Are Nuclear Rockets "Cheating"?

[magnemoe]

Yeah, true, there is nothing by definition that forbids using a oxidixer agent ( most likely liquid oxygen ) as propelling agent in a NERVA. But the efficiency of a NERVA is a inverse function of the molecular mass of the propelant ( better said, it is proportional to the average speed of the molecules of the propeller, that for a certain fixed temperature is a inverse function of the square root of the molecular mass of it ( IIRC ) ) , thus using LO2 is probably more than a order of magnitude worse than using LH2 ( depends of exaust temp and some other stuff ) and it is even worse with other ( heavier ) oxidizing agents. But besides that, it is really a bad idea to put anything in the proximity of a gaseous superheated oxidizer ... we know that because we use that exact same process to purge impurities from steel and to in general burn stuff to not even ashes:

http://www.americancombustion.com/image/photoelement/pj/index.35085.jpg

http://www.americancombustion.com/image/photoelement/pj/pyretron_flame35087.jpg

Now think on that inside your NERVA ( not only on the outside ) ... remember that your typical NERVA will need to have a large heat condutivity ( otherwise the nuke reactor would not be able to heat the propeller ), that normally means metal alloys that react pretty well with oxygen at high temps ( well, there is always aluminium nitride, but even that will react with oxygen above 1400 ºC ), or worse in the models that have propeller gas passing direcly on the core as moderator/cooler ( uranium and plutonium burn quite well ... ). In other words, if you use a oxidizer inside a NERVA you will most likely burn it to uselessness ...

Were was an idea of adding oxygen to the hot hydrogen in the engine bell, you would not run it trough the reactor but you can add it afterwards like an afterburner on a jet.

Like a afterburner It would increase trust but reduce ISP. Not cost effective unless you have free oxygen from mining ice for hydrogen anyway.

[cantab]

I think my record was 14 LV-N's on a single ship, which with attendant fuel give about 4500 m/s of delta-V with an 85 ton payload, enough to reach Jool without needing to aerobrake was the plan. Not sure if that falls into the too many nukes category.

There are the charts that show the lowest-mass engine for a given delta-V, TWR, and ship mass, and indeed engines other than the LV-N are widely competitive.

As for compressed hydrogen, I believe it's among what were once called the "permanent gases", that can't be made liquid by compression at room temperature. For all materials, at sufficiently high temperature or pressure the distinction between gas and liquid disappears and you just get a "supercritical fluid" - for the permanent gases room temperature is "sufficiently high".

[r_rolo1]

The critical point for molecular Hydrogen is roughly -240 ºC and 12,8 atm. So a canister of hydrogen at room temperature and, say, 15 atm of pressure will be a supercritical fluid. The issue is that supercritical fluids can still effuse through solids like gases do and molecular hydrogen is the worse offender possible in terms of possible effusion due to it's small molecular size, especially under pressure. TBH I would prefer it liquid ...

P.S In a side reading on the sources of the wiki link, I discovered that Venus atmopshere ( atleast in it's lower layers ) is actually in supercritical state for the main components ( carbon dioxide and nitrogen ) ... so technically we could even say that Venus has a ocean instead of a proper atmosphere ( and on a tangent, it would be a perfect place to do supercritical fluid extractions, like the one used for making decafs )

Edited August 24, 2014 by r_rolo1

[magnemoe]

Twenty six?? Replace those a single mainsail, and he's actually got more* delta-v, and more acceleration, since he just lost like 50 tons of engine!

What was he sending to Eve? The CN Tower? :S

* in most situations that might ever reasonably occur. I'd whip out the old abacus and figure out what those situations are, but I'm overcome with 'meh' about that..

Yes, this is a good point too. People have to drop the 'spam LV-N' stuff if they want effective rockets..

My old Jool lander used 24 but was planned to have 30 however 6 of them failed.

More normal but large ships like this orbital shipyard has 8, yes its kind of overkill however I will send it to Jool loaded with supplies to build a base.

Its docked to a miner at the bottom.

