Dark Matter Drive

[angelfromanotherpin]

Contemplating a space opera game, right now I'm thinking about propulsion issues.

Realistic spaceships are unfortunately subject to the rocket equation, which is a big spoilsport in terms of cool sci-fi action.* The usual solution is some sort of reactionless drive that directly converts power into thrust, but this comes with its own unfortunate implication. Specifically, anything with such a drive readily becomes a relativistic kill weapon of incredible power. At that point, all ships are (or should be) strictly controlled by governments and there's no room for all the fun smuggling and privateering that I'd like to include.

My proposed workaround is this:
1) One theory of dark matter is that it is mass that exists in some weird dimensional fold of our universe. If normal matter is stuff that sits on top of the rubber sheet of spacetime, dark matter is stuff that clings to the underside. Both deform the sheet in similar ways, but they don't otherwise interact.
2) The NQR (not-quite-reactionless) drive uses some sort of field to push off against dark matter, so it doesn't have to carry reaction mass. It just uses the galactic dark matter halo, slightly like how a boat's propeller uses water.
3) Crucially, the NQR drive's field also experiences drag from the surrounding dark matter halo, which increases with velocity, so that all ships have a maximum NQR speed. I think it also takes a while to shut the field down, so while you can NQR up to speed, turn it off, and then coast, you don't get to coast at your top NQR speed.

Broadly, this plays into a number of the old 'space is an ocean' tropes, (though certainly not all of them). I'm curious what more physics-minded people think. Is it at all plausible? Are there implications I might have missed?

Also, does the density of the dark matter halo matter much? It should be thicker towards the core and thinner out by the rim. Could one reasonably say that this reduces both reaction mass and drag proportionately and they balance out? Or would things get funkier than that?

*I love sci-fi action that accounts for the rocket equation, I just don't want to have to produce any.

[Occluded Sun]

Are there dark matter currents? What are the effects of changing the momentum of the dark matter?

Is this just a convenient way to handwave the drive systems for spacecraft, or are you interested in exploring the consequences of the postulated world you've defined?

[angelfromanotherpin]

I think the halo mostly just rotates with the galaxy. There are noticeable currents around black holes, probably? I don't think there's much consequence to pushing the dark matter around, I doubt anything on the scale of space fleets is going to affect something on the scale of an interstellar medium.

I mostly want to have an explanation for cinematic-style space propulsion that doesn't contradict known physics. (Thus, dark matter, because it's so unknown a quantity.) But if my explanation has unforeseen consequences, I do want to dig into them, for world-building if nothing else.

[Josh_Kablack]

Um what am I missing?

The issue that anything sufficiently fast to cross interstellar distances at human-lifespan relevant times is sufficiently fast to serve as a relativistic kill weapon still exists in settings that ditch the rocket equation and use implausible fantasy drive technologies.

The usual sci-fi workarounds for such in settings are

• Ditching space ships in favor of some form of teleportation or information beaming transport.
• Some sort of Gate / Wormhole that allows shortcuts through real space
• Some sort of Hyperspace that bypasses and doesn't interact with normal space.
• Some sort of Warp Drive / Space Folding / Wrinkling that compresses local space inside larger real space
• Some sort of life-extension technology (Spice, Hypersleep Pods, Consciousness Transfer, etc)
• Accepting it and writing about generation starships.
• Accepting it and limiting the points of travel in the setting to less than interstellar distances.

Now I think you are trying to set things up so that the Dark Matter Drag of your NQR drive somehow slows things down and limits the potential for relativistic kill vehicles -- but that runs into the issue that such vehicles are then not fast enough to traverse interstellar distances before old age, so I'm really not getting your solution here.

[Username17]

The only realistic FTL at the moment is the Alcubierre Drive. That requires an enormous amount of mass in your ships, which in turn means that using them as relativistic kill vehicles is probably impractical. You can't make a lot of Alcubierre ships, so you also can't use them as missiles. Also, since they are moving through their warp bubbles at sub-luminal velocities, they aren't actually all that great as relativistic kill vehicles.

Plus side: you get giant "mother ships" that move between star systems and then spit out smaller ships that move around inside star systems and land on planets and fight it out in space and shit. So like the Star Destroyer model, which is awesome.

