Details on how M-Drives work

[Grav_Moped]

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.

"Reactionless" means that the drive does not operate by pushing exhaust in the direction opposite the intended direction of travel. Examples are the apocryphal Dean Drive, "Gravity Drives", or in a Traveller context, Maneuver Drives in rule sets later than 1977.

You may be thinking of "Inertialess" in which the drive reduces or eliminates the apparent mass of objects within its zone of effect, or "inertial compensation" which is a zone effect neutralizing the effects of acceleration -- which isn't necessarily intrinsic to the propulsion system.

 

[whulorigan]

"Reactionless" means that the drive does not operate by pushing exhaust in the direction opposite the intended direction of travel. Examples are the apocryphal Dean Drive, "Gravity Drives", or in a Traveller context, Maneuver Drives in rule sets later than 1977.

Just an observation I made in a different thread several years ago:

I think part of the problem is that everyone keeps referring to the M-Drive (or "Thruster Plate") as "Reactionless". To my knowledge, only under MT/DGP:SOM is it explicitly "Reactionless". (DGP:SOM describes the Thruster Plate as being based on the Strong Nuclear Force, and employing some type of interaction that causes the plate to "push-off of itself". This interpretation was not carried forward into successive editions of Traveller).

CT post-1981 is rather ambiguous on the nature of the M-Drive (CT-era DGP considered it a gravitic-based system).

Both T4 & T5 explicitly describe the M-Drive as employing a gravitic-based interaction. Its thrust-efficiency also drops off to 1.00% beyond 1000 diameters under these rulesets.

The reason that is significant is that in any given star system there are plenty of gravitating bodies to react against employing a gravitic-based system, so it is not necessarily a "reactionless" system, any more than a NASA probe utilizing a gravitational slingshot or gravitational braking maneuver is a reactionless interaction. It is just that the gravitating bodies that the vessel is interacting with are so huge compared to the vessel itself, that the momentum change of the bodies in the star system are unmeasurably small.

So is part of the mechanism of the M-Drive (however it actually works) picking up momentum from other bodies in the system, which in reaction lose an equivalent amount of momentum? The power-plant only needs to power the M-Drive to do whatever it does. It doesn't have to entirely account for all of the momentum change - part of that can come from the bodies in the star system.

 

[Grav_Moped]

That's a fair interpretation. It's "reactionless" since it doesn't produce an exhaust plume or kick away things directly behind the drive, but it's also "not reactionless" because it does push "something(s)".

 

[Spinward Flow]

Reactionless = Thrust without Exhaust

 

[whulorigan]

Reactionless = Thrust without Exhaust

Well, no, not necessarily. That's the point I was making in my prior post above. Expelling reaction mass is ONE way of being a reaction-drive, but there are other ways of "reacting", as noted above.