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ClF3 as a replacement for Oxidizer


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The propellants in KSP: Liquid Fuel, Oxidizer. (Sorry Monopropellant, you are for another discussion). The classics, designed to be unambiguous.

However, the KSP community has always tried to put a definition on them. Liquid Fuel was easy - liquid hydrogen. It can run as a high efficiency vacuum engine, or as a jet engine along sea level. As a first stage or in a NERVA. Liquid Hydrogen checked all boxes.

Oxidizer, on the other hand, has stayed in debate if not given up on. Liquid Oxygen feels like a good candidate, but fails in one area: hypergolic-ness (Okay, that’s the dumbest way to say that).

What I mean is that to keep with KSP’s engine style of instant ignition and many starts and stops, the propellants probably have to hypergolic. (If you aren’t a rocket nerd, that means they ignite instantly upon contact). Unfortunately Oxidizers that are hypergolic are quite rare, and almost all of them only are with fuels like hydrazine.

But then I found one: (really a family of propellants, but I settled on one.) fluorine or one of its chemical siblings, chlorine trifluoride. These are hypergolic with pretty much anything (It doesn’t care if it’s liquid hydrogen or flippin concrete!) Also, with proper treatment, ClF3 can be stored in tanks made of many different metals. It even has a high ISP!

Why isn’t this miracle propellant being used? Well it turns out that government officials don’t like exhausts of hydrochloric acid and hydrofluoric acid in the air. Kerbals? They love going to space more than anything! Who cares? Even in the original KSP, there is a green goo on the launchpad. Guess what is a yellowish-green? ClF*.

All in all, a real propellant that has the magical aspects of oxidizer: Instant ignition and good efficiency.

Why not?

flamma est ubi?

*Of course it isn’t ClF3! The concrete would be on fire!

Edited by KeranoKerman
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Actually, KSP's propellants are pretty good match for UDMH and NTO, with monoprop being straight hydrazine. Notice how close their densities are, their OF ratio is nearly 1:1 by volume and they need no cooling or heating to stay liquid. The actual density also seems to match those propellants.

NTRs can run on any propellant, including UDMH. Jet turbines can run on just about anything combustible. Of course, making an UDMH-burning jet engine isn't good for ground crew, but it's Kerbals we're talking about. Liquid hydrogen is just about the worst match for LF you can think of, and ClF3 doesn't fit oxidizer, either. Neither does liquid oxygen, when you think about it. I hope KSP2 does a better job of, at least, separating kerolox, hydrolox and storables.

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2 hours ago, Dragon01 said:

NTRs can run on any propellant, including UDMH. 

You make a lot of valid points, but running UDMH in an NTR definitely can’t make 1000 ISP, or probably even 400. Also, jet engines could run or UDMH, but I doubt the efficiency can barely get to the numbers in KSP.

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Don't look at the stats, they're all over the place. In Making History, all engines run on LFO, despite some being hydrolox, some kerolox, and some storable (though admittedly, Titan engines had kerolox variants, as well). Stats make a half-hearted attempt at replicating the performance values of the real thing, but paired with very much non-real fuels, it's a trainwreck. 

An NTR using UDMH would be somewhere in the neighborhood of 400s of Isp. Jets, on the other hand, work very differently, and would likely have a similar efficiency with UDMH (especially since KSP's engines are modeled on ones that run on kerosene).

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2 hours ago, Dragon01 said:

I hope KSP2 does a better job of, at least, separating kerolox, hydrolox and storables.

I hope it doesn't. Managing different fuel types just adds more busywork without adding any meaningful gameplay or design challenges.

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Two people describe the joys that is ClF3, John Clark and Derek Lowe. Both of their opinions on the substance can be found here: https://blogs.sciencemag.org/pipeline/archives/2008/02/26/sand_wont_save_you_this_time. I think anything that has combustion products that include hydrogen fluoride can really given the foot note treatment and left on the shelf...  preferably somebody else's... in a different building.

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5 hours ago, Brikoleur said:

I hope it doesn't. Managing different fuel types just adds more busywork without adding any meaningful gameplay or design challenges.

