Enlighten me. What is a reactionless drive ?

[weasel fierce]

Allright, fill in a basic piece for me. What, exactly, is a reactionless drive ?

Maybe its due to me not having english as my main language, but could someone explain what the basic concept is ?

 

[veltyen]

Allright, fill in a basic piece for me. What, exactly, is a reactionless drive ?

All motion that we currently know of involves an equal reaction. For example the exhaust gases of a rocket move opposite the direction of thrust, a car tyre moves the ground via friction, and each of these is balanced against each other.

This has to do with conservation of momentum and conservation of energy.

A reactionless drive is one without a reaction, to move forward you do not need to push something back. It is "magic" to current physics, as it breaks a whole bunch of well tested theories.

 

[weasel fierce]

ah allright. That makes perfect sense then.

So in a Traveller sense, that would permit a starship to move and stop at will, basically ?

 

[veltyen]

ah allright. That makes perfect sense then.

So in a Traveller sense, that would permit a starship to move and stop at will, basically ?

Not quite. It allows it to accelerate without throwing material (Reaction Mass) overboard. It still might have other limits. Since it is magic you don't need reasons.

 

[Fritz_Brown]

Since it is magic

No, no, no. :file_28: It is not magic, it is handwavium! Quit confusing people, veltyen!

 

[Psion]

Not quite. It allows it to accelerate without throwing material (Reaction Mass) overboard.

FWIW, those not familiar with rocket science might not understand the importance of this, but when it comes to space travel, reaction mass considerations are very important. On the ground or in the air, it's not a big deal since you can just push against the ground or scoop up more reaction mass in the air. In space, you have to bring it with you.

It still might have other limits. Since it is magic you don't need reasons.

FWIW, in Robert Forward's book Indistinguishable from Magic, he proposes a conjectural way that reactionless drives might work. While I've adopted this as my pat explanation for thruster plates IMTU, gravitics still remain black-box-magic tech.

 

[aramis]

It's Newton's third law: For every action there is an equal and opposite reaction.

SO, to throw yourself forward with 1200kg of force (1sec, 1200kg 1m/s), you need to apply 1200kg of force in the opposite direction.

With a rocket, you take, say 100kg, and kick it 1200m/s out the back, to get your 1200kg rocket going 1m/s forward. Or you take 1kg, and send it out the back at 1200m/s.

Reactionless drives have to impart accelleration without the reaction.

IMTU, they use "the universe" as their reaction mass. Accellerate the universe at 1E-100m/s or so to move your 1200kg rocket forwars at 1m/s

Allan Dean Foster's Gravity Drive is also technically reactionless, but in looking at it, it actually has a sliht gravity differential, and thus is accelerating the interstellar medium. But I also believe it shouldn't work....

 

[tjoneslo]

There are a number of handwaves that count as "reactionless" drive systems, depending upon how much science you want in your magic.

There is the gravity drive, and probably the most frequently used one from the Traveller perspective. You create a gravity source in front of the ship, and it "falls" into it to provide "thrust"

A version of this is the Alcubierre drive, which compresses space in front of the starship and expand it behind, moving the ship through space without accelerating it. This is a classic Star Trek style warp drive. And it may work, even if Traveller doesn't allow you to go FTL with it. Variations of this are also know as a "pseudo-velocity" drive, since the ship doesn't have any real velocity, only apparent movement.

The Studderwarp drive, a short range, rapid cycling teletport, from 2300AD is also a reactionless drive.

There is a Dean Drive which involves changing rotational momentum into linear momentum. If you can do this, a rotating mass provides the thrust through it's understanding of physics. Using this system, the drive shaft on your car can provide enough power to let it fly. Which is strange because the engine won't provide enough power to let it fly. Many of the more magical reactionless drives are similar in theory, if they have any more theory than a black box, to this.

 

[tinker123]

why should gravitics be reactionless?

