# How to effectively and realistically blockade a star system?

Many Scify movies, games, and books depict massive space fleets blockading entire star systems during times of war. This is always depicted as forming a straight line across traveled space. While this is an efficient way to establish a blockade on Earth using naval forces, it is inefficient to do this in space, where the enemy fleets and relief forces can simply go around your blockade in any of many different possible entrance and exit points.

I have two questions:

• What is the minimum number of ships that would be required to create an effective blockade of a star system the size of our a solar system?

• How would an opposing fleet have to position these ships to block (or at least monitor) all possible entrances and exits?

For this assume that inter-system travel can take a year because FTL is by the very laws of physics impossible. Also assume that anti-matter has been successfully mass-produced and can be used as fuel.

Assume also the star system of the attacking force has a similar amount of metals as our system-----meaning the number of ships would be similar to what our solar system can build.

EDIT:

Technology- the technology level of the participating species is similar. They have invented warp technology that allows the participants to go faster than light to a certain point (e.g. Earth/Alpha Centuri travel time would take a month.

If it helps to clarify, I am mainly trying to prevent external intervention, rather than the escape of the defense forces. The reason for the whole system being blockaded rather than blockading a single planet is native military forces are present on all rocky planets in the system and some moons. Native "navy" is also a pain, although it isn't very strong it is using guerrilla tactics in asteroid belts.

• Closest system to us is Alpha Centauri at 4.3 light years away....inter-system travel takes far longer than a year without FTL travel. Heh, it takes 4.3 years at light speed as is. Does the attacker have any speed advantage, or shall we assume both can travel at nearly the same speeds? – Twelfth Mar 9 '15 at 16:25
• @Twelfth it may take less in more dense area of a galaxy. – Anixx Mar 9 '15 at 16:28
• Systems aren't directly blockaded - when every point of interest in a system is blockaded, the system is said to be effectively blockaded. The minimum number of ships required would be based on the distribution of these points and the interception effectiveness of the ship who are blockading. However two ships should effectively be able to blockade a planet - each monitoring a hemisphere, covering the other's blindspot - assuming excellent interception capability. – Scott Downey Mar 9 '15 at 16:31
• @anixx - not really, our solar system and Alpha Centauri are actually quite close on a cosmic scale. And even then...at 1% the speed of light, we're looking at 7 to 8 years before you hit the Oort cloud and about 150 years before you come out of the oort cloud and out of our solar system. Space is stupid huge. – Twelfth Mar 9 '15 at 16:39
• Historically, the way to effectively blockade a wide open space has been to not even bother trying; instead, create a more narrow choke point (gate, bridge, harbor, etc.) and blockade that. If you have no FTL, your logical choke point is whatever planet or space station you don't want people reaching. In a different sci-fi setting, a "jump gate" would be an ideal choke poiont. – Mason Wheeler Mar 9 '15 at 19:59

Well, at least 6 would be needed. One on each of the ordinal points. After that it would really come down to effective sensor scanning. How effective are the sensors looking for the ships. And how effective are the techniques the blockade runners use to hide from the scanners. How far out can the ships be detected. And how far away do they need to be interdicted to be considered 'effective'.

Spotting them will be easier than intercepting them, so having spotters and then a few fleets that reside inside the system and can be moved around to intercept anyone that is coming in system would be better. You should have at least 3 fleets for intercept, and how many in each would depend on the expected size of the threat.

Need one to intercept, a back up for an attack from the opposite side of a feint and a 3rd to protect the inner system, should the other two fail, or just be drawn away.

