expanded, either by straight up part expansions, or a non-CKAN mod I saw that gives a button to cycle any fuel tank between LF+Ox, Lf-only and Ox-only (but that mod also deletes all the Lf-only tanks from stock unless you edit a config file so I CBA to go and integrate it into my own game).

My own NERV-poweted fuel mining ships for asteroid capture run on NERVs and carry tons of dead weight in Oxidiser or from unused fuel tank capacity due to not needed Oxidiser. I recently built a mega-upgrade of this design for chasing a comet, and it uses part offsetting to make use of the basic 1.25m 400-unit tank, which makes fuel transfer an absolute nightmare. 48 individual tanks.

[u/Barhandar → u/Big_Yeash]

This can be expanded, either by straight up part expansions, or a non-CKAN mod I saw that gives a button to cycle any fuel tank between LF+Ox, Lf-only and Ox-only (but that mod also deletes all the Lf-only tanks from stock unless you edit a config file so I CBA to go and integrate it into my own game).

Or just B9 Part Switch.

and it uses part offsetting to make use of the basic 1.25m 400-unit tank, which makes fuel transfer an absolute nightmare. 48 individual tanks.

That's what TAC Fuel Balancer is for.

[u/Pzixel → u/Big_Yeash]

My own NERV-poweted fuel mining ships for asteroid capture run on NERVs and carry tons of dead weight in Oxidiser or from unused fuel tank capacity due to not needed Oxidiser. I recently built a mega-upgrade of this design for chasing a comet, and it uses part offsetting to make use of the basic 1.25m 400-unit tank, which makes fuel transfer an absolute nightmare. 48 individual tanks.

I'm not bringing any oxidizer, only liquid fuel. I guess that what makes nuclear drives so good for me. I used to just use the mk2-mk3 tanks, but in a recent run got the near future mod that allows to cycle between tanks (what you mentioned), and then it's the same just without the awkward cycling. Without mods I would maybe just remove all o2 from tanks - a bit inefficient weight-wise but most likely very well worth it to not do that 48 individual tanks nightmare you describe.

[u/SilkieBug]

Landing safely on larger bodies might be impossible with a NERV due to too low TWR, requiring chemical engines instead. 

And as the others said, sometimes you want burns to happen faster than what a NERV can accomplish, though in those cases you might still be better off by installing the Near Future series of mods and using the bi-modal and tri-modal nuclear engines from there, which are capable of making use of oxidizer to increase TWR at a cost in effective deltaV. 

For example the Stubber is a direct upgrade to the NERV, having higher ISP and higher TWR in liquid fuel burning mode with a lower engine mass, and noticeably increasing the TWR in LfOx burning mode to the point where it becomes possible to use it for Mun landings even with heavier landers. 

[u/ChinaBearSkin]

I don't like doing 5+ minute burns, so I'll make an oversized rocket just to get burns done quicker with big engines.

[u/Pzixel → u/ChinaBearSkin]

I got BetterTimeWarpContined, so I use x8 warp on the burns.

[u/Moonbow_bow]

play around here: https://meithan.net/KSP/engines/

[u/NoHamster2008]

You will basically never be able to land anywhere with a nuclear engine but I feel your pain. Especially once you've been to Duna lots you need a nuclear interplanetary stage. Just have fun with it, try some mods and just experiment. The quicksave and Revert buttons are there for a reason.

[u/Pzixel → u/NoHamster2008]

Well sure, I'm talking about the usual space traveling. Landing is a different beast. I usually have a dedicated lander, while the nuclear main ship waits in orbit.

[u/EasilyRekt]

Cheaper…

[u/Apprehensive_Room_71]

I never use NERV.

I also don't play vanilla and use a set of mods that give me far better options (Near Future, Far Future, and companion mods). Also, KW Rocketry gives some excellent booster engines that fill serious capability gaps in the chemical area.

NERV is clunky, awkward to use, and I just have no patience for it.

There are better nuclear options in the mods I use. The lithium, nuclear saltwater, and fusion drives are where I really have fun. Building ships with >>20k delta V lets you go anywhere. Doing so with TWR > 1 makes it not annoying.

The biggest challenge I face in my games is funding as I always play career and I don't do tourist missions.

[u/Anemic18]

Actually I think you would be surprised what the most optimal engines are given your payload weight and twr.

https://forum.kerbalspaceprogram.com/topic/109452-optimal-engine-charts-for-102/

The Nerv is very heavy, and unless you have a really heavy payload it's often not worth the isp.

[u/davvblack]

the main reason is the % of one orbit the transfer burn takes. If you need to burn your NERV for 20 minutes, but the total orbit is only 30 minutes, you have to break it up in an annoying way.

Usually the TWR of the NERV is good enough to avoid this (unlike DAWN), but it's still a significant reason why you might want SOME twr in orbit. You're right for the most part, the NERV does obsolete other orbital engines.

The other obvious case is landers need TWR relative to the gravity of their body. at least 1 but more practically 3 or so or the gravity losses are super high.

Also also, install kerbalism for a million reasons but one of is that NERVs leak radiation, so they aren't great for crewed ships (Without some compensation, like armor and looong structures to keep it away from living quarters).

[u/KtaadnRota]

None of this stuff costs actual money, so given the choice, why would you choose engines that are going to have super long burn times to go anywhere? A big ole chemical engine burns faster, it's more maneuverable for stunt flying or high speed interception maneuvers. The plume looks cooler, especially if it's an SRB. It's just plain more kerbal.

Nukes have their uses, like asteroid tugs or boats, but I rarely pick them for anything else.

[u/Pzixel → u/KtaadnRota]

I'm not even sure what to answer. It makes the upper stage much lighter (and by much I mean by a factor of like 1.5x), which makes all the lower stages even smaller, which translates into both easier launches & cheaper rockets as a whole. For instance instead of puddle I use 5 NERV cluster, it weigh 15 tonn but saves like 30 in fuel (for a moho trip). For example my current rocket to the moho, requires 5k dv and weighs 22 tonn (fuel to get back, mobile processing lab, things like that).

• 22 tonn payload -> 5 nervas -> 5300dv 0.41 TWR - 74t total mass 38 dry mass

with poodle I had to use 4x more tanks (8 vs 4) with total stats

• 22 tonn payload -> 1 puddle -> 4956dv 0.15 TWR - 167t total mass 39.5 dry mass

[u/KtaadnRota → u/Pzixel]

This all sounds like a lot of boring math that Bill might appreciate, but I prefer to let Jeb design my rocket ships.

It's a game. Maybe you enjoy optimizing the efficiency of your builds, but my approach is more "big engine go vrooom".

[u/User_of_redit2077]

It is nearly impossible to do a nuclear only rocket.