Overflown by a grav vehicle

[McPerth]

As I understand it, gravitics do not just nullify gravity but also push against the gravyty well (and so the mass, usually a planet) to achieve lift, more akin repulsors (just nullifying gravity will not allow them to fly, at least on vaccum).

If this visión is right, what happens to the ground below? May this pushing damage people just below the grav vehicle if it flies just overhead of them?

My take is that this effect strangly diminishes with distance (mostly by being dispersed as a cone), being negligible above a certain alttitude.

Of course, that will make flying above inhabited cities forbidden under that minimal distance, and will carry certain risks for passerbys near "grav parkings"...

This also would allow military craft to affect infantry just flying over them at low altitudes (a way to just overrun them).

Thoughts?

 

[CosmicGamer]

Thoughts?

Nooo!! My brain already hurts (have a headache) don't make me think.

Too late.

I'm no science whiz so there are lots of forces that I don't understand.

Take magnetism. Have a really powerful magnet and you could pull a crowbar. Pass your hand in front and you don't have the same effect.

I'd say the grav science does not currently exist so there is no way to know. No matter how much we discuss it we are still just guessing.

Go by what you think the rules intend. I don't think it's specified so you're free to do what you want. Myself, since the rules do not discuss any strategies or dangers from such, I'd say it doesn't have an effect.

Just the thoughts off the top of my throbbing head.

 

[Reban]

Well it depends...

The explanation of CG varies over the different editions the closest thing to what you are describing is Gravitic Displacement from FF&S for TNE.

The GD field is described as displacing the potential energyof the mass of a vehicle towards the nearest gravity source. This allows a massive object to rise and hover but a ship massing 1000tons hovering a dozen meters above the ground will have the same effect as if it were resting on the ground and will crush anything underneath. At higher altitudes the effect on the ground will be more dispersed with a bigger but shallower fotprint. It specifically allows grav tanks to make crushing attacks but also makes it vunerable to pressure mines.

See p. 76 FF&S TNE


But remember that's a variant of normal Traveller CG. I've always thought it worked in a manner closer to that described for Thrusters which "grab on" to the gravity well for propulsion.

I think of CG vehicles moving a bit more like airships with negative buoyancy and helicopters. That is, a CG vehicle with CG on sits there with mass but no weight until either a separate thrust agency or modulation of the CG field imparts thrust.

IMTU a CG vehicle in a gravity field floats in an invisible field. When the pilot pushes his stick forward the shape of the field is changed and rapidly cycled to impart momentum. To steer the field shape is altered.

The effects are similar to helicopter downwash as the rapidly cycling CG field causes atmosphere to move within the area of the field effect so someone standing close (not just below) will find their hair and clothing moving or flapping and debris may be kicked up. The same person will hear a high pitched whine.

I think you've identified the main considerations of displacing mass such as not flying above cities, restricted landing and parking. For smaller grav vehicles in cities there won't be free flight like aircraft but there maybe skyways, structures that the CG can push against. These may look just like modern roads and maybe duel use for ground vehicles.

Starports may also have much larger paved and reinforced areas besides the usual landing pads. In military terms bunkers become important to shelter in, as do pressure plate activated mines. Depending on how you handle it, "bear traps" for CG vehicles might be possible. Not as in the "tank falls in a hole" style but as in, the ground gives way and the tank suffers an unexpected change of altitude.

 

[McPerth]

The explanation of CG varies over the different editions the closest thing to what you are describing is Gravitic Displacement from FF&S for TNE.

The GD field is described as displacing the potential energyof the mass of a vehicle towards the nearest gravity source. This allows a massive object to rise and hover but a ship massing 1000tons hovering a dozen meters above the ground will have the same effect as if it were resting on the ground and will crush anything underneath. At higher altitudes the effect on the ground will be more dispersed with a bigger but shallower fotprint. It specifically allows grav tanks to make crushing attacks but also makes it vunerable to pressure mines.

See p. 76 FF&S TNE

Never read FF&S. In fact, I was closer to the torches and pitchforks party when TNE appeared, and I have only the basic book.

