Dark Matter Drive

[angelfromanotherpin]

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.

Now this is a real issue, although I think your numbers are a touch off. A million seconds is 11.5 days, frex.

Let's assume a ship that travels at 3 Mkmph, which is ~0.28% of lightspeed. That'll travel the average distance from Earth to Mars (~225 Mkm) in ~75 hours/3 days. It will also travel the average distance from Earth to Uranus (2.5 billion km) in ~833 hours/34 days. That seems perfectly adequate in terms of relative travel times between very near and very far destinations, but the issue of combat at such speeds is still problematic.

Now, I could get into a thing where ships that mutually wish to engage slow on approach to fight, ships that mutually don't wish to engage... don't engage, and when one ship wants to engage and the other doesn't they get into a chase where their relative velocities are probably much more reasonable. But I'm not going to, because 3Mkph is actually still stupidly fast and I don't want to deal with the attendant issues.

My new thought is to give up on solar empire stuff, set the DMD speed limits super low, and have an FTL method be the preferable means of getting around even in-system.

[Omegonthesane]

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.

PROTIP: when the premise of the thread is "I am absolutely committed to doing X. How do I do X?", and you think of replying "You don't", you're probably not making a sensible contribution unless X is something like "give all my money to Donald Trump".

[erik]

PROTIP: when the premise of the thread is "I am absolutely committed to doing X. How do I do X?", and you think of replying "You don't", you're probably not making a sensible contribution unless X is something like "give all my money to Donald Trump".

Meh. The customer isn’t always right. Sometimes the best help you can give is to convince them to choose another path.

[Pariah Dog]

PROTIP: when the premise of the thread is "I am absolutely committed to doing X. How do I do X?", and you think of replying "You don't", you're probably not making a sensible contribution unless X is something like "give all my money to Donald Trump".

Meh. The customer isn’t always right. Sometimes the best help you can give is to convince them to choose another path.

Not bothering to host this elsewhere because lazy right now.

[Username17]

My new thought is to give up on solar empire stuff, set the DMD speed limits super low, and have an FTL method be the preferable means of getting around even in-system.

I genuinely don't see how this is in any way different from just giving people some 1G constant acceleration and being done with it. Once people are a fair distance out of orbit, they can't stand and fight each other because they are moving too fast. While they are in orbit they can't accelerate fast enough to escape combat with each other for quite a while.

What does warping to FTL bring to the party that just accelerating doesn't already do?

-Username17

[DSMatticus]

I really do not understand the premise of this thread.

Interstellar space is big. The nearest star is 4 light-years away. The nearest star is a shithole and nothing lives there. Smugglers are never going to make the Sol<->Proxima Centauri runs without superluminal magical sci-fi bullshit because every run is a decade or more of their life. The only Sol<->Proxima Centauri runs are civil projects. Having cryotechnology does not substantially change this. People still have to touch base with society at some point, and ten years is a long fucking time for things to change and for their entire purpose to have been invalidated. Ten years is enough time for 8 years of Obama and 2 years of Trump. Ten years is apparently long enough for an entire planet's geopolitical stage to fucking implode.

Intrastellar space is still not small. If you fling a Boeing 747 out into space at .025% the speed of light, it will take a little over a month to make it to Mars, a little under three years to make it to the Kuiper cliff, and it's still going to be able to hit its destination like a nuclear weapon by just not decelerating, or, if you apply magical space drag, by just not turning off its engines. Okay, I feel like my Boeing 747 example is breaking down. A Boeing 747's engines would not work in space. You get the point. It's a reasonable size for a space vehicle intended to haul goods.

Also I super hope my math on this is mostly right, but whatever.

Point is, relativistic kill is not a consequence of reactionless drives. It's a consequence of anything that 1) makes space traversible in timeframes humans consider practical, and 2) isn't magical sci-fi bullshit that makes relativistic kill impossible. You add space drag to your setting. Okay? I just won't turn off my engines, and I'll keep going at the speed that got me to Mars in a month, and I'll still hit like a nuke. You need magical sci-fi bullshit, or you need really slow space travel, and in the context of a sci-fi tabletop roleplaying game really slow space travel is big disempowering.

