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Cooling LV-N "Nervas" - it's not rocket science

Kobymaru

By Kobymaru
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Kobymaru

Kobymaru

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Abstract (= tl;dr)

No, LV-N's are not nerfed. Use 3 Big S - Elevons 2 or 8 Small Delta Wings for cooling.

Introduction

KSP 1.0.2 is hard [citation needed]. One of the factors contributing to "hard-ness" is the so-called "nerf" of the LV-N nerva enginge, which now produces significant amounts of heat that usually leads to an overheating of craft components, if the craft was not built to handle high heat production.

In this post we show that the changes to the heat physics are NOT a "nerf", but instead require care in constructing heat-handling crafts while still allowing the LV-N to be a viable engine for low-thrust, high-ISP transfer burns.

The LV-N engine produces "a lot" of heat. In fact, it produces 31104 "magical units" of Heat. This heat will gradually heat up the engine itself and everything that is connected to it, and if it is allowed to build up will eventuall overheat and explode (either the engine itself or the attached parts). There are two ways to counteract that: one can use "heat sinks" - parts that have a high "thermal mass" or heat capacity. The problem with those is that even the deepest sink can fill up and burn times can only be very finite. Another possibility to counteract the heat is to get rid of it. But how? In the atmosphere, you use convection to give off the heat to the atmosphere. But there is no atmosphere in deep space! Instead, one has to rely on "blackbody radiation". Blackbody radiation is a real physical phenomenon where an object (no matte which!) is heated up, he radiates off the thermal energy as electromagnetic radiation.

The "Stefan-Boltzman" equation describes the blackbody radiation at high temperatures:

j = k * T^4

Where j is the energy radiated per surface area, T is the temperature and k is some constant. To put it in words, "The hotter an object, the more more more more radiation it gives off".

From this equation we can already see, that in order to get good radiation values, we need to get our "radiators" as hot as possible without them exploding. Most (if not all!) interesting parts for this have a temp. tolerance of 2000 K. We will use them at their maximum temperature in order to cool our nuclear engine!

In reality, the total amount of energy radiated also depends on the surface area. Since we don't have good measures for that in the game, we have to determine them experimentally.

We used the Thermal Data from the Debug Menu to read out thermal data, and Hyperedit to manipulate thermal conditions of the crafts.

Methods

The Mods used for this Test are:

Hyperedit, Version 1.4

A craft with a central core (Made from a full Kerbodyne S3-7200 Tank) and several surface-attachable structural parts was used to thest the radiation the parts give off. The craft is depicted in Figure 1 and is available for download from here: https://dl.dropboxusercontent.com/u/1414175/ksp/Heattest-Stuff.craft

Using the Debug Menu (alt-F12), thermal Data display was enabled.

The craft was "launched", then Hyperedited into an Orbit of 100011 m. Then, the "Heat Editor" feature of Hyperedit was activated and the temperature of the central tank was locked to the current ambient temperature.

Sequentially, all surface attachable parts were locked, and then their temperature raised to just below 2000 K (which is the maximum Temp. for all parts that were used). The "Rad Flux" field was recorded for each part.

A second craft was used to confirm sustainable burn capability. It consists of an LV-N, an FL-T400, a Mk1-Lander Can and 3 Big S 2 - Elevons. It is depicted in Figure 2 and is available for download from here: https://dl.dropboxusercontent.com/u/1414175/ksp/Nuketest-Big%20S%202.craft

The craft was launched, and Hyperedit to an orbit of 300000 m. Infinite Fuel was turned on using the Debug Menu. No heat manipulations were performed.

oDUKO5I.jpg?1

The craft used for the radiation testing.

Xgc0qp5.png

Figure 2: Sustainable burn test. Note that the test was started at 5s after craft launch, and the burn time at this time is about 14 minutes, which corresponds to about the maximum recommended burn time for tranfer burns in KLO.

