Are LV-N's worth the trouble?

[X-SR71]

I believe we should be being able to drain Oxidizer and increase LF by the same amount on any tank, maintaining its appropriate mass ratio. It would make sense, right?

[The Yellow Dart]

I think squad should provide a special tank or two just for LV-Ns, one of those bare frames with spherical liquid fuel tanks mounted inside the frame. It should have very high drag and very low impact tolerance and a slightly better dry weight than all other tanks. This would solve this problem of having to use aircraft fuselage for space craft and add more interesting and varied visuals to the game.

[Kuu Lightwing]

Performing the a task with LV-N requires X propellant, doing the same with ordinary engines requires Y propellant. Comparing two solutions to the same problem is an entirely reasonable proposition. What's unreasonable is that he stops right there and doesn't account for the lifter that's needed to get them into space in the first place.

The problem is how you present that comparison. If it wasn't "these are the same vessels, but this one only slightly more efficient and has less thrust", completely forgetting that it's using significantly less propellant than the other vessel.

[ArmchairGravy]

Updated OP with an equal fuel mass comparison. This comparison shows a 70% increase in dv using LV-N's and liquid fuel fuselages versus rocket fuel and LV-30's. I would say a 70% increase in dv is worth the heat management. If I use the 1.25m liquid fuel tanks and stage them off as they empty, this will improve the dv even more. I am back on board the LV-N bandwagon!

Edited May 13, 2015 by ArmchairGravy

[The Yellow Dart]

And if you don't want to have to worry about overheating, just limit thrust back to about 60% or less. I think of long burn times as "go get a snack" time so low thrust doesn't bother me.

[Mister Spock]

Thanks for updating the OP. Very helpful.

So if I don't have a fuel-switcher mod, what fuel tanks should I be using with the LV-N? Airplane tanks?

[Laie]

So if I don't have a fuel-switcher mod, what fuel tanks should I be using with the LV-N? Airplane tanks?

As of now, yes. Don't insist on calling it "airplane" tanks, though. Mk3 parts make sleek ships and look good in space. Sometimes they remind me of art deco cathedrals or 1930 high-speed trains, but that happens to be a look I like.

[Mister Spock]

Fair enough. They do look cool!

[Caelib]

This topic has been beat to death in the past two weeks, but my thoughts on the matter are the that engine still has some usefulness, but it is now very limited. I will only use it in group of two or more with the 2.5m tanks. Right now, they are just a hassle to use with their low TWR, lack of a LF-only 2.5m tank and heat management.

[ArmchairGravy]

This topic has been beat to death in the past two weeks, but my thoughts on the matter are the that engine still has some usefulness, but it is now very limited. I will only use it in group of two or more with the 2.5m tanks. Right now, they are just a hassle to use with their low TWR, lack of a LF-only 2.5m tank and heat management.

I looked back a couple of pages and didn't see anything specifically about LV-N's. Didn't mean to beat a dead horse.

[Francois424]

I usually edit my craft files after completed, to replace Oxidizer by LF.

I will continue to do so until I either start using a mod (Procedural fuel tanks ? or was it Procedural Fuels?) or until Squad gives me pure LF tanks.

Until then I will not accept this tempering with fuel tanks as 'cheating'

But yeah, LV-N still pretty useful =)

[KerikBalm]

Updated OP with an equal fuel mass comparison. This comparison shows a 70% increase in dv using LV-N's and liquid fuel fuselages versus rocket fuel and LV-30's. I would say a 70% increase in dv is worth the heat management. If I use the 1.25m liquid fuel tanks and stage them off as they empty, this will improve the dv even more. I am back on board the LV-N bandwagon!

What is with the X200-32 tank mounted at the bottom in both cases?

You do know that fuel will feed from that to the engines, even if the fuel lines only go to the orange tank... right?

I think you should get more than 70% increase...

