The U-G3901 is a combination hyperspace tap and gravity manipulation system wrapped around a ram augmented jet engine. The resulting design is more complex than a traditional gravity-based engine, using heavy-duty force fields to direct mass in front of the engine into an intake system, then using intense gravity-based distortions to accelerate that mass out the rear of the engine to produce thrust. Unlike a conventional Bussard Ramjet, the reaction mass is not used for fuel, and as such doesn't impart drag to the ship.
The U-G3901 has the same problems as other gravity based drives, in that its efficiency is directly related to how close something is to push off from. This is solved in part by the front intakes, and in part by having tanks of reaction-mass on hand to inject into the engine when the interstellar medium is not dense enough for the needed amount of thrust.
The intense gravity-warping effects from the engine can also be harnessed for use in shields and FTL travel. The production model U-1 was designed with this engine in mind, and leverages the gravity-warping effects in this manner.
Four G3801 hyperspace taps are built into the drive system, and produce the staggering amounts of power needed to run the device. Excess power can also be diverted away from the engine to power other systems.
The U-G3901 design was being toyed with as an idea as early as YE 35 as part of the C4 and C5 development program. A single, massive version of this engine was originally intended to power the C4 cruiser, a fast moving manned starship. By YE 39 the C4 design still hadn't quite materialized, and the engine was instead scaled down for use in the production version of the U-1 Prototype Variable Mecha. The resulting design was no longer the size of a starship, and instead was more suitable for small craft.
The U-G3901 uses an active cooling system, and has both an intake for cold coolant and an exit port for warm coolant. Warm coolant can also be injected directly into the engine's intake to expel it from the engine entirely rather than passing it on to an external radiator. Red tubing indicates an insulated 'hot' coolant line while a deep blue indicates 'cold'.
Intense gravity warping technology, Hyperspace taps, and the complex field geometry required to maintain them all require technology more advanced than can be repaired by hand. In the event of damage to the engine, the entire assembly needs to be re-manufactured before it can be used again. Though the gravity manipulation panels and hyperspace taps are discreet components, they need to be fine tuned to work together. Replacing them by hand can lead to a catastrophic failure of the engine.
Complex field geometry is used to keep the engine running. While most engines use some form of thermal-rocket, the U-G3901 uses gravity manipulation to achieve the desired effect. Instead of exploding from pressure, a catastrophic failure of the engine can lead to the engine being torn apart and Spaghettified. Alternatively a failure in the constriction forces of the engine can cause the engine to crush itself, resulting the entire assembly and everything in close proximity being turned into a metallic gravy-like substance.
The engine was designed to fail gracefully, and will always attempt to crush itself instead of explode if possible.
The rear of the engine contains a pair of gravity manipulation devices similar to those found in the starship, however these panels are used for directing the flow of reaction-mass out of the engine, and can be used for thrust vectoring.
The entire assembly can also be used to generate an omni-directional anti-gravity field, and this is generally used to levitate the assembly along with the connected vehicle off the ground for take-off.