How efficient are solar sails / mirrors? I'm imagining a spacecraft that doesn't have propulsion of its own, just loads of thin sails. Back home, a giant array of lasers shoots light at it, to push it forward. When it needs to decelerate, it breaks into two pieces, the bigger piece continuing to accelerate forward while the smaller piece is decelerated by light reflected from the bigger piece.
My next post is about this. You get a similar constraint because even the best reflected surface let through some photons (~1%), which means your acceleration is still capped by heat dissipation. So you can get more than 1%c with a very large array of lasers /solar focusers in established systems.
Almost no ideas of "breaking up into two pieces" ever work in new systems due to the distances needed to decelerate.
Another approach to solar sails would be to use a megastructure to accelerate the solar sail bearing craft to relativistic speeds on a trajectory passing by many stars in succession. Each close stellar passage would be used to slow down the spacecraft and maneuver the vehicle onto a course towards the next star (via a gravity assist and solar sailing). After enough close stellar passages, the spacecraft would be traveling slow enough to decelerate and orbit its destination star (which might not be known at the start of the journey if the trajectory passes through an arbitrarily opaque region of space).
The main drawback of this approach is that as you increase your velocity, the journey becomes increasingly risky, since there's only a limited amount of stars reachable via maneuver before the galaxy runs out. If deceleration cannot be completed before the spacecraft reaches the edge of their galaxy, then it's almost certainly damned to a slow death in intergalactic space. A gravity assist from a neutron star, black hole or white dwarf could be used to extend a vehicle's time within the galaxy, but there's no guarantee that one will be accessible. To increase the number of reachable stars, it is best to aim for the core of the galaxy, since galaxy's stellar density is highest there.
No mirror is 100% reflective, some light is always absorbed.
Yes, but the nice thing about mirrors is that the bigger the surface area for reflection, the bigger the surface area for radiation.
and also bigger surface area for hitting something while travelling at close to the speed of light, so there's other issues there. magnetic shields? furl the sails when coasting?
Breakthrough Starshot or similar I assume?
The Starshot concept envisioned launching a "mothership" carrying about a thousand tiny spacecraft (on the scale of centimeters) to a high-altitude Earth orbit for deployment. A phased array of ground-based lasers would then focus a light beam on the sails of these spacecraft to accelerate them one by one to the target speed within 10 minutes, with an average acceleration on the order of 100 km/s2 (10,000 ɡ), and an illumination energy on the order of 1 TJ delivered to each sail. A preliminary sail model is suggested to have a surface area of 4 m × 4 m. ...
The fleet would have about 1000 spacecraft. Each one, called a StarChip, would be a very small centimeter-sized vehicle weighing a few grams.[1] They would be propelled by a square-kilometre array of 10 kW ground-based lasers with a combined output of up to 100 GW.[25][26] A swarm of about 1000 units would compensate for the losses caused by interstellar dust collisions en route to the target.[25][27] In a detailed study in 2016, Thiem Hoang and coauthors[28] found that mitigating the collisions with dust, hydrogen, and galactic cosmic rays may not be as severe an engineering problem as first thought, although it will likely limit the quality of the sensors on board.[29] ...
The light sail was envisioned to be no larger than 4 by 4 meters (13 by 13 feet),[1][55] possibly of composite graphene-based material.[1][6][38][41][48][56] The material would have to be very thin and be able to reflect the laser beam while absorbing only a small fraction of the incident energy, or it will vaporize the sail.[1][6][57] [to your point @Pasha Kamyshev]
The project was announced on 12 April 2016 in an event held in New York City by physicist and venture capitalist Yuri Milner ... Milner placed the final mission cost at $5–10 billion, and estimated the first craft could launch by around 2036.
Yes this has been suggested before
If we don’t see other civilizations because [...] they expand at only a small fraction of the speed of light
That seems backward to me. If a civilization expands at a small fraction of the speed of light, they should both reach a large fraction of space and reach us much later than their light reaches us, making them maximally visible.
