Designing a fictional FTL drive

[The Disintegrator]

Hello all,

I've got a scifi pet project that I've been on and off working on for several years now, mostly consisting of writing some stuff, going back and deleting it, and never getting very far with it. Over time it's evolved from being about space pirates to going to a more typical military crew once I got over Pirates of the Caribbean. Anyhow I'm on my latest push to write this thing, and I've realized I need to get serious about figuring out this FTL problem.

Knowing full well that an FTL drive goes against physics, at the same point I'd like to be able to be able to use settings outside of the Sol System. Other than this I intend on doing my best to stay within the realms of real science. Now taking the advice of Atomic Rockets, I've decided to determine what I want this drive to do, and then insert the technobabble of how it works later. So here's what I've got figured right now;

- I'd like to give the FTL drive an average speed of about one light year per hour for most civilian travel, and an average speed of two light years per hour for military travel.
- Should be sufficiently expensive enough where most civilian spacecraft are reliant on one way transporters on the edge of solar systems in the same way as mass relays in Mass Effect. But they should also be reasonably affordable where military craft above the size of a fighter are commonly equipped with them.
- Spacecraft should still have to fly around star systems at slower than light speeds to allow for really cool space battles, give some warning of incoming enemy attacks, and to prevent nuclear warheads from simply appearing at their targets. I'm thinking go with a classic beyond the orbit of Pluto type or requirement for Sol for example.
- No FTL on the surface of a planet, so no Stargates.
- It should take more power to send a larger ship through FTL, but I would prefer not to have spacecraft that can use their fuel source as a weapon of mass destruction. So definitely nothing involving antimatter.

Now my thought on how to solve a lot of this is to go with the typical approach of gravity disrupts FTL. So with these considerations in mind I was hoping to get some ideas on the implications of this drive. Aside from making Einstein roll in his grave, FTL tends to really break universes, so I'm hoping to conceptualize some of the unintended consequences before I start making things fly faster than they should.

Also, if anyone has any other suggestions on how this should be tightened up I'd love to hear it.

[gigabytelord]

How does FTL inherently go against physics?

All the major SF FTL drives I know of generally get around this issue by 'diving' into another type of space e.g. hyperspace, warpspace, slipspace, so on and so forth.

Or by wrapping themselves in some sort of bubble which allows the space to move and simply take the ship along for the ride, I'm looking at you Alcubierre drive...

All of these ideas are plausible given a long enough time and sufficient technological advancement.

[Stark]

FTL is unrealistic in the extreme. Luckily, its fiction so it doesn't matter.

I think you're going about this the right way, because its only important to define what it can do and how that shapes what people will do with it. For instance, you want early warning so you can't jump insystem. Some people use all kinds of stuff (ie inbound FTL being very obvious, requiring chokepoints, requiring receiver stations, or whatever) to achieve the same effect, and the only important thing to worry about is how the way travel works impacts on what you want to do with the story. The downside of combining 'realism' with outsystem jumps is that it may lead to amazingly long travel times (which may not be a problem) in boring areas.

Just remember the Freespace Test. If everyone can jump at any time huge distances with high accuracy repeatedly, maybe its stupid to have them driving really slowly so convoy battles can happen. :V

[Formless]

Do you have some idea of what the plot is going to be like? The characters? The themes? Something more concrete story wise than "Pirates of the Caribbean Ripoff In Space?" Or "Generic Military Men Fighting Inadequately Explained War In Space?" We might be able to help with flavor, but that would require you know what tone this story is supposed to have in the first place.

One idea you could use to distinguish this from every TV or Movie space opera ever; remember that stars have immense gravity wells. Depending on how you want to justify it, the FTL could even be an instantaneous jump, like the Terraport system in Schlock Mercenary. But when the gravity well of an object and the FTL drive interact they create some sort of event horizon that's distance X from the source of the gravity. The ship could have to draw a straight line between itself and the target system, and if there are gravity wells between you and the target you have to make the trip in multiple jumps with STL travel in between-- military ambushes and choke points could happen at these pit stops. When they finally reach their stellar destination they arrive at the edge of the system and have to coast in with conventional engines. Then there is a natural delineation between military vessels and civilian ones based not on their FTL drive, but on the fact military ships need more powerful impulse engines anyway.

