maneuver drive limits

[Matt Wilson]

Howdy folks.

What sorts of setting constraints are there on how reactionless drives work, so that ships can't easily be turned into kinetic WMD?

I would like gas giant refueling to be a thing, but getting to, say, Jupiter without taking months or years would require something very close to a "flip halfway" trip a la CT book 2, i.e. near-constant thrust.

A typical ship's pp has enough fuel for 4 weeks. If you could jump into a system far enough out and drop off some old rickety 2g cutter and let it thrust toward a planet for 4 weeks, I think it'd reach about 15% C before it tunguskas its target. 4 gigatons, maybe? That would be bad. So there must be some sort of governor...

Thoughts? I'm sure in the last 50 years that this has been discussed to death, but I wouldn't know where to start looking.

 

[Badenov]

Howdy folks.

What sorts of setting constraints are there on how reactionless drives work, so that ships can't easily be turned into kinetic WMD?

I would like gas giant refueling to be a thing, but getting to, say, Jupiter without taking months or years would require something very close to a "flip halfway" trip a la CT book 2, i.e. near-constant thrust.

A typical ship's pp has enough fuel for 4 weeks. If you could jump into a system far enough out and drop off some old rickety 2g cutter and let it thrust toward a planet for 4 weeks, I think it'd reach about 15% C before it tunguskas its target. 4 gigatons, maybe? That would be bad. So there must be some sort of governor...

Thoughts? I'm sure in the last 50 years that this has been discussed to death, but I wouldn't know where to start looking.

Well, space traffic control will see this ship coming days out, and if it doesn't respond on radio, the Navy gets dispatched, and if it poses a danger, the Navy can board it and stop it, or blast it out of space, without too much effort long before it gets anywhere near space traffic. So that's not a real danger on any planet with a minimum sort of TL9+ infrastructure. A little below that, ground-based radar will track the target in, at least, but much of those low TL planets are fairly low populations, and like Tunguska, will strike in the middle of noplace and no one will care much until 50-100 years later.

 

[Matt Wilson]

You would need some impressive thrust to catch a ship that's been accelerating for 3 weeks, yeah?

 

[Badenov]

You would need some impressive thrust to catch a ship that's been accelerating for 3 weeks, yeah?

To catch, sure. But to put something in its path? Super easy. And if it's moving that fast, it can't turn quickly, so the path is super predictable. And then the only thing that hits atmosphere is some debris that burns up on re-entry.

 

[Spinward Flow]

So there must be some sort of governor...

The basic problem you're running into is that there's (very little!) "friction drag" in the (relative) vacuum of space ... so there's not a lot to prevent long accelerations from achieving "malicious" weaponization.

As @Badenov cites, this is what system defense and "routine patrols" are for ... to keep everyone "honest" in their maneuvering.

The whole thing isn't that different from being issued a weapon driver license for a lethal weapon ground car vehicle. Used "improperly" vehicle speeds are perfectly capable of inflicting lethal damage (just from the kinetic energy/sudden stop on impact) demolition derby style, but that tends to not be the common everyday use/occurrence with such vehicles. A similar thing happens with space craft ... although the relative speeds (and thus, potential energies) are going to be far far FAR higher.

You would need some impressive thrust to catch a ship that's been accelerating for 3 weeks, yeah?

That ... or a kinetic impactor (think, missile) to "knock it off course" ...
Elasticity of collision (Y/N?) then becomes a fun variable to play with.

In Classic Traveller, there are Repulsor Bay weapons that can be used to "push missiles away" from a craft, enabling an anti-missile role. Given enough warning time, something similar could be done to "nudge" an incoming (hostile) powered maneuver drive kamikaze drone craft off course (hopefully, just enough!) to be able to MISS its intended target/impact point. If the planetary body is missed entirely, then system defense can either "chase it down" to deal with it later/elsewhere ... or ... just simply let the hyperbolic orbital trajectory "throw the craft out of the planetary system" and let it become a piece of interstellar "rogue" space junk drifting between the stars.

So if you MISS your shot with such a drone, there won't be a second chance to hit your target.