You also have the nuclear tugs, this is moving a mobile base to the Mun, the orange tank is fuel to refuel the base and other projects at Mun. Tug returns to LKO to be refueled.

And I agree that the LV-N is overused, however if you start getting up to some payload size and some dV requirements its pretty much the only way to go.

Take an 18 ton payload, one LV-N and 720 fuel give you 2500 m/s and 0.2 in twr. two LV-N gives 2300, 0.38 twr and stage weight 13.7 ton.

Replace the two LV-N with a T-30 and you need 1800 fuel to get 2300 m/s, weighing 23.7 ton or 10 ton heavier. The Poodle is a bit better but you save less than 90 fuel.

Let us reduce the dV requirements, to 1200 or and things looks better for the chemical engines, a T-30 and 720 fuel gives that and weight 10 ton, 360 and a LV-N weight 7 and have more dV.

[r_rolo1]

You said it all, magnemoe. For certain values of needed dV and payload mass ( and of desired thrust, I would add ) , the LV-N is the best choice, even if in large numbers. But for lower values of dV needed and payload mass ( and of higher desired thrust ) other engines win due to the already mentioned weakensses of the LV-N : weak thrust and a quite big mass for that same thrust. You don't need a 26 LV-N transfer stage to Eve unless you are going to make a Eve Space center there, for a example

[m4v]

What is outrageous about the nuclear engine is that is manufactured by Jebediah's junkyard, WTF.

[Dangerous_Beans]

I'd love to see something like project prometheus with a biomodal nuclear reactor.

Nuclear thermal for an ejection purn, switching to some form of ion drive for a braichistichone followup, making the travel time shorter.

Possibly even make a trimodal one where the nuke helps provide thrust during liftoff to orbit

http://www.projectrho.com/public_html/rocket/enginelist.php#ntrsolidcore

this is one of the things i like about the Kerbal Space Program Interstellar mod. you have separate reactors and thermal rockets, so you can have nuclear reactors powering thermal rockets on take off/landing, then shut down the NTRs for low thrust electrical propulsion while in space, and during the downtime the reactors can power the craft. i had a flying, self fuelling nuclear power station for beamed microwave power that took off on thermal rockets (4m/s), and cruised on plasma (~1m/s). could go nearly anywhere. though weighing 600 tons made it a pain in the arse to land.

on topic: given the focus on career mode and research, wouldn't it make sense to have some early parts that get eclipsed by better parts later on? if you have to spend 3k science to unlock engine x, why does it need to be balanced against engine y you start off with? there is no need for a spear-man to be competitive against a battle tank in civ.

[NathanKell]

There's another factor no one's been mentioning so far: In real life, nuclear engines use LH2. LH2 is..."not dense" is putting it mildly. You need 14x as much of it for the same mass of fuel. That imposes its own penalty (in terms of drag and heavier tanks and unwieldyness).

Oh, and regarding LV-N thrust: KSP's liquid rocket engines have poor (1/3 to 1/4 the) TWR, compared to real life. The LV-N, however, has as-good-if-not-better. (Let's not talk about the insanity that is the KSP ion engine, off by about 3-4 orders of magnitude!). So while it's true that in real life they had 3-800kN NTRs under development, they also massed 6-15 tons.

[Jett_Quasar]

There are lots of compromises in the game that make it playable yet still remain fairly realistic. The nukes are a great addition to the game in my opinion, and aren't nerfed any more than anything else.

JR

[Streetwind]

L-VN is one of few examples of reverse nerf in game actually. Real life NERVA had superior thrust, and so would be very overpowered in the game - there would be no point in using any other engine outside of the atmosphere.

Oh, and regarding LV-N thrust: KSP's liquid rocket engines have poor (1/3 to 1/4 the) TWR, compared to real life. The LV-N, however, has as-good-if-not-better. (Let's not talk about the insanity that is the KSP ion engine, off by about 3-4 orders of magnitude!). So while it's true that in real life they had 3-800kN NTRs under development, they also massed 6-15 tons.

