Vehicle Design: Vehicles that Tow a Load

[R_Kane]

[This is part 1 of 3]

Hey there folks. I was on my lunch break at work, when out of the blue I wondered how one would design a vehicle intended to pull or push an external load; tug-boats, tow trucks, or helicopter cargo-lifters for example. I doubt such vehicles will be needed all that often, but once that idea got stuck in my head, I couldn’t get rid of it.

After looking through the vehicle design rules, I didn’t see any way to model this with the rules as given, so I thought about it for a couple hours and came up with some ideas.

I’m sure that this idea isn’t the most “realistic” model, but I feel it is easy enough to be used quickly and still provides a reasonable sense of the effects of towing a load.

For the rest of my message I use the term “tow” to mean any method of moving a load beyond a vehicle’s own vl…could be pushing, pulling, lifting

Step 1: Determine vehicle chassis size

Step 2: Determine what you would like the vehicle’s Max Speed to be
No matter how much Total Thrust is generated by the vehicle’s Drive Train and Power Plant, this step determines the absolute maximum speed the vehicle will ever travel

Step 3: Determine Tow Capacity - how large a load (vl) you would like the vehicle to be able to move (including the vehicle’s own chassis vl) at Max Speed

Step 4: Calculate the vehicle’s Tow Ratio
= Tow Capacity / Vehicle’s vl

Step 5: Calculate the Thrust needed to move the vehicle’s vl at Max Speed
= Thrust Required (for vehicle’s chassis vl) * Max Speed

Step 6: Calculate the Thrust needed to move the vehicle and tow load at Max Speed
= Results from Step 5 * Tow Ratio

Step 7: Determine the Drive Train/Power Plant combo that results in a Total Thrust equal to the result in Step 6
You may want to generate a few EP’s beyond this value needed for this Total Thrust for systems/equipment other than the Drive Train. Alternately, you could install two separate Power plant systems; one for the Drive Train and the other for the other systems that require EP’s.

Also note, all these EP’s are earmarked for use in towing loads and the vehicle will draw power (EP) as needed from the power plant to support towing a load. If all of these EP’s are not being utilized (pulling a smaller than Standard Tow, for example) any unused EP are NOT considered “extra EP”, and therefore not used to determine Agility. If you want to have Agility, you must generate EP’s above and beyond what is needed to power towing a load at Max Speed.

[continued in next post]

 

[R_Kane]

[part 2 of 3]

Some formulas and definitions -

Tow Capacity: the total amount of vl a vehicle can move at Max Speed, includes vehicle’s own chassis vl

Tow Ratio: a relationship between the amount of vl a vehicle can move at Max Speed (which includes its own chassis vl) and its own vl
= Tow Capacity / vehicle’s chassis vl

Standard Tow: the amount of vl a vehicle can push/pull, not including its own chassis vl
= Tow Capacity – vehicle’s chassis vl

A tow vehicle CAN tow more than its rated Standard Tow, but this will result in a reduction of Max Speed, and by derivation, max accel.

The Max Speed while towing an oversize load is found using the following formula:

Max Spd (oversize load)
= Max Speed (standard) * [Standard Tow vl / (Oversize tow vl – Standard Tow vl)]

I would recommend that Referee’s judge that a vehicle cannot move an oversized tow if the Max Speed is ever reduced below 1 kph by an oversized tow load. Unless you desire to let vehicles have Max Speeds in mph as opposed to kph.

[design example in next post]

 

[R_Kane]

[part 3 of 3]

As an example I designed a TL 8 tug boat which I call the “Super Tug”, since it can pull such an enormous load at a fairly reasonable speed.

Step 1. The Super Tug is based on a 20,000 vl standard chassis.

Step 2. I think a normal Max Speed of 100 kph is reasonable for a Tugboat.