On the contrary, one of the main advantages of the Grabby Aliens model is that it explains why we don't see alien civilizations: by expanding at an appreciable fraction of the speed of light, the time between the moment they become a galactic expanding civilization and the moment they reach us is relatively small, giving low prior probability of us noticing them the moment we turn our eyes to the sky (thus explaining away the Fermi question).
i see my phrasing was a little confusing. What i meant is that under the following conditions: a) aliens start very far away and b) they only expand at a tiny faction of the speed of light (say 1/ 100 000), they would need something like 10 billion years to colonize the milky way. Depending on what types of things they like to build in their systems we may or may not notice them until they were close. I am planning to write more posts on why "expansion" is likely much slower than mere "travel"
Basically, the reason Grabby Aliens just doesn't seem true is that the physics constraints on travel, let alone expansion seem pretty strict and the assumption of "expand at c" is basically impossible. Getting more precise constraints would be useful for thinking about the paradox more rigorously.
I don't feel like I misunderstood your phrasing:
- One of the main arguments in favor of the Grabby Aliens model is that it explains why we don't see aliens.
- Under your model, we should see aliens unless there's some reason they're not visible from far away.
- Therefore, one argument against your model is that it does not solve the Fermi question.
Thus goes the argument for Grabby Aliens:
- Assuming interstellar civilization moved very slowly compared to the speed light, there should be a bunch of visible aliens.
- There aren't a bunch of visible aliens in the sky (Fermi question)
- Therefore if there are interstellar civilizations, they probably move quite fast relative to the speed of light.
You can say, "well, let's discuss a bunch of possible reasons aliens are not visible", which is basically the literature on the Fermi paradox, but it doesn't solve it in the elegant way the Grabby Aliens model does.
So I guess your take on this is to compare the probability of expanding at c being possible based on this article's arguments, and the probability of Grabby Aliens model based on its own arguments?
This is not a model that fully explains the Fermi paradox, this is a model that talks about interstellar travel and what is possible. What is possible is an input into any model of the Fermi paradox, but is not a full model by itself. The reason why this make Grabby Aliens extremely suspect is that the entire assumption of "expanding a c" or 0.5c is not rooted in any plausible physical mechanism that can aid such expansion. Also travel is different from expansion, in so far that traveling to star system is not the same as "colonizing it" or making it viable to send ships forward from it.
"Grabby aliens" doesn't "solve" anything because it is based on the wrong assumption. Regardless of how elegant it's logic is, the wrong assumption disproves the whole model, which is not the same thing as somehow making my explanation (aliens are too slow) the correct one. There could be others, but we have to stick to what is actually possible.
Over inter-galactic distances a civilisation can easily expand at c with slow acceleration, so this doesn't discount grabby aliens but over larger scales.
it seems strange to me that you are assuming such low mass fractions (reaction mass as a fraction of total mass)? You mention 60% as if this is at the limits of physics, and 10% as more conservative. But much more extreme mass fractions are easily achievable, simply by using staging:
- the Saturn 5 rocket was around 95% fuel by mass at launch
- similarly, Starship is about 92% fuel on takeoff. these kind of numbers are necessary for reaching orbit
- speculative nuclear-powered interstellar missions, like Project Longshot to Alpha Centauri, would also have a mass fraction of around 90%
"the amount of power we need to emit is given in watts per kilogram and the amount of power we can emit is given in watts per square meter" -- this seemingly has another implication you don't mention: that a miniaturized spacecraft should work better than a large one from a heat perspective. it also seems worthwhile to miniaturize interstellar spacecraft as much as possible just to economize on the energy needed to launch them. hence many sci-fi books (and papers like Bostrom's "eternity in six hours") portray some extremely compact nanotechnological payload.
"overall efficiency: 90%
exhaust velocity: 0.17c
acceleration: 0.001g
reaction mass: 10% of the spacecraft mass...
...The exhaust velocity is still enormously beyond anything we can currently produce as a conventional rocket and the efficiency is far beyond any efficiencies of known tech."
arguably not true! fission fragment drives, per wikipedia, have "exhaust velocities of 3% - 5% the speed of light and efficiencies up to 90%", and NASA has been studying a design for such a rocket since 2023. though I'm not sure if you could accomplish the .001g acceleration or reaction mass fraction with fission fragment tech.
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.
