Rethinking the nuke engine, where can we take it from here?

[Geschosskopf]

Define "large". I'm currently under the impression that the point of breakeven comes at something like 5t of payload to Duna, or 3t to Jool. That is, pretty much anything larger than a one-person lander will benefit from nukes.

From what I've seen, it's not so simple. Check out this Jool flotilla of mine (1st post of this thread). The only ship that really and truly needed a nuke transfer stage was the large station module (the SLAC). But I wanted a 2nd nuke tug out there so I send another anyway with the drone airplanes---they'd have been better off using a chemical transfer stage. I also experimented using a nuke for a SCANsat probe to compare directly to an identical probe with a chemical stage. The chemical stage was way better. Even the mostrous PERV Carrier (last ship in the post) has a 3.5m chemical transfer stage.

The recent changes to the other engines really impact the usefulness of nukes. Back in the "20s", the nuke was the only viable choice for transfers beyond Duna because we had no chemical alternatives. But then they changed the Poodle so it's become my go-to 2.5m transfer engine for anything smaller than massive space station modules. And with 1.x, they've changed the 3.5m KR-2L "Rhino", by emasculating its atmospheric Isp, from its previous role as a lifter engine into a very nice transfer engine for most reasonably large payloads. In addition to this, we've now got the whole Ore thing, so that even playing stock it's no longer necessary to carry fuel for the return trip from Jool or wherever. That's a biggie when it comes to the utility of nukes. And finally, there's the question of TWR to avoid age-long transfer burns---chemical rockets win hands down here.

So, if you're planning on refueling at the destination (or not return at all), "big payload" means really big, like a huge mothership or large station module. For anything smaller, chemical rockets are better than nukes. Especially because if you are refueling at the destination, you'll likely be sending a flotilla so will have many burns to do and you want them as short as possible.

OTOH, if you're doing a round trip on your initial load of fuel, the fuel needed for the homeward transfer is actually part of your payload for the outward transfer. Thus, yuu've still got a "big payload", even though the actual mission equipment is probaboly going to be rather spartan and light. So here, Isp wins out and nukes are probably the best answer, at least for anything further away than Duna. Duna's dV requirements are only marginally higher than Minmus' so chemicals work there just fine for round trips.

Bottom line is, Squad put a lot of effort into tweaking all the engines so that each would have its own niche with very little overlap. Thus, much of the LV-N's pre-1.x territory has now been occupied by other engines. This is especially true now that we can refuel at the destination, which greatly extends the applications of chemical transfer engines.

It took me a while to grok this. Like most other long-time players, I was in the habit of seeing the LV-N as pretty much the only option for interplanetary trips, so was appalled by the heat problems it now has. But now that I've gotten more experience with 1.x, I've realized that I can do just as well, if not better, with chemical rockets for most applications I previously relied on LV-Ns for. Thus, the issue of the LV-N's heat has shrunk in my mind from a huge, game-breaking thing to a minor inconvenience, and I can easily get rid of even that with ModuleManager.

[waterlubber]

I play 1.0. The heat is not an issue. Just turn conductivity in the debug menu up to 100--10x the normal.

[foxkill2342]

The LV-N works fine. Just empty the oxidizer and add wing parts.

[Laie]

So, if you're planning on refueling at the destination (or not return at all), "big payload" means really big, like a huge mothership or large station module.

Where part count prevents this...

Bottom line is, Squad put a lot of effort into tweaking all the engines so that each would have its own niche with very little overlap. Thus, much of the LV-N's pre-1.x territory has now been occupied by other engines. This is especially true now that we can refuel at the destination, which greatly extends the applications of chemical transfer engines.

To a large degree, it has become a question of preferences. Laythe exploration, you say, one-way trip? That's ~2500m/s, amirite? I quickly pieced together a few vessels...

