Hovertanks

[TJP]

I've been thinking about the plausibility of hovertanks. I admit they are a nifty idea and I like them, but how realistic are they?

Nowadays, hovercraft aren't used on dry land that much. They are mostly water vehicles or used for amphibious landings, AFAIK. They require huge amounts of power to operate and aren't very well controllable on dry land (especially slopes/hills and woodland present problems to them).

When you start putting armor on them (as I'm talking about AFVs here), you only compound the problem of power to weight ratio of hovercraft.

Also, in none of the illustrations of hovertanks that I've seen, there is no sign of the "thruster," i.e. the propellor/fan/whatever that pushes the hovercraft forward. One could argue for vectored thrust fans in the bottom of the hovercraft, but I'm not convinced they could push the vehicle very effectively. (Aside: It may be that they were left out for artistic reasons, but that does not rhyme well with the hard sci-fi theme of 2300AD.) Whatever the case, that thruster is a big achilles heel of a hovercraft, as are the air intakes for the "lifters." By the 24th century, even the dumbest missile could home in on these and cripple the hovertank with a fraction of explosive power required to blast through the "real" armor.

Well, with all these problems I can't say why we would change tracked tanks into hovertanks. The speed of hovercraft is their only advantage, but you don't need to do 200 km/h with a tank and even the modern tracked tanks can do 100 km/h, which is more than adequate. The maneuverability department presents some unique advantages for the hovertank (with vectored thrust a hovercraft could slide sideways, for example), but there are disadvantages equally unique (e.g. as they glide, they are difficult to control effectively), so it all evens out.

I admit, converting hovertanks into conventional tracked ones does take away some of the "coolness factor," but if one wants to stay true to the hard sci-fi theme, it might be a sensible thing to do. Hovercraft could still be used for light recon vehicles, perhaps.

This all IMHO and YMMV, of course.

 

[Ellros]

Read the "Hammer's Slammers" series for some interesting ideas on how hovertanks work in a futuristic environment.
Fusion powerplants can provide a lot of power, so can overcome the thrust to weight ratios.
And you are correct, the fans are gimballed within their housings in most cases to provide thrust.

 

[Uncle Bob]

Hammer's Slammers are a lot of fun, but I am not impressed by his use of technology. Given unlimited energy hovertanks are possible, but there are other uses for that much power.

I'm with TJP here. And one thing he didn't mention was the enormous signiture (dust and noise).

One bit of gear I always liked in T2300 were the ducted-fan VTOLs. I think what will replace the tank will be a heavily armed VTOL gunship, kind of a combination of the Apache and the Harrier.

 

[Secrect Cow Level]

I've always had some issues with hovertanks as well. It seemed to me that one good hit on the shroud surrounding the hoverfan would cripple the tank. My guess is the designers at GDW were in love with the concept of gravtanks in TRAVELLER and wanted something akin to them in 2300.

Most likely hovertanks would be used for fast strikes on static targets like buildings and for attacks coming from sea. Also, according to the Aurore Sourcebook, it is cheaper to ship the lighter hovertanks offworld than it is to ship heaver tracked tanks, and mobility is more useful on the frontier battlefield than slower tracked tanks.

The Aurore Sourcebook does detail one heavy tracked tank; the French CC-21 main battle tank, so tracked tanks are still in use in 2300AD, but usually only on the core worlds.

 

[Fulminata]

Drake wrote an essay on the subject of why air cushioned vehicles replaced tracked AFVs in the Hammer's Slammers universe. I think it could be summarized as "hovercraft don't throw treads"

 

[BMonnery]

Originally posted by TJP:
I've been thinking about the plausibility of hovertanks. I admit they are a nifty idea and I like them, but how realistic are they?

Nowadays, hovercraft aren't used on dry land that much. They are mostly water vehicles or used for amphibious landings, AFAIK. They require huge amounts of power to operate and aren't very well controllable on dry land (especially slopes/hills and woodland present problems to them).

When you start putting armor on them (as I'm talking about AFVs here), you only compound the problem of power to weight ratio of hovercraft.

