Hovertanks

[Lancefighter]

Generally in either case, the recoil does not compare at all to the recoil of a cannon or railgun.. so uh >.>

[Tren]

I know, just bringing out more facts that reality has to burden us.

[3drts]

They are so small and light because their velocity makes them energy states, not matter. If they hit something, they will explode in a very big way.

What is it with railguns and people not understanding the basics. Pretty much nothing in the above statement is accurate.
A railgun projectile is still a heavy piece of mass, traveling at high velocity. It just uses EM fields to accelerate the slug to a high velocity than is possible with gunpowder, or any high pressure gas.
It is no more an "energy" weapon, than the hypothetical "rods from god" would be an exotic energy weapon - "Rods from God" being rod like slugs dropped from orbit, no propellant, just gravity.

Railgun projectiles are composed of matter, they do have mass, and velocity can never make something an "energy state", nor affect how small or light it is.

Next quote:

(Even lasers produce a small recoil impulse because the photons have a momentum)

It would be true, if photons had mass.

He wasn't confusing it with a particle accelerator, which produces recoil between that of a railgun and a laser.

High powered lasers are a viable means of spacecraft propulsion, in theory.
Photons may not have "mass" as we think of it, but they do have "momentum". They are affected by gravity, and conservation of momentum is in effect, photons do produce recoil.
This is why light sails work, and are actually put into use. With a nuclear/fusion reactor, they are viable extra solar propulsion.
Their acceleration sucks though
See this website which serves as a guide for "hard" Sci-Fi writers:
http://www.projectrho.com/rocket/rocket3c2.html#photon
"PHOTON DRIVE. The exhaust is not a stream of matter. Instead it is a beam of electromagnetic radiation, basically a large laser. The advantage is that it has the maximum possible exhaust velocity and thus the highest specific impulse. The disadvantage is the ludicrously high power requirements... In other words, one lousy Newton of thrust takes three hundred freaking megawatts!!"
Now, note that 1 newton will accelerate 1 kilo at 1 m/s^2
For a 20 ton(20,000 kg) hover tank, firing a three hundred MEGAwatt laser for a full second, the recoil would push (ignoring friction) the tank 1/20,000 m/s or 0.00005 m/s which is 0.0000118 mph.
Needless to say, one needs not worry about the "recoil" from a laser, unless you are an interstellar spacecraft, with a fusion/Anti matter reactor, firing an insanely powerful laser/light beam continuously, for decades.
(can't find the page on Solar "Light" sails vs solar "wind" sails, but the force from light is and order of magnitude or three greater than the force of the ions from the solar wind, will edit this post later if I find it).

RE: Jet Engines
Yes, they could be mounted, but they would not be practical for primary propulsion.
Think of the percent of a jet fighter's loaded weight that is just fuel...
Moving around a heavy tank, with jet turbines, with only thrust (M1 abrams, and most modern tanks use turbines, but they use the shaft power for propulsion, this is much better than pushing air).
One could do like the B-36, and mount turbines for "combat power", the B-36 kept its turbines off, until it got to the target area, then when it go to the target area it turned on its turbines and went to maximum speed/ its "dash" speed.
You simply couldn't move an armored hover tank around much with jet turbines, before running out of fuel.
Not to mention, jet turbines are more susceptible to damage.

Re: Verin and his "Logical hovertank"
Yes, that looks reasonable, but it is not a tank.
There is a difference between a Tank (lets say an MBT, the only form of modern "Tank", cruiser and destroyer tanks are WWII and earlier relic classes), and a "fighting vehicle" (lets say an Infantry fighting vehicle like an M-2 Bradley, or a BMP-1/2)

Its gun and grenade launcher would be woefully inadequate to go against armored tanks, and its armor looks rather light, vulnerable to RPGs and small man portable AT launchers (many of which are inadequate against modern armor, and at best can get mobility kills, the newer generations have top attack profiles to target the weaker top armor)

[TVR]

... About the railguns: they provide less recoil for the same amount of kinetic energy compared to ordinary cannons ...

Completely and utterly false.

I'm so sorry Skrim, for not meticulously explaining Newton's Third Law earlier.

For every force, there is an equal and opposite reaction

For every projectile accelerated out of a Newtonian cannon, there is equal amount of kinetic energy that is applied backwards to the entire cannon assembly.

For amount of force f = 10 N, the equal and opposite force can be represented as -f, for 10 N in the opposite vector.

The difference between two projectiles with the same kinetic energy, one with high velocity, low mass, and narrow diameter, and the other with low velocity, high mass and a broad diameter, besides higher velocity requiring less leading, is an entirely different concept known as pressure.