Minus side: there isn't much room for player characters to have their own Alcubierre ships. It's basically a Star Destroyer, not a Firefly. So a small group of protagonists are never going to "own" one.

-Username17

[angelfromanotherpin]

FTL is a separate thing. The issue is that reaction drive spaceships can't jaunt around even inside a solar system without needing a huge amount of reaction mass, especially if they want to land and take off again from planet-sized bodies. An actually reactionless drive that can get you from ground to orbit in any reasonable amount of time has enough thrust to become an RKV very quickly. Then you don't have space fleet battles, you have the equivalent of nuclear detente, with all the Cold War dynamics that implies.

[Username17]

FTL is a separate thing. The issue is that reaction drive spaceships can't jaunt around even inside a solar system without needing a huge amount of reaction mass, especially if they want to land and take off again from planet-sized bodies. An actually reactionless drive that can get you from ground to orbit in any reasonable amount of time has enough thrust to become an RKV very quickly. Then you don't have space fleet battles, you have the equivalent of nuclear detente, with all the Cold War dynamics that implies.

The rocket equation is actually extremely generous if the exhaust is very light and very fast. So if you're shooting lasers or plasma jets out the back, the amount of reaction mass you need to expell is pretty small. A 1G acceleration gets you to Mars in a couple of days and is entirely within the realm of the possible if you have fusion reactors. Such vessels are essentially powered by hydrogen bombs, but they aren't more dangerous ramming at high speed than they are just lobbing an actual hydrogen bomb at the problem.

Making solar empire stuff go on where people aren't particularly inclined to ram their interplanetary ships into things is not vrry hard. All you need is to posit that the space ships are significantly more expensive than nuclear weapons, which is a very easy sell if the ships have nuclear engines.

Now a separate issue is that nuclear war as we understand it is already a 'get wrecked' scenario. We do not have a good explanation for how war between factions that have access to nuclear weapons can be anything other than a rocks fall everyone dies situation. But reaction drives that'll get you to Jupiter and back in the timeframe of a cruise ornaval shipment doesn't make that any worse.

-Username17

[angelfromanotherpin]

My space travel calculator tells me that to travel the theoretically shortest distance between the earth and mars using the theoretical output of a fusion rocket requires a roughly 3:5 fuel/payload ratio. That's about as short a trip as you would ever want to take, and a round trip still requires 1.56x the weight of your ship in fuel, and it doesn't account for climbing out of either planet's gravity well. Also, liquid hydrogen is actually super bulky to carry, taking up over 14 times as much space as the same weight of water, so any even interplanetary ship would have to be mostly a giant fuel can. Substantially less of a giant fuel can than our current spacecraft, but not enough less.

My current model for keeping space combat in the pre-ICBM mold is just to say that ECM has advanced faster than ECCM, and so everybody has to fight at embarrassingly close range. Bombing a planet means parking a fleet in orbit, because long range missiles are just going to go off course to a humiliating degree.

[Wumpus]

My current model for keeping space combat in the pre-ICBM mold is just to say that ECM has advanced faster than ECCM, and so everybody has to fight at embarrassingly close range. Bombing a planet means parking a fleet in orbit, because long range missiles are just going to go off course to a humiliating degree.

How does that follow? It's trivial to calculate where a planet will be N days from now - with a computer and optical telescopic measurements - and likewise where a point on the planet will be, at least to the level of "aim a very high velocity rock and hit a city" - and it's not like you can jam an incoming rock traveling at 0.15C

[angelfromanotherpin]

How does that follow? It's trivial to calculate where a planet will be N days from now - with a computer and optical telescopic measurements - and likewise where a point on the planet will be, at least to the level of "aim a very high velocity rock and hit a city" - and it's not like you can jam an incoming rock traveling at 0.15C

I was responding to the question specifically of nuclear weapons, presumably delivered by some sort of missile.

But in any case, if someone starts accelerating a rock in-system, they can be intercepted by a fleet long before it gets up to speed. And if the rock-throwers could win a fleet battle, they wouldn't need the rock. If they accelerated the rock from another system, they're stupid, because it won't get there for at least 30 years and who knows what will happen in the meantime, and in any case the target has literal decades to notice and deal with it.