Seems to me that adds about 7.8 extra tons of fuel to the LV-T30 situation, and another 7.8 tons of dry mass to the LV-N situation (unless its empty in both cases, or you've toggled the green arrows to red Xs so that no fuel flows from it)

Also, I don't think those radial attached LF fusalages will feed fuel to the LV-Ns... I don't see any fuel lines, and I'm pretty sure fuel won't flow through the surface contact (LV-N fuel flow rules aren't like the jet engines)

[ArmchairGravy]

What is with the X200-32 tank mounted at the bottom in both cases?

You do know that fuel will feed from that to the engines, even if the fuel lines only go to the orange tank... right?

I think you should get more than 70% increase...

Seems to me that adds about 7.8 extra tons of fuel to the LV-T30 situation, and another 7.8 tons of dry mass to the LV-N situation (unless its empty in both cases, or you've toggled the green arrows to red Xs so that no fuel flows from it)

Also, I don't think those radial attached LF fusalages will feed fuel to the LV-Ns... I don't see any fuel lines, and I'm pretty sure fuel won't flow through the surface contact (LV-N fuel flow rules aren't like the jet engines)

The grey tank is lander fuel. My final version used an adapter in place of the grey tank, and just used the Mk. 3 tank which still gives me plenty of dv for a Duna return. I'm going to mess about in sandbox with staged 1.25 tank designs. Since this is my 1st Duna mission in 1.0.2 I want to ensure a stable design, even if it's not as efficient.

[Caelib]

I looked back a couple of pages and didn't see anything specifically about LV-N's. Didn't mean to beat a dead horse.

Here's the thread I was referring-to: http://forum.kerbalspaceprogram.com/threads/120067-Rethinking-the-nuke-engine-where-can-we-take-it-from-here

[Temeter]

Btw, about said heat management: The last ships engine mounts exerbate every issue with heat you can get. Was the standard way of doing things and why lots of people complained about heat.

1. The part directly about the LVN will be the 2nd hottest part of your ship (aside from the engines), and small plane tanks have a low thermal mass, meaning they will heat up a lot faster than big tanks. Can be risky to mount nukes to small tanks.

2. The girder segment between middle tank and engine tanks acts as a thermal insulator, meaning most of the heat will stay at the engine section. You want the heat to spread through all the ships tanks (especially the big central one), which are the parts with the highest thermal mass and therefor tank+radiate lots of heat. Connecting the tanks directly (radially) together will solve that issue.

3. Suggestion no.2 will result in all of your tanks getting hot, so you want to insulate the utility parts (probe cores, science stuff, solar panels) which only survives 1200K and has low thermal pass (otherwise the'y just blow up). One way to do this is to put them on girders or behind a inline stack of batteries.

Edited May 13, 2015 by Temeter

[Renegrade]

Yet the basic design has been run for many hours on a test stand. I'd say NTRs are a lot more real than, say, RAPIERS or little green men.

The parts of NTRs have been stand-tested. And yeah, the RAPIER is a whole new level of #lolfake. I think only the precooler of the SABRE has been tested, the rest of it has not.

For those reasons, it seems liquid methane would be the preferred propellant for long duration missions...

You could start with H2 for the initial burn...

but then suppose you're going to mars or something... its still a long journey. For orbital insertion and return to Earth, you'd probably use liquid methane, which gets you only 644s from a realistic solid core design (Gas core designs could do much better).

That would require a core temp of 3200k, which is 'may be eventually attainable', not the 2800K of 'extensively tested' ( http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910012833.pdf ) - you're looking at 606 specific impulse at 2800K. Also, isn't methane STILL a gas/cryo fluid (it's essentially liquid natural gas)? What's the mass ratios on tanks for methane? I imagine they aren't as terribad as the mass ratios for compressed H2 or liquid H2 (which are again, less than 1:1 for long-term storage), but I doubt that they're competitive with say hydrazine/NTO type tanks.

But here, I think a LANTRN concept could work, and work with ISRU.

You store water/ice.

You use the reactor to hydrolyse the water, and generate a lot of O2 and LH2.

You run the LH2 through the reactor in typical NERVA fashion, and inject the O2 after the reactor as an "afterburner".