Doesn't this assume constant, unrelenting acceleration? If the radiators can only dump 1/n the heat of the propulsion system, wouldn't it be possible to burn for one second and coast for n-1 seconds to bleed off the rest, repeating ad infinitum? The only constraint to max speed in both situations is drag from the interstellar medium, and with the higher effective constant acceleration you still run into that issue, albeit at a higher top speed.
Depending on the distance involved, the constraint is not just "max speed," the constraint is also how long it takes to get to that speed. If n is 100 000 as is in the example, and you need to accelerate on average at g / 100 000 = 1/ 10 000 m/s^2, it would take you 3 * 10^10 seconds to get to 1% c, which is around 950 years. For shorter hops of around 5 light years, this time dominates the cruise time at 1%c, which means lower exhaust speeds are actually faster due to lower heat dissipation needs.
Now, if you are going inter-galactic, you can use extremely low acceleration, but at that point it can just be continuous too.
Bursty acceleration is worse than continuous acceleration in all ways. The needed heat dissipation, fuel supply and top speed is the same in both cases, but the bursty engine has a higher peak power than the continuous engine, meaning it is heavier and harder to build. Existing rocket engines are very bursty because they have to maneuver quickly while close to planets. In the interstellar void, you're better off running continuously. Unless of course you have an engine that is intrinsically bursty, like an H-bomb, of course.
>but the bursty engine has a higher peak power than the continuous engine, meaning it is heavier and harder to build
Yes-but... that's an engineering problem, not a fundamental physics/"tyranny of the rocket"-type problem as presented in the OP
Interesting write up!
Do you have a kindle edition of your book, or otherwise sell it in open ebook format? I'd be interested to buy it if so.
I'm thinking that maybe you could focus first on launching an initial payload directly at a target, the payload is focussed entirely on surviving a crash landing, and internally contains an autonomous robot in charge of creating the infrastructure for a more comfortable landing.
One way to do that might be to target a super red giant - because they are huge but mostly vacuum, then cutting a chord across and might allow deceleration of "only" a few thousand g, something modern electronics can already be engineered to survive, allowing the payload to enter orbit around a the star.
yes, i have a kindle edition: https://www.amazon.com/Will-Stars-Contact-Pasha-Kamyshev-ebook/dp/B0FTTJ1F2Z
Yes, being able to send robots first is a likely strategy, but i am assuming humans (or aliens if we consider the Fermi paradox) will come at some point.
Generally speaking, using gravitational forces to slow down a ship in the solar system (reverse slingshots) can only subtract on the order of 1-10 km/s, which is much less than 3000 km/s = 1%c Yes, larger stars can do better, and black holes can be used for basically arbitrary deceleration, but you run into too many other problems with heat if you get close enough to either of those.
Robots and electronics can survive more Gs, which makes things easier and does open up some really creative solutions for slowing down. Maybe the big ship has a tiny ship inside of it which can be spun inside a particle accelerator along a particle stream and thus shot in the appropriate direction.
Thanks for the link, It doesn't work for me because I'm in the UK Amazon store.
Yes we'll want to send people, but robots could e.g. build a laser to slow down human ship.
I didn't mean to use gravitational slingshots, but to literally pass through the nearly vacuum of a super red giant, and use the few particles there are to air break.
Oh, just checked the UK market and it is available (i remember setting that up), but doesn't show up in searches for some reason (ugh, amazon). Thanks for the heads up.
https://www.amazon.co.uk/Will-Stars-Contact-Pasha-Kamyshev-ebook/dp/B0FTTJ1F2Z/
I have thought about the same thing, mostly "vibe-engineering" with Claude, there are things such as aneutronic fusion by inertial confinement, such as heavy ion beans, and directing fusion products (which carry most of the energy) by using magnetic fields, but then the recurring problem are the prompt gamma rays, which penetrate deep, so ablative shielding doesn't work. The fission version of that (using the products as exhaust) is the fission fragment rocket, but is probably worse. The ultimate problem seemed to be not the heating from the engine, but the ablation by interstellar dust, One way I thought to solve the problem is by using lasers to vaporize/ionize the dust, capture it with a magnetic field, as in a Bussard ramjet or a mass spectrometer, and then use the material to continuously reconstruct your shield.
