Details on how M-Drives work

[Straybow]

If it were expressed as gravity, then the warp drive is only good at low sublight, because of the potential for tidal stress to spaghettify the ship. Even still, 10 PSL means mars in 2-4 hours.

I dunno about that. Even a small acceleration, applied over a very long time, can reach near c. If the gravity effect has the benefit of scooping up or deflecting debris in the path it could be feasible as an interstellar sublight engine. Deceleration may be a problem if you have to move the gravity effect to the rear... maybe it can create a negative curvature. Or you create a big gravity effect behind the ship to slow down, and a little gravity effect in front as a shield.


But that would be a different game.

 

[aramis]

I dunno about that. Even a small acceleration, applied over a very long time, can reach near c. If the gravity effect has the benefit of scooping up or deflecting debris in the path it could be feasible as an interstellar sublight engine. Deceleration may be a problem if you have to move the gravity effect to the rear... maybe it can create a negative curvature. Or you create a big gravity effect behind the ship to slow down, and a little gravity effect in front as a shield.


But that would be a different game.

Indeed, too different.

There's also the "You can't push a surface down then fall into the void" thanks to thermodynamics. Clark and Asimov both held that against the drives in the Flinx/Commonwealth series.

The why low sublight is that since you're moving mid to high sublight with a gravity well, you need it ahead, not centered upon yourself. Why? because that gravity well isn't protecting you, while a moving bubble of spacetime is likely to make the Interplanetary or interstellar media have a "crust" of particles on the front of the bubble.

 

[kilemall]

Indeed, too different.

There's also the "You can't push a surface down then fall into the void" thanks to thermodynamics. Clark and Asimov both held that against the drives in the Flinx/Commonwealth series.

The why low sublight is that since you're moving mid to high sublight with a gravity well, you need it ahead, not centered upon yourself. Why? because that gravity well isn't protecting you, while a moving bubble of spacetime is likely to make the Interplanetary or interstellar media have a "crust" of particles on the front of the bubble.

I suppose it would look like this Comet Empire main 'ship'-


https://www.youtube.com/watch?v=5HQ6ZRUnCB0

 

[Straybow]

I know I've read stories that used gravity screens, based on Mach's Principle, but I can't remember the author. The idea is that inertia is the cumulative effect of gravitational influence of the whole universe that resists motion. If a small fraction of that influence can be "screened" or blocked by superdupersmartypants tech, then the vessel would be accelerated (freefall) in the opposite direction at hundreds or thousands of Gs.

 

[Werner]

How about a force field that blocks virtual particles in the quantum foam from going in one direction but not the other. It takes some energy to maintain this force field and the more energy applied, the more virtual quantum foam particles it blocks and the greater the force applied to the Starship causing it to accelerate. How's that?

 

[Grav_Moped]

I assume the "Dean Drive" for my maneuver drive, and require that the power plant number match the maneuver drive number on all ships. For small craft, I go with the Dean Drive as well, along with a power plant of Type A, depending on the G-rating.

Edit Note: The Dean Drive should be considered Tech Level 6 or 7, as it was developed in the late 1950s to early 1960s.

Coming back to this on a very cold read...
Maybe the Dean Drive only works if you cheat by using it in an artificial gravity field (part inside the field, part outside it). Still flips the bird to thermodynamics, but it provides a handwave (it works because of anti-gravity magic) instead of a lampshade (it shouldn't work, but what if it did?).

...I have the M-drive creating a gravitic bubble around the ship which has the primary effect of causing the ship to 'weigh much less' then it would normally against local space-time.
...
Then the heat coming off the fusion reactor powering this drive is ejecting superheated and when actually moving super-accelerated gasses, as such the nozzles on the end of the ship are more exhaust stacks then reaction jets.

Most of the time the ship is just venting heat, but when it's time to move the exhaust IS pushing the ship in a reaction mode, just that the ships 'weighs' grams on the kilogram and a lot less reaction mass is needed to create vector.
...

Now try the other end of it: anti-gravity magic doesn't just make the ship "weigh" less, but also makes the reaction mass "weigh" more as it's leaving the exhaust nozzle. It can (and probably does) revert to its "actual mass" once clear of the ship, but that doesn't matter since it's already had its effect on the ship.

Yes, we're ignoring thermodynamics again. Oh well.

 

[Baroun Tardis]

Wrote this a long time ago as a reply to a thread on the Traveller Mail List....
https://docs.google.com/document/d/19gYlG-VC5vrNtbDaXypoAFDm68i6VFSraJKeNUjmsFM/edit?usp=sharing

Basically an IMTU on how power plants, M drive and J drive work.

