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oiQMYuz.jpg

 

Tank design is often conceptualized as a balance between mobility, protection and firepower.  This is, at best, a messy and imprecise conceptualization.  It is messy because these three traits cannot be completely separated from each other.  An APC, for example, that provides basic protection against small arms fire and shell fragments is effectively more mobile than an open-topped vehicle because the APC can traverse areas swept by artillery fires that are closed off entirely to the open-topped vehicle.  It is an imprecise conceptualization because broad ideas like "mobility" are very complex in practice.  The M1 Abrams burns more fuel than the Leo 2, but the Leo 2 requires diesel fuel, while the omnivorous AGT-1500 will run happily on anything liquid and flammable.  Which has better strategic mobility?  Soviet rail gauge was slightly wider than Western European standard; 3.32 vs 3.15 meters.  But Soviet tanks in the Cold War were generally kept lighter and smaller, and had to be in order to be moved in large numbers on a rail and road network that was not as robust as that further west.  So if NATO and the Warsaw Pact had switched tanks in the late 1950s, they would both have downgraded the strategic mobility of their forces, as the Soviet tanks would be slightly too wide for unrestricted movement on rails in the free world, and the NATO tanks would have demanded more logistical support per tank than evil atheist commie formations were designed to provide.

 

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So instead of wading into a deep and subtle subject, I am going to write about something that is extremely simple and easy to describe in mathematical terms; the top speed of a tank moving in a straight line.  Because it is so simple and straightforward to understand, it is also nearly meaningless in terms of the combat performance of a tank.

 

In short, the top speed of a tank is limited by three things; the gear ratio limit, the power limit and the suspension limit.  The tank's maximum speed will be whichever of these limits is the lowest on a given terrain.  The top speed of a tank is of limited significance, even from a tactical perspective, because the tank's ability to exploit its top speed is constrained by other factors.  A high top speed, however, looks great on sales brochures, and there are examples of tanks that were designed with pointlessly high top speeds in order to overawe people who needed impressing.

 

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When this baby hits 88 miles per hour, you're going to see some serious shit.

 

The Gear Ratio Limit

 

Every engine has a maximum speed at which it can turn.  Often, the engine is artificially governed to a maximum speed slightly less than what it is mechanically capable of in order to reduce wear.  Additionally, most piston engines develop their maximum power at slightly less than their maximum speed due to valve timing issues:

 

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A typical power/speed relationship for an Otto Cycle engine.  Otto Cycle engines are primitive devices that are only used when the Brayton Cycle Master Race is unavailable.

 

Most tanks have predominantly or purely mechanical drivetrains, which exchange rotational speed for torque by easily measurable ratios.  The maximum rotational speed of the engine, multiplied by the gear ratio of the highest gear in the transmission multiplied by the gear ratio of the final drives multiplied by the circumference of the drive sprocket will equal the gear ratio limit of the tank.  The tank is unable to achieve higher speeds than the gear ratio limit because it physically cannot spin its tracks around any faster.

 

Most spec sheets don't actually give out the transmission ratios in different gears, but such excessively detailed specification sheets are provided in Germany's Tiger Tanks by Hilary Doyle and Thomas Jentz.  The gear ratios, final drive ratios, and maximum engine RPM of the Tiger II are all provided, along with a handy table of the vehicle's maximum speed in each gear.  In eighth gear, the top speed is given as 41.5 KPH, but that is at an engine speed of 3000 RPM, and in reality the German tank engines were governed to less than that in order to conserve their service life.  At a more realistic 2500 RPM, the mighty Tiger II would have managed 34.6 KPH.

 

In principle there are analogous limits for electrical and hydraulic drive components based on free speeds and stall torques, but they are a little more complicated to actually calculate.

 

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Part of the transmission from an M4 Sherman, picture from Jeeps_Guns_Tanks' great Sherman website

 

The Power Limit

 

So a Tiger II could totally go 34.6 KPH in combat, right?  Well, perhaps.  And by "perhaps," I mean "lolololololol, fuck no."  I defy you to find me a test report where anybody manages to get a Tiger II over 33 KPH.  While the meticulous engineers of Henschel did accurately transcribe the gear ratios of the transmission and final drive accurately, and did manage to use their tape measures correctly when measuring the drive sprockets, their rosy projections of the top speed did not account for the power limit.

