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  1. #61
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    13th February 2016
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    Quote Originally Posted by rocketsam2016 View Post
    Oh yeah I know I'm fine, the sanding worked well. I just want a pair of calipers if I can find the right ones
    I found a set of Starett 6" digital calipers with 1" digital micrometer on ebay many years ago. They've been working well. There are so many on the market these days.. I'm not sure what would be the smart choice. I'd go for stainless steel with a known brand name over the cheap plastic ones. Digital over Dial over Vernier.

    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  2. #62
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    31st August 2016
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    131
    With the MMT rings sanded so that the assembly fits, next up was cleaning/sanding where the fin roots will touch and placing foam board dams as discussed earlier in the thread. I had planned to set them 1/4 outside the fin root, but that looked big so I made it 3/16.

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    Heres how the epoxy will be placed:

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    As you can see I placed the dams radially instead of parallel with the fin, which Im regretting now since the extra width near the outer body tube isnt all that useful for strength. Im seriously debating not filling the chambers up all the way since its going to make a mess when Im trying to fine tune straightening the fin. The good news is that even if I fill them all the way, its only 6oz total of epoxy to do all the fins which isnt as bad as I was worried it would be when I realized that I shouldnt have placed them radially.

    Heres my math for how much epoxy it will be in case it is useful for anyone. There was no need to do these calculations so precisely but I like math. Sorry for the mixed units, but google doesnt care :-)

    The dams are placed 3/16 + 3/16 (fin) + 3/16 = 9/16 apart on the MMT. This represents 5.7% of the MMT exterior diameter:

    (9/16) / (pi * 79mm) = 0.057

    Thus, the block of epoxy will be 5.7% of the area between the MMT (79mm exterior diameter) and the outer body tube (99mm interior diameter), minus the area of the fin root (which is 3/16 wide and 10mm tall)

    ((99mm/2)^2 - (79mm/2)^2)*pi * 0.057 - 3/16in * 10mm = 111mm^2

    The combined root chord length of the fins is 13.5, so for each pair of fins Ill be using 38mL of epoxy:

    111mm^2 * 13.5 = 38mL

    We have 3 fins and rocketpoxy weighs 1.5g/mL, so the total weight of epoxy will be:

    38mL * 3 * 1.5g/mL = 171g = 6oz

    Tonight Im planning installing the MMT assembly and possibly doing my first fin. Woot.


  3. #63
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    22nd August 2015
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    Rhode Island
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    It's looking great, Sam. I'm going to copy this method on my next FG build.
    NAR #100940, RIMRA & CMASS
    L1 - 4/17/16, Tyrannosaur (by Binder Design), Loki H144
    Dual Deploy - 11/12/16, Tyrannosaur + Eggtimer TRS, Loki I405

  4. #64
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    31st August 2016
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    Quote Originally Posted by BDB View Post
    It's looking great, Sam. I'm going to copy this method on my next FG build.
    Thanks! Ask me again once I'm done though :-) Negatives that I can see so far

    - It took more time than I expected to cut out the little foam board dams
    - A lot of the added weight isn't doing much good. The most egregious part is the middle (halfway between the MMT and the body). The only benefit I can see that region adding is that we've formed a solid epoxy block around the fin base that adds a compressive/mechanical force against the fin pivoting or being ripped out. It does nothing much though to help adhere the fin to the MMT.
    - If I don't manage to completely fill the chamber with epoxy there will be a gap at the top and it'll have been no better than just filling it up a third of the way.
    - I anticipate a giant mess of squirting epoxy when I insert the fin if I try and fill it all the way, especially when I try and adjust the fin angle. I'm worried this will be even more of a mess with the rig I'm using to ensure the fin is straight
    - There is going to be quite substantial force resisting me adjusting the angle of the fin since the fin is flush with the centering rings and the MMT and is therefore a well-sealed piston. I may actually chamfer the front and back of the fin root tab so that epoxy has at least some path to take instead of just squirting out the body tube