There is the issue of jumping above the galactic plane to get anywhere instantly (including other galaxies!), but then you can play with such issues like the thickness of the galactic plane, the existence of dark matter, and the gravity well of the milky way itself. You can tie this in to all sorts of interesting aspects of cosmology and astronomy that don't traditionally get much love in space operas.

[Stark]

He's just asking for advice around how design elements of limitations of FTL travel impact storytelling, not storytelling advice. So long as he avoids making the action he wants impossible or inadvisable, he'll be fine.

[Guardsman Bass]

- I'd like to give the FTL drive an average speed of about one light year per hour for most civilian travel, and an average speed of two light years per hour for military travel.
- Should be sufficiently expensive enough where most civilian spacecraft are reliant on one way transporters on the edge of solar systems in the same way as mass relays in Mass Effect. But they should also be reasonably affordable where military craft above the size of a fighter are commonly equipped with them.

Just make it so that compact FTL engines are really expensive. Military craft can afford them because the military doesn't care if it has to spend 50 million space dollars per engine, but there's no way that most civilian craft can or will want to do that, particularly when the fixed FTL spots are much cheaper to use.

- Spacecraft should still have to fly around star systems at slower than light speeds to allow for really cool space battles, give some warning of incoming enemy attacks, and to prevent nuclear warheads from simply appearing at their targets. I'm thinking go with a classic beyond the orbit of Pluto type or requirement for Sol for example.

I think some hand-waving might be good in this situation. Just say that there's an "[insert name] sphere" around each type of main sequence star where you can't form an FTL transit point, so you have to go outside of it to do that - and that means basically beyond the outer solar system of most stars. If you want something that's real, you could say that FTL isn't possible inside of the "heliosheath" of a solar system, which would be 80-100 AU from the Sun in the case of our solar system (but that also means your ships have to travel a long ways to get into a solar system, even if you have super-rockets).

- No FTL on the surface of a planet, so no Stargates.

It'd be covered under the above, I think.

- It should take more power to send a larger ship through FTL, but I would prefer not to have spacecraft that can use their fuel source as a weapon of mass destruction. So definitely nothing involving antimatter.

Some type of super-fusion or fission rockets would work for that. I'd probably just go with "fusion reactors" and leave it at that, since that would also let you handwave some pretty good sublight rockets to get around solar systems.

[Formless]

He's just asking for advice around how design elements of limitations of FTL travel impact storytelling, not storytelling advice. So long as he avoids making the action he wants impossible or inadvisable, he'll be fine.

I know that, but I simply feel obligated to steer people away from the ordinary (i.e. Star CraftTrekWars) cliches if possible. Knowing more about what kind of story he wants to tell will help figure out how the FTL device fits into the larger picture, by which I don't mean the setting. Almost anyone can make a setting these days, but most of them have no story potential (at least no inherent potential) in my experience.

[Connor MacLeod]

Knowing full well that an FTL drive goes against physics, at the same point I'd like to be able to be able to use settings outside of the Sol System. Other than this I intend on doing my best to stay within the realms of real science. Now taking the advice of Atomic Rockets, I've decided to determine what I want this drive to do, and then insert the technobabble of how it works later. So here's what I've got figured right now;

I think what you mean is you want it to have a semblance of 'plausibility' and internal consistency. I tend to draw a distinction between that and 'real science' or 'realism' because otherwise people get confused when you say oyu want 'real science' and FTL.

That said the closest to 'plausible' and 'FTL' in the same sentence (at least amongst the Hard scifi crowd) is stuff involving wormholes. A few of the more flexible (those that feel 'hard' means 'science doesnt' expressly forbid it) have proposed alcubierre drives might fit into that category, but there is some dispute to this (although in my experience lots of 'hard' sci fi concepts get disputed at one point or another.) You want to avoid tachyonic like the plague at the very least, because that has some nasty stuff that gets into time travel and all other unpleasant shit.

- I'd like to give the FTL drive an average speed of about one light year per hour for most civilian travel, and an average speed of two light years per hour for military travel.

