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Purpose of new propellants


ChubbyCat
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I’m having trouble understanding what the purpose of the new propulsion methods are in KSP 2. From what I can gather, Orion nuclear pulse engines allow you to get to places like Jool in just a few dozen blasts or so, but why is that so much better than just burning methalox for a bit to get there? What is the purpose of metallic hydrogen? Can it burn for a long time? Does it make you accelerate very quickly? If anyone could explain to me since I’m dumb it would be much appreciated.

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They're all based on real world tech or realistic design studies that are at least feasible. So the question becomes why would space agencies and engineers want different engine designs? It comes down to power and efficiency. Same reason you wouldn't want to go interplanetary with just RCS thrusters: slow acceleration, anemic thrust, inefficient. An ancient VW beetle will technically get you from here to there, so why improve? Better technology leads to more power or more efficiency or both. More efficiency means less fuel needed which means more payload for the same craft mass.

You can't just keep adding fuel tanks to a mainsail and expect everything to work out ok... that might be doable for a trip to jool if all you're hauling is a command pod, but if you want to go to another star and bring the stuff you need to start an entire civilization... you'd need a small moon's worth of propellant and many years to burn it. It's not viable, hence the better tech.

If you want to experience why, probably check out to near future mods by nertea. Lots of good stuff in there. You will have little luck making it to another star system with chemical rockets.

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2 hours ago, ChubbyCat said:

I’m having trouble understanding what the purpose of the new propulsion methods are in KSP 2. From what I can gather, Orion nuclear pulse engines allow you to get to places like Jool in just a few dozen blasts or so, but why is that so much better than just burning methalox for a bit to get there? What is the purpose of metallic hydrogen? Can it burn for a long time? Does it make you accelerate very quickly? If anyone could explain to me since I’m dumb it would be much appreciated.

Specific Impulse.

https://wiki.kerbalspaceprogram.com/wiki/Specific_impulse

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The rocket equation determines how much delta-V you have. Increasing ISP directly increases delta-V; while chemical rockets tend to have ISPs of anywhere from 250-350 seconds for kerosene/liquid oxygen and possibly up to 500s for liquid hydrogen/liquid oxygen, a fission-powered nuclear thermal rocket can easily hit 900s and electric propulsion (e.g. ion thrusters) can reach a few thousand seconds.

A fusion drive like the Daedalus has a theoretical ISP of several hundred thousand seconds of ISP, maybe even millions.

The Orion nuclear pulse engine isn’t necessarily efficient in terms of ISP, but for sheer brute force there’s not much comes close to propelling your spacecraft with a series of uncontained nuclear explosions.

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To be honest, the only thing that beats the Orion drive for lack of subtlety in rocket propulsion is the Nuclear Salt Water Rocket.

The fuel is simply a solution of something like Uranium Bromide in regular old Water. Of course, it's not just ordinary Uranium, it's enriched uranium, which is where the energy comes from. The water is just the reaction mass, I suppose you could use Ammonium or something else other than water assuming it has good nuclear properties (doesn't slow down Fast neutrons that much).

It's like an Orion drive, but there are no "pulses". There's just one CONTINUOUS NUCLEAR DETONATION, and the rocket surfs the blast wave.

It's like a series of firecrackers under a tin can (orion drive) versus an actual rocket motor (NSWR).

Of course, we haven't yet figured out how to contain such an awesome level of power dissipation in a way that doesn't itself almost immediately vaporize from the fact that it's required to be mere inches from nuclear fire, but that hasn't stopped people from using them in science fiction.

Nertea's "Far Future Technologies" mod has 2 examples of Nuclear Salt Water Rocket engines, one based on "low enriched" fuel, and one based on "high enriched" fuel (for simplification of game play and resources, they both run on the same fuel).
The high-enriched one has a very high specific impulse, and a very high thrust, both at the same time. Somehow, I still find myself clustering multiple engines to get enough thrust to accelerate my interplanetary missions at several G's because I like fast transfers but I don't like having to wait an hour for a burn to finish (because I'm trying to minimize the amount of time I play the game "doing nothing" because I'm waiting for something to be finished, be that time-warping to the destination on a slow transfer or waiting for the burn to finish on a fast transfer).