 

[Grav_Moped]

Wrote this a long time ago as a reply to a thread on the Traveller Mail List....
https://docs.google.com/document/d/19gYlG-VC5vrNtbDaXypoAFDm68i6VFSraJKeNUjmsFM/edit?usp=sharing

Basically an IMTU on how power plants, M drive and J drive work.

Nice framing device for the exposition, and a decent framework for the handwaves. Thanks!

 

[shaunhilburn]

I am customizing the Cepheus Engine for my Trav game. Since how an M-drive works can have mechanical effects in game I decided to codify for my players.

I treat it as a reactionless rocket. It doesn't negate or create gravity; it's an electrical device supported by the powerplant that produces thrust. Its reaction and colossal amounts of heat are exhausted into "M-space", unseen and unnoticed by observers in normal space.

If you take the ship tonnages as actual metric tonnages instead of volumetric "displacement" tons (I have reasons to believe this was the original intent), then M-drive performance can be modeled like any propulsion system that produces thrust and consumes fuel.

Performance like anything described in the rules requires an fantastically high specific impulse of around 15 million, minimum. This gives the worst performing small craft (shuttle and slow pinnace) a classic burn & turn range of 1 AU. Of course, any of the vessels can travel greater distances with a burn-coast-brake voyage. Knowing thrust and specific imupulse, you can determine lots of things.

Where W = fully loaded "wet" mass metric tonnage, G = acceleration in Gs, Isp = secific impulse, and H = operating hours, then

consumption (tons) per hour = W·G/(Isp/3600)

and

total consumption (tons) = W·G·H·3600/Isp


For example, a 20 (metric) ton launch has a "wet" mass W = 22 tons when loaded with 1 ton of fuel and 1 ton of passengers, luggage, and supplies.
F = ma, so with 1 G acceleration, it has a thrust rating of 22 "tonnes". Full throttle fuel consumption "mdot" = 22/(15 million/3600) = 0.0044 tons per hour.
1 ton of fuel gives it an operating time of 227 hours, and a burn-turn range of 10.97 AU.


You can plug this into a spreadsheet and do apples-apples comparisons for the various LBB2 vessels. Interesting facts emerge.

 

[kilemall]

Now try the other end of it: anti-gravity magic doesn't just make the ship "weigh" less, but also makes the reaction mass "weigh" more as it's leaving the exhaust nozzle. It can (and probably does) revert to its "actual mass" once clear of the ship, but that doesn't matter since it's already had its effect on the ship.

Yes, we're ignoring thermodynamics again. Oh well.

Fear not, we can dump all that heat stuff with the exhaust and thereby also create the infamous HG-77 plume o'death.


I tend to think of gravitics as being more nullification/push then attractive/mass increase, due to the relatively weak floor plates vs. no tractor functionality vs. the short-lived repulsors. But there is an elegance to your 'solution'.

 

[Grav_Moped]

Fear not, we can dump all that heat stuff with the exhaust and thereby also create the infamous HG-77 plume o'death.


I tend to think of gravitics as being more nullification/push then attractive/mass increase, due to the relatively weak floor plates vs. no tractor functionality vs. the short-lived repulsors. But there is an elegance to your 'solution'.

Oh, but the fun we could have with remedial Kzinti Lessons for everyone! :rofl:

And seriously, I'm pretty sure that's what LBB2 drives were meant to be, at least in '77. Until someone thought about the implications... and then we got relativistic planet-busters for a couple of editions until that was figured out and nerfed.

Maneuver drives are a problem. Just sayin'.

 

[acgetchell]

That's not what I've seen...

... the XYZ set remains flat even though gravity curves motion in it; some argue that that is due to compactified additional dimensions in a parallel brane.

We don't know for certain how Alcubierre's theory would resolve, but it may not be as gravity. And the good Dr. doesn't think it is a gravitic effect.

If it were expressed as gravity, then the warp drive is only good at low sublight, because of the potential for tidal stress to spaghettify the ship. Even still, 10 PSL means mars in 2-4 hours. Alan Dean Foster used the idea of a generated well ahead of the ship, but his was way more of a thermodynamics violation than Traveller's Fusion Plants.

Alcubierre's drive isn't expected, if it works, to result in acceleration, merely motion while activated, dropping you back to inertial space when turned off, with the same moment and vector as when initiated relative to where you started, but translated along a line by having moved the circumscribed volume of space.

This can be problematic; you need to drop out at a point that will result in falling into a relatively good vector; if you can't, you'll need to make multiple short warps in the right spots to bleed off the velocity via gravitation, but then you're accelerating something else.

This is incorrect.

First, as was stated earlier, relative acceleration = space time curvature (via the Riemann tensor), that’s the content of the geodesic equation.