 

As a tank moves, power from the engine is wasted in various ways and so is unavailable to accelerate the tank.  As the tank goes faster and faster, the magnitude of these power-wasting phenomena grows, until there is no surplus power to accelerate the tank any more.  The system reaches equilibrium, and the tank maxes out at some top speed where it hits its power limit (unless, of course, the tank hits its gear ratio limit first).

 

The actual power available to a tank is not the same as the gross power of the motor.  Some of the gross horsepower of the motor has to be directed to fans to cool the engine (except, of course, in the case of the Brayton Cycle Master Race, whose engines are almost completely self-cooling).  The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat.  As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.

 

Once the power does hit the drive sprocket, it is wasted in overcoming the friction of the tank's tracks, in churning up the ground the tank is on, and in aerodynamic drag.  I have helpfully listed these in the order of decreasing importance.

 

The drag coefficient of a cube (which is a sufficiently accurate physical representation of a Tiger II) is .8. This, multiplied by half the fluid density of air (1.2 kg/m^3) times the velocity (9.4 m/s) squared times a rough frontal area of 3.8 by 3 meters gives a force of 483 newtons of drag.  This multiplied by the velocity of the tiger II gives 4.5 kilowatts, or about six horsepower lost to drag.  With the governor installed, the HL 230 could put out about 580 horsepower, which would be four hundred something horses at the sprocket, so the aerodynamic drag would be 1.5% of the total available power.  Negligible.  Tanks are just too slow to lose much power to aerodynamic effects.

 

Losses to the soil can be important, depending on the surface the tank is operating on.  On a nice, hard surface like a paved road there will be minimal losses between the tank's tracks and the surface.  Off-road, however, the tank's tracks will start to sink into soil or mud, and more power will be wasted in churning up the soil.  If the soil is loose or boggy enough, the tank will simply sink in and be immobilized.  Tanks that spread their weight out over a larger area will lose less power, and be able to traverse soft soils at higher speed.  This paper from the UK shows the relationship between mean maximum pressure (MMP), and the increase in rolling resistance on various soils and sands in excruciating detail.  In general, tanks with more track area, with more and bigger road wheels, and with longer track pitch will have lower MMP, and will sink into soft soils less and therefore lose less top speed.

 

The largest loss of power usually comes from friction within the tracks themselves.  This is sometimes called rolling resistance, but this term is also used to mean other, subtly different things, so it pays to be precise.  Compared to wheeled vehicles, tracked vehicles have extremely high rolling resistance, and lose a lot of power just keeping the tracks turning.  Rolling resistance is generally expressed as a dimensionless coefficient, CR, which multiplied against vehicle weight gives the force of friction.  This chart from R.M. Ogorkiewicz' Technology of Tanks shows experimentally determined rolling resistance coefficients for various tracked vehicles:

 

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The rolling resistance coefficients given here show that a tracked vehicle going on ideal testing ground conditions is about as efficient as a car driving over loose gravel.  It also shows that the rolling resistance increases with vehicle speed.  A rough approximation of this increase in CR is given by the equation CR=A+BV, where A and B are constants and V is vehicle speed.  Ogorkiewicz explains:

 

 

Nevertheless, the description of the rolling resistance coefficient provided by

equation 10.2 is adequate for many purposes and in the common case of tanks
fitted with tracks having double, rubber-bushed pins and rubber pads and running
on hard, smooth road surfaces the constant A, which is in effect the rolling
resistance coefficient at low speeds, may be taken as 0.030 and B as 0.0009. With
all-steel, single-pin tracks the constant A is typically 0.025 and with tracks having
sealed, lubricated pin joints with needle bearings A has been as low as 0.015

 

It should be noted that the lubricated needle bearing track joints of which he speaks were only ever used by the Germans in WWII because they were insanely complicated.  Band tracks have lower rolling resistance than metal link tracks, but they really aren't practical for vehicles much above thirty tonnes.  Other ways of reducing rolling resistance include using larger road wheels, omitting return rollers, and reducing track tension.  Obviously, there are practical limits to these approaches.

