Optimization Of Battlemech Mass For The Planned Mission

Battlemech designers have operated for centuries on the basic assumption that making a battlemech heavier increased its ability to carry armor and weaponry, and therefore its performance in toe-to-toe combat. This also comes with the assumption that certain weight classes of battlemech are sufficiently better at certain missions than other weight classes - that light mechs make better scouts, for example - that it overrides the reduced combat potential.

When considered in more depth, however, it becomes apparent that the key feature of the battlemech in these cases is not its weight class, but its mobility. An Ostscout makes a better scout than an Atlas, not because of its lighter weight, but because of its superior speed and jump-capability.

As a result of this, we end up with mech designers increasing a mech's weight to increase its combat potential, while keeping the speed the same to maintain the machine's appropriateness for its mission. At the extreme of this mentality, we have the CGR-1A1 Charger, which is classified as an "assault mech" by weight, but, in combat, is barely a match for most light mechs.

The reason for this is that all components of the machine either remain the same weight (as the cockpit does) or scale linearly with increasing mech weight (as the internal structure does), with one exception - the power plant. A quick examination of engine power/weight ratios reveals that an engine's weight increases exponentially with the amount of power it produces, and hence, its ability to drive a mech of linearly increasing weight at a constant speed. Because of this, there comes a point where increasing a mech's weight results in a greater increase in the weight of the engine, and thus a net loss of useful mass.

Dissection of the the Charger design confirms this. Over 84% of the Charger's mass is in the engine, internal structure, and other unavoidable subsystems that do not directly contribute to the combat performance of the machine, leaving only 12.5 tons for weapons and armor. Compare this to the DRG-1N Dragon, which is 20 tons lighter overall and has the same maximum speed. The Dragon's engine, skeleton, et cetera, occupy slightly under 52% of its mass, leaving 29 tons for arms and armor - 16.5 tons more than the 20-ton-heavier Charger has available. Comparison of the combat records and reputations of the Charger and the Dragon reveals that this is more than just a paper advantage.

These figures lead to the startling conclusion that it is possible, in some cases, to increase the combat potential of a mech by reducing its overall mass. The question, then, becomes exactly where the break-even point is, beyond which increasing the mech's mass is no longer a profitable endeavour. Examination of the tonnage required for for each combination of mech weight and speed results in the following table. Cross-referencing the desired mech speed with the tonnage reveals the tonnage that remains after addition of all the essentials. The maximum of this value for each speed is highlighted in red. Any mech lighter than the weight that results in this value can be profitably increased in weight. Any mech heavier should have either its mass or its speed reduced.

Note that reducing speed will always, except in trivial cases where the mech is already extremely light and relatively slow, result in a increase in available mass, but carries with it its own combat penalties in reduced maneuverability. Determination of the appropriate speed for a mech should be based on its intended mission, which is beyond the scope of this document, but the information in the tables can be used to make decisions on this as well.

Speed/Weight Optimization with Standard Fusion Engines
Walking Movement (× 10.8 km/h)
12345678910111213
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a
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50.000.000.000.000.00
104.504.504.004.003.503.503.002.502.002.000.500.00
159.008.508.508.007.506.505.004.504.003.002.501.500.50
2013.5013.0012.5011.5011.009.008.007.006.004.502.000.50
2518.0017.0016.5015.5013.5012.0010.509.006.504.001.00
3022.0021.5020.0018.0016.5015.0012.009.506.502.00
3526.5025.5023.0021.5019.5016.5013.509.503.00
4031.0029.5027.0025.0022.5018.5014.006.50
4535.5033.5031.0028.5024.5020.0012.000.50
5039.5038.0034.5031.5026.5020.008.50
5544.0041.0038.5033.5028.0018.00
6048.5045.0042.0036.5029.0014.00
6552.5049.0045.5039.0028.005.50
7057.0053.0048.0041.0026.50
7561.5057.0051.5042.5022.00
8065.5061.0054.5042.5012.50
8570.0065.5057.5042.50
9074.0069.0060.5041.00
9578.5073.0063.0037.50
10083.0076.5065.0030.50

There are some pieces of data in this table that should be taken note of. The first is that our example mech, the Charger, is well over the ideal weight for a 50 km/h-walking-speed mech. At the other extreme, we have to drop the mech weight clear down to 20 tons before we find another mech with as little tonnage available at that speed as the Charger has. This would imply that the 20-ton FOX-1A Fox is an even match for the Charger in combat. Anecdotal evidence seems to indicate that this is, in fact, the case. The Dragon, on the other hand, and not coincidentally, is exactly at the ideal weight at that speed.