Also, in none of the illustrations of hovertanks that I've seen, there is no sign of the "thruster," i.e. the propellor/fan/whatever that pushes the hovercraft forward. One could argue for vectored thrust fans in the bottom of the hovercraft, but I'm not convinced they could push the vehicle very effectively. (Aside: It may be that they were left out for artistic reasons, but that does not rhyme well with the hard sci-fi theme of 2300AD.) Whatever the case, that thruster is a big achilles heel of a hovercraft, as are the air intakes for the "lifters." By the 24th century, even the dumbest missile could home in on these and cripple the hovertank with a fraction of explosive power required to blast through the "real" armor.

Well, with all these problems I can't say why we would change tracked tanks into hovertanks. The speed of hovercraft is their only advantage, but you don't need to do 200 km/h with a tank and even the modern tracked tanks can do 100 km/h, which is more than adequate. The maneuverability department presents some unique advantages for the hovertank (with vectored thrust a hovercraft could slide sideways, for example), but there are disadvantages equally unique (e.g. as they glide, they are difficult to control effectively), so it all evens out.

I admit, converting hovertanks into conventional tracked ones does take away some of the "coolness factor," but if one wants to stay true to the hard sci-fi theme, it might be a sensible thing to do. Hovercraft could still be used for light recon vehicles, perhaps.

This all IMHO and YMMV, of course.

I've done some work on hovertanks and the power:weight ratios are acceptable, depending on the ground. On level ground like deserts, grassland etc. the hovertank does have sufficient power to maintain lift and full speed. On rough ground they would loose a lot of air from the plenum and require much more power to stay aloft (this is covered in the travel rules).

Over very rough ground (boulders etc.) the hovertank would have to use their vectored thrust jets to cross, at a much slower speed and with more fuel consumption (assumidly from an afterburner).

The hovertank tactically is a cross between a tank and an attack helo. She can go hull down in good ground and fight from there like a tank, or can use her vectored thrust to mount pop-up attacks like an attack helo.

Indeed, since the main AT weapon carried by hovertanks are missiles (ranges of 8-10km vs ~2km for the MDC), the helo vs helo model probably suits hovertanks better.

The speed of the hovertank is a crucial thing. Engagements are fought at 8-10km using missiles, dodging enemy missiles (which are usually top attack and so will smash through) and shooting them down.

The following articles (which I co-wrote 2 of) may be of interest to you:

http://www.users.globalnet.co.uk/~dheb/2300/EG/MontEG.htm
http://www.users.globalnet.co.uk/~dheb/2300/EG/cav/CavEG.htm
http://www.users.globalnet.co.uk/~dheb/2300/USMM9.htm

Bryn

 

[Uncle Bob]

First of all, these designs all look like fun, and I am certain they are in line with the 2300 technical architecture. My following comments should be taken as a criticism of that architecture, not the designs.

Not having access to “canon” equipment design rules, and being too lazy to study fluid dynamics, I decided to make some generalizations with 1980s era hovercraft. I found data for the USN’s ACAV, the commercial version of the US Army’s LACV-30, and a smallish passenger ferry, the AP.1-88. I compare the Cavalier, the M9B4 and the Templar APC (the Montgomery link is dead today).

The Good. The three known ACVs have available power of from 32-45 KW/ton, much less than I thought. The T2300 vehicles have available 28-44 KW/ton, which is more reasonable than I thought at first.

The Bad I think we can take it that the power needed to maintain the plenum will be basically proportional to the weight of the vehicle. The pressure inside the plenum in RW ACVs is 1.5-5 Kilo Pascals, in T2300 vehicles (I may have improperly measured vehicle size) seems to be 12-14 KPa. This increase might be somewhat justified by improved skirt materials, but it will take more power to make up the leakage, since the leakage will be higher velocity (about 20% more power, I think.) Also, being smaller, there is a higher proportion of leaky periphery requiring more power. I am going to ignore issues of pressure loss crossing terrain, except to point out that only the LCAC is expected to cross anything but calm sea and smooth beach and it already has more power/weight than any of the T2300 vehicles.

The Ugly The T2300 hover vehicles move at 180-200 KPH, the RW at about 65-70 KPH. We can ignore rolling resistance (being a hovercraft), but wind resistance will be about 24x as high at that of the current vehicles. Figure about ¼ the frontal area and ½ the drag coefficient, the T2300 still requires about 3x the power of the RW AP.1-88.

