[Spaceships] Reactionless Drives, Pseudovelocity, and NOT breaking a setting (much)

[Ulzgoroth]

Originally Posted by malloyd

This might be a good example of the sort risks you run into with these things. If you assume you are "safe" from an abuse because planetary gravity wells are not portable, you may end up in trouble when a clever player attempts to build an equivalent potential well generator with a pair of (much more portable) charged spheres.

Fair point. So, a drive that can generate potential energy of any kind, not just gravitational, in excess of its input plus acceptable free-energy margin (because small amounts of free energy may break physics and have setting implications but won't break planets in useful time scales) is an RKV-threat unless you have an aspect that prevents it operating when moving at relativistic speeds.

(Though this is premised on considering photon drives plus free power to constitute a setting problem.)

Originally Posted by malloyd

Probably. Actually I suspect a preferred frame may be necessary for a reactionless drive to make sense at all - the relationship between your momentum and kinetic energy will not transform the same way between different frames as the energy consumed by your drive - so if it's "safe" in one frame, it isn't in others. Of course that's a contradiction - it's the same system - so relativity is going to have to give somewhere. With a reaction drive, the relative momentum and kinetic energy of both your ship and your exhaust change, and not in the same way (their relative velocities are necessarily different relative to any other frames you are considering), allowing for some cancelling out.

The dramatic disagreement about how much net change in energy the drive is creating even at much-less-than-c speeds does seem to suggest that, at least for the simpler models, and a lot of the less simple ones can't even be described without overtly using a preferred frame.

But more specifically, I meant an external preferred reference frame. A reactionless drive in the most elementary conception might be preferring the inertial frame in which it is at rest at any given moment, but I think it's worth drawing a distinction between that and situations where the drive cares about your motion relative to, say, nearby major gravity wells.

Having your drive somehow limited relative to something akin to Kerbal Space Program spheres of influence is a (fairly basic) starting point for trying to make it less geocidal.

[Varyon]

Uncertain if this will be of any use, but here's the paradigm I'm considering for a Space Opera setting (which is probably softer-science than Vicky's likely going for, but may be informative).

First off, hyperspace exists and has something of an atmosphere, called aether. Hyperspace contains "gravitic shadows" (or similar jargon) that correspond perfectly to the location of matter in our "layer" - but at a fraction of the scale (I haven't yet decided on an actual scale, but for our purposes we'll say things in hyperspace are 1/1,000,000th the size). Transitioning into hyperspace makes the rest of the universe essentially shrink from your perspective, after which point you can travel through the aether; whatever speed you perceive yourself as moving at, you're actually moving a million times faster relative to the rest of the universe. You can't exceed - or even really get close to - c relative to hyperspace, but you can exceed it relative to normal space.

That out of the way, there are three methods of travel, all of which use the same drive*. To travel between systems, you need to transition into hyperspace, so that you can exceed c. Objects in hyperspace have minimal interaction with those in real space - they can be detected by special, well, detectors, but the fact that they're moving faster than the speed of light means by the time you detect them, they're gone (or have already transitioned back into real space). You technically maintain your real space velocity while traveling through hyperspace, but the only way out of hyperspace is coming into contact with the gravitic shadow of a sufficiently massive object (typically, a star), which dumps you back into real space whether you'd like it or not and sets your velocity to match that of the "anchor" (if this change would destroy energy, the excess is dumped into hyperspace; if it would create energy, this is instead drawn from hyperspace).

To travel within a system - particularly after transitioning out of hyperspace, which puts you a pretty hefty distance from anything interesting - vessels use a "Boost" mode. This involves a partial transition into hyperspace - the vessel doesn't change in size and still (almost) fully interacts with real space, but it lacks proper substance, passing through matter like a ghost. This transition allows it to change to any speed up to a sizable fraction of c (I'm currently leaning toward 0.9c) instantly, but this is pseudovelocity. It can only be maintained for a short period of time, after which the vessel must drop out of Boost and rest for a bit. Dropping out of Boost requires a nearby "anchor," typically a fairly large object, with which the object automatically matches velocity (again, energy is transferred to/from hyperspace to effect this). The pilot doesn't get to choose what his anchor is - rather, it is simply the nearest object (or collection of objects, provided they are close enough together) of sufficient mass. In fact, getting too close to a potential anchor will actually force the ship out of Boost - a sufficiently large vessel (or fleet) can use this to interdict a smaller vessel (or fleet).