 

To calculate power losses due to rolling resistance, multiply vehicle weight by CR by vehicle velocity to get power lost.  The velocity at which the power lost to rolling resistance equals the power available at the sprocket is the power limit on the speed of the tank.

 

The Suspension Limit

 

The suspension limit on speed is starting to get dangerously far away from the world of spherical, frictionless horses where everything is easy to calculate using simple algebra, so I will be brief.  In addition to the continents of the world not being completely comprised of paved surfaces that minimize rolling resistance, the continents of the world are also not perfectly flat.  This means that in order to travel at high speed off road, tanks require some sort of suspension or else they would shake their crews into jelly.  If the crew is being shaken too much to operate effectively, then it doesn't really matter if a tank has a high enough gear ratio limit or power limit to go faster.  This is also particularly obnoxious because suspension performance is difficult to quantify, as it involves resonance frequencies, damping coefficients, and a bunch of other complicated shit.

 

Suffice it to say, then, that a very rough estimate of the ride-smoothing qualities of a tank's suspension can be made from the total travel of its road wheels:

 

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This chart from Technology of Tanks is helpful.  A more detailed discussion of the subject of tank suspension can be found here.

 

The Real World Rudely Intrudes

 

So, how useful is high top speed in a tank in messy, hard-to-mathematically-express reality?  The answer might surprise you!

 

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A Wehrmacht M.A.N. combustotron Ausf G

 

We'll take some whacks at everyone's favorite whipping boy; the Panther.

 

A US report on a captured Panther Ausf G gives its top speed on roads as an absolutely blistering 60 KPH on roads.  The Soviets could only get their captured Ausf D to do 50 KPH, but compared to a Sherman, which is generally only credited with 40 KPH on roads, that's alarmingly fast.

 

So, would this mean that the Panther enjoyed a mobility advantage over the Sherman?  Would this mean that it was better able to make quick advances and daring flanking maneuvers during a battle?

 

No.

 

In field tests the British found the panther to have lower off-road speed than a Churchill VII (the panther had a slightly busted transmission though).  In the same American report that credits the Panther Ausf G with a 60 KPH top speed on roads, it was found that off road the panther was almost exactly as fast as an M4A376W, with individual Shermans slightly outpacing the big cat or lagging behind it slightly.  Another US report from January 1945 states that over courses with many turns and curves, the Sherman would pull out ahead because the Sherman lost less speed negotiating corners.  Clearly, the Panther's advantage in straight line speed did not translate into better mobility in any combat scenario that did not involve drag racing.

 

So what was going on with the Panther?  How could it leave everything but light tanks in the dust on a straight highway, but be outpaced by the ponderous Churchill heavy tank in actual field tests?

 

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Panther Ausf A tanks captured by the Soviets

 

A British report from 1946 on the Panther's transmission explains what's going on.  The Panther's transmission had seven forward gears, but off-road it really couldn't make it out of fifth.  In other words, the Panther had an extremely high gear ratio limit that allowed it exceptional speed on roads.  However, the Panther's mediocre power to weight ratio (nominally 13 hp/ton for the RPM limited HL 230) meant that once the tank was off road and fighting mud, it only had a mediocre power limit.  Indeed, it is a testament to the efficiency of the Panther's running gear that it could keep up with Shermans at all, since the Panther's power to weight ratio was about 20% lower than that particular variant of Sherman.

 

There were other factors limiting the Panther's speed in practical circumstances.  The geared steering system used in the Panther had different steering radii based on what gear the Panther was in.  The higher the gear, the wider the turn.  In theory this was excellent, but in practice the designers chose too wide a turn radius for each gear, which meant that for any but the gentlest turns the Panther's drive would need to slow down and downshift in order to complete the turn, thus sacrificing any speed advantage his tank enjoyed.

