Rethinking the Nuke -- for real this time

[Tankman101]

I'd also prefer if there was a dedicated nuclear propellant, and the ability to tweak a tanks' contents as needed. One might even call it "Hydrogen" and make it a lot less dense than fuel... I think that, too, would be a lot easier to sell than the half-empty tanks we have now, even if it was objectively just as bad or even worse. I'll add that to the OP if you don't mind.

Honestly I think it's fine that we have the abstraction of liquid fuel, and adding a single fuel for a single engine is a bit silly (Though I can accept it with ion engines). While the normal engines may have an ISP nearish to Ker-LOX, the fuel cells that have been added seem to imply Hydro-LOX. In either case, the use of liquid fuel is fine for the games purposes. The game really needs switchable fuel tanks though.

[Brotoro]

Let's look at some renditions of engineering designs for NTR spacecraft (ranging from the 1968 Integrated Manned Interplanetary Spacecraft design to more modern examples)...

One thing you will notice in common with all of these designs is a lack of radiator wings. Because the NERVA and related NTR engine designs DON'T NEED THEM.

If people want to add 'radiator' wings or panel-van bodies or whatever to their spaceships because they think it's cool, I'm fine with that (I don't like to rain on other peoples' parades) ...but if we can't build a proper-looking NTR-powered spacecraft, then KSP has implemented something incorrectly.

[NonWonderDog]

Just to add, the real nerva had 333.6 Kn of thrust, 850 isp, weighed 34 metric tons when empty, and was about 10 m wide and 40 something m long. So maybe moar thrust for our tiny scaled down nerva wouldn't be unrealistic.

Common mistake: you're quoting the numbers for an entire Nerva-driven rocket stage, including a hydrogen tank the size of the space shuttle's. The bare-bone engine would have been more like 7t.

Not only that, but there wasn't really anything like "the real NERVA." There were about a dozen completely different ground test engines, but they never built a free-standing flight-weight version. The weight you sometimes see quoted for, say, NERVA XE-Prime (18 tonnes) apparently includes a good amount of mounting and support structure, tankage, instrumentation, and extra-heavy radiation shields. Flight weight was supposed to be around 7-8 t for the 245 kN, 847 Isp, 2.6 m diameter final design.

BUT, I have no idea if the 7-8 t includes radiation shielding. I think the ground shields for testing were 4.5 t, but any flight shield would be much lighter.

Modern alloys are lighter. The 25 klbf PEEWEE-derived engine in the NASA reference Mars mission was supposed to weigh something like 3.3 t, produce 111 kN of thrust at 900 Isp, and have a 1 m thrust chamber (and a 1.7 m diameter engine bell). That's perfectly Kerbal-sized, but nothing like that was ever actually built.

Sources are hard to come by, but here's a 20 year old NASA review: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930017729.pdf

Edited May 18, 2015 by NonWonderDog

[Wolf Baginski]

One big political difference: the Kerbals don't seem to have ever built nuclear weapons. I was around in the cold war days, and there is evidence of a form of PTSD developing from being alive then. Not common, I misunderstood some things at the time and that maybe insulated me, but I lived a couple of miles from where there were live missiles on open launch pads, and nuclear-armed bombers were always flying overhead from nearby airfields.

We don't want the politics in the game, or getting argued about in the forums, but I had to live half my life under the threat of nuclear obliteration. Here we are, a quarter century after the Berlin Wall came down, and that threat hasn't really gone away.

I can accept a bit of reputation loss if an LV-N ends up as debris on Kerbin's surface, and the same idea for an RTG. But I don't want the Kerbals to be weighed down with the history we have.

[KerbonautInTraining]

On Monday, May 18, 2015 at 2:48 PM, Brotoro said:

One thing you will notice in common with all of these designs is a lack of radiator. Because the NERVA and related NTR engine designs DON'T NEED THEM.

(I can't get rid of the images on mobile)

True, but their fuel tanks need them. I realized this the other day. By using half-empty rocket fuel tanks and radiators you can (very roughly) simulate the struggles of storing liquid hydrogen.

