A Recurrent Nova Engine or (RNE) is a mega-structure that utilizes repeated artificial novae as a power source. This power source most commonly uses white dwarfs as a core. Research into using other objects such as neutron stars or black holes are ongoing. Such objects are the crowned jewels of any civilization to create them. The sheer quantity of the energy produced dwarfs even the largest of dyson swarms.
The idea of using Novae as a power source comes to almost every species just as the idea of englobing stars in Dyson Swarms does. Most species, however, simply never grow to control enough energy to even seriously consider the technology. Only the largest of civilizations are able to create them. Currently, they are used for the mass-production of elements beyond iron that a fusion reaction would absorb energy for. The power of the average star is used within seconds to satisfy the large demand of civilizations for heavy elements. For most of history, heavy elements were simply harvested from moons or asteroids. Having this technology is a game-changer on the galactic stage.
Currently, there are fifty-two of these objects in service around the known universe. They are all publicly owned with companies renting out energy and space on the device to create rare isotopes or large quantities of transuranic elements at a far cheaper rate than around a star. Even so, they have yet to turn a profit for the government's owning them due to the ludicrous price of these objects. These devices are the Kardashev III civilization's equivalent to an early Kardashev II's stars.
The RNE is a large sphere mostly made up of power-harnessing devices. This sphere is the largest type of megastructure commonly built at 1 light-year across. Like the power-generation devices commonly built around stars, a RNE can be a solid sphere or a dense collection of power collectors and habitats. In the center of this sphere is a white dwarf. Any white dwarf would work, although the white dwarves usually mass between 0.75 to 1.25 M☉. While designs vary, these objects are usually built with several layers in order to absorb as much energy as possible.
This device is typically located near the center of galaxies to allow for the easiest power transfer to other areas. The large size requires large numbers of automated asteroid and debris defense lasers to constantly vaporize potentially harmful objects. Communications across RNEs often require faster-than-light communication for the coordination of power collectors and visiting craft.
RNEs require extensive supplementary infrastructure around them. After several firings, the white dwarf in the center is considered "spent" and has to be replaced. The white dwarf is then surrounded by a harness and accelerated out of the RNE as a new one is pushed into the center. This process usually takes 25 years using the most advanced propulsion methods. After the spent white dwarf is removed, it is often divided into two smaller dwarfs to be used as fuel or as a new core.
The device creates power once every 7-8 months when an object with the mass of a brown dwarf is slowly dropped onto the white dwarf. The extreme gravity of the white dwarf will cause the hydrogen to rapidly begin fusing, creating a Nova. This requires pure hydrogen gas (specifically protium) to be as efficient as possible. Only 1/10 of the fuel will be fused, so most of it will be collected and used again.
The Nova itself takes around 25 years to get to the sphere itself. Were the sphere much smaller, it would be vaporized by the regular novae. This device is able to create up to one billion times the energy output of a small Dyson Swarm within a year, although it takes at least a dozen millennia to set up, while a Dyson Swarm takes decades at the longest.
Compared to objects such as black holes, RNEs make up a fairly small percentage of the population. They are usually mostly automated residentially powerhouses used for the synthesis of materials. These objects always represent the majority of a civilization's production capacity (if they choose to build these objects). Supermassive black holes make up the next largest block for production capabilities with regular stars making up the smallest average production capability cumulatively. Due to large galactic-scale wars such as the War of the Ancients, most of the objects in the vicinity of an RNE are equipped with weaponry. Most have massive military bases integrated onto the surface of the objects.
The most important RNEs are those which house large populations. Nocturne is an example of an RNE used not only for production, but for housing. These objects offer enough power for entire civilizations, causing an interesting form of civilization in which the entire population entirely centered into the area of one star system. Dotskgard is an example of such a civilization.
There are only a handful of significantly different RNEs in the universe which count as a variant. The most common of these variants is the RSE, a Recurrent Supernova Engine. These objects dwarf even the RNEs the same way a supergiant dwarfs a red dwarf. The most notable RSE is the object Mili. Despite construction beginning on this object over 75,000 years before the present day, the object is still only halfway complete. Already, the object is generating enough power on its own to make its nation qualify as a Kardashev III civilization despite being only ten light years across.