Results

The radiation data, along with part mass and radiation flux per mass data is shown in Table 1

Table 1: Parts, their masses and radiation fluxes. "Base Width" is dimension of the surface attachable side. Darkest shade of Yellow in "Radiation Flux" column: highest radiation. Darkest shade of Green in "Flux per Mass" Column: best Flux per Mass Ratio.

Figure 3 shows an overview of the Flux to Mass ratios.

[TABLE]

[TR]

[TD]Name[/TD]

[TD]Mass[/TD]

[TD]Radiation Flux at 2000 K [?][/TD]

[TD]Base Width [m][/TD]

[TD]Flux per Mass [?/kg][/TD]

[/TR]

[TR]

[TD]Structural Panel 1x1[/TD]

[TD=align: right]75[/TD]

[TD=bgcolor: #fffef9, align: right]800[/TD]

[TD=align: right]1[/TD]

[TD=bgcolor: #f3faf7, align: right]10,7[/TD]

[/TR]

[TR]

[TD]Structural Panel 2x2[/TD]

[TD=align: right]300[/TD]

[TD=bgcolor: #fff6da, align: right]3060[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #f3fbf7, align: right]10,2[/TD]

[/TR]

[TR]

[TD]M-Beam 200 Pocket[/TD]

[TD=align: right]188[/TD]

[TD=bgcolor: #fffffc, align: right]556[/TD]

[TD=align: right]0,2[/TD]

[TD=bgcolor: #feffff, align: right]3,0[/TD]

[/TR]

[TR]

[TD]M-Beam 200[/TD]

[TD=align: right]375[/TD]

[TD=bgcolor: #fffdf5, align: right]1070[/TD]

[TD=align: right]0,2[/TD]

[TD=bgcolor: #feffff, align: right]2,9[/TD]

[/TR]

[TR]

[TD]M-Beam 650[/TD]

[TD=align: right]80[/TD]

[TD=bgcolor: #fffffd, align: right]535[/TD]

[TD=align: right]1[/TD]

[TD=bgcolor: #f9fdfb, align: right]6,7[/TD]

[/TR]

[TR]

[TD]Modular Girder Segment[/TD]

[TD=align: right]125[/TD]

[TD=bgcolor: #fffffe, align: right]466,12[/TD]

[TD=align: right]0,66[/TD]

[TD=bgcolor: #fdfefe, align: right]3,7[/TD]

[/TR]

[TR]

[TD]Modular Girder Segment XL[/TD]

[TD=align: right]375[/TD]

[TD=bgcolor: #fffefa, align: right]766,75[/TD]

[TD=align: right]0,66[/TD]

[TD=bgcolor: #ffffff, align: right]2,0[/TD]

[/TR]

[TR]

[TD]Elevon 1[/TD]

[TD=align: right]50[/TD]

[TD=bgcolor: #fff8e4, align: right]2350[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #bde5d1, align: right]47,0[/TD]

[/TR]

[TR]

[TD]Elevon 4[/TD]

[TD=align: right]40[/TD]

[TD=bgcolor: #fffae9, align: right]1952[/TD]

[TD=align: right]1[/TD]

[TD=bgcolor: #bae4cf, align: right]48,8[/TD]

[/TR]

[TR]

[TD]Elevon 5[/TD]

[TD=align: right]80[/TD]

[TD=bgcolor: #fff4d4, align: right]3525[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #c1e6d4, align: right]44,1[/TD]

[/TR]

[TR]

[TD]Structural Wing Type D[/TD]

[TD=align: right]25[/TD]

[TD=bgcolor: #ffffff, align: right]333,13[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #eff9f4, align: right]13,3[/TD]

[/TR]

[TR]

[TD]Wing Connector Type C[/TD]

[TD=align: right]100[/TD]

[TD=bgcolor: #ffe089, align: right]9012,4[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #7dcba5, align: right]90,1[/TD]

[/TR]

[TR]