You clearly didn't try any mod that actually does it. The design challenges it add are very much meaningful, especially if you implement boiloff, as well (as those mods typically do). That goes double if you want to do ISRU, which will be a big part of KSP2. Also, you learn something about how rockets work. [snip]

Edited by Guest
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There is actually a video out in the wild (YouTube) of some crazy french chemists from back in the day dumping ClF3 into stuff for fun. Plexiglass explodes...

No one is ever going to use ClF3 as a rocket oxidiser any accident you have - even a tiny leak goes from being an inconvenience with a normal oxidiser to being a catastrophic inferno which you can’t extinguish with ClF3. 
 

Also it can’t really be “safely” stored yes copper or steel containers which have been pretreated can contain it “safely” - but if the protective metal fluoride layer ever gets damaged (a small bump might do) you suddenly have an uncontrollable fire. 
 

Basically it always turns into an uncontrollable fire, it’s just a question of how long it takes.

It is fantastically entertaining as a chemical but absolutely useless - though not quite a useless as the fantastically named FOOF (the sound it makes in contact with everything) - dioxygen difluoride.

Source: have phd in chemistry and unhealthy interest in things normal chemists don’t want to work with.

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IIRC, a while back someone did work out what kind of propellant LFO was, based on the ISP, although I don't recall what the answer was.  In my headcanon though, it's equivalent to a hypergolic, like Aerozine-50/Nitrogen Tetroxide, as it doesn't simulate any of the traits of a cryogenic, like boil-off or extreme cold.  I use the Cryogenic Engines mod to add those.

Edited by Capt. Hunt
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9 hours ago, Dragon01 said:

You clearly didn't try any mod that actually does it.

I’ve tried it. I didn’t like it, and I would appreciate it if you stopped making assumptions like this, it’s not the first time you’ve done it with me.

I’m also quite willing to be convinced, but that’s not going to do it. Assume I’m not at all interested in realism or being educated and try again from a different tack.

I care about gameplay. Show me that a feature you want adds interesting, meaningful, and fun gameplay (and does not add rote, repetitive gameplay, a proliferation of parts, or draconian restrictions to part suitability) and give it another shot. 

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Ah, ClF3.  Let's see what John D. Clark said in his autobiography Ignition! (1972)  Chapter 6, "Halogens and Politics and Deep Space", pp73:

Spoiler

So ClF3 it had to be. Otto Ruff had discovered the stuff in 1930 (as he had also discovered the majority of the compounds listed above) and the Germans had done a little work with it during the war, and so quite a lot was known about it. The efflorescence of fluorine chemistry sparked by the Manhattan Project led to studies in this country, and the Oak Ridge people, among others, investigated it exhaustively during the late 40's and early 50's. So it wasn't exactly an unknown quantity when the rocket people started in on it.
 

Chlorine trifluoride, ClF3, or "CTF" as the engineers insist on calling it, is a colorless gas, a greenish liquid, or a white solid. It boils at 12° (so that a trivial pressure will keep it liquid at room temperature) and freezes at a convenient —76°. It also has a nice fat density, about 1.81 at room temperature.


It is also quite probably the most vigorous fluorinating agent in existence—much more vigorous than fluorine itself. Gaseous fluorine, of course, is much more dilute than the liquid ClF3, and liquid fluorine is so cold that its activity is very much reduced.
 

All this sounds fairly academic and innocuous, but when it is translated into the problem of handling the stuff, the results are horrendous. It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water —with which it reacts explosively. It can be kept in some of the ordinary structural metals — steel, copper, aluminum, etc. —because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes. And even if you don't have a fire, the results can be devastating enough when chlorine trifluoride gets loose, as the General Chemical Co. discovered when they had a big spill. Their salesmen were awfully coy about discussing the matter, and it wasn't until I threatened to buy my RFNA from Du Pont that one of them would come across with the details.