Pressure = Force / Area, it is what separates a punch with a knife, and punch without one.

[Lancefighter]

f22s loaded weight is around 30,000kg. It holds about 8k kgs of that in fuel...

so yeah, 25% or so... but keep in mind it also has a 1:1 trust to weight ratio. Im fairly sure no tank needs that much trust...

And for the record, I am aware that most jets do not work on pure thrust alone, this is merely the best way for a fighter to work. Gas turbine engines /are/ jet engines....

[Tren]

It is no more an "energy" weapon, than the hypothetical "rods from god" would be an exotic energy weapon - "Rods from God" being rod like slugs dropped from orbit, no propellant, just gravity

By any chance are you refering to Fletchettes?

[psychopompos]

i cant remember where i say it, but hovercraft have been used militarily as light troop transport where boats & helo's where impractical but there was still too much water for standard infantry ground war.

putting light guns & missile/rocket systems on hovercraft is possible.
since the round recoil would still have to overcome the chassis/hull inertia which could be compensated for between rounds by the prop system.

also, the skirt based hover system isnt the only only available, the jet based ground effect would be much more powreful.
not to mention would allow traversal of terraign like the stone desert area of the sahara more effectivly then tracks or wheels which are prone to shredding or buckling in said terrain.

however, heavily armoured, they are not.
nor are they capable of traditional tank tactics.

but keep in mind it also has a 1:1 trust to weight ratio. Im fairly sure no tank needs that much trust...

to follow on from that, as i said about the rocket ground effect. what may only have a .5 thrust to weight ratio 500 meters up, could have a have stable lift force at 50cm with no change to equipment thanks to the ground effect.

[TVR]

... however, heavily armoured, they are not ...

Ground-effect vehicles can support heavier armour than track-based propulsion, as the pressure is dissipated along the entire base of the vehicle, versus just the width of the treads.

However, this would come at the price of further reducing the abyssal acceleration and deceleration provided by a turboshaft propeller.

Turbofan, turbojet, turboprop, etc. engines cannot be mounted on an amphibious vehicle! The sea water mist will both corrode an unfiltered jet engine and cause engine stalling from incomplete combustion.

[Lancefighter]

Turbofan, turbojet, turboprop, etc. engines cannot be mounted on an amphibious vehicle! The sea water mist will both corrode an unfiltered jet engine and cause engine stalling from incomplete combustion.

Turboshaft engines are commonly used in applications which require a sustained high power output, high reliability, small size and light weight. These include helicopters, auxiliary power units, boats and ships, tanks, hovercraft, and stationary equipment.

>.>
edit:quote fail

[TVR]

I am amazed.

You totally managed to miss the fact that I specifically mentioned turbofan, turbojet, and turboprop, and any other unfiltered jet engines only.

If you were to read my first post, which your first reply demonstrates that you didn't:

... what is wrong with jet engines? ...

You would have noticed that I explained exactly why an exposed jet engine design wouldn't work:

... The actual exposed propulsion mechanism can't be a jet-engine, as proximity to salt-water will easily cause engine stalling, electric motor, turboshaft, or rocket are feasible alternatives. ...

I really recommend reading at least the entire post before replying.

[Lancefighter]

Im not a big fan of reading comprehension, as might be noted by the repeated other people telling me to read entire posts before posting.....

Now, while your busy berating me for not reading.

[allcaps]
TUROSHAFT IS A TYPE OF JET ENGINE
[/allcaps]
edit for random clarity.

Theres no such thing as a filtered jet engine. The mere fact of filtering the intake of a jet engine lowers the output by a factor of 3*. Its the same reason we dont put bird-proof cages in front of jet engines - it hampers productivity more than it helps.

* statistics may be pulled out of my ass

[3drts]

... About the railguns: they provide less recoil for the same amount of kinetic energy compared to ordinary cannons ...

Completely and utterly false.

I'm so sorry Skrim, for not meticulously explaining Newton's Third Law earlier.

For every force, there is an equal and opposite reaction

For every projectile accelerated out of a Newtonian cannon, there is equal amount of kinetic energy that is applied backwards to the entire cannon assembly.

Completely and utterly false.

The recoil is imparted according to the laws of momentum, which is simply Mass * Velocity.
Kinetic energy is 1/2 Mass * Velocity^2
Because the Velocity term is squared, a high velocity low weight projectile with the same KE as a high weight lower velocity projectiles, will result in less recoil than the heavy projectile.
When a gun fires, the guns KE is much less than that of the bullet.