[TD]Small Delta Wing[/TD]

[TD=align: right]50[/TD]

[TD=bgcolor: #ffecb5, align: right]5780,6[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #57bb8a, align: right]115,6[/TD]

[/TR]

[TR]

[TD]Big S - Elevon 1[/TD]

[TD=align: right]150[/TD]

[TD=bgcolor: #ffeebe, align: right]5115,83[/TD]

[TD=align: right]2[/TD]

[TD=bgcolor: #d0ecde, align: right]34,1[/TD]

[/TR]

[TR]

[TD]Big S - Elevon 2[/TD]

[TD=align: right]230[/TD]

[TD=bgcolor: #ffd666, align: right]11517,7[/TD]

[TD=align: right]3[/TD]

[TD=bgcolor: #b8e3ce, align: right]50,1[/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD]Heat produced by LV-N [?][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[TR]

[TD=align: right]31104[/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[TD][/TD]

[/TR]

[/TABLE]

pubchart?oid=728538586&format=image

Figure 3: Overview over the Flux to Mass ratio

Conclusion

Table 1 shows the vales for radiated energy per part at 2000 K. Judging from this chart, we can determine that a "Big S - Elevon 2" is the part with the highest radiation value. As confirmed by Figure 2, a burn with an infinite duration is indeed possible - albeit at almost maximum temperature.

However, when constructing radiators for actual ships, using 3 Big S Elevons might not be the most economic choice. Table 1 along with Figure 3 shows that using 6 or 8 "Small Delta Wing"s might be the more economic choice in terms of additional mass.

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Jonboy

Jonboy

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Very thorough explanation. This is definitely useful information. Personally I haven't even come close to overheating nukes on my normal low-TWR interplanetary burns.

The longest burn times for most of my craft are in the 7-10 minute range and I always split these into two burns: one to boost apoapsis up past the Mum, and another to escape velocity once you come back to peroapsis. This way each burn is only 4 or 5 minutes long. With those burn times I haven't even come close to overheating, even with no heat sinks or radiators. So the supposed "nerf" hasn't affected my operations at all.

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Jonboy

Jonboy

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Just because you can make it viable by gluing stuff to it, that doesn't mean it wasn't nerfed, it just means you found a way to cope with the changes.
Just because you can make it viable by gluing stuff to it, that doesn't mean it wasn't nerfed, it just means you found a way to cope with the changes.

Yeah, this is why some kind of simple radiator part should have been included with 1.0.

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KerikBalm

KerikBalm

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I don't carry enough LV-Ns to have OH-ing problems.

I don't do 14 minute burns, I perapsis kick for anything longer than a 4 minute burn.

The last few times I went to Mun/Minmus... I had 1-2 perapsis kicks before the final burn, which obviously gives time for things to cool down.

The biggest problem I see, is that things don't seem to cool down unless they are in physics range.

If I do a perapsis kick, and the next PE isn't for another 5 hours, I may switch to another vessel, go back to the spaceport and do something there.. etc...

Then in the tracking station I timewarp until its time to PE kick, and find that my craft is as hot as it was at the end of the burn, which was 5 hours earlier (in game time).

That could be fixed... I really don't want to just have to watch a craft as it cools.

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WOODY01

WOODY01

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With 4 gigantor solar panels and rockomax fuel tank I can usually burn for around 10-12 mins. I'll have parts glowing but nothing explodes.

The only times I had some trouble was burning to circularize around Moho when I made the encounter. Something overheated and exploded and I lost control of the ship and I could not stop the engine. It ran for awhile after and eventually more things started exploding. That was about 5-6 minutes until the first explosion and I only had 4 smaller solar panels on that rocket. The other time was my first design post 1.0 with a LV-N. It was a smallish probe with two solar panels, things started overheating rather quickly.

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Brotoro

Brotoro

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There were changes made to the engine that made it less efficient (having to carry around extra mass in radiator parts or limiting how you can attach the engine)...that's the definition of a 'nerf.'