Trust me, even at several G's of acceleration, and spending more than 10km/s on both the ejection and injection burns for the transfers, those "High enriched" based NSWR engines have more than enough performance that I should be able to visit every planet in the stock Kerbal solar system without refueling a single time.
But if that's not good enough, they ALSO include the infrastructure you need to make more fuel from just Ore (in multiple steps, first make Enriched Uranium from ore, then transfer that to the 2nd machine, that 2nd machine takes Ore and Enriched Uranium to make nuclear salt water rocket fuel).

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Because methalox won't be practical for even the "shortest" interstellar transfer.

And I'm going to clarify what I mean by practical here.

Yes, in theory you could loft enough methalox into orbit to complete a transfer to another star. Especially when you have a minmus/mun colony, but the sheer quantity of tankage would make the craft fragile under acceleration, and frame rates would likely be measured in minutes per frame...

And by the end you still have the biggest issue, which is turning the whole thing around and cancelling enough velocity to capture at the other star.

Now most of these issues can be solved by clever staging, and the final craft that reaches the destination very well could be over 1/32 the size of the original. 

But that's a significant amount of time, money and resources you used both IRL and in game to achieve the same result using a much smaller and more manageable Orion or more advanced craft would've accomplished in a shorter time.

As for the rest? Energy density is king when you want to optimize the amount of mass per volume of fuel you have to carry. 

If I want to make a small, compact lander. There's very little that beats RP-1 and LOX, but as mentioned above I might not be able to find them at my destination (let's say jool). I know I'll find plenty of hydrogen and oxygen at laythe in the form of water, so hydrolox for my lander might be preferred despite the bulky nature.

Duna I know I'll find plenty of CO2 to make methane though, so methalox is actually viable the whole way.

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Others have mentioned the ISP advantage, but what really interests me is the ability to strategically leverage through a series of fuel economies depending on where we chose to explore and what we find while prospecting. The thing about a fuel system that you're mainly supplying through ISRU is that its not just a one-off decision made in the VAB on Kerbin. You need satellites and probes to find it, harvesters and processing plants to gather it, and tankers and depots to move it to where you need it. Ideally at any given stage you'd have tankers and personnel busses and tugs and landers all running off as few main resources as possible so they can all share the same supply infrastructure. It's just not efficient to try to do everything at once. You're better off to focus and say "Okay Im moving from methalox and PV to NERVAs and nuclear power early, because the weight efficiency is worth all the overhead of building mining and transportation equipment for H2 and Uranium." Or maybe you decide to stretch out your use of methalox longer or rely on light-weight solar powered Ion thrusters until you can leverage into He3 and fusion to produce MH and skip NERVAs almost entirely. Ether way there's going to be a tendency to consolidate your fuel systems for pure efficiency reasons--because adding another fuel source requires building a whole system of harvesting and delivery infrastructure to support it. 

Edited by Pthigrivi
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Great answers in this thread. All of the engines have upsides/downsides that make them more or less suitable for particular applications, and that becomes especially true when you start talking about interstellar trips (in the real world, delta-V to Mars is about 4 km/s; delta-V to Alpha Centauri is more like 20,000km/s). So obviously the interstellar-class engines are pretty specialized for that application, but are by and large not great for interplanetary travel because they can take a while to get up to speed. 

But one thing that I like a lot about the new progression, which @Pthigrivi put quite nicely, is that different fuels are derived from different resources, and what you discover and harvest has an effect on what you are able to build. I'm especially excited to see how people use non-optimal engines in creative ways when they have to work within unusual resource constraints.  

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Put it this way, imagine playing KSP and the only fuel we ever had was Monoprop. Build a Jool mission using only that, and it gets quite hard, you have to build pretty big to get just a few kerbals there. Now imagine a "new" fuel gets unlocked, called "Liquid Fuel" + "Oxidizer". Some of these "new" LFO engines don't produce more thrust than the Monoprop engine, and clusters of Monoprop engines can still provide the same thrust as those big LFO engines. So why use LFO then?

The answer to that can also be used for questions like, "why use the NERVA engine?", or "why use the ION engine?" or the orion drive, etc...   They all provide a bigger payload capability when using the same mass launcher.

In the end, some are either cheaper, have better TWR, better ISP, run on a fuel that you are already mining, and stuff like that makes it so that most fuel types will always have a place

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16 hours ago, Nate Simpson said:

But one thing that I like a lot about the new progression, which @Pthigrivi put quite nicely, is that different fuels are derived from different resources, and what you discover and harvest has an effect on what you are able to build. I'm especially excited to see how people use non-optimal engines in creative ways when they have to work within unusual resource constraints.  