2500-2600m/s with a 60t payload:

Engine(s)       total vessel mass    acceleration
-------------------------------------------------
six nervas         133t                  0.34g
one poodle         152t                  0.21g
three poodles      165t                  0.58g
one rhino          175t                  1.46g

...with a 40t payload:
six nervas          98t                  0.47g
one poodle         105t                  0.30g
two poodles        107t                  0.60g
one rhino          123t                  2.07g

Even at the relatively tame requirement of 2500m/s, the Nerva becomes the most fuel-efficient engine at something like 7t, and is definitely superior at 10t. BUT: the other solutions are totally affordable, do the transfer burn more quickly, and are a lot simpler. The one rhino solution is three parts if you discard it, ten parts if you don't. I can slap that together in one minute. The six nerva tug is nowhere as easy to design.

[herbal space program]

After endless hours of trying, I finally got a 1.0.2 SSTO to the Mun and back with a Kerbal and without refueling (yeehah!). I used a ship that weighed 47 tons at takeoff and had 4 RAPIER engines and one nuke. I will post an album (and maybe a challenge) tomorrow. I used the nuke as an afterburner both while breaking the SB and at the top of my air-breathing envelope, then milked about another 3000 deltaV out of it as an OMS and lander engine. I don't think there is any combination of engines that could do this that does not involve the nuke. It is indispensable when you need both long-range OM capability and a TWR that can land you on all the smaller bodies. That nuke burned almost 10 tons of fuel on the way there and back. The next most efficient engine, the LV-909, would have required 23.5 tons of fuel to generate the same total impulse, and more still to generate the same deltaV. You just can't do it without the nuke.

Edited June 8, 2015 by herbal space program

[Geschosskopf]

To a large degree, it has become a question of preferences. Laythe exploration, you say, one-way trip? That's ~2500m/s, amirite? I quickly pieced together a few vessels...

...

Even at the relatively tame requirement of 2500m/s, the Nerva becomes the most fuel-efficient engine at something like 7t, and is definitely superior at 10t. BUT: the other solutions are totally affordable, do the transfer burn more quickly, and are a lot simpler. The one rhino solution is three parts if you discard it, ten parts if you don't. I can slap that together in one minute. The six nerva tug is nowhere as easy to design.

Yeah, there's a lot of personal preference involved, certainly.

As you say, multi-nuke tugs are more challenging to build these days due to the unrealistic heat system. Putting the engines on I-beam or girder outriggers like we used to do doesn't work very well because the outriggers explode. OTOH, putting them directly on the center tank feeds more heat into that while reducing its ability to absorb heat, plus imposes constraints on the shape of the rest of the vehicle. Plus, of course, lotsa parts involved.

The sole attraction of the LV-N is its Isp, and the only real reason to care about that is if you're making such a long trip that a chemical rocket would need a prohibitive amount of fuel. A "prohibitive amount of fuel" is also largely a matter of personal taste. It depends on how much you can get off the ground in 1 launch, whether or not you want to be bothered with docking stuff in LKO prior to departure, and ultimately on how many parts your system can tolerate in 1 place. Looming over this decision are the issues of what TWR you find acceptable, whether you're going to refuel or not, and the costs of various alternatives if you're playing career.

The fact that we can even mention "LV-N" and "personal preference" in the same sentence illustrates how nuke engines have gone from being virtually indispensible to being a just another tool in the toolbox. There are now viable alternatives to nukes for many applications. That IMHO is the biggest change to the LV-N. It's not so much that the LV-N has changed itself, but that it's now set in a different context. Realizing this and carefully examining the alternatives to nukes, rather than trying to make the LV-N work in all situations, is where the rethinking should be going on (as well as Squad rethinking the unrealistic heat system).

[Acet]

In KSP you're probably never going to face a burn larger than, say, 4km/s (braking at Moho)

It's not the dV of the burn that counts for heat generation, it's the throttle level plus time length.

I've had things like 10 minute burns with 4 NERVAs at max while pushing a Class D asteroid into a Kerbin aero-capture trajectory, and while nothing blew up, seeing pretty much every component on that ship showing a high temperature alarm was enervating, to say the least.