I can't think of any reason why you can't plausibly handwave this. I mean since FTL isn't really an established 'science' the criteria dictating how fast or slow omething goes just doesn't exist. Say you had some sort of 'wormhole' drive magic - you might postualte some sort of limitation that dictates how fast the drive could travel. Or maybe the hops are instantanoues, but onyl over a short distance. If you want an 'effective' speed of 10 LY per hour, I'm guessing travel between systems is less than a day. Sio you could say that your drive can make a hope over X distance, but with a delay between jumps for whatever reason (dissipate heat, recharge the drive, whatever.) You could even scale the speeds somehwat - further hops are possible, but they are less efficient and require far longer to cool down/recharge, compared to the hops resulting in the 'average' 1 LY per hour you are aiming for. Alternately, the drive takes a long time ot 'build up' to the jump. maybe whatever mechanism it uses to achieve the jump takes awhile to form safely (it can form faster, but only at great risk), or it may require some sort of calculations to safely plot. you could even use a combination of any number of factors to pull it off.

Or maybe if the drive system operates over a given distance, you have to take a certain amount of time in realspace to let the crew 'recover' or drive them insane, kill them, or whatever. Maybe even then you have to have your crews in some sort of hibernation or cryogenics with each 'jump' to help minimize effects (people who make jumps conscious may be driven insane/killed much faster or over much shorter periods of time.)

Another interesting possiblity might be, to actually have the drive be 'instantaneous' from the POV of people onboard, but a variable lenght of time (due to various factors) passing for the jump to be achieved in realspace.

In any case, military drives might allow better performance because of better cooling/better power storage/faster recharge/better powerplants/etc.

- Should be sufficiently expensive enough where most civilian spacecraft are reliant on one way transporters on the edge of solar systems in the same way as mass relays in Mass Effect. But they should also be reasonably affordable where military craft above the size of a fighter are commonly equipped with them.

That would again argue for 'wormholes' I think. Funny enough the novelizations of the Wing commandre movies had a second novel that had a drive system that could create artificial 'jump points' rather than relying on fixed ones.

Bear in midn that this creates that 'choke point' - because it invokes a certain predictability in interstellar travel it becomes easy for enemies to defend against attacks or attack your position. In such a case wormholes may not be 'quite' what you want, but rather something where your ships only have certain 'safe' routes they travel along in their drive (although this is usually used with FTL involving jumping to naother dimenison, and I'm not sure how this fits in the plausibility category.)

- Spacecraft should still have to fly around star systems at slower than light speeds to allow for really cool space battles, give some warning of incoming enemy attacks, and to prevent nuclear warheads from simply appearing at their targets. I'm thinking go with a classic beyond the orbit of Pluto type or requirement for Sol for example.

Distance isn't going to be the sole factor here. Alot of it will depend on how your sublight engine performance works out and how long oyu want people onboard ship to travel from entry/exit points to their destination. It doesn't sound like you're envisioning ships ripping about the galaxy at any significant fractions of lightspeed, so I'd expect getting 'fast' warheads that could simply zip in without being detected or reacted to is going to be difficult at best and not a big issue.

- No FTL on the surface of a planet, so no Stargates.

There's no reason for your hyptohetical FTL to allow that if you don't want. If you're establishing the whole 'emerge some distance away from planet' sorts of things that would tend to rule this out as well. OR if you don't like that you could say that doing it on planet, even for small objects requires impossible quantities of energy to achieve. Or it may have dangerous side effects (generating large amounts of waste heat, or dangerous radiation, or whatever.)

- It should take more power to send a larger ship through FTL, but I would prefer not to have spacecraft that can use their fuel source as a weapon of mass destruction. So definitely nothing involving antimatter.

This shouldn't be difficult to handwave either, since again its not something that numbers exist for anyhow. You don't even have to have it scale linearly really - big ships could be more effective combat wise, but also possess vastly inefficient drives that make them slower.

Now my thought on how to solve a lot of this is to go with the typical approach of gravity disrupts FTL. So with these considerations in mind I was hoping to get some ideas on the implications of this drive. Aside from making Einstein roll in his grave, FTL tends to really break universes, so I'm hoping to conceptualize some of the unintended consequences before I start making things fly faster than they should.

I dont think you have to say 'gravity doesn't make it work'. you could postulate any sort of magic barrier doing it. I mean if you're going with FTL to begin with, saying that some other handwavy something or other prevents it from working within a certain distance of a planetary body or large mass of any kind isn't that far fetched. You dont' really even need to explain why it happens, just that the limitation exists. (Saying that 'gravity' causes it could possibly hamstring you further down along the line if you envision some osrt of gravity whatsit or other for exmaple.)

[Connor MacLeod]

I think I may have to clarify that choke point thing if its too confusing. I wasn't happy with how I phrased it.