I don't understand why any drive needs to be reactionless. The reaction may not be local, in the sense of pushing away burnt rocket fuel. When you drive around in an air/raft, using gravitics in a planet's gravity, the drive doesn't have to be reactionless - you could easily visualise it "pushing" against the planet.

Out in space, you still feel the pull of planets' and stars' gravity, it just tails off with the square of the distance (IIRC?). So why can't the manoeuvre drive just "push back" against the nearest star or other large mass? There's less to push against (fewer gravitons arriving), so it needs more energy input to achieve the same push. That's why spacecraft manoeuvre drives are so much bigger than grav belts or air/raft motors.

 

[aramis]

That is implied in MT's drive types, and skirted around in FF&S 1&2.

 

[Fritz_Brown]

When you drive around in an air/raft, using gravitics in a planet's gravity, the drive doesn't have to be reactionless - you could easily visualise it "pushing" against the planet.

But, if it pushes against the planet, it only goes one direction - straight up.

it just tails off with the square of the distance (IIRC?)

That's mostly right - it's the inverse of the square. So, twice the distance means one quarter the force. At an altitude (above the Earth) equal to the radius of the Earth, you get .25G; at 3 radii (4 from the center) you get .06G; at geostationary altitudes you get about .0278G; at the moons altitude it becomes about .00033G (I'm assuming you're not on or near the moon).

so it needs more energy input to achieve the same push.

But, it needs progressively more and more - 3-6 orders of magnitude more! The advantage of being in deep space is that you could push any direction - unlike in a gravity well.

 

[Valarian]

I don't understand why any drive needs to be reactionless. The reaction may not be local, in the sense of pushing away burnt rocket fuel. When you drive around in an air/raft, using gravitics in a planet's gravity, the drive doesn't have to be reactionless - you could easily visualise it "pushing" against the planet.

Out in space, you still feel the pull of planets' and stars' gravity, it just tails off with the square of the distance (IIRC?). So why can't the manoeuvre drive just "push back" against the nearest star or other large mass? There's less to push against (fewer gravitons arriving), so it needs more energy input to achieve the same push. That's why spacecraft manoeuvre drives are so much bigger than grav belts or air/raft motors.

It also implies that in DEEP space, your m-drive won't work as there's not enough energy available in the drive. Watch out for those deep-space misjumps. :file_23:

 

[TheEngineer]

Reaction

Hi Tinker !

Indeed, drives don't need to be reactionless.
Anyway as soon as a drives performance does not depend on the mass of the moved object, it just really means the drive works reactionless.
Now thats true for CT/MT drives, just as Aramis noted.

In MT its a bit weird, as you could use mass-independant starship antigrav maneuver units as well as mass-dependant anti-grav modules to somehow move your ship...

Regards,

TE

 

[Daneel Olivaw]

Old school, mostly...

IMTU, there is no such thing as a reactionless drive, but I like the idea of experimental or prototypical drives using antigravity to create a reaction against the positive gravitational fields of planets, stars or the galaxy itself.

 

[Straybow]

My take on thrusters and antigrav

I see two possibilities for "reactionless" drives. They work by directly manipulating the fabric of space, producing a gravity-like gradient by artificial means. A vessel "rides" the manipulated space like a surfer whose downward motion on the wave face is effectively canceled by the forward motion of the wave itself.

In the first case, the drive mechanism creates a dense field that only accelerates the drive itself (and perhaps a small zone of space around it). The ship must then structurally transmit this generated force, just as it would for a reaction drive. The occupants feel the full force of accelerations.

The second case is for a broad field that encompasses the whole ship, and thus the structure and occupants feel no net forces acting upon them except for irregularities or internal gradients in the generated field.

Now there would be two variants of each of these. A drive can be directional, only able to produce thrust in a very narrow directional cone. I imagine the first type of drive to be limited in this way, it just fits my way of thinking about it better.