Great TWR, the Isp will be about 647s

While the tankage would be MUCH better in this scenario, you'd need a lot more auxiliary equipment. Generators, to produce electricity (nuclear reactors produce ZERO electrical power on their own), associated thermal transfer fluid and piping and radiators, electrolysis equipment, compressors (and more radiators), a small tank for temporary hydrogen storage, additional cooling equipment for the core (you aren't using liquid H2 anymore at 20K), etc etc.

What it all boils down to this: there's no such thing as a free lunch. You either have to sacrifice dry mass in terms of tankage, or dry mass in terms of auxiliary equipment, or specific impulse in terms of less efficient fuels.

And one cannot ignore mass ratios. They're every bit as important as specific impulse.

Size 3 tanks (kerbodyne ones): 9:1.... they no longer have worse mass ratios than the 1.25 and 2.5m LFO tanks

Yeah, the rocket LF/O parts are now all 9:1 - including the tiny ones (Oscar/Round-8). I guess that was easier than building a sensible progression to them.

We need standard cylindrical LF only tanks... unless the spaceplane tanks get nerfed, then they should be between 8-9:1 mass ratio

the LF-only tanks should be 8:1. Squad has established a clear pattern that LF-only/spaceplane parts are to have a poorer mass ratio than rocket LF/O parts.

Until then I will not accept this tempering with fuel tanks as 'cheating'

Sorry, but it is. Editing of save files is pretty much the poster child example of cheating. Squad has specifically designed the game such that LF-only tanks have poorer mass fraction, replacing the oxidizer with liquid fuel eliminates that unless you adjust for an 8:1 ratio.

[Mister Dilsby]

Sorry, but it is. Editing of save files is pretty much the poster child example of cheating. Squad has specifically designed the game such that LF-only tanks have poorer mass fraction, replacing the oxidizer with liquid fuel eliminates that unless you adjust for an 8:1 ratio.

Editing save files, yes. Modding parts fairly, no.

Consider the Mk2 LFO and LF fuel tanks. Both mass 0.5t empty. The LFO tank holds 360 units of LF and 440 units of oxidizer. The LF-only version holds 800 units of LF. Note that 360+440=800 -- clearly, Squad intended for units of O and units of LF to be volumetrically interchangable in the Mk2 series.

Based on that equivalency, I would consider it fair to mod any MK2 LFO part to LF by replacing units of O with units of LF on a 1:1 basis. Thus a properly modded LF-only MK2 bicoupler should be exactly like the LFO version, except it carries 400 units of LF instead of 180 units LF and 220 units O.

ETA: OK, I think I see what Renegrade's on about now. There is no such equivalency in the Mk1 parts. The LF fuselage is .15 empty and carries only 150 units LF. The closest size LFO tank, the FL-T200, is .125 empty and carries 90 units LF + 110 units O2 = 200 units! Somehow LF must not fit well into cylindrical spaces. In any case I think my reasoning stands w.r.t. the Mk2 parts.

Edited May 13, 2015 by Kuzzter
Corrected fuel unit count

[X-SR71]

the LF-only tanks should be 8:1. Squad has established a clear pattern that LF-only/spaceplane parts are to have a poorer mass ratio than rocket LF/O parts.

Wait, what's the reason that LF-only tanks should be a 8:1 ratio? I mean, I can understand the current MK plane parts having a 8:1 ratio due to them being structurally different - they are differently shaped to generate lift. But I can't see why a cylindrical shaped tank should hold less propellant if it's full of LF instead of LF+O. Both already weight 5kg per unit afterall.

[Mister Dilsby]

Wait, what's the reason that LF-only tanks should be a 8:1 ratio? I mean, I can understand the current MK plane parts having a 8:1 ratio due to them being structurally different - they are differently shaped to generate lift. But I can't see why a cylindrical shaped tank should hold less propellant if it's full of LF instead of LF+O. Both already weight 5kg per unit afterall.