Step 3. I made this an uber-tugboat that has a Tow Capacity of 250,000 vl

Step 4. This results in a Tow Ratio of: 250,000 vl / 20,000 vl = 12.5

Step 5. This chassis requires 20 TH per 1 kph, so to have a Max Speed of 100 kph, it would need to generate 2000 TH.

Step 6. In order to move its Tow Capacity at 100kph,the vehicle needs to generate a Total Thurst of: 12.5 * 2000 TH = 25,000 TH

Step 7. To generate 25,000 TH the Super Tug has 50 water(surface) Drive Train Units (20 TH per Unit per EP applied) and 25 turbine Power Plant Units (1 EP per Unit)
(20 TH /EP * 50) * 25 EP = 25,000 TH

So, now we have a 20,000 vl tugboat capable of pulling a Standard Tow up to 230,000 vl (Tow Capacity – Vehicle’s chassis vl) at a Max Speed of 100 kph.

But what if Super Tug needs to pull a stranded ocean liner (3,000,000 vl)?

While towing the ocean liner the Super Tug will have a Max Speed of:
100 kph * [230,000 vl/(3,000,000 vl – 230,000 vl)] = 8.3 kph

which results in a Max Accel/decel Rate of .83 kph

Not too shabby, considering the ocean liner is 150 times the size of the tugboat. In fact the Super Tug can pull a load of 23,000,000 vl before its Max Speed drops below 1 kph.

[end posting]

Any comments? Feedback is always welcome.

-Roger

 

[tjoneslo]

This is similar to the system written for TA#3, On the Ground. The thing you've forgotten is that your vehicle needs a reenforced hull to withstand the increased demands of the weight being towed or the vehicle will simply shred itself.

During the vehicle design, add a "Towing Capacity" item, which will increase the cost of the hull and size of the "controls" (which includes the frame). Calculate the max speed based upon the larger hull size, with no excess speed nor agility if you are not towing the weight.

Your system is incorrect in that if I want to move a 270,000 vl vehicle (20,000 + 250,000) at 100kph, I should require 27,000 TH. Your system only requires 25,000TH. Why the difference?

You can simplify the overload tow calculation by using the following:
Thrust /((Vehicle VL + Towing vl)/1000))

 

[R_Kane]

Your system is incorrect in that if I want to move a 270,000 vl vehicle (20,000 + 250,000) at 100kph, I should require 27,000 TH. Your system only requires 25,000TH. Why the difference?

Actually, it is not 20,000 + 250,000..it is 250,000 total (towed load + vehicle volume)...so the tow is 230,000 and not 250,000

 

[R_Kane]

This is similar to the system written for TA#3, On the Ground.

D'oh!

The thing you've forgotten is that your vehicle needs a reenforced hull to withstand the increased demands of the weight being towed or the vehicle will simply shred itself.

I did think about that...this morning in the shower ...my original post was based on just a couple hours thought, so i knew I'd probably miss something

You can simplify the overload tow calculation by using the following:

Good point...again, see last comment

How about designing the tows theselves ..trailers, barges, etc

I would guess it is as simple are doing a regualr design except with no drive trains and power plants...unless it is a mobile command post trailer, or similar, then you would add a power plant or batteries for EP's...

Any guesstimates on when TA3 is due out?

-Roger

 

[R_Kane]

Oops, left this question out of my last post

During the vehicle design, add a "Towing Capacity" item, which will increase the cost of the hull and size of the "controls" (which includes the frame).

How much does this reinforcment of the frame increase cost? Does this increase the volume of the base vehicle?

-Roger

 

[tjoneslo]

Originally posted by R_Kane:

How about designing the tows theselves ..trailers, barges, etc

I would guess it is as simple are doing a regualr design except with no drive trains and power plants...unless it is a mobile command post trailer, or similar, then you would add a power plant or batteries for EP's...

As you suggest, simply build as vehicles without drivetrain. You can add a power plant or batteries for power consuming equipment, or simply draw from the towing vehicle.

Any guesstimates on when TA3 is due out?

Real Soon Now. It's in final edit and layout.

