## How grav drives may work.

Discuss the Traveller RPG and its many settings
Condottiere
Warlord Mongoose
Posts: 8927
Joined: Mon Sep 23, 2013 8:23 pm

### Re: How grav drives may work.

I'm imagining something similar to magnetic repulsion, which is where you can see instability of the components, and some kind of electromagnetic attraction that just overcomes the repulsion sufficiently so that the components remain a little bit apart but can't break loose from each other.
F33D
Duck-Billed Mongoose
Posts: 1645
Joined: Tue May 29, 2012 1:13 pm

### Re: How grav drives may work.

CosmicGamer wrote:
To me, this is extremely poor role playing.
That's because you are an extremely poor RPer.

Seriously though, you just constructed a Strawman.

In the Trav era, knowing basically how a grav driver operates would be like a current day person knowing how an internal combustion engine works, basically. NOT internal medicine!
Last edited by F33D on Mon Jun 15, 2015 4:29 am, edited 1 time in total.
F33D
Duck-Billed Mongoose
Posts: 1645
Joined: Tue May 29, 2012 1:13 pm

### Re: How grav drives may work.

Tom Kalbfus wrote: Gravity attracts all objects proportional to their mass and inversely proportional to the square of their distance from the center of the attracting mass.
Yes, yes. We all know this basic data. That doesn't answer the question.

Tom Kalbfus wrote:If we have antigravity I'll assume that it pushes proportional to the pushed object's mass and ...
You are restating what you already wrote, and what is already knows. So, HOW does your AG drive work?
Tom Kalbfus
Cosmic Mongoose
Posts: 2521
Joined: Sun Feb 09, 2014 8:56 pm

### Re: How grav drives may work.

F33D wrote:
Tom Kalbfus wrote: Gravity attracts all objects proportional to their mass and inversely proportional to the square of their distance from the center of the attracting mass.
Yes, yes. We all know this basic data. That doesn't answer the question.

Tom Kalbfus wrote:If we have antigravity I'll assume that it pushes proportional to the pushed object's mass and ...
You are restating what you already wrote, and what is already knows. So, HOW does your AG drive work?
Why, do you want to build one in real life? I assure you, if I knew the secrets to a gravity drive, I'd have it patented and become very rich! I can only tell you how it would behave, I don't know how to build one, or how to change the Universal Gravity Constant for a local area, but if I could, that is what it would do. You might as well ask, 'how do I cast a magic missile spell or a fireball spell?" or "how does a flying carpet operate?" I think when you have rules for something and you have it obey those rules consistently, that makes it more credible, but if I knew the secrets to many science fiction devices, they wouldn't be science fiction!
F33D
Duck-Billed Mongoose
Posts: 1645
Joined: Tue May 29, 2012 1:13 pm

### Re: How grav drives may work.

Tom Kalbfus wrote: Why, do you want to build one in real life? ...
No. Based on how we know gravity works, how would your anti-gravity work? Either you have decided or, you haven't. Simple.
Tom Kalbfus
Cosmic Mongoose
Posts: 2521
Joined: Sun Feb 09, 2014 8:56 pm

### Re: How grav drives may work.

F33D wrote:
Tom Kalbfus wrote: Why, do you want to build one in real life? ...
No. Based on how we know gravity works, how would your anti-gravity work? Either you have decided or, you haven't. Simple.
Simple, you change the gravity constant in the equation to a value which will create the desired gravitational force you want with the mass you have. How you change the gravitational constant, I don't know, that is why its called future tech.
locarno24
Cosmic Mongoose
Posts: 3163
Joined: Mon Jun 13, 2005 7:46 pm
Location: Wildly Variable

### Re: How grav drives may work.

Simple, you change the gravity constant in the equation to a value which will create the desired gravitational force you want with the mass you have. How you change the gravitational constant, I don't know, that is why its called future tech.
Provided you can alter the sign as well. But even then, if you want to go "thataway", you need an object nearby. Using antigravity which pushes or pulls against other objects falls down in a deep space environment.