• I believe this is the best answer, for a too broad question. – Jorge Aldo Mar 9 '15 at 19:03
• If your only argument for 6 is full "sensor coverage" of the system, why 6 instead of 4 at the points of a tetrahedron formation? – Sparr Mar 9 '15 at 21:13
• Two would be sufficient, if we're worried about escapees trying to hide behind the star or any other object. – Aurast Mar 9 '15 at 21:26
• Your answers all seem to ignore orbital mechanics. Parking on the 4 points of a tetrahedron would necessitate constant fuel burn (which is inefficient for long term, the definition of a blockade). A more realistic option would be to park at L3 L4 L5 of a planet/moon system. Depending on how sensors work, the pole should be covered by all of them. – Aron Mar 11 '15 at 1:58
• If your fleet could respond quickly enough at that range...then the correct answer would be 1 ship. – Aron Mar 11 '15 at 2:06

## Points, not area

You can't blockade the 3d "perimeter" of a solar system in any meaningful way. The immense distances involved, as well as the fact that the area you need to protect grows as the square of the blockaded sphere makes it completely, utterly futile.

Almost all of space is empty. It would be more practical to blockade the few actual important points that matter - all the planets - rather than a whole system. This is in the exact opposite to the 2D situation, where maintaining a single perimeter takes less units than multiple smaller encirclements and thus the opposite choice is reasonable.

• Actually, also in 2D many very small circles have smaller circumference than one really big one. – Paŭlo Ebermann Mar 9 '15 at 21:50

Not Possible.

Think about the purpose of a blockade - you're trying to cut off your target from outside relief and supplies. On Earth that usually means trade goods, food, and medical supplies.

The problem is that none of those really apply to interstellar travel. Food is right out - there's no way to transfer enough in bulk to be significant. And given the expense of interstellar freight, you aren't going to have trade goods or supplies like on Earth. Interstellar trade will be in knowledge, and small tech samples - instead of shipping tons of drugs or metals, you'll instead trade the knowledge of how to make that drug, along with the necessary support (schematics for machines to produce it, for example).

And the problem there is that no blockade is 100% effective. Blockades are meant to stop bulk shipping, and there's no such thing as bulk shipping between stars. Effectively, all interstellar trade is closer to being smuggling in terms of how difficult it is to stop - space is big, ships are small. A blockade can slow things down because ships will need to run cold and use ballistic trajectories to avoid detection - but there's no way they can stop it entirely, assuming approximately equal technology.

• I am just looking for a way to prevent enough supplies getting through to support the native inhabitants' army. As max brooks said, an army needs bullets, beans and bandages... – the_OTHER_DJMethaneMan Mar 9 '15 at 17:06
• Interesting points-- if you've reached the technological point to even consider a blockade, wouldn't there be far more effective ways to achieve the same end? – David Schwartz Mar 9 '15 at 23:33
• I disagree with the idea that shipping goods won't be important in a space-faring society. Plausibly, there will be very high-quality goods that aren't needed in great quantities but can only be manufactured on particular planets with rare resources, such as machine parts made out of metals with unusual properties that make them essential for their uses. A single ship could transport enough of these parts to make quite a profit off the trip, and you can't simply transport a few parts to show a planet without the right materials how to make them. – Kevin Mar 10 '15 at 21:05
• @Kevin: that's partially my point. You'll see small, rare goods traded, but that's closer to "smuggling" than bulk shipping if you're comparing to earth scenarios (not that it's illegal, I'm just using smuggling as an analogy of difficulty of detection and stopping). Blockades are fine vs bulk shipping, but they're going to have a hard time stopping low-quantity, high value goods. – Dan Smolinske Mar 10 '15 at 21:14
• @HostileFork: I don't mean it in a utopian way whatsoever. I just don't think it will be cost effective to transport significant goods between stars, which leaves knowledge/tech samples. Nothing to do with software, just how rough interstellar travel will be. – Dan Smolinske Mar 12 '15 at 6:19

This answer really depends on technology level. If the race being blockaded was capable of obtaining high speeds in any direction from the blockaded planet...then it's pretty much impossible to enforce a full blockade. At 1 % the speed of light, it takes well over 100-150 years to get to what we'd consider out of our solar system. By the time a ship is detected and caught by the blockade, odds are 2 or 3 generations have came and gone...will the blockade still be in effect?