This is huge, it's going to shape how the game is going to be played. 

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25 minutes ago, Master39 said:

This is huge, it's going to shape how the game is going to be played. 

I was thinking the same thing immediately after seeing that the NERV had a “uranium” requirement to build, where players would be using less efficient engines for even some medium-sized missions simply because they didn’t have access to uranium on the colony they were building from. I didn’t even consider that you might not be able to access some fuels which could also shape ships. Now, on top of least expensive, lightest, and smallest crafts there will be a new superlative for the easiest to produce, and mono prop only ships might start being designed. Exciting times. All we need now is a more precise release window, but the team is doing an amazing job with system integration and testing, so keep it up, we’re supporting you all!

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IMO KSP 1 "monopropellant" covers many engines where you'd be far more likely to be using Hypergolics, such as that "Puff" engine.
High thrust monopropellant engines are simply not practical IRL because the larger the monopropellant thruster is, the harder it is to ensure both that all the propellant is decomposed by the catalyst bed AND simultaneously that the catalyst bed which decomposes the propellant does not itself melt under sustained operation due to the catalyst itself having to exist in what is essentially a rocket engine combustion chamber.

This means that to me, even the O-10 "Puff" monopropellant engine at a whopping 20 kilonewtons is testing the limits of plausibility. IMO 5 kN is much more reasonable and likely to be achievable in the real world.
EDIT: Put another way, assuming you don't think KSP 1 "Monopropellant" also covers hypergolic fuel mixtures, the O-10 Puff engine has too much thrust for a monopropellant engine, but if the same engine had only 5 kN of thrust (and 1/4 the mass to keep the TWR the same), it would be much more reasonable, because what happens when a spacecraft needs a lot of thrust out of just monopropellant in real life is that you see clusters of smaller thrusters being used, instead of one large one. However, if you assume it DOES cover hypergolic propellant mixtures, the O-10 Puff is right in the sweet-spot of small-to-medium scale hypergolic rocket engines such as used as the apogee kick motor of geostationary satellites and small upper stages of rockets such as Fregat and Briz-M.
(edit ends)

Naturally, these limits of Monopropellant technology limit both maximum thrust and maximum specific impulse to relatively low levels, whereas Hypergolic thrusters of similar thrust levels have no such issues, start reliably many many times just like Monopropellant engines if they don't have turbomachinery (some are capable of thousands of ignitions or more between needing to be inspected, let alone serviced or replaced), and offer growth options towards generally higher specific impulse and thrust (for example, the main engines of both the US Titan II "ICBM-turned-satellite launcher" and the Soviet/Russian UR-500 Proton "we built a mega ICBM but now we don't need it so we'll use it to launch big things into space" both use hypergolic fuels in their first and 2nd stages, which are both quite high thrust and far outside the realm of anything Monopropellant can do feasibly)

I mean the only way I can think of to make a Monopropellant rocket engine that scales well to high thrust levels is to switch away from using solid catalyst beds and instead use liquid solutions of catalytic agents, at which point you've essentially made a hypergolic rocket engine with extra steps and worse performance because part of your propellants (the catalyst solution) doesn't provide any energy to the reaction. I mean there might be a way to arrange things so that the reaction is energetic enough that it self-sustains, in which case you would be able to drastically reduce the amount of catalytic solution you carry on the rocket, but at that point you're still leaving performance on the table compared to what you're almost doing, which is "RP-1+ LOX with TEA-TEB igniter fluid used to start it".

In other words, if you're going to use a liquid propellant rocket, other than very very specific situations there's very little reason to not use some form of 2-component fuel combination.
Not even RCS thrusters are immune to this. The Space Shuttle and the SpaceX Dragon capsule both use/used bipropellant hypergolic attitude control thrusters.
Yes, the Shuttle's RCS was in fact hypergolic, not monopropellant. The reason is that the Shuttle Orbiter is BIG and quite MASSIVE, and because of that it needs the extra performance margin that hypergolic attitude control thrusters (and OMS engines) have over monopropellant ones. As a secondary matter, because they needed the OMS engines, they were already forced to have both components of hypergolic propellants on board, so the matter of making everything use the same propellants (instead of the RCS only using Hydrazine but the OMS using Hydrazine + N2O4) simplified the plumbing on that already massively complicated vehicle.

