Grav vehicle movement
- Thread starter Peter the Swede
- Start date
"Essentially, a "single" thruster (more likely to actually be three in a parallel array) is able to produce full thrust off axis by (IIRC) 30°, and drops smoothly to 25% in a 10° cone. (so (180-(30+10))/75=140/75=1.86, so 1.86°/-1%)... which means the horizontal leg is sin(30°)=0.5 x rated... while the vertical is cos(30)=0.86 x rated. So, minimum thrust is 1.15x weight (not mass - see below) for helicopter like efficiency."
Where is this formula from? I couldn't find it in a quick scan through the MT Referee's Manual or FFS.
"So, minimum thrust is 1.15x weight (not mass - see below) for helicopter like efficiency.
...
Why Weight? because thrust needs to overcome gravity. Acceleration is mass limited, but lift is weight."
That implies that the thrust a thruster can produce varies with the gravitational field that it's in: less if it's on the moon, much more if it's on a 40,000 km diameter pure iron body. By implication, a thruster would produce zero thrust in zero-G. I think that the logic is flawed.
Quoth FFS:
"CG lifters do not provide thrust and cannot physically lift a craft or vehicle. Instead, they neutralize most of the gravitational attraction of a world (approximately 99% ...) ... usually CG is used only as an adjunct to the ship's thrusters. By neutralizing most of a world's gravitational field, a ship with only 1G of thrust can still escape the world's gravity well.
Note that CG does not reduce the mass of the ship, and so a 1G thruster will still only produce 1G of acceleration; CG merely negates the gravitational vector of a world."
"Movement of lift vehicles is based on their thrust. The G rating of a lift vehicle is its total thrust in tonnes divided by 10 times its hull displacement (in displacement tons)." In FFS, grav tanks generally use HePlaR thrusters, which clearly care about mass, not weight. (I don't take MT to be the ultimate canon here, I'm just recapitulating what it says.)
Where is this formula from? I couldn't find it in a quick scan through the MT Referee's Manual or FFS.
"So, minimum thrust is 1.15x weight (not mass - see below) for helicopter like efficiency.
...
Why Weight? because thrust needs to overcome gravity. Acceleration is mass limited, but lift is weight."
That implies that the thrust a thruster can produce varies with the gravitational field that it's in: less if it's on the moon, much more if it's on a 40,000 km diameter pure iron body. By implication, a thruster would produce zero thrust in zero-G. I think that the logic is flawed.
Quoth FFS:
"CG lifters do not provide thrust and cannot physically lift a craft or vehicle. Instead, they neutralize most of the gravitational attraction of a world (approximately 99% ...) ... usually CG is used only as an adjunct to the ship's thrusters. By neutralizing most of a world's gravitational field, a ship with only 1G of thrust can still escape the world's gravity well.
Note that CG does not reduce the mass of the ship, and so a 1G thruster will still only produce 1G of acceleration; CG merely negates the gravitational vector of a world."
"Movement of lift vehicles is based on their thrust. The G rating of a lift vehicle is its total thrust in tonnes divided by 10 times its hull displacement (in displacement tons)." In FFS, grav tanks generally use HePlaR thrusters, which clearly care about mass, not weight. (I don't take MT to be the ultimate canon here, I'm just recapitulating what it says.)
OK, found it in the Starship Operator's Manual. Not sure off-hand if it originated somewhere else. Relevant text:
"Thruster plates can attain full propulsion only in a single direction, usually due aft, but can be used to provide thrust in any direction. Outside of an arc formed at about 20 degrees from the primary direction of thrust, propulsion drops off fairly quickly. When directed to provide thrust in a perpendicular direction, the plates can generate about 25% of their normal thrust. If directed to provide negative thrust and cause the ship to 'back-up', plate output drops to about 10% of full thrust. Thus, a craft with a 3G maneuver drive can apply a maximum braking force of 0.3G without changing its attitude."
However, later it states:
"So far, attempts to scale down thruster plate technology to sizes useful for craft below about 20 tone displacement have proven unsuccessful. The power-to-thrust ratio of such small thruster plates is so poor that straight anti-grav technology is preferable."
Note that the diagram appears to show the same angle directly ahead for the 10% thrust region as directly behind for the 100% region, so another 40 degree total angle. The diagram does not appear to be drawn correctly, however; the rear "100% arc" appears to be about 20 degrees total, while the text states that it should be 40 degree total: 20 degrees on either side of the axis.
I'm not sure how that "straight anti-grav technology" is supposed to work.
Note that it never says that the drop-off from 100% to 25% is linear, only that it is "rapid."
Going by your concept, assume that available thrust at 20 degrees off axis needs to provide enough "lift" to counter the weight of the vehicle in a 1G gravity well. If the thruster plate is pointed straight "down," at 20 degrees, you get 94% of your thrust vertically and 34% horizontally. I'm not sure where your definition of helicopter-like efficiency comes from, but if you require 50% thrust in the horizontal plane, well, you can't do that and not shoot upwards ...