For travel near an anchor (and for combat), the vessel's drives will actually generate real velocity. The change from the vessel's velocity absent the drives is limited - in general, space fighters have performance comparable to WWII-era fighter planes, capital ships have performance comparable to WWII-era battleships.

*It's essentially the same technology, but there are actually two different drive designs. Short range vessels, like fighters, use fully reactionless drives that have pseudoatmospheric handling and burn through a lot of energy. Long range vessels, like capital ships, instead use semi-reactionless drives (that is, reaction drives with physically impossible delta-v efficiency) that have a much lower top speed and acceleration but burn through far less energy.

[Fred Brackin]

Originally Posted by Johnny1A.2

That's not how you'd do it. With the Bergenholm, what you'd do is take a brick or a rock or a ball of cotton, put on a ship, and let that ship accelerate inert until it's pressing against c, relative to Prime Base. That wouldn't be hard for ships like those of the later Lensman books. It doesn't matter if it takes a week to do it.

.

A month if you use the 11Gs Velantians build for when inert. You'd need 40-50Gs for a week.

Even regular Patrol ships may have more thrust than they can stand to use while inert. It could well be that the 5Gs most Patrol ships can pull while inert is because of organic limits and not propulsion,. There's no benchmark I can think of that would tell you how fast a drone ship could accelerate.

I'm not even sure you can run a bergenholm inside another bergenholm and your carrying ship and that ship would have to match intrinsics with Earth before it would be allowed to land.

There's probably a customs inspection by spy-ray or sense of perception is not physical boarding just to make sure the hold isn't full of duodec which would make your multi-megaton fireball without bergenhom tricks.

You'd have to be satisfied with blowing up the Port of New York anyway. Nobody but the Patrol lands at Prime Base, No more than commercial jetliners land at Andrews AFB.

[Emerald Cat]

Originally Posted by Varyon

First off, hyperspace exists and has something of an atmosphere, called aether.

This is an interesting approach. It sounds like weapons would work more or less normally in this setting. Political borders would arise naturally. I favor "shortcut" type hyperspace based comms and FTL myself because it allows the PCs to go interesting places while preventing world breaking shenanigans. And also neatly resolves the Fermi Paradox in a way that doesn't preclude galactic civilizations.

Building a reactionless drive that doesn't violate the (known) laws of physics is difficult.

But it is quite feasible to prevent "true velocity" reactionless drives from breaking the setting with two simple rules:

• Energy and Angular Momentum are Conserved: you must pay an energy cost equal to the change in KE of your ship + PE due to changing your location. Also makes accelerating anything to c exorbitantly expensive.
• Velocity is Continuous: you can't just instantly jump to the desired velocity. You must accelerate. This helps reactionless drives mesh with human intuition while making them harder to weaponize.

[Ulzgoroth]

Originally Posted by Emerald Cat

Energy and Angular Momentum are Conserved: you must pay an energy cost equal to the change in KE of your ship + PE due to changing your location. Also makes accelerating anything to c exorbitantly expensive.

This isn't possible.

If your drive conserves linear momentum (and doesn't use some kind of un-counted virtual velocity), it's not a reactionless drive, it's a reaction drive. If it conserves angular but not linear momentum it's...almost certainly mathematically impossible.

But you definitely cannot conserve energy, because the change in KE of your ship is a reference-frame dependent value. Picking up 1 m/s from rest is only 1/3 as much kinetic energy as increasing from 1 m/s to 2 m/s, but those are descriptions for the same thing from only very slightly different points of view.

[Anthony]

Originally Posted by Ulzgoroth

But you definitely cannot conserve energy, because the change in KE of your ship is a reference-frame dependent value.

You can as long as long as it doesn't change -- teleportation conserves both KE and linear momentum -- but you probably can't do so while also conserving angular momentum (which teleportation fails to conserve, unless it has a corresponding reaction mass).

[Ulzgoroth]

Originally Posted by Anthony

You can as long as long as it doesn't change -- teleportation conserves both KE and linear momentum -- but you probably can't do so while also conserving angular momentum (which teleportation fails to conserve, unless it has a corresponding reaction mass).

True. You definitely can't conserve KE with a 'true velocity' reactionless drive. A drive that produces displacement by means other than real velocity can conserve KE so long as it leaves your velocity vector unchanged.

[acrosome]

I've thought about this exact problem A LOT, and the best answer I've ever come up with is in agreement with Roger:

A 2300AD-style stutterwarp, which loses efficiency in steeper gravity wells.

