Some Shower-thoughts on the LV-N Nerf

[MalfunctionM1Ke]

Hi Guys,

yes those are rather strange thoughts I am having while showering but I want to share them nontheless.

http://wiki.kerbalspaceprogram.com/wiki/LV-N_%22Nerv%22_Atomic_Rocket_Motor

Here you can find the Wiki-Article about it's real-life counterpart:
https://de.wikipedia.org/wiki/NERVA

Firstly:
The LV-N is essentially a Fission-Reactor that generates great amounts of heat and If you run a gas over it, it will expand and generate some thrust with a very high specific Impulse.
If my idea is correct, then why does the Engine heat up, while you are firing it? There is no combustion process happening and the gas is actually carrying away the heat from the reactor into space.
So in fact the engine should cool down while it is working and heat up while it is not, right?
Maybe there are control rods build into this reactor to give it some more control, other than just fuel-valves releasing the gas into the reactor chamber.

Secondly:
If you think about the fuels that we are using (Liquid Fuel / Oxidizer) you will see that those are actually only placeholders for the vast variety of fuels that are used in the real world.
BUT, the ISP's and Thrust that we are getting out of those fuels in our regular rocket engines do actually tell you that the Liquid-Fuel is something between Hydrazine and Kerosene. [citiation needed (I read that somewhere on here)]
Lets, go with Kerosene for this, because we also burn Liquid Fuel in our Jet Engines.
Kerosene is by far anything but cryogenic and the actualy Nerva-Engine used Liquid Hydrogene because a cryogenic fuel expands tremendously when you run it over a incredibly hot surface.
So my though was... wouldnt it be more efficient to run the LV-N with the Oxidizer we use in the game? Because that has to be liquified Oxygen at -200°C while Kerosene would freeze at around -50°C
Not sure if the Oxygen wouldnt eat away everything it touches at those temperatures.

Would be great if to get some brainstorming on this from you guys
 

[Wallygator]

Agree completely.

The thermal issue is bogus and needs to be fixed pronto.  The ISP was a design decision (I think) and should be reversed to provide a mode realistic value. And the fuel? Why not!

So it's agree x 3. ;-)

However, those shower thoughts... Need to fix that also!

 

Edited December 14, 2015 by Wallygator

[DBowman]

Firstly - I think you are correct here. An NTR would have control rods that damp the chain reaction, when you want to burn you withdraw them and let the chain reaction get going, once it's at operating temperature you start pushing fuel through it, you keep it at optimal temperature by tweaking with the control rods and fuel flow rate, when you are near done with the burn you have to tail off by using the rods to damp the chain reaction and pushing enough fuel to bring the temperature down. I believe they didn't ever 'just shut the engine off' but always 'spooled it down' to cool it safely and as a side effect generating sub maximum thrust that you would have to account for as part of the burn. The engine should never over heat, I think the overheating is there for 'game play balance'.

Secondly - I believe with the NTR the lighter the molecule you are heating the higher the ISP you will get out of it, so you'd expect O2 to get higher ISP than Kero. On the other had Kero may dissociate in the engine to C & H which would be lighter and therefore better than than O2 or O. Also using hot oxidizer might oxidize the engine - I cannot recall if they had plans for coatings etc that would allow using oxidizing fuels. A NERVA running on CH4 has an ISP of 644, kero would be less that that. Again I think they picked the number for 'game play balance'.

[problemecium]

A common mistake made by people in this community is to try and equate KSP's fuels with anything that exists in real life. We can safely claim that the properties of LiquidFuel are similar to those of Hydrazine and Kerosene, and even that they were probably used as a basis, but we shouldn't presume that LiquidFuel needs to be treated the same in any other way.

My hypothesis regarding the current state of the LV-N is that SQUAD evaluated the possibilities and decided that this was a case in which realism had to take a back seat to balance. If a NERVA goes into production in real life, it will in all probability completely outmode all other vacuum engines (save for ion engines on probes) due to its awesome Isp. So anyone running a real space program would, for the purposes of their spacecraft, quit investing in chemical rockets altogether, and they would be reduced to fond memories. SQUAD doesn't want this happening in the game. Some of us would probably like a KSP where we can slap an LV-N on a mothership and instantly get 25km/s of delta-V, but then again we'd probably also like an Unobtanium Hyperengine that gives 10,000 thrust and infinite efficiency. But we must concede that while fun for a few minutes, it would ruin the game.