If the race being blockaded is more at our tech level...the only way we can really get to speeds that could reasonably exit our solar system over the course of 1 life time is by using other planets gravity to sling shot you through the solar system. This really limits the number of points the solar system can be exited through and keeps us to a 2-D plane bound by the positions of the planets in the solar system. These routes could quite easily be blockaded by a smaller number of ships...calculate all the feasible sling shot routes out of the system and blockade each route (at this point, you are basically setting up a blockade at every planet. The key thing the blockading force will need is a really good scanner / detection method so that ships attempting to leave the system can be intercepted.

SO the answer really depends on tech level. If the race being blockaded has somewhat advanced propulsion and could launch ship in any direction from their planet and still get to where they wanted...then no, this isn't feasible. If they are lower tech, then there is a relatively limited number of points we could leave the solar system and making the blockade feasible as only a few points (pending planetary positioning) would really need to be blockaded.

Though I must ask the question...what exactly are they blockading? The only thing I can think of this being an effective tactic for a blockade on this scale is preventing the blockaded planet/system from colonizing elsewhere.

• @BenVoigt - not true...planets momentum changes. en.wikipedia.org/wiki/Gravity_assist : This explanation might seem to violate the conservation of energy and momentum, but the spacecraft's effects on the planet have not been considered. The linear momentum gained by the spaceship is equal in magnitude to that lost by the planet, though the planet's enormous mass compared to the spacecraft makes the resulting change in its speed negligibly small. These effects on the planet are so slight (..) that they can be ignored in the calculation. – Twelfth Mar 10 '15 at 0:40
• You mean "planet's kinetic energy changes". My earlier comment said nothing whatsoever about momentum (for good reason). – Ben Voigt Mar 10 '15 at 0:47
• @BenVoigt ah yes, you are correct there. In any case, the speed of the planet adds to the speed of whatever is doing the sling shot maneuver and apparently reduces the planets kinetic energy in doing so. The formual is kinda simplistic, initial speed of the spacecraft = v and planet velocity = U. The spacecraft starts at V prior to the sling shot, and v + 2U after the sling shot. Atleast thats what I gathered from the link. – Twelfth Mar 11 '15 at 0:50
• @Twelfth Only by a completely insignificant amount, though. I actually asked about this on Space Exploration as What is the effect of gravity slingshots around Earth on Earth's rotation and orbit time, and is this effect worth considering? – a CVn Apr 13 '16 at 18:43

It depends on several things.

1. What are you actually blockading, and why? Do you really need to blockade the empty space in the system? Why? It is likely to be more efficient and effective to blockade or protect all the relevant planets, space stations, and heavily escort any regular spaceship traffic, than to try to deny entry to the entire system.

2. If you do need/want to blockade the entire system, and you say you have FTL warp drive, then it becomes very material what the speed of your ships is, and what the range and speed of your detection and communication technology is. If your ships have sufficient speed and maneuverability and they can detect ships at great distance (and especially, if they can identify ships at great distance, and/or communicate at great distance quickly), then you only need as many ships to cover the area you want to cover, spaced out by the effective interception distance this provides. Of course, you'll also have tactical details to consider, such as how big a force you want to meet each type of intrusion, and if the detecting ships are doing interception as well as detection, how much redundancy/replacements do you need to fill in the detection net when a detecting ship moves to intercept something. Also, what is your plan for reacting to attacks as opposed to just intercepting trespassers. How well you can use weapons while moving at warp speed is also important for details of how you operate. So would any details of warp travel itself - whether it requires time to start and stop, and so on.

3. How effective a blockade do you need? If you're a control freak who must stop everything, then clearly you need a lot more than if you just want to discourage most trespassers. Even a few armed ships may be plenty to discourage people from attempting trespass, if they have options that don't risk being attacked at all, and if warp drive is available, they may have many other options, and so choose not to test your blockade. You might even be able to just declare a system blockaded, but devote zero ships to actually doing it, because scanning to prove a blockade is there might be something that few or no people might even attempt, and it might be difficult to detect an actual blockade anyway without testing it in a risky or expensive way.