By the way, that burn was maybe 300-400 m/s dV - it's just that the asteroid was about 400t in weight.

[regex]

It's not the dV of the burn that counts for heat generation, it's the throttle level plus time length.

I keep forgetting that there are people who choose efficiency over sanity or use MechJeb to perform burns. No craft I build has a TWR less than 0.25.

[Acet]

That ship had great TWR until docked with the 400 ton asteroid.

After that, it had a TWR of about 0.01

Considering that a significant amount of the dV spent in an asteroid redirect is to actually go out there and capture the asteroid before it gets into Kerbin's SOI, doing a ship capable of having a TWR of 0.25 AFTER an extra 400 tons are added to it is actually self-defeating since it would require a lot of tonnage in engines which would have to be dragged around useless until the asteroid is reached.

The whole purpose of my big asteroid capture missions is to get nice juicy fuel depots in Kerbin orbit, not to try and beat the Kerbin record of most LF+Ox consumed per-second.

[herbal space program]

I keep forgetting that there are people who choose efficiency over sanity or use MechJeb to perform burns. No craft I build has a TWR less than 0.25.

Sometimes you've got no choice.

[regex]

Sometimes you've got no choice.

You always have a choice. Besides, it's not at all tough to build a craft that can handle multiple nukes burning for 20 minutes straight and clearly Acet knows how to handle the heating.

The LV-N heat will get nerfed, though, I'll pretty much guarantee it. It'll get marked down in KSP history in the same category as stitch-strutting floppy joints and spaceplanes before the RAPIER engine as some arcane process that provided an "engineering challenge" only a few people were willing to tolerate and be proud of. I suppose we should be happy that a knee-jerk multiplication will likely only last two interim versions.

[herbal space program]

You always have a choice. Besides, it's not at all tough to build a craft that can handle multiple nukes burning for 20 minutes straight and clearly Acet knows how to handle the heating.

The LV-N heat will get nerfed, though, I'll pretty much guarantee it. It'll get marked down in KSP history in the same category as stitch-strutting floppy joints and spaceplanes before the RAPIER engine as some arcane process that provided an "engineering challenge" only a few people were willing to tolerate and be proud of. I suppose we should be happy that a knee-jerk multiplication will likely only last two interim versions.

I think I stepped into something here without looking upthread enough. I'm not arguing that the nukes' heating should be nerfed, just that for some applications, like moving whole asteroids or seeing how far you can get in the Kerbolar system with a single-fueled SSTO, you're going to have to deal with some pretty low TWRs. I suppose you could always choose not to do such missions.

Edited June 8, 2015 by herbal space program

[regex]

I think I stepped into something here without looking upthread enough. I'm not arguing that the nukes' heating should be nerfed, just that for some applications, like moving whole asteroids or seeing how far you can get in the Kerbolar system with a single-fueled SSTO, you're going to have to deal with some pretty low TWRs.

The whole argument stems from the nuclear engine's heating. No one would be discussing TWRs or acceptable burn times if the excessive heating weren't at the core of the issue.

[herbal space program]

The whole argument stems from the nuclear engine's heating. No one would be discussing TWRs or acceptable burn times if the excessive heating weren't at the core of the issue.

Yes I got that after reading up the thread. Sorry to jump in without enough information. Anyway, even at TWR<0.25 there are only a few situations where you'd really to burn more than 3 minutes at a time, like braking into Moho. For high deltaV Munar and interplanetary transfer burns, they can easily be done in multiple stages.

[Red Iron Crown]

Yes I got that after reading up the thread. Sorry to jump in without enough information. Anyway, even at TWR<0.25 there are only a few situations where you'd really to burn more than 3 minutes at a time, like braking into Moho. For high deltaV Munar and interplanetary transfer burns, they can easily be done in multiple stages.

