The reason why a “low” reaction mass ratio of 60% is more reasonable with high exhaust speeds (~0.86c) compared to a chemical rocket ratio has to do with E=mc^2 and how relativistic exhaust velocities “lose more mass” than the actual mass out the back of the ship. And so, the 60% has to be both “actual reaction mass” and the mass allocated to “pure energy”. Given that the only way to store “pure energy” is anti-matter and containing anti-matter has a ton of overhead, the complexity of fuel tank is way higher than simply attaching a jug of chemical fuel.
Yes, my “more realistic” design is roughly based on the idea of a fission fragment drive, except for fission we use fusion. It’s hard to estimate the max reaction mass available to such designs. Most but not all fission mass can be made to travel at 3-5%, for fusion it is a mix of speeds from 5-17%. Upon thinking about it some more, 10% might in fact be too low, especially if we use stages and drop parts of the ship along the way. So, yes as I mentioned, this is not the very “top of the tech tree”. However, due to the heat constraint and the low acceleration mentioned in the article, getting more fuel to get to a higher max speed might not be that useful for shorter hops (5 light years). If it takes 95 years (3 * 10^9 seconds) to get to 10% c (3 * 10^7 m/s^2) at 0.001 g(0.01m/s^2), then you are way past the half-way point of the trip and you can’t decel. There is a nice point somewhere between 1%c and 10%c, but i haven’t taken the time to compute it yet.
So yes, the proposed design of 10% is not the “max speed,” for rockets, especially for long journeys, just a sample estimate of what a civilization with far better, but not perfect physics knowledge would be able to do.
“this seemingly has another implication you don’t mention: that a miniaturized spacecraft should work better than a large one from a heat perspective.” Well, it’s a little complicated. We are not radiating heat from the entire surface of the ship, rather we use some sort of external radiator to do so. Given the same exact shape, surface area decreases compared to mass, however as mass increases we can construct more and more elaborate shapes (think tree-like radiators).
Also i am assuming the spacecraft has to carry humans (at least on some missions) and nuclear powered engines may need a certain minimum size anyways.
The ship already needs to have a mirror as sail to be as close to perfectly reflecting as possible, mirrors get more momentum and absorb the least heat. Yes, it is possible to setup a system of mirrors to improve efficiency, that would certainly cut down on the total amount of light you need to collect.
I looked into black-hole related mechanics for this article and decide to not include them because they are very counter-intuitive and not as helpful as you might expect. Even if you don’t get wrecked by tidal forces, and you operate at the orders of 10-200*event horizon distance, the main issue with black holes is that it’s hard to actually leave. If you have enough speed to properly cancel out centripetal acceleration to a 1G level, this is very unlikely to be enough speed to have escape velocity at that orbit and what you’ll get is a larger orbit around a black hole.
Black holes can be used to boost rocket efficiency burns and there are theoretical frame dragging effects near rotating black holes that have been hypothesized to exist, but have not been experimentally verified. If they are real, they can be used by rockets to steal further momentum.
Larger stars that are not black holes might have a sweet spot of mass where helical orbits are vastly superior to circular ones, but i haven’t done the math to properly verify this.