It happened at their Shreveport, Louisiana, installation, while they were preparing to ship out, for the first time, a one-ton steel cylinder of CTF. The cylinder had been cooled with dry ice to make it easier to load the material into it, and the cold had apparently embrittled the steel. For as they were maneuvering the cylinder onto a dolly, it split and dumped one ton of chlorine trifluoride onto the floor. It chewed its way through twelve inches of concrete and dug a threefoot hole in the gravel underneath, filled the place with fumes which corroded everything in sight, and, in general, made one hell of a mess. Civil Defense turned out, and started to evacuate the neighborhood, and to put it mildly, there was quite a brouhaha before things quieted down. Miraculously, nobody was killed, but there was one casualty — the man who had been steadying the cylinder when it split. He was found some five hundred feet away, where he had reached Mach 2 and was still picking up speed when he was stopped by a heart attack.


This episode was still in the future when the rocket people started working with CTF, but they nevertheless knew enough to be scared to death, and proceeded with a degree of caution appropriate to dental work on a king cobra. And they never had any reason to regret that caution. The stuff consistently lived up to its reputation. Bert Abramson of Bell Aircraft fired it in the spring of 1948, using hydrazine as the fuel, NACA and North American followed suit the next year, and in 1951 NARTS burned it with both ammonia and hydrazine.


The results were excellent, but the difficulties were infuriating. Ignition was beautiful —so smooth that it was like turning on a hose. Performance was high —very close to theoretical. And the reaction was so fast that you could burn it in a surprisingly small chamber. But. If your hardware was dirty, and there was a smear of oil or grease somewhere inside a feed line, said feed line would ignite and cleverly reduce itself to ashes. Gaskets and O-rings generally had to be of metal; no organic material could be restrained from ignition. Teflon would stand up under static conditions, but if the CTF flowed over it with any speed at all, it would erode away like so much sugar in hot water, even if it didn't ignite. So joints had to be welded whenever possible, and the welds had to be good. An enclosure of slag in the weld could react and touch off a fire without even trying. So the welds had to be made, and inspected and polished smooth and reinspected, and then all the plumbing had to be cleaned out and passivated before you dared put the CTF into the system. First there was a water flush, and the lines were blown dry with nitrogen. Then came one with ethylene trichloride to catch any traces of oil or grease, followed by another nitrogen blow-down. Then gaseous CTF was introduced into the system, and left there for some hours to catch anything the flushing might have missed, and then the liquid chlorine trifluoride could be let into the propellant lines.


It was when the stuff got into the motor that the real difficulties began, for a chlorine trifluoride motor operates at a chamber temperature close to 4000 K, where injectors and nozzle throats have a deplorable tendency to wash away, and unless the materials of which they are made are chosen with considerable astuteness, and unless the design is very good, the motor isn't going to last long. The propellant man liked CTF because of its performance, and the engineer hated the beast because it was so rough on motors and so miserable to handle. Although he had to learn to live with it, he postponed the learning process as long as he could. It is only recently, as the customers have been demanding a better performance than can be wrung out of IRFNA-UDMH, that CTF has been the subject of much intensive, large scale, testing.

In short, like many of the halogens and exotics, the advice is simple "Don't use, not worth it."

Edited by Jacke
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16 hours ago, Brikoleur said:

I’ve tried it. I didn’t like it, and I would appreciate it if you stopped making assumptions like this, it’s not the first time you’ve done it with me.

I’m also quite willing to be convinced, but that’s not going to do it. Assume I’m not at all interested in realism or being educated and try again from a different tack.

I care about gameplay. Show me that a feature you want adds interesting, meaningful, and fun gameplay (and does not add rote, repetitive gameplay, a proliferation of parts, or draconian restrictions to part suitability) and give it another shot. 

If you dont care to be "educated" i dont understand why you're playing a physics simulation space game to begin with. If you want to play with only LF & Oxidizer so be it.... the kerbal system is the same and most the parts will be the same. Anything they add makes its easier for modders to play around. Most of the complaints i see from you could easily be modded to your liking with a word processor. Yes, squad has made it that easy to mod ksp! The game play mechanics it could benefit are too much for me to list. One of those would be signifigant "design changes".