 

[tjoneslo]

Originally posted by R_Kane:
Oops, left this question out of my last post

During the vehicle design, add a "Towing Capacity" item, which will increase the cost of the hull and size of the "controls" (which includes the frame).

How much does this reinforcment of the frame increase cost? Does this increase the volume of the base vehicle?

-Roger

Yes. For example, your 20kvl Super Tug which can tow 230kvl would need to be build on a 250kvl hull, which cost kCr250, plus the controls would be 50kvl and cost kCr125, meaning you can't build a 20kvl tug strong enough to haul 250kvl. Your base SuperTug would have to be much larger.

Your example step 7 is incorrect as well, but only because the THB text is worded badly. Each DTU provides the rated thrust (e.g 20 TH) and requires 1 EP. To provide 25,000 TH requires 1,250 DTU (1,250 * 20 = 25,000), which takes 31,250vl, costs Cr156,260 and requires 1,250 EP. The Turbine for that would be 2,500vl, Cr125,000.

This puts a much smaller limit on the size of a trailer or barge the vehicle can tow.

 

[R_Kane]

Yes. For example, your 20kvl Super Tug which can tow 230kvl would need to be build on a 250kvl hull, which cost kCr250, plus the controls would be 50kvl and cost kCr125, meaning you can't build a 20kvl tug strong enough to haul 250kvl. Your base SuperTug would have to be much larger.

Hmm...I'm not seeing this...take a present day tug boat for example; it does not have a chassis/hull big enough to "carry" a large tow, it just has massive pulling power. Granted, like you posted before, the tugboats chassis would have to be reinforced to accomodate the stress put in the frame by the towed load. But I don't see that the boats chassis would have to be big enough to carry the load..inthat case it would just be a big self-propelled vehicle.

Please, correct me if I'm getting a different impression than you are trying to present.

Your example step 7 is incorrect as well, but only because the THB text is worded badly. Each DTU provides the rated thrust (e.g 20 TH) and requires 1 EP. To provide 25,000 TH requires 1,250 DTU (1,250 * 20 = 25,000), which takes 31,250vl, costs Cr156,260 and requires 1,250 EP. The Turbine for that would be 2,500vl, Cr125,000.

OK, that woudl make a difference...I was reading the book that a DT unit provides x TH per each EP applied to it...not that each DTU requires 1 EP...yes indeedy that will change things...

Thanks for your feedback and comments, I do appreciate them. Even if what I came up with will be moot when TA3 arrives...

-Roger

 

[tjoneslo]

For example, we want the super-tug to be able to haul 230kvl at 100kph. In the base vehicle design, in order to haul around 230kvl, you need a control size of 46kvl, or a frame weight/volume if you prefer. Plus you need to support the engine to haul 230kvl at 100kph which would be 28.75kvl. This then becomes a recursive process, which ends up with a vehicle size of 120kvl, a frame of 70kvl (120kvl frame is 24kvl, plus the 46kvl for the cargo = 70kvl). The drivetrain is sized to haul 350kvl (120kvl vehicle + 230kvl cargo), at 43.75kvl.

The way I suggested doing this was to add a line item to the vehicle design spreadsheet titled "Towing Capacity". This volume needs to be accounted for in the Vehicle size, Frame Size, and drivetrain performance characteristics. The volume doesn't need to be accounted for for Armor, Enviornmental controls or count for the maximum size of weapons or turrets mounted. Use the smaller size for the AC and SI calculations.

 

[R_Kane]

OK...I get what you are saying now...maybe I was just being dense before

-Roger

 

[tjoneslo]

No, its good. I'll double check the text in TA3 to make more sure the explination is clear.

 

[manley_t]

I think Tugs are very viable -- and I think space tugs would make more sense than internal cargo holds ..... but that's just me.

In CT I used to always create space tugs. The problem is that you need to have engines and fuel to haul the 'pods' built into the tug itself. This basically means you're riding an engine strapped to a fuel tank......