I found it extremely hard to actually find useful info on the NERVA project engines, since most websites just blindly parrot the never built NERVA-2's projected on-paper stats, likely a case of someone who doesn't understand what they're looking at copying info from someone else who didn't understand what they were looking at. That's what causes popular misunderstandings like the one of Scotius here. That's just not the case. No such engine was ever built. It could maybe have been built if the program hadn't been terminated, but that still doesn't guarantee it would have posted those exact specifications.

There was only one single NTR in the entire world ever built in full flight configuration - please correct me if I'm wrong here, Nathan - and that was the NERVA-EX. And stats for that thing are incredibly hard to find on the internet. I bet it's in some written records and books somewhere, but I don't have access to those. I still spent a good two hours looking this up a few months ago and came up with something like... I don't remember too well, 330 kN of thrust with 11.9 metric tons of weight? With Isps reported anywhere between 750 and 850. Sounds about right, because I calculated its TWR to be around 2.8 back then. Which means that both Isp and TWR are actually pretty much spot on those of the LV-N. If the numbers I found are correct, then the LV-N is (intentionally or unintentionally) modeled right off of the NERVA-EX.

Which of course leaves the possibility of me having found false information. God knows there's more than enough of that out there. Nathan, you seem to have more engines memorized than a teenage girl knows Justin Bieber songs - can you confirm or deny my findings above? I'm really interested in a second opinion.

[Renegrade]

TL;DR - Yes, sometimes LV-Ns are better, but not always. Don't spam them.

You also have the nuclear tugs, this is moving a mobile base to the Mun, the orange tank is fuel to refuel the base and other projects at Mun. Tug returns to LKO to be refueled.

That's a lot of solar panels~

And I agree that the LV-N is overused, however if you start getting up to some payload size and some dV requirements its pretty much the only way to go.

Take an 18 ton payload, one LV-N and 720 fuel give you 2500 m/s and 0.2 in twr. two LV-N gives 2300, 0.38 twr and stage weight 13.7 ton.

Replace the two LV-N with a T-30 and you need 1800 fuel to get 2300 m/s, weighing 23.7 ton or 10 ton heavier. The Poodle is a bit better but you save less than 90 fuel.

Let us reduce the dV requirements, to 1200 or and things looks better for the chemical engines, a T-30 and 720 fuel gives that and weight 10 ton, 360 and a LV-N weight 7 and have more dV.

These numbers don't match up exactly exactly with my own figures past the first example. Are you including tankage and engine in the 'payload' mass?

Here's my numbers:

1xLV-N, 18 ton payload. Total dry mass is 21.25 (including 1 ton of tankage using stock tanks and 2.25 tons of engine). +8 tons for fuel. TWR is 0.209, giving a total of 2507 dv for 29.25t.

2xLV-N, Dry=23.5t, Fuel=8/1t (fuel mass/tank mass), TWR=0.401, DV=2046, Total=29.25t

1xLV-T30, Dry=21.75, Fuel=20/2.5, TWR=0.558, DV=2142, Total=39.25t

1xPoodle, Dry=22.4375, Fuel=19.5/2.4375, TWR=0.568, DV=2163, Total=39.5t

(Going loosely on the configurations you gave, 720 fuel being 8t, 1800 fuel being 20t, adjusting other non-nukes to match ~2100dv).

Here's a couple of alternatives, starting with the 'what is twr?' configuration, moving on to the 'let's spam nukes' and finishing with some TWR oriented designs

1xLV-909, Dry=20.75, Fuel=18/2.25, TWR=0.140, DV=2160, Total=36.5t

4xLV-N, Dry=28, Fuel=10/1.25, TWR=0.699, DV=2117, Total=37t

8xLV-N, Dry=37.6875, Fuel=13.5/1.6875, TWR=0.988, DV=2139, Total=49.5t -- #worsethanskipper #lv-n_spam

1xSkipper, Dry=23.75, Fuel=22/2.75, TWR=1.541, DV=2154, Total=43t -- I am good friends with Mr. Oberth

1xMAINSAIL, Dry=27.25, Fuel=26/3.25, TWR=3.058, DV=2143, Total=50t -- I AM HERMANN OBERTH

40x48-7S, Dry=25.125, Fuel=25/3.125, TWR=2.603, DV=2150, Total=47t -- guh #nerf_plox (a 50-engine version of this is better than the mainsail edition while still being lighter)

(dv values are truncated due to lazy, and also for safety)

Oh gods, Dave Kerbin infected me with the spreadsheet.