Basically the idea is that if you have a very 'small' wormhole (or wing commander style jump point, or however you want to envision your 'point to point' type transfer) it can create a predictable zone of entry or exit that the enemy can anticipate and defend against. This is precisely the case in certain novels like David Weber's STarfire, or the Miles Vorkosigan novels (minefields, fixed defenses, placed around the 'wormhole'.) Obviously the bigger the 'area' it occupies, the harder it is to predict entry/exit points, and the bigger your defenses have to be. Chokepoints are thus not inevitable (which is what my previous statement implied I believe) but its still an issue that can be addressed.

Alternately, you might have a drive that can potentially travel anywhere (That is, move in any direction, enter/exit space at theoretically any location) but for practical reasons faces certain limitations. For example you might have certain 'fixed' routes that are safer or more predictable that are often used by civilian ships, but military ships (because of better drives, better information, or because they are simply toughter) are not restricted to those routes (or can venture/stray from them more readily than civilian ships can.) Likewise, such limitations or risks may prohibit the other issues you wanted to avoid - ships getting too close to planets (or not wanting anything resembling a stargate, etc.)

Again you dont have to go with one specific reason, you could combine any number of them.

[The Disintegrator]

All of the feedback is much appreciated. I completely understand wanting to steer people away from the cliches of the big Star_____ franchises. I love those franchises dearly, but I don't want to just make a copy of them. Anyhow, to give an idea of what I plan to do with the story...

The basic premise I'm working with is aliens attack a geology research outpost, and the military wants to find out who and why. The main character is the mechanic that's assigned to the transport being used to transport the investigators. After the investigators find that their identities have been compromised they're forced to use the mechanic as their guy on the ground. So from there they chase some leads, explore some places both within human territory, and in alien territory, and then eventually find themselves involved in a political conspiracy. Sorry if that's pretty vague, but it's really early on in the writing process so there's not really much set down yet.

To give an idea of the themes and atmosphere it'll likely take, it's mostly inspired by my day dreaming when I was an aircraft mechanic on the active duty side of the Air Force (hence the main character's occupation). A lot of the inspiration is pretty much what if the C-17 I was working on could fly to other star systems. And also a lot of inspiration from the oddities of the military, stuff like bureaucracy, bizarre human behavior, and strange events that took on even stranger stories. So the story will be about serious thing such as preventing a war, but I intend on keeping the tone mostly light hearted. I suppose the closest thing I can think of to compare the tone to would be Eureka, where it's not really a comedy but it should have some good laughs.

But moving back on to FTL specific stuff, I'm liking all the ideas suggested. Pushing the boundary of where the FTL drive can be used does indeed mean there could be extended travel times, so I suppose they're going to need some pretty wickedly powerful engines. Although I wonder if laser propulsion could be used to get spacecraft going without having to equip every civilian cargo ship with engines powerful enough to traverse a system in a reasonable amount of time.

[Jaepheth]

You could have the FTL work by manipulating gravity and any nearby mass not going along for the trip will tear the ship apart as it accelerates/decelerates. So ships have to be well clear of any large bodies of mass when entering or leaving FTL. Pluto orbit distances in the case of the sun, and maybe 100s or thousands of miles of distance away from any station or ship when jumping or stopping. Just take Newton's formula for gravity and plug in the mass of your ship and the sun's mass and calculate what the force would be at around Pluto's orbit for r. You can also add a fudge factor introduced by your FTL engine if you want. Keeping that as the maximum amount of force an accelerating ship can take will let you stay consistent in determining how far away from a star/planet/ship/comet you have to be in order to enter/exit FTL.

Even the mass of the ship and cargo could have an effect on this, so you can't accelerate too fast past C without your ship tearing itself apart. Military ships would be built with expensive materials allowing it to shrug off stresses that would tear a cheaper ship apart; thereby allowing it to go twice as fast.

[Zwinmar]

One of the ideas I like is jump gates (I.e. EVE) provides the choke points you wanted. Just an idea

[Formless]

Sorry if that's pretty vague, but it's really early on in the writing process so there's not really much set down yet.

Quite alright, general ideas is all I need. I don't want you to spoil the whole plot, after all.