Its worth looking at a big, H bomb or anti-matter orion drive from this thermal perspective. The reason it works despite needing to dissipate such vast amounts of waste heat is that the inefficiency-generated heat mostly goes into warming the non-fuel components of the reactor (specifically, blasting those components into multi million degree plasma). More goes into heating the pusher plate surface or sail to extreme temps, where it is removed by ablation (plate) or radiation (sail) without having to be moved to a radiator by pumps. Brb with actual math.
The radiators still need to dissipate heat from the shock absorbers, but at 1g this power is of scale (ship mass * relative velocity of ship and plate squared) not (ship mass * relative velocity of ship and exhause squared)
What about intergalactic photolithography? You can do some wild things with suitably tailored electromagnetic waves (holograms, EMPs). With a galactic-sized metalens seems like you might be able to print bootstrapping probes on the surfaces of planets in another galaxy. Even if the conditions on the planet has to be juuust right, there's a lot planets with a lot of surface areas.
I think it's fundamentally impossible to predict where planets will be with sufficient accuracy over multi year timescales. Even assuming you knew the exact centre of mass and size of every other body in the solar system, just variance in the solar wind causes multi cm level uncertainty.
I agree you're pretty blind and will have large uncertainties about the planet is, but I don't think you don't need to know exactly where planet is. E.g. suppose you sent a picosecond columnar flash of light to a star in a distance galaxy -- a solar-system sized wallpaper pattern of tiny 1 square mm MEMS chips. Since the light is columnar, you don't care about focal depth. Since the flash is a picosecond, you don't care about the velocity of the planet. Since the wallpaper pattern is of a tiny 1 square mm MEMS chips repeated the size of a solar system, you don't care about horizontal translation. You do care about the angle you hit that patch of the planet (head-on with the light or oblique) and the chemistry of what you hit but I don't think hitting some part of the planet in focus is an issue[1]?
- ^
I don't think bright fast columnar planar flashes are the best way to manipulate the surface of an alien planet (I bet you can be more clever; you have a 3d + time wavefield to work with), but I think it shows how you can hide away uncertainties in the location and velocity of the planet.
Writing truly hard science fiction, such as Will of the Stars means contending with the laws of physics as they actually are, rather than as we would like them to be.
In particular, I am going to assume that the speed of light is a real constraint and that the various tropes about FTL (wormholes, warp drives, and so on) are not feasible. Given this assumption, some futurists have modeled the speed of interstellar expansion as approaching the speed of light. The idea is that sufficiently advanced technology, intelligence, and engineering could eventually allow humanity, or another species, to colonize worlds almost as quickly as light can travel between them.
However, if we look at the laws of physics as well as the economic and physical incentives that govern expansion across the stars, there are many constraints on interstellar travel that appear long before we reach the speed of light.
Why is this important? Correctly estimating the speed at which a high-tech civilization can spread across the stars changes our perspective on the Fermi paradox. If we don’t see other civilizations because they are confined to individual star systems, or because they expand at only a small fraction of the speed of light, then interstellar expansion itself may be a very difficult but ultimately conquerable “Late Filter.” It also changes how we should search for alien civilizations and what kinds of technological signatures we should expect them to produce.
Understanding the physical constraints on interstellar travel, colonization, and expansion also changes our perspective on what our own civilization could look like in an optimistic future. Given how much visions of the future inform the present, it is worth making those visions more physically precise.
There are three broad ways of accelerating a spacecraft through interstellar space: pushing off against an external megastructure, using light or beams of particles from a large source, and carrying reaction mass in a rocket. It is worth distinguishing between two types of interstellar travel: travel between two already established star systems, and travel from an established system to a completely new one. In both cases, a spacecraft needs to accelerate to its cruising velocity and then decelerate at the destination. But unless the destination already has infrastructure capable of braking the spacecraft, such as a laser sail or a magnetic megastructure, the final deceleration has to be performed by the spacecraft itself. This makes the use of rockets inevitable for the sake of expansion from one previously settled system into a new one.
Relativistic Rockets
At relativistic velocities of either the rocket or the exhaust velocities, we need to use the relativistic rocket equation.