Secondly, curvature equals mass (via the stress-energy tensor), that’s the content of the Einstein field equations.

There is no difference between acceleration in a rocket or falling towards a massive object, that’s the content of the Equivalence principle.

One solution of the EFEs is the Kerr metric, which describes rotating, uncharged, spherically symmetric objects such as planets, stars, and spinning black holes.

Another solution is the Alcubierre metric, which describes warp drives. In the various warp metrics (e.g. Alcubierre, Natario), the traveller is at rest with respect to an asymptotically flat space time which is in turn encapsulated by the warp metric. Various numerical solutions show effects similar to extreme relativistic velocities with respect to warping the forward and aft views of the outside universe to cones along the axis of travel. The main difference between less than c and FTL is the onset of Cauchy horizons (i.e. Closed Timelike Curves).

All solutions share certain mathematical similarities, such as lapse and shift functions which describe timelike and spacelike “coordinate changes” along the 3+1 foliated hypersurfaces of the Arnowitt-Deser-Misner formalism (which is one of the few ways to numerically calculate GR).

Of course the laws of physics don’t care which metric you pick (diffeomorphism invariance) as long as you are consistent in applying transformations. So vector and spinor quantities aren’t generally absolute. In particular, the requirement for asymptotic flatness means you are sitting in a patch of universe without connection to any other, so there’s no notion of conserving moment or vector (except Killing vectors).

 

[coliver988]

(I think we need an exploding head emoticon response...)

 

[Spinward Scout]

I am customizing the Cepheus Engine for my Trav game. Since how an M-drive works can have mechanical effects in game I decided to codify for my players.

If you can get your hands on MegaTraveller's Starship Operator's Manual, it's explained in there. Good luck getting your hands on a hard copy, tho. It was over $100 on eBay last I saw.

 

[Spinward Flow]

There's a very simple explanation for how the M-drive works.
So simple that it can be said in a single word.

MAGNETS

 

[Condottiere]

Off hand, there seem to be two variants:

1. One that rejects gravity.

2. One that pushes against it.

 

[whulorigan]

Off hand, there seem to be two variants:

1. One that rejects gravity.

2. One that pushes against it.

The Thruster-Plate M-Drive system in MT/DGP:SOM is explicitly described as a Strong Nuclear Force based interaction (i.e. it is independent of gravity) and the plates "push off of themselves" - i.e. a true "reactionless" (and physics-bending) propulsion system. Note that in MT Gravitic Drives (TL10) and Thruster Plates (TL11) are different systems.

T4 and T5 both describe the Thruster-Plate/Maneuver Drive as a gravity-based interaction with the gravitational fields produced my massive bodies in the star-system.

 

[aramis]

Off hand, there seem to be two variants:

1. One that rejects gravity.

2. One that pushes against it.

There are, canonically, 4 different gravitic effects:
1) disconnection (partial) from local gravity, i.e.: flattening local space-time
2) a repulsion effect against a local gravity well (diagravitic, in the same way a magnetic field repels diamagnetic materials)
3) a "reactionless thrust" against the device
4) a reactionless generation of acceleration in the vicinity of the device but relative to the device

Canon does not detail the proportions of 2 & 3, and only TNE and later have #1 explicit, 4 is treated as part of the M-drives in CT, but separate (Inertial compensation and artificial gravity), and often divided into two commponents. (Which implies the off "deckward" forces require additional units installed)

TTNE sets "contragravity" to 98% of mass nullified within (IIRC) 10 diameters. It lacks 2 & 3...
CT has 3 & 4 in the M-Drive.
MT has 2, 3, & 4, but a clear ratio difference between early and later.
T4 has all 4.
T5 I forget.

 

[Condottiere]

As I recall science fiction tropes, reactionless means that no acceleration effects are felt, so it allows incredible speed by the spaceships involved without harming the contents.

It's basically my simple interpretation after glancing through Tee Five's starship design sequence, in that if the (space)craft is immersed in a gravitational field, it can reject gravity (or at least it's motors), while thrusters somehow anchor themselves to the gravitational field and push against it; anchor, since pushing would imply a single direction away only being possible.

I'm not sure if this is somehow contradictory, or if you can reconcile any of it, whether within an edition, or in relationship to all of them.

In terms of efficiency, whether one, ten, a hundred or a thousand diameters, you could close an eye.

Tractor and repulsor beams undoubtedly are based on this technology, but manoeuvre drives are an internal effect, whereas tractor and repulsor beams would be an external effect, though you could use the repulsor effect to float on gravity much like it was water, though how closely this is related to magnetism, I couldn't say.

 

[mike wightman]

If it's a reactionless drive and there is no acceleration felt then why do you need a separate acceleration compensation system?