 

[Badenov]

Our TL7/8 Planetary Defense office is tracking stuff that might hit us in 2032. I think tracking is not going to present a problem in the future. https://science.nasa.gov/planetary-defense/

 

[Matt Wilson]

To catch, sure. But to put something in its path? Super easy. And if it's moving that fast, it can't turn quickly, so the path is super predictable. And then the only thing that hits atmosphere is some debris that burns up on re-entry.

Ah, good point. Thanks! One thing I appreciate about default Traveller is that the "bean counting" of supplies is all in one big chunk with jump fuel, rather than tracking "burns" and similar, so I would prefer to keep it as is.

 

[Matt Wilson]

As @Badenov cites, this is what system defense and "routine patrols" are for ... to keep everyone "honest" in their maneuvering.

Yeah, more good points. and I don't know how hot maneuver drives are compared to rocket plumes, but you could probably just set up a few satellites here and there for cheap, and spot anything within a few light minutes.

Thanks chums, this is good stuff to think about.

 

[whulorigan]

Well, to begin with, a quibble - depending upon how you are choosing to understand your M-Drive system operation:

1. Propellantless - Propellantless Drives are often mistakenly referred to as Reactionless Drives because they do not operate by carrying and expelling reaction-mass for propulsion (and thus appear to be reactionless to casual observation). But many Field Propulsion Systems are properly Reaction Drives, it is just that they do not employ carried reaction mass for propellant. Modern examples include the spacecraft gravitational slingshot maneuver (the spacecraft gains or loses momentum at the expense of the orbital momentum of the world it is gravitationally interacting with), or electromagnetic propulsion systems taking advantage of an external electromagnetic field (think coil-gun, rail-gun, mass-driver, or maglev).
2. Reactionless - Reactionless Drives appear to violate the Laws of Newtonian Motion as we understand them, as they produce an Action without a corresponding Reaction. This may be as a result of a higher-level physics interaction that we do not yet know about (i.e. it is compensated for, but not in an immediately visible way to common observation (e.g. dimensional, currently unknown force interaction, etc.).

In most versions of Traveller (MegaTraveller M-Drive Thrusters excepted), the M-Drive and G-Drive are in the first category, interacting thru the gravitic interaction with the masses in space around it (which can gain or lose momentum in proportion to what is lost or gained by the ship with the M-Drive).

Likewise, in most versions of Traveller (MegaTraveller M-Drive Thrusters excepted), the M-Drive and G-Drive (due to the above consideration) are limited in a non-linear fashion to the gravitational field strength of the local gravitating system in which they find themselves to "react against". Per RAW (supposedly due to quantum mechanical considerations in regions of sufficiently low field-potential), CG-Lifters are only viable out to about 1.0 diameter [i.e. 1 radius above a planetary surface]), G-Drives are fully functional out to about 10 diameters, and M-Drives to 1000 diameters, in each case dropping to .01 efficiency beyond that range. The Interstellar sublight propulsion engine known as an N-Drive (NAFAL), is only rated in tenths of a "g" per engine-factor but is functional to about 1/8 lightyear (51 weeks acceleration) radial-trajectory focused.

Well, space traffic control will see this ship coming days out, and if it doesn't respond on radio, the Navy gets dispatched, and if it poses a danger, the Navy can board it and stop it, or blast it out of space, without too much effort long before it gets anywhere near space traffic. So that's not a real danger on any planet with a minimum sort of TL9+ infrastructure. A little below that, ground-based radar will track the target in, at least, but much of those low TL planets are fairly low populations, and like Tunguska, will strike in the middle of noplace and no one will care much until 50-100 years later.

You would need some impressive thrust to catch a ship that's been accelerating for 3 weeks, yeah?

The possible way around this is to find an uninhabited system (or "body") close to the target system and do all of your accelerating there for as many weeks as required, and then "Jump" to the 100-diameter limit of the target-system on a terminal trajectory (since momentum is notionally preserved thru jump). At Earth's 100 diameter limit, a vessel coming out of Jump at 0.1c constant velocity will reach Earth in 40 seconds.

This gives a reason for deep-space patrols of such systems and bodies.

