I've just got 14 minutes out of the nuclear engines. Stock parts

[juanml82]

This is the spaceship

Now, as you may remember from 0.90, nuclear engines also overheated there if you ran them at 100% throttle. So I've put that thing together, limited the thrust to 92% which is where they wouldn't overheat in 0.90, hyperedited to a 125 km orbit and hit "Z"

Fourteen minutes later, the jet fuel tanks blew up

Now, I think there are issues here anyway. First, I'm not sure real nervas overheat as much. Second, I was keeping kerbals inside modules heated at over 1000°K. Either they have awesome insulation (which should be on the engines then) or they should be dead. Third, radiators are still needed for stuff like mining, for instance.

But I don't think nuclear engines are broken by any means nor that Squad should release a hotfix now or anything else. 14 minute long burns are more than enough for nearly all situations (maybe not circularizing at Moho, though) and if a burn needs even more time, then it's usually a better idea to split it into several burns anyway because of the inaccuracy of such a long burn.

TL;DR It's not a broken game mechanic. It's 1.0 asking as to stop doing things as we used to and expect the same results.

[SkyRender]

The thing is, as far as engines go, it IS a broken mechanic because no sane engineer would OK sending up an engine that is prone to melting itself (or worse, its fuel supply container) within minutes of ignition and well before its fuel supply is spent. One of the main reasons why nuclear rocket motors never had any launches into space in the real world was because they couldn't get the heat dissipation levels down to an acceptable amount. Were the project ever revived, you can be damn sure they'd have to solve that issue before they got put into use.

[andruszkow]

From what I had the time to grasp in the chat earlier today, a heat generation nerf is to be expected for the Nuclear Engine in the next update.

[lajoswinkler]

We don't need heat generation nerfing. In fact, I'd crank it up!

What we need are radiators as a special part. It's absurd to run a nuclear reactor in space and not have them. No ship will ever be able to do it.

Small, medium and large radiators. I expect to see them in the next bigger update.

[dryer_lint]

The chat? What chat!? :0

[Brotoro]

From what I had the time to grasp in the chat earlier today, a heat generation nerf is to be expected for the Nuclear Engine in the next update.

Really? Squad said this...or is this wishful thinking?

[metl]

So where's the problem? Have you tried it with using something to help dissipate the heat?

[Brotoro]

We don't need heat generation nerfing. In fact, I'd crank it up!

What we need are radiators as a special part. It's absurd to run a nuclear reactor in space and not have them. No ship will ever be able to do it.

No. Nervas were designed to run in space with no radiators. When the reactor is on, the fuel running through the Nerva picks up the heat from the reactor core and carries it away in the exhaust (which has a lower temperature than chemical engine exhaust). After the reactor is shut down, fuel continues to be run through the engine as the core cools down (leading to a thrust tailoff that should be modeled in KSP instead of overheating that would not occur).

If you want a reactor in space for generating lots of electrical power, then YES, you are going to need radiators.

- - - Updated - - -

I've just got 14 minutes out of the nuclear engines...

OK. And how long can you run the engines when the tanks are nearly empty?

[SkyRender]

I did some testing on this just now and discovered a few interesting things... 1.) solar panels are horrible heat radiators: the Gigantor XLs only bleed off about 1 degree per second of heat over the rest. 2.) What you hook up directly above your LV-N has a HUGE impact. An engine precooler, paradoxically, will allow the engine to overheat super-fast. By contrast, a Mk3 Fuel Fuselage Short section will cause the engine to overheat much more slowly (as in, over 20 times slower than on a precooler; it was kinda nuts).

[lajoswinkler]

No. Nervas were designed to run in space with no radiators. When the reactor is on, the fuel running through the Nerva picks up the heat from the reactor core and carries it away in the exhaust (which has a lower temperature than chemical engine exhaust). After the reactor is shut down, fuel continues to be run through the engine as the core cools down (leading to a thrust tailoff that should be modeled in KSP instead of overheating that would not occur).

If you want a reactor in space for generating lots of electrical power, then YES, you are going to need radiators.

Nobody said this is "the NERVA", but I'm not so sure it was designed to work without radiators.

Nuclear fission reactor creates such enormous amounts of heat during active controlled fission, way more than expected quantities of hydrogen can absorb and remove, but that's not the main issue. What's with the heat generation after the chain reaction is stopped? All those fission products will lead to melting unless the heat is removed.

