Reactors and Power Systems

One of those awkward things about space travel is that you need to bring everything with you. You have no air, no food, and critically, no power. Fortunately, being a member of a spacefaring civilization does give you a few options as to how you can provide power for your ships. As always, a ship designer has to make tradeoffs and choose what power supply(ies) to use, sometimes you just have to accept a suboptimal solution.

You might note that solar panels aren’t mentioned here. That’s primarily because not only are they passive, they’re also very dependant on what kind of star is nearby and how close you are to it. For some applications it’s the perfect solution, but here we’re looking at power sources and storage systems for mobile ships, which are expected to be travelling into areas where solar just isn’t viable.

RTG Generators
The humble RadioThermal Generator is a staple of spacecraft, one of the first major power technologies to be used for long term power generation, and fundamentally one of the most reliable. While never a source of substantial power generation, an RTG guarantees a stable, long term source of power that can function for years with little, if any maintenance. Powered by radioactive decay, the average RTG is good for about five years of continual power before its raw output starts to taper off.

RTG systems haven’t been used as a shipboard primary power source for decades, but they are still found in long range probes, lifeboats, and navigational buoys. Some ships will pair them with battery arrays, using the RTG for lower regular draws and the batteries to cover the occasional need for higher power output.
Nuclear Fission
For years, the basic nuclear reactor was the standard power source for use in space. While surpassed by more efficient and powerful systems, the basic nuclear reactor  soldiers on in a variety of uses, playing on its basic robustness and comparative simplicity. In space, one of the biggest challenges remains heat dispersal, which requires a substantial amount of support equipment not required with some other systems. Nuclear reactors require continual maintenance and monitoring while in operation, but on average only require a full refuel every five to eight years on average.

While a variety of isotopes can be used as fuel, each has its own challenges. In addition to different reactor geometry, some require radically different fuel handling systems and coolant, potentially adding to the complexity. The various isotopes also have different power densities and different mean times between refueling, which means a designer can have substantial flexibility in the design of the actual power plant.

While not used for shipboard power generation on most modern designs, the basic fission power plant still sees substantial use planetside, taking advantage of the surface (and any atmosphere) to easily sink out waste heat. Space stations in more remote applications also use the fission plant, trading the additional challenge of heat dispersal for substantially easier fuel logistics.

“Cold” Fusion
Long understood (and proven) to be a technical impossibility, Cold Fusion proved to actually be a viable power generation technology, alibet with a few fairly substantial  caveats. In broad terms, “Cold Fusion” replicates the base fusion reaction of a star at room or near-room temperatures thanks to the presence of a catalysing agent. A small amount of energy is used to kickstart the process, but once running it provides a net positive flow of energy until the fuel source is shut off.

Unfortunately Cold Fusion has several drawbacks. The first is a hard limit on how much power can be extracted from a single reactor. While more substantial than a RTG or even a conventional combustion-turbogenerator combination, it cannot be scaled up, only extended in parallel. For larger power draws a proper fusion reactor is preferable, as fusion reactors get exponentially more powerful for a given volume compared to the linear increase from additional cold fusion reactors. The other major problem is that cold fusion requires a catalyst. For the deWulf, that catalyst is a uranium isotope in liquid suspension. Worse, it’s not a proper catalyst but is instead slowly contaminated as the cold fusion reactor operates, requiring it to be eventually refreshed.

Even with these restrictions, the Cold Fusion generator is a popular secondary or backup power generation system (especially since it can be fed with basic hydrogen/proteum common with most modern fusion plants). Often times a major ship refit will include the addition of one or several Cold Fusion plants to handle additional power draw, instead of doing a full refit and replacement of the ship’s fusion plant with a higher rated model.
Thermonuclear Fusion
While cold fusion is a good first step, it is for the most part a side path. Certain concepts and are applicable, but by and large the core technological underpinnings of full on fusion. Still, the development of a full up fusion engine is mainly a challenge of engineering, not proving its scientific viability (the giant balls of fusion commonly known as stars prove it is possible). Initial reactor designs used raw hydrogen, but follow on designs moved on to deuterium/helium-3 and then to protium/boron as a primary fuel.

While large and complex, fusion engines generate phenomenal amounts of power. Better still, power output is a function of volume; a reactor twice the size actually generates something closer to four times the power output. Engineering challenges place an upper limit on the size of the reactor, mainly due to the challenges of building a sufficiently strong containment chamber. Still as material technology improves power densities follow along, helping the base technology remain viable even as power demands increase.

deWulf reactors tend to be heavier and have a somewhat lower power to volume ratio, but are more durable and easier to maintain. Krak reactors generally have an excellent power to volume ratio, but sometimes have production defects that make them quite unsafe. By contrast, Ibizan designs still use deuterium/helium-3 as a fuel. This trades a lower power density for an ability to do “wilderness refuelling”, presuming you have some onboard harvesting and refining equipment.

One final note about fusion plants, something shared with their more primitive fission counterparts, is the ability to run above their rated power level. While upgrades can be done to improve a safe baseline (a common upgrade is more modern containment coils that generally buy you another 5-8% more power), it is possible to bypass the feed safeties and feed in more fuel and divert some of the additional power to reinforce the containment system. This however is a balancing act, and if not carefully done can result in more power being generated than the containment system can handle. This kind of containment breach is almost always explosively fatal to the reactor, the operator, and whatever the fusion reactor is installed inside.

LiHy – Lithium Hybrid
Building off of conventional battery technology, the Lithium-Hybrid is the latest generation of classical chemical battery technology. Well understood, durable, and  efficient, virtually all ships in service make use of LiHy batteries as their primary emergency backup power source. Some designs have large battery rooms that centralize the backup power systems, but most ships instead go with a distributed model where small battery rooms are spread throughout the ship. This trades the efficiency and simpler engineering that comes with a single battery compartment for a more survivable solution.

The main challenge with LiHy batteries is that they are by definition hazardous. They are made of highly reactive chemicals and store large amounts of energy in an enclosed space. Fire or damage can cause the batteries to discharge or fail; the former can cause local system damage and crew injuries. The latter can result in explosions that can cause serious structural damage to a ship. For this reason, most civilian ships have their backup LiHy arrays installed in the interstitial space between the outer and inner hull. That way, any catastrophic damage caused by the arrays will vent outside the ship, reducing damage to their onboard systems and crew spaces.

 

Fusion (Lithium)
In some applications, LiHy batteries have several large drawbacks: Their maximum storage is dictated by their chemical composition, their structural design puts a soft cap on the amount of raw power that can be generated at any one moment, and their very nature makes them explosively vulnerable to battle damage. Enter the Fusion Battery. Instead of relying on a purely chemical reaction, the Fusion Battery stores power by fusing two elements into a third, heavier element. When power is required, the reactor draws down on the reserve of stored reaction mass, fissioning it and harvesting the power from the split atoms. While the Fusion Battery requires a substantial amount of power to charge, it provides an equally substantial amount of power in return. As a bonus, the energy storage element is substantially more stable than a chemical battery array. One flaw is that the charge/discharge cycle is not 100% efficient, and continued use causes a slow but steady loss in the fusion battery’s total capacity. Fortunately, this is easily fixed by topping off the base feedstocks.

This technology has only been introduced in the last few years for the deWulf, and the most common element used as a storage medium is lithium. While a chemically reactive element, it is easily producible with commonly available elements. Its relative nuclear instability substantially lowers the challenge in fissioning it back to its raw elements, which makes the battery substantially simpler compared to heavier, more inert elements. At present, the technology is only used in deWulf fleet units, as it is still bulky and expensive. But as the technology continues to mature, it will both become cheaper and more widespread.

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