I've been quite happy with my FANTASY HACK rules as blog posts (rather than the way that I had done it—as a discrete Google doc or PDF) and have given some serious thought over the years to coming up with a similar rule-set for space opera. In fact, I gave this some serious thought before FANTASY HACK actually existed; FANTASY HACK is specifically designed to utilize my rules for a D&D analog, but before that I'd been using various m20 options for various other fantasy settings, and more. But before I even thought of FANTASY HACK, I'd already modified m20 into STAR WARS REMIXED and laid out the very first high level exploration of what AD ASTRA would be.
In the way that FANTASY HACK was evolved from my DARK•HERITAGE and later CULT OF UNDEATH m20 rulesets into something that is specifically "more D&D-like", AD ASTRA (unofficial nickname SPACE OPERA HACK) is an evolution of my STAR WARS REMIXED m20 ruleset that springs from something more narrow into something a bit more broad. I prefer "broad" to "generic" because I refuse to believe that D&D is "generic" fantasy—rather, it's a fairly specific fantasy in many, many ways that is defined by what the rules say the setting is like—which is not generic. But it is certainly broad, and many settings can fit into the confines of what the rules imply the setting to be like. The same is true for AD ASTRA—although I do have a vaguely Star Wars meets Traveller overtly space opera setting that goes along with the rules, I actually envision that as being little more than a sample; much as Timischburg is the sample setting for FANTASY HACK (or for that matter, how The Grand Duchy of Karameikos is the sample setting, only lightly touched upon until it got it's own splatbook later, of the B/X rules.)
As D&D brings with it setting assumptions and any setting that uses those rules pretty much has to buy into those assumptions, the same is true for AD ASTRA. It doesn't shy away from pre-Campbellian (or post-Starwarsian, if you prefer) "non-hard" sci-fi concepts such as psionic knights and space wizards—but it doesn't go the way of Warhammer 40,000 by being so overtly fantasy oriented that it has space elves and space orks, for instance. It's space opera. It's Star Wars and Leigh Brackett and Flash Gordon and E. E. "Doc" Smith and Edgar Rice Burroughs and Dune, etc.
What are the setting assumptions, then? They include, among other things, the notion that old-fashioned melee combat is still something common. People still have sword-fights in space, because it's still useful to use swords in many situations, and the idea of people reverting completely to firearms or vehicles to fight just isn't as much fun. This isn't actually as unrealistic as many people think—people survive gunshot wounds all the time in real life, of course—and modern military and police still use bayonets (pole-arms), shield walls (especially in riot police) and tactical tomahawks and machetes, etc. This is reinforced in the rules by virtue of the fact that the damage of firearms is not inordinately better than that of melee weapons. Not only that, the psionic knights are able to create psionic shields that make them less vulnerable to projectiles, and many of the technological armors do the same.
There is no interstellar communication technology other than sending someone as a courier with news, messages, and information through the bulk, or Outer Darkness. The Outer Darkness is a large extra dimension, as described in various brane cosmology theories, and by using complicated mathematical formula and literal "magic" space wizards can pierce the boundaries of the membrane (or known universe), travel directly through Outer Darkness, and reemerge within the known universe at a given point. The point or course would be to travel somewhere that you could not reach by conventional means, as in travel to another star system, or extremely distant travel within the same star system. Travel through Outer Darkness means traveling outside of the normal observed universe, including its 4th dimension of time, so there are some unusual effects of traveling through the bulk—one of which is that no matter how far you are traveling, it seems to require several days (as observed from within your ship) to do so, and the amount of time that passed within the observed universe also seems to rarely be less than about a week. There are limits to how far in real space such a bulk shortcut (sometimes called a jump, or bulk-cut) can take you, beyond which the amount of mathematical variables that can adversely affect the trip becomes suicidally unpredictable.
This means that traveling jump ships become something similar to an interstellar pony express at its fastest, which is the only way of connecting vast distances across the setting. This relative disconnectedness means that large central governments are relatively ineffective, and local processes of culture, politics, religion and more dominate rather than broader, galactic ones.
And although aliens exist, this is still a humano-centric setting. In fact, at least a third of the humanity, which is most of the characters you'll see, are the descendants of former northern Europeans and Americans—later joined by smaller contingents of Russian and Chinese settlers. The other two thirds are, of course, humans but not of original Earth extraction, so they represent completely foreign and alien ethnic groups, often with unusual physical features that developed as a response to particular environmental conditions from which they originated.
Anyway, this "menu" page serves as an introduction and author's note—a bit more esoteric and "inside baseball" in terms of content than the one I created for FANTASY HACK, but eh, whatever. As I make the actual rules posts that make up the content of the game, I'll be sure and include links here, so that this becomes your one-stop shop for AD ASTRA rules content.