 

So why would a tank be designed in such a strange fashion?  The British thought that the Panther was originally designed to be much lighter, and that the transmission had never been re-designed in order to compensate.  Given the weight gain that the Panther experienced early in development, this explanation seems like it may be partially true.  However, when interrogated, Ernst Kniepkamp, a senior engineer in Germany's wartime tank development bureaucracy, stated that the additional gears were there simply to give the Panther a high speed on roads, because it looked good to senior generals.

 

So, this is the danger in evaluating tanks based on extremely simplistic performance metrics that look good on paper.  They may be simple to digest and simple to calculate, but in the messy real world, they may mean simply nothing.

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Nice article Collimatrix! I learned quite a bit. 

 

And considering this: "The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat.  As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.".

 

Does this mean that a hybrid electric system is more efficient? 
The efficiency of a normal electric engine is more than 90%, same for a generator, frequency modulator and transformer. The loss from resistance in the cable should be close to zero, considering the length. 

I may not be a engineer (yet), but from my understanding, making a hybrid-electric propulsion system would be simpler and easier than a current day tank transmissions. But when it comes to cooling i have no idea how that would turn out in a tank, since most electric engines are self-cooled. Or how any mayor component would act for that matter inside a tank, I am more used to the industrial standard equipment. 

 

I could draw up something for you, but the steering system would be quite basic, considering I have no expertise in that field. 

 

Oh and, from what I remember, the CV9030C2 is actually 35-36 ton, and it uses rubberband tracks, so a 40 ton vehicle might be able to use rubberband tracks in the future. 

 

 

Mvh
Xoon.

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Nice article Collimatrix! I learned quite a bit. 

 

And considering this: "The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat.  As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.".

 

Does this mean that a hybrid electric system is more efficient? 

The efficiency of a normal electric engine is more than 90%, same for a generator, frequency modulator and transformer. The loss from resistance in the cable should be close to zero, considering the length. 

I may not be a engineer (yet), but from my understanding, making a hybrid-electric propulsion system would be simpler and easier than a current day tank transmissions. But when it comes to cooling i have no idea how that would turn out in a tank, since most electric engines are self-cooled. Or how any mayor component would act for that matter inside a tank, I am more used to the industrial standard equipment. 

 

I could draw up something for you, but the steering system would be quite basic, considering I have no expertise in that field. 

 

Oh and, from what I remember, the CV9030C2 is actually 35-36 ton, and it uses rubberband tracks, so a 40 ton vehicle might be able to use rubberband tracks in the future. 

 

 

Mvh

Xoon.

 

Interesting.  I knew the new Turkish IFV used band tracks, and broke 30 tonnes, but I did not realize that CV90s did as well.

 

And yes, I think today a diesel-electrical drivetrain would have better efficiency than a mechanical one.  There would be about 90% efficiency converting the output of a diesel motor to electricity, and about 90% going from electrical back to mechanical in the motors.  There would probably still have to be epicyclic final drives, and those are about 97% efficient, so the final efficiency would be about 79%, or about a 7% gain in power at the sprocket over the best mechanical systems.

 

There would be some secondary advantages too.  The diesel-electric would probably provide better initial acceleration because the drivetrain would have a lower moment of inertia.  It would also be possible to separate the main engine and the drive motors with almost no internal space penalty.  You could, for example, have the diesel engine in the back and the drive sprockets up front, and instead of a big, bulky power shaft taking up lots of room under the turret, relatively thin wires could snake around the turret basket.  I have no idea why you would want to do this, but you could.

 

I think it's only fairly recent that electrical motors have become this efficient, however.  The diesel-electric drives that Ferdinand Porsche was so fond of were significantly less efficient than their purely mechanical counterparts.

 

Yuri Pasholok posted a neat thing about comparing T-28, T-29, and T-35 suspensions and how they affect top speed: http://tankarchives.blogspot.ca/2014/02/suspensions.html

 

Ooooh, this is very nice.  It even gives detailed figures on the strength and movement range of the suspension!