So you need magical sci-fi bullshit. And in order to choose the magical sci-fi bullshit that's best for you, you need to know what your criteria are. You say no relativistic bombardment, I see 'low velocities (at least near planets)'. You say no space jousting, I see 'instantaneous acceleration/deceleration, velocity caps.'

You probably just want some kind of space-folding you can apply arbitrary rules to. Accelerates near instantly, has a maximum speed, velocity is 'virtual' instead of 'real' so no relativistic kill, acceleration is instant so ships move w.r.t to one another locally based on their maximum speed (drive quality) instead of zipping by one another and struggling to turn around only to zip by one another again.

EDIT: I suppose the real lesson here is that fast-moving objects turn into nukes well before you get into relativistic math, and the name 'relativistic kill vehicle' is a superhuge misnomer unless you're talking about going full Chicxulub.

[maglag]

My new thought is to give up on solar empire stuff, set the DMD speed limits super low, and have an FTL method be the preferable means of getting around even in-system.

I genuinely don't see how this is in any way different from just giving people some 1G constant acceleration and being done with it. Once people are a fair distance out of orbit, they can't stand and fight each other because they are moving too fast. While they are in orbit they can't accelerate fast enough to escape combat with each other for quite a while.

What does warping to FTL bring to the party that just accelerating doesn't already do?

-Username17

It's not about warping to FTL doing anything extra, it's simply accelerating doing too much.

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.

FTL warp skips any complicated acceleration maths.

I guess that being able to add some magical bullshit that makes FTL warp not pratical for relativistic kill missiles is another nice perk.

[Trill]

maglag, please fix those tags

[Lokathor]

Dark Matter drives and other "fake" medium drives (Hyperspace or whatever) can be limited to physics situations people are comfortable thinking about by saying that (1) your craft has drag against the fake medium, (2) the intensity of the drag is inversely proportional to how close you are to a gravity well.

So, the closer you are to a planet, or even a star, the lower your top speed becomes, but if you get out between the stars your top speed goes through the roof and you can get to another star system in reasonable time. Even if you sit in interstellar space and launch something at your enemy at "top speed", by the time it gets to the solar shell it begins to slow down, and by the time it gets to an actual planet it's going no faster than a fighter jet. So you can crash space ships into buildings at fighter jet speeds (which is dangerous enough, and does mean you need to have your ship registered in all civilized zones), but that's probably not the best use of military resources.

[violence in the media]

I know this question is contrary to the stated thread goal, but should a space game cater to the same old tropes about fighter jocks and dogfights that we're used to? Especially if you're trying to be plausible with your science in zipping around the stars? Wouldn't it be more interesting to require the players to do or think about things differently because they can't fall back on Top Gun, Firefly, or Star Wars tropes?

Like, what is wrong with having a game where, if the players want to travel between solar systems, they have to be a part of a government or corporation or something? They can space pirate to their hearts' content in a single system, but they'll never personally command the means to travel to another system outside of some organizational hierarchy. Why is it so bad to have intercepting a ship in deep space be a thing that simply doesn't happen? I mean, I don't think that's really even a thing in Star Trek/Wars (though I am not an expert). The Empire doesn't wreck the Rebel fleet while it's in hyperspace transit. Trek doesn't really have people intercept ships at warp speed, though I think they occasionally have chases from the same start point. I guess I'm just trying to say that, even in the media, it seems like ships only engage when someone slows down to look at something, and someone else encounters them there.

Maybe think of interstellar or interplanetary travel more like teleporting, and less like scrambling fighter jets? If I teleport from New York to LA, you aren't going to try and intercept me from Houston by teleporting to Indianapolis.

[angelfromanotherpin]

I know this question is contrary to the stated thread goal, but should a space game cater to the same old tropes about fighter jocks and dogfights that we're used to?

This one should, because it's what the players requested.

[Hicks]

Then they may he better serviced playing 1930s sky pirates with mothership zeppelins



I mean, the Macon and Akron were totally real things that you can base a campaign on:

[kzt]

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.

If you have a fusion reactor you carry your fuel as water, not cryo fluids. Denser and far safer.

