L1/L2 2.6" Madcow DX3 Build

Mendal said:

For what it's worth my 4" dx3 only required .8 grams of black powder to separate the drouge. I have not yet tested the main, but I think I'm only going to fly with one or two shear pins as I do not think it should require more then that and at most I'm thinking 1.2 grams bp. All this is in a 4" rocket. At 2 grams in a 2.6" rocket I think you are over charging. Have you ran a bp calculator to figure out how much bp you need?

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Mendal, yes, I have used 3 different bp calculators to estimate what I would need. It's possible that we have slightly different situations here, but even so, I'm surprised that .8g provided separation for you. You must have had really good containment and a fast burn. That's awesome! I'm guessing that you didn't have shear pins on your drogue separation. I may not need them or use them, but I'd like to test them. Based on those same online calculators, for your rocket, 3.9 ID and around what, maybe 21" pressurized length in front of the centering ring, it looks like a .8g charge would result in only about 6.5psi, which would be far too small to break my shear pins. In addition, I have a 54mm motor mount here, rather than a 38mm (like yours?) so the pressurized airframe length (when I'm flying a 38mm motor with an adaptor) is a bit longer than yours (although still a smaller diameter).

[It was interesting comparing these two. I had realized the Super DX3 was fatter, 4" vs 2.6", but I hadn't realized it wasn't scaled. It's only 3.5" longer! I could still fit it in the back of my car!]

As I mentioned, I figured about 25 psi is a good target. But let's do the math, as I haven't myself yet. The max shear strength (read 'required breaking force') for a #2-56 nylon machine screw is approximately 46lbs (https://www.feretich.com/rocketry/Resources/shearPins.html) and I have 3, so 138lbs (minimum would be 31*3=93lbs). The internal diameter of the rocket is 2.56", which is a cross-section area of 5.15". 138lbs / 2.15in^2 = 26.79 lbs/in^2. Looks like I may have actually underestimated my target pressure a bit. (The minimum required force may be as low as 18.06psi.)

I'm not going to get into actually deriving the psi developed from a particular volume (weight) of bp, I'll leave that to the calculators, but I estimated about 28" worth of airframe volume to fill, resulting in a required charge of approx. 1.85g. I rounded up to 2g to account for coupler friction and to get the chute out. It all comes down to the actual ground testing though. Will it break the pins? Will it break the harness? Will the whole thing fly across my back yard? Weather dependent, we'll see later this evening.

For what it's worth, I use 1.5 on both sides of my 2.6" screech, and they're solid deploys. I don't pin the booster however I imagine it'd work. My payload bay is stuffed full of a 4' rocket man and 15 foot of 9/16 TN, with a 12" nome in there as well.

I've used 3 4-40 pins and broke it with that.... I try to use 2-56 now.

Charges are packed in vials, and stuffed and packed with wadding, then closed and taped shut.

I was targeting 12-14 psi, the chamber length was about 10 inches. The MMT is 18" in this rocket so it consumes most of the booster section and the AV bay eats up another 4".

I have to ground test the main separation with shear pins before our next launch on the 18th, as I am going to go for dual deploy.

I did not have pins on the drouge but I will on the main, but i really fail to see why I would need more then say one or two.

The Payload section is also about 13 inches (19" - 3" AV Bay - 3" Nosecone = 13"), I think I'm going to target about 16 PSI with 1.27 grams of BP.

Dave, I gotta ask. There's a picture on your site of your 54mm Screech newt to a (loki?) 54/2550 case. (posted 3/16/16)
How in the world do you fit recovery gear in there? Drogue-less?

So the ejection test was 50/50. The larger charge (2g) worked well, cleanly separated and extended about 7-8 feet. Shock cord never fully extended, but drogue chute analog quite cleared and mostly unwrapped/unfolded. It was very slightly blackened with bp residue along one edge. Just about text book, based on what I've seen from other test videos I've seen on the site anyway. The smaller .75g main charge failed to shear the pins. I'm going to re-try tomorrow with a 1g charge.

Video's were taken and will get posted as soon as I have time / remember how. :wink:

MCriscione said:

Dave, I gotta ask. There's a picture on your site of your 54mm Screech newt to a (loki?) 54/2550 case. (posted 3/16/16)
How in the world do you fit recovery gear in there? Drogue-less?