I'm with you--if you carried the Mk2 logic down to Mk1, then there should be an LF-only equivalent of the FL-T200 that carries 200 units LF and weighs .125 empty. Such a part would obsolete the poor LF fuselage which weighs more and carries less. I don't think Squad thought that through when designing the parts...anyone can run their game how they wish, of course, but in my game I'll follow Squad's lead and stay consistent within part families: that is, I'll mod as I described above within the Mk2 family, but if I make any longer 1.25m (Mk1) cylindrical tanks I'll keep the same ratio as the Mk1 fuselage.

[Brotoro]

I always thought the aircraft fuselage tanks had a lower mass ration of fuel because the fuselage was meant to be structurally more robust that fuel tanks for rockets (which are designed on the edge for minimum mass).

[Renegrade]

Based on that equivalency, I would consider it fair to mod any MK2 LFO part to LF by replacing units of O with units of LF on a 1:1 basis. Thus a properly modded LF-only MK2 bicoupler should be exactly like the LFO version, except it carries 300 units of LF instead of 135 units LF and 165 units O.

Although the numbers and procedure for deriving them seems off (you seem to come to an understanding of my point later in the post though), the concept here happens to be correct.

The LF/O mk2 is 180+220 LF/O (total 400) for a total of 2t fuel. It's dry mass is 0.29, and therefore it's wet mass is 2.29. Wet:dry ratio of approximately 8:1 (it's slightly less on the bicoupler for whatever reason, ~7.9:1)

The appropriate LF-only version would be 400 LF (2t), again with a dry mass of 0.29. That would give it an 8:1 ratio again.

ETA: OK, I think I see what Renegrade's on about now. There is no such equivalency in the Mk1 parts. The LF fuselage is .15 empty and carries only 150 units LF. The closest size LFO tank, the FL-T200, is .125 empty and carries 90 units LF + 110 units O2 = 200 units! Somehow LF must not fit well into cylindrical spaces. In any case I think my reasoning stands w.r.t. the Mk2 parts.

That's the concept there, although I generally convert/use mass figures directly (as the units of fuel are rather weird - they're allegedly liters but that gives a density higher than titanium and implies the tanks are actually the size of soda cans)

Wait, what's the reason that LF-only tanks should be a 8:1 ratio? I mean, I can understand the current MK plane parts having a 8:1 ratio due to them being structurally different - they are differently shaped to generate lift. But I can't see why a cylindrical shaped tank should hold less propellant if it's full of LF instead of LF+O. Both already weight 5kg per unit afterall.

If you want to bring hardcore reality into it, yes, a cylindrical shaped tank should hold MORE of one type of propellant than two types as it would not require any internal walls then, HOWEVER, you then get into the concepts of things like "NTRs 'burn' hydrogen which is wildly impractical to store in any sort of moving vehicle, doubly so for mass-sensitive ones like rockets".

I always thought the aircraft fuselage tanks had a lower mass ration of fuel because the fuselage was meant to be structurally more robust that fuel tanks for rockets (which are designed on the edge for minimum mass).

That used to be the sole thing in the old days (plane parts had, and still do have, generally higher impact ratings), but now that LV-N "Nervs" are treating LF-only tanks as some sort of hydrogen, additional balance factors have to come into play.

People have to remember that even liquid hydrogen has a density around 70kg/m³; if you could survive the 20K temperature, swishing your hand around in there would feel like a really thick gas rather than a liquid (air is 1.2kg/m³, water is 1000kg/m³, and iron products around 8000kg/m³). KSP isn't some hardcore simulation, but it does try to aim for what Harv called "authenticity" - matching the broad strokes of reality without getting into the nitty-gritty and excruciatingly boring attention to detail. The broad stroke of NTRs is that their tankage will SUCK, no matter what you do, which is why the Nerv is LF-only, and all the stock LF-only tanks are slightly worse than LF/O tanks.

Well, plus the LF fuel is fueling the still-insanely-OP jet engines....but that's a different thread.

Random fun fact: based on my postage scale and some quick measurements with some pepsi cans, a pepsi can has a mass ratio of 29:1~

[Mister Dilsby]

Although the numbers and procedure for deriving them seems off (you seem to come to an understanding of my point later in the post though), the concept here happens to be correct.