With Dragon (and therefore Dragon 2 as well), the reasoning was basically down to the secondary concern on the Shuttle (it makes the plumbing simpler).
They already had the SuperDraco thrusters using hypergolic fuels, so the question of what to use for the RCS was simple to answer because if the attitude control thrusters can use the same fuel combination that the SuperDraco ascent abort system thrusters use, then you don't have to carry around two sets of fuel tanks to make both work, instead you can simply make one (slightly larger) set of fuel tanks that will cover the needs of both systems in the worst-case scenario, plus a safety margin. Doesn't hurt that originally they also planned to use the SuperDraco thrusters to propulsively land the Dragon capsule if they didn't need to use them during an abort (which would help speed up turn-around time because they wouldn't have to re-pack the parachutes, something that takes a long time when you have to record and review every step of it to make sure it was done right so that you are absolutely sure that they'll work right when they have to work).

 

OK so long story short, I don't see Monoprop being particularly valuable except around low gravity worlds, as I don't expect them to be making any larger engines that run on the stuff and the specific impulse is just horrible anyways.

Whenever possible, I try to use the Vernor RCS thrusters, because it reduces the number of different resources I have to carry around on the vessel.

I wouldn't be against the idea of re-naming Monopropellant to a single "Hypergolics" resource, and additionally I hope that we see RCS thrusters that run on all of the new (and old) fuel mixtures that rely on a chemical (not nuclear) reaction to produce thrust. So that would be both "Methalox" and "Hydrolox" RCS thrusters. Additionally, I hope that we're not limited to only the "single nozzle" Vernor-style bi-propellant RCS thrusters, and instead have both single and configurable multi-nozzle RCS block variants available.
Perhaps MH or Xenon+Electricity RCS too, but that would be more about simplifying the plumbing than it would be about the much higher specific impulse (and thrust in the case of MH) that either would provide.

 

EDIT 2:
To expand on what others have been saying, I don't think that Monopropellant would have the "simplest to construct" rocket engines. Instead, that title goes to (you guessed it) Hypergolic engines. Hypergolic engines have no "special materials" required in their construction, however Monopropellant engines ALWAYS need a small amount of rare materials (usually platinum-group metals) for the catalyst bed.
Of course, if you happen to wrangle a large enough example of the right kind of asteroid, you'd likely be able to mine it for all the platinum-group metals you could ever possibly need. Of course, since Dres is also basically an overgrown asteroid, it should also be rich in such materials.
So Monopropellant vessels may in fact be common, but around Dres and asteroid bases, not so much around Minmus.

Edited by SciMan
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Pointless how? They made concessions to realism in that Minmus is now glassy, why wouldn't they do the same with respect to things like the very much more important actual propellants they're telling you the engines use?

I mean, I suppose "A rose by any other name", but at the same time these words have very specific meanings, and I thought I was suggesting a title that better fit what the propellant was actually being used for.

IRL you don't often see a monopropellant thruster capable of a full 1000 Newtons of thrust, yet that's exactly what you get with most of the RCS thrusters in KSP.

For reference, the hypergolic bipropellant Draco thrusters on Dragon 2 are only capable of 400 newtons each. That's a little less than half the thrust of a typical KSP RCS thruster.

And yet it's plenty enough to give the Dragon 2 capsule full control in space.

The SuperDraco thrusters (at 71 kN each, or 142 kN for a pair, with the Dragon 2 capsule having 4 pairs) are ONLY used during an Abort situation, and they too are still Hypergolic Bipropellant thrusters.

 

I mean surely you must have more to say about it than just "pointless"? You gotta explain what you mean at the very least.

And even if I think your explanation for why it's pointless is valid, I still think that there should be another type of fuel called "Hypergolics".

Or maybe we just do it differently, say we say some engines use "Monopropellant plus Oxidizer" as a valid fuel combination, to represent hypergolic fuel combinations?

My point is that just "monopropellant" doesn't explain the whole thing well enough. At best, it's the "Lies to children" version of the truth.

Edited by SciMan
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On 12/22/2021 at 6:50 PM, Jefftheguyperson said:

Will different propellants be more/less expensive to handle in Career mode? Such as toxic propellants being more difficult to handle safely compared to simple hydrogen and oxygen?

