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Hi there. I'm a recently separated USAF jet engine mechanic, and a current Physics student. I have a long time interest in propulsion systems, and have learned a great deal about them even outside of the few engines I've had wrench time on. I bring all of this up mostly to brag if I'm honest, but also hopefully to lend my opinion (and it is just an opinion) a little more credence. I know the feedback the devs are looking for currently is probably on performance and bugs at the moment, but I can't help but already make gameplay suggestions, lol. For all I know, the following suggestions are already planned, but I thought I'd chime in if not. KSP 2 is introducing different fuel types into the game (beyond solid, liquid, and xenon), which I think is great. KSP 1 oversimplified fuel types, which is great if you're new to rocketry, but opens up exploits, like unrealistic densities when it comes to LH2 derived engines. However, I think KSP 2 could be implementing these different fuel types into the game a little bit better in the future. Currently, it looks like SSTOs are more difficult now that jet engines and NERVAs do not share fuels. And, SSTOs SHOULD be more difficult than in KSP1 imo, SSTOs are anything but easy IRL... However, I think they should be more difficult in a different way than simply not sharing fuel types... Because in reality, these engines should be capable of sharing fuel types... In short, I think KSP2 developers should consider introducing more fuels, and, make some engines multifuel capable, either in VAB, or in the field (as well as make well researched decisions on which engines are multifuel, how, and why). RP-1/JP-1 to JP-8/Kerosene/Diesel/Alcohol/Gasoline, etc: Wholly Too Much Information About RP-1 One fuel simply absent is a generic heavy hydrocarbon based liquid fuel. I understand the choice to, at least starting off, make most engines "Methane" powered, as most rocket and jet engines can be made to run off of Methane, at least in theory. It's a good every-man fuel. In reality though, its lower density and cryogenic requirements often make larger hydrocarbon fuels cheaper and easier to use. It's why my own plane (The KC-135) was powered by JP-8. Methane is rising in popularity mainly because it provides better performance than larger hydrocarbons, while not being quite as demanding storage wise as LH2, but there are still plenty of reasons to use old fashioned hydrocarbons. There are probably hundreds of iterations of the "cheap storable hydrocarbon" fuel, but KSP 2 could easily simplify this concept down to say, some fictional fuel called "LP-1", that serve as a universal stand in for all of these variants... Because they all have similar chemistry, similar density, similar performances, etc... For example, one could conceivably throw RP-1 (a rocket propellant) into a jet engine, adjust its specific gravity on the fuel control, and it might run just fine (in fact, I believe that was one of the military requirements in the specifications for RP-1, that it could be used to run existing military jets in a pinch). One reason to choose this fuel in game is that, although it's dirtier and less efficient (lower ISP) than Methane or LH2, it would also be far denser (smaller tanks for the same mass of fuel), and in the future career mode, it could also be a lot cheaper than any fuel requiring cryogenics (like Methane or LH2). Less efficient fuels like these heavy hydrocarbons can remain competitive against the higher performance stuff, because the lower tankage requirements has the potential to save on dry weight. Jet Engines (and some rocket engines): Igntition! (Truly, one of the books of all time) Jet engines are generally indiscriminate about what they will burn. John D. Clark even states as such on page 32 of his book Ignition. It's actually one of the advantages of jet engines, as in WW2, it allowed Germany to use low quality fuels (pretty much just diesel) in high performance aircraft. As such, it would make sense in KSP 2 for jet engines to be able to burn either Methane, or this fictional LP-1... However... It should also be able to burn LH2. Although it is an uncommon fuel for jet engines, it is completely conceivable to run a jet engine off of LH2. In fact, running hydrogen fed jet engines was an important stepping stone in jet engine development. Hans Von Ohain ran his first jet engine in 1937... Off of gaseous hydrogen. He chose this fuel because it doesnt leave coke behind (it has no carbon in it), and as a gas, it's already atomized before even entering the engine. Even liquid hydrogen, is easier to atomize than anything based on hydrocarbons, because of its very low boiling point. Using hydrogen, he was able to get his first prototypes up and running, before working out the issues on using hydrocarbon fuels. He briefly talks about it in the book The Jet Age on page 33 (an awesome primary source that collects accounts from the greatest jet