[Username17]

My space travel calculator tells me that to travel the theoretically shortest distance between the earth and mars using the theoretical output of a fusion rocket requires a roughly 3:5 fuel/payload ratio. That's about as short a trip as you would ever want to take, and a round trip still requires 1.56x the weight of your ship in fuel, and it doesn't account for climbing out of either planet's gravity well. Also, liquid hydrogen is actually super bulky to carry, taking up over 14 times as much space as the same weight of water, so any even interplanetary ship would have to be mostly a giant fuel can. Substantially less of a giant fuel can than our current spacecraft, but not enough less.

My current model for keeping space combat in the pre-ICBM mold is just to say that ECM has advanced faster than ECCM, and so everybody has to fight at embarrassingly close range. Bombing a planet means parking a fleet in orbit, because long range missiles are just going to go off course to a humiliating degree.

Reasonably sure that you're reading that wrong. Mars is 76 billion meters away and if you accelerate at 1g until you are halfway there and then decelerate at 1g the rest of the way you get there in 49 hours. During that period you'll have accelerated a total of 1.7 million meters per second, but the theoretical exhaust of D+3He fusion reactors comes out at 26.6 million meters per second. So the entire trip there and back should take less than 13% reaction mass. To the point that I am not even bothering to take the natural log of wet and dry mass and am instead just considering wet and dry mass to be the same.

Basically, the rocket equation is only tyranical if your exhaust is coming out at less than 1 percent of the speed of light or you're hoping to get between stars. If you're willing to putter about in the solar system on journeys that take a few days and your power source is a portable sun, equal and opposite reactions are fine.

-Username17

[OgreBattle]

Does a human get squished at that level of acceleration, what does it feel like?

[erik]

Does a human get squished at that level of acceleration, what does it feel like?

You tell me. You are experiencing 1g right now.

[OgreBattle]

Ah 1g was mentioned, read it too quickly. Knights of Sidonia has a colony ship traveling like that with the direction they're moving from being "down". they have 'earthquakes' from fast decelerating to avoid collisions and attack that causes residential parts of the structure to get damaged, people get squished from falling stuff, etc.

[Orca]

Traveller:2300 had a 'stutterwarp' drive which moved the ships around without giving them inertia, and which allowed them to see the universe around them. The drive moved the ship in and out of their version of hyperspace many times a second. That much worked in the game, a shame the setting wasn't better for running games in.

[Username17]

If you're really concerned about reaction mass, don't shoot the products of the power plant out at all. Make energy, then use that energy to power a mass drive that then hyper accelerates an ion trail to a significant portion of the speed of light. The plasma jet has an essentially negligible mass and its inputs into the rocket equation are basically "lol wut." Deuterium fusion's theoretical output of 88 trillion joules per kilogram of fuel means that if you run it all into an ion jet your fuel and ejection mass is a rounding error on the crew's personal affects allowance.

-Username17

[angelfromanotherpin]

Reasonably sure that you're reading that wrong.

Your model uses different assumptions from mine. For instance, mine gives a D+3He fusion exhaust speed of about 1/3 of yours. But even with your much more generous model you're still only:
1) Achieving an uneventful round trip between two very close planets.
2) Not actually achieving that because 1g acceleration doesn't get you off the ground at origin.

It takes roughly as much fuel to get to Earth orbit as to get to Mars from that orbit. If you want to do any maneuvering along the way (such as an unplanned rescue or a dogfight) you need more. And every time you need more fuel, you need more fuel to carry it. It iterates up fast.

Doing extremely basic space adventure shit like 'fly to nearby dangerous planet and away again, including a short space action sequence in there somewhere' turns even an optimistic reaction ship into an appendage on its fuel tanks. Somewhat less basic shit like 'venture into the far reaches of the system to search for the mysterious Planet Nine' is much worse.

Finally, even if reaction drives were practical, I don't ever want to have to do acceleration math at the table to see if one ship catches another before it reaches its destination. I don't ever want to have to do acceleration math at the table at all.

[Username17]

Gravity Wells.

