Nukes [Heat up and push out at high speed and pressure] Fuel Only

[Whirligig Girl]

Now that we have tweakables (since 0.23!), can we make Nukes burn fuel only?

Edited December 25, 2014 by GregroxMun

[ddenis]

Never thought about it before, but yes, NERVa should use only Liquid fuel.

[Kegereneku]

I'm okay with this, but I would prefer an handy way to recognize LF-only tank visually to come with it.

[zxczxczbfg]

My personal theory is that it's a hybrid nuclear-chemical rocket, not just a nuclear thermal rocket, so the oxidizer is necessary.

[Anquietas314]

My personal theory is that it's a hybrid nuclear-chemical rocket, not just a nuclear thermal rocket, so the oxidizer is necessary.

Given the temperatures typically found in a nuclear reactor, you really don't want oxidizer anywhere near that thing (this is less of a problem for rockets since the exhaust carries most of the heat away from the engine itself).

[Starman4308]

I suspect the reason why this doesn't happen is because Squad wants pure LF tanks (essentially jet fuel fuselages) to remain relatively heavy, and LF/O tanks to be more in-line with space travel. If you make LV-Ns consume only liquid fuel, you either force them to use mass-inefficient fuselages (or half-empty LF/O tanks, also mass-inefficient), or destroy Squad's ability to balance jet fuel tanks separately with LF/O tanks.

[Anquietas314]

I suspect the reason why this doesn't happen is because Squad wants pure LF tanks (essentially jet fuel fuselages) to remain relatively heavy, and LF/O tanks to be more in-line with space travel. If you make LV-Ns consume only liquid fuel, you either force them to use mass-inefficient fuselages (or half-empty LF/O tanks, also mass-inefficient), or destroy Squad's ability to balance jet fuel tanks separately with LF/O tanks.

Alternatively, you could just make them require a special fuel type which solves both problems.

[LABHOUSE]

The cfg file mentions this, they do not want to implement this yet.

[Vaporo]

The cfg file mentions this, they do not want to implement this yet.

Exact text from the file:

// Yes, I know this is wrong. NTRs don't actually burn fuel and oxidizer, but we don't want to jump into making separate tanks for the two yet. 

I assume that we will eventually get unique fuel tanks (containing hydrogen?) for the LV-Ns.

Edited December 20, 2014 by Vaporo

[Anquietas314]

The cfg file mentions this, they do not want to implement this yet.

It's worth considering that the nuke engine was added quite a while ago, it's possible squad decided not to implement it at all (or forgot about it)

EDIT: It's also possible whoever implemented the nuke engine is no longer on the team.

Edited December 20, 2014 by armagheddonsgw

[CaptRobau]

Given the temperatures typically found in a nuclear reactor, you really don't want oxidizer anywhere near that thing (this is less of a problem for rockets since the exhaust carries most of the heat away from the engine itself).

Meet the LANTR nuclear thermal engine which does inject oxidizer into the mix, to create a higher TWR in exchange for lower Isp (still higher than normal chemical rockets).

Edited December 21, 2014 by CaptRobau

[NERVAfan]

I suspect the reason why this doesn't happen is because Squad wants pure LF tanks (essentially jet fuel fuselages) to remain relatively heavy, and LF/O tanks to be more in-line with space travel. If you make LV-Ns consume only liquid fuel, you either force them to use mass-inefficient fuselages (or half-empty LF/O tanks, also mass-inefficient), or destroy Squad's ability to balance jet fuel tanks separately with LF/O tanks.

Using mass-inefficient fuselages/tanks makes sense, though. A NTR using pure hydrogen would have much more volume per mass of propellant than even a hydrogen/oxygen chemical rocket, much less a hydrocarbon/oxygen one.

[Anquietas314]

Meet the LANTR nuclear thermal engine which does inject oxidizer into the mix, to create a higher TWR in exchange for lower Isp (still higher than normal chemical rockets).

Technically yes, but the trick is that the hydrogen is heated separately in the reactor, then the oxygen is added afterwards (presumably in a way that does not allow the oxygen itself to get heated before reacting). I suppose you could say the LV-N in KSP is based on that, but at least to me it seems like the idea is that everything's just shoved through the reactor.

[Pthigrivi]

I like this. I dont think its a problem that the LF tanks are heavier, the LVN is pretty OP anyway.

It would change the game a lot though. Id guess that 90% of my fuel after leaving Kerbin goes through LVNs.

[Bill Phil]

Nukes don't burn fuel. They hear up propellant. So... Burn on doesn't apply here.

[CaptainKipard]

Why are the tanks heavier? This doesn't seem to make sense.

[thereaverofdarkness]

Alternatively, you could just make them require a special fuel type which solves both problems.

I think the going opinion is that the liquid fuel is Kerbosene (Kerbal Kerosene equivalent) and the oxidizer is the Kerbal equivalent of Dinitrogen Tetroxide. These are nice and cheap, and store well, but the more expensive and powerful rocket fuel is liquid hydrogen and liquid oxygen. Perhaps late in the tech tree could be an unlock for the option to use the kerbal equivalent of liquid hydrogen and oxygen, which would be far more expensive but would provide significantly more power and delta-v for the same mass.