The second variant depends on multiple emitters or "thrust plates" to create the gross gradient on which the vessel "rides." Acceleration can be induced in any line touching two emitters or plane touching three emitters. Rotational or linear accelerations perpendicular the line or plane are very limited. A three dimensional area around at least four emitters (roughly tetrahedral in configuration) could "flatten out" the local space (ie, traditional antigrav). Again, limited acceleration is provided by biasing the field against the gravity well.

 

[tinker123]

But, if it pushes against the planet, it only goes one direction - straight up.

Surely not - our my legs would only let me bounce up & down, not walk

I picture air/raft gravitics in a similar way - the plates push against the planet's gravity field, and the driver angles the plates so the vehicle slides forward. The same as a helicopter hanging under its rotor disc. All steering is by angling the anti-grav plates, and stopping distance is quite long if you accelerate constantly to the half-way point.

Clearly there is a reaction, and the lift applied to the air/raft very slightly pushes the planet downwards.

I'm not sure about spacecraft manoeuvre drives being unrelated to mass, either. If that were true, why do we have to put bigger ones in to ships with higher-volume hulls? I assumed this was based on an average density calculation that makes them more massive.

I agree that reaction against planetary or steller masses (or even the average centre of mass of a galaxy) would work badly in deep space, and enjoy a major power boost near a planet. Maybe that solves the mystery of how to get a 1G-capable ship to lift off from an earth-like planet???

 

[TheEngineer]

...
I'm not sure about spacecraft manoeuvre drives being unrelated to mass, either. If that were true, why do we have to put bigger ones in to ships with higher-volume hulls? I assumed this was based on an average density calculation that makes them more massive.

Well, according to the CT/MT rules the actual weight of a construction does not interact with the performance of the drive.
As You noticed just the volume is used.
So, the 1g drive provides the same performance, regardless if fuel tanks and cargo bays are full or empty

Regards,

TE

 

[tinker123]

Well, according to the CT/MT rules the actual weight of a construction does not interact with the performance of the drive.
As You noticed just the volume is used.
So, the 1g drive provides the same performance, regardless if fuel tanks and cargo bays are full or empty

Yes, I noticed that, but since all my RPG buddies at the time were physicists (and an astronomer) we just assumed that was because any rule that adjusts performance based on loading would be too complicated for most players to bother with. So we treated the quoted result as the worst performance when fully loaded, and allowed a bit of improvement with low fuel / no cargo. Of course, if ships have superdense hulls (as some have suggested) then that's another reason to leave everything else out of the calculation, because the hull has so much more mass than the fuel & cargo.

Of course, I'd love to re-do the page that shows turn-round at half-way to destination with constant acceleration, so that the acceleration is a little bit higher on the second leg, because the fuel is a little bit lower. Except the sums would be too hard for me, and would depend on the relative mass of hull & fuel...

 

[Fritz_Brown]

Surely not - our my legs would only let me bounce up & down, not walk

The problem with that image is that you aren't pushing against the force of gravity to walk - you're using the friction coefficient of the sole of your shoe and the pavement to go forward. If you push against gravity (and gravity only) you go straight up.

You could do the sliding downhill thing by putting in two points, and turning one down, so the vehicle slides off the "hill" created by the other one. But, that's almost the same as saying it produces a gravity well into which the vehicle slides - which would be a reactionless drive. Sorta, anyway.

The whole point of reactionless drives was to avoid calculator geeks from spoiling the fun with mass and fuel onboard calculations.

 

[Ptah]

The problem with that image is that you aren't pushing against the force of gravity to walk - you're using the friction coefficient of the sole of your shoe and the pavement to go forward. If you push against gravity (and gravity only) you go straight up.
....

Who says it has to be a dot product of the vectors? Why not a cross product, your drive magical anti-graivty repulision vector cross the local gravity vector puts you on a new vector perpendicular to both. Then you just slew your drive plate/vector around to up/down/sideways.