[Coga19000]

Hmm, though your intentions are good, I do have some objections on most of the suggestions posted here:

1. Making the engines heavier and more powerful is currently my favourite idea as of yet. As log as the two values balance themselves so the NERVs don't suddenly become good enough for lifters, it seems like the most logical, simple, and effective idea, which will also impact all game modes.

2. I don't find the game mode idea too intuitive; adding a game mode for a single part/set of parts (even as a simple option) seems too much, and I don't see any reason why the two futures cannot complement each other in some way. If anything, at least make it an option to disable either nukes or ISRU, or both, but without making them mutually exclusive.

3. Both the spooling and the unthrottleability are not only generally bad ideas for gameplay in paper, but also will not affect too much in practice outside of the aforementioned landers (spooling doesn't matter when you're in orbit, and throttlability is even less impactful for what is the least powerful liquid engine in game)-and I haven't even seen too many nuclear landers around. And it doesn't even make sense compared to real-life: fission reactors are usually on all the time, and are notoriously hard to completely stop and then restart in such a compact size as a nuclear engine. If one were to actually restart a fission reactor during flight, it would take quite some time to start up, which would be plain annoying for everyone. And since NERVAs do not rely on bipropellant production, but on ventilation of fuel over the reactor, they are in fact the ONLY currently conceived engine that is both FULLY throttleable and IMMEDIATELY throttlable.

4. Reputation costs seem logical to me, but a realistic and integrated addition would be much more complicated than that; the initial reputation penalties should happen for the first flights carrying a nuke, with diminishing penalties, until an X amount of flights; afterwards, penalties would incur for every time a nuke was fired in the atmosphere (possibly with also diminishing results) and a HUGE one every time a nuke blew up in the atmosphere.

My opinion on the fact is not that overheating is a bad idea, but that it is implemented in reverse of how it should be. What I think should happen, is that the engine should not overheat when fired (since heated fuel is expelled from the vehicle), but instead when it is NOT fired, where the reactor keeps heating up, but the heat remains contained in the vehicle. For anyone not interested in radiator spam, two options should be added:

1. A new toggleable option, Active Regenerative Cooling. During that, the engine's waste heat is reduced (but not completely), but a small trickle of fuel is lost over time, without producing thrust. (Alternatively, you could also use Oxidizer for the process, which would solve the half-full tank corundrum.)

2. Shutting down the engine would cool down the reactor, which would essentially spool down the engine, but differently than the way suggested. A shut engine will still produce waste heat, but at a much smaller rate (smaller than regenerative cooling, probably). The process will not be instantaneous, but will take some time (from 5 to 15 minutes, or even more). Activating the engine would be the same process backwards, during which the engine would either not work or fire at a reduced specific impulse. This way, the engine on a vessel without a crap load of radiators would in fact have to spool up/down the engine, but only through interplanetary transition intervals, not through every single time the engine is fired.

Oh, and if a constant power generation output is added to the engine (which I'm supportive of), then that output should either stop or greatly diminish.

[jwbrase]

11 hours ago, Coga19000 said:

 

 

3. Both the spooling and the unthrottleability are not only generally bad ideas for gameplay in paper, but also will not affect too much in practice outside of the aforementioned landers (spooling doesn't matter when you're in orbit, and throttlability is even less impactful for what is the least powerful liquid engine in game)-and I haven't even seen too many nuclear landers around. And it doesn't even make sense compared to real-life: fission reactors are usually on all the time, and are notoriously hard to completely stop and then restart in such a compact size as a nuclear engine. If one were to actually restart a fission reactor during flight, it would take quite some time to start up, which would be plain annoying for everyone. And since NERVAs do not rely on bipropellant production, but on ventilation of fuel over the reactor, they are in fact the ONLY currently conceived engine that is both FULLY throttleable and IMMEDIATELY throttlable.