Another interesting datum is that the ideal weight at 40 km/h, the standard speed for most heavy and assault mechs, is actually a plateau across the 75, 80, and 85 ton ranges. This would imply that a BattleMaster's 10-ton weight advantage over a Marauder is nearly irrelevant, as both have the same available mass for weapons and armor. Frequent matchups between the two in real-world situations and in testing have also proven this true.

The introduction of extralight engines and other advanced construction materials alters this picture somewhat. Because the masses of most of the advanced materials scale linearly with mech tonnage, the effect of their use is minimal, though not non-existent. XL engines, like their heavier counterparts, scale exponentially in weight, however, and their effect is outlined in the next table.

Speed/Weight Optimization with XL Engines
Walking Movement (× 10.8 km/h)
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50.250.250.250.250.250.000.000.000.00
104.754.754.504.504.254.254.003.753.503.502.252.001.751.501.251.001.000.500.25
159.259.009.008.758.508.006.756.506.255.755.505.004.503.002.501.751.000.25
2013.7513.5013.2512.7512.5011.0010.5010.009.508.757.006.255.254.002.50
2518.2517.7517.5017.0015.5014.7514.0013.2511.5010.258.757.003.750.75
3022.5022.2521.5020.0019.2518.5016.5015.2513.7511.507.753.50
3527.0026.5024.7524.0023.0021.0019.5017.5013.759.752.75
4031.5030.7529.0028.0026.7524.2522.0017.7512.502.75
4536.0035.0033.2532.0029.5027.2522.7517.00
5040.2539.5037.2535.7532.7529.5023.2511.75
5544.7542.7541.5038.5035.7530.2520.75
6049.2547.0045.5042.2538.5030.50
6553.5051.2549.5045.7539.7528.50
7058.0055.5052.5049.0041.25
7562.5059.7556.5052.0041.25
8066.7564.0060.2553.7538.75
8571.2568.5064.0056.00
9075.5072.5067.7557.50
9580.0076.7571.2558.00
10084.5080.7574.5056.75

The curve for the new Light engines, designed to reduce the mech-killing vulnerability of a bulky engine extending into the side torsos, is a compromise between the more conservative standard engines and the higher-performance but vulnerable XL engines.

Speed/Weight Optimization with Light Engines
Walking Movement (× 10.8 km/h)
123456789101112131415
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a
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50.120.120.120.120.12
104.624.624.254.253.883.883.503.122.752.751.381.000.620.25
159.128.758.758.388.007.255.885.505.124.384.003.252.500.750.00
2013.6213.2512.8812.1211.7510.009.258.507.756.624.503.381.880.00
2518.1217.3817.0016.2514.5013.3812.2511.129.007.124.882.25
3022.2521.8820.7519.0017.8816.7514.2512.3810.126.751.62
3526.7526.0023.8822.7521.2518.7516.5013.508.382.38
4031.2530.1228.0026.5024.6221.3818.0012.124.25
4535.7534.2532.1230.2527.0023.6217.388.75
5039.8838.7535.8833.6229.6224.7515.88
5544.3841.8840.0036.0031.8824.129.88
6048.8846.0043.7539.3833.7522.25
6553.0050.1247.5042.3833.8817.00
7057.5054.2550.2545.0033.88
7562.0058.3854.0047.2531.62
8066.1262.5057.3848.1225.62
8570.6267.0060.7549.25
9074.7570.7564.1249.25
9579.2574.8867.1247.75
10083.7578.6269.7543.62

The high-density Compact engines, on the other hand, are designed to conserve space at the cost of power plant tonnage. They shift the optimal curve in the other direction, reducing the available useful mass at any given tonnage/speed point, but making extra room in the center torso of the mech. Compact engines, unfortunately, are a woefully inefficient way to accomplish this, because engines are usually the heaviest single component of a 'mech, but have relatively little bulk compared to advanced structural materials that represent less weight, and still less weight savings, such as Endo Steel internal structure or Ferro-Fibrous armor.

Speed/Weight Optimization with Compact Engines
Walking Movement (× 10.8 km/h)
12345678910
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a
g
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5
104.254.253.503.502.752.752.001.250.500.50
158.758.008.007.256.505.003.252.501.750.25
2013.2512.5011.7510.259.507.005.504.002.500.25
2517.7516.2515.5014.0011.509.257.004.751.50
3021.5020.7518.5016.0013.7511.507.503.75
3526.0024.5021.2519.0016.0012.007.501.50
4030.5028.2525.0022.0018.2512.756.00
4535.0032.0028.7525.0019.5012.751.25
5038.7536.5031.7527.2520.2510.50
5543.2539.2535.5028.5020.255.75
6047.7543.0038.5030.7519.50
6551.5046.7541.5032.2516.25
7056.0050.5043.5033.0011.75
7560.5054.2546.5033.002.75
8064.2558.0048.7531.25
8568.7562.5051.0029.00
9072.5065.5053.2524.50
9577.0069.2554.7517.00
10081.5072.2555.504.25

For example, the WGT-1LAW/SC Wight uses a Compact engine to achieve a 50 km/h walking speed and provide enough internal space to use bulky double heat sinks, bulkier Endo Steel internal structure, and still bulkier Heavy Ferro-Fibrous armor.