In any case, what you say describes exactly what I meant (just clearer).

But remember that's a variant of normal Traveller CG. I've always thought it worked in a manner closer to that described for Thrusters which "grab on" to the gravity well for propulsion.

But thrusters are about 3 TLs above gravitics, and the grav plates in vehicle design (at least for MT, that's the one I'm more familiar wih) are not thrusters, just grav plates (and lose capacity as they go farther the gravity well).

Also, again in MT rules, thrusters, being a kind of maneuver drive, cannot work in vehicles/crafts under 20 dton, while many grav vehicles are under that tonnage.

I think of CG vehicles moving a bit more like airships with negative buoyancy and helicopters. That is, a CG vehicle with CG on sits there with mass but no weight until either a separate thrust agency or modulation of the CG field imparts thrust.

But airships depend on the atmosphere for this buoyancy (being lighter than air, Arkimedes law makes them float), while grav works even in vacuum, where no airship will float. Even , in MT ,where maneuver drives are not available under 20 dton, many extra atmospheric crafts under that tonnage work on gravitics.

IMTU a CG vehicle in a gravity field floats in an invisible field. When the pilot pushes his stick forward the shape of the field is changed and rapidly cycled to impart momentum. To steer the field shape is altered.

The effects are similar to helicopter downwash as the rapidly cycling CG field causes atmosphere to move within the area of the field effect so someone standing close (not just below) will find their hair and clothing moving or flapping and debris may be kicked up. The same person will hear a high pitched whine.

Again, what then when there's no atmosphere?

I think you've identified the main considerations of displacing mass such as not flying above cities, restricted landing and parking. For smaller grav vehicles in cities there won't be free flight like aircraft but there maybe skyways, structures that the CG can push against. These may look just like modern roads and maybe duel use for ground vehicles.

Starports may also have much larger paved and reinforced areas besides the usual landing pads. In military terms bunkers become important to shelter in, as do pressure plate activated mines. Depending on how you handle it, "bear traps" for CG vehicles might be possible. Not as in the "tank falls in a hole" style but as in, the ground gives way and the tank suffers an unexpected change of altitude.

Agreed in all of this.

See also that, NOE being forbidden in most planets, Avionics would be quite rares (military crafts aside).

 

[whulorigan]

I utilize versions of all of the above technologies that Reban described, usually becoming available at increasingly higher TLs.

IMTU:

Basic Contragravity Lifter (CG-Lifter / "floater") @ TL9 - Nullifies gravity, but still requires independent motive force to move vehicle. At this TL, artificial gravity on craft must still be produced via a spin habitat or acceleration, with no Inertial Compensation. Spacecraft drives are still conventional reaction-mass thrust agencies (e.g. Fusion-Torch Rocket).

Gravitic Lifter @ TL10 - As Reban described above. This allows basic lateral motion thru a bias in the field (e.g. air/rafts and grav cars), but has a flight ceiling. Fast lateral motion (e.g. Speeders) still requires an outside thrust agency for propulsion. Spacecraft drives still require conventional reaction-mass thrust agencies (e.g. HEPlaR Rocket ["R-Drive"]).

Artificial Gravity/Inertial Compensators @ TL10.

Gravitic Displacement (GD) Lifter @ TL11 - As described in TNE's FF&S, which is also the basis for High Efficiency Gravitic Spacecraft Drives ("G-Drive"). Fast Gravitic Propulsion available thru this agency, but its use is regulated below certain altitudes due to the "crushing effect" on objects directly underneath (diminishing with and spread out over a wider area with increasing altitude).

M-Drive/Thruster @ TL12 - This uses the Space-Warp Principle of riding the curvature of spacetime for spacecraft propulsion.

This gives a bit more "flavor" for each TL, and also gives reasons to use applications of different aspects of the technology. For example, craft with GD-Propulsion (TL11) will usually want secondary CG-Lifters or Gravitic Lifters when they are near a world surface. (Craft and shipbuilders obviously can always import higher TL components for their craft from off-world, of course).