engineers from the jet engine's early history). Again, generally, hydrocarbons are simply more practical to run in jet aircraft. But, some modern jet engines are designed to run off of LH2. For example, scramjets require the fast atomization properties of LH2, due to how little time intake air spends inside the engine before being exhausted. And, the Sabre engine (the engine the Rapier is based off of) also is designed to run on LH2 for precooling purposes (I'll get to the Rapier in a minute). In short, I think players should be able to select between LP-1, Methane, and LH2 to run their jet engines off of. This would require a lot of thought from players on whether they want low cost and ease of storage of LP-1, the high efficiency but poor storage of LH2, or the middle man of Methane. Jet engines are generally aircooled, which means the main consideration in adapting a jet engine to run on different fuels would be the fuel pump, fuel control, and injectors... Which on my engine at least, wouldn't be that difficult to change out. The bigger concern would be going from normal hardware to something that can handle cryogenics, rather than the physical act of mixing the fuel and air properly. Maybe the fuel lines would need to be changed out for insulated ones as well. But, even in the field, those are all line items a qualified mechanic could change within a few shifts. Also for LH2 in particular, a service engine would likely need to be made out of slightly different alloys, to combat hydrogen embrittlement. Perhaps these changes could be reflected in the fuel efficiency, cost and weight of the engine in the VAB in game (maybe one day give them slightly different models depending on fuel type). Perhaps the player could select one that could be multifuel in the field (meaning, could be refueled with any of the three fuel types in situ), but also be the most expensive and heaviest version (since it would likely require multiple fuel pumps and some sort of adaptive injector). Rocket engines could also be selectable, but should maybe require a lot more thought from the devs on which fuels, which engines, and why. Many of them are designed with active cooling or special turbopumps that rely on a specific fuel. For example, I don't think an RS-25 (the Vector engine in game) could be very easily adapted to run on LP-1 due to its active cooling and special preburning closed cycle turbopump. By the time you redesigned that into an LP-1 engine, it would basically be a totally different engine anyway, sharing few traits with the original. So perhaps an engine like that should be limited to LH2, and maybe methane. Point is, while in theory rocket engines should be as simple as jet engines to adapt to different fuel types, in practice, it may be really hard, and therefore, not worth doing in KSP 2 for many types of rocket engines (particularly the more advanced and realistic ones. Perhaps multiple fuel options could be restricted to OG engines, like the LV-T30, LV-T45, Skipper, and Mainsail, since those engines are more fictional anyway... Would also give some reason to use them more often). The Rapier Engine: Wholly Too Much Information On The Sabre Engine (Upon which the Rapier engine is based) The Rapier Engine in particular is currently running the wrong fuel. I think one could be built to run on Methane, but above the other jet engines, it is specifically designed to run on LH2 in real life (the Sabre engine), to take advantage of the cryogenic LH2 for precooling. This is one of the reasons why Skylon is so big, that SSTO needs to store LH2 to run the Sabre engines. Jet engines get most of their energy out of a fuel by having a difference in temperature between intake air, and exhaust. The greater this difference, the more power and efficiency the engine has. Engines are made out of non-fictional materials however (kind of a dumb way to put it, but it's apt), which means there's only so hot they can get before they literally just melt or fail inside. This places an upper bound on exhaust temperature. However, the intake air can also get pretty hot as a lower bound for this temperature difference. The act of forcibly compressing air through ramming (which basically every jet engine does except for Scram engines, which can't afford to do that very much) increases intake air temperature before it even has a chance to be burned, reducing efficiency and thrust. This problem gets worse and worse the faster you go, as shock heating at supersonic, and then hypersonic speeds, increases intake air temperature to the point where it's as hot as the exhaust... This is the end of the road as far as normal jet engines are concerned, as there is no way to burn fuel at those intake temperatures, without the exhaust just melting the engine. The Sabre engine aimed to solve this problem with the use of a precooler. Instead of using ram compression to compress intake air, it instead uses an extremely efficient heat exchanger between cryogenic fuel (liquid