My argument. A 45 ton hovertank will need about 2-2.5 MW to maintain hover over open terrain or gentle seas 1-2. It will also need 2-2.5 MW to achieve 200 kph. These minimums are not technology driven, so w can expect them to stay the same in 2300. Thus we need a 4-5 MW powerplant, not a 2 MW.

That is a lot less power than I thought, but I take comfort that we have not yet discussed cornering, complex terrain, slopes and side slopes, or noise and dust (signiture).

I have heard some talk about using ducted fans. I have been assuming large diameter low-pressure fans, which are more efficient than small-diameter, high-pressure fans at low velocity. As for jumping an AFV (leaving ground effect briefly), judging from helicopters and Volanters, you need a close to 200 KW/ton (for a large fan, probably 300 for a ducted fan). For a 45-ton tank, that would require about 85 MW.

Any holes you can knock in my reasoning would be appreciated.

 

[Nurd_boy]

did anyone mention the vectored movement? like sliding on ice, even if you chnge facing, one still continues in the last direction until force is applied...wich means piloting one is difficult without proper training.....

 

[Ellros]

I could see that Russian system being useful at long ranges, but not shorter. The radar and defensive weapons would be very vulnerable to machine-gun and light cannon fire. But it's an interesting concept nonetheless.

IE... a Bradley can use its 25mm to strip that radar off, then send a TOW into the tank.

 

[TJP]

First, thanks for the links Bryn. Excellent articles. Love the amount of detail you've poured into the descriptions (great illustrations!). By providing design and service history etc., you've also provided "personality" to the designs, that makes the vehicles much more "real," they're more than just numbers on paper. (The good ol' Desert Rats are still going strong!)

In addition to the points mentioned above by Uncle Bob and Nurd_boy, a hovercraft's disadvantage in a sci-fi setting (featuring other planets than Earth) is that it's susceptible to variations in atmospheric pressure. While it's relatively painless to make a tracked (or wheeled) vehicle that works in vacuum environment, one cannot make a hovercraft that works in vacuum. In a less extreme example, a low atmospheric pressure planet would make hovercrafts lose lifting power and so possibly "grounding" them, while tracked vehicles would not be similarly affected. OK, such planets might also have lower gravity, but not necessarily.

Speed is a good defense (dodging etc. to not get hit), but stealth is even better. If you're not detected, you don't have to dodge since noone's firing at you. Also, I'm not completely sold on the idea of dodging missiles. I believe if a tank gets targeted by a missile it's best defense is in decoy dispencers and point defense (by 24th century targeting computers should be smart enough to be able discern incoming missiles from all the other stuff flying about in a battlefield - but it's not a case of installing the CIWS/Phalanx system of US carriers on tanks; plus if the missile is not a top-hitting one, there's a real danger of firing on near-by accompanying infantry with your PD minigun in the process; counter-missiles might be good tho'). Dodging is really the last defense I would depend on. Better to not get fired on in the first place, hence stealth is life. IMHO it's a lot easier to make a stealthy tracked tank than a stealthy hovertank.

Just my 2 cents.

PS. Should we move to the issue of combat walkers (or man-portable plasma guns) next?

 

[Uncle Bob]

Originally posted by Ellros:
I could see that Russian system being useful at long ranges, but not shorter. The radar and defensive weapons would be very vulnerable to machine-gun and light cannon fire. But it's an interesting concept nonetheless.

IE... a Bradley can use its 25mm to strip that radar off, then send a TOW into the tank.

Well the system is armored vs small arms, but probably not against cannon.

But what do you expect the 125mm gun on the T-90 is doing while you are peppering it with 25mm? Also, remember the TOW has a 3.75 km range, the 125mm has a 4 km range (5 km w/ Refleks missile), and the 25mm is only accurate to about 1-1.5 km.
Tanks with Arena also have thermal imagers.

 

[Uncle Bob]

The latest in point defence: the Boeing SLID
http://www.boeing.com/defense-space/missiles/slid/slid.htm

And the Russian version, Arena. Maybe not as good (losses are only reduced by 30-40%) but already in service.
http://armor.vif2.ru/Tanks/EQP/arena.html

BTW, let me plug Vasiliy Fovanov's Modern Russian Armour page, where I got that information.
http://armor.vif2.ru/

 

[BMonnery]

Originally posted by Uncle Bob:
First of all, these designs all look like fun, and I am certain they are in line with the 2300 technical architecture. My following comments should be taken as a criticism of that architecture, not the designs.