[Hicks]

No? You would not do that. 88.79% of water's mass is useless, corossive oxygen that is nothing but dead weight in a fusion reactor. And I thought this was about duterium/tritium fusion, not proton/proton fusion, which is way harder. All that oxygen would have to be electrolized out, and it would be useless as reaction mass.

[maglag]

No? You would not do that. 88.79% of water's mass is useless, corossive oxygen that is nothing but dead weight in a fusion reactor. And I thought this was about duterium/tritium fusion, not proton/proton fusion, which is way harder. All that oxygen would have to be electrolized out, and it would be useless as reaction mass.

The crew needs oxygen so it's far from useless dead weight.

[kzt]

If you have fusion reactors that can drive ships to Saturn in weeks energy is a solved problem for the society. Your fuel factory can build your water out of an appropriate mix of tritium and deuterium.

Though you might be better off using separate tanks for heavy water and super heavy water, depending on what else you might want to do with the water. You shouldn't ingest very much tritium at all but deuterium is not particularly toxic, though you can't live on it alone as weird metabolic disorders will arise. But you can swim in it and ingesting moderate amounts is not thought to be harmful.

[SeekritLurker]

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.

This is essentially the Gundam model - in which magical science-particle fields are spread in battlefields such that space engagements must be fought at visual range, because radar and radio don't work within those fields.

Such a thing does help with most guided missiles, but wouldn't help with the ICBM problem - if you're shooting at a city on the moon, it's not going to move, so you can just aim it with math.

It would, on the other hand, make dedicated bombers a thing for payload delivery.

[Josh_Kablack]

Okay, I have some time, so lemme do some high school physics here:

I'm not going to get close to relativistic speeds, I'm not going to worry about the rocket equation.

It is 54. 6 million km from Earth to mars at their closest point. That's as close as any two planets get in our solar system.

Let's assume 1 g acceleration for half that distance. In realistic (if over simplified physics) you would plan on applying thrust to decelerate after that.

1g == 9.81 m/s

S==1/2 Acceleration * ( time squared )

27,300,000,000 == 9.81 / 2 * ( t * t)

27,300,000,000 == 4.905 ( t * t)

5,565,749,235 == t * t

74,604 == t rounded to whole seconds

So that's a bit less than 21 hours until your turnaround point in realistic (if simplified physics).

More importantly, you can plug that value back in to the simple equation

Velocity == acceleration * time

to get a top speed of:

V = 9.81 * 74,604

V = 731, 865 meters / second

Now, we can plug that in to the formula for Kinetic Energy:

KE = 1/2 mass * velocity squared

KE = mass /2 * 535,626,378,225

Now mass is going to vary considerably, but for the sake of argument, lets say you have a ship as light as a 2008 Tesla Roadster which is 1305 kg. That gives you a ship which is also a projectile that has 349,496,211,791,812 joules of kinetic energy, which is equivalent to roughly 83.5 kilotons. While that's well short of contemporary H-bomb yields, it is still over 5 times the force of the Hiroshima bomb.

If instead, you go with the more plausible mass of a 747, let's say 184,000 kg (approx empty weight) that gives us: 49,277,626,796,700,000 joules, which is about equivalent to about 11.8 Megatons of force. If you bump that up to the maximum takeoff weight for the heaviest-lifting variant of the 747, you get pretty close to the yield of the Tsara Bomba.

None of this is relativistic. None of this is worried about the rocket equation.

But, it means that if your fictional handwavium does not prevent vehicles from reaching speeds of 732 km/second then space-convertibles become potentially as devastating as atomic bombs, and space-airplanes become potentially as devastating as thermonuclear devices.

So what happens if we slow things down to prevent this?

Let's say your setting's handwavium allows instant acceleration, but to only a fictional space-drag limit of 75 km/second. (about one-tenth of where we got before)

It now takes 8 days, 10 hours, 13 minutes and 20 seconds to get from Earth to Mars at the closest point in their orbits.

But the space-roadster is down to 0.877 kilotons of kinetic energy, and the empty weight 747 is down to about 124 kilotons. That's much less effective, but still mass destruction territory.

However, with those assumptions, you are now looking at 3 months to traverse the closest distance between Earth and Jupiter.

None of this is relativistic. None of this is worried about the rocket equation.

