RTG and NERVA decay

[DerpenWolf]

As we all ( hopefully) know pretty much everything has a half life, especially radioactives. What I'm wondering is should RTGs and NERVAs become less effective as they age? Maybe with RTGs slowly putting out less and less power over the years and maybe (not sure what would happen in the nuclear engines case so please give other suggestions if this is wrong) the NERVA could get less efficient? (It would get an efficiently and atmosphere Isp boost). This could potentially help balance out the op nature of the two parts a bit however I'd rather here what you think!

[phoenix_ca]

http://forum.kerbalspaceprogram.com/threads/80570-How-long-do-RTGs-last

[Sirrobert]

There is no part degration in this game. Why would 2 specific parts have to be any different?

[Kasuha]

I think it's almost irrelevant whether the NUK or LV-N will degrade to 50% output in 50 years or whether it's eternal. Most players don't have their space programs any close to that long so only a few might "appreciate" the feature.

[SRV Ron]

In reality, the nuclear engine total power output would be designed to match the amount of fuel carried for that mission. One can assume similar in KSP even for those crafts used as tugs and refueled on a regular basis.

Similarly the RTG are designed for an expected half life of useful power output. Voyager nuclear battery is still putting out enough power to continue its mission far beyond what was expected of it. One can make the same assumption for a KSP mission even if the craft is flown for 50+ years in Kerbal time.

[Red Iron Crown]

I don't consider RTGs or LV-Ns overpowered. The RTG's eternal output independent of solar conditions is balanced by its horrible electricity/mass ratio. LV-Ns high vacuum Isp is offset by its unrealistically low atmo Isp and poor TWR. I don't think either part needs to be nerfed, especially when the proposed nerf wouldn't affect >90% of missions that use them.

[Random Tank]

I don't consider RTGs or LV-Ns overpowered. The RTG's eternal output independent of solar conditions is balanced by its horrible electricity/mass ratio. LV-Ns high vacuum Isp is offset by its unrealistically low atmo Isp and poor TWR. I don't think either part needs to be nerfed, especially when the proposed nerf wouldn't affect >90% of missions that use them.

I think (if they ever get round to it) just changing the NERVA's fuel (to what is it, hydrogen?) would be a big enough 'nerf' in itself, because then the fuel wouldn't be interchangeable; but you haven't changed the rockets stats at all...

ALso, 2 rockets that use completely different fuels are very difficult to compare against each other!

[Red Iron Crown]

I'd rather just see LV-Ns move to using liquid fuel without oxidizer, seems simpler to implement and I don't think we've been told definitively what liquid fuel actually is. The high Isp with the LV-N implies liquid fuel is hydrogen, but most other indicators make it seem like kerosene.

[CaptRobau]

At the moment decay isn't that necessary, as voyages aren't very long in the Solar System we have now. But it might warrant attention in the future if more planets are introduced. Just like the semi-major axis of Earth is the basis for the Astronomical Unit (AU), the distance of Kerbin can also be used for a Astronamical Unit of its own. The solar system of KSP is scaled in pretty much the same way as that of real-life. Mars is 1.5 Earth AU, Duna 1.5 Kerbin AU, Jupiter 5 Earth AU, Jool 5 Kerbin AU, etc. The only outlier is Eeloo. That Pluto analog is only 6 Kerbin AU from Kerbol, while Pluto is 39 AU from the Sun. If the Kerbol system is expanded I imagine, we'll see Eeloo move as well. If Eeloo were positioned at the same relative distance as Pluto is IRL, it'd take on average 49 years to get there. Then the half-life of the RTG suddenly has an impact.

For nuclear thermal rockets there is no reason to model half-life decay. Nuclear reactors use Uranium-235, which has a half-life of hundreds of millions of years, compared to 87.7 years for Plutonium-238 (which is used in RTGs). What nuclear reactors like those in a nuclear thermal rocket do have is a fuel cycle. Nuclear fuel in a reactor gets irradiated, determined by its use (want a lot of power, the nuclear fuel gets irradiated more quickly and vice versa). After a while the fuel gets so irradiated that it's no longer usable to get a nuclear reaction. This you could quite easily simulate in KSP already, by having nuclear thermal rockets use a NuclearFuel resource. After a few missions it'd run out. You could use that as a way to put a limit on the life of nuclear-powered space tugs, or through adding something like mining mining you could replenish your NuclearFuel.

[technicalfool]

I think keeping the old RTG as an infinite trickle-charger is a nice game-friendly feature. On the other hand, the more powerful reactors in the KSP Interstellar mod do show how vastly more powerful but finite energy sources can be used in the game, without breaking it.

So keep the RTG as a trickle charger to make sure you don't end up with a dead probe, and have some more powerful generator for keeping the full array of instruments and systems online?

[Sirrobert]

Good ideas technicafool.

Not like you can use RTGs to power your ion probe anyway

[Kasuha]

Good ideas technicafool.

Not like you can use RTGs to power your ion probe anyway

You can, it will be just a bit intermittent.