Additionally, the mass of air moving out the end of the barrel itself results in a significant proportion of the recoil effect.
Sure, you can add a muzzle brake, and reduce recoil, but the gasses expelled from the gun still result in a net increase in recoil force, although a muzzle brake makes that contribution much lower.
A rail gun, with no gasses behind the projectile, will have less recoil, even if it is firing the exact same projectile at the exact same speed as a gas/gunpowder gun.

Lance Fighter- according to wikipedia, the F-22 is more like 19.7 tonnes unloaded, with 8.2 tonnes of fuel- 30% of the weight is fuel.
Anyway, consider the weight of an M1-Abrams - 61 tonnes, with only 1,900 liters of fuel (anyone know a general conversion of liters to kg for general fuel? tank turbines run on diesel, JP-8, etc)
Any tank that can climb up a 45 degree incline, can exert a force of at least 70% its weight (we can't really call this thrust though, but it is the nearest analogy)

Turbines are just less efficient, particularly at low speeds, and true "tanks" are very heavy from all their armor, and large fuel tanks are a liability - not to mention they increase the size and profile of the tank, and make it require even more armor.

Any sort of hover tank would likely be running a turbine to power the air-cusion to start with (the landing craft we have today are turbine powered), and would get the greatest thrust for the engine size and a given fuel consumption by using the shaft to turn props at the back - as our landing craft do today.
Anything powered by combustion, I would have to assume would have a fan for primary propulsion.
Its a simple consideration of thrust to weight ratios (thats why the F-35 with its lift fan thats driven by the turbine shaft beat out the X-32 with its direct thrust concept- the X-32's thrust to weight was much worse than the X-35's in hover mode, despite using the same turbine, and being lighter).
Its why helicopters may be driven by turboshaft engines, but don't look like harriers(which only hover when low on fuel, and no ordinance)
Thrust to weight, and fuel consumption, rule out jet propulsion.
Jet turbines- yes if the air jet isn't primarily for propulsion, but rather a "beneficial" side effect of turning the shaft that drives the other machinery.
Supplementary jet turbines for short use during combat, is all I will concede

As far as turbojets on seaplanes:






They can be mounted on seaplanes, their intakes are usually elevated, and they need special materials because of corrosion, and if they ingest too much water, they don't work, but these problems are not and were not insurmountable.

Tren:
When I said rods from god, I was not specifically refering to flechettes, nor would any generic flechette fit the catagory described by "Rods from god"
http://www.popsci.com/scitech/article/2004-06/rods-god

http://en.wikipedia.org/wiki/Kinetic_bombardment

The point is that just like theoretical rail guns, these would be very high velocity projectiles, that don't use gunpowder to attain that velocity - it doesn't make it any more of an "energy weapon" than a standard explosive gas powered cannon

[Tren]

Ah, they work on the basis of original flechettes, dropped from planes in WWI

[psychopompos]

Ground-effect vehicles can support heavier armour than track-based propulsion, as the pressure is dissipated along the entire base of the vehicle, versus just the width of the treads.

"can in lab" =/= "can in world"
for practicality sake it would need to be more agile to make it viable as a combat vehicle.
which means light/medium Armour & slats are the most defense you get.
you also have to give it certain ground clearance ability.

However, this would come at the price of further reducing the abyssal acceleration and deceleration provided by a turboshaft propeller.

like i said, wouldn't have heavy armour.
but its speed over given terrain types would be exceptional.
deltas, swamp, stone desert,

Turbofan, turbojet, turboprop, etc. engines cannot be mounted on an amphibious vehicle! The sea water mist will both corrode an unfiltered jet engine and cause engine stalling from incomplete combustion.

they are flying those engines through sand storms. which is essentially pouring liquid glass through them.
not to mention that when you reduce the weight, you reduce the wash.

hovers would by no means be the force they are in wz current, nor would you get them on anything bigger then a cobra(unless you have a troop transport) but in the right circumstances they could be quite decisive.

[TVR]

... TUROSHAFT IS A TYPE OF JET ENGINE ...

The good news is that you are absolutely correct...

The bad news is that you've completely missed the point.

Turbofans/Turbojets/Turboprops are reaction engines, they intake air directly and exhaust the combined oxidized product directly out of the rear of the engine as propellant.

Turboshaft is a type of internal combustion engine, since it is located inside the vehicle, it can use regular intake filters and dehumidifiers as necessary, and also as with other ICEs, it drives a shaft which powers the exposed propellers on the rear and bottom of a GEV.

... When a gun fires, the guns KE is much less than that of the bullet. ...

Newton's Third Law states that the force (ie, the Kinetic Energy) must be equal, otherwise it would violate the conservation of momentum.

... they are flying those engines through sand storms. ...

Operating through (for landing/take-off) != Operating in for extended periods of time (10s of hours during extended Ops., as expected from an computer-controlled vehicle)