The post above you is a necropost, so the thread is pretty much dead, but I don't think this is quite true: you can instantaneously vary the mass flow rate by quite a bit, but you have to worry about over or under cooling the reactor by doing so. If you dump on too much propellant without increasing reactor power first, you'll lose ISP and thrust for a given mass flow setting as the reactor re-equilibrates at a lower temperature. And if you suddenly yank back throttle without reducing reactor power, you'll briefly see an increase in ISP and the thrust delivered at the new throttle setting will climb for a bit (unless you've yanked back all the way to zero mass flow), until the reactor overheats and the engine fails.
 

[KerbonautInTraining]

6 hours ago, jwbrase said:

The post above you is a necropost, so the thread is pretty much dead

Oh, crap. I was searching randomly and found this. Forgot to check the date.

[Nich]

 I 100% support  spool down and if the fuel is cut then the heat will be bled off to the craft.  I like the idea of a delayed start but this just adds annoyance and anyone can preheat there Nuke 30 seconds or a minute before the burn. An ISP decrease when  spooling down makes sense and is a really good idea but I don't know if it's worth the effort of implementation 

[magnemoe]

On 17.5.2015 at 1:05 AM, Kelderek said:

I agree that the overheating is lame - especially when you consider the abysmal TWR that leads to extra long burn times anyway. I can think of a couple more options for balancing:

1. Adjust the funds cost. Make it 10x more than any other engine.

2. I don't know enough about how the engine is meant to work, but to me the word "nuclear" implies radioactive which leads me to think of Uranium as a possible resource. This is a whole different can of worms, but I think it would be cool if resources could be used as building materials. If that were possible then you could have uranium required to construct the engine and the usual LF to fuel it.

Apart from that, I also like your idea to just make it much more massive.

Make it heavier, might as well make it an 2.5 meter part but with more trust, might just as well increase its TWR.
Now add build in radiators on its sides, to increase the width so its diameter is at least 2.5 meter. this to make it harder to mount it radial, other option is to have it radiate a lot of heat so you don't want anything close. 
Increase the cost a lot. 

Now its not an good selection for an lander but better for huge interplanetary ships and larger mun/ minmus- kerbin tugs and tankers. 

Reputation penalty then crashing on Kerbin is ok but require more coding for a special case unless they can use lost kerbal functionality. 
Spooling would also be nice but require more coding too. 
 

[Buster Charlie]

When are they going to nerf jet engines ? They don't seem balanced when compared to gasoline piston driven propellers...

 

I don't honestly understand why the nuclear rockets need to be "balanced " by making them artificially bad.

Being a sandbox game, if players feel they're not fun, don't use them. 

 

I know mech jeb is a mod, but people decide not to use it, some people decide to use it. Nobody said "let's make mechjeb 25% of the time purposely destroy the rocket to make it balanced.

[Laie]

In before the lock: this thread may be old, but not *that* old -- things have barely changed in the time since.

I find it interesting that the newcomers seem to approve of the "just make it bigger and heavier" approach, which wasn't exactly greeted with enthusiasm the first time around.

[RP1IsSuperior]

when NTRs were actually appreciated as the best way to get to the inner planets

[Staticalliam7]

On 2/22/2016 at 5:02 PM, Coga19000 said:

Shutting down the engine would cool down the reactor, which would essentially spool down the engine, but differently than the way suggested. A shut engine will still produce waste heat, but at a much smaller rate (smaller than regenerative cooling, probably). The process will not be instantaneous, but will take some time (from 5 to 15 minutes, or even more). Activating the engine would be the same process backwards, during which the engine would either not work or fire at a reduced specific impulse. This way, the engine on a vessel without a crap load of radiators would in fact have to spool up/down the engine, but only through interplanetary transition intervals, not through every single time the engine is fired.

reminds me of how in SR2 nuclear engines take forever to spool up to full thrust

[Arugela]

Would it be unrealistic for nukes to not just be engines, but also have the ability to act like batteries and enhance other engines instead of actually firing it's own. Not sure how real nukes would have worked. What about a mode where it delivers fuel to other engines. But you need alot. It could be an ISP enhancer. Not worth the weight on it's own, but in the right circumstances it could make other engines more worth using.

