Thursday, May 28, 2009
I've been interested in tires for military vehicles for a long time. That may sound like an odd sort of interest (well, if you've been reading this blog for long, you know I have lots of interests like that!), but it stems from my own military service, and seeing at first hand the demands placed on such tires.
You see, military tires have a much harder time than most 'civilian' models. They get shot at, blown up (both directly, by land mines beneath them, and at a distance, by improvised explosive devices or artillery and mortar shells that land near them), and driven at insane speeds over rough terrain as the vehicle drivers and crews pursue an enemy, or hunt for cover, or try to reach a designated point as quickly as possible to meet some operational requirement. The vehicles they're mounted on are routinely overloaded to a mind-boggling extent, under-maintained (wheel alignment? What wheel alignment?), driven in a way that no self-respecting owner would ever treat his personal vehicle, and expected to (literally) 'soldier on' beneath the burden of such abuse for years on end. The tires end up taking most of the pounding.
In the South African military, we got used to a rather harder ride than usual out of our tires, because they were half-filled with water. This was a very useful trick to minimize the effect of landmine blasts. When a mine went off beneath the wheel, the water instantly damped down the flash and blast effects, making injuries from that cause much less severe. (Of course, tire life was reduced by about 50%, and the wheels rusted badly, but these were regarded as minor irritants compared to the benefit in reduced casualties. Tires and wheels could be replaced relatively cheaply. Dead, maimed and injured soldiers couldn't.) Added to the mine-resistant design of South African armored fighting vehicles such as the Buffel, Casspir and Ratel (which, along with some Rhodesian vehicles, were the first in the world so designed: many of their features are now included in the MRAP's being fielded by the US military), this trick saved more than a few of us.
You can imagine, therefore, that my interest was piqued by a recent post by the indispensable Al Fin, who wrote of a new 'airless tire' being developed by a Wisconsin firm. Intrigued, I delved deeper, and found the Web site of Resilient Technologies LLC. This company is developing a new concept of automotive tire for the US military. Prototypes are already under evaluation on Humvees of the Wisconsin National Guard.
Instead of conventional sidewall construction, the so-called 'NPT' (for 'non-pneumatic tire) comprises a honeycomb structure of hexagonal cells, supporting a solid tread. It's made of a mixture of polymers, details of which aren't being made public at this stage. Here's a prototype mounted on a Humvee.
As you can see, there's no sidewall at all. The tires are said to be capable of carrying a load of 3,850 pounds each, and are built for a service life under all operating conditions of at least 15,000 miles. (That may sound dismally low compared to civilian auto tires, but remember, the military versions will run at all speeds, under massive overloads, on all surfaces - or none - and in any weather. Under such conditions, 15,000 miles is pretty good!).
Under load, the tires deform, but the cells prevent them from flattening out completely. Here's a picture of a prototype of the new tire under full load.
Resilient Technologies claims that this form of tire construction will be impervious to deflation by penetration or explosion (from bullets, shrapnel, nearby blasts, etc.). No word on how it'll fare when running over a landmine, but I daresay they'll be testing for that as well at some point. They say that even if up to 30% of the honeycomb cells are severed by flying shrapnel, the wheel will still function.
I'm not sure how this open structure would perform in thick mud or deep snow. Would the mud or snow build up inside the honeycomb cells, forming a solid bar as it dried or froze, and preventing the cells from deforming under load or providing cushioning to the suspension? I guess Resilient Tech has thought about that, and will be testing it with their prototypes. Perhaps some form of flexible sidewall will be needed, to keep that sort of thing out of the cells.
Resilient says that they expect to be able to ship a production unit by 2011, and that it should cost about the same as current Humvee tires. If it's a success, I presume more models and sizes will be developed to fit other vehicles. I'd be interested to see whether this technology could be used for aircraft tires. Over- or under-inflation can cause burst tires on take-off or landing, or contribute to heat buildup that can spark fires in the undercarriage. On some specialist 'bush' aircraft, such as the Piper Super Cubs used in Alaska, their 'tundra tires' are so fat, low-pressure and delicate that some models can't be used on hard surfaces like concrete or tarmac, for fear they might burst. If this new technology can do away with the need for tire inflation altogether, it might be a significant safety enhancement for many aircraft.
There's also the intriguing thought of eventual civilian use. Imagine a tire that would never puncture, never need new valves or inner tubes, and wouldn't have a nice neat sidewall to scuff against the pavement! Even better, it's made of polymers, which can be produced from recycled materials as well as coal, oil, and the newer algae-based synthetic fuel industry. When the tires wear out, they can themselves be recycled into new ones. This would save an enormous amount of rubber. I don't know whether this technology will lend itself to high-performance sporting tires, but for vehicles like my pickup, it should work just fine. I hope so, anyway!
Thanks to Al Fin for alerting me to this story, and good luck to Resilient Technologies. I hope they can make this work. If you're interested, you can read more at the company's Web site, or in articles here and here.