In the original Star Trek, for instance, one starship could effectively blockade a planet (or an entire system, if there was no determined attempt by multiple simultaneous trespassers), because it could go to warp speed very quickly, travel hundreds of times the speed of light, had no real fuel/energy constraints, IIRC it could detect ships across an entire system, could fight and maneuver at warp speed, and was far more powerful than most interlopers. Even so, a single ship could be baited away from a target by a decoy.

This is a similar problem to a naval blockade, but in three dimensions. If you naively considered blockading an enemy coastline you might think you can't do it, there's thousands of miles of coastline, they can launch from anywhere. But in reality there are only a few discrete points on the coast that vessels worth blockading will launch from: ports. Areas with the combination of a good harbor and the facilities to load and unload large cargo and fuel and maintain ships. In reality, you only need to blockade a few points along the coast.

Similarly for space flight, you only have to blockade the major launch and cargo facilities. Depending on the technology maybe it's space elevators, maybe it's space ports, maybe they're just conveniently close to industrial and trade centers.

The other reality is there are only so many places major cargo ships will go. If you know their major trading partners, you can position ships along their route to them. If they go around you, you have still succeeded in slowing down their trade. One cargo ship making two 10 day trips carries as much as two ships taking 20 days.

Finally, what is the point of your blockade? If it's a commerce war, rather than blockading all cargo vessels you may wish to simply destroy the enemy's commercial fleet. This was the strategy of the German U-Boat campaigns of WWI and WWII. It doesn't matter how much cargo you have if you don't have the ships to move it. Once the enemy's commerce fleet is destroyed, your military vessels can move on to doing something more important.

What about a blockade of the military? During the Napoleonic Wars, the British bottled up the French Fleet keeping it divided. This allowed the British to freely use the seas without having to protect convoys with large military escorts. Individual vessels which slipped by were annoying, but not a threat to the war effort. This was only possible because the British fleet was measurably more powerful than the French, each blockading British fleet could overpower the French fleet coming out to fight it. The French fleets could not unify to destroy the scattered blockading British fleets.

However, the fact that the French Fleet existed caused the British to devote far larger numbers of ships to the blockade than they may have lost simply fighting them. This is known as a Fleet In Being. It means the blockading force is better off destroying the opposing fleet than blockading it. Much of the naval strategy of WWI was devoted to each side trying to divide and destroy the other's fleet. In reality, they mostly sat and watched each other. The inferior German fleet, merely by continuing to exist, prevented the British from mounting a major invasion of Northern Europe to outflank the trench lines.

So generally you want to destroy the fleet rather than spend the war blockading it.

Which you are doing depends on whether it's going to be a close, distant or loose blockade. Each has their advantages and disadvantages. Close blockades take a lot of ships, and you asked for the minimum number of ships, so it will have to be a distant or loose blockade.

A distant blockade would use smaller, cheaper means to watch for cargo vessels (spies, small ships, long range sensors, satellites) and send military vessels after them. Since cargo ships tend to be slower than military vessels (for fuel and cost efficiency) this tactic can work.

A loose blockade would feature your blockading ships hiding nearby and outside of sensor range to lull the enemy into a false sense of security. They come out, and are detected and destroyed.

The minimum number of vessels to implement a loose or distance blockade of a solar system with the goal of dividing and destroying the enemy fleet (military or commercial) depends on...

• The number of enemy ports
• The number of their trading partners
• The relative speed of their vessels vs yours
• The relative size and quality of their military vs yours
• The quality of your sensors

Can't give a specific number.