Consider maybe giving the aliens the ability to either break or (preferably) bend the usual rules for the technology humans use. Maybe their ships can jump closer to a star than human vessels can, for instance. Don't tell the audience what rules do and do not apply to them, at least until later in the story; that way you give them an aura of mystery and some extra reason for the military to be suspicious of their behavior. It could also tie into the conspiracy, with the humans either looking to get their tech, or to overcome their advantages, or maybe a reversal where the aliens see advantages in the human way of doing things and are willing to do shady dealings to get a hold of our technology. And those are just some of the ideas that I thought of off the top of my head; you don't have to decide right away how you want to play it, if you chose to do something like this at all.

But moving back on to FTL specific stuff, I'm liking all the ideas suggested. Pushing the boundary of where the FTL drive can be used does indeed mean there could be extended travel times, so I suppose they're going to need some pretty wickedly powerful engines. Although I wonder if laser propulsion could be used to get spacecraft going without having to equip every civilian cargo ship with engines powerful enough to traverse a system in a reasonable amount of time.

It can, but be aware that any laser which could be used this way could also be used as a weapon. Even if the main laser lacks the cohesion over a distance necessary to do significant damage on its own, it could still propel a missile towards the inner solar system which not only acts as a kinetic warhead in its own right, but also a focal point for the laser simply by carrying a suitable combat mirror or lens. This is, in fact, the true Kzinti Lesson, as the humans in Niven's work were using laser arrays to propel solar sail ships (well, that and fusion drives which should lack weapon grade range, and photon drives which are just silly inefficient). I would suggest instead having civilians maybe use some sort of slingshot technology to give them a boost. It wouldn't have to be gravity based if that would screw with the FTL, but electromagnetism is a stronger physical force anyway. This could also potentially give it a more omnidirectional boost capability than a laser, by slinging people out of the sun's gravity if they have to make a pit-stop in their way to another solar system.

[jollyreaper]

There's good news and bad news. The bad news is that pretty much any good, literary stardrive you can imagine has already been invented, you won't likely come up with anything new. The good news is you can pick and choose from what's out there to lend your story the exact flavor you want.

http://www.projectrho.com/public_html/r ... StarDrives

The depth you go into with the details depends on the story you're telling. If the ship your action takes place on is a blockade runner and the story is about the crafty captain using wits and guile to slip past revenue cutters, the how's and why's of your ships are very important. If you're telling an Agatha Christie murder mystery and the ship is just the setting, the details might not matter beyond "The trip takes a week and we have to discover the murderer before we reach port."

[Formless]

You're doing it wrong, jollyreaper. Not that this TVTroper bullshit surprises me anymore. That list is, and was always intended to be, too generic to say that "every stardrive you can imagine has already been invented". First off, not everything in that list has probably ever been used in an SF story, so its a moot point. Second, that list doesn't include all the variations of time travel, which one would have to include to truly be representative of FTL in fiction. Thirdly, one need merely invent something new to prove that statement is a crock of shit, hence showing where your creativity is pitiful. And lastly, the devil is in the details, so merely cherry picking from other people's work and ideas will come off as ripping off other writers if you don't do your own work.

Take my idea for instance. I start with a jump drive and compare it to the Terraport. However, I then distinguish it from Schlock Mercenary, which I see as having missed potential by designing it with too few or artificial limits. So my drive now has to have line of sight (notably, something which Atomic Rockets doesn't mention in conjunction with jump-drives and instantaneous travel), and include a built in limitation related to gravity and distance from stars so that there are obstacles between the jumper and the target. That makes all the difference in the world, since it thus eliminates the kind of planet to planet insertions that happen all the goddamn time in the aforementioned comic, as well as other things.

Basically, directing someone to Atomic Rockets when they stated their familiarity with Atomic Rockets in the OP is adding no substance whatsoever to the thread. Idiot.

[jollyreaper]

Keeping it classy.

Firstly, Atomic Rockets is a huge site. It's entirely possible to be familiar with it and still find new, hidden gems.

Secondly, everything has already been done before. Line of sight teleport has been used as a limitation in magic systems. I'm pretty sure I've heard of it in FTL systems where there's no steering once you hit the button.

Thirdly, it's entirely possible that a system someone just made up has already been done before. But that doesn't mean you can't have your own take on it.

Fourthly, it's always important to have something to say. It's no good to simply recreate something somebody else did. We already have the original. But there's a very long tradition of taking what you like from different sources and cobbling them together into something with your own unique stamp on it.

I never did understand what all the fuss was over tv tropes, everyone hating on it. Then I finally found out about the creepy pedo contingent on the anime sections. Ok, that's pretty bad. As for being superficial, it's still a start. It's like with an encyclopedia, not the definitive work on a subject but can at least point you in the right direction to explore.

