Possible acceleration systems

[The Architect]

During flight, the ship needs to monitor the environment (although I don't think I need to get into details here), be able to make some level of course correction, resist to erosion, defend itself against hostiles. It can brake and accumulate energy by deploying a forward magsail towards the end. Ideally, it would run as fast as a Conversion drive or some drive of equivalent efficiency would permit. Ideally somewhere between 0.5 and 0.8c

Agreed, you probably don't need to worry about the details environmental monitoring on general principles, unless it plays a role in the story. Note that if the ship is potentially engaging in combat while in flight, being able to monitor the environment and detect incoming enemy ships/weapons could play a very important role in the story that could result in you needing/wanting to delve into this aspect of things more.

You're absolutely right about that. There is surely a page in the EG about that, I'll use it as a starting point in case it mentions sensors I don't know yet. I would imagine that at interstellar distances passive sensors would be the most important. I'll look this up in more detail

Course corrections in the realm of STL interstellar flight are doable, but take a long time. Given the already established use of magsails and such, your best bet might be to configure the magsail to interact unevenly with the galactic magnetic field to allow the ship to turn. Such turns will be very gradual (think years to decades to accomplish, depending on how much you want to turn), but in principle the ship could do a 180 given enough time. You can also in principle use this to generate electricity onboard. Like magsail based braking, this will also tend to slow the ship a bit, so this isn't something you can just use without limit. But its doable and the details may or may not be something you need to do a deep dive on.

As I understand it at the moment, only very minor course corrections can result in making the position of the starship impossible to figure out if it temporarily shuts down its drive and uses asymmetric magsail drag to change course by a random tiny fraction and then wait until the search volume becomes very big. Of course, the pursuers can also go stealthy and it becomes a game of cat and mouse, which could make for interesting peripeteia in the story. For example the starship could deploy decoys matching its drive signature to make the pursuers reveal their positions and drive them off until it's out of range of their detection and can start accelerating a little without being detected, and increase acceleration slowly as it falls away. Of course this would only work on a small number of pursuers. I would then have one pursuer with a very different signature manage to sneak up very close and send a miniature unit able to land on the starship, survive on the hull and dodge potential defences, get inside and remain dormant until later in the story when it is activated.

Combat at interstellar speeds in a STL framework is not a topic that comes up a lot in SF. I/OA can offer some notions (and also contribute to the OA projects info on this since this sort of thing also applies to OA), but I'd also recommend seeing if Sevoris has some thoughts on it or can point you at relevant posts on the ToughSF blog. One of the biggest single factors may be the amount of kinetic energy wrapped up in the ships and how that can be used (or have to be defended against) in combat. Put another way - one of the most effective weapons in this kind of fight could be throwing a bag of gravel (or even just a bunch of glitter or a lightsail) overboard and slowing it enough so that your opponent then runs into it at high speed with an effect like a bunch of high explosives going off in their face.

This is an interesting subject to delve in indeed. It is touched on in the Space Warfare, Modern EG article. I'll dig the matter some more. See if Isaac Arthur has anything interesting. I'm also digging the ToughSF blog, which has a lot of interesting articles.

Magsail braking can work very well, and works better the faster the ship is moving. I can point you at an article by Robert Zubrin about this. See HERE and the references at the end. Note that decel via this method starts out at multiple gravities and then drops off over a period of years (assuming high initial speed) and ceases to be effective once the ship slows down to about one half of one percent of the speed of light (1500km/s). After that you need some kind of other system to get down to 'interplanetary speeds' - rockets or some combo of rockets and other things probably.

In the OA setting, we presume that the Y11k versions of magbrakes can also operate as a ramscoop and collect/slow down/package and store interstellar material during the decel phase so that it can be used as reaction mass once the magbrake stops being effective. We don't have any hard numbers on this to share, but it seems doable. Using the energy generated during decel to make antimatter is a new (and fun) variant, but I'm not sure how much electricity you could actually extract or how much amat you might be able to make this way based on the energy available. That might be possible to calculate in terms of what the absolute physical limit is although the details could be left vague. And not sure you want to/need to get so much into the weeds to even calc the absolute limit for purposes of this story. Basically, it's an editorial choice on your part mostly.

I think I'll leave the details vague and just mention the drive can somehow convert interstellar dust into amat during the decel phase and then store it to use later as reaction mass to brake to interplanetary speeds.

In terms of the ship's velocity (.5-.8c) - Beamrider tech can do this in principle as long as you can keep the beam focused long enough. My preferred method is the 'smart dust' approach proposed by G. David Nordley in which a 'beacon laser' shines on the dust (which is actually a complex nanotech built construct) which in turn ejects parts of itself to push itself back into the focus of the beam. This system has limits (exactly how limited is open enough that you can partly make an editorial choice in this area), which generally mean the ship will want to accelerate hard at first and get up to cruise speed quickly while still close enough for the beams to work.

In terms of OA, for reaction drive ships with ramscoops, we describe using boostbeams to get the ship up to 'ramming speed' (the speed where the ramscoop works), then switching over to rocket drive using the scoop until the scoop is no longer effective (ramscoops have their own issues), then coasting until it is time to deploy the magbrake to slow down. This method has the option of only needing to get the ship up to ramming speed (generally ranging from 1% to 5% of c depending on who you ask) rather than all the way up to cruise speed. So your boostbeam energy and accelerator systems could be vastly smaller than those needed to get the ship all the way to .5-.8c.

I would like to avoid taking too much from OA, but at the same time I find the magsail to be a powerful symbol for its obvious analogy with old ship sails so I had started trying to come up with alternate designs using the magsail. So there would be 2 acceleration systems, the initial one and the one with a working ramscoop design (conversion drive equivalent, with amat probably).

I would then need to make a choice for the first acceleration system. I would like to avoid using OA's boostbeam and could look into Nordley's design if I want to have a sail used during that phase as well. The drawback is that it would make me have to either describe a complex system that has no bearing over the story or handwave it, which the readers may not appreciate either. I could alternately use something like an amat engine module that would detach itself at 0.1c when the magsail / CDrive-amat-equivalent starts functioning, and then fit another purpose (interstellar space surveillance). The advantage here I think is that the general concept of antimatter is simple enough to understand for the reader and if I reuse it in various forms, that will be one less thing the reader will have to learn about in order to understand the story, or put simply it will lower the conceptual load for the reader. I want to make sure I don't choke them with a conceptual fire hose.

In terms of drive specifics - Assuming you don't have conversion drive based on magnetic monopoles, you could use something involving antimatter (there are lots of designs). If you really want to push the limits of amat tech, I'd refer you to the q-mirror article HERE. Q-balls are a real think in RL theoretical physics and if you wanted to use a Q-mirror you could basically match OA monopole conversion capabilities or even exceed them in some respects depending on the details.

That's interesting, I'll have a look at that and see if there would be some payoff in making it matter in the story. If not, that will be useful for me to have a better sense of how things operate in this department