So even if you limit the setting to the inner planets, and accept that any interplanetary trip takes over a week (and some take multiple months), you still need to keep spacecraft smaller than jumbo jets and/or make them so expensive that rogue states and criminal organizations find atomic weapons cheaper options.

[Username17]

None of this is relativistic. None of this is worried about the rocket equation.

Pretty much this. Getting to other planets in the solar system does not require relativistic speeds and it does not require particularly interesting amounts of reaction mass or fuel. If you generate energy and spit out a high energy trail, the amount of lost mass is trivial and if you're getting the energy from atomic power the fuel mass is trivial as well.

Nevertheless, ramming into planets hits with the force of nuclear weaponry. It just does. That's the inevitable result of things moving fast enough to meaningfully travel from one planet to another. But it's not really all that much better if you aren't moving at reasonable in-system speeds or even aren't "moving" at all. Terminal velocity is like 320 KPH if you just fall onto Earth's surface, and pushing large rocks into such a position is not especially energy intensive.

If you want people to not go ramming into surfaces of planets, you aren't going to meaningfully get there on the end of the speed limitations of the bullshit engines on your space ships. Virtually any speed including "zero" speed would potentially be enough to make such attacks viable. You could have ground bases good at deflecting incoming rams. You could have ships be enough more expensive than nuclear missiles that it isn't worth it. All kinds of things. But no amount of speed reduction is going to meaningfully impact the calculus.

-Username17

[kzt]

The pretty obvious response is that if you approach within X distance of something someone cares about at a velocity and vector they they don't like they will try very damn hard to kill you and convert your ship to a cloud of ionized gas, or at least to a cloud of debris that isn't on a Constant Bearing, Decreasing Range trajectory.

And they can track you trivially. The thermal output from terrawatt fusion reactors can be passively detected from Alpha Centauri. 'So follow our very simple rules of the road and we won't have to kill you.'

[erik]

If for some reason you don’t want kinetic weapons of mass destruction then you could have some sort of tractor beam defense arrays that can be powered by huge power sources typically only able to be based on planets. With maybe lesser versions on capital ships so they have some defense against a single rail gun but not against tons of em.

Kzt I don’t think traffic cop is the answer. Do they have ftl sensors? Otherwise the traffic ticket is going to come years after the fact. And it just feels heavy handed.

[Username17]

Kzt I don’t think traffic cop is the answer. Do they have ftl sensors? Otherwise the traffic ticket is going to come years after the fact. And it just feels heavy handed.

If you have people moving around the solar system with simple 1G acceleration plasma jets, they are going way slower than the speed of light, and ground based telescope arrays should indeed have a trivial time seeing all of them moving about the solar system. Any of them on a collision course could be identified as doing that hours in advance, and it wouldn't seem heavy handed or unreasonable for them to have things they could do about it. A nuclear bomb can be deployed from orbit much faster than a ship can build up a nuclear bomb's worth of kinetic energy with modest acceleration.

-Username17

[erik]

I was meaning having someone in Alpha Centauri watching Sol travelers and interdicting there seemed a bit unreasonable. Maybe I misread that comment tho and it was meaning just to emphasize how detectable travelers are.

Assuming that, then yeah, I'm on board with having people at the bottom of gravity wells being invested in tracking potential threats and dealing with them one way or another.

[kzt]

yeah, it was just how detectable they are. (using a somewhat different telescope than used in the example below)

http://www.projectrho.com/public_html/r ... detect.php

An example from there:
"A Russian Oscar submarine has a mass of about 15,000 metric tons. Say it was accelerating at a tiny one-tenth of a g (As = 0.1). A chemical rocket has an Isp of around 450 seconds, an ion drive has 21,000 seconds, and a steady-state plasma has about 30,000 seconds.

"This means the maximum detection range of the chemical Oscar is about 1.2 billion kilometers (7.7 AU), and both the ion Oscar and the steady-state plasma Oscar is 25 billion km (167.4 AU). For purposes of comparison the distance between the Sun and Pluto is about 40 AU."

[Thaluikhain]

Hmmm...is this not assuming either our solar system or one like it?

Could you perhaps not fudge things a bit by setting it elsewhere where the distances are more to your liking? Instead of traveling from inner planets to outer ones, perhaps from one moon of a gas giant to another, say?