Anyhow, the usefulness of these different configurations are variable. The heavy TWR designs may be bigger and heavier for the same-ish dv, but they'll be able to exploit the Oberth effect more. That's something that can't be universally calculated. In high Minmus orbit, it means little, but in low Eve orbit, means quite a bit.

Note how that as the LV-N clusters are approaching 1 TWR, they're sucking eggs, to put it mildly. Just as heavy as chemical designs overall, yet with worse TWR and same-ish DV. I can assure you that the 8x version will cost a vast fortune compared to the Skipper edition. Plus the fairings will probably kill an engine or two.

The low TWR NTRs are better than the low TWR chemical engines, so unless you're worried about nuclear proliferation or irradiating your kerbals (pfft, they enjoy radiation. It's an essential part of their diet in fact), you're better off there.

The correct answer for deep space / interplanetary isn't always "SPAM NUKES". That sort of suggests to me that they ARE balanced, or relatively so.

Personally, I generally don't use LV-Ns due to their awkwardness, low thrust, and now cost (although I've always been kinda conscious that it would be much more expensive than a chemical engine and that they use a rare/dangerous resource which may be in rather limited supply), but in the few instances where I have, they generally propelled small, 1.25m, single-engine 7-8km/sec one-man re-usable explorer vessels around the Kerbol system..

@NathanKell - Another thing about low density fuels is the size of the tankage. I built a RealFuels-powered lander with LH2/Lox, and it was huge, floppy, and awkward (and had a fair amount of boil-off before reaching Mun orbit) compared to the heavier but compact Aerozine 50/n2o4 design I finally settled on. I have to say I enjoyed that playthrough, although LONG FUEL LIST IS LONG. Hehe.

@Dangerous_Beans - I wish that later parts DID eclipse earlier ones. Note how we're talking about LV-T30s and LV-Ns in the same stats block. I'd love to see a better progression with upgrades to existing parts from tech nodes (ala RealFuels) and just straight up replacements (like T15/T30 in BTSM) scattered throughout.

@m4v - Haha, good point! Not really reassuring is it? Oh well, at least it wasn't "found lying by the side of the road"...

@all regarding NTR mass - I think a lot of the published stats include the weight of the associated tankage for the imaginary Saturn S-N...

[juanml82]

There's another factor no one's been mentioning so far: In real life, nuclear engines use LH2. LH2 is..."not dense" is putting it mildly. You need 14x as much of it for the same mass of fuel. That imposes its own penalty (in terms of drag and heavier tanks and unwieldyness).

Oh, and regarding LV-N thrust: KSP's liquid rocket engines have poor (1/3 to 1/4 the) TWR, compared to real life. The LV-N, however, has as-good-if-not-better. (Let's not talk about the insanity that is the KSP ion engine, off by about 3-4 orders of magnitude!). So while it's true that in real life they had 3-800kN NTRs under development, they also massed 6-15 tons.

Well, liquid fuel rockets also use LH2. Now, my understanding is that the ratio of fuel and oxidizer in the game is very wrong, but AFAIK, there is no reason to think that a Mainsail and a LV-N should use different fuel.

As for the ions, it's a matter of gameplay. They are almost useless as they are. IMHO, they need to change the mechanics for ions to work. They have to be used while the ship is on rails and with a different system than maneuver nodes.

[Streetwind]

Well, liquid fuel rockets also use LH2. Now, my understanding is that the ratio of fuel and oxidizer in the game is very wrong, but AFAIK, there is no reason to think that a Mainsail and a LV-N should use different fuel.