The LF/O mk2 is 180+220 LF/O (total 400) for a total of 2t fuel.

You're right, I was going from the Wiki parts list (did not have the game open at the time) which lists the Mk2 LF/O bicoupler at 135+165. Will correct my post.

[Archgeek]

Heheh, no need to turn down the thrust. My tests have shown that it's not all that hard to get something that'll never get hot enough to blow anything up, due to logarithmic heating and cooling curves.

This modified drive assembly was run from MET 5:00 to 50:00, and was forced to run to there after running out of fuel at around 10:30.

Everything's nearly in perfect balance with hundreds of kelvins to spare.

The rhino's on board as an escape kicker motor from the drive's original design as an Eve transfer stage, but it serves as an oddly good radiator once out of oxidizer. It's not obvious from the picture, but the conduction and radiation flux numbers have been converging, such that nothing other than the NERVAs are going to even hit 1800 kelvin.

The key seems to be either using an immediate tank a size larger than the LV-N, or to apply radiating fins. A simpler experiment not pictured showed that a single NERVA cannot blow up a single mk1 fuselage if it has two gigantors or eight of the midsize extendable panels attached. Six might also work. Things may look hairy, but the hotter things get, the faster they cool.

[KerikBalm]

Also, isn't methane STILL a gas/cryo fluid (it's essentially liquid natural gas)? What's the mass ratios on tanks for methane? I imagine they aren't as terribad as the mass ratios for compressed H2 or liquid H2 (which are again, less than 1:1 for long-term storage), but I doubt that they're competitive with say hydrazine/NTO type tanks.

Not all cryofluids are the same.

For example, Xenon is stored as a cryofluid... and can be stored for a long time.

Melting points at 1 atmosphere:

Xenon: 161.4 K

O2: 90.2 K

H2: 20.3 K

CH4: 90.7 K

N2: 77.3 K

Storage of CH4 should be pretty much as easy as storing O2. Its a lot easier to keep something a liquid that boils at 90K, than 20.3 K (and obviously, pressure can be increased by a lot in the tank). Both are easier to store than liquid Nitrogen, which we can already store for pretty long periods of time with minimal boiloff.

Additionally, there's the simple molecular size to take into account.

CH4 is much much much more viable for long term storage than H2. (Plus, Titan is already storing large amounts, ready for use... you don't find LH2 lakes anywhere... do you?)

While the tankage would be MUCH better in this scenario, you'd need a lot more auxiliary equipment. Generators, to produce electricity (nuclear reactors produce ZERO electrical power on their own), associated thermal transfer fluid and piping and radiators, electrolysis equipment, compressors (and more radiators), a small tank for temporary hydrogen storage, additional cooling equipment for the core (you aren't using liquid H2 anymore at 20K), etc etc.

Indeed, however, as far as electricity generation, the ship may already have a need of that

See the "bimodal NTR" section here:

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore (scroll down a bit)

Power generation can be used for life support, small electric thrusters (hall effect, ion, etc) for attitude control and precision adjustments, etc.

That Auxilliary equipment could be left at the destination (hence the ISRU comment).

You do your ejection burn using LH2 and O2... You arrive.... you mine ice, and convert it with equipment that is laready there, you convert it to LH2 before you leave.

If the site has no ice to mine, you can still leave the equipment there, and bring your own ice as return fuel.

Carrying that equipment back and forth each time would indeed be extremely inefficient.

the LF-only tanks should be 8:1. Squad has established a clear pattern that LF-only/spaceplane parts are to have a poorer mass ratio than rocket LF/O parts.

...

Sorry, but it is. Editing of save files is pretty much the poster child example of cheating. Squad has specifically designed the game such that LF-only tanks have poorer mass fraction, replacing the oxidizer with liquid fuel eliminates that unless you adjust for an 8:1 ratio.

I think you're making a logical fault here.

Squad has establish a clear pattern that spaceplane tanks have worse mass ratios than rocket tanks.