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9 hours ago, harrisjosh2711 said:

Most of the complaints i see from you could easily be modded to your liking with a word processor.

If that's the case, then the same is true in the other direction. I have zero objections to realism mods, on the contrary I have a tremendous admiration for them even if I don't want to play them myself. What we're discussing here is how fuels should work in the base game. 

9 hours ago, harrisjosh2711 said:

The game play mechanics it could benefit are too much for me to list. One of those would be signifigant "design changes".

There would certainly be significant design changes, but whether those are beneficial to gameplay or not is a different matter, and I contend that on balance they are not.

Yes, it would be an additional constraint which would lead to different designs, and dealing with those constraints would be fun in some cases.

However, it would also significantly restrict the suitability of engines for particular purposes, which would be further restricted when you take ISRU into consideration. For example, if you have a high-power kerolox engine, and there's no kerosine on Eve, then that engine is not suitable for use as a launch stage from an Eve base supplying fuel with ISRU.

One of the things I like best about KSP is the way it's possible to combine parts in innumerable ways to create craft that perform jobs the part creators never even imagined. The Lego thing. I don't think there is a single engine designed specifically for an Eve launch stage, but there are several that work for the job because of their characteristics. If you're proposing a feature that significantly limits what parts can be used for what purposes, this will significantly reduce the range of possible, viable designs in the game. Tighten these constraints enough, and the Lego-ness is as good as lost: the viable designs will be tightly constrained and closely similar, just like with RL spacecraft. 

Again: I am aware that realism superfans would want it this way, and as I've said many times I'm all for having mods like Realism Overhaul that accomplish this. However I am very much against taking the base game in this direction. As far as I'm concerned it is right now in a very, very sweet spot that balances constraints and creativity, and any move out of that in either direction should be treated with extreme caution. KSP is so successful precisely because it hits this sweet spot, and it wouldn't take much to shift it out of it. 

I don't want that, which is why I'm so negative about many of these proposed changes (fuel types, life support, n-body physics etc). As I said I'm willing to be convinced but my default position is "if it isn't broke, then don't fix it," and core KSP gameplay is very much not broke. 

And, once more, if you want to convince me, make an argument about gameplay: how does the change you want enhance gameplay? What, if any, negative gameplay consequences would it have? How would you deal with those consequences?

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4 hours ago, Brikoleur said:

And, once more, if you want to convince me, make an argument about gameplay: how does the change you want enhance gameplay? What, if any, negative gameplay consequences would it have? How would you deal with those consequences?

First of all, the change extends the gameplay. LFO would still be there, with more accurate names, as hypergolics. It's just that it would, as a "starter" fuel, have low stats, at the cost of not having to deal with new cryo mechanics. Kerolox/Methalox (just one is needed, they're too similar to both be of use) would be the next step, introducing a small boiloff to the oxidizer, able to be removed just by using insulated tanks. Hydrolox would be the next incremental step, the best fuel/oxidizer combination in game, but at the cost of needing an insulated tank to work at all, needing a large volume, requiring either an additional part (condenser, part of the ISRU part lineup) to eliminate boiloff, or special, very heavy and EC-hungry tanks that would also need radiators. On the other hand, both jet and rocket engines using them would be much more efficient, and once you've mastered using hydrogen, you could move on to nuclear rockets and all those sci-fi things. Many people are concerned that the new interstellar drives are going to render every other engine obsolete. If you want to avoid the "fusion drive+oil drum=instant torchship" problem, there need to be additional complexities involved in using the advanced engines, and this is one of them.

You'd have three sets of tanks: normal, insulated and ZBO. You have three sets of fuel/oxidizer combos, steadily increasing in performance, at the cost of increasing complexity. Note that each tank would be able to hold two or three mixes, or be configured as a single tank. It's basically what Nertea did in his mods (although methalox is an optional patch). All engines which are now very OP, like the Vector or Nerv, would be balanced by requiring the fuel that is the hardest to use, and also require additional mass to be carried in form of ZBO tanks and radiators. This means they wouldn't always be the best choice anymore, and some designs would be better served by a small hypergolic enigne with a big nozzle.