Liquid hydrogen is remarkably lightweight, partly just because of a low molar density but even the mass needed for a given amount of thrust isn't so high. So if the LV-N burned liquid fuel only, it would be much more efficient once you unlock liquid hydrogen, provided you are willing to pay the extra cost for that. And why wouldn't you? LV-Ns are also rather expensive.

[Starwaster]

Technically yes, but the trick is that the hydrogen is heated separately in the reactor, then the oxygen is added afterwards (presumably in a way that does not allow the oxygen itself to get heated before reacting). I suppose you could say the LV-N in KSP is based on that, but at least to me it seems like the idea is that everything's just shoved through the reactor.

It would make more sense to heat it up before injecting it into the exhaust, and you can essentially do that for free by passing it through the nozzle in the same manner as regenerative cooling. Otherwise you're suffering from an unnecessary loss of performance

Why are the tanks heavier? This doesn't seem to make sense.

I think they're talking about pure LF tanks which are heaver, presumably on the grounds that they're airplane parts and need the extra structural reinforcement. If you look at LF only tanks you'll see they're not only heavier but that they also have increased crash tolerance. But, you're right, for rocket parts it wouldn't make sense. (although, if you were to get into using LH2 as it should be done, then you're using larger fuel tanks and so they're more massive because of that. But I don't think that was what was meant?

[Laie]

Why are the tanks heavier? This doesn't seem to make sense.

In real life, they're much bulkier because of the low-density fuel, and have to be insulated as well.

By the way, the Mk2 & Mk3 fuselages have&hold the same weight, be they fuel-only or LF/O. The last fuel-only tank that's still heavier than an LF/O tank is the simple drum.

But one thing that's still missing from this discussion: what's the benefit of fuel-only LV-Ns? They'd make craft design and mission planning a lot more difficult. Where's the upside?

[ddenis]

But one thing that's still missing from this discussion: what's the benefit of fuel-only LV-Ns? They'd make craft design and mission planning a lot more difficult. Where's the upside?

Why and how fuel-only LV-Ns would make craft design or mission planning more difficult?

I don't see any reason for mission planing getting any harder.

Benefits... a bit more logical and realistic engine. And more dV for the same weight of fuel. It's questions not about benefits in craft design, mostly, but how players see it.

Edited December 25, 2014 by ddenis

[Laie]

Why and how fuel-only LV-Ns would make craft design or mission planning more difficult?

I don't see any reason for mission planing getting any harder.

Because (unless you go nuke only) you'll have parts of your mission that require oxidizer (presumably the landing) and parts that don't (presumably the transfer). Right now, fuel saved during landing can make up for a botched transfer, and the other way round. This will no longer be possible. Actually, if your landing works better than expected, you end up carrying extra weight (now-useless oxidizer).

I don't know if "more difficult" is the right term, but having to plan for two kinds of fuel consumption would certainly provide you with more opportunity to fail.

[Kegereneku]

But one thing that's still missing from this discussion: what's the benefit of fuel-only LV-Ns? They'd make craft design and mission planning a lot more difficult. Where's the upside?

That's a very good question. We shouldn't make change just for the sake of more realism if it only complicate the gameplay needlessly.

Myself I would suggest that combined with "Deep Space Refueling" feature you could create an potentially interesting game-mechanic where it is easier to obtain LF-only for such propulsion than LF and Oxidizer.

However it is not strictly needed in any way and kind of low priority to me.

[ddenis]

Because (unless you go nuke only) you'll have parts of your mission that require oxidizer (presumably the landing) and parts that don't (presumably the transfer). Right now, fuel saved during landing can make up for a botched transfer, and the other way round. This will no longer be possible. Actually, if your landing works better than expected, you end up carrying extra weight (now-useless oxidizer).

I don't know if "more difficult" is the right term, but having to plan for two kinds of fuel consumption would certainly provide you with more opportunity to fail.

Yeah, "more difficult" sounded not right for me. I agree with: "different", "feels not right" or "strange", I would even agree with "a bit harder" or "quite harder" but nothing unbearable. Or another term.

But I would argue with the first part. I play with mods which shows dV. Usually I build landers in such manner, that they have just right amount of fuel to land and get to orbit and then I can abandon it without docking. But stock KSP limit players ability to plan due to absence of dV information.

If we could dump fuel/oxidizer in the KSP, then players wouldn't have problems with carrying extra weight (now-useless oxidizer). On trip to somewhere player have full lander in any case and it's a dead weight even if main engine uses the same fuel, because taking fuel/oxidizer from lander we can take too much, that the one wouldn't be able get into orbit again or land.

having to plan for two kinds of fuel consumption would certainly provide you with more opportunity to fail.

It could.

Edited December 25, 2014 by ddenis

[Whirligig Girl]

Now I'm not suggesting resources, but in this image, look at where it mentions NTRs.

[NERVAfan]

I think they're talking about pure LF tanks which are heaver, presumably on the grounds that they're airplane parts and need the extra structural reinforcement. If you look at LF only tanks you'll see they're not only heavier but that they also have increased crash tolerance. But, you're right, for rocket parts it wouldn't make sense. (although, if you were to get into using LH2 as it should be done, then you're using larger fuel tanks and so they're more massive because of that. But I don't think that was what was meant?

Well, what I was saying is that since KSP uses an "abstract" single type of LiquidFuel, the heavier airplane tanks could represent both structurally-reinforced kerosene tanks and less-volume-efficient hydrogen tanks.