However, the Wight's Compact 175 ton-rated engine masses 10.5 tons, 3.5 tons more than the 7 tons of a standard fusion 175. The Wight's Endo Steel internal structure weighs 1.75 tons, and saves the same, half the weight of standard internal structure for a 35-ton 'mech. The Wight carries 5.5 tons of Heavy Ferro-Fibrous armor, which provides actual protection equivalent to 6.1 tons of Ferro-Fibrous, or 6.8 tons of standard armor, so the Heavy Ferro-Fibrous saves only 1.3 tons compared to standard armor, or a mere 0.6 tons compared to regular Ferro-Fibrous.

The weight savings from the Endo Steel and Heavy Ferro-Fibrous combined are less than the weight cost of the Compact engine, yet the Endo Steel uses almost five times as much space as the Compact engine saves, and the Heavy Ferro-Fibrous uses fully seven times as much.

The Wight could gain more actual protection by using a standard engine, regular Ferro-Fibrous armor that doesn't need the extra room provided by a Compact engine, and just using the 3.5 tons saved to put on more armor — and other equipment, because it only takes 1.5 tons of additional armor to exceed the maximum capacity of the frame.

But it's worse than that. The 210 ton-rated fusion engine that the Wight would need to have a 60 km/h walking speed masses only 9 tons. It requires a larger gyro, but even with that, a 210-rated standard fusion engine weighs in at 0.5 tons less than the 175-rated Compact engine, which is only a little less than the 0.6-ton difference between using Heavy Ferro-Fibrous and regular Ferro-Fibrous, so for only a minor sacrifice in armor, the Wight could be faster with a standard engine. But even that turns out to be unnecessary. The more powerful engine allows more of the Wight's heat dissipation to be internal to the engine, and not having to allocate space external to the engine for one of the bulky double heat sinks saves as much space as using a compact engine does, so using the heavier, less powerful, more expensive compact engine provides literally no advantage. Simply by using a standard engine, the Wight could be faster, keep its Heavy Ferro-Fibrous armor, and still have a half-ton left to use for other purposes. And it would be cheaper, too.

The experimental, super-light XXL engines, like the XL engines, have the effect of shifting the curve of optimal weights further out, making faster, heavier mechs practical, as outlined in the next table. Again, the effects of other experimental materials are negligible for our purposes here.

Speed/Weight Optimization with XXL Engines
Walking Movement (× 10.8 km/h)
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50.330.330.330.330.330.170.170.170.170.000.000.00
104.834.834.674.674.504.504.334.174.004.002.832.672.502.332.172.002.001.671.501.170.00
159.339.179.179.008.838.507.337.177.006.676.506.175.834.504.173.673.172.672.001.17
2013.8313.6713.5013.1713.0011.6711.3311.0010.6710.178.678.177.506.675.673.502.000.00
2518.3318.0017.8317.5016.1715.6715.1714.6713.1712.3311.3310.177.675.672.67
3022.6722.5022.0020.6720.1719.6718.0017.1716.1714.6711.839.004.67
3527.1726.8325.3324.8324.1722.5021.5020.1717.3314.6710.00
4031.6731.1729.6729.0028.1726.1724.6721.5018.0011.50
4536.1735.5034.0033.1731.1729.6726.3322.50
5040.5040.0038.1737.1734.8332.6728.1720.50
5545.0043.3342.5040.1738.3334.3328.00
6049.5047.6746.6744.1741.6736.00
6553.8352.0050.8348.0043.6736.17
7058.3356.3354.0051.6746.17
7562.8360.6758.1755.1747.67
8067.1765.0062.1757.5047.50
8571.6769.5066.1760.50
9076.0073.6770.1763.00
9580.5078.0074.0064.83
10085.0082.1777.6765.50

It may be noted that even the greatly improved efficiency of the XXL engine leaves the Charger overweight, though only by 5 tons, with a mere 175 kilograms less available mass than the 75-ton ideal weight. A margin that small could be made up by utilizing Endo Steel or other advanced construction materials that would provide a slightly larger advantage for a heavier mech.

These same principles apply to vehicles as well, but the differences in vehicle construction and the effects of various vehicle motive types on power train efficiency, the specifics are highly variable. These are covered in this appendix.