To provide a forward thrust, as F33D says, you either have to create a gravity well you can fall into or create a gravity hump you can slide off.

Since we don't really have any experience with 'positive gravity', I'd assume you create a well 'ahead' of you. You fall towards this mass. The locus of the virtual mass either moves ahead as you do or gets 'reprojected' further ahead still. The latter feels less like trying to pull a metal car along by waving a magnet on a stick out of the window, but frankly any reactionless drive is going to feel wierd if you try to apply newtonian physics to it, so I say what the hell, don't poke the plot-holes.

If you can bend space to create gravity-to-order, that also explains why you can have consistant 1G inside a traveller spaceship - you and the ship are both in free-fall, so you're weightless relative to each other, and if you can create a fake gravity well sufficient to throw a kilotonne starship forwards at 6G, a 1G internal field can only be a challenge of engineering, not fundamental science.

One thing that I do find interesting - which I don't recall ever hearing discussed - is the interaction of drive fields and/or using drive fields in an atmosphere. If you imagine a traveller drive as producing a 6G gravity well ahead of you, then theoretically craft near one another should interact - to the point of being able to slingshot around one another - and the aerodynamics of a Type S with a point gravity well ahead of it flying through a jovian's atmosphere must be a bit bizzare.
Understand that I'm not advocating violence.
I'm just saying that it's highly effective and I strongly recommend using it.
simonh
Duck-Billed Mongoose
Posts: 1527
Joined: Thu Aug 10, 2006 3:56 pm

### Re: How grav drives may work.

Tom Kalbfus wrote:...So the gravity drive works by altering the Universal Gravity constant around the vehicle causing its mass to repel gravitationally the Moon's surface....
If you were to change the gravitational constant in a field around avehicle, it would reduce the apparent mass of the vehicle for the purposes of calculating the pull of the moon on the vehicle. You would only be changing the value G in the equation. That makes things very simple, you just decide by what factor you want to change G. To cancel out gravitational pull alltogether you just change G to zero. To accelerate away from the moon at the rate you would normally accelerate towards it, you just change G to be a negative value with the same magnitude as it normally has psitive, so -6.674×10−11 N⋅m2/kg2 instead of 6.674×10−11 N⋅m2/kg2.

However your descriptions seems to be assuming that the attractive force of garvity would still be in action, at the same time that you are also manipulating G to cause a repulsive effect. The value of G can't be two different values at the same time in the same place though. Either the mass of the vehicle is being attracted by the moon, or it's being repulsed. Having both happen at the same time to the same mass due to the same force doesn't seem to me to make much sense, unless I'm totally and completely missunderstanding your explanation.
Many of the Repulsorlift vehicles in Star Wars operate in this way, including the racing pods piloted by Anakin Skywalker in the Phantom Menance, those required jet engines as well as antigravity to keep it off the ground.
If a grav drive changed the gravitational constant in the space around a vehicle, the pull of gravity on the vehicle and it's occupants would be reduced. If the vehicle floated, the occupants would also float as well and would experience reduced gravity. There'd be nothing to hold them into their chairs other than perhaps seat belts. That's not what is depicted in the Star Wars universe.

Simon Hibbs
Check out StarBase, the open source science fiction campaign mapping application.
Condottiere
Warlord Mongoose
Posts: 8927
Joined: Mon Sep 23, 2013 8:23 pm

### Re: How grav drives may work.

I think for the racing pods there's a general directional repulsive force away from the components, which might not effect the planet's gravity effect either on them or their passengers.
dragoner
Duck-Billed Mongoose
Posts: 1716
Joined: Sat Sep 29, 2012 8:37 pm
Location: Indiana, US

### Re: How grav drives may work.

locarno24 wrote:
You fall towards this mass.
Why not have your mass push you? It is only a question of geometry then.