When you get far enough from Sun (200-many-zeros-and-a-little), solar panels cease to work completely. At that area, RTGs are the only sustainable power source.

[Iron4venger]

Since theres nothing else beyond Eelo, theres no point in having an infinite powersource because u cant do anything with it there (there nothing).

But if you go to jool with a probe and you plan to make a long expedition there, then RTGs with diminishing power ouput adds a layer of mission planning that I think is great. Ofc nothing too overkill (u lose 1 power output per day is waay to high) but just a little lower each day makes the mission much more interesting I think.

And in the Kerbol System even at Jool range Solar pannels are still functional. In the old days, The trick with solar pannels in Jool was that ions were very slow, and you need a ton of energy to use it, but since ions were upgraded, solar pannels were not needed anymore beyond Dres in you have even a single RTG (even a couple) due speed increase in ion engine.

[LethalDose]

As stated by others, the time line it would take for RTGs to lose productivity to any meaningful degree would be much longer than 99.9% of the vehicles built with them are around. Most players will typically crashed or outdated or recovered long before the RTG's reduced power output would become a meaningful game mechanic.

Hence, infinite RTGs are a reasonable game mechanic, even if they aren't realistic.

I'd rather just see LV-Ns move to using liquid fuel without oxidizer, seems simpler to implement and I don't think we've been told definitively what liquid fuel actually is. The high Isp with the LV-N implies liquid fuel is hydrogen, but most other indicators make it seem like kerosene.

This is a change I'd love to see. There was a comment in the NERVA's part CFG that basically said "Yeah, we know it's wrong, wait till tweakables and we'll fix it"... Never happened. I hope we'll see it change with refueling being a possibility in future updates.

[Laie]

I'd rather just see LV-Ns move to using liquid fuel without oxidizer, seems simpler to implement and I don't think we've been told definitively what liquid fuel actually is. The high Isp with the LV-N implies liquid fuel is hydrogen, but most other indicators make it seem like kerosene.

People would be clamoring for large fuel-only tanks in that case. I'm not sure whether I'd enjoy having around another line of tanks; but if you went that far, you could also declare them to hold hydrogen. Then again, that could cause trouble with the planned ISRU -- making hydrogen is much easier than synthesizing hydrocarbons. I'm not even going into the option of having both Kerosene and Hydrogen fuel -- I don't think that would ever happen. All things considered, I don't think the current Nervas are that bad: you need to bring up X amount of something that will be consumed at the appropriate ISP. That's the most important part, anything else is cosmetics.

However, I'd like to toss out the option that LV-Ns might be limited to (say) ten hours of operation. While RTGs only tap into decay heat, LV-Ns have actual nuclear fuel that is being consumed. Ten hours go a long way, so you'd only have to worry about it if you put up reusable infrastructure. Having to retire your transfer stages every couple of trips would be a nice touch; but the player needs to be made aware of it in some way, or it will make for a horrible gameplay experience.

[Red Iron Crown]

That post was from 6 months ago, before we knew ISRU in some form was coming. The lack of LF only tanks has been mitigated somewhat with the SP+ parts, and will likely become more so with the new Mk3 parts in the pipeline (assuming there are LF only tanks planned).

Honestly, it might be better to have the NTR burn any kind of fuel, with varying Isps for them.

[Laie]

That post was from 6 months ago, before we knew ISRU in some form was coming.

Yes, I know. That was one thing leading to another: the notion of fuel-only tanks opened the posibility that they might as well hold a different kind of fuel, which in turn raised the question of how to integrate a new fuel into the game.

I like the idea of having kerosene and H2 burners; the ISP 320-360 engines would burn kerosene, the deep-space engines would use hydrogen (and may go up to 450 if you will). That would kinda-sorta bypass the ISRU problem: you only get to make H2 in space, but that's also all you need in space, so no worries. But it would make rocket design more involved. Key problem: how do you prevent newbies from sticking kerosene engines to a hydrogen tank?

Fuel-only Nervas already pose similar problems, only worse: with only one type of generic "fuel" that may be used by all engines, mission planning will become quite difficult. How should a delta-V calculator tell which amount is to be burned in what engine? Just look at what happens when you put jets and rocket motors on the same vessel, and now imagine having to deal with that all the way to Jool and back. I'm opposed to fuel-dependent variable ISP for the same reasons.

MK3 tanks would be too small, by the way. And if the deadweight/fuel ratio isn't about as good as that of ordinary tanks, Nervas will become pointless pretty quickly.

[Red Iron Crown]

MK3 tanks would be too small, by the way. And if the deadweight/fuel ratio isn't about as good as that of ordinary tanks, Nervas will become pointless pretty quickly.

The new Mk3 profile will supposedly be large enough to enclose 2.5m parts in the cargo bays, so I think tanks in that shape will be sufficiently large for non-absurd missions. That's a good point about the mass ratio though, hopefully they're no worse than the 3.75m tanks are now.