 

[Condottiere]

If it's unguided, all it needs is a nudge in another, harmless, direction.

If it's guided, aim a mass driver at it, and keep firing.

 

[AnotherDilbert]

What sorts of setting constraints are there on how reactionless drives work, so that ships can't easily be turned into kinetic WMD?

Some editions have limits to how far from a star or planet M-drives work, say 1000 diameters, see e.g. T5. Otherwise, none...

I would like gas giant refueling to be a thing, but getting to, say, Jupiter without taking months or years would require something very close to a "flip halfway" trip a la CT book 2, i.e. near-constant thrust.

That is the general assumption, so very high velocities are a thing.

A typical ship's pp has enough fuel for 4 weeks. If you could jump into a system far enough out and drop off some old rickety 2g cutter and let it thrust toward a planet for 4 weeks, I think it'd reach about 15% C before it tunguskas its target. 4 gigatons, maybe? That would be bad. So there must be some sort of governor...

No speed governors, just physics...

Presumably M-drives are constant thrust (as specified in FF&S), not really constant acceleration, so acceleration will slow down closer to C. Constant acceleration (implied constant mass) is a simplification to get the simple neat acceleration, distance and time formulae and avoid the rocket equation and differential equations.

Note that:

CT HG'80, p17:
Tech level requirements for maneuver drives are imposed to cover the grav plates integral to most ship decks, and which allow high-G maneuvers while interior G-fields remain normal.

So, if you really wanted to build a 20 G drone, you probably can...

Thoughts? I'm sure in the last 50 years that this has been discussed to death, but I wouldn't know where to start looking.

Yes, of course...

Under the rules it can be done. You can even accelerate somewhere else and jump into the target system at close to C. If you don't like it, just house-rule it.

 

[Grav_Moped]

Well, to begin with, a quibble - depending upon how you are choosing to understand your M-Drive system operation:

1. Propellantless - Propellantless Drives are often mistakenly referred to as Reactionless Drives because they do not operate by carrying and expelling reaction-mass for propulsion (and thus appear to be reactionless to casual observation). But many Field Propulsion Systems are properly Reaction Drives, it is just that they do not employ carried reaction mass for propellant. Modern examples include the spacecraft gravitational slingshot maneuver (the spacecraft gains or loses momentum at the expense of the orbital momentum of the world it is gravitationally interacting with), or electromagnetic propulsion systems taking advantage of an external electromagnetic field (think coil-gun, rail-gun, mass-driver, or maglev).
2. Reactionless - Reactionless Drives appear to violate the Laws of Newtonian Motion as we understand them, as they produce an Action without a corresponding Reaction. This may be as a result of a higher-level physics interaction that we do not yet know about (i.e. it is compensated for, but not in an immediately visible way to common observation (e.g. dimensional, currently unknown force interaction, etc.).

In most versions of Traveller (MegaTraveller M-Drive Thrusters excepted), the M-Drive and G-Drive are in the first category, interacting thru the gravitic interaction with the masses in space around it (which can gain or lose momentum in proportion to what is lost or gained by the ship with the M-Drive).

Likewise, in most versions of Traveller (MegaTraveller M-Drive Thrusters excepted), the M-Drive and G-Drive (due to the above consideration) are limited in a non-linear fashion to the gravitational field strength of the local gravitating system in which they find themselves to "react against". Per RAW (supposedly due to quantum mechanical considerations in regions of sufficiently low field-potential), CG-Lifters are only viable out to about 1.0 diameter [i.e. 1 radius above a planetary surface]), G-Drives are fully functional out to about 10 diameters, and M-Drives to 1000 diameters, in each case dropping to .01 efficiency beyond that range. The Interstellar sublight propulsion engine known as an N-Drive (NAFAL), is only rated in tenths of a "g" per engine-factor but is functional to about 1/8 lightyear (51 weeks acceleration) radial-trajectory focused.





The possible way around this is to find an uninhabited system (or "body") close to the target system and do all of your accelerating there for as many weeks as required, and then "Jump" to the 100-diameter limit of the target-system on a terminal trajectory (since momentum is notionally preserved thru jump). At Earth's 100 diameter limit, a vessel coming out of Jump at 0.1c constant velocity will reach Earth in 40 seconds.