IMTU: http://dragonersdomain.com/forum/viewto ... f=35&t=209
Tom Kalbfus
Cosmic Mongoose
Posts: 2521
Joined: Sun Feb 09, 2014 8:56 pm

### Re: How grav drives may work.

simonh wrote:
Tom Kalbfus wrote:...So the gravity drive works by altering the Universal Gravity constant around the vehicle causing its mass to repel gravitationally the Moon's surface....
If you were to change the gravitational constant in a field around avehicle, it would reduce the apparent mass of the vehicle for the purposes of calculating the pull of the moon on the vehicle. You would only be changing the value G in the equation. That makes things very simple, you just decide by what factor you want to change G. To cancel out gravitational pull alltogether you just change G to zero. To accelerate away from the moon at the rate you would normally accelerate towards it, you just change G to be a negative value with the same magnitude as it normally has psitive, so -6.674×10−11 N⋅m2/kg2 instead of 6.674×10−11 N⋅m2/kg2.

However your descriptions seems to be assuming that the attractive force of garvity would still be in action, at the same time that you are also manipulating G to cause a repulsive effect. The value of G can't be two different values at the same time in the same place though. Either the mass of the vehicle is being attracted by the moon, or it's being repulsed. Having both happen at the same time to the same mass due to the same force doesn't seem to me to make much sense, unless I'm totally and completely missunderstanding your explanation.
Okay, one more time.
1) The gravitational constant G is assumed to be not constant, it can be manipulated in much the same manner nuclear forces can be manipulated by a nuclear dampner thus preventing nuclear explosions through atomic fission

2) The value of G at the location of the grav vehicle is different from the value of G over most of the Moon, which assumes the value the Universe gives to it, but within the field surrounding the grav vehicle the value of G is the opposite sign and of a different amount that the rest of the Universe, the gravity field around the grav vehicle is repulsive, meaning that it pushes away all matter. The gravity field of the Moon still attracts all matter. Now at the boundary where the value of G changes, the gravity fields don't. That is if you are within the changed G field you feel a repulsive gravitational force coming from the grav vehicle, that is if you are holding your position with respect to the gravity field, you feel your weight pushing away from the vehicle, the closer you get to the vehicle, the heavier you feel, also your various body parts would also be repelling each other within this field, as your body and all other objects would possess repulsive gravity fields in proportion to their mass. If you go outside the G change field, you no longer feel this self repulsion of your body, but you still feel the repulsive gravity effects coming from the grav vehicle. The G value only affects the gravity field where it originates, at the mass of the object.

For simplicity sake lets assume the grav vehicle is in the shape of a sphere 10 meters in radius. Lets say at the surface of this sphere the antigravity is 4-g, the effects of the changed G value ends inside the vehicle, so everything at the skin of the vehicle and outwards is normal. A person grabbing onto handles at the surface of the grav vehicle would feel 4 times his weight pushing outward. if he lets go, he will fall away from the vehicle at 40 meters per second squared, by the time he reaches 20 meters from the center of the sphere, he is only falling away at 10 meters per second squared, by the time he reaches 40 meters away from the sphere, he is accelerating outwards at 2.5 meters per second squared and so on.

Now lets say there is a natural planet who's surface gravity is 1-g and who's radius is 6,400 km, at 12,800 km an object is accelerating toward the planet at 2.5 meters per second squared, then it reaches 6,400 km from the planet's center, it is falling at 10 meters per second squared, just before it hits the planet's surface.

Now lets suppose we have a grav vehicle 10 meters in radius generating a repulsive gravitational force of 4-g at its surface, and with its antigravity field on it falls towards the planet at 2.5 meters per second, that is because the planet is not within range of the vehicle's antigravity field, but the vehicle is within range of the planet's gravity field, the amount of push the vehicle gives the planet is negligible, but the attractive force of the planet on the gravity field is 0.25 g, so it falls. Now what happens if the planet is Earth is the grav vehicle enters the atmosphere, and atmospheric friction slows it down, the antigravity field pushes away the atmosphere, but not by enough to create a balloon like bouyancy, so it continue to fall with atmospheric friction slowing it down. as it approaches the planet's surface, we'll assume ocean, it hits the water and goes under, but in this case the water is denser that the grav vehicle so it floats to the top, and once it reaches the surface it rises above the ocean to a height of 10 meters, this is the distance at which the antigravity vehicle pushes on the ocean surface at the same rate as the Earth's gravity is pulling on the grav vehicle, so it continues to bob up and down until atmospheric friction causes it to settle at 10 meters where it hovers. if you push down on the sphere it will repel the ocean surface harder ad rise, if you raise it higher, then it will fall back towards the ocean and bob some more until it reaches its equilibrium distance. If you want to go higher, you can generate a repulsive force of 16 gs at the sphere's surface and it will rise to 20 meters above the ocean's surface, it also pushes away surrounding air in all directions creating a bubble of less dense air, if it does this enough, it will rise like a balloon until the density within the repulsive grav bubble equals the density of the thinner air surrounding it, much as an ordinary balloon would do. Passengers would need grav plating to counteract the effects of this antigravity field as it would also affect them!
simonh
Duck-Billed Mongoose
Posts: 1527
Joined: Thu Aug 10, 2006 3:56 pm

