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r circuitry of the solar mppt system i used the ltc spice and built a singe solar cell model i then combined 10 of these to be a 10 cell array then i used the ltc7804 test jig as the basis for a full up simulation so instead of regulating the output voltage it now regulates the input voltage the op amp on the left side inverts amplifies and offsets the cell array voltage and adapts it to the ltc7804 1 2v reference the two voltage sources on the left side vtargmppt and voffset will be combined into a low power precision dac from whatever microcontroller i choose to drive this mess one could thevenize the resistance and voltages to form a single source but since no significant current should flow into the node of the op amp the resistance probably does not really matter basic power path is decided now to make a couple more decisions and prototype in the real world with a set of real cells for deciding the mppt tracking point there are several choices random path search percentage of voc what i chose last time table of v vs cell temp i ll play with all three modes i m hoping the cell temp vs v works well the data sheet says vmppt is 5 8v at 25c and is 1 84 mv deg c on the airframe front i ve also been playing with basic aircraft layout using xflr5 to design the airframe while a solar wing really wants to be rectangular the improvement in minimum sink with an additional tapered panel is so large that its probably worth doing even if it has no solar cells winglets have same effect but shadow top side cells so that does not work i really need to get a good target weight i m comfortable with so i can have a target design and then determine gram watt sort of sensitivities to make proper power weight trades an example trade would be to add cells to the tailboom hard to balance as all the weight is aft and so will likely need added wt forward the weight and drag acts 24 hrs a day added power is only useful for 12hrs and then requires added battery wt to use the excess power overnight adding panels in the taper tips are at differnt angle solar incidence than the main wing cells so they need their own mppt tracker does it make sense to add a tracker for 15w of cell running at 1 5 to 2 v so efficiency will be poor with the ltc7408 it has synchronous reftification so that should help at low voltages inductor will be smaller as well speaking of inductors i ve gotten proper aluminum magnet wire quoted at 2 20 ft and 1kft minimum order so i ve 3d printed some air core toroid forms and wound un insulated aluminum craft wire on these made some light inductors trades trades optimizations as far as the eye can see more later posted by paul breed at 12 03 pm 79 comments email this blogthis share to x share to facebook share to pinterest saturday september 17 2022 solar powered planes in 2004 i built a solar powered rc plane you can see pictures and read about it sees some pictures and relive its sad end here old web page from 2004 i m going to try this project again in the 18 years since this flew a number of things have improved microscopic autopilots for rc planes are cots mro 5gm etc i no longer have to build my own long range rc control and telemetry links are cots crossfire elrs dragonlink i no longer have to build my own the solar cells have improved the new sunpower c60 cells are 6 5 gm 3 5w 22 efficient and flexible the a 300 cells i used 18 yrs ago were 12 gm 3w 20 efficient and pretty rigid old cell left new cell right the new cells are pretty flexible on the old plane i had 3 rows of solar cells on each wing each row was wired in series to make about 6 5 volts i then built a 6 channel mppt see wiki mppt the schematic for one channel of my mppt the full 6 channel unit this mppt is setup as a boost converter so the system bus or battery voltage must be above the voltage of the solar string it also allows solar strings of varying size to feed a common bus ie i can have a 4 cell panel on the tail a 4 cell panel on the wing tips and a 2x 10 cell panels on each main wing there are lots and lots of interesting tradeoffs in this space at minimum power the wing drag is primarily induced drag one reduces induced drag by making the wing have a very high aspect ratio and an elliptical lift distribution if you want the absolute minimum power you want a really high lift airfoil like s1210 selig s1210 yet look how thin the trailing edge is and how hard that would be to build lightly heavily under cambered airfoils also have really high pitching moments meaning the airfoil wants to pitch forward and this must be resisted put this airfoil on a really long high aspect ratio wing and its not unheard of for the wing tip to fail in a downward direction the airfoil tip tries to twist forward twisting the wing enough that the wing tip on light structure is now lifting down and bad things ensue one could choose an airfoil designed for flying wings now or very low pitching moment like mh60 much easier to build