    On the plus side:
    - Using a dowel to add internal fillets to such a small space going nearly 15" into the rocket would be brutal and hard to do well!
    - No drilling holes in the frame for injection
    - No trying to inject rocketpoxy (I'd rather not invest in another epoxy, I like this stuff)
    - Compared to injection, it's OK and even preferable to have some gaps between the fin root and MMT and centering rings

    Overall, I'd be thinking harder about whether to use this method if it was on a leaner and meaner rocket or a rocket that had marginal stability. The Frenzy is a heavy, draggy and very stable beast though so the extra weight isn't the end of the world.

  5. #65
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    31st August 2016
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    131
    Holy cow the foam board added a ton of friction. I got the mmt assembly about 75% of the way into the body tube before realizing I wasn't going to make it.Fortunately I hadn't put any epoxy in yet. My full body weight applied through a pole wasn't enough to get it to even budge to get out, nor was trying to jump to apply my weight. I had to put a bulkhead down agiant it and use a mini sledge hammer with force transfered through a metal pole to get it moving.

    I cut down the foam board so that the top edges were angled and this time got it in. I put a solid epoxy bead at the top centering ring. The foam board dams might be a wee bit off center but they are close enough. Off to ice my bruise hands from trying to push it out... Good news though is my mmt is installed! And friction alone apparently would be enough to keep it in place for a lot of motors...

  6. #66
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    31st August 2016
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    Picture of the rear CR. Only showing since it's kind of cool how you can see the dams and centering rings from the inside of the MMT.

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    I also went ahead and attached the aeropack retainer. Smart move would've been to wait in case I screwed the fins up badly and/or needed access through the rear centering ring for some reason but I was feeling enthusiastic :-)

  7. #67
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    31st August 2016
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    Time to attach fins! Cleaned and sanded (still dubious about whether I'm passing the water break test, but I cleaned and sanded the snot out of this thing with 100 grit)
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    I estimated 14mL (21 grams) of rocketpoxy would be needed for a single rear fin and ended up using around 24 grams, not bad! I originally was trying to get the rocketpoxy into a syringe to get it cleanly into the slot but that was a pain and it turns out it wasn't so bad to just pour it in from the cup. Next time I think I'll put saran wrap on my jig since the most stressful part of this was worrying about accidentally gluing my jig to the fin or body.

    As you can see my jig is inspired by other folks on the forum. Two boards of particle board with a beveled angle cut perfectly straight on each of them. I lined them up then drilled 5/8" (I think) holes for dowels to keep the boards perfectly lined up no matter the spacer size. I use 3 #8 screws to hold it tight around a spacer (one of the forward fins), and then keep it tight to the rocket with rubber bands. First time using it and it seems to work great! I wish I hadn't tried to size it to work with everything from 29mm to 98mm tubes. If I'd just sized it for 3" plus (aka used a steeper angle and a thicker or doubled board) then I could've had more clearance between the jig and the rocket, minimizing the risk of getting it glued on as well as making it more powerfully hold the fin straight. Oh well.

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  8. #68
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    Instead of saran wrap use clear or brown packing tape the it can be attached to the jig and epoxy wont stick.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  9. #69
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    Quote Originally Posted by rharshberger View Post
    Instead of saran wrap use clear or brown packing tape the it can be attached to the jig and epoxy wont stick.
    Thanks! Used this tip on the second fin.

    I decided to figure out my recovery system loads. Sorry for the brain dump. My understanding is that the rating of kevlar and nylon cords is the likely failure load, while the WLL or SWL ratings on lifting eyebolts is 10% of the failure load. Here's the full stack from fin can to nose cone:




    Note that I'm using a deployment bag, so so there is either no attachment (free bag) or a relatively week one (tethered bag) between the main parachute and the pilot chute + nose cone. This means I think that it's pretty hard to have all that much force on the nose cone bolts themselves.

