## How grav drives may work.

Discuss the Traveller RPG and its many settings
F33D
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### How grav drives may work.

Condottiere's thread on ship design and his proposal for a tractor beam got me thinking about how to do it so it complies with what we definitely know about gravity.

While I thought that one would need to create a grav well behind an object to "push" it (as there are no "gravitons" or their opposite) I came up with a way to do it that conforms to known physics. This also would work for deep space grav drives.

As you see from this first image, gravity is caused by a mass making a "dent" in the fabric of space. Other masses then slide "down" the slope. So one could create an artificial well in front of a ship so as to get "grav drive" without "pushing" against an existing grav well.

A way to either push a ship using anti-gravity or, make a repulsor beam would be by making a "raised" area in the fabric of space. See next image below. The opposite of a gravity well. Objects would then slide away (go downhill) from the raised area of space.

BrianSmaller
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### Re: How grav drives may work.

F33D wrote:Condottiere's thread on ship design and his proposal for a tractor beam got me thinking about how to do it so it complies with what we definitely know about gravity.
I just wave my hand and that explains everything.
wbnc
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### Re: How grav drives may work.

When I was spending some time working n a game word and its technlogy, I had to do some brain work to come up with a method of creating anti-grav without tossing physics out completely.

My version was a device that could slow or speed up the vibrations of quantum strands causing space to expand or contract. by creating rapidly cycling bubbes in space time the structure of a shi would be pushed away from the center of a field of minute points of expanded space.
To avoid the object from being drawn back toward the bubble as it collapsed. a new field of bubbles would be created just ahead of the previous field of bubbles.

Since the bubbles would be microscopic in size and of very short duration the system woud not require the sort of energy draw that would require detonating stars or anti-matter to create. This however requires a large number of "plates" arranged around the hull of the ship and in areas where you wanted to generate artificial gravity to keep things from floating about.

The obvious problem with drives like this, or at least the ones that I added t keep them from being wonder widgets. Is that they create a wake of gravitational turbulence, and a great deal of electromagnetic noise. Both of which interfere with sensors and can make a ship a bit rocky when working in an existing gravity gravity field.
Also the plates had to be properly aligned and coordinated to function properly. Also the plates would have to be securely attached/reinforced or they would pull free or buckle a structure if the system became erratic.

To create an acceleration compensater the plates would be controlled to create a cancelling force against any sudden accelerations. slight delays, and power restrictions limit the effectiveness of the compensater, and cause the crew to feel a fraction of the forces generated by sudden changes in direction, and velocity.
F33D
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### Re: How grav drives may work.

BrianSmaller wrote:
F33D wrote:Condottiere's thread on ship design and his proposal for a tractor beam got me thinking about how to do it so it complies with what we definitely know about gravity.
I just wave my hand and that explains everything.
Yes, that works until you get very educated & creative players. Then it doesn't work so well.
F33D
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### Re: How grav drives may work.

wbnc wrote:When I was spending some time working n a game word and its technlogy, I had to do some brain work to come up with a method of creating anti-grav without tossing physics out completely.

My version was a device that could slow or speed up the vibrations of quantum strands causing space to expand or contract. by creating rapidly cycling bubbes in space time the structure of a shi would be pushed away from the center of a field of minute points of expanded space.
To avoid the object from being drawn back toward the bubble as it collapsed. a new field of bubbles would be created just ahead of the previous field of bubbles.

Since the bubbles would be microscopic in size and of very short duration the system woud not require the sort of energy draw that would require detonating stars or anti-matter to create. This however requires a large number of "plates" arranged around the hull of the ship and in areas where you wanted to generate artificial gravity to keep things from floating about.

The obvious problem with drives like this, or at least the ones that I added t keep them from being wonder widgets. Is that they create a wake of gravitational turbulence, and a great deal of electromagnetic noise. Both of which interfere with sensors and can make a ship a bit rocky when working in an existing gravity gravity field.
Also the plates had to be properly aligned and coordinated to function properly. Also the plates would have to be securely attached/reinforced or they would pull free or buckle a structure if the system became erratic.

To create an acceleration compensater the plates would be controlled to create a cancelling force against any sudden accelerations. slight delays, and power restrictions limit the effectiveness of the compensater, and cause the crew to feel a fraction of the forces generated by sudden changes in direction, and velocity.
Pretty interesting ideas in there. As an aside, recent String theory explains why gravity from objects in our universe would effect somewhere like jump space. Marc got that one right.
BrianSmaller
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### Re: How grav drives may work.

F33D wrote:
BrianSmaller wrote:
F33D wrote:Condottiere's thread on ship design and his proposal for a tractor beam got me thinking about how to do it so it complies with what we definitely know about gravity.
I just wave my hand and that explains everything.
Yes, that works until you get very educated & creative players. Then it doesn't work so well.
My other rule is to only play with uneducated thickos with no imagination.
ShawnDriscoll
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### Re: How grav drives may work.

F33D wrote:Condottiere's thread on ship design and his proposal for a tractor beam got me thinking about how to do it so it complies with what we definitely know about gravity.
We still don't know how gravity works, or what it even is. Einstein came up with some ways to explain his theories of it. And that's it.