• Problem with distant blocade: I use a gravity assist to deflect out of the star's orbital plane. Now you need far more delta-v to intercept than I needed to escape in the first place. You might well be able to catch me, but now your military vessel's in the wrong plane and very low on fuel, and I can always launch another. – Joshua Mar 10 '15 at 2:45
• @Joshua We don't have the technological parameters defined to know whether your scenario is plausible, but distant blockades will let ships through. That's ok. In your scenario, the clever blockade runner cannot change course without consuming a lot of fuel. A well run distant blockade will guess their destination based on the trajectory and send a ship along that route or near that destination to intercept. – Schwern Mar 10 '15 at 3:05

So if you are talking about a planet then blockading the planet doesn't really require much in the way of ships; it just requires preventing supplies from reaching the planet surface; which can be accomplished by filling low planetary orbit with debris large enough and dense enough to be dangerous to pass through.

With the existence of antimatter, using some decent sized rocky asteroids as the base material to create the cloud would be good; it would be challenging for the defenders to prevent without turning the attackers converted asteroid into a kinetic kill weapon; which would be a very bad thing for them. Added anti-matter missiles in slightly higher orbits than the cloud itself could help make things slightly more challenging, as well as taking down the cloud harder for the defenders (as attempts to do so could create EMP bombs).

Of course, No FTL means no interstellar warfare for anything remotely like us (see John C. Wright's sci-fi novels); Even interstellar trade in anything but information becomes, challenging. The easiest way to have interstellar warfare and blockades is to have limited FTL via stable wormholes or jumpgates, something of that sort. Which given your edit seems to be a possibility, if that is the case then only those particular points need to be defended. Or in the case of needing a clear exit from FTL, mining the established FTL exit points would also work, which could be (Star Wars IV) putting debris in the exit point, to actual missiles, depending on the magic of FTL being used.

• There's an issue with filling low planetary orbit with rocky debris: those rocks will encounter air resistance and then fall to the ground since they are without the ability to make course corrections. Depending on the size of the individual pieces of rock used in the cloud, those falling debris could then turn into missiles and those that survive re-entry could potentially have a devastating impact on the local area where they land. – Ellesedil Mar 10 '15 at 17:21
• That does depend on the size of the rocks used, which would depend on the ability of ships to shield themselves from such impacts, which would also impact how well planets and cities could shield themselves from such impacts; I mean currently to prevent space travel from earth the cloud could be made up of solely things that would burn up well before reaching the surface. – John_H Mar 10 '15 at 17:34

I believe I disagree with all answers so far on how one would go about this: let's assume that interstellar trade is a thriving business because hey, if you have unlimited free energy (some dark matter engine-contraption) then there isn't really a barrier to supply and demand - there will always be a buck to be made by taking ore from a mineral rich world and giving the inhabitents some tasty food in return.

Given the above parameters, the difficultly of hitting anything which can travel at 40c (speed of light) and the relativistic physics nightmare that would be, and the sheer volume of space you are trying to cast a net over, I see only two solutions, one very much realistic and the other imaginative but within the boundaries of the universe you have created:

1. imaginative solution: using four or more ships, encapsulate the system in a shape (in the case of four ships, a triangular based pyramid) and distort all space in the plane of the four triangular faces of the shape. If the space-time distortion in these faces could be large enough, anything travelling through them should be ripped to pieces and any distress signal should become static garbage. Think of it as a planar black hole. How the four ships producing these distortions survive their creations is beyond me, and the rate of energy production to sustain the distortions would have to be truely phenomenal, but were this to be effectively implimented the four corner ships would become like border crossings.
2. the realistic solution: the blockading race/species disincentivises any attempt or desire to break the blockage through unimaginable brutality and a fearful reputation. Something along the lines of "should any individual attempt to leave the system, or any goods from another system be accepted by the system, we'll kill every newborne in the system for the next 25 Earth years". Then follow through with your threat if required, but always allow enough to survive that the stories of your race/species travel between the stars as nightmare/legend amongst all races. Think The Dark Knight Rises, only much, much worse. In this way you needn't actually blockage anything at all, the local government will be obliged to police all ships leaving their planet surface for you for fear of the reprisal, and you need only observe, which shouldn't be difficult given the distortion any ship leaving the system at or near to warp would create.