If immersion-breaking is the problem, I personally think of it this way: the engine contains small nuclear fuel rods that do not generate significant heat individually (they may also be in insulated, thermally-controlled containers). When activated, it moves them all close together and runs propellant over them. Not only do they start to generate heat faster, but because the propellant is thermally conductive and has significant mass, as it rubs around the inside of the engine it heats up the whole thing. It takes some of the heat with it, of course, so we can surmise that a closed-circuit reactor made from an LV-N would simply overheat much faster.
Or perhaps it's actually a fusion engine. It only says it's an "atomic rocket motor" in the KSP lore. As discussed earlier, its resemblance, even if intentional, to the real NERVA is no basis for extrapolation. Fusion is "atomic" just as much as fission is. So perhaps the LV-N really contains a fusion reactor that produces huge amounts of heat while running and none while off.

[KerikBalm]

LV-Ns use LH2 for best results.

Squad decided against adding another fuel type and the resulting changes that would be needed (extra tank parts, or something in the VAB to tweak tank contents) which would complicate things (and in their opinion) without really improving gameplay.

Many people have complained about the overheating issue... The fact is that a realy nerva solid core design would have a core around 2700-3000 K when in operation.... but the mass flow would be high enough that heating when thrusting would not be a problem.

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

"A useful refinement is the Bimodal NTR.

Say your spacecraft has an honest-to-Johnny NERVA nuclear-thermal propulsion system. Typically it operates for a few minutes at a time, then sits idle for the rest of the entire mission. Before each use, one has to warm up the reactor, and after use the reactor has to be cooled down. Each of these thermal cycles puts stress on the engine. And the cooling process consists of wasting propellant, flushing it through the reactor just to cool it off instead of producing thrust.

Meanwhile, during the rest of the mission, your spacecraft needs electricity to run life support, radio, radar, computers, and other incidental things.

So make that reactor do double duty (that's where the "Bimodal" comes in) and kill two birds with one stone. Refer to above diagram. Basically you take a NERVA and attach a power generation unit to the side. The NERVA section is the "cryogenic H2 propellant tank", the turbopump, and the thermal propulsion unit. The power generation section is the generator, the radiator, the heat exchanger, and the compressor.

Warm up your reactor once, do a thrust burn, stop the propellant flow and use the heat exchanger and radiator to partially cool the reactor to power generation levels, and keep the reactor warm for the rest of the mission while generating electricity for the ship.

This allows you to get away with only one full warm/cool thermal cycle in the entire mission instead of one per burn. No propellant is wasted as coolant since the radiator cools down the reactor. The reactor supplies needed electricity. And as an added bonus, the reactor is in a constant pre-heated state. This means that in case of emergency one can power up and do a burn in a fraction of the time required by a cold reactor."

 

For balance I would propose the following:

Like the ISRU and drills, the LV-N has an optimal core temperature (3,000 K), but starts cold. (core temperature being different from internal and skin temperature).

When you run the engine, the internal temperature goes down, and the thrust&Isp is a factor of the core temperature.

If you have no radiators, and don't use the engine until the core temperature reaches 3000k, the core to internal heat flow is high enough that the nerva will explode (or possibly the part it is attached to).

Therefore to use it without radiators, you must start your burn before the core has warmed up all the way (lets say around the time it only produces 400 Isp?) and you must start spooling down the reactor to a lower temperature before you finish your burn... so that at the start and end of your burn you obtain lower thrust and Isps... and its harder to get precise burns (maybe you supplement with chemical engines to finish off the burn precisely).

Or, you need a lot of radiators to keep the engine from exploding as you heat the core to 3000K before a burn, and as you cool down the core after a burn.

Thus the engine's drawbacks would be that you don't get thrust on demand like a chemical rocket, but you need to warm and cool the core after each burn.

You would need lots of radiators for short burns... but for a long continuous burn, you may find it more efficient to ditch the radiator mass, and just start the burn before the core reaches its maximum/optimal temperature.

 

 

Edited December 14, 2015 by KerikBalm

[Streetwind]

3 hours ago, MalfunctionM1Ke said:

Firstly:
(...)
So in fact the engine should cool down while it is working and heat up while it is not, right?

Secondly:
(...)
Not sure if the Oxygen wouldnt eat away everything it touches at those temperatures.

Firstly:

Engines have a module which produces an amount of heat that scales with the thrust output. Engines do not have any other way to do anything with heat.