Unfortunately, even if the drop-off is linear, figuring the correct angle for buoyancy-neutral maximum thrust is annoyingly complicated. I worked up a quick spreadsheet and I think that you get 50% lateral thrust at about 27 degrees off-axis.
"Thruster plates can attain full propulsion only in a single direction, usually due aft, but can be used to provide thrust in any direction. Outside of an arc formed at about 20 degrees from the primary direction of thrust, propulsion drops off fairly quickly. When directed to provide thrust in a perpendicular direction, the plates can generate about 25% of their normal thrust. If directed to provide negative thrust and cause the ship to 'back-up', plate output drops to about 10% of full thrust. Thus, a craft with a 3G maneuver drive can apply a maximum braking force of 0.3G without changing its attitude."
However, later it states:
"So far, attempts to scale down thruster plate technology to sizes useful for craft below about 20 tone displacement have proven unsuccessful. The power-to-thrust ratio of such small thruster plates is so poor that straight anti-grav technology is preferable."
Note that the diagram appears to show the same angle directly ahead for the 10% thrust region as directly behind for the 100% region, so another 40 degree total angle. The diagram does not appear to be drawn correctly, however; the rear "100% arc" appears to be about 20 degrees total, while the text states that it should be 40 degree total: 20 degrees on either side of the axis.
I'm not sure how that "straight anti-grav technology" is supposed to work.
Note that it never says that the drop-off from 100% to 25% is linear, only that it is "rapid."
Going by your concept, assume that available thrust at 20 degrees off axis needs to provide enough "lift" to counter the weight of the vehicle in a 1G gravity well. If the thruster plate is pointed straight "down," at 20 degrees, you get 94% of your thrust vertically and 34% horizontally. I'm not sure where your definition of helicopter-like efficiency comes from, but if you require 50% thrust in the horizontal plane, well, you can't do that and not shoot upwards ...
Unfortunately, even if the drop-off is linear, figuring the correct angle for buoyancy-neutral maximum thrust is annoyingly complicated. I worked up a quick spreadsheet and I think that you get 50% lateral thrust at about 27 degrees off-axis.
FF&S and TNE contra-grav is completely different to that in CT and MegaT.
TNE went to a new concept whereby grav only nullified 99% of mass. Thrust needed to be provided from another source. That could be HEPLAR, or ducted fans, or even jet engines.
This is vastly different to the CT/MegaT concept of a vectored thrust CG drive that propelled the vehicle as well as lifting it.
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But you're right that the text in MT/SoM that Aramis was quoting is clearly specified as applying to thruster plates on starships.
TNE went to a new concept whereby grav only nullified 99% of mass. Thrust needed to be provided from another source. That could be HEPLAR, or ducted fans, or even jet engines.
This is vastly different to the CT/MegaT concept of a vectored thrust CG drive that propelled the vehicle as well as lifting it.
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But you're right that the text in MT/SoM that Aramis was quoting is clearly specified as applying to thruster plates on starships.
Some responses.
See, here is where not being a scientist helps, I can and do treat the Manuever/Gravitic drives as magi-tech so I can totally see a g-tank spinning on its axis. Why wouldn't it?
So, some how being able to move one way and shoot another is the reason for a turret, yet if the g-tank can vector move, again this is no issue and again my g-tank egg is still safe. And yeah, maybe I should have sketched out that I see the Grav tank as more an egg (possibly flattened) with a hole in one end for the non-meson equipped tanks and completely enclosed for the meson tanks. So, in my g-tank there isn't the same problem of having weak points in the armor for road wheels and engine radiators and such like, though I suspect there are still going to be a possible few depending on how the sensors on the exterior connect to the interior. As to sensors, in all honesty, I suspect those will be off hull and be drones and trooper feeds, which allows even more stealth for the tank. But then I don't see them as sitting passively anywhere, I see them as patrolling and laying waste to enemy armor/air while being able to take a couple of solid hits while doing it. A veritable land battleship.
mike wightman
SOC-14 10K
Note that in T4 battledress evolves into egg shaped battlepods at high TLs (Emperor's Arsenal).
Regardless of shape, you still have to choose how to distribute armor. You can have equal armor all around, but you have to give up something in exchange: speed, armament, fuel capacity, whatever.
If I have the same tank, but put %50 of my armor on the front (the side the gun points out of), my frontal armor is at least twice your frontal armor. That's a significant advantage.
Now, if you're saying, well, I can move in any direction equally well, and I'm ignoring aerodynamics or anything else*, then I can't argue with you. You've decided that's how you want things to work. However, that won't translate to the OTU or many peoples' ATUs. You have dropped out of any broader discussion, however.
* Which means that you can have frontal armor just fine, since you'll just point it in the direction of the enemy while you move sideways or whatever.
That wasn't five minutes!
I think you and others really just want it to look like 20th Century tank because you are both familar with it (you know how to fight it since it fights like real world tank) and think it looks cool (which it does). Operationally, once gravitics hit the scene as the standard of motivating tanks the old tactics die. If we are going to start with new doctrine we might as well start with a new tank design.
Oh, I'll dump speed and it being any size but fanarking huge (for a two man vehicle). Big gun, lots of armor, twinned PPs (one for movement and one for weapons), and smooth lines.