There are no rocket LF only tanks

There is no difference in the mass ratios of the spaceplane parts if they are LF only or LFO tanks.

Imagine an alien coming to earth, and the first swimming things it finds in the sea are whales and dolphins.

They conclude all tetrapods need to breath gaseous O2

They conclude all swimming things need to breath gaseous O2 <- ....... those swimming things needed O2 because they were mammals, not because they were swimming

Likewise, those LF only tanks are 8:1 because they are spaceplane parts, we can't conclude they are 8:1 because LF only tanks are supposed to have worse mass ratios than LFO tanks.

Now... if the spaceplane tanks were say... 7:1 for LF only, and 8:1 for LFO.... then it would make sense that if rocket LFO tanks were 9:1, then rocket LF only tanks should be < 9:1

That is why I suggested a value between 8 and 9...

- Not that it really matters for most purposes... when your payload fraction gets really high (as it is with SSTOs and nuke tugs), the tank mass ratio doesn't have much of an effect

ie, with an SSTO, the payload may be 37% of starting mass, if your tanks comprise 10% of the remaining 63% of the mass, or 12% of the remaining 63% of the mass, it doesn't affect your dV all that much.

So... in a way... the 4.6:1 mass ratio for just using emptied LFO tanks could still be balanced.

Imagine a nuclear tug pushing a payload to escape velocity from LKO.

~1000 m/s is needed. 9.81*800* ln(X) = 1000

ln(X) = 0.12742

X = 1.1359

Only 13.6% of your mass needs to be fuel

For a 100 ton payload, at a 9:1 tankage ratio

(100+9x)/(100+x) = 1.136

100+ 9x = 113.6 + 1.136x

7.864x= 13.6

x = 1.73 tons of tanks.... 15.6 tons of tanks+ fuel

For an 8:1 ratio, its simple 6.864x = 13.6

x= 1.98 tons of tanks ...... 15.85 tons of tanks+ fuel

The mass of your tug that imparts a dV of 1,000 m/s to a 100 ton payload goes from 15.6 to 15.85... or rather, total craft mass foes from 115.6 to 115.85...

Not a huge difference (and this is ignoring engine mass and such, which has pretty much the same effect as increasing payload mass, making the comparison even closer).

If we instead look at a required dV of 2,000 m/s, and a 9:1 vs 4.6:1 mass ratio:

2000/9.81/800 = 0.2548 = ln (X); X = 1.29

(100+9x)/(100+x) = 1.29

100+ 9x = 129 + 1.29x

7.71x = 29

x= 3.76

3.76 tons of tanks... 33.84 tons of tanks+ fuel (less than an orange tank)

vs a 4.6 tankage ratio:

100+ 4.6x = 129 + 1.29x

3.31x = 29

x = 8.76 tons of tanks... 40.3 tons of tanks+ fuel

So for a craft that needs a dV of 2,000 m/s, and has a mass of 100 tons excluding fuel tanks, the total mass at a 9:1 ratio is 133.8 tons, the total mass at a 4.6:1 ratio is 140.3

140.3/133.8 = 1.0486....

Less than a 5% change in craft mass, when the fuel tank mass ratio is decreased by 48.9%

Now... this will get worse and worse as you increase the required dV... because of the non-linear nature of the rocket equation, but with the LV-N's Isp, and the dVs required in the kerbin system - the tank mass ratio can be pretty bad without affecting things much.

Its also why ion drives work well in game, even though their tankage ratio is horrible too.

The ability to use spaceplane parts at an 8:1 mass ratio, means LV-Ns will remain top-dogs, despite this fuel change.

[Brotoro]

This may involve overly esoteric engineering...but I can't see why I couldn't fill both the liquid fuel portion of a rocket LFO tank AND the oxidizer portion of the tank with liquid fuel. The config files say both liquids have the same density, so I expect I'd get the same total number of units of liquid. And, of course, if it's a purpose-built rocket liquid fuel tank without the need for the extra bulkhead(s) and feed lines required for holding two propellants, I'd expect the mass ratio of the tank to be even better than an LFO rocket tank.