So, as many of you likely know, Toyota is planning to tow the Endeavour for the last portion of its trip to the California Science Center. This is a great publicity stunt, and I'll be there to cover it, but I'll admit, I suspect that what we'll be seeing isn't the full story.
First of all, there's the question of what rig the Shuttle will be on when it's being towed. The rig that's getting the Shuttle to the Science Center you may note has a total number of zero trucks pulling it. That's because the rig itself is made up of four independent tractor units that provide the motive force.
So, if this is the rig that will be used when the Tundra is hitched up, we can reasonably conclude that Toyota has proven that the Tundra is the equal to the work that can be done by zero trucks. Impressive!
Perhaps the tractor units will be powered down, or maybe throttled back? Honestly, I don't know yet, but if this is the rig used, I'll be sure to try and find out. If these tractor units are in place during the Tundra's tow, then I would take anything that happens with a large, delicious grain of salt.
Now, it's possible they'll remove the tractor units and place the Shuttle on a different, unpowered rig (this has been suggested in their materials); if so then the demonstration will be significantly more impressive. The stock towing capacity of a 5.7 L V8 Toyota Tundra (with towing package) is listed on Toyota's own site as 9000 lbs— though I've also seen a figure of 10,000 lbs. That's a lot, but it's nowhere near the 172,000 lbs of orbiter and however heavy the towing rig itself is. All told, for the full, motorized rig, it comes in near 300,000 lbs. Maybe that'll be less with another towing rig, but it's still likely in the 200,000 lbs range.
So, that's a tidy 190,000 lbs or so over the rated capacity of the truck. Can it do it?
Probably. I don't think Toyota would even bother if the truck would just sit there, revving until something catches on fire. Hell, I once towed a Dodge Duster up a hill in my '73 Beetle, so I know, for at least short periods of time, you can tow surprising amounts of weight with fairly modest-seeming machinery.
We can compute drawbar pull (the actual amount that can be physically pulled by the truck) fairly easily. The equation is:
Drawbar Pull = Torque x Gear Reduction ÷ Tire Radius - Rolling resistance of surface
Now, I don't know all the variables, but based on some rough estimates and calculations (~400 lb-ft of torque, 18" tires, 10.1 gear ratio, etc) that comes to around 2500 lbs of drawbar pull for the Tundra. This is rough, so let's be super-generous and say it's 3500 lbs. To compare this number to something we know, one of those little tugs made by TUG that pulls commercial airliners around airports has a drawbar pull rating of 10,000 lbs. And it gets that insanely high number with a 3.1 L diesel engine making 87 HP and 199 lb-ft of torque, which should show how important the gearing is.
So, if one of those tough little TUGs is the sort of thing normally used to do this sort of job, I'd think something would have to be up with the Tundra pulling the Shuttle. I'm sure it'll do it, but I'll be curious to see how they manage it— can the truck be temporarily overtaxed, will a lesser drawbar pull be sufficient, anyway (wheels do help a lot— think about how you can pull a car on a flat surface— or how people can pull big things on wheels), or will the rig be (at least partially) powered?
I'll find out whatever I can and let you know!