There is no 2300 design system, they're all modified FF&S designs. To prove the system I tried backengineering the canon vehicles using FF&S, and got a reasonable fit with a couple of alterations (using the materials from NAM for example, Advanced Composite has a density of 2 and AV6 per cm)

Not having access to “canon” equipment design rules, and being too lazy to study fluid dynamics, I decided to make some generalizations with 1980s era hovercraft. I found data for the USN’s ACAV, the commercial version of the US Army’s LACV-30, and a smallish passenger ferry, the AP.1-88. I compare the Cavalier, the M9B4 and the Templar APC (the Montgomery link is dead today).

I'm not a fluid dynamisist, but I dug out the H/C air cushion formulae about a year back and made a spreadsheet:

http://www.geocities.com/littlegreenmen.geo/ACV.xls

The Good. The three known ACVs have available power of from 32-45 KW/ton, much less than I thought. The T2300 vehicles have available 28-44 KW/ton, which is more reasonable than I thought at first.

The Bad I think we can take it that the power needed to maintain the plenum will be basically proportional to the weight of the vehicle. The pressure inside the plenum in RW ACVs is 1.5-5 Kilo Pascals, in T2300 vehicles (I may have improperly measured vehicle size) seems to be 12-14 KPa. This increase might be somewhat justified by improved skirt materials, but it will take more power to make up the leakage, since the leakage will be higher velocity (about 20% more power, I think.) Also, being smaller, there is a higher proportion of leaky periphery requiring more power. I am going to ignore issues of pressure loss crossing terrain, except to point out that only the LCAC is expected to cross anything but calm sea and smooth beach and it already has more power/weight than any of the T2300 vehicles.

Surprisingly (or not) the size of the plenum makes a huge difference. The air pressure needs to be equal to the ground pressure of the plenum, so 40 tons divided over 40 sqm is only 10,000 pascals of pressure and at 40% efficiency 1.05MW is required to maintain the plenum.

The Ugly The T2300 hover vehicles move at 180-200 KPH, the RW at about 65-70 KPH. We can ignore rolling resistance (being a hovercraft), but wind resistance will be about 24x as high at that of the current vehicles. Figure about ¼ the frontal area and ½ the drag coefficient, the T2300 still requires about 3x the power of the RW AP.1-88.

My argument. A 45 ton hovertank will need about 2-2.5 MW to maintain hover over open terrain or gentle seas 1-2. It will also need 2-2.5 MW to achieve 200 kph. These minimums are not technology driven, so w can expect them to stay the same in 2300. Thus we need a 4-5 MW powerplant, not a 2 MW.

I assume this is a M-9, so about 0.95MW is required for the hover. Another 0.5MW (roughly) in forward accelleration gives about an accelleration of 11 ms-2, just over 1G (at 100% eficiency). More than this will create crew problems (imagining a tank crew getting G-LOC). I can't comment on drag though...

That is a lot less power than I thought, but I take comfort that we have not yet discussed cornering, complex terrain, slopes and side slopes, or noise and dust (signiture).

Signature will be a huge problem, with similar signature to helicopters (those who've been in an LZ will know what that's like). The best bet is for a sprint and stop approach. Slopes will cause major problems, as will woods etc. and here is where the jump jet capability comes in. With vectored thrust from the duct fans you can produce enough thrust to hover without the plenum, making it a classical "helicopter", although the rules state that speed is quartered and fuel consumption increased 10-fold ISTR.

I have heard some talk about using ducted fans. I have been assuming large diameter low-pressure fans, which are more efficient than small-diameter, high-pressure fans at low velocity. As for jumping an AFV (leaving ground effect briefly), judging from helicopters and Volanters, you need a close to 200 KW/ton (for a large fan, probably 300 for a ducted fan). For a 45-ton tank, that would require about 85 MW.

Duct fans are more efficient at subsonic velocities and are canonical (the diagrams in GVG shows them but never mentions them).