 

[McPerth]

I'm no science whiz so there are lots of forces that I don't understand.

Same can be said about jump drives, and there are pages and pages of discussion about it in this same board :CoW:...

 

[Reban]

But thrusters are about 3 TLs above gravitics, and the grav plates in vehicle design (at least for MT, that's the one I'm more familiar wih) are not thrusters, just grav plates (and lose capacity as they go farther the gravity well).

Also, again in MT rules, thrusters, being a kind of maneuver drive, cannot work in vehicles/crafts under 20 dton, while many grav vehicles are under that tonnage.

Sorry. What I did not mean was "CG is just smaller lower tech Thrusters". What I meant was that that CG and thrusters are part of the same "field effect" science of contra gravity. They have different mechanisms and actions but the underlying science is the same. I also failed to make it clear this is an IMTU interpretation.

But airships tpened on the atmosphere for this buoyancy (being lighter than air, Arkimedes law makes them float), while grav Works even in vacuum, where no airship will float. Even , in MT ,where maneuver drives are not available under 20 dton, many extra atmospheric crafts under that tonnage work on gravitics.



Again, what then when there's no atmosphere?

This was an analogy not a direct comparison. i could have equally used a submarine.

What I'm trying to say here is that gravity is the medium instead of atmosphere or water. So imagine a gravity field (hard to do). Now the CG vehicle applies power to its CG system and achieves equilibrium with local gravity. In effect it floats. Now apply a little more power and the CG vehicle becomes positively buoyant or overcomes the force of gravity.

Gravity is the medium that CG works in, vacuum or atmosphere are only of secondary relevance.

See also that, NOE being forbidden in most planets, Avionics would be quite rares (military crafts aside).

Avionics covers a lot more than just terrain avoidance required for NOE flight. Terrain avoidance might actually be more important for your craft, it's just that the terrain you are avoiding is not the hard sort that you want to avoid impacting rather its the fragile and squishy stuff you want to avoid crushing.

 

[McPerth]

This was an analogy not a direct comparison. i could have equally used a submarine.

And the question would yet be there, just changing atmosphere by water. If the grav drives are only counter-gravity, they may make you weightelss, but, unless something other (be it Arkimides Law in water or atmosphere, or any kind of rockets) will not make you flight, and even less maneuver.

To really achieve flight, you need some kind of force pushing you against anything, and, as I understand gravity, this is gravitational force pushing against a gravity well, but this force might interfere on anything (or anyone) being in its path.

To compare it with an helicopter, it attains its flight by pushing air downwards until its push overcomes the helicopter weight, but this same air interferes into anyone on its way in the form of wind, and effect that disperses as the helicopter is farther from you (higher altitude). That's more or less how I see gravity drives.

Avionics covers a lot more than just terrain avoidance required for NOE flight. Terrain avoidance might actually be more important for your craft, it's just that the terrain you are avoiding is not the hard sort that you want to avoid impacting rather its the fragile and squishy stuff you want to avoid crushing.

Well, IIRC NOE is defined as dodging tres, and avionics in MT are mainly to allow you higher speed NOE flight.

If so, and if the grav force is dangerous enough at such altitudes to make them forbiden in inhabited places, avionics would be quite rare in civilian vehicles (moslty those designed for civilized zones, as the car you usually see in any Earth city).

 

[atpollard]

As I understand it, gravitics do not just nullify gravity but also push against the gravyty well (and so the mass, usually a planet) to achieve lift, more akin repulsors (just nullifying gravity will not allow them to fly, at least on vaccum).

If this visión is right, what happens to the ground below? May this pushing damage people just below the grav vehicle if it flies jus overhead of them?

My take is that this effect strangly diminishes with distance (mostly by being dispersed as a cone), being negligible above a certain alttitude.

Of course, that will make flying above inhabited cities forbidden under that minimal distance, and will carry certain risks for passerbys near "grav parkings"...

This also would allow military craft to affect infantry just flying over them at low altitudes (a way to just overrun them).

Thoughts?