Core KSP gameplay is very much broke - balance is vague and very confused, there are not enough engineering choices to make and optimums are rather obvious. If you want a pimped up KSP1, well, Squad is working on just that, and for little cost on your part. Modders already did a lot in that area, as well. I want KSP2 to be different. More complex, deeper, better thought-out. In particular, hydrogen engines need to move out of "cheaty" to "difficult, but awesome" category. High reward for high effort is good gameplay.

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1 hour ago, Dragon01 said:

I want KSP2 to be different. More complex, deeper, better thought-out. In particular, hydrogen engines need to move out of "cheaty" to "difficult, but awesome" category. High reward for high effort is good gameplay.

OK now you're talking my language. 

How would you avoid the problem of the increased constraints forcing the player into increasingly tightly constrained paths? That's my main issue with this particular feature. If it's done realistically, then there's pretty much just one way to build a rocket for any given purpose. For example, consider the RV-1 "Cub" from Making History. This is based on the RD-107, which is a launch-stage engine that runs on kerolox, correct?

Thing is, I absolutely love to use this engine as a primary thruster for somewhat exotic applications -- a light Duna VTOL spaceplane, for example:

AvUlazo.jpg

Or the orbiter stage for a reusable Eve launch system:

qUrlMiF.png

Now, both of these missions are predicated on the idea of having in-system ISRU that I can use to refuel them. I don't think you can produce kerosene with an ISRU, because it's a petroleum derivative. This means that I simply could not use this engine for this purpose. 

This leads to two possible scenarios:

(1) There is no replacement for it, which would mean that entire categories of craft would no longer be viable, and the craft we could build would be that much more uniform and that much less creative.

(2) There is a replacement for it -- namely, another small side-mounted high TWR engine which does run on a fuel/oxidant mix which can be produced with ISRU

I don't like either of these options. I don't like (1) because the thing that I most enjoy about KSP is the way we can combine parts in creative ways to solve problems the creators of the game never thought about, in ways they never thought about. And I don't like (2) because that would lead to a proliferation of nearly identical parts, which defeats the purpose of having multiple fuel types in the first place, and lead to the dull chore of having to verify that you're using the right variant for the job every time.

So: how would you solve this problem? Or do you perhaps not consider it a problem at all?

Edited by Brikoleur
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Assuming that you have access to carbon dioxide and water, then you certainly can make kerosene in-situ.

Electrolyse water to generate hydrogen and oxygen. Reverse water gas shift reaction (carbon dioxide plus hydrogen gives you water plus carbon monoxide), gives you carbon monoxide, carbon monoxide mixed with hydrogen is syngas (short for synthesis gas) which can be converted to a whole bunch of things including kerosene.

Or, if you want to run a methalox engine, electrolyse water to generate hydrogen, use a Sabatier reactor to convert carbon dioxide and hydrogen to methane and water.

So, assuming that Duna is at least a rough Mars analogue with water ice and atmospheric carbon dioxide available, you could totally make hydrocarbons there. Eve *shrug* - who the heck knows what Eve is supposed to be.

 

Edit. All of the above works on paper. In practice, the engineering would probably be quite challenging but fortunately KSP lets us abstract that away. 

Edited by KSK
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6 minutes ago, KSK said:

Assuming that you have access to carbon dioxide and water, then you certainly can make kerosene in-situ.

Electrolyse water to generate hydrogen and oxygen. Reverse water gas shift reaction (carbon dioxide plus hydrogen gives you water plus carbon monoxide), gives you carbon monoxide, carbon monoxide mixed with hydrogen is syngas (short for synthesis gas) which can be converted to a whole bunch of things including kerosene.

Or, if you want to run a methalox engine, electrolyse water to generate hydrogen, use a Sabatier reactor to convert carbon dioxide and hydrogen to methane and water.

Ok, so we've moved the issue from having multitudes of engines to having multitudes of ISRUs - or we just have a magic generic ISRU which can convert to anything.

