How TUDelft Circle overlooked Power-to-Mass
as key AWES performance figure-of-merit

June 23, 2020, post by Dave Santos
How TUDelft Circle overlooked Power-to-Mass as key AWES performance figure-of-merit

How did TUDelft and its AWES commercialization partners and spinoff ventures mistakenly overlook power-to-mass as the most critical design factor, when that's been axiomatic in Aerospace for over a century? How did a myth take hold that highest L/D was key, and scaling laws were comparatively neglected?

Miles Loyd is modestly proud and a bit mortified of his landmark paper. He told me in 2012 that had he known it would become so iconic in AWE, he would have worked harder on it. there were minor regrets, like ambiguously coining "lift mode" and "drag mode" for "groundgen" and "flygen" architectures, respectively. [Lang, Drachen Foundation, 2004] coined "flygen";"groundgen" added accordingly). The major shortcut in the paper Loyd made was to neglect mass, which, as an expert kite flier and aerospace technologist, he well knew as critical. Again, Loyd was knocking out a quick paper, that specifically noted its calculations omitted mass. Naïve developers overlooked the omission. Excess mass is parasitic of power.

Any AWES model or simulation will make whatever power you like, if you assume enough wind. The engineering trap is when required wind velocity design is far higher than most-probable wind [van der Lind, Makani]. After mass-to-power, the next figure of merit to a kiter is raw area. [Hardham, AWEC2011] presented Makani's in-house heuristic that a fine wing and equivalent power soft wing have a 1-to-10 area ratio, which matches KiteLab field experience. Good enough, soft kite engineers say, how about a hundred times the area? After all, Single-Skin fabric might weigh 1/100th a rigid wing by area. Highest power-to-mass is so critical for scaling that Kite Power's 100m2 LEI, an SS with pressurized frame, would already show quite diminished power-to-mass and require improbable wind merely to lift off. 

TUDelft's oversight was not inevitable. Their founding AWE scientist, former Astronaut and AWE pioneer, Professor Wubbo Ockels knew the power of SS, from his yacht's sails to the power and pilot kites he tested. Solid wings nowhere to be seen, never a generator in the air, Wubbo was characteristically wise. Single Skin had been known most potent by power-to-mass since the '60s [Rogallo, Barrish, NASA], and the NASA Power Wing of that time is still popular, with Cult-like status. It is seen everywhere from paraglider reserves to giant Polar exploration kite sleds. The Ultimate SS kite was developed by [Dave Culp, KiteShip] a radical three-line minimal surface with no bridle lines. Since the FAA required Pilot-in-Command operation, premature automation was not seen desirable.

For those who went for the high-complexity design approach, a perfect storm of engineering design failure came together. Complexity was layered on to patch rampant vulnerabilities of inherent system brittleness of high-mass, high-velocity, control-instabilities, single-line failure-modes, com-link dependence, eVTOL, and so on. Every venture that adopted these dependencies is disastrously short of a Minimum Viable Product (MVP), but many continue to lure massive investment without specific warnings. The developers are increasingly in a state of guilty-knowledge of unrealistic hype.

Google was most unwise and went all in for an over-scaled high-complexity kiteplane with generators; weak, expensive, and dangerous, and failed. All the AWES venture down-selects can be graded on a spectrum of power-to-mass from Google's M600 to Culp's SS. Loyd told me he kept his doubts to himself as Google went nuts with high-mass high-velocity, high-cost, high-risk engineering. So did Joby. Ampyx is the next major crash in AWE waiting to happen. Culp preached "pure rag and string" which pops right back up after crashing. Pure polymer kept at its max working load cannot be beaten by power-to-weight.

Finally, after decades of R&D, with millions spent ignoring power-to-mass design dependence [1], TUDelt and affiliated research is beginning to properly identify power-to-mass as critical. [2] Its probably too little too late for those AWE players.

Failure of premature over-scaling and over-complexity of marginal platforms is nothing new in Aviation R&D (see photos). "History does not repeat, but it rhymes."

AWE is in fact prevailing on the soundest principles long known to sailing, aviation, ship-kite, and kite-sport practice; plus aerospace engineering's deep KIS ethos. Its the most exciting moment for those players.