Something to consider is the magnitude of the actual forces involved.
Let's take a standard pickup truck as a starting point ... It weighs about a tonne and is loaded with about a tonne of cargo. So at first glance, the idea of a 2 tonne vehicle parking on your head sounds like an "ouch".

But the truck is about 2 meters wide and 5 meters long ... 10 square meters.
As a grav vehicle hovering a few mm over the surface, that comes out to only 200 kg per square meter. Standing up, a person is only about 0.5 meters wide by 0.3 meters deep ... 0.15 square meters. So the loaded grav pickup hovering a few mm over your head will exert a force of only 30 kg on your head and shoulders.

Unpleasant, yes.
A crushing weight, no.

If we assume that the grav force extends in a 45 degree cone, then the force varies with the square of the height.

So the same 2 tonne grav pickup at an altitude of 2 meters will spread its weight over an area 6 meters wide and 9 meters long ... 54 square meters ... for a force of 37 kg per square meter ... or 5.6 kg on the head and shoulders of a person standing 2 meters below the craft.

And the same 2 tonne grav pickup at an altitude of 10 meters (rooftop height) will spread its weight over an area 22 meters wide and 25 meters long ... 550 square meters ... for a force of 3.6 kg per square meter ... or about 0.6 kg on the head and shoulders of a person standing below the craft.

And just for laughs, the same 2 tonne grav pickup at an altitude of 100 meters will spread its weight over an area 202 meters wide and 205 meters long ... 41410 square meters ... for a force of 50 grams per square meter ... or about 7 grams on the head and shoulders of a person standing below the craft.

So it is a safety and comfort concern, but potentially a manageable problem.
Heavier vehicles will need greater minimum operating heights in populated areas.

 

[rancke]

But the truck is about 2 meters wide and 5 meters long ... 10 square meters.

As a grav vehicle hovering a few mm over the surface, that comes out to only 200 kg per square meter. Standing up, a person is only about 0.5 meters wide by 0.3 meters deep ... 0.15 square meters. So the loaded grav pickup hovering a few mm over your head will exert a force of only 30 kg on your head and shoulders.

Unpleasant, yes.
A crushing weight, no.

I'm reminded of something I saw in a childrens' magazine once about trucks that had partly inflated balloon-like tires instead of ordinary ones for crossing very rough terrain; one balloon tire for the entire front wheel and one for the rear. There was an accident and the truck rolled over someone, who took no damage at all.


Hans

 

[McPerth]

Something to consider is the magnitude of the actual forces involved.
Let's take a standard pickup truck as a starting point ... It weighs about a tonne and is loaded with about a tonne of cargo. So at first glance, the idea of a 2 tonne vehicle parking on your head sounds like an "ouch".

But the truck is about 2 meters wide and 5 meters long ... 10 square meters.
As a grav vehicle hovering a few mm over the surface, that comes out to only 200 kg per square meter. Standing up, a person is only about 0.5 meters wide by 0.3 meters deep ... 0.15 square meters. So the loaded grav pickup hovering a few mm over your head will exert a force of only 30 kg on your head and shoulders.

Unpleasant, yes.
A crushing weight, no.

If we assume that the grav force extends in a 45 degree cone, then the force varies with the square of the height.

So the same 2 tonne grav pickup at an altitude of 2 meters will spread its weight over an area 6 meters wide and 9 meters long ... 54 square meters ... for a force of 37 kg per square meter ... or 5.6 kg on the head and shoulders of a person standing 2 meters below the craft.

And the same 2 tonne grav pickup at an altitude of 10 meters (rooftop height) will spread its weight over an area 22 meters wide and 25 meters long ... 550 square meters ... for a force of 3.6 kg per square meter ... or about 0.6 kg on the head and shoulders of a person standing below the craft.

And just for laughs, the same 2 tonne grav pickup at an altitude of 100 meters will spread its weight over an area 202 meters wide and 205 meters long ... 41410 square meters ... for a force of 50 grams per square meter ... or about 7 grams on the head and shoulders of a person standing below the craft.