Now let's not divert the nice OP's thread into a pigfight, ok?

[Formless]

Keeping it classy.

Oh boy, condescension. Are you just trolling now, or have you forgotten what this website is?

Look, I would have a tiny bit more respect for you if you would at least bother to read the other posts in a thread to see if you have anything new to add, and refrained from posting if you clearly did not. But enough explaining myself.

Firstly, Atomic Rockets is a huge site. It's entirely possible to be familiar with it and still find new, hidden gems.

The OP stated he had read Atomic Rockets page on FTL. Hence, following simple logic only you could fail to follow, its reasonable to conclude that he read that list before asking for our input. This isn't a hidden gem on some page no one visits, its at the bottom of a page we know he has visited before.

Secondly, everything has already been done before. Line of sight teleport has been used as a limitation in magic systems. I'm pretty sure I've heard of it in FTL systems where there's no steering once you hit the button.

The difference between you and me:

Formless: "I feel obligated to steer people away from popular cliches." [understatement: I also have a known loathing for "Tropes" logic *]

jollyreaper: "There are no new ideas because everything I need to know [on this subject at least] has been brainstormed by Nyrath and co." Nyrath, who gave himself the full title of Nyrath the Nearly Wise.

Look, Nyrath isn't perfect, and he knows that not everything in sci-fi that exists or could exist is represented on his site. That's why the site exists in the first place. Again, this should be obvious. Besides, Fantasy and Sci-Fi are related, but the same. Fantasy doesn't generally deal with stories on an interstellar scale, so whatever fantasy magic has done with teleportation isn't really applicable here.

Thirdly, it's entirely possible that a system someone just made up has already been done before. But that doesn't mean you can't have your own take on it.

Fourthly, it's always important to have something to say. It's no good to simply recreate something somebody else did. We already have the original. But there's a very long tradition of taking what you like from different sources and cobbling them together into something with your own unique stamp on it.

Irrelevant. You are claiming that every FTL that can exist is represented in either existing fiction, or on that list. Please prove that claim, or withdraw it. At the very least, I want a source for your claim that line of sight teleporting limited by space-time obstacles (i.e. gravity wells) has already been done.

I never did understand what all the fuss was over tv tropes, everyone hating on it. Then I finally found out about the creepy pedo contingent on the anime sections. Ok, that's pretty bad. As for being superficial, it's still a start. It's like with an encyclopedia, not the definitive work on a subject but can at least point you in the right direction to explore.

* Its for similar reasons I am irritated with you right now. The inability to understand the relationship between creativity and inspiration, or the source of those things; plus the refusal to hear anything negative (and not just about the community's pedo- problem) meant that you would never see something called out as a ripoff, and everything is assumed to be an homage. If you are a troper, there are no coincidences: there are no simultaneous inventions. Its conspiracy logic, and a place I would never send an upcoming writer for tips and advice. It would be like telling an upcoming painter to paint something in both a cubist and a pointillist style at the same time. You can take inspiration from Picasso and Vincent van Gogh, but using both of their styles at once would take a master artist if its possible at all.

[Stark]

It's a cat fight over nothing, because he can have a thing consistent with his story, that fits, that creates compelling stuff he wants to do etc whether its old as the hills or super new. Avoiding obvious pitfalls like Freespace didn't doesn't require super original bullshit, and originality doesn't automatically make anything better.

I think it's pretty sad a guy showed up and wanted advice and the thread has turned into nerd cred posturing.

[Sleck]

- Spacecraft should still have to fly around star systems at slower than light speeds to allow for really cool space battles, give some warning of incoming enemy attacks, and to prevent nuclear warheads from simply appearing at their targets. I'm thinking go with a classic beyond the orbit of Pluto type or requirement for Sol for example.

I think some hand-waving might be good in this situation. Just say that there's an "[insert name] sphere" around each type of main sequence star where you can't form an FTL transit point, so you have to go outside of it to do that - and that means basically beyond the outer solar system of most stars. If you want something that's real, you could say that FTL isn't possible inside of the "heliosheath" of a solar system, which would be 80-100 AU from the Sun in the case of our solar system (but that also means your ships have to travel a long ways to get into a solar system, even if you have super-rockets).