Only if I shoot you in the front and really who lines up that shot as opposed to a nice turret line, flank or even rear shot? Or why I even have flying g-tanks in the first place.
Well, why can't a gravitic craft not move in any direction? It's not bound by roads, waves or atmo so I don't see why it doesn't spin on it own axis or fly obliquely or even sideways. Hell, that is the reason to make a nice smooth, rounded shape to decrease drag when it is in standard or thick atmospheres.
Please, explain using small words how a Trepedia is more areodynamic that a flattened egg shape. Seriously, because to me the average MBT with a turret and main gun, not mention antenna and AP guns all over it is drag magnet, and that is with the turret facing in the direction of travel, once it turns on an angle to the direction of travel the drag should be increasing.
I think you and others really just want it to look like 20th Century tank because you are both familar with it (you know how to fight it since it fights like real world tank) and think it looks cool (which it does). Operationally, once gravitics hit the scene as the standard of motivating tanks the old tactics die. If we are going to start with new doctrine we might as well start with a new tank design.
You run the risk of falling into the great weakness of the M1 Abrams ... it can't get to the battle in time to actually participate (Abrams is too heavy to transport, your egg will be too slow).
Or the weakness of the German WW2 tanks ... too difficult/expensive to build in large numbers ... killed by swarms of cheaper tanks (really big to have everything, means really expensive, too).
(The rest is just arguing tactics ... a topic about which I know far too little to really respond.)
I think the point here is that you can move in any direction but not all at the same rate.
Either drag or thrust will affect how fast you can travel in the direction of your choice or in the orientation of your choice.
Nice smooth rounded shapes aren't necessarily the best shape to decrease drag depending on what speed and through what medium you're moving.
Also armour thickness isn't the only way to achieve protection, hull shape also contributes to ballistic protection, but thats hard to model in most design systems.
Not bound by atmosphere? It's going to be affected by atmospheric drag.
As I discussed at length (painful length), OTU anti-grav doesn't work that way. It doesn't give thrust in any direction that you want. This inherently produces vehicles with defined fronts and rears.
That said, you're free to do anything you want in your TU. As I said, though, when you do that, you've going off into a different discussion that's incompatible with discussions about grav vehicle movement in the OTU and ATUs that use similar assumptions about grav propulsion.
Much better, the body in particular is a blunt-nosed delta. This provides better inherent aerodynamic stability, and lower drag, than a disc/flattened egg. (I'm not clear on how your "flattened egg" is oriented. Are we taking an egg, putting its long axis horizontal, squishing it flatter, and then pointing the pointy end at the enemy?)
In looking at some of the engineering assumptions in the OTU, its clear that grav vehicles maneuvering at high speed will do well to take advantage of aerodynamic lift to turn. The shape of the Trepida's hull is actually well-adapted for that (many of the other grav vehicles are not). Yes, grav vehicles should bank to turn when at high speed.
Yes, both the turret and the body have the narrow end fore and the wide end aft, then squashed.
It's not quite a delta, either - it's a pentagon with 2 right angles, IIRC. But it's awful close to a delta. And it and the Astrin use the same berths.
(Note to Self & Don: probably need to check the MT data in 101V for the whether the Astrin has the pictured hull extension in the actual stats.)
Having designed a 15mm range of Grav Armor (Topgun Grav Armor), and given it a bit of thought, I'll weigh in here.
My ultra-high tech designs can drop in from orbit, and climb back out of a planet's gravity well, as needed. They can drop in from orbit at Mach 3, and cruise smoothly at just under Mach 1 in level flight.
Each vehicle has a set of grav repulsors arrayed together on their undersides, which provide both lift, and vectored propulsion.
Generally, most designs of grav tanks permit the application of the greatest thrust to the rear of the vehicle, so that they can be propelled forward, quickly and efficiently.
There is some ability to provide thrust for rearward, or sideways movement, but less so than for forward propulsion. That, combined with the overall aerodynamic shape of the vehicles, affects performance.
As mentioned by another person, at low speeds, like NOE movement, Grav Tanks may move in any direction desired. However, they are primarily limited to forward flight at fast speeds, though they do have the ability to sideslip some, during that flight, as needed.
I hope that helps.
My ultra-high tech designs can drop in from orbit, and climb back out of a planet's gravity well, as needed. They can drop in from orbit at Mach 3, and cruise smoothly at just under Mach 1 in level flight.
Each vehicle has a set of grav repulsors arrayed together on their undersides, which provide both lift, and vectored propulsion.
Generally, most designs of grav tanks permit the application of the greatest thrust to the rear of the vehicle, so that they can be propelled forward, quickly and efficiently.
There is some ability to provide thrust for rearward, or sideways movement, but less so than for forward propulsion. That, combined with the overall aerodynamic shape of the vehicles, affects performance.
As mentioned by another person, at low speeds, like NOE movement, Grav Tanks may move in any direction desired. However, they are primarily limited to forward flight at fast speeds, though they do have the ability to sideslip some, during that flight, as needed.
I hope that helps.