 

In an old book on tanks I have back home, it claimed that the Challenger could beat the Leopard 2 cross-country due to a better suspension. Not sure how true that is.

 

That's a bit of a head-scratcher.  The chart from Technology of Tanks shows the Leo 2 as having more total suspension travel, so I am inclined to think that its suspension is generally better.  But there might be enough subtle differences in how they perform that aren't reflected in gross roadwheel deflection numbers that mean that you could set up some sort of test track with just the right number of rocks and hills of just the right size that a Chally would best the Leo 2.

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And yes, I think today a diesel-electrical drivetrain would have better efficiency than a mechanical one.  There would be about 90% efficiency converting the output of a diesel motor to electricity, and about 90% going from electrical back to mechanical in the motors.  There would probably still have to be epicyclic final drives, and those are about 97% efficient, so the final efficiency would be about 79%, or about a 7% gain in power at the sprocket over the best mechanical systems.

 

There would be some secondary advantages too.  The diesel-electric would probably provide better initial acceleration because the drivetrain would have a lower moment of inertia.  It would also be possible to separate the main engine and the drive motors with almost no internal space penalty.  You could, for example, have the diesel engine in the back and the drive sprockets up front, and instead of a big, bulky power shaft taking up lots of room under the turret, relatively thin wires could snake around the turret basket.  I have no idea why you would want to do this, but you could.

 

I think it's only fairly recent that electrical motors have become this efficient, however.  The diesel-electric drives that Ferdinand Porsche was so fond of were significantly less efficient than their purely mechanical counterparts.

 

The main advantages I find with electric engines is the use of regenerative breaks and its simplicity. The regenerative break will allow the vehicle to recover some energy when it uses its breaks. And by only having a shaft directly from the engine into the sprocket, compared to a gearbox, should make it much easier to repair and give it a longer life span. 

And of course, it can allow the vehicle to drive in reverse as fast as forward, which is a great advantage.

I am unsure of how much i would cost, the typical industrial electric engine is very cheap, but I do not know what a gearbox cost in comparison. 

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The Ferdinand tank destroyer and several other petrol electric and diesel electric AFVs had one electric motor per drive sprocket.  Typically, the problem with this arrangement is that the drive motors aren't exactly evenly matched in output, or at the very least that the resistance of the ground underneath each track isn't perfectly even, so driving straight forward is difficult, and the vehicle tends to veer to one side or the other.  Typical mechanical tank transmissions don't split their power for the two drive sprockets until the very end of the power train, so they usually don't have this problem (or have it as badly).  So some sort of mechanical clutch between the two motors for straight running would be desirable.

 

There's also an issue of motor sizing.  If the motors are being used for regenerative steering (one motor acts as a regenerative brake during steering, and the power is fed to the other), then they need to be sized to take some extra power above and beyond what they would take during maximum speed forward movement.

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There's also an issue of motor sizing.  If the motors are being used for regenerative steering (one motor acts as a regenerative brake during steering, and the power is fed to the other), then they need to be sized to take some extra power above and beyond what they would take during maximum speed forward movement.

Or you could have a battery to store the extra power, this system would however be more practical in a wholly electric system. 

If I am not mistaken, all armored vehicles should have a battery.

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Yes; a battery or capacitor or something would also help with initial acceleration because the diesel takes a bit to spin up.

 

Come to think of it, the drive motors probably need to be oversized anyway because turning in tracked vehicles has so much skidding, and consumes more power than going straight forward.

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That's a bit of a head-scratcher.  The chart from Technology of Tanks shows the Leo 2 as having more total suspension travel, so I am inclined to think that its suspension is generally better.  But there might be enough subtle differences in how they perform that aren't reflected in gross roadwheel deflection numbers that mean that you could set up some sort of test track with just the right number of rocks and hills of just the right size that a Chally would best the Leo 2.

 I suppose the difference could be due to the fact that Chally has a hydro suspension while Leopard II is torsion bar.  

