💜6purplemoons💜
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- Oct 17, 2025
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A lot of the ideas in this are inspired by both the Cryogenic Engines mod by Nertea and the more recent Chemical Technologies mod by Charle_Roger. They are both amazing mods, and I encourage anyone reading to try them out if they haven't already.
This proposal is a simple propellant system for KSA’s engines that adds the capability of variety and creativity within crafts. The goal here is to give each propellant its own distinct niche and create meaningful tradeoffs that are fun to interact with and to introduce them over time so as not to overwhelm new players. The following proposal focuses only on chemical propellants:
APCP (Solid Fuel Replacement)
Cheap, powerful, and simple.
Cannot be throttled or restarted.
Examples: Shuttle SRBs, UA1205, Aerojet 260.
LOX/RP-1 (LFO Replacement, Kerolox)
Dense, compact fuel.
Great thrust for launch stages but lower efficiency than cryogenics.
Examples: Merlin 1D, RD-107, F-1.
N₂O₄/N₂H₄ (Hypergolics)
A catch-all for storable bipropellants (UDMH, Aerozine-50, etc.) with reliable ignition and restarts but less efficient.
Examples: Apollo SPS, LR87, R-4D.
N₂H₄ (Monopropellant Replacement, Hydrazine)
Simple and versatile, weaker performance but requires no oxidizer.
Examples: Simple RCS thrusters.
LOX/LH₂ (Hydrolox)
Very high efficiency, but bulky tanks and potential boil-off issues.
Examples: RL-10, RS-25, J-2.
LOX/LCH₄ (Methalox)
Balanced density and efficiency between RP-1 and hydrogen.
Examples: Raptor, BE-4, Prometheus.
Some engines may have multiple part configs (similar to BDB's amazing system) where they may have options to use different fuels in exchange for changes in specifications such as efficiency, mass, thrust, etc. For example, the LR87, which not only had a kerolox version used in the Titan I and a hypergolic version used in the Titan II-IV but also a hydrolox version that was planned to be used in the S-II and S-IVB stages but ultimately not chosen.
In addition, similar to Chemical Technologies, I think it would be great if not only these fuel options were available but also options to mix different fuels and oxidizers in some engines. For example, in a kerolox engine, you could have the option (for some engines) to replace kerosene with hydrazine for higher performance (e.g., the RD-119).
Common objections:
“This will overwhelm new players.”
Complexity is gated by progression. Early career will still feel familiar with solids and kerolox, and more advanced fuels will only appear when players are ready.
“What about part clutter?”
You don’t really need to have any more parts than KSP does; just balance the engines with their propellants. Tank parts can be shared across multiple propellants (e.g., a generic liquid fuel tank can hold any of the liquid propellants).
This proposal is a simple propellant system for KSA’s engines that adds the capability of variety and creativity within crafts. The goal here is to give each propellant its own distinct niche and create meaningful tradeoffs that are fun to interact with and to introduce them over time so as not to overwhelm new players. The following proposal focuses only on chemical propellants:
APCP (Solid Fuel Replacement)
Cheap, powerful, and simple.
Cannot be throttled or restarted.
Examples: Shuttle SRBs, UA1205, Aerojet 260.
LOX/RP-1 (LFO Replacement, Kerolox)
Dense, compact fuel.
Great thrust for launch stages but lower efficiency than cryogenics.
Examples: Merlin 1D, RD-107, F-1.
N₂O₄/N₂H₄ (Hypergolics)
A catch-all for storable bipropellants (UDMH, Aerozine-50, etc.) with reliable ignition and restarts but less efficient.
Examples: Apollo SPS, LR87, R-4D.
N₂H₄ (Monopropellant Replacement, Hydrazine)
Simple and versatile, weaker performance but requires no oxidizer.
Examples: Simple RCS thrusters.
LOX/LH₂ (Hydrolox)
Very high efficiency, but bulky tanks and potential boil-off issues.
Examples: RL-10, RS-25, J-2.
LOX/LCH₄ (Methalox)
Balanced density and efficiency between RP-1 and hydrogen.
Examples: Raptor, BE-4, Prometheus.
Some engines may have multiple part configs (similar to BDB's amazing system) where they may have options to use different fuels in exchange for changes in specifications such as efficiency, mass, thrust, etc. For example, the LR87, which not only had a kerolox version used in the Titan I and a hypergolic version used in the Titan II-IV but also a hydrolox version that was planned to be used in the S-II and S-IVB stages but ultimately not chosen.
In addition, similar to Chemical Technologies, I think it would be great if not only these fuel options were available but also options to mix different fuels and oxidizers in some engines. For example, in a kerolox engine, you could have the option (for some engines) to replace kerosene with hydrazine for higher performance (e.g., the RD-119).
Common objections:
“This will overwhelm new players.”
Complexity is gated by progression. Early career will still feel familiar with solids and kerolox, and more advanced fuels will only appear when players are ready.
“What about part clutter?”
You don’t really need to have any more parts than KSP does; just balance the engines with their propellants. Tank parts can be shared across multiple propellants (e.g., a generic liquid fuel tank can hold any of the liquid propellants).
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