Anyone that says he knows how gravity works is lying to himself. Thus, we have sci-fi-ence so we can just make stuff up about it. Just because two people make up how gravity works the same way, it doesn't mean it's true. Just because a professor conned someone to fund his gravity project, it doesn't mean he's legit.
Last edited by ShawnDriscoll on Sat Jun 13, 2015 10:15 pm, edited 1 time in total.
F33D
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### Re: How grav drives may work.

BrianSmaller wrote:
My other rule is to only play with uneducated thickos with no imagination.

I do it for RP purposes. It allows a player to try to create good ideas of how to do stuff without me just shrugging my shoulders and telling the player to just "roll a die vs. attribute" to see if they succeed in screwing up the M-drive as an example.
Condottiere
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### Re: How grav drives may work.

Part of it is an exercise in speculation, sometimes to fit in with what we know or belief, and sometimes to fit in with game mechanics.
ShawnDriscoll
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### Re: How grav drives may work.

Condottiere wrote:Part of it is an exercise in speculation, sometimes to fit in with what we know or belief, and sometimes to fit in with game mechanics.
You have the right idea. The key is game mechanic, and how it fits in with a future setting one makes

It's when the "science fiction is real" or "junk science is true science" that the crowd mentality people see on their phones and believe as gospel, that Traveller referees look bad. Such game groups are where future bad referees come from.
dragoner
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### Re: How grav drives may work.

Tom Kalbfus
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### Re: How grav drives may work.

Lets take what we know about gravity and build a drive with that. Lets also assume that gravity can push as well as pull. Lets take an airless body such as the Moon. The mass of the Moon is 7.15x10^22 kg, the gravitational constant is 6.674×10−11 N⋅m^2/kg^2

The size of the Moon is 3476 km so its radius is 1738. Lets say we have a grav vehicle with a mass of 1,000 kg, so how much would it weigh on the surface of the Moon with its grav drive off? So m1 = 1000 kg, and m2 = 7.15x10^22kg, r = 1,738,000 m, and G = 6.674×10−11 N⋅m^2/kg^2

the result is 1579.766 newtons or kilogram meters per second squared, so divide this by 1000 kilograms and the grav vehicle will accelerate towards the Moon at 1.579766 meters per second squared until it hits the surface and stops. Now in order to make the grav vehicle levitate a repusive force of 1579.766 newtons needs to be exerted by the grav vehicle against the Moon's surface. So to keep it consistent, we need to generate -1579.766 newtons on the Moon's crust at say a distance of 10 meters, so how much mass would you need to generate this force at this distance using gravity? we would have to change the gravitational constant locally around the grav vehicle so that G = -157.9766 N⋅m^2/kg^2, m1 = 1000 kg, r - 10, we'll leave the Moon's mass out of the equation, as we are mostly repelling the Moon's crust under the craft and we get -1576.766 newtons at a distance of 10 meters above the Earth's crust, If it is above 10 meters then the Moon's gravity pulls it down because its greater, if it is closer, then the antigravity drive pushes it upward, and in a vacuum the gravity vehicle could bob up and down indefinitely. So the gravity drive works by altering the Universal Gravity constant around the vehicle causing its mass to repel gravitationally the Moon's surface. In an atmosphere a grav vehicle can fly higher by pushing away the air surrounding atmosphere, the air becomes less dense around the vehicle and it thus rises like a balloon. As the altitude increases the vehicle needs to push away a greater volume of atmosphere to ascend higher. The gravity drive only works by pushing up or down, and it pushes equally in all directions, it can't push a vehicle forward, you need another sort of drive to do that.

There may be a problem if the vehicle gets too close to the Moon's surface as the negative gravitational constant will also cause the moon's surface to repel itself and explode, So we want the gravity exerted to extend beyond the range where the gravity constant is altered.
F33D
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### Re: How grav drives may work.

Tom Kalbfus wrote:Lets take what we know about gravity and build a drive with that. ...
Other than the force equations, how does the drive you are talking about utilize gravity (space curvature)?
F33D
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### Re: How grav drives may work.

ShawnDriscoll wrote: Anyone that says he knows how gravity works is lying to himself.
I feel embarrassed for you.
ShawnDriscoll
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### Re: How grav drives may work.

Tom Kalbfus wrote:As the altitude increases the vehicle needs to push away a greater volume of atmosphere to ascend higher. The gravity drive only works by pushing up or down, and it pushes equally in all directions, it can't push a vehicle forward, you need another sort of drive to do that.
I read stories where a stasis bubble is put around a moon-type vehicle. The bubble negates any attraction to gravity. It may even literally stay where it is put. So the Moon may move on by underneath it. I don't remember what kind of propulsion the stasis bubbles needed to get them moving though. Something about turning knobs and the bubbles hyper-spaced in a given direction.

I like your idea though of the Moon surface exploding if too close. Tony Stark had repulsor technology for his suit. But then, he had a mini-fusion reactor to power everything.
CosmicGamer
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### Re: How grav drives may work.