### Re: How grav drives may work.

You seem to be assuming that only whole objects or their surfaces affect each other, but gravity functions at the atomic and even subatomic level. Every particle in every object can be considered separately.

If only the gravitational field around the vehicle is repulsive, every atom of the vehicle will repel every other atom of the vehicle just as much as they repel the planets surface. As the vehicle approaches another object, every atom in both the vehicle and the other object will repel each other.

If the magnitude of G were normal and only the sign were reversed, the strength of the effect would be tiny because the masses are tiny (compared to the mass of a planet or moon).

If the value of G were great enough to cause a strong enough effect to repel the vehicle from the surface enough to counteract it's weight, the force of repulsion between the components of the vehicle would also be very strong. Any effect great enough to push the vehicle away from the planets surface would also have to be strong enough to push the vehicle and its occupants apart. At a minimum the vehicle would need to be designed to withstand considerable stress from it's components and any occupants trying to powerfully repel each other.

Simon Hibbs
Check out StarBase, the open source science fiction campaign mapping application.
Tom Kalbfus
Cosmic Mongoose
Posts: 2521
Joined: Sun Feb 09, 2014 8:56 pm

### Re: How grav drives may work.

simonh wrote:You seem to be assuming that only whole objects or their surfaces affect each other, but gravity functions at the atomic and even subatomic level. Every particle in every object can be considered separately.

If only the gravitational field around the vehicle is repulsive, every atom of the vehicle will repel every other atom of the vehicle just as much as they repel the planets surface. As the vehicle approaches another object, every atom in both the vehicle and the other object will repel each other.

If the magnitude of G were normal and only the sign were reversed, the strength of the effect would be tiny because the masses are tiny (compared to the mass of a planet or moon).

If the value of G were great enough to cause a strong enough effect to repel the vehicle from the surface enough to counteract it's weight, the force of repulsion between the components of the vehicle would also be very strong. Any effect great enough to push the vehicle away from the planets surface would also have to be strong enough to push the vehicle and its occupants apart. At a minimum the vehicle would need to be designed to withstand considerable stress from it's components and any occupants trying to powerfully repel each other.

Simon Hibbs
This would tend to limit its altitude to a few tens of meters above the surface, because to ascend you would have to increase the value of G, and beyond a certain point the gravitational propulsion would overcome the molecular bonds and cause the grav vehicle to explode. Maybe magnetic fields originting from outside the G effect zone could compress a ball of very dense matter to achieve higher altitudes, and of course the balloon effect could be used within atmospheres.
BrianSmaller
Stoat
Posts: 53
Joined: Wed Sep 17, 2014 10:18 am

### Re: How grav drives may work.

I have read this thread with some amusement. Since I started playing Traveller in 1977 I don't think this question has ever come up in a game - i.e. - how does anti-grav work? It just does. Same way as the Jump Drive does.

Everyone I have played with both as player and GM accept the suspension of disbelief over the fantastical tech to allow the game to happen and accept that their characters know how it works but we don't. I do remember once in one game there was an air/raft with damaged grav generators that could only carry half the cargo. It was a plot device rather than a scientific equation. The plot was who gets left behind because they can't all travel in the air/raft.
Condottiere
Warlord Mongoose
Posts: 8927
Joined: Mon Sep 23, 2013 8:23 pm

### Re: How grav drives may work.

Probably that's the difference between D&D, Star Wars, Star Trek and Traveller.