As for jumping, you just need to put more than 1G of thrust downwards. For a 45 ton tank that's (45,000kg * 10ms-2) = 0.45 MW, although obviously you'll need more than this (extra for non-100% efficient and extra to boost the craft up).

Any holes you can knock in my reasoning would be appreciated.

and visa versa...

Bryn

 

[Jame]

Where does one get one of these hover-ACVs, anyway (and I do mean fully armed, so keeping it is another story)?

 

[aramis]

Ducted fan VTOLS will still gain bonuses from ground effect... and thus will probably stay near ground when cruising patrol. Effectively, a skirtless (and low-efficiency) hovercraft.

The biggest problem with the DF systems is vulnerability to particle intake; this isworsened in ground efffecct by the "Dusting" effect...

With FF&S like PP tech, such devices can be made feasable for up to a week of cruise.... or a day or two with major armaments (Both power requirement changes and loss of fuel capacity).

 

[Uncle Bob]

Well, I'm having fun. Thanks for the spreadsheet.

Originally posted by BMonnery:
I'm not a fluid dynamisist, but I dug out the H/C air cushion formulae about a year back and made a spreadsheet:
]http://www.geocities.com/littlegreenmen.geo/ACV.xlshttp://www.geocities.com/littlegreenmen.geo/ACV.xls[/i[/i]

Nice spreadsheet. I threw my real-world hovercraft at it and got a space of 4-6 cm for my vehicles. I think a 1 cm gap is optomistic anywhere but a skating rink, but YMMV.

I assume this is a M-9, so about 0.95MW is required for the hover. Another 0.5MW (roughly) in forward accelleration gives about an accelleration of 11 ms-2, just over 1G (at 100% eficiency). More than this will create crew problems (imagining a tank crew getting G-LOC). I can't comment on drag though...

I think you are getting force and energy confused. 0.5 MW will give you one megajoules of energy every two seconds. Applied at 100% efficiency (don't ask me how, see last paragraph) that will accellerate 45 tons to 24 Km/hr (neck-snapping stuff, like a Yugo), 53 kph after 10 seconds and 131 kph after a minute. This is ignoring wind drag which increases with the cube of velocity (I had some good notes about figuring drag from form, but that was three moves ago).

As for jumping, you just need to put more than 1G of thrust downwards. For a 45 ton tank that's (45,000kg * 10ms-2) = 0.45 MW, although obviously you'll need more than this (extra for non-100% efficient and extra to boost the craft up).

Uh, that is 0.45 meganewtons, not megawatts. A ducted fan needs about 20 MW to create 0.45 meganewtons of lift, a big fan (helicopter) about 9 MW

Hmm, that means your 0.5 MW horizontal thrust of the M9 applied through a ducted fan will give you a thrust of 0.011 meganewtons. That will give you an accelleration of 0.24 m/s2. After 10 sec thats 8.6 KPH, after a full minute 51 Km/hr. That is about 15% efficiency.
Or, looking at it another way, it is not nearly enough to keep you from sliding backwards off a 1:12 wheelchair ramp.

 

[El Darbo]

An interesting side note here:
The US navy, in co-operation with the Royal Navy Of Thailand are currently testing a new vehicle that is part hovercraft, part Ground Effect Vehicle for maritime patrol and attack.
If you have ever seen a pelican or other seabird scim along a wave without flapping for a great distance, this is the ground effect at work.
Apparently, from the pix in this mag. that I saw at the North Island NAS (sitting on a table in the waiting room of the base dentist), this thing uses ducted fans for lift/movement, and at a certain speed it can break free of the water surface and use these stubby wings to sort of hold it aloft, thus using less fuel than the brute force of just usinf the fans. It can hover to about 10m or so, but it sounded ungainly and unmaneuverable doing this. It looked like it had a ock-up of a chain gun or AC on it, but the article talked about surface to surface anti-ship missiles, and SAMs. Didn't sound liek they were going to use it as an AFV or APC, didn't look like the system could be robust enough.
Though not a true hovertank, this hybrid might be the direction of things. As I recall, the russians had a bunch of GEV designs too, but mostly as transports. I'll have to dig and see what they were.....

 

[Tzeentch]

Arena has not been deployed, it's being marketed in developmental form (it's not a final system) at the moment to fund additional research). The only deployed active protection system is the old Drozd.