A Recurrent Nova Engine or (RNE) is a mega-structure that utilizes repeated artificial novas 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 Penrose Spheres.
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 a regular star is used within seconds to create large portions of iron. Anything beyond that takes ludicrous amounts of power. For most of history, these materials were simply harvested from moons or asteroids. Having this technology is a game-changer on the galactic stage.
Currently, there are fourteen of these objects in service around the known universe. They are all publically 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.
The RNE is a large sphere mostly made up of power-harnessing devices. This sphere is the largest type of megastructure built at 16 light-years 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 they are usually 0.75 to 1.25 solar masses as to more easily move them to the proper location.
While designs vary, they are usually covered in large arrays of radiators. These allow waste heat to be dissipated by radiating away waste heat that cannot be used. Were it not for these radiators, the power-collection devices would overheat and most of them would end up vaporized.
This device is typically located near the center of galaxies to allow for the easiest power transfer to other areas. The large size requires quadrillions of automated asteroid and debris defense lasers to constantly vaporize potentially harmful objects.
Communications across the object need wormholes or some other form of FTL to allow for the coordination of the novae.
The object itself is usually accompanied by 50 or so red dwarfs. These stars cannot orbit the white dwarf, as it is much too far away and not nearly massive enough. These red dwarfs use various thrusters to remain in place. They supply power for when the device is not making power (which is most of the time).
The device creates power once every 7-8 days when the mass equivalent of a Jovian object 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 constant 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 over one millennia to set up, while a Dyson Swarm takes only a decade at the longest.