hydrogen) and intake air. This is pretty amazing technology actually, as it allows the engine to both compress AND cool the intake air, rather than trade off between compression and heating. Since it cools the intake air, it allows the engine to operate at airspeeds far higher than any other jet engine. This doesn't just have applications in the hybrid jet/rocket Sabre engine, in theory, adding a precooler like this to any suitable jet engine, should massively improve that engine's efficiency, thrust, and top speed. Unfortunately, the precooler in KSP1 doesn't do a whole lot besides act as a decent inline intake. KSP2 should probably rectify this by making the in-game precooler do what it says on the tin (cooling intake air to allow faster operation of jet engines, particularly for the rapier engine... In fact, the rapier probably shouldn't be able to go faster in airbreathing mode than the panther or whiplash, without a precooler). Considering all of this... The rapier engine should probably come standard as using LH2, and requiring an LH2 fed precooler to run at peak performance... Conveniently, this would mean it uses the same fuel as the nuclear engines, to the rejoice of SSTO builders (though also require the massive fuel tanks and the new low density LH2). As for the selectability of those two components... Well... Methane is still cryogenic to some extent, so maybe that would work. LP-1 wouldn't be cryogenic at all. However, even LP-1 ran through a precooler could be better than no precooler at all, due to the difference in heat capacity between any liquid state fuel, and the gaseous state of the intake air. That's actually one of the reasons why the precooler is so efficient IRL, it's not just that the LH2 is cold, it's also that it is in a liquid state, far denser than the intake air. So even tepid LP-1 could be used as a coolant against thousand degree intake air. So perhaps the Rapier and Precooler could be considered multifuel as well in game... But there would have to be some performance penalties for using less or non-cryogenic fuels. Perhaps the devs could reach out to Reaction Engines to ask them directly whether or not the precooler/Sabre concept could even remotely work with anything other than LH2 to confirm. NERVA: Wholly Too Much Information About Nuclear Rocket Propellants The nuclear engines are perhaps the most multifuel capable engines IRL, of the engines on this list. In nuclear engines, the "fuel" is actually the fuel rods (Uranium or Plutonium), and what is exhausted is just propellant. A propellant is literally anything with mass, and in NERVA engines, are little more than a coolant for the core... So any fluid that can suitably act as a coolant, can be used in a NERVA engine. There is no need for combustibility for a NERVA propellant, so even completely inert fluids are usable (Helium, for example). The main reason LH2 is used is that, in any rocket, a higher exhaust velocity increases efficiency. That exhaust velocity is, at least in any thermal rocket (from chemical rockets to nuclear thermal rockets) a function of both the energy produced (heavily correlated to temperature) and the molecular weight of the exhaust... LH2 is the lowest conceivable molecular weight (unless you want to get into some crazy fuels like metallic hydrogen, plasmas, or laser light), which means that for the same operating temperature, hydrogen can be accelerated to the highest exhaust velocities possible in thermal propulsion. There's also a big reason not to use anything else. Most of the NERVA's efficiency is achieved purely through the use of such a lightweight propellant, and not by achieving high temperatures. The peak allowable temperature of the core of a nuclear rocket is actually often lower than what temperatures can be achieved in chemical propulsion (though the nuclear engines in game appear to achieve pretty ludicrous core temps)... This means that running something heavier, such as perhaps water, could leave the NERVA operating at as low, or lower ISP than high performance chemical rockets like the Vector (RS-25) engine, depending on the core temperature of the NERVA. Although LH2 is clearly the best option for a nuclear engine, there is still good reason to give a player the option to run suboptimal fuels. For example, maybe a player just wants fuel compatibility between engines and between other spacecraft, so they're willing to take a hit on engine efficiency if it means only dealing with Methane. I think the best reason for it is to ensure greater usability in deep space where fuel production is limited. Let's say you're busy colonizing the Jool system, and only have simple H2O or Methane mining set up, but no real LH2 production set up... A suboptimal fuel on a nuclear powered spacecraft is better than no fuel at all. In fact, I think having the nuclear engines have an in situ mutlifuel option would be pretty amazing, even if that version of the engine were ungodly heavy, if only because it gives the