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Yea this one kinda scares me. going to just be some 1/4" kevlar in there, but that's a lot of motor. We'll see how it goes.... probably next summer... I'm not up for mixing anything that size just yet.

Ejection charge testing is essentially complete. A 1g charge successfully sheared the payload/nosecone pins, when the .75g charge didn't. Again, it looked like good extension of the nose cone from the body without excessive force applied. Here's a video with clips from all three tests.

[video=youtube;azM2wL71UuY]https://www.youtube.com/watch?v=azM2wL71UuY[/video]
https://youtube.com/watch?v=azM2wL71UuY

I'd like to get some feedback from anybody that watches the video regarding the energeticness (is that a word?) / force exerted and if I should further increase the charges for the actual flight. I'm aware of some posts in other threads that discuss successful ground testing and subsequent failures during actual flight.

Drogue looked fine, but if it were my rocket I'd look for a slightly stronger main.

Looks like it will work our fine. Don't forget in flight both section will move in opposite directions when charge fires. Right now the booster can't move its held in place by the ground.

Tom, thanks for your response!

Crossfire, I agree with your conclusion, but not with your justification.

Thanks to physics, we know that by grounding the booster, the payload/nosecone gets all the energy from the ejection charge (reduced by friction and shearing the pins) as kinetic energy. In an in-flight ejection the energy would be distributed (proportional to mass) to the two. Effectively no difference vs. the video except for frame of reference.

I've watched the video a number of additional times. It seems like the drogue ejection got pretty good extension/separation, and the 'drogue' is clearly out and separated from the chute protector. In addition, the motor(s) that it will fly on will take up more space than what I plugged up, slightly reducing the volume even further. I think that one's fine. The main ejection does seem to be a bit short, but it's also 'full extension. I've placed the nose cone on the third loop (about 2/3 along the length) with the main at the far end. Thus the total rocket descending will be main-nose-payload/av-drogue-booster. If I add anything more to the main, I'm concerned it will just cause more snap-back. Should I have used a longer harness? It's about 12' total length with the third loop at about the 8' mark. I know I've heard of a lot of folks using much longer.

Ever get a chance to get this in the air? I'm nearing the end of my own build, about to ground test shortly. What parachute are you planning on using?

Banzai88 said:

Ever get a chance to get this in the air? I'm nearing the end of my own build, about to ground test shortly. What parachute are you planning on using?

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Hey Tom, I'm glad to hear that your build is coming along nicely. I've been watching your updates from the shadows and enjoying your build. Sadly, I've been stuck, grounded, due to the local club canceling their August HPR launch, due to field conditions (it was full of hay!). My other chance with MMMSC in Maine this weekend was a schedule conflict for me, so now I'm aiming for the next CMASS HPR launch on Sept. 17th in Amesbury, MA. Hopefully the weather will cooperate, and I assume the hay is up by now. We'll just have to see.

Since my last update here, I've been quiet, partly due to the launch cancellation being a bit of a bummer. However, the LPR launch two weeks ago re-energized me a bit and I started moving again. I've done final ground testing for charges and settled on 1.2g main / 2g drogue. I've got the camera shroud all painted up and mounted, though I still have the rail buttons left to put on. About the same time as the last post, I had a (probably somewhat oversized) Par58 thin-mil main chute on order from Top Flight Recovery. It arrived prior to the canceled launch date, but only just. I had planned to use another Par18 that I already had as a Drogue.

Then I started getting interested in chute design. And toroidal chutes. And started talking with AlphaHybrids. And got, perhaps, in a bit too deep. It's possible that I might have ordered some 1.1oz fabric from https://ripstopbytheroll.com and some nylon cord from amazon, and stopped at the in-laws to borrow a sewing machine. But trust me, if that happened, the I'd probably have a couple brand new hand made chutes to show off here in another couple days or so. If I can only find some way to photograph them nicely.

I have to admit that I'm at a point of analysis paralysis on length and material of recovery harness(same problem I have on just about every rocket!) and parachute. I didn't weigh the parts before assembly, so my OR sim was much lighter than I anticipated(my fault). Re-reading the various 2.6 Screech/DX3/Tomach threads, I find that the weight I'm coming in with is about in line with real world, so I just had to adjust my thinking on getting a 36"ish parachute and recovery harness in an 11" payload bay and accept a 20fps landing speed on this size rocket.