a very light structure with this trailing edge and the torsional rigidity of the wing will matter a lot less the minimum power airfoil assuming an infinite wing is where cl 3 2 cd is maximum for the 1210 109 for the mh60 its 68 so the minimum sink is roughly proportional to weight 1 2 min power is thus weight 3 2 so for a 1000gm airplane you could add a couple hundred extra grams and be even so if one wanted to fly over night using the potential energy of the aircraft as a battery sink rate is only effected by wt 1 2 where if you wanted to fly all night at constant altitude via battery then assuming the battery is 30 to 50 of the mass reasonable assumption then each extra gram of structure is bad by an additional requirement for more battery like i said a really complex trade space looking at the professional aerospace for hints clues is not really helpful helios was a flying wing that failed due to complex aerodynamic structural and turbulence interactions see report here the solar impulse and airbus zypher both chose a more traditional aircraft platform posted by paul breed at 11 11 am 46 comments email this blogthis share to x share to facebook share to pinterest friday february 22 2019 creating new things in the real world musk tesla spacex etc i m a space geek much of my social interaction is with other space geeks a lot of these people worship what spacex and elon have accomplished at the same time i ve been more and more convinced that tesla is going to fail this causes some significant personal cognitive dissonance the following is my attempt to work this out making a new thing is hard you will fail over and over again you will fail edison tried thousands of light bulb filaments before he found one that worked one of the really cool things about spacex is that they let you see them fail the blooper reel of landing attempts is outstanding attempting to do something new and hard will have failures trying something new in engineering or in business will have failures because of spacex we now know that first stage reuse can make sense we now know that the iterate fast model of engineering works for at least some of the aerospace world supersonic retro propulsion works you can given the right engineering team built rockets for a tiny fraction of what the big aerospace primes charge all of this is new information in the world due to spacex efforts everything above this line is engineering we still don t really know that the spacex way will work in a business sense recent layoffs and recent failed funding raise make this more questionable than it was 12 months ago its clear that the business case for the large aerospace primes does not work absent billions in direct government subsidies from an engineering stand point spacex has been somewhat innovative spacex have either surpassed or are approaching 50 of the total world wide launches in my opinion if ancillary things like tesla don t take it out spacex will survive long term and the world will be a better place because of that see my earlier dialog on why space matters the space business has been run for a long time by cost plus contractors working for governments this is not a place where innovation thrives the car business on the other had has been a brutally competitive market with microscopic margins fought over by efficient international behemoths its a hard brutal world tesla managed to crack open that market and with the s and x they delivered a well received high end quality product that changed the perception of the world now every one of those behemoths is planning an ev just having a shot at this is a huge achievement some how the engineer elon that allows us to see spacex failures has been unable to admit to the world that their are also business failures by any unbiased measure solar city was weeks from failing when tesla bought it this burdens tesla with a millstone that makes the difficult task of breaking into the brutally competitive car market even harder watching the engineer elon do magic things at spacex i greatly admire what he they have accomplished watching the business man elon commit what looks very much like fraud to save a failing business i do not admire business ethics matter and used to matter to elon see his several year old tweet about debts and personal responsibility for solar city debits some how the pressure to not have the business fail has pushed pushed elon to do things that cross the ethical line in a way that has caused me to loose my respect for business man elon the big ones solar city purchase and fraudulent solar roof presentation model 3 production guarantees on the eve of big tesla bond raise screwing over his devoted customer base with the crappy quality m3 and things like not refunding deposits the 420 funding secured tweet and subsequent response the flipping the bird at the sec taunting tweets the recent guidance on tesla that just does not pass the smell test the fraudulent guarantees in the buffalo solar city factory