What do you mean by "fuel flowing through the engine"? LH2 absorbs it, then what? Where's the heatsink?

Liquid hydrogen is a terribly pathetic material for picking up heat and there's no enough of it in a olympic pool sized vat which could cool down a typical heat generation after shutdown of any nuclear engine. Radiators are absolutely neccessary.

[NathanKell]

Brotoro, while I 100% agree with you on NTR design, the same could be said of liquid engines. And yet they generate lots of heat in KSP too. At this point I'd say it's dev choice regarding using overheat as a mechanic, although re: the strength of the NTR's overheat, well, we'll have to see.

[Brotoro]

Nobody said this is "the NERVA", but I'm not so sure it was designed to work without radiators.

What you said was "It's absurd to run a nuclear reactor in space and not have them. No ship will ever be able to do it."

So all I need is a single counterexample. If you don't like the NERVA design, you can look at newer designs.

I expect those ship designs must have radiators on them somewhere because they have refrigeration units to prevent boil off of the LH2 (and because those nuclear engines they have there are "bimodal", designed to run at low power to generate electricity in addition to being used as nuclear thermal rocket engines). But I assure you, this ship would not have all the attached tankage and structure glowing hot when its engines are in use like nuke-equipped ships in KSP 1.0 have now.

Edited May 1, 2015 by Brotoro

[juanml82]

OK. And how long can you run the engines when the tanks are nearly empty?

Half tanks, 8 minutes, which something like 2100 d/v

1/10 tanks, 5 minutes, around 1700 d/v

I think these times should be improved using mk2 tanks instead of the mk1 jet fuel tank. But the mk1 one looks cooler.

[Aerindel]

Yeah, its still not good enough. I built almost that exact same ship and tried to use it and its pretty much useless as a spaceship since even if you do get a longer burn time your actual thrust is so low its pointless.

[lajoswinkler]

What you said was "It's absurd to run a nuclear reactor in space and not have them. No ship will ever be able to do it."

So all I need is a single counterexample. If you don't like the NERVA design, you can look at newer designs.

http://i.imgur.com/cslvsjN.png

I expect those ship designs must have radiators on them somewhere because they have refrigeration units to prevent boil off of the LH2 (and because those nuclear engines they have there are "bimodal", designed to run at low power to generate electricity in addition to being used as nuclear thermal rocket engines). But I assure you, this ship would not have all the attached tankage and structure glowing hot when its engines are in use like nuke-equipped ships in KSP 1.0 have now.

Refrigeration units? OK, where's the heatsink? Refrigerators just transfer heat from one place to another (producing more heat in the process). Ultimatively, heat needs to be dumped as radiation into space.

I'm pretty sure those are just hypothetical concepts and not actual developed blueprints.

Yes, the reactor would not be glowing as it was about to melt. But that's not what I was talking about.

[Brotoro]

Refrigeration units? OK, where's the heatsink? Refrigerators just transfer heat from one place to another (producing more heat in the process). Ultimatively, heat needs to be dumped as radiation into space.

I'm pretty sure those are just hypothetical concepts and not actual developed blueprints.

Yes, the reactor would not be glowing as it was about to melt. But that's not what I was talking about.

Umm... yes, that's why I said there must be some radiator in that design somewhere. If the tank isolation is good, they won't have to dump a lot of waste heat to keep the hydrogen cold. But what the design does NOT have giant radiator wings to cool down its nukes. The reactors of the NTRs are cooled by their fuel. And, yes, these are design concepts by people showing how nuclear propulsion systems that we could build today can be used for Mars missions. These were not made by people playing games. Their primary concern is the nuclear propulsion system.

[juanml82]

Yeah, its still not good enough. I built almost that exact same ship and tried to use it and its pretty much useless as a spaceship since even if you do get a longer burn time your actual thrust is so low its pointless.

Well, nukes always had low thrust. Try it this way

This has an initial twr of 0.31 and (at 92% throttle) will last for 11:20 minutes or so when starting at full tanks - that's a 3050 m/s burn and, unless they are to capture in a body without atmosphere, such burns are usually better done by splitting them anyway, because they are too inaccurate if done all at once.