As for the ions, it's a matter of gameplay. They are almost useless as they are. IMHO, they need to change the mechanics for ions to work. They have to be used while the ship is on rails and with a different system than maneuver nodes.

You misunderstand. Chemical rockets can use LH2/LOX, a mixture which burns (detonates, in fact) upon igniting it.

Nuclear thermal rockets use LH2, period. It's just a gas. It does nothing. It merely gets piped through the reactor, where it picks up heat, thereby cooling the reactor and taking its heat energy into itself. The gas is then vented out back through a nozzle. There is no combustion happening.

[Red Iron Crown]

You misunderstand. Chemical rockets can use LH2/LOX, a mixture which burns (detonates, in fact) upon igniting it.

Nuclear thermal rockets use LH2, period. It's just a gas. It does nothing. It merely gets piped through the reactor, where it picks up heat, thereby cooling the reactor and taking its heat energy into itself. The gas is then vented out back through a nozzle. There is no combustion happening.

A decent approximation of this in KSP could be done by changing the LV-N to use Liquid Fuel only, without oxidizer.

[ferram4]

I'd argue that KSP's rockets don't use an LH2 + LOX mix based on not only the wrong mixture ratio, but also the large density of LF. It's probably something closer to kerosene + LOX in density (in actuality, it's slightly denser than kerosene), though the lack of boiloff makes me somewhat partial to a mixture of either UDMH, Aerozine 50, or UH 25 as LF with NTO as O, since there is no boiloff at all. Kerosene, UDMH, Aerozine 50 and UH 25 are not the kind of things you use for nukes because the molecules are so heavy (which reduces exhaust velocity). Of course, lighter molecules also means lower desnsity.

As long as the NTRs with the current stats run on LF with the current density of LF, they will have the performance of running on pure LH2 but using fuel with the density of kerosene, which basically means there are never any drawbacks to using them.

[KerikBalm]

Drawbacks: poor TWR, high weight....

Other engines in the game probably use LH2, given the ISPs appraochign 400... should we nerf them too?

http://en.wikipedia.org/wiki/Project_Timberwind

Sadly, this is unsourced, and I think the numbers are switched on the diameters...

Vacuum specific impulse: 1000 s

Thrust to Weight Ratio: 30

Note, that LH2 doesn't need to be used, you could use a heavier gas (it gets you more thrust too!, just less ISP)

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Solid_Core

Going to a closed cycle gas core would get ISPs >1,500 s

One could then easily go with CH4 or NH3, and still get the ISP desired (and a pretty awesome TWR)

[magnemoe]

TL;DR - Yes, sometimes LV-Ns are better, but not always. Don't spam them.

These numbers don't match up exactly exactly with my own figures past the first example. Are you including tankage and engine in the 'payload' mass?

Here's my numbers:

1xLV-N, 18 ton payload. Total dry mass is 21.25 (including 1 ton of tankage using stock tanks and 2.25 tons of engine). +8 tons for fuel. TWR is 0.209, giving a total of 2507 dv for 29.25t.

2xLV-N, Dry=23.5t, Fuel=8/1t (fuel mass/tank mass), TWR=0.401, DV=2046, Total=29.25t

1xLV-T30, Dry=21.75, Fuel=20/2.5, TWR=0.558, DV=2142, Total=39.25t

1xPoodle, Dry=22.4375, Fuel=19.5/2.4375, TWR=0.568, DV=2163, Total=39.5t

(Going loosely on the configurations you gave, 720 fuel being 8t, 1800 fuel being 20t, adjusting other non-nukes to match ~2100dv).

