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Horse Bones

Horse Bones Photo Credit: Clipart.com

Thanks to the popular book and movie-adaptation Seabiscuit, horse racing has been in the news a lot lately. But this so-called "sport of kings" has captivated fans for centuries. One reason is the grace and agility of the horses themselves—when they're running at top speed, they look as if they're flying down the track. In this Science Update, you'll hear how studying horses may help engineers improve human flight in air and space.


Transcript

Boning up on aircraft parts. I'm Bob Hirshon and this is Science Update.

Running puts a lot of stress on a horse's leg bones. But according to Andrew Rapoff, a mechanical engineer at the University of Florida, their bones easily withstand that pounding, even in places where you'd expect them to fail.

For example, bones have natural holes in them where blood vessels pass through. Instead of being weaker around these points, the bones almost never fracture there.

So now, Rapoff and his colleagues are studying horse bones to get new design ideas for air- and spacecraft parts.

Rapoff:

Holes exist in aircraft structures and spacecraft structures all over the place. Conduits and hydraulic lines have to be run through structures that penetrate different internal substructures in wings, for example. And those holes represent potential sources of failure.

The researchers found that the bone's composition varies around the holes so that it's denser and stronger wherever the stresses are higher, and less dense where it needs to be more flexible.

Rapoff:

Bone seems to have given the weight-optimized solution, or the minimum-weight solution to these discontinuities.

The researchers are adapting what they've learned from horse bones for use with a wide range of materials. For the American Association for the Advancement of Science, I'm Bob Hirshon.


Making Sense of the Research

This may seem obvious, but from an engineering standpoint, putting a hole in something makes it weaker. The trouble is, sometimes you need a hole. For example, an airplane or a spacecraft might need holes in certain parts for wiring, fuel or water lines, air flow, or any number of other reasons.

To make up for the weakness caused by the hole, engineers usually reinforce the area around the hole with extra material. The drawback to this strategy is that it adds weight, which can be costly. For example, engineers generally estimate that cutting a pound of weight from an airplane saves about ten pounds of fuel. And the savings can really add up over time. This issue is even more critical in space flight, since launching stuff into orbit is far more expensive than flying it on a plane—but even slight structural weaknesses in spacecraft can have catastrophic consequences.

Rapoff and his colleagues focused on the bone around a specific hole called the foramen, found in the third metacarpus bone in the horse's leg. This bone bears the brunt of a racehorse's punishing gait, usually with no problem. And studies have shown that even when it does fracture, it doesn't break near the foramen, although that's what you might normally expect.

When they studied the bone in detail, they discovered its secret. It turns out that the bone isn't uniform throughout. Rather, the internal structure and composition of the bone around the foramen is arranged specifically to shunt the pressure away from the foramen, and toward stronger areas. This compensates for the weakness of the hole without adding excess weight. The researchers then copied this structural pattern in a polyurethane model of the horse bone, and found that it resisted stress much better than another model which lacked that kind of structure.

This research represents a growing field in science called biomimetics. Simply put, biomimetics is the science of studying and copying nature to solve mechanical problems. Other biomimetic projects include attempts to make artificial spider silk (actually one of the strongest flexible substances known), and robotic lobsters that can explore the ocean floor.

Now try and answer these questions:

  1. Why are scientists looking for better ways to make up for holes in planes and spaceships?
  2. What's so special about this particular horse bone?
  3. What was the key difference between the two polyurethane bone models that the researchers tested?
  4. Can you think of other things in nature that human engineers might like to copy?

For Educators

Mechanical Engineering Magazine published Thoroughbred Structures, a feature on Rapoff's research.

NASA's Virtual Skies Biomimetrics site has information and links about the use of biomimetics in flight.

Discover...Anatomy, on The Royal Institution of Great Britain "Inside Out" site, investigates the skeletal system.


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