    The weakest link in the main stack (everything under the main chute) is the 9/16 nylon (1500lbs) that came with the kit, followed by the kevlar y harnesses (1700lbs). 1500lbs seems quite enough though - even if this thing ends up 20lbs, that'd be good for a 75G deployment event, and realistically I'll come in well below that for burnout weight, not to mention several of those pounds will not be hanging under the main. Im not sure if the load ratings for the fruity chute swivels are failure loads or 10% failure loads, but the main chute is rated at 1500lbs and that's the one that could potentially see a massive load in an anomalous event whereas the drogue swivel just can't get loaded up all that much.

    With this logic (1500lb target and WLL of links/bolts are 0.1 * failure load), I really don't have to get fancy with my choices of quick links or eye bolts for the nose cone. I'll get rated stuff for sure and used forged eye bolts, but it's nice to know that even 1/4" hardware will be far stronger than the target strength.


  10. #70
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    On most quick links and eyebolts the WLL is actually a 5:1 ratio ( referred to as a safety factor of 5) so a 100lb wll has a 500 lb failure limit (iirc my basic crane and rigging classes) BUT that number is only valid if pulled in line with the long axis of the eyebolt or quicklink, any off axis strain and the WLL goes down and rapidly and only shoulder style eyebolts are rated for off axis loads.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  11. #71
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    31st August 2016
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    Quote Originally Posted by rharshberger View Post
    On most quick links and eyebolts the WLL is actually a 5:1 ratio ( referred to as a safety factor of 5) so a 100lb wll has a 500 lb failure limit (iirc my basic crane and rigging classes) BUT that number is only valid if pulled in line with the long axis of the eyebolt or quicklink, any off axis strain and the WLL goes down and rapidly and only shoulder style eyebolts are rated for off axis loads.
    Yup you are absolutely right. Glad I'm doing a build thread :-)

    I've been thinking about size+quantity of shear pins for the nose cone, and I'm leaning towards 3 4-40 nylon pins.The nose cone is going to be something like 2.25 pounds with hardware and tracker all installed. I want the pins to hold up to 50Gs, which should be pretty conservative considering my drogue is small for this size rocket (15"), my drogue shock cord is 25' of tubular nylon and I will have redundant altimeters firing the drogue at apogee. The only real way I could get remotely close to 50Gs would be if the drogue shock cord tangled badly during apogee separation, and even then I'm hoping it wouldn't hit that. This means the pins need to hold 112.5lbs of force. Rocketmaterials.org tells me that I'll achieve 115lbs on average with 3 4-40 nylon shear pins. In one test these failed at 99lbs, so if I wanted to be super conservative I could go with 4 of them. That feels like overkill though, and even with a 99lb shear that would survive 44Gs.

    So, to get 112.5lbs of force plus a 50% safety margin on a 4" rocket I need 13.4PSI from the main ejection charge which should be fine, though I did the math out and this force applied to a 2.25lb nose cone will accelerate it at 75Gs! Hopefully my home brew tracker mount holds :-) In practice I'll ground test until I have a reasonably energetic separation of the nose cone and then have my backup main charge be sized 20-25% larger.

  12. #72
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    13th February 2016
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    Quote Originally Posted by rocketsam2016 View Post
    Yup you are absolutely right. Glad I'm doing a build thread :-)

    I've been thinking about size+quantity of shear pins for the nose cone, and I'm leaning towards 3 4-40 nylon pins.The nose cone is going to be something like 2.25 pounds with hardware and tracker all installed. I want the pins to hold up to 50Gs, which should be pretty conservative considering my drogue is small for this size rocket (15"), my drogue shock cord is 25' of tubular nylon and I will have redundant altimeters firing the drogue at apogee. The only real way I could get remotely close to 50Gs would be if the drogue shock cord tangled badly during apogee separation, and even then I'm hoping it wouldn't hit that. This means the pins need to hold 112.5lbs of force. Rocketmaterials.org tells me that I'll achieve 115lbs on average with 3 4-40 nylon shear pins. In one test these failed at 99lbs, so if I wanted to be super conservative I could go with 4 of them. That feels like overkill though, and even with a 99lb shear that would survive 44Gs.