Another way to blockade a system would be to let the enormity of space do the blockading for you. As @Twelfth mentioned, if you have to rely entirely on slower-than-light engines it's going to take a long time to get anywhere. Additionally, if the civilization is accustomed to using FTL engines, they probably won't have many ships with strong sub-light engines.

So how do we force them to use sub-light engines instead of their FTL engines? Use something like the interdiction field from the Star Wars EU. If space travel is common (and the existence of FTL drives makes that likely), then it's reasonable to assume that one of focuses of military research will be to impede FTL travel. A way to block FTL travel would make for a very effective blockade, especially if their interdiction range is pretty large (on the scale of 1 AU), and even more so if this is the first time that the interdiction technology is used.

How it would play out:

Enough ships show up in the system to blanket the system in interdiction fields. Each ship would have some escorts, but they wouldn't need too many. The ships would be designed to be self-sufficient for years at a time, and for the most part the escorts would be docked with it. In short, they'd be functioning like space stations for the duration of the blockade.

If the interdiction technology was previously unknown, the blockade would have quite a while before they'd need to worry about any threats whatsoever. As I said earlier, existing ships would be set up to rely heavily on FTL engines. It would be reasonable to expect that the civilization being blockaded wouldn't even have ships capable of easily reaching the interdiction ships, so if they wanted to take down the blockade they'd have to build a new fleet designed to be able to reach them using only sub-light engines.

Even a fleet able to reach an interdiction ship would be at a severe disadvantage. If the fleet is small enough to avoid detection, the escorts would be able to handle it. A large fleet would be at a severe tactical disadvantage - the interdiction ship will know well in advance that they are coming and be able to have a fleet of their own come in to counter them. The blockade's fleet would enter at some point through the interdiction blanket, having the interdiction ships temporarily turn off the fields while the fleet passed. The fields would also be arranged with overlaps in such a way that there would be a way to block the incoming enemy fleet while still allowing the friendly fleet to arrive unimpeded. So after months (and months) of slow travel to get to an interdiction ship, the enemy fleet will find that the friendly fleet has already arrived. Additionally, that friendly fleet will be fresh and will have been put together with a knowledge of what they'd be up against (so they could bring ships that would be most effective against the enemy fleet).

What would they do about ships trying to take the slow route through the system? Not much.

Thanks to the blockade, it will be much more costly for ships to enter or leave the system. It will be a long trip and any ship that attempts it knows that they will be watched the entire way. They have no way to know whether or not they'll make it through. In order to make it risky for people to attempt such runs, the blockaders need to catch some of them. A fleet could either be waiting for them near the edge of the interdiction blanket, or use the strategy I mentioned in order to show up deep inside the blanket. There is plenty of time for either strategy because of how long the trip will take without FTL. When they get caught, there are a number of things that could happen - if they resist capture they would probably be blown to smithereens, but if they don't resist they may have their ship seized for the duration of the blockade or just have their goods taken.

Any fleet traveling through the interdiction blanket could count on a blockade fleet meeting them at some point during their journey. This goes for both incoming and outgoing fleets.

To answer your two specific questions, the interdiction ships would be spread fairly evenly throughout the solar system to form a nice blanket. The number needed depends on the range of the interdiction fields. A sphere that extends to the orbit of Pluto (40 AU) has a volume of 64,000 AU^3, so if the interdiction field covers a 1 AU radius (resulting in a little over 4 AU^3 volume) you'd need about 16,000 ships to fully cover that sphere. However, without FTL you'd need gravity assists to leave the sphere, and you can only get those in the plane of the solar system. That gives an area of ~5026.55 AU^2 to cover, with each ship able to cover ~3.14 AU^2, giving full coverage with 1600 ships. I'd bump that up to 5000 to explain overlap and having some above and below the plane to ensure that they can't easily escape that way. If the coverage of a single field is a radius of 10 AU you get a 100x improvement in area coverage, allowing you to do it with 50 ships.