player a lot of capability in fueling it with basically anything they can find out in deep space. One could literally just melt ice from comets, and with no electrolysis or croygenics for LH2 production, run straight water to get back to Kerbin, then switch over and refuel with LH2 in LKO for the outbound trip. I found a 2016 research report which shows predicted ISPs for different propellant types, LH2, NH3, H2O, and CO2, at different core temperatures. This graph peaks at just over 1100 ISP for LH2 at a core temperature of 3600k. The bigger nuclear engine in KSP 2 achieves 1450 ISP, which suggests a pretty insane core temperature of more like 4,000k. Although this chart doesn't go that high, we can still make some educated guesses. Methane has a molecular weight of about 16 g/mol, which should give it an ISP slightly better than NH3 (which has about 17 g/mol). Now, the two nuclear engines in the game are at 900 ISP and 1450 ISP with LH2 (if I remember correctly). Going off the graph in this document, it would be reasonable to have a methane propelled version at 500 and 800 ISP respectively... Which honestly, is nothing to sneeze at. Even H2O propelled versions (if H2O is ever added as a resource) would manage 350 and 500 ISP respectively (even worse, but better than nothing, and still just outperforming most chemical rockets... Though in a much heavier engine than any chemical rocket). Then LP-1 could be used too, though I'd imagine its ISP would be awful (maybe as bad as monopropellant)... That'd be the sort of thing a player would just use in an emergency, like if they're willing to throw ANYTHING in the core to get back home. That reminds me, it would be nice to use oxidizer as well, as a way to use unspent oxidizer... Though that might not be the best idea IRL, given most oxidizers, particularly liquid oxygen, HAPPILY react with metals in high temperature environments in the absence of fuel to react with... Perhaps that could be a funny feature though, like, use oxidizer in a nuclear engine for an emergency, but take care not to use it too long, or the engine blows up. BTW, this is sort of in-line with the performance of methane and water nuclear thermal rockets from the game Children of a Dead Earth, which is another realistic space sim that does allow for different propellants on nuclear thermal engines. One final advantage of allowing for different fuels on nuclear engines, is heavier fuels may actually increase the TWR of the engine (it's easier to get higher mass flow rates out of heavier fuels, if for example, the propellant pumps and injectors being used are more volume flow rate limited than mass flow rate limted). This could mean that these otherwise very heavy nuclear engines could be more useful in landing in taking off from worlds using Methane, and then switch to LH2 for long hauls between planets. ---------------------------------------------------------------------------------------------------- Anyway, this has been a long winded case for multifuel engines. If you've managed to drag yourself this far through my word vomit, then I'd be interested to hear what your thoughts are as a player. Do you think this would add to player choices, dilemmas, and improve game-play, or do you think multifuel engines only needlessly complicate things? Personally, I like the Dilemma of... Do I rot in deep space for eternity, or dare use the forbidden propellant (pure oxidizer) in the 10 ton nuclear power plant, rolling the dice on getting back home alive? Or detonating in a mini Chernoby that can be seen from Kerbin...

Greetings all. I noticed a rather strange (and briefly alarming) occurrence during my game Monday night, and wanted to ask for input on what might be going on. Also, please note I am still playing v 1.02 by choice (and always immediately turning off Temperature Gauges to work around the memory leak), simply because I can't abide the universe's physics changing mid-game. I realize I am intentionally using Nuclear Engines with a pre-radiator game. I had launched my 100+ tons Duna lander (w/ 8 Nuc engines) into low Duna orbit, and then let it be while I waited for the transfer window to Kerbin. After ~ 100 days I returned to the ship, finding it on the dark side of Duna. It was obviously on the dark side, because in the dark the WHOLE SHIP was glowing cherry red. Every thermometer on the vessel was reading +900 degrees. I watched the temperature climb, expecting that after an orbit or two it would hit 1000 and just explode. However, as soon as the ship orbited into Sunlight, the temperature immediately started falling. I was later able to burn out of orbit, and am on my way back home. My question is, is there some aspect of the ship's exposure to sunlight that could rationally explain what I witnessed, or is this just an earlier version bug? My initial search for a known bug did not yield an obvious hit. (I do have pictures, hopefully I will have time to post later this week in the "What did you do Today" thread)