So it was either this, or no heat simulation for the LV-N at all. Since high heat output generates an interesting engineering challenge for the player, clearly that is the better choice, right?

 

Secondly:

Apart from the fact that KSP oxidizer is not supposed to represent any actual realworld substance - yes, pure oxygen would absolutely corrode the engine away rapidly at those temperatures.

[MalfunctionM1Ke]

Wow, this Thread really got some very interesting facts and ideas.

Thanks Guys, keep it coming if there is more

[Hannu2]

2 hours ago, KerikBalm said:

When you run the engine, the internal temperature goes down, and the thrust&Isp is a factor of the core temperature.

If you have no radiators, and don't use the engine until the core temperature reaches 3000k, the core to internal heat flow is high enough that the nerva will explode (or possibly the part it is attached to).

Therefore to use it without radiators, you must start your burn before the core has warmed up all the way (lets say around the time it only produces 400 Isp?) and you must start spooling down the reactor to a lower temperature before you finish your burn... so that at the start and end of your burn you obtain lower thrust and Isps... and its harder to get precise burns (maybe you supplement with chemical engines to finish off the burn precisely).

This would be very interesting detail in engineering point of view. However, there should be suitable tools for timings for precise maneuvers. I suspect that it would be far too complicated for most players and against Squads oversimplified philosophy. But it would be interesting mod with MechJeb plugin.

I think that it would be impossible to balance real world fuels and KSP's small world. Real ISP values would make KSP too easy. Maybe it would work better with RSS, but it have other problems considering playability. Real nerva would also immediately make all large chemical engines completely obsolete. It would have huge thrust, huge TWR, huge ISP and reasonable price (if not increased by artificial political decisions). I would not like that in KSP. Unlocking the technology is only beginning phase of my missions and I do not want to get an engine which would make gravity as negligible perturbation.

[KerikBalm]

"I suspect that it would be far too complicated for most players and against Squads oversimplified philosophy"

Note really, we could specify a "safe" core temperature, where one can have the LV-N sit at that temperature for a long time without overheating (Say.... 10 minutes). Then you specify a "spool up" and "spool down" time. If at 2000 K (many parts have at a 2000K tolerance.. although some parts like the mk1 pod for example have much lower internal heat tolerance), it won't over heat for 10 minutes, and it takes 5 minutes to spool up/down, its obviousy not going to be a problem to get your core to that temperature, burn when you want (which cools the internal temp), and then abruptly stop the burn and spool down the reactor.

If you want precise burns, you only heat the core to that temperature, and you can use it pretty much on demand, just at reduced Isp.

If you have a long burn,

Before the burn, you heat up the reactor.... you know the spool up time/time to reach 3000k... (5 minutes for example)... and spool it up 5 minutes before the maneuver. Of course, under this system, without radiators, it will overheat before reaching 3000k if you don't run propellant through it... so you watch the overheat bar go higher and higher, and then you start to throttle up to keep it from overheating (or just go full throttle right away), then you don't need to worry about the temperature again until you approach the end of the burn.

When you've got 1-2 minutes left on your burn, you start spooling down the reactor... then you finish the burn at ~500 Isp instead of 800 (or whatever, the numbers are just an example).

Or you carry around enough radiators to handle the heat flux... like with the ISRU, you'll know (or be able to find out though minimal experimentation) how many radiators you need to handle a reactor sitting there idle at 3000k core temperature (although this would change depending on if you are at kerbin/moho/eeloo).

 

Such a system would be of minimal concern on an interplanetary vesse/tug... but it would make them poorly suited for lander engines.

I've already stopped using LV-Ns as lander engines though... I don't like the idea of my kerbals climbing up and down a lander just a meter away from a hot LV-N.

While LV-Ns may not spray radioactive exhaust... the LV-N itself is still something you'd want to stay far far away from

[Randazzo]

These sound like great ideas for mods.

Just sayin'.

[Wallygator]

Having a hi ISP late game engine like a our little KSP nuke is not a game or balance breaking idea.

It was a real-life space programme breaking idea due to bureaucracy and politics.  Kerbal's do not yet have those two things.

It should cost ALOT.  It might even have to be a bit larger.  I might even have a limited duty cycle.  It might need to be later in the tech tree. But it should still represent achievable levels of performance with respect to the physics.  Today, they are disposable debris at the end of a mission - maybe they should become retainable orbital assets cherished by a player one they achieve them in their career.