Here's a couple of alternatives, starting with the 'what is twr?' configuration, moving on to the 'let's spam nukes' and finishing with some TWR oriented designs

1xLV-909, Dry=20.75, Fuel=18/2.25, TWR=0.140, DV=2160, Total=36.5t

4xLV-N, Dry=28, Fuel=10/1.25, TWR=0.699, DV=2117, Total=37t

8xLV-N, Dry=37.6875, Fuel=13.5/1.6875, TWR=0.988, DV=2139, Total=49.5t -- #worsethanskipper #lv-n_spam

1xSkipper, Dry=23.75, Fuel=22/2.75, TWR=1.541, DV=2154, Total=43t -- I am good friends with Mr. Oberth

1xMAINSAIL, Dry=27.25, Fuel=26/3.25, TWR=3.058, DV=2143, Total=50t -- I AM HERMANN OBERTH

40x48-7S, Dry=25.125, Fuel=25/3.125, TWR=2.603, DV=2150, Total=47t -- guh #nerf_plox (a 50-engine version of this is better than the mainsail edition while still being lighter)

(dv values are truncated due to lazy, and also for safety)

Oh gods, Dave Kerbin infected me with the spreadsheet.

Anyhow, the usefulness of these different configurations are variable. The heavy TWR designs may be bigger and heavier for the same-ish dv, but they'll be able to exploit the Oberth effect more. That's something that can't be universally calculated. In high Minmus orbit, it means little, but in low Eve orbit, means quite a bit.

Note how that as the LV-N clusters are approaching 1 TWR, they're sucking eggs, to put it mildly. Just as heavy as chemical designs overall, yet with worse TWR and same-ish DV. I can assure you that the 8x version will cost a vast fortune compared to the Skipper edition. Plus the fairings will probably kill an engine or two.

The low TWR NTRs are better than the low TWR chemical engines, so unless you're worried about nuclear proliferation or irradiating your kerbals (pfft, they enjoy radiation. It's an essential part of their diet in fact), you're better off there.

The correct answer for deep space / interplanetary isn't always "SPAM NUKES". That sort of suggests to me that they ARE balanced, or relatively so.

Personally, I generally don't use LV-Ns due to their awkwardness, low thrust, and now cost (although I've always been kinda conscious that it would be much more expensive than a chemical engine and that they use a rare/dangerous resource which may be in rather limited supply), but in the few instances where I have, they generally propelled small, 1.25m, single-engine 7-8km/sec one-man re-usable explorer vessels around the Kerbol system..

@NathanKell - Another thing about low density fuels is the size of the tankage. I built a RealFuels-powered lander with LH2/Lox, and it was huge, floppy, and awkward (and had a fair amount of boil-off before reaching Mun orbit) compared to the heavier but compact Aerozine 50/n2o4 design I finally settled on. I have to say I enjoyed that playthrough, although LONG FUEL LIST IS LONG. Hehe.

@Dangerous_Beans - I wish that later parts DID eclipse earlier ones. Note how we're talking about LV-T30s and LV-Ns in the same stats block. I'd love to see a better progression with upgrades to existing parts from tech nodes (ala RealFuels) and just straight up replacements (like T15/T30 in BTSM) scattered throughout.

@m4v - Haha, good point! Not really reassuring is it? Oh well, at least it wasn't "found lying by the side of the road"...

@all regarding NTR mass - I think a lot of the published stats include the weight of the associated tankage for the imaginary Saturn S-N...

My calculations was for the transfer stage without the 18 ton payload, done with mechjeb in the vab.

And you are right about twr close to 1, in this setting the LV-N is hopeless, you will not want it for an Tylo lander as an example. An exception might be an hybrid engine on a large lander where the LV-N is used alone for the deorbit and circulate burn

Hybrid engines also have other uses, my best example is the Kethane miner who mine Minmus then return to LKO, it has to have enough trust to take off from minmus, fully loaded, then burn back to LKO, and go back then the fuel is delivered. two LV-N and a skipper works nice here, Skipper is used for Minmus takeoff and perhaps the burn to LKO if you don't want to wait, on return and landing on Minmus the ship is light and can run on nuclear alone.