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 I suppose the difference could be due to the fact that Chally has a hydro suspension while Leopard II is torsion bar.  

 

That is the most salient difference, although how that would favor the Chally in terms of speed is puzzling.

 

There's a secondary effect of suspension mentioned in that article you put up on your blog where the damping provided by the suspension effectively increases the rolling resistance of the tank when moving over uneven terrain.  The suspension is essentially a big collection of springs, and springs want to act like harmonic oscillators.  Harmonic oscillation would not be good, because it would mean that the tank would pitch nose up and nose down over and over, so there are usually dampers of some sort on the suspension to prevent this tendency.

 

What happens when the tank is moving over rough terrain, then, is that some of the kinetic energy of the tank moving forwards is converted into the harmonic pitching motion.  This harmonic pitching motion is then damped out and converted into waste heat.  So, the higher the damping coefficient, the more of the kinetic energy of the tank gets converted into waste heat, which effectively raises the rolling resistance coefficient.

 

Steel springs have very little built-in damping because they work extremely well as springs; that is, almost all the energy put into deforming them can be extracted from them as kinetic energy when they are released to rebound.  So the Leo 2 has auxiliary friction dampers on the first two road wheels (I think) and the last.  This is pretty standard for tanks with torsion bar suspension.  Hydropneumatic suspension usually doesn't require auxiliary damping because all the fluid sloshing around inside loses a fair amount of energy as turbulence, and because compressing the air inside it loses some energy as waste heat (it isn't perfectly adiabatic).  So, absent more specific numbers, I think that the Leo 2 is going to have lower damping losses than the Chally.

 

But what may be the case is that they have different suspension frequencies, so like I said, it might be possible to choose a course with just the right size of rocks and rises that favors the Chally.

 

But I don't understand this sort of suspension behavior particularly well, so someone else may be able to come up with a better explanation of what's going on.

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ok, one question regarding tracks:

 

In the article you mention that a lower ground pressure reduces the loss of power at difficult terrain (soil, snow, lose gravel ect.). And as I understand, on paved road you want tracks that are as narrow and light as possible to reduce the mass of the track. So does this mean that there is a trade off between higher road speed and off-road speed? 

 

And to take it example to the extreme, let's say give a tank extremely wide tracks, with the center center to contact ratio of 1, would the size of the track cancel out the advantage of lower ground pressure?

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ok, one question regarding tracks:

 

In the article you mention that a lower ground pressure reduces the loss of power at difficult terrain (soil, snow, lose gravel ect.). And as I understand, on paved road you want tracks that are as narrow and light as possible to reduce the mass of the track. So does this mean that there is a trade off between higher road speed and off-road speed? 

 

And to take it example to the extreme, let's say give a tank extremely wide tracks, with the center center to contact ratio of 1, would the size of the track cancel out the advantage of lower ground pressure?

 

I had seen claims that narrower tracks work better on pavement, but not in anything remotely scholarly.  I'm not exactly sure what the mechanism would be whereby wider tracks would cause more friction.  Sure, they have more contact area, but the force is also spread out over a greater contact area.  Friction is just normal force times mu.  So unless there's something that I'm missing, I don't think wider tracks would perform any worse on pavement ceteris paribus.

 

That said, tracks are a surprising percentage of the weight of an armored fighting vehicle; usually around ten percent.  Tracks don't last all that long either; usually a few thousand miles.  So making them narrower if possible means significant weight and logistical savings.

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You get diminishing returns when it comes to battery size vs storage. Not sure you'd get the efficiency you're after. 

Nice article Collimatrix! I learned quite a bit. 

 

And considering this: "The transmission and final drives are not perfectly efficient either, and waste a significant amount of the power flowing through them as heat.  As a result of this, the actual power available at the sprocket is typically between 61% and 74% of the engine's quoted gross power.".

 

Does this mean that a hybrid electric system is more efficient? 
The efficiency of a normal electric engine is more than 90%, same for a generator, frequency modulator and transformer. The loss from resistance in the cable should be close to zero, considering the length. 