    So, to get 112.5lbs of force plus a 50% safety margin on a 4" rocket I need 13.4PSI from the main ejection charge which should be fine, though I did the math out and this force applied to a 2.25lb nose cone will accelerate it at 75Gs! Hopefully my home brew tracker mount holds :-) In practice I'll ground test until I have a reasonably energetic separation of the nose cone and then have my backup main charge be sized 20-25% larger.
    Hmm.. interesting discussion and good thinking.
    In my level 2 build, I'd considered pin sizing and number with a 25G deceleration peak due to apogee booster separation shock cord extension. It was a guess. Why did you chose 50Gs?

    I just weighed my same nosecone, bulkhead, mounting ring, tracker mount, Eggfinder TRS, 950mAh 2S LiPo and 1/4" SS shouldered eyebolt... and they came out to 2.25 lbs just like you have. However, that did not include the weight of the parachute, chute protector and shock cord. I think they should be included in the mass that would decelerate and act against the shear pins. My planned main chute, protector and shock cord total 16.3 oz or 1.2 bs. This increases the weight to 3.45, say 3.5 lbs. At 25Gs that would be 87.5 lbs... or, for 3 shear pins, 29.2 lbs per pin. That's about the average shear for a 2-56 nylon screw. Up until reading your post, I haven't thought too carefully on what size pins I should use in the payload section.. and was hoping to use three 2-56 nylon screws. This is definitely something that needs to be considered more carefully.

    I was thinking that it might be possible to start with three 2-56 shear pins and fly some smaller motors with the same apogee deployment charges. If three 2-56 pins prove insufficient and the apogee charge causes the nosecone to separate, the altitude wouldn't be too extreme for a descent from apogee under the main. If the 2-56 shear screws proved too small, then I think it would be possible to simply drill and tap the same holes used for the 2-56 screws for 4-40 screws. The respective major and minor thread diameters are very close.
    Last edited by kevinkal; 5th February 2017 at 05:00 AM.
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  13. #73
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    Here's a spreadsheet a TRF'r did that I find pretty useful, it does Static Port calculations, BP estimates and Shear Pin forces.

    Black Powder - Shear Pin Calculator Rev4.xlsx
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  14. #74
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    13th February 2016
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    Quote Originally Posted by rharshberger View Post
    Here's a spreadsheet a TRF'r did that I find pretty useful, it does Static Port calculations, BP estimates and Shear Pin forces.

    Black Powder - Shear Pin Calculator Rev4.xlsx
    I like the sheet. The Black Powder - Shear Pin calculator sheet assumes that there's no vent hole in the parachute compartments.. and thus with pressure decreasing with altitude, the internal pressure builds up and acts on shear pins. I plan to drill a single or more 1/8" vent holes in the booster and payload sections per Madcow recommendations to vent the parachute compartments.
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  15. #75
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    Quote Originally Posted by kevinkal View Post
    I like the sheet. The Black Powder - Shear Pin calculator sheet assumes that there's no vent hole in the parachute compartments.. and thus with pressure decreasing with altitude, the internal pressure builds up and acts on shear pins. I plan to drill a single or more 1/8" vent holes in the booster and payload sections per Madcow recommendations to vent the parachute compartments.
    That may be true, but my experience has been that vent holes have little effect on the force of the ejection charge. Like you I use a vent hole in all my MPR/HPR rockets payload bays.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  16. #76
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    31st August 2016
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    Quote Originally Posted by kevinkal View Post
    Hmm.. interesting discussion and good thinking.
    In my level 2 build, I'd considered pin sizing and number with a 25G deceleration peak due to apogee booster separation shock cord extension. It was a guess. Why did you chose 50Gs?
    Fear of the apogee shock cord tangling and stopping short, and/or the larger backup charge if the primary fails. Where I fly, if the main opens at apogee the rocket is almost certainly lost for good to a high tree. A few threads suggest people routinely see events in the 20G plus range, but it does seem that many folks think that 25G should be fine:
    http://www.rocketryforum.com/showthr...786#post669786
    http://www.rocketryforum.com/showthr...853#post752853
    http://www.rocketryforum.com/showthr...700#post307700
    http://www.rocketryforum.com/showthr...42#post1445142

    Hmm. Maybe I'll go with 4x #2. That would hold against 35-40G.

