[WIP] Fuel and ISRU rebalance mod [FIR]

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Fuel and ISRU rebalance mod [FIR]


Source: MIT AeroAstro presentation "Utilizing Molten Regolith Electrolysis Reactors to Produce
Oxygen on the Moon", 2015. The Installation shown above is designed to make 10t of oxidizer per year.


What is this mod about?

Brief summary:

FIR is a mod that rebalances prices of stock fuel (cheaper) and mixture mix used by stock engines/fuel cells (3:1 instead of 1.1:0.9). Additionally it makes behaviour of stock ISRUs a bit more realistic (can only produce oxidizer from ore + work much slower + other tweaks). The mod aims to approximate KSP stock behaviour to reality and thus be a middle ground between the stock game and mods like RO/RealFuels.

Detailed description:

This is a mod that rebalances couple of things that  limited my gameplay and at the same time the learning value of KSP.

1) stock ISRU can only produce oxidizer now. And it is a very slow process (however faster than in reality). ISRUs, scanners and ore tanks are moved earlier in the tech tree.


Oxidizer only - Making oxidizer of any kind from ore (regolith) is broadly researched in many working papers concerning both Mars and Moon. It's decisively simpler than making all parts of propellant and can be done virtually anywhere by 'just' heating feedstock to a high temperature (in a process called Molten Regotlih Electrolysis). It also doesn't require any inputs except energy. Conversely making both oxidizer and fuel out of ore or even water is quite complicated (taking into account all requirements that accompany off-world manufacturing) and energy intensive process - to such an extent that even known papers about it seems to not get past theoretical stage (G. Landis and others). For now the most advanced in testing, simplest and reliable technology assumes making only oxidizer part of fuel.

Lower processing speed - the most advanced MRE ISRU designs currently tested (on the picture above) are able to make about 10t of oxidizer a year assuming continuous operation (on the picture above).In comparison a small stock ISRU is able to churn out the same amount of oxidizer in an hour (0.55 unit/s * 5 kg per unit * 60 * 60 = 9.9t). Therefore my mod is going to rebalance small and normal ISRU's LOX production to about 28t and 160t per Kerbin year (1.2 and 6.6 jumbo tanks accordingly).

Increased weight - in reality MRE regolith>oxidizer making system (so ISRU+tanks+power source) has a weight efficiency of about 5-7 (measured as a weight of oxidizer production per year divided by ISRU system weight). Therefore meeting production quota above (28/160t) would require much heavier ISRU units. Again I'm happy with approximation to reality so the mod make small ISRU weigh 2t and large 8t. Moreover large ISRU is much more efficient so there is a reward for bringing larger ISRU to the planet's surface.

Lower technology requirements - by moving tech requirements earlier the main difficulty is moved from researching the technology to actually making it work. So the player would need to send and ISRU system in advance to produce fuel (so similar to assumptions of some current MARS/Moon missions). It should create interesting situations when player can utilize ISRU before upgrading science compound in a career game. Therefore with careful planning he can extend the range of lower tech rockets and reach further planets so the gameplay could be more interesting in early career compared to late game.

Energy demands for small isru remain unchanged at the moment. Standard ISRU however has 4x higher EC demand (4x bigger size and 50% better efficiency = heating more regolith to a higher temperature).

2) Changed fuel mixture ratio for stock tanks, engines and fuel cells to 3:1 (from 1.1:0.9). This is a compromise as to what stock LiquidFuel might be since sometimes it acts as RP-1/Kerosene (jets, big engines) and sometimes as liquid hydrogen (LV-N).


3:1 ratio seems to be a good compromise and generalized mixture ratio that encompasses different fuel types:

Typical oxidizer to RP-1 mixture ratio = 2.56:1 (RD-180 2.72:1)
Typical oxidizer to methane mix ratio = 3.2:1
Typical oxidizer to liq hydrogen mix ratio = 5:1 *(from 4:1 to 8:1)

So we can see that no matter what popular fuel mix we take they always use more mass of oxidizer than fuel.

This means that player can refuel now a significant portion of a fuel (75% of mass) in the form of oxidizer. Also 3:1 ratio means that it's easier to make quick calculations by seeing whether you have more than 3x times more oxidizer than liquid fuel.

3) Liquid fuel, oxidizer, monopropellant and solid fuel are much cheaper now. So player using SSTO or recovering stages gets much higher cash bonus than before.


I didn't like how my big SSTOs or rockets still used quite a lot of fuel even though they were recoverable. In stock KSP - depending on your design - fuel costs can comprise even 10-20%+ of a spacecraft or almost 40% of a stock tank. In reality fuel costs are about 0.4% in case of a space shuttle so very little compared to space vehicle costs. Since the parts' total cost remains the same, the parts' dry cost is relatively higher while fuel price is

So now player that invests a lot more time to build recoverable parts or/and fly SSTOs is rewarded with higher percentage of recovered funds.