Afterall KSP is billed as being driven by a sense of real physics (I will not entertain any discussions about the actual words on the website - you know what it says).  What really grinds my metal backside is while the game engine is based on real physics, we have parts (the nuke engine for example) that are not so much.

[A35K]

Very interesting thoughts. In fact, the cooling down while the engine is running is exactly how the thermal rocket nozzles work in KSP Interstellar (which are basically the same principle of the LV-N, except the reactor is external). Unfortunately, you cannot run them on oxidizer only even in Interstellar, but you can use a wide variety of different fuels. I'm not sure how accurate this is, but in the mod, LiquidFuel provide lowest thrust but highest ISP, while MethaLOX provides highest thrust (mainly due to combustion), but this significantly lowers ISP.

I think this system should be implemented in stock, especially, as others have mentioned, the necessity for radiators on LV-N equipped ships to keep them cool when the engine is NOT running, and as you run fuel over it it should cool down. Maybe they could introduce some sort of nozzle heating, which would be the part that actually heats up while the engine is running, but that may be a bit too complex.

Edited December 14, 2015 by A35K

[NathanKell]

Not sure why Oxidizer is assumed to be LOX...

[panzer1b]

What i dont want are some overly complicated mechanics for burns.  I have nothing against some alterations, but i feel that all engines in KSP should be simple, easy to use, and not have any annoying or difficult to use mechanics or said engines are gonna just be rarely if ever used.  Right now (although i know its not realistic), the nuke is fairly well balanced, lowish TWR, very heavy at 3t, and has a heat problem (even if that isnt how real NTRs work), which make it a good choice but not the only viable engine either. 

Regardless of what anyone says, the current state of the game is both fun to play and fairly well balanced with almost every engine having its use, even if its a minor one.  Im not saying there arent engines right now that are either very situational or that i myself have no use for, but in a general sense no engine is pointless, and there are clear advantages/disadvantages of using a nuke compared to say ions or LFO systems.

If the nuke did get any spool-up/spool down mechanics then it should ONLY be with regards to ISP.  It would be cool if nukes started out at a somewhat lowish ISP or thrust when spooling up, and then increase its ISP to the maximum when at operating temperature.  This would make it more realistic in that the nuke is good for its usual useage of long burns, but suffers in both thrust and ISP when cooled down making it bad for short range bursts, unless you preheat it or something (but ofc then we'd need the ability to engage some form of heater perhaps for balance uses electricity or some of it).  This way itd be interesting, either pick a cold start, or use electricity (or something else) to heat it up before the burn.

that said, please no spool-down mechanic, i dont want to have some mechanic that forces preplanned maneuvers and shutting down the engine before hand, especially sine the stock game relies on trial and error, i refuse to have to quickload more then i already have to, and there is no way to preplan a maneuver that advanced.  If we had some method of determining how long the spooldown would ofccur, then id accept it, but we need to have it done in such a way that new players would be able to understand it reliably, and that it doesnt become a utter pain to use that engine in the first place.  Im all for some more advanced mechanics with the nuke, but please make sure that the changes add to the game, make it more fun (and in my opinion realism for the sake of realism is not always fun) and arent just extra tedious or excessively complicated.  As i always say, realism for the sake of realism is not fun, realism when it adds to the game, opens up new options, and adds a challenge that isnt purely tedious or annoying is good, all else better stick to mods and not be forced upon those that do not find 100% realism enjoyable especially since when i tried realism overhaul, i found it to mostly limit my ability tyo go anywhere, and do anything fun per say, so not for me.

[Draconiator]

"Hey Jeb?" 

"Yes Bob?"

"I was thinking we make a rocket fuel derivative out of shampoo.   I dunno, it would save us from bringing our own provisions on missions."

"...While normally I would say yes...I do not like the thought of dowsing my head in rocket fuel.  I may be crazy but I'm not THAT crazy."

[KerikBalm]

Well panzer... the spool down mechanic would only be in place if you don't have radiators... radiators would basically become mandatory for LV-Ns if you want to have power on demand.

Otherwise, if you are OK with ISP and thrust starting low and "spooling up", then I don't see what the problem is if you need to start "spooling down" ~30 seconds before the end of a burn... we do have a burn estimator anyway.

We could make it fairly forgiving... for example, make it safe to use with no spool up or down at 600 Isp and 45 kN.

