Article: 1930
\nBy Robert Krausert, January 2010<\/p>\n
Carl Hamilton asked:<\/strong> Ray Stoner commented:<\/strong> Figuring out the actual strength of the bulk plate is beyond me and I’ll leave it to others. The most science I’ve applied to it, is “this looks good enough”. The glue joint is where I’d focus my efforts.<\/p>\n I’ve used as little as a 1\/4″ thick Baltic Birch bulk plate in a 4″ ~20 lbs bird without failure.<\/p>\n Ray Stoner commented:<\/strong> Steve Cutonilli commented:<\/strong> I will bet anyone on this list that the best measure to mitigate stresses to hard-points on your airframe is to use moderate length shock-cord (like we’ve heard of already) AND z-fold shock cord segments then wrap them with tape (figure a dozen segments typical on apogee cordage) – upon deployment the energy absorbed is significant.<\/p>\n Or, attach a separate chute to each airframe segment and don’t worry about it (applies primarily to big massive rockets where hard-point stresses really factors more than normal).<\/p>\n Ken McGoffin commented:<\/strong> And this can also reduce the tangle of shock cord in the rocket.<\/p>\n John Armitage commented:<\/strong> The staggered shock absorbing is similar to, for those of you also in construction, a shock absorbing fall arrest lanyard for a body harness. The Flat nylon is Z-folded then stitched so that the stitches tear thru as the load is arrested.<\/p>\n Also makes loading easier. I’ll bundle all the shock cords ahead of time, keeps ’em nice and neat.<\/p>\n Bob Yanecek commented:<\/strong> This allows minimal force to begin unfolding but ever increasing force as the unfolding continues.<\/p>\n If all z-folds pull loose, then next time around start with extra stitching with a goal of having that last one or two z-folds intact after fully deployed.<\/p>\n This allows you to ‘measure’ just how much force was exerted during deployment.<\/p>\n With this technique, you can quite accurately determine force\/stitch required to pull it loose.<\/p>\n I do something similar with blue tape and Kevlar but this does not allow much control over the force required to pull any particular z-fold loose. I do try and tape the begeesies out of the first z-folds and always look to see if they pull loose or not.<\/p>\n My rational is that I’m using 1\/8″ braided Kevlar and it’s kind of tough to stitch accurately (and still get 20’+ in 2″ of 38mm airframe).<\/p>\n Greatest risk is if your harness is too short. Either the shock of hitting the end locates a weak link or the recoil allows the parts to crash back together with a myriad of results. I recovered once with the weirdest hole in my upper airframe before (thanks again Keith!), determining the hole matched the eyebolt in my fin can.<\/p>\n Ken McGoffin commented:<\/strong> But I can fly the same rocket on the same motor under the same conditions and end up with differing numbers of unopened bundles. It’s not an exact science but it is good practice IMO.<\/p>\n Ken McGoffin commented:<\/strong> Article: 1930 By Robert Krausert, January 2010 Carl Hamilton asked: Is there some easy way (i.e. for a non-materials engineer) to determine, roughly, how strong a bulk plate is going to be given a material, thickness, and diameter? Is there an easy way to determine how that strength will be effected by various holes drilled […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1903,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":{"ngg_post_thumbnail":0},"_links":{"self":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1930"}],"collection":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1930"}],"version-history":[{"count":1,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1930\/revisions"}],"predecessor-version":[{"id":1931,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1930\/revisions\/1931"}],"up":[{"embeddable":true,"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=\/wp\/v2\/pages\/1903"}],"wp:attachment":[{"href":"https:\/\/northwestrocketry.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1930"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
\nIs there some easy way (i.e. for a non-materials engineer) to determine, roughly, how strong a bulk plate is going to be given a material, thickness, and diameter? Is there an easy way to determine how that strength will be effected by various holes drilled through the bulk plate? By strong, I mean, “How much force can be applied to the bulk plate through an attached U-bolt before it fails?” Is that shear strength?<\/p>\n
\nYour bulk plate will generally be weaker than the glue joint in the body tube. I don’t believe I’ve ever seen a bulk plate fail in a “normal deployment”, nor have I seen many fail in a high speed deployment. It’s usually the recovery harness, or the ‘chute. Virtually all of the failures I’ve seen are in the glue joint between the bulk plate and the body tube.<\/p>\n
\nThe bulk plate will usually be STRONGER then the glue joint.<\/p>\n
\nIt all comes down to chasing the weak sister in your system. I like Ray’s description of the glue joint going first – well, yeah except some of us always use a sliver of coupling tube on the load side of the bulkplate to increase the glue’s shear resistance and then one wonders why the eye-bolt yielded next.<\/p>\n
\nMy favorite. I use this even on a lot of model rockets. The whole idea is to absorb the kinetic energy of separation after the halves have separated a ways. The idea is NOT to bring the halves to a sudden, jerked halt with a short length of unyielding Kevlar or to bounce them back together with rubber bands or bungee cords.<\/p>\n
\nI also like using the blue painter’s tape. I’ve actually had recoveries where all of the tape had not torn thru.<\/p>\n
\nKeith Stormo uses this ‘stitch’ technique with great results. Key to the theory is to start (last z-fold to release) with multiple stitches, then ramp down the # of stitches as you continue with the Z-folding.<\/p>\n
\nI like to see one or two bundles remaining at recovery. More, I’m wasting shock cord length. No remaining bundles, I figure a bit more energy should have been absorbed to keep stresses from shock down.<\/p>\n
\nYeah in my posts about this I was talking about taped Z-folds, small to mid-size rockets. Not terribly precise. Using it in a large and very expensive rocket stitching would be much better.<\/p>\n","protected":false},"excerpt":{"rendered":"