4) - for people using Atomic Age mod by porkjet - I've changed fuel mix and isp value for LANTERN engine in Atomic Age Nuclear Propulsion mod by porkjet:

Using stats from this page:

and reducing isp to stock values, I've increased slightly isp in LOX augmented mode to 500isp instead of 455. In reality isp for 3:1 fuel mix is expected to be in range of 631-647s. Isp for LF mode only remained unchanged (720s). LOX augmented mode uses now 3:1 fuel mix.

Therefore LANTERN engine has now the highest isp (500) from all engines that can be refuelled with an oxidizer (since LV-N can't be refuelled with new ISRU mechanics).

Aside from more challenging and (imho) interesting gameplay this rebalance increases educational value for KSP; a player:

- can see that oxidizer is making a bulk of the rocket fuel (similar to real world)
- observes that ISRU can extend range of the rocket by making only oxidizer however implementation of it requires planning ahead (similar to real life). Also it's rather not viable as a part of a spaceship due to system mass (heavier) and rate it produces oxidizer. The main purpose of it is now to sit on a surface of a planet and produce small amounts of oxidizer for the long time.
- can note that fuel costs in rockets are very small thus researching and developing reusability is a key point to savings and extending the space program.

And the last point - if you still reading :) -this mod will be a basis for my next mods which involve low-tech/low-isp alternative isru fuels/propulsions that were researched extensively due to the ease of implementation compared to more advanced/complicated and therefore less viable solutions.

No download link yet, the project is on the drawing board at the moment.

I welcome all suggestions and discussion about the mod and ISRU stuff in general:)

Edited by riocrokite

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Is there going to be an option to produce liquid fuel later down the tech tree?

If not, can you include an option to use oxidizer in the NTRs? Less isp, more thrust due to higher molecular weight.

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Yes and yes,

the idea is to have a basis with only oxidizer isru. This would give an area for other mods to have other types of isrus that can make particular fuels (at the cost of mass / EC usage / general complexity)

NTR with this option is already supplied by LANTERN engine in Atomic Age by porkjet - it has two modes. Conversely since 3:1 ratio NTR is covered by Atomic Age I could add different fuel mix for second mode for stock LV-N like 5:1 (so even higher thrust and lower isp about 450ish) so bigger portion of fuel could be refuelled.

However I have some issues with LV-N. It has such a good Isp. In effect other engines - even with refuelling oxidizer option - can't match LV-N effective isp. So I'm back to drawing board and trying to rethink the concept.

Ideally there would be 3 basic fuel types: liquid fuel (RP-1 - kerosene), LH2 (for LV-N) and methane. Plus oxidizer and other stuff that we have atm so monoprop, solid fuel and xenongas.

1) Liquid fuel would be basic stuff, hard to make on other planets (would need . It would be used by almost all stock engines (+ jet engines). Standard isp, couldn't be easily produced off-world.

2) LH2 - highly efficient fuel however tanks would be heavier (to compensate for storage issues), something along Nertea's concept. It would be used by LV-N (so effectively it would have less effective isp since the tanks are heavier). Other engines that could be used -> cryoengines mod by Nertea. ISRU production: electrolysis of water (insane energy amounts so probably would need a good nuclear reactor to do this). Water could be found usually near north/south pole (so it would be challenging to get there) or as an ice on distant planets (tylo? eeloo?).

3) methane - isp similar or a bit better than LF (higher tech requirements though), similar tanks mass. Cheapest fuel, maybe also cheaper engines and tanks (lower chamber pressure) + slightly better isp than LF (+3.5%). Would need to get some new engine models for that. It would have the best refuelling options: duna - similar to real life concepts: bring LH2 and react with CO2 from atmosphere, so you get about 4 times more ch4 than brought lh2 + all oxidizer part. According to Zubrin calculations you would get 106.7 tonnes of CH4/LOX from 5.8t LH2  that you bring. 2nd option - produce from water and Dunian atmosphere. 3rd option (and it gets interesting here) - methane could be found on other planets for example you could refill methane straight from eve's oceans (similar to Titan's lakes) or scoop it from gas giants (in stock only Jool but in other mods there are more of those types of planets). Methane clathrates could be also found underneath surface in more distant planets (eeloo? dres?). Maybe also on the floor of Laythe ocean?

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The LANTERN engine's not quite what I meant.

What I meant was have the NTR have an option to 'burn' oxidizer instead of liquid fuel.

Then again, hot oxygen is corrosive as Hades. So you've got good reason (other than gameplay) to say no to that.


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Got it,
yah it would be possible, haven't researched that option yet. Still my main issue is the op-ness of LV-N compared to other engines. So for now I'm stopping dev of this version of the mod and moving to something a bit different (vintage rocket type mod - simulates space exploration to 2010', so probably no LV-N at all (by design, harder gameplay and more in-line with current designs like mars direct etc) however more refuelling options and more efficient high-tech (or high-toxic) engines that reach about 450isp (or even 500isp) in the end game.

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working on custom tanks for the new vintage rockets mod; going to do a lot of conceptual work so tanks are consistent along with all the piping and stuff. Example below:


I borrowed and modified an engine from Ven's mod to see how it might look together, each color is a separate part.

Edited by riocrokite

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