It could work where the number of engines it enhances reduces the effective ISP gain. So you have to balance the number of engines set to fire with nukes compared to how many nukes you use. The nukes could go into a special mode, but the engines have to be set to also receive the bonuses so that you can control it. Then you can have nukes fireing in normal mode after helping enhance ISP of other engines. Although I assume the nukes enhancement would only work primarily in low gravity and be reduced heavily by gravity reducing effects.  But it could be good for rapier builds with a single nuke giving the engines more fuel efficiency.

https://www.energy.gov/ne/articles/6-things-you-should-know-about-nuclear-thermal-propulsion

Would it be realistic for any form of nuclear propulsion to enhance a different engine instead of shooting it outside of it's own bell? Since it's more energy dense it could help a different engine type. I assume it could just be diverted out a different hole instead of the bell and shot out a different nozzle. Maybe combined in the bell or similar of a different engine as added propellant or force.

This could allow chemical and nuclear engines with an added fuel efficiency or thrust. In KSP terms you could enhance your RL+LF rockets with LF engines.

If it added 50% ISP at max that would be a huge fuel saver. Not sure if it would make the game to easy though. I wonder how many rockets could be enhanced before it drops efficiency too much assuming any.

and since KSP is at 1/10th scale. What about a 5% gain.

That makes a 295 ISP engine run at 309.75 ISP. That is a realistic/manageable little jump for KSP isp values.

If this were realistic would it effect the ISP/fuel use of LF or LF+OX in other engines? I'm guessing LF+OX plus extra LF fuel use. But I have no idea honestly. Or would the LF not need to be used on the nuke in such a situation/mode?

Edited December 21, 2021 by Arugela

[Stelari]

I truly think it would be pretty unrealistic for nowadays nukes to be used as batteries

[Arugela]

I prefer the funny option. Make it super tiny like a hand popper. then make it release smelly gas like a small fart. Try to remove the radiation problem! 8D Perfect nuke.

NVM, just figured it out. You meant the nuke engine. ><

I put up a weird idea about making it enhance other engines for 5% gain in isp up to so many engines or ISP or whatever is appropriate. This would be a mode. leave the rest the same. In this mode it does or doesn't work normally(probably not working itself.Although I could see either.(maybe it reduces it's own efficiency or is given a fuel limit that drains.)) and enhances so many other engines. These other engines must be set to accept the enhancement for fine control. Then you can turn this off to use them normally at the appropriate height.

I saw a video where it sound like it was 50% more efficient than normal engines and could enhance them in real life. Given this game is 10% of real life reduce it's enhancement to 5% gains in ISP.

I haven't played in a while but I have always liked the nukes balance. Just make it do more. Or give it a limited internal radioactive fuel. This could also go with the existing fuel. then LF enhances LF+OX. This could slightly help a lot of ships I've made with multiple engine types. Or enhance staged stuff while it's still staged.

I think it made 295 ISP 310 ISP approximately. It's a nice small bump for launches and even longer burns for things like rapiers and all other engines. I think the 340 becomes 357 and 350 becomes 367.5, but this all comes with varied weight gains from the nuke obviously. So, limited gains depending on the engines at hand.

Nuke  would not enhance other nukes. Unless that is realistic.

NVM, I just posted this 2 posts up. DERP.

Maybe for the internal nuclear fuel. Make a 1/10 of the normal realistic amount and have all actions remove more fuel. The more you enhance more engines the faster this reduces it's internal stuff and possibly produces huge amounts of heat. This could add heat to whatever is realistic. Itself, other engines, the entire ship starting from it's starting position(a few cheaty methods could be used to avoid though.)

You could also add more heat if it's realistic and radiation on crashes you ahve to pay to clean up. They could be fairly expensive and either go away or just block off an area for long term usage. Making their use risky on the ground on kerbin.

BTW, how much would a real nerva's nuclear fuel last and be usable. How would that translate into this game? If it had internal fuel to go with the LF how would it work? IE the part heating up the hydrogen in this video. I'm assuming the hydrogen is LF by comparison.

 

Edited January 1, 2022 by Arugela