[DannySwish]

I don't know if this has been said yet, but another way that the current LV-N's could be considered as "cheaty" is that they use regular old LiquidFuel, rather than actual nuclear fuel. Most of the time, this probably wouldn't make a big difference (unless the use of real nuclear fuel requires certain equipment, weighs significantly more, etc.), but could be considered an unrealistic benefit to ships that use other regular boosters, since you can transfer the fuel between these without a problem.

[magnemoe]

I don't know if this has been said yet, but another way that the current LV-N's could be considered as "cheaty" is that they use regular old LiquidFuel, rather than actual nuclear fuel. Most of the time, this probably wouldn't make a big difference (unless the use of real nuclear fuel requires certain equipment, weighs significantly more, etc.), but could be considered an unrealistic benefit to ships that use other regular boosters, since you can transfer the fuel between these without a problem.

Think an nerva can run a long time as in months before you have to change the fuel rods, yes this is burn time and it would never be an issue in KSP, probably not in real world either unless you used it for a weekly shuttle to moon for decades.

[Sky_walker]

TL;DR - Yes, sometimes LV-Ns are better, but not always. Don't spam them.

Someone made a comparison charts of the engines while ago and it came out that LV-N is better for vast majority of test cases. Like: 99% of cases if you're making an interplanetary flight.

[KerikBalm]

Basically, there are only 4 engines you should use in the game if part count and cost are not an issue:

Turbojets: for use in the atmosphere of Kerbin/Laythe

48-7s: for use when high thrust is needed

LV-Ns: for when a high TWR is not needed, but you do need decent amounts of thrust

Ions: for when a very low TWR is acceptable.

All the other engines have very marginal uses if part count is not an issue

[Red Iron Crown]

I don't know if this has been said yet, but another way that the current LV-N's could be considered as "cheaty" is that they use regular old LiquidFuel, rather than actual nuclear fuel. Most of the time, this probably wouldn't make a big difference (unless the use of real nuclear fuel requires certain equipment, weighs significantly more, etc.), but could be considered an unrealistic benefit to ships that use other regular boosters, since you can transfer the fuel between these without a problem.

Nuclear engines can use regular rocket fuel (or any other fluid, even water) for propellant. They additionally use fissionable material in the engine core, but that lasts for years or decades. They're different from chemical rockets in that the propellant doesn't provide the energy, just reaction mass.

Someone made a comparison charts of the engines while ago and it came out that LV-N is better for vast majority of test cases. Like: 99% of cases if you're making an interplanetary flight.

Tavert's mass optimal engine charts. I have this in my bookmarks and refer to it regularly, it's a great resource if you're trying to maximize efficiency.

[r_rolo1]

Basically, there are only 4 engines you should use in the game if part count and cost are not an issue:

Turbojets: for use in the atmosphere of Kerbin/Laythe

48-7s: for use when high thrust is needed

LV-Ns: for when a high TWR is not needed, but you do need decent amounts of thrust

Ions: for when a very low TWR is acceptable.

All the other engines have very marginal uses if part count is not an issue

So exactly what would you use in a dense non-oxigenated atmopshere around a planet with bigger grav pull than Kerbin? Turbojets do not work in those conditions and all the others you mention are quite unsuitable ...

[Red Iron Crown]

So exactly what would you use in a dense non-oxigenated atmopshere around a planet with bigger grav pull than Kerbin? Turbojets do not work in those conditions and all the others you mention are quite unsuitable ...

Check tavert's charts linked in my post above. It depends on the desired delta-V and payload mass, but the answer is likely the 48-7S or aerospike, or the SLS engines when you get to big payloads *and* high dV requirements. The aerospike is a specialist engine really only all that useful on Eve for larger payloads.

[r_rolo1]

My comment was a bit tongue in cheek, but thanks for the tavert charts link But TBH I am somewhat surprised that the 48-7S still gives a fight in that regard ... that is a sure sign that the engine needs rebalalncing. And it is a good thing that the aerospike is a specialist engine nowadays... I assume you still remember the times where all you saw was aerospikes and nuke engines , because 1 aeropsike could get 10 FT-L 400 and a mk1 capsule out of the launchpad to LKO if you dropped empty tanks in between