If you want to use it at 800 Isp and 60 kN, then be prepared to spool up/down, or use radiators.

[Wallygator]

Make the spool mechanic an element of the difficulty settings.  Job done.

[SuicidalInsanity]

With the magic of MM patches something similar could be done - want 'hard mode'? Add the reactor spool MM patch. I've done some preliminary experimenting, and I'm fairly sure that the desired behavior - Core heat mechanic with decreased thrust while the reactor is cold - can be done with ModuleResourceConverter (for core heat production and generating a placeholder ThermalEnergy resource), ModuleEngineFX (uses the ThermalEnergy resource and LF for thrust, with the HeatProduction value set to something like -400) and ModuelCoreHeat (ModuleresourceConverter supplies a steady source of heat while active, ModuleEngineFX's HeatProduction reduces that heat while firing the engine)

[panzer1b]

1 hour ago, KerikBalm said:

Well panzer... the spool down mechanic would only be in place if you don't have radiators... radiators would basically become mandatory for LV-Ns if you want to have power on demand.

Otherwise, if you are OK with ISP and thrust starting low and "spooling up", then I don't see what the problem is if you need to start "spooling down" ~30 seconds before the end of a burn... we do have a burn estimator anyway.

We could make it fairly forgiving... for example, make it safe to use with no spool up or down at 600 Isp and 45 kN.

If you want to use it at 800 Isp and 60 kN, then be prepared to spool up/down, or use radiators.

i think this would be fair.  itd make radiators actually useful for similar vessels to now, and itd be both forgiving enough for those that dont want radiators (or well there will be that penalty), and all itd do for me personally is make me add a small radiator or 2 to a nuke powered ship.

Btw, why would radiators allow instant on demand starting?  Im no expert on NTRs, and all i know is they convert heat from some source (usually a nuke) into kinetic energy by heating propelland as it passes through.  So why would a radiator actually allow one to enable/disable on demand.

Edited December 14, 2015 by panzer1b

[KerikBalm]

Well, in reality, for practical purposes it wouldn't allow instant on demand starting. You wouldn't have the control rods fully retracted so that the reactor is producing maximum power at all times, even if you could cycle enough coolant through it (and then through radiators) to keep it from melting down.
 As in the bimodal example, you would use it to generate electricity, so the core would be "warm" but not "hot", but also not "cold". Spool up/down time would be reduced.

The basic idea is that when you crank up your reactor output, and heat the core to ~3000K, you are generating a lot of heat, and the surrounding structure cannot take it. You need something to take that heat away and cool the surrounding material (in this case, the fissile core can withstand those temperatures... 3000k being about as hot as we can make a core without it melting...)

Also in addition to warming up the core, there is also the matter of the reaction rate not changing instantly.

If your core is at 3000K, but the fission power production is very low... when you start passing cryogenic fluid through it, the temperature will drop until the power output equals the heat flow into the fluid passing through it.

So... when your NTR is producing maximum thrust at maximum Isp, its got a hot core and a high rate of fission.

If you suddenly stopped flowing cryogenic H2 through it... it wouldnt just sit there at 3000K, as it still has a very high power output. It would rapidly heat up when the cryogenic fluid stops flowing. So when you stop passing "liquid fuel" through it, you need to pass something else through it to keep it from overheating while you wait for the fission rate to decreases/the power output todrop/the core to cool.

This something else would simply be "coolant". You would route the coolant through the reactor, then to radiators where it will cool off, and then back into the reactor again.
You could then abruptly shut off the fuel flow through the engine at will, as long as you start running coolant through the engine and radiators as soon as you shut off the fuel flow.

As far as I can figure, the spool up is less of a problem. You start the fission reaction, as fission power increases, the core heats up, when the core is hot, send fuel through it. I'm not sure what the heat flow situation is like... maybe the stuff surroundng the core will melt if the core sits at 3000K? maybe it won't, but the core will exceed 3000K if you don't start running fuel past it. If the former, then you've got to start using fuel before the core is hot enough/ at its maximum power output

If you don't send fuel through it, it will overheat... you can't keep it at a hot temperature and high fission rate for very long before you need to pass something through it to keep it from melting. Radiators would allow you to cycle fluid through it while it sits there hot with a high power output.

 

Basically, you can use liquid fuel to keep the core from overheating until the liquid fuel is gone, or you can "cycle" coolant through the core and then radiators to keep it cool indefinitely.