    Quote Originally Posted by kevinkal View Post
    I just weighed my same nosecone, bulkhead, mounting ring, tracker mount, Eggfinder TRS, 950mAh 2S LiPo and 1/4" SS shouldered eyebolt... and they came out to 2.25 lbs just like you have. However, that did not include the weight of the parachute, chute protector and shock cord. I think they should be included in the mass that would decelerate and act against the shear pins. My planned main chute, protector and shock cord total 16.3 oz or 1.2 bs. This increases the weight to 3.45, say 3.5 lbs. At 25Gs that would be 87.5 lbs... or, for 3 shear pins, 29.2 lbs per pin. That's about the average shear for a 2-56 nylon screw. Up until reading your post, I haven't thought too carefully on what size pins I should use in the payload section.. and was hoping to use three 2-56 nylon screws. This is definitely something that needs to be considered more carefully.
    I'm not sure you need to include the parachute or shock cord. Sure they get decelerated, but they compress quite substantially and a 25G+ event is only going to happen because of a very sharp hit to the end of the shock cord. I imagine that by the time the parachute flies forward and compresses enough to apply much force, the nose cone itself has already been stopped so you can use max(nose cone weight, parachute weight) rather than the sum of them.

    Quote Originally Posted by kevinkal View Post
    I was thinking that it might be possible to start with three 2-56 shear pins and fly some smaller motors with the same apogee deployment charges. If three 2-56 pins prove insufficient and the apogee charge causes the nosecone to separate, the altitude wouldn't be too extreme for a descent from apogee under the main. If the 2-56 shear screws proved too small, then I think it would be possible to simply drill and tap the same holes used for the 2-56 screws for 4-40 screws. The respective major and minor thread diameters are very close.
    I don't think this kind of testing would answer the question. I fully intend to design the apogee separation to not hit the end of the cord hard or even at all. I need the pins to be strong enough though to handle abnormal but non-catostrophic events like using the backup charge or a having a tangle in the apogee shock cord.

  17. #77
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    With the 3 rear fins done, I used a heavy duty aluminum angle to line up the first of the front fins and epoxy it in. Holy cow these pockets take a lot of epoxy. I think it took about 50g!! If I ever do pockets like this again I will place the dams differently (angled in) or possibly use a dowel to get a concave shape since I think a lot of the weight just isn't doing much at all to increase strength. I nearly aborted and only filled it partly up but decided to stick with the plan after confirming that these big old fins result in a very stable rocket that can handle a heavy rear.

    Click image for larger version. 

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  18. #78
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    One thing that surprised me was that despite using a large (1/8" material, 1.5" wide, 30" long) aluminum right angle and large clamps, it took some fiddling to get the fore fin truly aligned with the aft fin. It wasn't egregiously out of alignment, but enough so that I could easily see rocking of a second straight edge held higher up on the fin and it definitely would have caused spin. I didn't expect this to be possible since the right angle was clamped tightly, but fortunately I noticed and was able to add more clamps and adjust things until the second straight edge showed perfect alignment top to bottom. Counter to my intuition, I had the best results by clamping the right angle near the bottom of the fin, presumably since this made for a longer lever arm against which to clamp. I think the variation comes from the fact that the groove that came pre-cut in the body tube is a little wider than the fin and therefore allows for some twisting.