morass treatment of personnel at both spacex and tesla you can only run in start up panic mode for so long before it destroys people i have many space friends that have left spacex because of the relentless pressure it is my opinion that tesla is very close to failing and will fail in the next 24 months it is my hope that this does not destroy spacex elon has generated significant personal liabilities in his tesla dealings and if tesla fails the only real asset elons owns is spacex spacex will be a very different and less dynamic place when its owned by plaintive s lawyers rather than elon i find the whole affair sad i wish elon had stuck to spacex and just focused there i know that some of my followers that see the spacex brilliance and subscribe toe the saint elon can do anything view will rush in to defend tesla i don t want to rehash that argument here lets us just agree to disagree and we will know who is correct in 18 months or so posted by paul breed at 3 03 pm 352 comments email this blogthis share to x share to facebook share to pinterest saturday december 22 2018 low low low cost liquid thrust chambers the very premise of what i m trying to do is low cost as a primary design goal one has to question all of ones assumptions i personally fired the first ever 3d printed regenerativly cooled liquid rocket engine ever fired in the world 2009 i ve done a lot of work in that space the 3d aluminum costs are coming down its cool high tech and in many ways awesome its also the single most expensive hardware component on my rocket by a factor of 3 for the long term business case to work i need to get the single 6 24 foot tube cost ready to fly under 2k a die cast aluminum inner with a saddle jacket outer made from injection molded saddle and thin wall extruded aluminum outer is a rocket motor with a cost of under 200 00 so for the next generation of motors i m going to abandon the 3d printed motors the first rocket motor i designed my self and the 2nd liquid rocket motor i ever fired was saddle jacket and this generation will be as well 113 sec fireing i ll use machined aluminum done on my cnc lathe mill as a stand in for di cast aluminum and the saddle will either be machined aluminum or possibly machined polyethylene as a stand in for injection molded posted by paul breed at 5 14 pm 101 comments email this blogthis share to x share to facebook share to pinterest saturday december 15 2018 fusion360 cad beg i m tryingto convert from rhino to fusion 360 the following was done in rhino i m trying to learn how to do the same in fusion i can easily draw this in fusion a extruded cylinder with a hole in it i can also draw my o ring groove in fusion in rhino i next draw two spheres blue and green then i generate the line where the spheres intersect the cylinder inner intersection lines have been removed for clarity i did this twice and the blue green colors swapped sorry for the confusion i then use the sweep2 command in rhino to make a solid from o ring profile and the two rails this is correctly tangent to the cylinder surface and has the correct o ring breaks radis etc then subtract the solid and i have my o ring grove so my fundamental fusion 360 issues i can t draw the spheres based on points only dimensions the lower dimension is easy to derive its the hole radius the oring offset the outer sphere is a lot harder as the oring groove sides are not straight and while i can place a point in the appropriate place fusion won t let me draw a sphere to the point that problem i could work around with some triganometry but the 2nd problem i can t seem to solve i need to create a pair of lines to sweep the pattern these lines need to be the intersection of a sphere and the cylinder i can t seem to create this geometry no matter how i try advice welcome posted by paul breed at 6 47 am 50 comments email this blogthis share to x share to facebook share to pinterest monday november 19 2018 0 018 matters i had a batch of end caps machined for the carbon tube tank this was 4 outer 0 035 metal liner 0 009 poly bag and 0 004 gap this was all carefully measured and tested it works well i thought i could reuse these end caps in the 4 035 wall tube alas i neglected to account for the bag the 0 009 on both sides or 0 0018 in diameter is enough that the caps don t seal correctly grumble grumble i could line the aluminum tube with the bag and continue not a huge deal but i thought i had enough for 3 sets of flight hardware now i have zero i need to machine new ones so even with the loose fit i thought i d assemble the tank and hydro test one the end cap retention failed at 420psi the cap retaining screws failed in shear in hind sight i did not use enough screws according to the screw stress calculations they should have failed at 390 psi so all in all its nice to actually be building and assembling hardware again so mariellen s garden got its first dose of hydro test water in about 4 years wednesday morning update rebuilt the test tank used more ...