The return leg might be problematic. With the central tank empty and the outer mk2 tanks full, it won't make it to 6 minutes, but that's still a 2000 m/s burn

[Brotoro]

Brotoro, while I 100% agree with you on NTR design, the same could be said of liquid engines. And yet they generate lots of heat in KSP too. At this point I'd say it's dev choice regarding using overheat as a mechanic, although re: the strength of the NTR's overheat, well, we'll have to see.

Yes, we wait and see. But I'm hoping the devs decide to dial back on the heating. I also hope they give us some cool radiator parts, because more parts means more fun and more design options. If we don't want to stick our nuclear engines right on a big tank, it would be nice to have heat pump pipes to carry the heat away to some massive heat sink and/or radiator.

[Aerindel]

The return leg might be problematic. With the central tank empty and the outer mk2 tanks full, it won't make it to 6 minutes, but that's still a 2000 m/s burn

Which is still pretty crippled compared to the interplanetary ships I used to make. As the ultimate KSP engine it still leaves a lot to be desired. If it stays this way then there really needs to be a bigger better version further down the tech tree. I refuse to believe that space exploration is as forever crippled as KSP (1.0) would have us believe. I'm not asking for warp drive here but we should be able to pull off 2001 style missions with large ships and multiple landers.

[magnemoe]

What you said was "It's absurd to run a nuclear reactor in space and not have them. No ship will ever be able to do it."

So all I need is a single counterexample. If you don't like the NERVA design, you can look at newer designs.

http://i.imgur.com/cslvsjN.png

I expect those ship designs must have radiators on them somewhere because they have refrigeration units to prevent boil off of the LH2 (and because those nuclear engines they have there are "bimodal", designed to run at low power to generate electricity in addition to being used as nuclear thermal rocket engines). But I assure you, this ship would not have all the attached tankage and structure glowing hot when its engines are in use like nuke-equipped ships in KSP 1.0 have now.

More fun, you do not want to heat liquid hydrogen much as its then stop beeing liquid, yes the liquid hydrogen is also used to cool the outer parts of the reactor before it enter the core.

And as you say you tapper off the flow after shutdown to cool it down.

The refigurators are to cool the hydrogen so it don't boil off during the trip to mars, one other option is to use the nerva only for the Mars burn or use other fuel in them for the later part.

[juanml82]

Which is still pretty crippled compared to the interplanetary ships I used to make. As the ultimate KSP engine it still leaves a lot to be desired. If it stays this way then there really needs to be a bigger better version further down the tech tree. I refuse to believe that space exploration is as forever crippled as KSP (1.0) would have us believe. I'm not asking for warp drive here but we should be able to pull off 2001 style missions with large ships and multiple landers.

Is it? The only place you'll need a 2+ km/s burn to return to Kerbin from is Moho, and that's problematic already because a typical 3,5-4,5 km/s circularization burn is already out of the question (fancy maneuvering can reduce that by a lot, or so people on these forums claim). Jool could require that much - but only if you aren't using Tylo to give you a gravity assist. I don't think Eeloo requires so much delta-v for the trans Kerbin injection but even if it does, you can easily split the burn.

It could be problematic if you want to return with a large amount of equipment but why should you? After all the effort in delivering whatever cargo you delivered to Jool, what's the point in return it back? Head back with kerbals and the upper stages of the landers (which are way lighter than the vehicles you took there to begin with) and leave everything else in the Jool system, to be used by future expeditions.

[Geschosskopf]

Nuclear fission reactor creates such enormous amounts of heat during active controlled fission, way more than expected quantities of hydrogen can absorb and remove, but that's not the main issue. What's with the heat generation after the chain reaction is stopped? All those fission products will lead to melting unless the heat is removed.

What do you mean by "fuel flowing through the engine"? LH2 absorbs it, then what? Where's the heatsink?

Liquid hydrogen is a terribly pathetic material for picking up heat and there's no enough of it in a olympic pool sized vat which could cool down a typical heat generation after shutdown of any nuclear engine. Radiators are absolutely neccessary.

Hmmm, what makes you say there needs to be a "heatsink", as in a solid object that absorbs heat? There's no necessity for such a thing. Anything cooler than the heat source will absorb heat, and that something can be nothing (as in vacuum).

The ability of a substance to absorb heat is defined by its specific heat. The higher the specific heat, the more energy it absorbs to change 1 degree of temperature (whether F, C, or K). Specific heat can be defined as at a constant pressure (Cp) or constant volume (Cv). So let's look at specific heats.