In reality, a compact reactor of highly enriched fissile material at a very high fission rate would consume enough fissile material and make enough byproducts, that you couldn't just leave it "on" for days on end.

KSP does not model consumption of any fissile material, there's no need for reprocessing of the contents of a LV-N... so in KSP+ this proposal there's no reason you couldn't leave the LV-N on indefinitely with radiators (unless you wanted to go even more in depth, and make LV-Ns consume liquid fuel and "blutonium").

The thing is that those radiators would then be always in use dumping reactor heat, and wouldn't be available for cooling other things. So you'd probably want to turn off your LV-Ns if you have something else on your vessel that needs radiators to cool it when it in use.

Edited December 15, 2015 by KerikBalm

[Commander Zoom]

Or if you want your kerbals to perform an EVA anywhere where they can see the engines, and vice-versa...

(Those things get 'hot' in more ways than one.)

Edited December 15, 2015 by Commander Zoom

[Arugela]

Having my LVN's do double work as electrical generators would be a dream. It would excuse the weight to no end. I could dump all of my little thermic generators and lighten up my vehicle a little and my parts count alot!

They would make perfect mining vessels. All that extra electricity for RCS, drills, and ISRUs.

If they do 5 electricity while outputting thrust how much would they do in electricity mode?

Slow startups for LVNs would be fine too. It's easy to do on the runway or on the way up to orbit etc. It would add some character to launches too.

Edited December 15, 2015 by Arugela

[Tw1]

We have already got spooling for jets, I wonder if the same sort of thing could apply to the Lv-N? It would keep things simple. 

For the trouble, we could be compensated with a slight un-nerf...

[cantab]

The small KSP system actually I think counts against the LV-N, by reducing the delta-V requirements to get anywhere. In real life the NERVA was considered as an upper stage engine for a Saturn V style rocket, I've virtually never seen them used on KSP launchers because you don't need that much delta-V at the low TWR the LV-N does well. Last I checked you could go as far as Duna and the chemical and nuclear options would be similar in overall mass, and that was before the LV-N gained weight.

All this talk about spooling and excessive thermal management seems a bit complicated, I reckon it could get annoying. What I think would be better to make the LV-N stop being so dominant would be the same thing that limits it in real life - it uses liquid hydrogen and that boils off. Fuel boiloff could be implemented quite easily in KSP already I think, just make the fuel tank slowly consume its own fuel.

To that end KSP would of course need to adopt multiple fuels. I don't though want a zillion fuels and tiny differences in mixture like you get in Realism Overhaul. What I think would be about right:

CryoFuel: Has properties similar to liquid hydrogen, including boiling off. Alone, runs the LV-N and the RAPIER in jet mode.
CryoOxidizer: Has properties similar to liquid oxygen, again including boiloff. Burns with CryoFuel in highly efficient chemical engines and the RAPIER in rocket mode.
Liquid Fuel: Has properties similar to kerosene and hydrazine. Alone runs the jet engines, other than the RAPIER, but also now runs the RCS jets and some of the smaller chemical engines.
Oxidizer: Has properties similar to nitrogen tetraoxide. Runs most of the chemical engines along with Liquid Fuel.
Monopropellant: Is now abandoned in the interests of simplicity.

In this vision we've only one more fuel than before, and yet I feel there's considerable new richness added to the game. Engines great for the ejection burn are no longer also suitable for orbital insertion at your destination. Not needing separate monoprop tanks will I think be a welcome simplification of craft designs, and make RCS more practical, at present it seems under-used. ISRU becomes more relevant than ever - the only way you're flying back from Jool on an LV-N is if you mine some hydrogen when you're there. There's the natural option for engines using Liquid Fuel and CryoOxidizer too, which would fit between CFCO and LFO engines in efficiency, and might logically be some of the most powerful in the game.

On the downside, we'd probably need a fuel tank switcher in stock to keep the parts list manageable.

[m4v]

If you want NERVAs to be realistic then you should take their main disadvantage too, such as killing anyone that isn't in the right end of an operating nerva several kilometers away. A nerva is a nuclear reactor that is unshielded, it has a shadow shield for protect its crew, but anyone that isn't the front of the engine is gonna get cooked.

While I bet most realism advocates would welcome such handicap, I don't believe the more casual player base will like that their most efficient rockets can now casually kill the crew of another vessel during a rendezvous, so you have to make other compromises.

Edited December 15, 2015 by m4v