Megastructures can speed up interstellar travel, but they have to be HUUUUGE

In this blog we consider using megastructures to speed up an interstellar ship and whether we can beat the estimates provided by rockets. Previously I spoke about rockets, (also on substack) which can reach on the order of 1-10%c and really struggle beyond that either due to the rocket equation, energy density or heat dissipation issues reducing the potential acceleration. Megastructures need to be used in “established” systems only and have to be used to both speed up and slow down ships. Megastructures have very high theoretical limits, so I will attempt to give a sense of scale required to accelerate a ship around to 0.1c. I am going to consider the following structures: light or laser arrays, mass drivers, proton beams and giant slings (yes, really). Light array or lasers. A light array is a structure that focuses sunlight from a particular area into a beam that then drives the sail. It is similar to a laser, but can be a lot more efficient, since one doesn’t have to go through a an intermediate step of light->electricity -> light. This means the sail can be smaller and still receive a substantial amount of light. We consider a focusing light array of A square meters than can exert all of its pressure onto a nearly perfectly reflective solar sail that moves a ship of mass M. The light array is at certain distance from the Sun, where the light exerts a certain amount of pressure. Around H portion of light is transferred to the surface as heat. There is a big issue of this setup, which is light diffraction and dissipation over large distances. I am basically going to ignore this effect entirely and assume that we can solve this technologically by creating advanced lenses along the pathway which refocus the light into a point (this may or may not be possible). The nice part of this setup is that the bigger the array, the more momentum one can transfer onto a ship. So, hypothetically you can move a ship arbitrarily fast if you focus enough of a stars energy onto its sail. The problem is once again heat dissipation. No surface is perfectly reflective. Some amount of light will always get inside the surface and become heat. While the amount of reflectivity of a surface is much higher than engine efficiencies, it is still not 100%. If we wish to accelerate a ship of mass M at an acceleration of 1 G, the amount of total light power we would need is: (m * g * c) / 2 and the amount of heat needed to be dissipated is (m*g*c) *h /2. Plugin in m = 1 kg, g = 10m/s^2 and h = 0.001 (a very optimistic reflective surface), we get heat = 1.5Mw Once again, this is beyond anything we could plausibly expect radiators to handle, but if we drop acceleration to 0.01 g (0.1 m/s^2), we get a reasonable amount of 15 kw per kilogram of spaceship mass, which gets into a theoretically realistic range (see previous post). The issue with such a small acceleration is that the amount of time that the light needs to be active becomes immense. The distance that this acceleration needs to be active is given by the formula of (v^2 / 2 a), which for v = 0.1c and a = 0.1m/s^2 is equal to (0.1 * 3 * 10^8) ^2 / (0.2) = 4.5 * 10^15 meters, which exactly 1000 times the distance between the sun and Neptune. This term v^2 / a shows up everywhere in the concept of megastructures and gives you a sense of the problem. We don’t just need a hefty laser with a few lenses floating around in the solar system, we would need to extend this system of lasers and lenses far into interstellar space. Another important concern is how large each laser need to be or consider how big the surface area needs to be to get this level of laser pressure from sunlight. At m = 10 ^8 kg (around the size of an aircraft carrier) and acceleration of 0.01g (0.1ms^2), we need about 10^7 N of force. The solar pressure around earth is around 10^-5 N per square meter, which means we need about 10^12 square meters= 10^6 square kilometers. This means a laser or a solar lens would need to concentrate around 1000 by 1000 km worth of sunlight on an aircraft carrier sized ship to move it at around 0.01g. If we a just considering one specific mirror, this seems doable, however, this level of sunlight pressure needs to be maintained from a specific point in the solar system outwards till around 1000 times the distance to Neptune. Mass drivers Image is NOT to scale. For our purposes, the ring would has to be WAAY larger. Mass drivers is basically “train tracks in space.” There are tons of potential designs for such train tracks. Coilguns have a nice property that they don’t create much heat in the ship and don’t have to deal with rail deformation. Railguns are another option, though large material and electrical pressures mean they are harder to keep using over and over again. Current maglevs are similar to coil guns, regular train tracks are more similar to railguns. The design space is rather large, so we’ll consider the general issues core to every design. Using many mass drivers is not