  19. #79
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    Looking good. How difficult was it to get the Rocketpoxy to pour into the fin slots?
    Regarding alignment, do you think that more epoxy could have been captive in one side of the fin slot than the other and, being trapped, made it difficult to get the fin to align?

    I'm still slowly working on my build proposal, but thanks to your thread, I've been thinking about the pinch points in detail. I still don't know what path I'll take for fin mounting...
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  20. #80
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    Quote Originally Posted by kevinkal View Post
    Looking good. How difficult was it to get the Rocketpoxy to pour into the fin slots?
    Regarding alignment, do you think that more epoxy could have been captive in one side of the fin slot than the other and, being trapped, made it difficult to get the fin to align?

    I'm still slowly working on my build proposal, but thanks to your thread, I've been thinking about the pinch points in detail. I still don't know what path I'll take for fin mounting...
    Pouring is easy, especially once I gave up trying to be super tidy and just poured up and down the slot. The epoxy is viscous enough that as long as the bead you pour isn't far far wider than the slot, it ends up almost entirely pulled down into the slot, and the remainder is easy to scrape off with a popsicle stick (and clean up with rubbing alcohol once you have it all in.)

    I sanded a 45 degree transition at the front and back of the fin tabs to let epoxy have a path from side to side. I don't think the epoxy getting trapped affected alignment at all, I'd be more worried that you could create air gaps with alignment adjustments, and as I mentioned earlier, having an air gap at the top of the chamber almost entirely defeats the purpose of pouring to the top in the first place....

    I'm really flummoxed on the alignment though. I've done 2 of the 3 forward fins now, and tried to be even more careful on the second. I used a level though to go back and check after it was all dried (I laid the rocket perfectly level with the fins sticking out horizontally and checked whether the fin was level in the axis of the rocket body). The aft fins are almost perfect so my jig is good, but both the fore fins are off a wee bit. It's not off by much - I can't see it by eye, but it's just amazing to me that my extremely straight, very rigid aluminum angle clamped at 5 points to the fins is failing to achieve perfect alignment between the aft and fore fins. I'm seriously considering using my jig to attach the 3rd fin, though that will be annoying making sure it isn't offset at all from the aft fin. Given that 2 of the 3 are done though and not quite perfect, I may just do the third with the same aluminum angle and be even more cautious. Again, it's not observable by eye so I do think I'm getting pretty close, but I'm not optimistic I'll get a spin-free flight for video :-)

    The other thing is that the fins are a very tight fit into the slots before sanding, but doing a suitably aggressive sanding prep to the fin tabs adds some slack space. Better that than poor adhesion, but if I wasn't sanding then the slots themselves would entirely dictate the direction the fin points.

  21. #81
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    The last one went on straighter - I used smaller clamps and clamped with less force. I think I may have been bending the aluminum angle a bit. Nonetheless I'm also convinced now that maybe there is slight deviation in the fin slots. Regardless it's done now, and as I said before the deviation is not observeable just by looking so I think this is good enough.

    On to the external fillets. I'm aware that fillets ideally have a radius between 4% and 8% of the root fin chord. I already have a lot of epoxy in the rear of this rocket, and my understanding is external fillets are more about aerodynamics than strength, so I'm going to go on the low end.

    The frenzy has a split fin with a combined root cord of 13.5". This would suggest a fillet with a 0.54" radius. Alternatively, I could size it to the larger fin which is 8" long and would yield a fillet with radius 0.34". Anyone have thoughts on how to size for a split fin?

    Finally, I believe the volume used per fillet is:
    13.5in * r^2 * (1-pi/4)

    Or for the whole rocket:
    6 * 13.5in * r^2 * (1-pi/4)

    With a 0.5" radius, that would be 71ml or about 106g of epoxy. Oooooof. Maybev I'll go with 0.34" radius

  22. #82
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    Posts
    131
    First fillets on! These split fins with the rear fin joining the body at 90degrees take some care to get nice fillets. Mine are.... not terrible but not amazing. I went with a 0.5" radius since I saw a number of other similarly sized split-fin builds using that. Before doing so I checked my weighting by putting 3.5oz of washers in to simulate the 0.5" fillets and loading up the rocket with everything else that I have (chutes, tracker, shock cords, hardware, etc. everything but the AV bay guts). Fully loaded the CG was at 56.5" from the nose which happens to be the same as the open rocket file provided by Madcow. The CP is at nearly 76", and I used OpenRocket to confirm that no 75mm motors would take it below 2 calibers of stability. Additionally, the AV bay guts are well ahead of the CG and will further help the balance.