And it's all silly, because NTR engines do not produce copious amounts of heat like this (anybody can learn how NTR's work...it's not secret information...you just need to put in a little effort). Having them overheat like this is Bad Rocket Science, Squad.

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softweir

softweir

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Posted
Just because you can make it viable by gluing stuff to it, that doesn't mean it wasn't nerfed, it just means you found a way to cope with the changes.
I shouldn't have to glue wings all over a non-atmospheric craft just to use an engine.

Look at it another way: a RL nuclear engine would need radiators to keep temperatures low enough. The only quibble is that there is no purpose-built part to do this! (Yet. We may get one later.)

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Levelord

Levelord

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As of yet we don't have dedicated radiator parts in the game and it's unreasonable to have to stick ~5+ fins and whatnot on the NERVAs because it significantly adds to the part count (~5 fins per engine... especially so on larger ships that require 2 or more engines for burns). This already slows down the game framerate and bloats interplanetary ships with unnecessary wing parts.

OP, this is not considered a 'fix' when you have to sacrifice game performance to give the NERVA reasonable burn times within the game.

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regex

regex

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Nukes are working just fine for me; no overheating troubles at all.
I wouldn't say any troubles myself, but they're easily overcome. This craft had six extra parts added over the previous Poodle tug, four wings and those two girder segments (I added those small deltas after the fact for "roleplaying" because the entire craft gets quite hot, they're not at all needed), and burns its entire fuel supply out in 30 minutes without anything exploding. Yes, I modded those parts to handle liquid fuel only because it's stupid that we don't have that ability in stock.

If I limited burns to normal transfers and braking burns, I'd probably only need to add those girder segments, if anything. It gets 8.5km/s on four nukes. The "problems" with nukes are quite easy to overcome.

Yes, I can do this bone-stock with four engines as well, the single mod isn't a huge advantage.

uznIK5fh.jpg

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problemecium

problemecium

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Posted (edited)

I'm with worir here - it IS Rocket Science!

But I'm also with the others here who are wondering why 1.0 introduced parts that generate large amounts of heat and didn't include any radiators. They did include radiators on solar panels, yes, but really that's unrealistic (they'll absorb heat from the Sun; a radiator oriented parallel to the rays instead, rather than perpendicular as in a solar panel, would be more effective).

BTW I've also found that a simple magic sauce for keeping LV-Ns unexploded is to just thrust limit them to 73.5%. Sure, it's a bit of a thrust nerf in effect, but A: SQUAD believed they needed a nerf, and their case is pretty solid, and B: once the large jet fuel tanks are unlocked the changed LV-N chemistry turns into a massive boost. The way I see it, LV-Ns have just gotten the opposite treatment from what ion engines got a few versions ago - made about 2/3 as strong but twice as efficient. In a general sense anyway,

Edited by parameciumkid
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SciMan

SciMan

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Posted (edited)
No... it's heat transfer.

No, it's Thermodynamics!

To be honest, the whole way that heat works with LV-N engines is, without overstatement, 100% BASS ACKWARDS.

Not only do Nuclear Thermal rockets NOT work that way, but if an engine NEEDS radiators, they should ALWAYS be part of the engine that needs them. If that requires a re-do of the model, SO WHAT? It's not like there's a lack of people making mods for this game, or that they would be unwilling to make a replacement model for the LV-N.

If you always need part B to use part A, make them both into part C. If you always need radiators to use the LV-N, the LV-N model should have radiators.

This is a complaint about design decisions made by Squad that are IMO poorly thought out, but well executed.

Edited by SciMan
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m4v

m4v

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A RL nuclear engine would NOT need radiators to keep temperatures low enough.

I fixed that for you.

Nuclear engines would also need shielding and spew a lot of neutron and gamma radiation, why is that people are so vocal about their unrealistic overheating but quiet about their unrealistic lack of radiation?

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