If the species is sufficiently advanced then a "time dilation device" ala what the Asgard did to the Replicators in the Stargate TV series.

However, this was only part of the Asgard's plan. They used a Time dilation device to slow down time within a radius of 0.16 light years by 10^4 power.

EDIT: You end up making the species contained within the time dilation expend so much resources to escape that it simply becomes uneconomic to do so; or it takes them so long to escape that your own species has time to complete their own goals.

• Welcome to the site! Just a reminder to check out the help center and the tour. While this answer appears to be satisfactory with an explanation it borders the 'not-enough-description' line that usually earns down votes. In the future it is best if you include some length and more detailed description. If you need an example check Robb Watts' answer as well as any of the higher-voted answers. ;) – the_OTHER_DJMethaneMan Mar 11 '15 at 13:05

There is little or nothing to be gained by blockading empty space itself. With the massive difficulties involved it may be infeasible to establish a blockade around an entire star system. However, any reasonable military objective that could be achieved by blockade could be achieved by blockading the inhabited planets in the star system.

I think this is the only way to do it: establish blockade in low orbit around planet. It is not possible to hide during boost phase without effectively magical technology. Anything that tries to boost out of the atmosphere is immediately declared a target and shot down. Of course, you're now within target range of planetary surface defenses.

It comes down to what your weapons' performance is like and what the blockade-runner drives are like.

If you have very long range missiles that majorly outclass ship drives then a single ship can impose a blockade. A blockade runner shows up on infrared, launch a missile at it. It doesn't matter that it's a tail chase, the missile will run it down eventually. There's no hiding your heat in space, they're sure to spot you.

On the other hand, if you're limited to beam weapons (quite possible in a world where point defense is a far easier mission than long range attack, or a world where drives are acceleration-limited to low values--a missile isn't much good if it can't overtake) and the blockade runners have drives as good as yours it's going to take an awful lot of ships (too many to be feasible) if you're actually trying to blockade a star system and not merely a planet in it.

Thus you need to set some weapons performance parameters for this to be at all answerable.

Its impossible because FTL

So earth to Alpha Centuri is about 1 light year so to cover it in a month you would be going at 12 * c where c is the speed of light. This is ignoring all the very interesting relativistic problems that occur at near speed of light and keep you from going over the speed of light.

To make a blockade work you need to reliably detect blockade runners and then get between them and their destination or at least within firing range before the reach the safety of their destination.

How do you detect something? You bounce something off it usually photons. Photons are bound by the speed of light often so they move slower than FTL.

If an blockade runner comes barreling in a 12 *c then the photons coming off it won't reach a defender until after the runner has passed, even if the runner passes within feet of the defender. It is impossible to detect the approach of an FTL ship until it has already passed. Worse still the defender can't stop him because any projectile, laser or signal is slower than the speed of light.

You can't. You really really Can't, well not without creating a Dyson Sphere out of battle ships, what you blockade is strategic targets, how many vessels you need to do that depends on relative sensors, acceleration, Delta V and numbers.

Two

You would have two robot cannon ships orbiting the sun in a tight orbit. Each has a hemispheric viewpoint with the sun at its back. You need two, not 1 because otherwise a ship could sneak in keeping the sun between itself and the patrolling cannon ship.

I assume with the future tech possessed by this crew that the cannon ships can fire FTL projectiles or beam weapons such that any ship spotted entering the system can be rapidly fired upon and destroyed.

In the story I imagine, the blockade runner learns that the 2 cannon ships have hemispheric viewpoints but only hemispheric. They are not back to back: there is the sun and a little space between them - which means there is a (rotating with the orbit) blade thin path between the rotating hemispheres thru which a ship can sneak, and get right behind one of the robot cannon ships and board it. Then they turn it around and shoot the other robot cannon ship through the star.