F33D wrote:
BrianSmaller wrote:I just wave my hand and that explains everything.
Yes, that works until you get very educated & creative players. Then it doesn't work so well.
This seams a bit cruel. There are plenty of people that are educated & creative in a vast number of things other than physics and space science. They are not uneducated or uncreative because they do some "hand waving" of the things they are not able to explain. Not everyone wants to spend years analyzing the game and figuring everything out before they play. How long have you been at it and you are still figuring things out?
F33D wrote:I do it for RP purposes. It allows a player to try to create good ideas of how to do stuff
A character with real world medical knowledge explaining accurately and with detail how their character with no experience or education in this field does emergency surgery with misc items found in the common area. The gun bunny security officer with a low IQ and EDU and no engineering, science, or mechanical skills somehow coming up with innovative solutions because the player is the one with the ability.

To me, this is extremely poor role playing.
Last edited by CosmicGamer on Sun Jun 14, 2015 2:12 pm, edited 1 time in total.
Tom Kalbfus
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### Re: How grav drives may work.

F33D wrote:
Tom Kalbfus wrote:Lets take what we know about gravity and build a drive with that. ...
Other than the force equations, how does the drive you are talking about utilize gravity (space curvature)?
Gravity attracts all objects proportional to their mass and inversely proportional to the square of their distance from the center of the attracting mass. If we have antigravity I'll assume that it pushes proportional to the pushed object's mass and inversely proportional to the distance from the center of the antigravity hill, which is a gravity well turned upsidedown. Since the Moon's gravity field is much larger, the gravity from it diminishes much less with increased distance that the antigravity field from the antigravity drive. So in my example, the antigravity drive pushes away at one sixth gee at a distance of 10 meters, if you were to move closer to 5 meters for example, it would then push you away at four sixths of a gee, if you were to move to 20 meters it would repel at 1/24th of a gee, but over all of these distances from the surface of the Moon, the force of gravity from the Moon would remain approximately the same. The grav vehicle would reach an euilibrium against the Moons pull at about 10 meters above the Moon's surface as its antigravity field is much shorter in range than the Moon's gravity field. Now if you wanted to increase the Grav Vehicle's height above the surface, you would have to quadruple its antigravity output and hence need four times the power to do so. Both the antigravity drive and the Moon's gravity obey the inverse square law of diminishment with distance, only the distance at which the moon's gravity quarters is much greater than that of the antigravity drive. the Antigravity drive is only good at pushing against objects that are in range, it could just as easily push on the surface of an asteroid who's surface is 10 meters away, and the ship would accelerate away from it at 1/6th gee, but as it drew further away that acceleration would decrease until it was about nothing at all. A gravity drive is no good for moving about in deep space only against pushing against other nearby objects. Where ever the object is, the antigravity drive will push the ship in the opposite direction from it.

So tell me, is this how a grav vehicle in Traveller works? I think it would need another propulsive device to push sideways while it is pushing against the main mass below it, in order to achieve orbit. 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.
Tom Kalbfus
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### Re: How grav drives may work.

ShawnDriscoll wrote:
Tom Kalbfus wrote:As the altitude increases the vehicle needs to push away a greater volume of atmosphere to ascend higher. The gravity drive only works by pushing up or down, and it pushes equally in all directions, it can't push a vehicle forward, you need another sort of drive to do that.
I read stories where a stasis bubble is put around a moon-type vehicle. The bubble negates any attraction to gravity. It may even literally stay where it is put. So the Moon may move on by underneath it. I don't remember what kind of propulsion the stasis bubbles needed to get them moving though. Something about turning knobs and the bubbles hyper-spaced in a given direction.

I like your idea though of the Moon surface exploding if too close. Tony Stark had repulsor technology for his suit. But then, he had a mini-fusion reactor to power everything.
Ideally the change in the Universal Gravitational Constant would only affect the grav vehicle itself, or a greater part of the mass thereof, not including the crew passengers and payload. I think actually changing the mass of the grav vehicle would violate the principle of the conservation of mass energy, and with the current grav constant, the mass would have to be negative to produce a negative gravity field, so we change the gravity constant instead. I don't know if its possible to do locally, but who knows?
Condottiere
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### Re: How grav drives may work.

As I recall, the racing pods seem to be a marvel of repulsing and attracting energies that allowed independent movement, but remained loosely connected to each component and away from the ground.
Tom Kalbfus
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### Re: How grav drives may work.

Condottiere wrote:As I recall, the racing pods seem to be a marvel of repulsing and attracting energies that allowed independent movement, but remained loosely connected to each component and away from the ground.
There was a definite limit to how high it could levitate off the ground. Basically antigravity substitutes for wheels in that case. Traveller Grav Vehicles on the other hand can fly, can hover, they just can't travel in space far away from a gravitational body. So grav vehicles need something to push against, as it doesn't carry its own reaction mass. A grav vehicle would therefore be classed as a reaction drive, the reaction mass is the world's surface that it is pushing against or even the atmosphere itself, but it needs to push against something. A maneuver drive in some cases is a reactionless drive, it manages to accelerate a vehicle without pushing against anything, or perhaps it pushes on dark matter itself, or perhaps it pushes on virtual particles.

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