I went so far down the rabbit hole with parachutes last year that I bought my own serger and regular sewing machine. I have only flown a few pieces of ripstop that haven't been under my machine(a few Fruitys) in about a year now, flat, parabolic cupped, or elliptical up to 42" and as many as 12 gores. Working on my first torroidal now. I have this bad habit of color matching main and drogue to the colors of the rocket! Ripstopbytheroll is a great place, I get order turn around in about a day from them. I'm still working on getting machine settings dialed in for 1.1oz, it's a little hard to work with compared to 1.6 or 1.9. I use 125 and 250lb Dacron kite string for shrouds.

Banzai88 said:

I have to admit that I'm at a point of analysis paralysis on length and material of recovery harness(same problem I have on just about every rocket!) and parachute. I didn't weigh the parts before assembly, so my OR sim was much lighter than I anticipated(my fault). Re-reading the various 2.6 Screech/DX3/Tomach threads, I find that the weight I'm coming in with is about in line with real world, so I just had to adjust my thinking on getting a 36"ish parachute and recovery harness in an 11" payload bay and accept a 20fps landing speed on this size rocket.

I went so far down the rabbit hole with parachutes last year that I bought my own serger and regular sewing machine. I have only flown a few pieces of ripstop that haven't been under my machine(a few Fruitys) in about a year now, flat, parabolic cupped, or elliptical up to 42" and as many as 12 gores. Working on my first torroidal now. I have this bad habit of color matching main and drogue to the colors of the rocket! Ripstopbytheroll is a great place, I get order turn around in about a day from them. I'm still working on getting machine settings dialed in for 1.1oz, it's a little hard to work with compared to 1.6 or 1.9. I use 125 and 250lb Dacron kite string for shrouds.

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I'm thinking that if I was to do it again (or if it will see much flying, I'll be changing over from the 9/16" TN that came with the rocket to 1/4" TK instead for weight and space savings (especially for the main). And yea, I agree, 20fps is a bit high, but survivable, especially if you don't have much pavement around. I'd hoping for somewhere around 15fps +/-2 depending on wind conditions, I'll go larger or smaller.

I too have had trouble with the 1.1. There was a lot of bunching until I got my pressures set right. I don't know about others, but I've been using a high foot pressure and mid-range thread tensions and that got it laying pretty flat with 8 stitch/in. straight stich. Zigzag on the other hand is a total mess, with nothing really working. I've tried pressure, tension, needle and bobbin changes, even backing but I still get tunneling even on the narrowest widths. At one point I got close to dialed in, or so I though, and had a couple small samples, but I came back a few minutes later and they had started to tunnel too, although only to a small extent. I just can't get zigzag to lay flat, especially over an edge. It's probably due to the machine I've got, which is an old drop-in bobbin, with minimal bottom tension adjusting (allegedly factory set, but too high for this fabric).

I found that 275 paracord is a bit heavy/stiff for the application, and may replace it on the one chute I used it on. Have you found the kite line easy to attach? I was concerned that it might be such a small diameter that sewing it could be problematic. (to hit is centered with the needle). Or is that a hand stitch job?

I'm thinking on going with the 1/4" TK, although I do have some 3/8 flat nylon that I'm wanting to try. I usually land on dirt or sod, so 20fps isn't all that bad, but I would prefer a little softer, and that means either larger or more efficient. Since I'm not willing to invest in a Fruity torroidal, and I haven't mastered making a functional one that I trust yet, I'm sitting here amazed that David has managed to stuff "a 4' rocket man and 15 foot of 9/16 TN, with a 12" nome in there as well"!

Yeah, I hear ya on getting the settings not only right, but consistent! I write everything down, to include thread type and needle used, foot tension, and thread tension. I learned from reading the kitemaking forums(they use the same ripstop that we use) that different threads will work differently, too. I'm currently using the Gutermann MARA 70 from ripstop, and it's been working rather well with the thinnest needle I can get locally (an 11, I think). My mother has a machine that costs more than any used car that I've ever bought, with more adjustments than the space shuttle, that I plan on using to try to dial in 1.1 stitching with. My mid level machine just isn't very consistent and I get the same tunneling that you describe. As for the serger, that's such a pain to dial in on the 1.1 that I usually set it and tune it and only reset to 1.6/1.9 settings if I have a larger project. On the heavier ripstop, the serger is a godsend! So fast and easy and clean.