Gravity wells are a big problem, but not one that is solved with warp drives or dark energy drives or whatever. Your system of moving between planets has very different needs than your system of moving onto and off of planets. Getting to Jupiter requires a long time of modest acceleration or some sort of science fiction device that compresses distance. Getting out of Earth's gravity well takes less than ten minutes but requires enormous amounts of acceleration that entire time.

Like, you can have an Alcubierre Drive that will allow you to compress space and warp your mothership to Alpha Centauri in a few days. But that does not in any way reduce the needs for booster rockets or planet side mass drives or space elevators or some fucking thing to get all the pieces of that mothership into space in the first place.

The only way you can get into space and travel around the solar system in a reasonable amount of time is if you science fiction yourself some anti-gravity. If you can directionalize gravity using some sort of bullshit science fiction black box, you can have your ship fall off the Earth in a few minutes and then fall to Mars in a few days. Otherwise your interplanetary drive and your gravity well escape system are so different in needs that they are not answered by the same technological declarations.

Finally, even if reaction drives were practical, I don't ever want to have to do acceleration math at the table to see if one ship catches another before it reaches its destination. I don't ever want to have to do acceleration math at the table at all.

Why would you ever have to do acceleration math? Things moving between planets are all moving so quickly that any combats are essentially jousting matches - you get exactly one shot as you zip by each other. The exact number of relative meters per second is something the characters have to care about, but the players and authors sure as fuck don't. When your velocity relative to the Earth/Mars reference frame is more than two thousand times the speed of sound, even a tiny difference in vector means that ships will not meaningfully be staring at each other in deep space for longer than is required for a pre-calculated flyby attack.

On the flip side, ships in orbit might as well be stationary. They are all moving at essentially the same speed against a common reference frame and they can jolly well send hailing frequencies at each other and have Star Trek style communications and have pitched battles and shit.

-Username17

[angelfromanotherpin]

Why would you ever have to do acceleration math? Things moving between planets are all moving so quickly that any combats are essentially jousting matches - you get exactly one shot as you zip by each other.

1) I'm uninterested in space battles that are solely jousting matches. If your propulsion system creates that, I'm not interested in it. If spaceships have top velocities and cruising velocities - like sea ships – that's much less of an issue.
2) Even if a space battle joust was the only form of space battle, acceleration math would be needed to determine if there was a joust at all. The PCs are at point A trying to get to point B in ship X, can the enemy starting at point C in ship Y intercept them before they get there? If I have the luxury of arbitrarily determining those inputs, I can of course just declare the answer, but PCs are gonna PC and who knows what the situation will devolve into? Many of those inputs are things the players should be able to know in advance.

Reaction drive ships don't fit my wants, and I'm not going down this tangent any further.

[Username17]

Why would you ever have to do acceleration math? Things moving between planets are all moving so quickly that any combats are essentially jousting matches - you get exactly one shot as you zip by each other.

1) I'm uninterested in space battles that are solely jousting matches. If your propulsion system creates that, I'm not interested in it. If spaceships have top velocities and cruising velocities - like sea ships – that's much less of an issue.

No. the only alternative to jousting matches in the void between worlds is "no deep space combat period." Uranus is 3 trillion meters away. That's Trillion, with a T. Now that's not a huge problem with a constant acceleration drive - at 1G of acceleration you get there in 24 days. It's fine. And I don't have a huge problem if you have some sort of science fiction bullshit where you warp to top speed and maintain it for a few weeks. But think about that for a moment - you're looking to spend a million seconds traveling three trillion meters! That's an average speed of about 3 million meters per second - one percent of the speed of fucking light. That's over Mach eight thousand. The idea that you could do anything in terms of meaningful naval maneuvers and broadside exchanges to someone when you're both traveling at that speed is completely ridiculous. A 1 degree difference in vector would put an extra 52 kilometers between the two vessels every second.