    Click image for larger version. 

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    My intention is to do a full fillet around the back of the rear fin, but that takes multiple passes due to gravity. As you can see the back looks pretty bad right now, but we'll see if it works out.

  23. #83
    Join Date
    13th February 2016
    Location
    Gilroy, CA
    Posts
    426
    Looking good! Looking forward to your first flight report!
    Have you planned a color scheme? I'm thinking something similar to the stock scheme.. since I purchased the stock vinyl decals... I thought of yellow and red vs. white and red .. not sure.
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  24. #84
    Join Date
    31st August 2016
    Posts
    131
    Quote Originally Posted by kevinkal View Post
    Looking good! Looking forward to your first flight report!
    Have you planned a color scheme? I'm thinking something similar to the stock scheme.. since I purchased the stock vinyl decals... I thought of yellow and red vs. white and red .. not sure.
    Thanks! I haven't decided yet on the color scheme, and the large amount of snow in my backyard means I don't have to decide soon :-). My wife points out that my most recent three rockets were neon (orange, orange and pink respectively), so perhaps I should keep with the trend and go neon green. I'm pretty sure I'm going to do a split color though, perhaps neon green up top, black switchband, and white on the bottom? Or perhaps metallic silver for the switchband to match the nose tip. The nice thing about black for the switch band is it would mask the seams as well as make my switches (black slides mounted underneath) less visible.

  25. #85
    Join Date
    31st August 2016
    Posts
    131
    I have an eggtimer tracker going in the nose. I built this assembly out of plywood (laser cut) for my last rocket. There are two parts - a battery cage and a mount for the tracker. Screwing them together locks the battery in the cage:
    Click image for larger version. 

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    This gets inserted through a home-shaped hole in the nose cone bulkhead. The weird-shaped hole means that I can insert and then rotate the assembly 90 degrees and it is tightly held in place with no need for screws or parts that can get lost. I wanted this bulkhead to be removable though in case something breaks off inside or I want to change my tracker cartridge design, so I epoxied a 54mm/98mm centering ring to the end of the nose cone coupler, and then use two 1/4" flanged blots to hold the bulkhead on, and I use a pair of eye nuts on the other end to hold the bulkhead on and attach a Y kevlar cord. The eye nuts each have a 400lb WLL, so they'll break at 4000lbs total (2000lbs each). Now the load isn't quite vertical, but they'll definitely be strong enough.

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    Tonight I'll glue this coupler into the nose cone, but I wanted to do all this stuff first since it's much cheaper to replace a coupler than a whole nose cone in case I screwed something up :-)

    Oh and I'll drill two small holes in the 1/4" bolts for a locking pin when I get a chance.
    Last edited by rocketsam2016; 13th March 2017 at 03:31 PM. Reason: Link to eye nuts

  26. #86
    Join Date
    13th February 2016
    Location
    Gilroy, CA
    Posts
    426
    Looks nice. I like the modular design, and ease of access. I see you have the stock antenna. I have the stock antenna too.. but am wondering if it would be wise to put on a higher gain external antenna for the L3 high altitude flight. Eggfinder says the range is something like 10,000 feet with the stock unit. That will be something to read up on for me. What are your thoughts on range?