Sort of. The first 950m/s or so can be split up into successive burns, after that you really need to do the rest in one big burn to have any kind of efficiency. For Jool that means >1km/s in one burn, could be trouble.

Though to be honest I haven't found LV-N heat unmanageable, it just takes some new design principles. An LV-N attached to a large Mk2 LF tank can burn through the entire fuel load at full throttle (>10 mins) without overheating even without adding wings for radiators, I use multiples of this for interplanetary stages and tugs.

[herbal space program]

Sort of. The first 950m/s or so can be split up into successive burns, after that you really need to do the rest in one big burn to have any kind of efficiency. For Jool that means >1km/s in one burn, could be trouble.

You can't take yourself to the top Kerbin orbit in stages, do say a 600m/s escape burn, until you're fairly to close to the desired ejection angle, then wait an hour or two on that trajectory before finishing? It doesn't seem like you'd lose too much in efficiency doing it that way, but then I haven't tried...

[Laie]

Though to be honest I haven't found LV-N heat unmanageable,

Don't forget that ships glowing white and red elegantly solves the problem of black-in-black screenshots.

[Red Iron Crown]

You can't take yourself to the top Kerbin orbit in stages, do say a 600m/s escape burn, until you're fairly to close to the desired ejection angle, then wait an hour or two on that trajectory before finishing? It doesn't seem like you'd lose too much in efficiency doing it that way, but then I haven't tried...

It's the wait that kills the efficiency, the vessel will be going much slower once away from the low Kerbin periapsis, and so will benefit less from the Oberth Effect.

It's certainly not impossible to use that profile (indeed, the common newbie interplanetary maneuver is to escape Kerbin and then figure out their transfer burn), but the dV costs go up significantly.

[herbal space program]

It's the wait that kills the efficiency, the vessel will be going much slower once away from the low Kerbin periapsis, and so will benefit less from the Oberth Effect.

It's certainly not impossible to use that profile (indeed, the common newbie interplanetary maneuver is to escape Kerbin and then figure out their transfer burn), but the dV costs go up significantly.

Yes I guess you're losing speed pretty quickly in that first hour after Kerbin periapsis, even if you're on an escape trajectory. I guess I'll find out if it's a problem anyway, As I'm going to see if my SSTMAB can make it to Leythe!

[NecroBones]

Don't forget that ships glowing white and red elegantly solves the problem of black-in-black screenshots.

So THAT's why they added black body radiation. It all makes sense now. lol

[xtoro]

For those who love the nukes, I use and highly recommend Atomic Age . It's a nice mod that adds 3 different sized nukes to the game which prevents me from having to mount multiple nukes to larger ships and allows smaller ships and probes to have a little nuke engine. It's also well balanced with the stock nuke.

Edited June 9, 2015 by xtoro

[Warzouz]

I kind of like the LV-N nerfing of 1.0.2. It's not the obvious and nearly only way to do interplanetary anymore. I'm discovering other engines. That's great. I would like to have even bigger and heavier LVN.

Heating mecanic is quite a good idea IF it's a manageable and understandable minigame. For now the rules of that game aren't clear for new players. But the "stupidest" thing to it is that you have to use irrelevant parts to optimize heat. If the LVN overheats, it's logical to have som kind of radiator, not to use wings parts in space or cubic struts in cargo bay...

KSP is a game, not a simulation. Everything has to be considered in the "gameplay" angle. For now I don't feel that the heat has a fun gameplay.

As others I've Hyperedit a transfer stage with dead payload into space to test a 4000+m/s / 16min burn. It didn't blew up but I must say I didn't get what I'm expected to do with that. I didn't have any clue if it would have blown or not. The whole heat stuff seemed pointless.

A game mecanic which has no "positive" gameplay should be removed. Maybe the solution to heat production would be more obvious (explicit heat capacity of parts and explicit heat radiation of parts). ex : your ship/stage produces X units/s of heat, radiates Y units/s and store Z units before parts starting to blow.