This also has the benefit that interstellar (and interplanetary) travel only requires power, which is presumably cheap. This allows for common scifi tropes such interstellar trade and warfare. (Because when you have to throw away remass that stuff gets too expensive.) Of course you still have to deal with getting out of gravity wells, which will still be relatively expensive, but having TL12 engineers as playthings greatly simplifies almost anything...

Beanstalks solve a lot of problems.

[Johnny1A.2]

Originally Posted by Ulzgoroth

Not sure that the use of the inertialess capability is contributing much to the relativistic bombardment problem. The ability to drive a payload to near-c speeds without using the Bergenholm is the foundation of what you're doing...

That's where you get the KE, yes. Where the Bergenholm comes in is the delivery system. A straight kinetic kill attack is pretty much futile in the Lensman universe, your .99999c object will be detected and diverted/destroyed long before it gets anywhere close to Prime Base or any other significant Patrol target, if it's just coming through normal space. There isn't even any point to it anyway, if you're using the direct approach you can get the same effect with a big total-conversion atomic bomb anyway.

But the Bergenholm lets you carry a relativistic kinetic kill weapon in your backpack, while travelling at whatever speed is convenient for you, or something of the sort. It may have a KE of .99999c relative to where you are, but it's harmless and motionless as long as the Bergenholm is running. Your dining room table centerpiece could have a KE equal to .99999c and it'll sit there looking pretty until the Bergenholm stops running.

Originally Posted by Fred Brackin

A month if you use the 11Gs Velantians build for when inert. You'd need 40-50Gs for a week.

Which wouldn't be that hard for the technology of Civilization/Boskone. But I just picked 'a week' for convenience, slot in whatever time it takes your inert drive to get you to whatever fraction of c is needed.

Even regular Patrol ships may have more thrust than they can stand to use while inert. It could well be that the 5Gs most Patrol ships can pull while inert is because of organic limits and not propulsion,. There's no benchmark I can think of that would tell you how fast a drone ship could accelerate.

Exactly.

I'm not even sure you can run a bergenholm inside another bergenholm and your carrying ship and that ship would have to match intrinsics with Earth before it would be allowed to land.

True, but that doesn't really help. You don't need to run one Bergenholm inside another. Once your attack-object is free, it's safe as long as it's within the radius of effect of any Bergenholm.

There's probably a customs inspection by spy-ray or sense of perception is not physical boarding just to make sure the hold isn't full of duodec which would make your multi-megaton fireball without bergenhom tricks.

True. But that's where the real determining issue kicks in, the skill and ability of the terrorist at disguising his Bergenholm that's keeping the kinetic-kill item safe. Hiding from a spy-ray survey would be no different that hiding from any visual survey, the real issue would be whether you could mask or jam the emissions of the Bergenholm to avoid tripping any sensors on the lookout for it. The stories are not definitive on that point.

Of course, a suicidal terrorist could also probably bypass the problem by using the ship itself as a KK weapon. Take a cargo ship, build it up to .9999c, go free, fly to Earth. When in proper position, turn off the Bergenholm just as if you were about to match intrinsics.

If you set it up right to the directions and so forth are right, your suddenly-inert ship will cover the few thousand miles to Earth's surface in a fraction of a second, with an effect sort of like a major meteor impact.

You'd have to be satisfied with blowing up the Port of New York anyway. Nobody but the Patrol lands at Prime Base, No more than commercial jetliners land at Andrews AFB.

From a terrorist POV, such an attack on civilian spaceports, if properly coordinated, could be very useful in disrupting economic activity. If nothing else, it would force the Patrol to divert resources in that direction.

But if you can disguise your Bergenholm well enough to get it past the spaceport, then you can smuggle in KK objects and plant them where you want, subject to your ability to disguise a Bergenholm.

[AlexanderHowl]

Pseudovelocity is always a problem. Even if you are able to neutralize the threat of relativistic weapons, there is still a lot of stuff that weapon can do. For example, ship A turns off its velocity two hundred kilometers from planet B along its orbital route, ship A detonates explosive bolts to release a 100 gigaton fusion device (a 20,000 metric ton cargo by our technology [minimum possible size is probably around 4,000 metric tons]), and then ship A turns on its velocity before anyone on the planet can figure out what is going on. In less than a space turn (20-seconds) the bomb could be detonated over any city on the planet. The bomb would have a thermal radiation radius of around 1,300 kilometers (5,380,000 square kilometers), which would create a firestorm powerful enough to kill everything within that radius. The amount of "free" energy gained by the bomb is insignificant compared to the energy released by the bomb when it explodes.