Or try the opposite - claim that FTL takes up ridiculous amounts of energy, and that close proximity to a star is required to power it. That makes stars essentially giant wormholes.
then you can have civilian solar collectors to send off commercial and personal travel, but have you military ships big and armoured enough to get close enough to make the jump by themselves.

[Batman]

And how is that supposed to work? Yes, stars release an incredible amount of energy....99.9999999999999999999999999999 percent of which never comes into contact with you, and that's when you're sitting practically on top of them. Unless you accept ridiculously resilient shielding yet somehow power generation that's probably outdone by modern day fission, even fusion is bound to be massively more powerful than what you could get off solar collectors.
Unless you want to use a technobabble 'we can use a serious fraction of the star's power despite virtually none of it actually hitting us' tap, of course.

[Knife]

You could have the FTL work by manipulating gravity and any nearby mass not going along for the trip will tear the ship apart as it accelerates/decelerates. So ships have to be well clear of any large bodies of mass when entering or leaving FTL. Pluto orbit distances in the case of the sun, and maybe 100s or thousands of miles of distance away from any station or ship when jumping or stopping. Just take Newton's formula for gravity and plug in the mass of your ship and the sun's mass and calculate what the force would be at around Pluto's orbit for r. You can also add a fudge factor introduced by your FTL engine if you want. Keeping that as the maximum amount of force an accelerating ship can take will let you stay consistent in determining how far away from a star/planet/ship/comet you have to be in order to enter/exit FTL.

Even the mass of the ship and cargo could have an effect on this, so you can't accelerate too fast past C without your ship tearing itself apart. Military ships would be built with expensive materials allowing it to shrug off stresses that would tear a cheaper ship apart; thereby allowing it to go twice as fast.

I developed an idea a few years back for a story that went the other way, a gravity type drive that used the gravity deformation on space time of a planet to break through to 'hyperspace', mostly due to power restrictions of human tech. Basically, you had to 'jump' from planet to planet and there was no deep space anything unless you snail drive it out there. It made orbital defenses as important as ships and craft.

[Simon_Jester]

And how is that supposed to work? Yes, stars release an incredible amount of energy....99.9999999999999999999999999999 percent of which never comes into contact with you, and that's when you're sitting practically on top of them. Unless you accept ridiculously resilient shielding yet somehow power generation that's probably outdone by modern day fission, even fusion is bound to be massively more powerful than what you could get off solar collectors.
Unless you want to use a technobabble 'we can use a serious fraction of the star's power despite virtually none of it actually hitting us' tap, of course.

It could just be that the hyperspace drive only works in a gravitational field of, say... 20g or higher.

That would do a pretty good job of explaining why nobody ever discovered it in modern times (about as good as we'd get by saying it only works in fields of 0.001g or lower); the physics don't actually work the same as we think but the deviations are only noticeable under certain conditions we can't easily access.

[Batman]

That would certainly be an unusual approach, ordinarily you have to get out of a gravity well for your hyperdrive to work. Having to be close to a star for the drive to work certainly...well, works, but I was commenting on how that's not really a good way to power the drive. Needing 20gs to operate is certainly feasible, as would be the drive requiring prodigious amounts of background radiation. Solar collectors for power? Not that it hasn't been tried, mind you (BT K-F drive & solar sails) but that doesn't mean it makes sense.

[Simon_Jester]

True. Sleck kind of handwaved the part where you convert the 'massive energy' of a star into useful energy to power your FTL-jump. "Close to star" does not mean "unlimited power," you're right about that.

[Sleck]

True. Sleck kind of handwaved the part where you convert the 'massive energy' of a star into useful energy to power your FTL-jump. "Close to star" does not mean "unlimited power," you're right about that.

You're already breaking the laws of physics by going to FTL in the first place - frankly, I don't see the difference between claiming it happens because gravity or because star or HYPERSPACE! As long as it's at least plausible, the purpose of anything in a story is to serve that story - Hyperdrive gets blocked by Interdictors, ysalamiri block Force. None of these delve into Hyperdrive equations, or ysalamiri biology. Nowhere does an X-Wing comic stop to present the interested reader with a physics formula so that they can figure out precisely how much mass is required to block hyperdrives or offers a ysalamiri autopsy report and accompanying lecture by an Imperial professor on the nature of the Force. It's a drive. Heat goes in, kinetic energy goes out. It is made of Phlebotnium, a rare alloy of Unobtanium and Wonderflonium. That is all a reader need know.