So it is a safety and comfort concern, but potentially a manageable problem.
Heavier vehicles will need greater minimum operating heights in populated areas.

TY, that was the kind of information I was looking for.

This reduces quite a lot the danger for bystanders on a city, but grav tanks are quite more heavy, and infantry might be prone (so adding surface, and hence force, to its effect).

Of course, as Reban said, if this force might be used against infantry it also makes them vulnerable to to presure mines. This may change tactics for grav tanks, as they must choose among flying high (making them more vulnerables) or take advantage of the terrain, flying NOE, but risking those mines effects...

 

[Limburger59]

Something to consider is the magnitude of the actual forces involved.
Let's take a standard pickup truck as a starting point ... It weighs about a tonne and is loaded with about a tonne of cargo. So at first glance, the idea of a 2 tonne vehicle parking on your head sounds like an "ouch".

But the truck is about 2 meters wide and 5 meters long ... 10 square meters.
As a grav vehicle hovering a few mm over the surface, that comes out to only 200 kg per square meter. Standing up, a person is only about 0.5 meters wide by 0.3 meters deep ... 0.15 square meters. So the loaded grav pickup hovering a few mm over your head will exert a force of only 30 kg on your head and shoulders.

Unpleasant, yes.
A crushing weight, no.

If we assume that the grav force extends in a 45 degree cone, then the force varies with the square of the height.

So the same 2 tonne grav pickup at an altitude of 2 meters will spread its weight over an area 6 meters wide and 9 meters long ... 54 square meters ... for a force of 37 kg per square meter ... or 5.6 kg on the head and shoulders of a person standing 2 meters below the craft.

And the same 2 tonne grav pickup at an altitude of 10 meters (rooftop height) will spread its weight over an area 22 meters wide and 25 meters long ... 550 square meters ... for a force of 3.6 kg per square meter ... or about 0.6 kg on the head and shoulders of a person standing below the craft.

And just for laughs, the same 2 tonne grav pickup at an altitude of 100 meters will spread its weight over an area 202 meters wide and 205 meters long ... 41410 square meters ... for a force of 50 grams per square meter ... or about 7 grams on the head and shoulders of a person standing below the craft.

So it is a safety and comfort concern, but potentially a manageable problem.
Heavier vehicles will need greater minimum operating heights in populated areas.

Excellent work!

So a grav car isn't going to crush you, whereas a grav tank may...if they are standing still. What if it flying towards you at 100 kph, two meters over your head. In addition to the pressure wave of displaced air, you're getting hit with 5.6 kg traveling at 100 kph. Damage is possible, getting knocked down probable, upsetting your aim or coffee cup a sure thing.

 

[Reban]

And the question would yet be thre, just changing atmosphere by water. If the grav drives are only counter-gravity, they may make you weightelss, but, unless something other (be it Arkimides Law in water or atmosphere, or any kind of rockets) will not make you flight, and even less maneuver.

To really achieve flight, you need some kind of force pushing you against anything, and, as I understand gravity, this is gravitational forcé pushing against a gravity well, but this force might interfere on anything (or anyone) being in its path.

To compare it with an helicopter, it attains its flight by pushing air downwards until its push overcomes the helicopter flight, but this same air interferes into anyone on its way in the form of wind, and effect that disperses as the helicopter is farther from you (higher altitude). That's more or less how I see gravity drives.

I still think we are misunderstanding each other, but thats okay Your version of gravity drives is good and I see others have fleshed it out with some calculations.

Using the comparison with a helicopter has confused me a bit. A helicopter flies by the rotor blades generating lift. The difference in air pressure above and below the blade sucks the blade up. The thrust generated is used to move the helicopter forward.

Gravity is an attractive force so to overcome it we have to generate a repulsive, or contra-gravity, force (just like your vision of a gravity drive). If the force of gravity is exactly equaled by the contra-gravity force the ship should be neutrally buoyant (it will neither sink nor rise). If you increase your contra-gravity force the ship should rise (or move away from the gravity source). If you reduce your contra-gravity force the ship will sink (or be attracted to the gravity source).

