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

[Ulzgoroth]

Originally Posted by 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.

Uh, nuclear planetary bombardment doesn't seem to be in any way a specifically pseudovelocity problem.

The supposition that pseudovelocity allows you to freely place things at rest relative to the sun is pretty hard on planets, though.

[Johnny1A.2]

Originally Posted by Ulzgoroth

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.

That's been my own approach. In my world, there are superscience drives that look like reactionless drives, that is, someone watching one would think he was seeing a ship accelerate for no visible reason, but in fact they are exchanging momentum with other bodies (albeit by superscience means). This means they can't accelerate indefinitely without limit.

When they operate in normal 4D spacetime, they produce normal velocity.

[AlexanderHowl]

It is not planetary bombardment that is the issue, it is planetary bombardment without potential of defense or detection.

[Fred Brackin]

Originally Posted by AlexanderHowl

It is not planetary bombardment that is the issue, it is planetary bombardment without potential of defense or detection.

How did pseudovelocity enable you o get an unpty-gigaton bomb 200 kilometers away from an inhabited planet without detection or the possibility of defense? doesn't it empower patrol and interception forces at least as much as attackers?

[dataweaver]

How about this:

When you use a Pseudovelocity drive in a gravitational field, there are two effects:

1. There's a limit to how much pseudovelocity you can have: the stronger the gravitational field, the less PV you can have; and the more actual velocity (technically, momentum) you have relative to the gravitational source, the less PV you can have. Put another way, the faster you would be going relative to a gravitational source, the further away you are when its gravitational field starts sapping your speed. Attempting to enter a gravitational field with excessive PV will automatically throttle down the PV to the limit determined by gravity and relative momentum.

2. As PV bleeds off when interacting with a gravitational field, so does the actual velocity differential. Note that this means that you won't lose as much PV as you would have if your actual velocity wasn't a factor in determining the gravity-imposed speed limit; but you will still lose a lot of PV and some actual velocity. The end result is that by the time your PV gets driven down to zero, your momentum will be low relative to the gravitational source.

The idea here isn't to prevent KK weapons; it's to ensure that planetary defenses will always have time to react. Accelerate an asteroid to near-lightspeed and attempt to position it to hit the planet using a PV drive, and you'll either bleed off most of that momentum in your attempt to approach the planet or the defenses will have, say, days to stop your approach.

The other idea is to provide a justification why ships always seem to arrive in the vicinity of a planet with velocities reasonably similar to the planet's velocity, so that you don't have to worry about the massive velocity differentials between, say, Earth and Mars when they're on opposite sides of the Sun: the PV Drive will act as a “brake” of sorts, adjusting your velocity (both actual and pseudo) to match your destination as you approach it.

Thoughts?

[David Johnston2]

Originally Posted by AlexanderHowl

It is not planetary bombardment that is the issue, it is planetary bombardment without potential of defense or detection.

Pseudovelocity doesn't make you invisible or unstoppable. It is not Relativistic Kill Vehicles.

[Emerald Cat]

Originally Posted by David Johnston2

Pseudovelocity doesn't make you invisible or unstoppable. It is not Relativistic Kill Vehicles.

Right. To clarify, this is because you don't gain any velocity when using a pseudovelocity drive by definition. Obviously, if you don't gain any velocity it won't work as a kinetic kill missile.

Reactionless drives without the pseudovelocity modification do have this problem. How exactly does one stop a relativistic kill vehicle? Blowing it up will only work at close range if the shrapnel is too small to cause damage to the planet. With enough lead time, you could theoretically use your own reactionless drive to divert the object.

But unless you have FTL sensors you would have to detect it accelerating to light speed. According to relativity, the speed of light is the fastest information can propagate. Once the ship is traveling near light speed, sensors would be detecting the ship as it arrives. If that happens, the defenders have lost.