I may not be a engineer (yet), but from my understanding, making a hybrid-electric propulsion system would be simpler and easier than a current day tank transmissions. But when it comes to cooling i have no idea how that would turn out in a tank, since most electric engines are self-cooled. Or how any mayor component would act for that matter inside a tank, I am more used to the industrial standard equipment. 

 

I could draw up something for you, but the steering system would be quite basic, considering I have no expertise in that field. 

 

Oh and, from what I remember, the CV9030C2 is actually 35-36 ton, and it uses rubberband tracks, so a 40 ton vehicle might be able to use rubberband tracks in the future. 

 

 

Mvh
Xoon.

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I'm gonna need you to propagate error there pal.

Interesting.  I knew the new Turkish IFV used band tracks, and broke 30 tonnes, but I did not realize that CV90s did as well.

 

And yes, I think today a diesel-electrical drivetrain would have better efficiency than a mechanical one.  There would be about 90% efficiency converting the output of a diesel motor to electricity, and about 90% going from electrical back to mechanical in the motors.  There would probably still have to be epicyclic final drives, and those are about 97% efficient, so the final efficiency would be about 79%, or about a 7% gain in power at the sprocket over the best mechanical systems.

 

 

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You get diminishing returns when it comes to battery size vs storage. Not sure you'd get the efficiency you're after. 

Efficiency in mass/volume and energy storage, or efficiency in the usage of power? 

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Both, sort of. 

 

First, you're speaking of using a diesel/electric system for a moving vehicle, right? Compression  diesel ignition engines are at best 33% +/- 4% efficient. http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.htmlWhich, honestly, is pretty damned good. Chemically stored energy is really the best bang for your buck.

 

Although chemically stored energy is really good, we're only getting around that percentage of usable joules out of it per unit mass. But that's actually really good, and it's mostly all usable. 

 

Using the joules we can steal from this engine, we can then power an alternator and charge up a battery, as you have suggested. This isn't a 100% efficient system, either. It's more efficient, but not 100%. Then we want to use that stored energy to power a vehicle. Again, electric engines are efficient, but not 100% efficient. 

 

And all this means is that were running in circles, bleeding energy from our primary source of energy. 

 

Also, how big will this battery need to be? In a submarine, size isn't that big of an issue, relatively. But on a land vehicle, size and weight are serious numbers to take into consideration. 

 

So I say, why not skip the middle management and use the source of your energy directly? 

 

Plus, if it was a better system it would have been done before. This tech isn't exactly ground breaking. 

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The power flow wouldn't be diesel->generator->battery->motors.  That's just dumb for the reasons you identify.

 

For primary propulsion it would be diesel->generator->motors.  There could be some sort of beefy alternator to power the electronics and turret drive and that sort of stuff (M60 did this IIRC), but you could also have a completely separate APU to run that stuff (Chieftain did this).  The APU has the added advantage that if the tank is defensively holding a position, the main engine can be switched off, but the tank can still have power for the turret, radios, et cetera while burning almost no fuel.

 

The battery would be recharged opportunistically; while the main engine is idling or during regenerative braking, and it would be used mainly to turn the main diesel over, or conceivably give the drive motors a little extra juice during the first few seconds of acceleration while the diesel generator unit is still suffering from turbo lag.  This, combined with the lower moment of inertia of the diesel-electric powertrain would give a meaningful improvement in dash acceleration (such as bounding from cover to cover) even if the sustained maximum speed were the same.

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Regarding the issue of electric motors causing the vehicle to wear of to the side. I have discussed it with some people and they suggest that instead of putting a clutch in-between the electric motors, you have a sensor on each motor, that senses when the tracks get different resistant and compensates. All you need is a controller circuit hooked into the system, which would be needed anyways if you want the fancy western T-bar for tanks. 

This would be a much cheaper alternative, and be more reliable. 

 

This should be how the set up would be for the main power (not including the control circuit):

Or2kxZG.pngThere probably should be a transformer somewhere in there, but I am to lazy to add it. 

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      Appendix 1- armor calculation
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