I think up to this point both our visions of gravity drives are the same we just differ on the details of their effects.

So the ship can move up and down in a gravity well. Where we differ is whether a gravity drive can be used to impart sideways movement. An outside thrust agency is one way of solving this, so fitting some kind of engine is required. Thats one solution, but suppose you can cycle the drive on or off really fast. Effectively the ship falls as gravity affects it again.

If we had fine enough control over this cycling of the gravity drive we could make the ship fall in a particular direction. IMTU this explains why CG drives aren't effective beyond the 10D limit.

Hopefully this might be of benefit to describing your gravity drive.

Well, IIRC NOE is defined as dodging tres, and avionics in MT are mainly to allow you higher speed NOE flight.

If so, and if the grav force is dangerous enough at such altitudes to make them forbiden in inhabited places, avionics would be quite rare in civilian vehicles (moslty those designed for civilized zones, as the car you usually see in any Earth city).

Its more about preventing the aircraft from coming into contact with things that will do it damage, trees, the ground, buildings, mountains.

For your gravity drive I imagined something that does the opposite i.e warns the pilot when his course will intersec with terrain that the ship might do damage to. I realize this isn't the way MT avionics work, but I still think that even in a civilian vehicle there might be a warning system in the cockpit that says "Terrain, terrain, TERRAIN!!!" like you find in a modern airliner.

Of course the main terrain avoidance equipment aboard any aircraft is the Mk1 Eyeball connected to the pilots brain.

 

[leo knight]

It's funny, Atpollard, I thought about this while watching "The Phantom Menace". Early in the film, Qui Gonn and Jar Jar dive under a hovering troop carrier. The thing obviously had some ground effect, as the foliage whipped around as it passed over them. Ever the nitpicker, I thought, "Why weren't they crushed?" OK, actually I thought, "Why, oh why, wasn't Jar Jar crushed?', but hey. Then it occurred to me that if the thing had some sort of anti-grav pusher-plate on the bottom ("repulsorlift" in Star Wars speak), the repulsive force would spread over the entire bottom of the craft, and perhaps spread out a bit as it gained altitude. Sadly, not enough pressure to smoosh Jar Jar.

On a weirder note, I found a book by an aerospace engineer named Paul R. Hill, titled "Unconventional Flying Objects". The story goes he saw UFOs not once, but twice, and intrigued by what he saw, tried to work out the performance specs for them as if they were aircraft. He supposed that they had some anti-gravity, repulsive force field driving them. He included eyewitness accounts of UFOs affecting objects on the ground: trees, cars, water, people, etc.

 

[CosmicGamer]

So a grav car isn't going to crush you

But I could see a parking lot needing a landing/launch area and then you drive into a parking spot because Atpollard's info (thanks) makes me think you could possibly knock a person, especially a child, over if you land or lift beside someone getting in and out of a vehicle.

 

[Amber Chancer]

It's a fascinating question you raise here, McP ... but I think it's important to keep a distinction between gravitics and thrusters.

A GravBelt is NOT a personal jet pack ... it's something altogether different.

There are some questions (this being one ... the way Jump Drive works being another) that I think it is best not to ponder too deeply. Just accept that they ARE, and move on to enjoying your adventure.

 

[xevus11]

I have always treated grav thrusters as micro acu-whatever (I use warp-drives in my universe, not jump drives) drives, essentially minorly warping space to "Pull" the vehicle in said direction, so getting very close to a vehicle can be very dangerous, but a meter or two away, and the affects are non-existent

 

[McPerth]

There are some questions (this being one ... the way Jump Drive works being another) that I think it is best not to ponder too deeply. Just accept that they ARE, and move on to enjoying your adventure.

You're a bit late to say how jump drives work should not be discussed, just accepted ...

This aside, imagine the implications it could have in role playing if the grav plates may affect (even at low level effect) people below (low flying grav vehicles used as anti-riot non-lethal systems, new dangers, producing mild accidents, with all its legal implications, etc...)