The Meaning of “Characteristic Velocity” for Tethers

On my earlier post where I first showed the Moravec mass ratio, I introduced the concept of the “characteristic velocity” of a material, without making any attempt to explain what, if anything, this might mean.

When I first read about characteristic velocity, or Vc, I thought it was a very strange thing to be talking about material properties in terms of a velocity. Looking at the equation simply makes me think that we’re talking about some kind of specific tensile strength of a material, and indeed, Vc is a manifestation of that, but a few years ago I found out that Vc actually has a physical meaning, and it actually has to do with velocity.

I found this when I attempted to derive an expression for an untapered tether, in other words, a tether whose cross-sectional area doesn’t change. That’s quite a bit different from the constant-stress, tapered tether we talked about in the Moravec derivation, because an untapered tether will have much less performance for a given tip velocity than a tapered tether.

The characteristic velocity of the high-tensile strength material is a basic figure-of-merit in high-strength tether design, analogous to the specific impulse of a rocket engine. Despite our material advances over the past few decades, the characteristic velocity of today’s materials still limits the performance of a tether system considerably.

When NASA had an active momentum-exchange tether research program, we looked as closely as we could at all of the materials that had the best characteristic velocity, and anything conceptual that might improve it.

Spectra 2000 is the trade name of ultra-high molecular weight polyethylene (UHMWPE), which is a very simple and strong polymer that consists of nothing more than a carbon backbone flanked by hydrogen atoms.

Zylon is the trade name for polyphenylene benzobisoxazole, or PBO. It has a more complex polymeric structure, and is called a “rigid-rod” polymer.

M5 is the trade name for polydiimidazo pyridinylene, or PIPD. It has a polymeric structure very similar to PBO’s, with some key differences. PIPD has hydroxyl groups on its structure, and a few amines and nitrogens that PBO doesn’t, that give M5 the ability to cross-link three-dimensionally and have the potential for greater overall strength.

Another interesting feature of PIPD is the fact that it has a fully pi-conjugated structure, which means that it has alternating single- and double-covalent bonds along its structure. This means, at least in principle, that PIPD might be made to be electrically conductive, since conductive polymers all exhibit pi-conjugated behavior.

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MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999

About Kirk Sorensen

MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999
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