For a rocket with initial rest mass m_0, final rest mass m_1, and exhaust velocity v_e, it is:
For example, suppose a rocket has an exhaust velocity of 0.86c and a mass ratio of 2.5, meaning 60% of the rocket is going to be used for reaction mass and energy. Then:
The cruise velocity would be about half of this number, which seems reasonably fast. But getting an exhaust velocity of 0.86c is a massive engineering problem.
Energy density
The energy required to accelerate a particle of rest mass m to velocity v is
where
At 0.86c, gamma is about 1.96 and kinetic energy is approximately 0.96mc^2
In other words, accelerating 1 kg of reaction mass to 0.86c requires kinetic energy roughly equivalent to the rest-mass energy of another kilogram.
It is extremely hard to store this much energy.
For comparison, the energy released by nuclear fission corresponds to roughly 0.1% of the original nuclear mass, while typical fusion reactions release an order of 0.1%–1% mass as energy, depending on whether we are counting fission igniters and other parts of the bomb. For matter-antimatter annihilation, the entire rest mass of the matter and antimatter can be converted into energy, meaning it is capable of required energy densities.
To get a sense of scale, take a look at the Tzar Bomba explosion. It had a yield of approximately 50 megatons of TNT, which corresponds to roughly 2.1*10^17 J of energy, which is about 2.3 kg of mass converted into energy, equivalent to the annihilation of 1.15 kg of antimatter with 1.15 kg of ordinary matter under the ideal conditions.
An aircraft-carrier-sized 100,000-ton interstellar spacecraft, with 60% of its mass as matter / anti-matter fuel (30 000 tons of anti-matter and the corresponding 30 000 tons of matter) has about 25 million times the energy of the Tzar Bomba.
So, while energy density is “in theory” a solvable problem, it is worth understanding the scales of energy involved before the reach the real constraint: heat dissipation.
Heat
Even with an arbitrarily good energy source, a rocket cannot turn all of its stored energy into useful directed propulsion. Some energy will inevitably end up inside the ship as excess heat.
How much excess heat does the spaceship produce? It fundamentally depends on the design, however transferring energy from stored fuel into electricity and into propulsion or just directly into propulsion is a process that necessarily not 100% efficient. We can’t make it 100% efficient, without violating the second law of thermodynamics. And while this law does feel much more “possibly violatable” compared to most other laws of physics, I am just going to estimate that the max efficiency we can get is ~90%. To see why this is a *very* generous assumption. It’s worth keeping in mind that existing nuclear reactors max out at around 50% efficiency, particle accelerators lose 99% of energy to heat (or more), meaning they are 1% efficient or less. Chemical rocket engines tend to either recycle heat well or lose the heat as part of the exhaust, rather than towards the ship, which can make them rather efficient, but this efficiency quickly disappears once the amount of heat that can be recycled is higher than is need to sustain reactions or higher than can be spent in temperature of the exhaust.
The 90% refers to the efficiency of the WHOLE system, not just efficiency of individual components.
Suppose a propulsion system has efficiency n. The fraction of its input energy that ends up as waste heat inside the spacecraft is H=1−n
To see the scale of the problem, consider a spacecraft of mass M accelerating at acceleration a, using exhaust velocity v_e with propulsion efficiency n and H=1-n ends up as waste heat.
For a relativistic exhaust, the required mass flow (f) is
The kinetic energy carried away by that exhaust per second is
Substituting the mass flow and simplifying
If only a fraction 1 - H of the propulsion system’s input energy becomes useful exhaust kinetic energy, the required input power is
The waste heat is therefore
This equation, being roughly linear in v_e captures one of the central problems with extremely high exhaust velocities: Increasing exhaust velocity saves reaction mass, but it requires enormous power for a given thrust.
Take the following example again:
spacecraft mass: M=1 kg
acceleration: a=10 m/s^2, approximately 1 g
exhaust velocity: v_e=0.86c
heat loss: H = 0.1
The resulting waste heat P_heat is around 200 MegaWatts per kilogram, which is enough energy to supply electricity to small city. And this is the heat we need to move *away* from the ship, not even the total propulsion power.