If you do want naval maneuvers to ever happen under any circumstances, reaction drives is where you want to be. Because at least then things are moving at handlable relative velocities at the beginnings and ends of their journeys. If things get to move to 3 million meters per second in any kind of reasonable time frame, space ships are never going to be stuck in a reference frame when they can meaningfully interact with each other for more than an instant. With reaction drives, ships that find themselves orbiting the same gravity well will have relative velocities that are (comparatively) close to zero and it takes them hours to pull away into something similar to ludicrous speed. If people hard warp into ludicrous speed, then battles just fucking end the moment anyone on either side turns on the hyperdrive. But if people have to putput their way into decent velocities over long periods, then they are stuck with each other for long enough to have shouting matches over the intercoms and shoot lasers at each other and shit. Indeed, 1G of acceleration is achievable by a sports car, so it enables the kinds of vehicle maneuvers that people can visualize and demonstrate with a plastic spaceship model making "zoom" noises with your mouth. At constant velocity, the numbers are all insane bullshit, so the only maneuvers possible are weird intangibles like "masking warp signature" and shit.

Another important issue is that constant acceleration ships make farther objects relatively closer. Which is to say that at 1G you are spending a day accelerating and a day slowing down to get to Mars over the weekend, and you're spending nearly two weeks speeding up and three weeks slowing down to get to Uranus in 24 days. That means that you get decidedly naval time frames to get to the close planets and to get to the far planets. If you were moving at constant velocity, a speed that would get you to Uranus in 24 days would get you to Mars in like 14 hours. A constant speed that would get you to Mars in 2 days would get you to Uranus in 78 days. Constant velocity space travel just fails to do the Solar Empire stuff right, because no matter where you peg the constant velocity, it makes the close planets too close or the far planets too far or both. And that's even before we get into the fact that Uranus' distance varies over time by six hundred billion meters, which is kind of a big deal for constant velocity but a relatively minor deal for constant acceleration.

2) Even if a space battle joust was the only form of space battle, acceleration math would be needed to determine if there was a joust at all. The PCs are at point A trying to get to point B in ship X, can the enemy starting at point C in ship Y intercept them before they get there? If I have the luxury of arbitrarily determining those inputs, I can of course just declare the answer, but PCs are gonna PC and who knows what the situation will devolve into? Many of those inputs are things the players should be able to know in advance.

This is the opposite of true. For mid-space interception, the only possible interception is by people who start broadly speaking "in front" of you. Trips take days, and no one is ever going to "catch" you unless they some sort of weird fast burn ship that has an above normal acceleration rate.

Or to put it into shit simple terms: people can joust you in space if and only if they could reach the place you are going to before you could. Either by having a faster ship or by starting at the planet or space station you are moving towards. That's it. You just compare travel times from where you are, and if it's less, you can space joust. Where the space jousting would actually take place is a complicated calculus problem involving multiple variables, but you also don't give a shit because space is a formless empty void and any space joust is going to take place millions of kilometers from anything anyone has ever or will ever care about.

And if you don't want people space jousting at all, just hand wave that mid-flight interceptions are too hard and you can't actually intercept people until they slow down to orbit a planet or if they intend to crash into a planet at speed then at least not until they are close enough to the planet in question that they have a profoundly limited number of vectors and thus literally all space battles take place near a gravity well one way or the other.

-Username17

[Mord]

Dude just handwave it. Real physics produces boring outcomes. Ignore it. Invent some magic drive that works the way you want it to, make the outputs clear, and move on to telling the stories.

Analogy: no one gives a shit how the Three Laws are enforced within the positronic brain; they care about the stories you can tell with that well-defined set of rules by which robots operate.

[angelfromanotherpin]

Dude just handwave it. Real physics produces boring outcomes. Ignore it. Invent some magic drive that works the way you want it to, make the outputs clear, and move on to telling the stories.

Alternate plan: I'm going to not do that and also tell you to fuck yourself.

[Ancient History]

In Space Madness! I handle solar empire stuff with a combination of chemical rockets, gravity polarizers, and Azathoth engines basically cribbing the most useful features off of Larry Niven's *Known Space* and Frank Herbert's *Dune* for purposes of making solar system travel viable but still preserve relatively vast distances and making travel between solar systems still a tricky proposition.

[Mord]

Alternate plan: I'm going to not do that and also tell you to fuck yourself.

I hate to break it to you, but any attempt to use realistic methods to achive unrealistic results, or, as you put it earlier, "an explanation for cinematic-style space propulsion that doesn't contradict known physics" is impossible and doomed to failure.

Good luck tilting at that windmill and thanks for the salt.

[Ghremdal]

You have to make new physics to woek with to make the setting cinematic. Ive got a thought or two that I will post later.