Probably not so much handling issues except radiation is likely to be an thing so no more strapping two nerv on the side of an hitchhiker module and more of the long spine style of ships you see in hard science fiction. 
Now getting the fuel will be the challenge, hydrogen and oxygen can be made from ice.  An orion require nuclear bombs, metalic hydrogen is likely much harder to make as in require an larger and more energy hungry factory but might not require all the special materials like uranium.  

Engine size is also an factor, very heavy engines is only practical for large ships so an mission to Jool deploying a base and an some landers with an orion makes sense, but the landers used mined hydrogen and oxygen as they can then be used over and over. 

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Landing with an Orion drive seems like a fantastically bad idea unless you build a very very specific kind of landing pad that's basically "the Orion drive's pusher plate but upside down and firmly embedded in the ground"

Said kind of landing pad would also be the only feasible way to LAUNCH an Orion drive craft directly, and not have to use some form of auxiliary propulsion method to get the vessel to a high enough altitude where it won't either obliterate the launch complex (if launched from a surface colony or KSC itself), or just kick up so much surface debris that it damages the vessel (if launched from bare ground).

The main reason that we see rockets launched from launch pads is because that's just the shape of the thing that can fuel and drain the rocket, and also not cause the rocket to suffer damage when being launched (Even just regular chemical rocket engines can be so loud that when the sound from them reflects off the ground and back up, those reflected sound waves can be strong enough to physically damage the rocket! This is one of many reasons why they have "water spray sound suppression" systems on most launch pads).

 

Of course, as far as sourcing the stuff, there's actually two kinds of Orion drives, and they have different fuel requirements.

The basic "Just make it work as simply as you can" type of Orion drive (and the least efficient) is the Pure Fission Orion drive. This uses pulse units that aren't much different from the Fat Man bomb, as they're probably "fission pit implosion" type devices.
The drive itself is pretty much just mundane materials, after all the most prominent part of it is the hydropneumatic shock absorber system used to couple the pusher plate to the rest of the vessel, and that's well known technology (just on a very very large scale) and the handling mechanisms for the pulse units are basically similar to that of a 5 inch anti-aircraft gun (rapid fire projectile dispensing mechanism).
The pulse units themselves are where the problems start. They would obviously require Uranium or Plutonium, some form of high explosives (probably doable if you can make decouplers and stack separators), and some electronics to handle safing and arming, fusing, and the timing of all the detonators to make the thing actually go off with a significant nuclear yield and not just make an expanding cloud of plutonium atoms and whatever is left over of the fusing and trigger explosives (probably doable if you can make probe cores). After all, we're talking about nuclear explosives here, while a dud wouldn't be a GOOD thing by any means, it's much preferable to the alternative of the pulse unit going off when you don't want it to go off.

 

The more advanced "Make it actually useful for doing things" type of Orion Drive is the Thermonuclear Fusion Orion drive. The drive itself is mostly similar to the Pure Fission Orion drive, but as you might guess, this uses pulse units that get most of their energy from Fusion, the fuel for which is Lithium Deuteride, and the trigger for which is a fission nuclear device similar to the pulse unit for the Pure Fission type Orion drive. The interesting thing here is that you can in fact make a "directional" thermonuclear device, which means that you can make a Thermonuclear Orion drive pretty efficient since with a well engineered pulse unit you can direct roughly 80% of the energy of the pulse unit in a direction where it will impact the pusher plate. If you want to look it up, this is called a "Casaba Howitzer" for some reason, but I'm pretty sure most of the "here's the math we did" kind of results for it are still highly classified, since we are talking about nuclear explosives after all.
The drive itself would therefore have similar construction requirements to the Pure Fission Orion drive, but the pulse units are again where it gets interesting.
For the sake of simplicity, we may as well say that all fusion fuel is the same. So it could be that the drive needs hydrogen isotopes, it could need Deuterium and He3, or it could need Lithium Deuteride, but the differences aren't really that large in the grand scheme of things because given enough research you could feasibly make any of the fusion drives use any of these combinations of fusion fuels. Granted, they already have the best fuel for the particular drive picked out, but the distinction isn't that relevant unless you're a nuclear physicist and not a rocket scientist.
So the pulse units would need all the things that the Pure Fission Orion pulse units need, and then a large quantity of the "Fusion Fuel" resource, whatever they decide to  call it.