Either way, it still sounds like a lot of extra busywork over the current stock system, for not much extra gameplay for most.

(Honestly, I often start designing my space career in current stock KSP to limit the number of fuels I have to balance, as I hate having to keep track of how much of each is at each refueling point...  I'll often either cut monoprop out and just use LFO RCS or cut out oxidizer and stick to nuclear and monoprop engines, depending on the playthrough.)

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Thanks @KSK, that answers my main objection to having more fuel types.

That still leaves my secondary one however: what's the point? If all fuel and oxidant types can be produced with ISRU, then what gameplay benefit do the additional fuel types bring? As far as I can tell the only outcome is that you have to check that your fuel tank type matches your engine type, and refuelling missions become more fiddly since you have to haul up the correct types of fuel and build your tanker to match it. That... does not strike me as fun, just fiddly. It still violates KSP's "Lego principle" -- that almost any parts are compatible with each other. (There is one outlier already, the DAWN electric propulsion system -- and tediously long burns aside, that's the main reason I almost never use it.)

There is the progression that @Dragon01 discussed, but that can be perfectly well abstracted out as it is now -- to thrust, weight, and Isp, with higher tech levels unlocking more powerful and more efficient engines. 

That said, if the ISRU obstacle is removed, I could live with multiple fuel types -- it wouldn't be a showstopper, just a minor feature I dislike. 

---> There is one way I think fuel variety could become an engaging and interesting gameplay element, but that would require an entire new system and level of abstraction. Namely, let us be the engine designers. Instead of engines, give us engine parts: nozzles, combustion chambers, pumps, housings. Let us combine them as we see fit to make our own engines. Naturally you could still have a bunch of pre-built engines giving a similar selection as we have now. Fuel type would fit into this type of gameplay very well, and would give us the flexibility to make exactly the kind of engines we want. But that's a big new system, an entire gameplay dimension in fact, and I'm not sure it's realistic to ask for something like that.

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55 minutes ago, KSK said:

Assuming that you have access to carbon dioxide and water, then you certainly can make kerosene in-situ.

Actually, because of ISRU, and Elon making methalox a household name, I'd go with methane for our hydrocarbon of choice. Fabricating higher hydrocarbons is a bother, and methane works just as well as kerosene, aside from being mildly cryogenic (about the same level as LOX). Methane can be found in Martian atmosphere, in Titan's oceans and atmosphere, as well as made by Sabatier reactors. It's a solid choice. 

Of course, for something like Making History it'd be questionable to see F-1 run on methane, but I think that a historical DLC could introduce additional fuels for the sake of accuracy. TBH, given how MH was received I don't think they'll try that one again, anyway (@CobaltWolf kind of blew it out of water before it was even a thing...).

42 minutes ago, Brikoleur said:

There is the progression that @Dragon01 discussed, but that can be perfectly well abstracted out as it is now -- to thrust, weight, and Isp, with higher tech levels unlocking more powerful and more efficient engines. 

True, but this has an impact on sandbox, as well. There has been strong opposition in this community to having an engine that is straight out better than another, with nothing else to differentiate it. Storables, particularly, have higher density than semi-cryogenics, and are better when space is at premium. They also have slightly higher thrust. ISRU would be a good gameplay consideration, actually. It can be done for storables, but would require more facilities than methane, oxygen, or LH2. It would, I think, make sense to restrict ISRU for the storable propellant engines, and make it work best with methalox and hydrolox. Of course, if boiloff isn't used as a balancing method, this could make the advanced engines too good.

Yes, adding more fuels would result in engine niches being duplicated, but for example, Cub's LH2 equivalent could have lower thrust, higher Isp, but cost more. This would require a lot bigger engine selection, which is a very good thing. I should remind you that right now, there is pretty much only one "correct" way to build a rocket for anything at all. Introducing additional fuel mixes would increase the number of available approaches to rocket building. Just look at how many different combinations there are in the real world. 

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4 minutes ago, Dragon01 said:

 I should remind you that right now, there is pretty much only one "correct" way to build a rocket for anything at all.