This gives a reason for deep-space patrols of such systems and bodies.

This assumes a Jump accuracy which may not be possible.
The best way to ensure a miss is to aim the vector and jump exit point exactly at the desired target...

 

[AnotherDilbert]

Well, space traffic control will see this ship coming days out, ...

Coming into the system at close to C any signals from the ship will come in just before the ship itself. You will not have days, but seconds to see the ship and react before it reaches you.

 

[Condottiere]

Oh ye of little faith.

There's the sudden drop off after a thousand diameters, that reduces acceleration of default manoeuvre drives to one tenth of a percent of listed potential.

 

[Matt Wilson]

Oh ye of little faith.

There's the sudden drop off after a thousand diameters, that reduces acceleration of default manoeuvre drives to one tenth of a percent of listed potential.

Interesting. Is that a thing across many of the versions?

It would make in-system travel a bit slower, I think, than the standard burn-and-flip times presented in the books, but check my math:

At 1g, you'd hit the 1000d limit going about 500-ish kps, which would make a trip from Earth to Jupiter about 17 days long.
12800000m x 1000 is 12,800,000,000 meters to the cutoff limit.
At 1 g (10m to make it simpler), the time taken in sec would be the square root of that / 5: 50596 seconds.
50596 x 10 = 505960 mps, or about 506 kps. In a day, that's about 44 million km. Avg distance to Jupiter is about 765 million km.

A 2g ship could get up to 715 kps, which would get you 62 Mkm per day. That's 12 days and change.

In comparison, if you can thrust the whole way per the chart in MgT, Jupiter averages about 6 and a half days at 1g.

I would think, if time is money for a free trader, they aren't going to consider skimming a GG unless they're really desperate. Then again, when isn't a free trader desperate? Hmm...

I like the cap, and it gives the drive a little more "flavor". It also makes a lot of in-system destinations far more practical via jump.

 

[whulorigan]

If it's unguided, all it needs is a nudge in another, harmless, direction.

That depends on how far out it is, the speed, and the gravitational attraction.

Notionally, yes. But you may not nudge it sufficiently, depending, or you may just end up putting it on a long elliptical, multi-stellar orbit trajectory to strike later.

 

[jec10]

TNE explicitly set out to resolve this by doing away with reactionless drives (except as an optional alternative tech in FF&S).

Of course it resulted in a lot of unintended consequences - gas giant refueling became nonsensical for most situations, HEPLAR thrusters themselves appeared to break several rules of physics, landing with a HEPLAR thruster alight would probably eviscerate the starport, etc, etc.

 

[Matt Wilson]

... or you may just end up putting it on a long elliptical, multi-stellar orbit trajectory to strike later.

"Well, it's FutureNPC's problem now."

 

[whulorigan]

Interesting. Is that a thing across many of the versions?

It would make in-system travel a bit slower, I think, than the standard burn-and-flip times presented in the books, but check my math:

At 1g, you'd hit the 1000d limit going about 500-ish kps, which would make a trip from Earth to Jupiter about 17 days long.
12800000m x 1000 is 12,800,000,000 meters to the cutoff limit.
At 1 g (10m to make it simpler), the time taken in sec would be the square root of that / 5: 50596 seconds.
50596 x 10 = 505960 mps, or about 506 kps. In a day, that's about 44 million km. Avg distance to Jupiter is about 765 million km.

A 2g ship could get up to 715 kps, which would get you 62 Mkm per day. That's 12 days and change.

In comparison, if you can thrust the whole way per the chart in MgT, Jupiter averages about 6 and a half days at 1g.

I would think, if time is money for a free trader, they aren't going to consider skimming a GG unless they're really desperate. Then again, when isn't a free trader desperate? Hmm...

I like the cap, and it gives the drive a little more "flavor". It also makes a lot of in-system destinations far more practical via jump.

That is 1000 diameters from any massive body. In the Sol system, you would need to calculate 1000 Solar diameters (which takes you out to about Saturn).

 

[whulorigan]

"Well, it's FutureNPC's problem now."

That's what Bureaucracies are for.