 

It is my belief that Orion drives of all types are not suitable for interstellar travel. They have a lot of thrust, but for a drive using nuclear physics to create thrust the specific impulse is not that great, especially if you use the Pure Fission Orion drive. Of course, that doesn't mean they're useless, in fact they're great (nearly ideal, the only downside is the difficulty of sourcing the pulse units) for interplanetary travel.
Oh and one thing I forgot to mention, Orion drives actually work BETTER inside an atmosphere, because that means they have more reaction mass available to heat up and project at the pusher plate (the atmosphere itself that is between the pusher plate and the pulse unit will contribute to thrust, so in a way Orion drives are one of the few kinds of drives that have increasing ISP as atmospheric density increases).

For interstellar travel, you need to step up to at least a Pure Fusion pulse drive, such as the Daedalus drive that we see in the very 1st KSP 2 trailer that was released.
Even then, you have to be specific about which kind of pure fusion drive you are talking about. A magnetic confinement fusion drive that injects plain old Hydrogen into the exhaust to boost thrust won't work, the specific impulse is not high enough. Again, great for interplanetary travel tho, and a good bit easier to get the fuel for it than for an Orion drive (but the Orion drive can move more payload at a time because it has much higher thrust).

Chemical propellants are really all about the CHON group of chemicals. Carbon, Hydrogen, Oxygen, Nitrogen. Same stuff that life as we know it is mostly made of.
Hypergolics and Monopropellant need a source of some Nitrogen containing ice, liquid, gas, or mineral (because Hydrazine is H4N2, UDMH is C2H8N2, and the oxidizer for both of them is N2O4, so really there's quite a lot of nitrogen involved unless you're saying Monopropellant is in fact hydrogen peroxide in which case all you would need is Water ice).
Hydrolox just needs Water ice (melt and electrolyze water to get H2 and O2 and then liquefy those gases to get LOX and LH2).
Methalox needs CO2 and Water (or some other source of Hydrogen, but Water makes the Sabatier reaction's equations and the equations of methalox rocket fuel balance out nicely).
Kerolox needs, well, pretty much it needs a planet that has crude oil and therefore likely abundant carbon-based plant life on it for millions of years. Alternatively with much more energy input you could create a synthetic longer-chain hydrocarbon from Methane feed-stock that acts like Kerosene/RP-1 as far as a rocket engine is concerned but is likely not quite the same when chemically analyzed.

So the way I see it, if you want an ISRU converter, you're going to have to source it from either Dres (lots of easily accessible Platinum-group metals needed for catalysts and that sort of thing) or Kerbin.

If you need a whole lot of metals for general building purposes, you're going to need to go to Moho (Mercury-like planet so small but highly metal-rich near the surface), or again maybe Dres or Ike or even Gilly.

Since they now said Minmus is glassy, if you want ceramics you're gonna want to go to Minmus.

Fusion fuels, you go to either a gas giant or a planet with a water ocean (IRL it's projected that there's enough deuterium in Earth's oceans to provide our energy needs for a couple thousand years (even factoring in growth in energy needs as time advances), but there's millions of times more in Jupiter's atmosphere, and Jupiter has Helium-3 and probably a little bit of Tritium as well).

As far as Uranium and Plutonium and things like that, the Mun would be a good place to look, if you can't find enough on Kerbin. But really, that stuff should be around on any rocky planet.

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This is why I hate / love KSP. Reading threading thinking if a had 2 Orions back to back to build a cargo cannon between planets?

Aft blast accelerates the tug and the cargo which then seperate so a forward blast pushes the cargo up to speed and stops the tag breaking orbit.

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On 12/27/2021 at 5:52 AM, magnemoe said:

Probably not so much handling issues except radiation is likely to be an thing so no more strapping two nerv on the side of an hitchhiker module and more of the long spine style of ships you see in hard science fiction. 
Now getting the fuel will be the challenge, hydrogen and oxygen can be made from ice.  An orion require nuclear bombs, metalic hydrogen is likely much harder to make as in require an larger and more energy hungry factory but might not require all the special materials like uranium. 

Hmm, that seems much more reasonable, I always thought some of the stock craft designs were a little silly with the NERVs like you mentioned (strapping Kerbals to ion engines is fun though). Handling nuclear fuels will certainly be an interesting challenge.

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