With D&D you can chalk it up to magic; with Star Wars it's the plot; with Star Trek it's pseudo-science; whereas with Traveller, you try to make sense of the underlying mechanics.
dragoner
Duck-Billed Mongoose
Posts: 1716
Joined: Sat Sep 29, 2012 8:37 pm
Location: Indiana, US

### Re: How grav drives may work.

BrianSmaller wrote:I have read this thread with some amusement. Since I started playing Traveller in 1977 I don't think this question has ever come up in a game - i.e. - how does anti-grav work? It just does. Same way as the Jump Drive does.
Internet stuff, people argue over nothing.
BrianSmaller
Stoat
Posts: 53
Joined: Wed Sep 17, 2014 10:18 am

### Re: How grav drives may work.

Condottiere wrote:Probably that's the difference between D&D, Star Wars, Star Trek and Traveller.

With D&D you can chalk it up to magic; with Star Wars it's the plot; with Star Trek it's pseudo-science; whereas with Traveller, you try to make sense of the underlying mechanics.

I have never played a game where a bunch of characters sitting around discussing physics sounded like a fun way to spend my real world leisure time when the same characters could be adventuring, uncovering nefarious Zhodani plots and being involved in lots of action.

If required it is "Make a Space Sciences (Gravatics) roll."

Different strokes and all.
simonh
Duck-Billed Mongoose
Posts: 1527
Joined: Thu Aug 10, 2006 3:56 pm

### Re: How grav drives may work.

It's not necessarily all just sophistry. A setting in which the only kind of gravitics is a null-g field that cancells gravity is quite different from the default Traveller assumption that gravitic drives are just a kind of thruster. It would be a setting much more like 2300AD, with actual thrust provided by things like jet and rocket engines, and where the only way to provide pseudo-gravity for spacecraft is through spinning ring sections and such.

Simon Hibbs
Check out StarBase, the open source science fiction campaign mapping application.
F33D
Duck-Billed Mongoose
Posts: 1645
Joined: Tue May 29, 2012 1:13 pm

### Re: How grav drives may work.

BrianSmaller wrote: I have never played a game where a bunch of characters sitting around discussing physics sounded like a fun way to spend my real world leisure time
Neither have I Brian (nor have I played in one really). But, I have played games where the players understand the physics of their game world like people understand the world they live in. Makes for a FAR better game than a player not knowing. The difference between role playing and roll playing... I keep dice rolling to an extreme minimum in all my games. Rolling a die instead of playing a role defeats the purpose of playing an RPG.
Tom Kalbfus
Cosmic Mongoose
Posts: 2521
Joined: Sun Feb 09, 2014 8:56 pm

### Re: How grav drives may work.

BrianSmaller wrote:I have read this thread with some amusement. Since I started playing Traveller in 1977 I don't think this question has ever come up in a game - i.e. - how does anti-grav work? It just does. Same way as the Jump Drive does.

Everyone I have played with both as player and GM accept the suspension of disbelief over the fantastical tech to allow the game to happen and accept that their characters know how it works but we don't. I do remember once in one game there was an air/raft with damaged grav generators that could only carry half the cargo. It was a plot device rather than a scientific equation. The plot was who gets left behind because they can't all travel in the air/raft.
What about a "light saber?" I've read a book that said a plasma arc confined by a magnetic and electric fields can act like a "light saber", so we don't get into trouble with Star Wars, we could call it a "Plasma sword". The main problem with a plasma sword is it would give off a tremendous amount of heat, and the wielder would need to be wearing an insulating suit to protect himself from all the heat given off by the glowing plasma sword, Probably you can cause damage to an unprotected opponent just by going near him with your plasma sword. So with that description in mind, would you allow them in your Traveller campaign?

### Who is online

Users browsing this forum: Sigtrygg and 20 guests