Landbased nuclear reactors are cooled by water. This is a closed, constant-volume system in the reactor. This water absorbs heat from the core, loses it in a heat exchanger, and goes back into the core. The heat exchanger heats up another water system, which turns to steam, spins turbines (losing heat), then goes through the big cooling towers (a type of radiator) to lose the rest (which is most of it). So, in this case, the ultimate "heatsink" is the atmosphere surrounding the cooling towers. As concerns what happens in the reactor itself, the elevant measure is the Cv for water, which is 4.19 kJ/kg K.

Nuclear engines are cooled by hydrogen, which expands out the nozzle to prodcue thrust. The engine runs at a constant pressure to achieve a given thrust. The relevant measure then is the Cp of hydrogen, which is 14.32 kJ/kg K. The heat sink is the surrounding vacuum, into which the exhaust carries off heat.

The interesting thing to note here is that hydrogen is 3.4 times better at absorbing heat than water. And that's saying a lot, because water has about the highest specific heat of anything commonly available at ambient conditions. But the problem is, liquid hydrogen has a density of only 70.85 kg/m^3 whereas water is 1000 kg/m^3 by definition. Thus, you can put 14 times more mass of water than you can liquid hydrogen in a tank of a given size. But because hydrogen is 3.4 times better at absorbing heat, that tank of water can only absorb 4 times as much heat as if it was full of hydrogen.

So at the bottom line, hydrogen is far from "pathetic" as an absorber of heat. Sure, you do need "olympic swimming pools" of it, but that's why those real rocket designs shown by Brotoro have so many friggin' huge tanks.

And here we run into a problem with the game, not hydrogen. Stock KSP doesn't have an LH2 resource, so has no proportionally huge LH2 tanks. Instead, we're just using "liquid fuel", which we suppose is kerosine that can be used by both jets and chemical rockets. Kerosine, FWIW, has a Cp of only 2.01, half that of water. If you want LH2 tanks, get the Near Future mod and you'll have your "olympic swimming pools", plus nuke engines that use it and a patch to make the stock LV-N use it, too.

So take from this that nuclear engines are designed to handle the heat they make, and can indeed use LH2 for that purpose because it actually works pretty good. And the real NERVA XE was ready for flight testing when the politicians pulled the plug on the space race. So let's have no more of that. In real life, the system works without overheating.

Now, back to the real issue, which is that the LV-N is broken. It does NOT need radiators, it needs its ridiculous and unrealistic overheating problem totally taken out, knocked in the head, and buried in an unmarked grave.

[allmhuran]

Umm... yes, that's why I said there must be some radiator in that design somewhere. [...] If the tank isolation is good, they won't have to dump a lot of waste heat to keep the hydrogen cold. But what the design does NOT have giant radiator wings to cool down its nukes. The reactors of the NTRs are cooled by their fuel.

I don't really know anything about real life nuclear thermal propulsion, so help me out here.

I gather that the idea is this: "We cool off the engine itself by dumping heat into the reaction mass. The hot reaction mass is obviously being discarded from the ship. So we can get rid of the engine heat via this mechanism."

My question, though, is: how much of the heat can you get rid of via this mechanism? Let's say the exhaust is at a temperature of 2000K. It is at that temperature because it has passed through the engine. then the engine itself is also at a minimum temperature of 2000K, yeah? Probably somewhat higher, unless the fuel cycles through the engine for quite a while.

So my question, then, is: how are you going to get rid of this 2000K worth of engine heat, ie, the heat left over after the equilibrium is reached between fuel and engine?

if we use some kind of heat pump to move even more heat from the engine to the fuel/exhaust (ie, beyond equilibrium), well, that doesn't seem to help, because the heat pump must also be on the ship, and in order to work it will generate even more heat than it pumps.

[Aerindel]

You get rid of the extra heat by radiating it away into space. Use some coolant like liquid sodium and pump it through thin vanes with lots of surface area.

[allmhuran]

You get rid of the extra heat by radiating it away into space. Use some coolant like liquid sodium and pump it through thin vanes with lots of surface area.

Sure, but this is what lajoswinkler is saying: we need dedicated radiator parts. Others are saying this isn't necessary, that's what I'm trying to understand.