Could you elaborate on that, with an example? I have a feeling we may be talking at cross purposes. My experience is that this is sort of true for lifter stages, but not true at all for orbiters, deep-space vessels, landers, or spaceplanes. (And FWIW I find lifters to be the least interesting type of craft to build.)

4 minutes ago, Dragon01 said:

This would require a lot bigger engine selection, which is a very good thing. 

Why would it be a good thing? One of the things that's annoying me a bit about KSP1 right now is the proliferation of very similar parts. Engines are still under control but there are a lot of fuel tanks; just finding the one I'm looking for is a bit of a hassle.

Again, as I said above, I would rather solve the part proliferation problem by breaking the engines themselves into a smaller number of parts, with variants, that you could then combine to build your own. Having very large numbers of highly similar parts is not a direction I'd like the game to to go.

 

Edited by Brikoleur
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9 minutes ago, Brikoleur said:

Why would it be a good thing? One of the things that's annoying me a bit about KSP1 right now is the proliferation of very similar parts. Engines are still under control but there are a lot of fuel tanks; just finding the one I'm looking for is a bit of a hassle.

Except that them being similar is a failure in Squad's part, and part of the problem I'm trying to fix here. Fuel tanks, for instance, should be visually distinct, according to their purpose (and in Nertea's mods, they are). Same with engines, most of them should have something unique about them, despite having an overall similar role.

There's another approach to reducing engine proliferation: cut down on the size range. Instead of having 1.25m engines, then the same, but a bit bigger 1.875m ones and so on, have a collection of varied engines in sizes from 0.625m to 2.5m, and handle specific needs by clustering. With KSP2, clusters of individual engines are already said to be the name of the game. IRL, there is exactly one engine that approaches 3.75m scale, for instance (the M-1), and that one never flew. Most real rocket engines end up in the 1.25m range after scaling them to KSP, and the ones that don't usually have nozzles several times the size of the rest of the engine.

10 minutes ago, Brikoleur said:

Could you elaborate on that, with an example? I have a feeling we may be talking at cross purposes. My experience is that this is sort of true for lifter stages, but not true at all for orbiters, deep-space vessels, landers, or spaceplanes. (And FWIW I find lifters to be the least interesting type of craft to build.)

A nuclear tug blows every space-oriented craft larger than a probe out of water, because the Nerv is just that good. If you're roleplaying anti-nuclear activists banning it, then it's the Vector. Only landers and spaceplanes have more than one viable design, and that's mostly due to aerodynamics being a tricky beast. Admittedly, part of the reason for all this is that KSP's toy solar system doesn't even really need all that extra performance LH2 can provide, but with more places to get to, this will stop being the case. With multiple propellants, even lifter design becomes much more interesting. 

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@KeranoKerman "Unambiguous" is not what I would describe LiquidFuel and Oxidizer as. They are abstract and are open to the player's imagination. Because of LiquidFuel's density it's actually primarily considered by the Realism Overhaul community, to be RP-1, a form of Kerosene. Meanwhile, its use in the one nuclear engine, the NERV, suggests that should be Liquid Hydrogen. Of course, it's far too dense to be Hydrogen, and Kerosene would utterly suck in an NTR. But here we are: abstract stock propellants being stretched across use cases far too wide for any one specific propellant to fit.

I will agree that Oxidizer is quite a gray area. Most, I think, tend to equate Oxidizer to LOX simply because "It contains Oxygen. It's liquid. So it must be Liquid Oxygen!" However, Kerbalism (and I think, KSP Interstellar) interpret it as HTP, which persists at room temperature for a generous but not indefinite time.

With a quick look at the Wikipedia page, I see that ClF3 is stored well in quartz vessels, and reacts quite well with Water. In such a world I imagine kerbals would have tanks made of diamond, and Water would directly replace LiquidFuel as the main fuel. I'm honestly concerned for what would go into an NTR that can operate on Fluorine (and not dissolve). Finally, I imagine that Kerbin wouldn't be green for long if Fluorine rocketry became a norm.

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