Getting rid of the heat
A spacecraft can ultimately dispose of its internally generated heat only by emitting electromagnetic radiation. Current ISS radiators emit around 14kW per 740kg or around 19 W/kg
The theoretical limits can be estimated using the Stefan-Boltzmann law:
where
P is radiated power, epsilon is the emissivity of the radiator,
σ=5.67×10^8 A is radiator area, T is radiator temperature in kelvin.
So, for a perfect blackbody, 1m^2 operating at 1000K (this is 730C or 1340 F), the total emissions are around 57kW. Note the kilowatts given out by the ideal radiators is already more than 1000 times better than the watts of current radiators, but is still not the megawatts that we need to emit __per kilogram__ of spacecraft mass in an interstellar journey. We are missing at least 3.5 orders of magnitude.
The megawatts we need to emit are not even per *radiator mass*. The kilogram of spacecraft mass has to include everything on board, such as fuel, useful payload AND radiators. In other words, the radiator budget has to come out of the % of useful payload. For example, if the radiators take up 10% of the mass of the ship, and fuel is 60%, the useful payload is now reduced from 40% to 30%. So, there is likely another order of magnitude missing.
There is also an very interesting engineering question in that the amount of power we need to emit is given in watts per kilogram and the amount of power we can emit is given in watts per square meter. How many square meters can we squeeze out of a kilogram? One NASA study considered a high-temperature radiator operating at 800 K and estimated roughly 4 kg of radiator mass per square meter for a particular design. As of now, we can’t even get 1 square meter out of a kilogram, and we are losing another 0.5 orders of magnitude (for a total of 3.5+1+0.5 = 5). So the factors of how much energy we need to dissipate and how much energy we can dissipate don’t match up by a factor of around 100 000.
Of course, the obvious solution is to run the radiators hotter. If the radiators can run at 5000K (close to the temperature at the surface of the sun), we can make up a factor of 625 and our radiators then need to “only” get 160 square meters of surface area out of a radiator kilogram using materials that don’t degrade being a blackbody over a few months when exposed to 5000K temperature, the vacuum of space, a slow but steady stream of high energy particles and gamma rays and, lastly 1 G of acceleration. Last one doesn’t sound bad until you realize how much mass has to structurally fit on extremely thin supports. Best known blackbody is graphite and it degrades at around 4000K. So, even with extremely generous assumptions we can’t create such a heat dissipation system, not to mention the radiators don’t reach ideal blackbody numbers anyway.
A more plausible example
Consider a much less extreme hypothetical propulsion system:
overall efficiency: 90%
exhaust velocity: 0.17c
acceleration: 0.001g
reaction mass: 10% of the spacecraft mass
radiators: 25% of the spacecraft mass
the amount of heat one needs to dissipate is 28.5 kw / kg of spaceship mass, or 104 kw / kg of radiator mass, which is within the theoretical limits for 1000K blackbody with 1 kg per 2 square meters.
The exhaust velocity is still enormously beyond anything we can currently produce as a conventional rocket and the efficiency is far beyond any efficiencies of known tech.
One very sci-fi approach to such a drive (which I might as well label the “Pasha drive”) would be using a cold fusion reaction, but somehow “aligning” all atoms in the fuel before hand so that the neutron stream shoots in a predictable direction instead of all over the place. Whether or not this is even physically possible depends on whether certain theories of “material atoms” are correct and we can use the underlying structure of the atom to reduce fusion entropy and thus out-engineer the currently uncertain mathematical models of quantum mechanics.
So, even this type of design (which still might not be possible) is likely indicative a civilization that is FAR, FAR better at fundamental physics than us, but is not at the “very top” of the tech tree.
How much delta-v does this have? Around 0.018c, enough for a cruising speed of around 0.9% c.
If we are going to a star system 5 light years away, we would take around 550 years to get there based on cruising speed + around 9 years to account for speedups and slowdowns.
Interstellar expansion is might be much slower than you would expect if you only considered general relativity as a constraint, such as in the grabby aliens model. While new engineering techniques could potentially over-ride some of these concerns, we could also find further fundamental limitations that make the bounds even more strict.