Atomic Annihilation Solid Antimatter Engine

More than anything, an interplanetary (interstellar) engine concept of enormous dimensions and exceptional efficiency for medium sized payloads.

As in the plans, it fuses liquid hydrogen with solid antihydrogen to obtain a nuclear annihilation reaction, using the energy released as a propulsion method. (This mockup has heavy thinkered engines).

Unlike metallic hydrogen engines (which can be smaller), these engines themselves need huge magnetic field currents to keep the reaction stable. Since in itself, antimatter, for obvious reasons, cannot even come close to ordinary matter. One of the reasons for their huge nozzle sizes, apart from the fact that every nuclear reaction releases huge amounts of residual radiation. These types of engines also come with considerable amounts of radiation protection.

I didn't do too much research as the precise (or estimated) performance characteristics of these engines are not entirely accurate or are unknown. Besides, it was difficult to take a correct model to do it, what took me the most time were the engine adjustments, chronologically it is not far from the current era in which SR2 is developed, so its performance is not superior to an ordinary NTR engine (but thrust buffed).

One of the most crucial decisions in the design itself was the color of the exhaust gases, although these engines would give more of a strongly irradiated and ionized plasma than fuel vapor. So more for technical reasons, the selected color was purple, more than anything because precisely it is hydrogen, something close to metallic hydrogen engines but also distant in operation.