    I was able to get a test flight on my Eggfinder TRS Saturday. It didn't go too well. Partly due to my forgetting how the TRS is configured, and partly due to the way I had armed the TRS before sliding the nosecone into the payload tube. It turned out that the act of sliding the nosecone with the tracker into the payload tube created enough pressure to trick the TRS into thinking it was recording a flight to 514'. The GPS position continued to report.. but the altitude remained stuck at 514.. something I didn't notice until launch. I was only using it for GPS position and baro altitude.. none of the deployment side was connected. As for position data.. I used an Eggfinder Rx connected to my laptop running Mapshere.. It worked fine until launch.. where the GPS altitude was not correct and the baro altitude (reported on the Eggfinder LCD) was stuck at 514'. The flight was straight up to about 5k, but under drogue then main, the rocket drifted downwind perhaps 0.4 miles. During this path, the GPS had a significant glitch in position.. jumping perhaps 0.2 miles.. but then settled back to something near reality. My poor rocket landed in a creek! The altimeter bay filled with water and sunk. The Eggfinder Quantum shut down due to this, but unbelievably the StatologgerCF continued to operate underwater! My nosecone with my Eggfinder TRS magically landed on a small island in the middle of this small creek and did not get wet! Whew. I have had similar experiences with position and GPS altitude with my basic Eggfinder Tx unit. I think the TRS should be better if I can get it setup correctly for the next flight.. and not set it up for flight mode until the nosecone is fully on first.
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  27. #87
    Join Date
    31st August 2016
    Posts
    131
    Quote Originally Posted by kevinkal View Post
    Looks nice. I like the modular design, and ease of access. I see you have the stock antenna. I have the stock antenna too.. but am wondering if it would be wise to put on a higher gain external antenna for the L3 high altitude flight. Eggfinder says the range is something like 10,000 feet with the stock unit. That will be something to read up on for me.
    Thanks! My only concern is whether it is strong enough. The first version sheared in two (plywood delaminated) in a flight in the fall, most likely during landing. The problem with laser cutting is it really weakens the plywood of small parts with heat damage.

    I got the "ducky" antenna for my receiver which will help things. My field's waiver is 10k feet though, so Cris has reassured me that I should be fine.

    Quote Originally Posted by kevinkal View Post
    I was able to get a test flight on my Eggfinder TRS Saturday. It didn't go too well. Partly due to my forgetting how the TRS is configured, and partly due to the way I had armed the TRS before sliding the nosecone into the payload tube. It turned out that the act of sliding the nosecone with the tracker into the payload tube created enough pressure to trick the TRS into thinking it was recording a flight to 514'. The GPS position continued to report.. but the altitude remained stuck at 514.. something I didn't notice until launch. I was only using it for GPS position and baro altitude.. none of the deployment side was connected. As for position data.. I used an Eggfinder Rx connected to my laptop running Mapshere.. It worked fine until launch.. where the GPS altitude was not correct and the baro altitude (reported on the Eggfinder LCD) was stuck at 514'. The flight was straight up to about 5k, but under drogue then main, the rocket drifted downwind perhaps 0.4 miles. During this path, the GPS had a significant glitch in position.. jumping perhaps 0.2 miles.. but then settled back to something near reality. My poor rocket landed in a creek! The altimeter bay filled with water and sunk. The Eggfinder Quantum shut down due to this, but unbelievably the StatologgerCF continued to operate underwater! My nosecone with my Eggfinder TRS magically landed on a small island in the middle of this small creek and did not get wet! Whew. I have had similar experiences with position and GPS altitude with my basic Eggfinder Tx unit. I think the TRS should be better if I can get it setup correctly for the next flight.. and not set it up for flight mode until the nosecone is fully on first.
    Oof! Yeah I'm not counting on getting a good track during flight. I'm tentatively planning to upgrade my receiver to connect with bluetooth to my phone but we'll see if I get to that. If I lose the eggfinder I may get a TRS to get the better altitude data, but the nice thing about the eggfinder is it is so small and so I can put it in even a 29mm rocket! That's actually why the top part of my assembly is shaped the way it is - I have plans to slide it into an Apogee Aspire with a smaller battery.


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