I use zig zag pattern for the shroud lines and set the stitch width to hit the shroud line. With both the 125 and 250 I can get all 3 stitches to hit the line. I run between 4 and 10% line length attachment, stitch from edge to center, leave the needle down on a center stitch, lift the foot, flip the project, lower the foot and stitch back to the edge. My test samples ALL tore the fabric and never once popped a stitch. I've had a few off-timed deployments that tested the stitching, too...all came through with no issues and have gone on to perform very well with no ill effects. It's basically the same Dacron cord used in cheap blinds. I get it from my local kite shop, but it can be had from Amazon or Ebay for about the same price.

This is interesting. I've been using the MARA 50 instead, which I understand is a bit lighter, and also reading those kite forums, including this page. I think the thread from ripstop is a great deal for the price. An easy add-on for a first purchase. I bought some #10 microtex (sharp point) needles at the local fabric store that seem to do better than the #12 universal points that I had started with (that got mixed results). The sharp point 10's go through the nylon so much more cleanly than the 12's ever did. It even sounds better, less 'pop' or something.

And yes, I too am amazed by Dave's packing skills. The whole thing makes me want to invent some weird kind of pneumatic parachute packer that will fold/roll/rap at higher pressure than my hands can achieve. I test packed the 58" parasheet into an approximately 2.25" dia x 4" long cylinder using just hand strength though, and that fits OK in the airframe with the 9" nomex around it and about 5" above it for all the TN and quicklinks and such.

By the way, I'm glad to hear that I'm not the only one making test samples to test to destruction! I've found similar results with the 275 paracord, even with just a single line of straight stitching down the center, though I prefer the triple zigzag. What I need now is a tower to do drop tests from to estimate decent speed / Cd of these homemade things prior to sending them up in a rocket. The lifeguard tower at the beach ain't going to cut it. Perhaps I can rig something up to a couple tall trees in my yard. I'll have to look into quick-releases or something if I can't actually stand next to it.

I have some #10 needles on order. The 11s work fine for me on the heavier stuff without issue, it's the light stuff that's such a pain. After walking down that road I advise people that the few thin mil chutes that you're going to need are well worth paying for!

For testing I took an old curtain rod and put it across two ladders. I sewed a rod pocket in the fabric, a band about 4 inches wide, and then sewed 5 feet of line to it. I used successively higher work out weights dropped from the top of a 10 foot ladder to test stitching and attachment length. Even with crappy stitching and single line, fabric always failed before stitching did, regardless of fabric weight. Amazingly, even WalMart basic thread performed well without issues.

Sorry, I may not have been clear. The shroud line to chute tests haven't been an issue, and I got similar results. It's the decent speed tests that I need to do next that I'm thinking about and trying to arrange. I haven't thought about it much yet.

Gotcha. There was a thread somewhere recently about the TFR chutes and their actual CD. I'll see if I can find it.

Actual measured descent rate as an output of flight data is one of the biggest reasons that I switched to flying the RRC3. For smaller birds I have an ALT2 that I use (when I remember that I have it with me!). Once I get actual rate, I adjust the CD of the parachute in my OR sim to match real world. Usually that means adjusting it upward dramatically.

EDIT: Found it. https://www.rocketryforum.com/showthread.php?42559-X-type-Drag-Coefficient&p=1609201#post1609201

I've been coming down on a four foot rocket man. 1/4" TK is likely enough..... I've been using 9/16 TN. may go 11/32 TK

real world decent rate-

J510W


K1127LB

DavidMcCann said:

I've been coming down on a four foot rocket man. 1/4" TK is likely enough..... I've been using 9/16 TN. may go 11/32 TK

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Thanks for the input Dave. I think you posted those descent rates for me before. I think I had asked what units the main rate was in, if it's in ft/s it seems quite high compared to recommended. If it's in MPH, well, yikes :shock:. I'm aiming for closer to 17/18 fps with the 'big' main parasheet from TFR.

I had meant to get some photos to post of the chutes that I've been making, but simply haven't had the time to get shots of them 'full' in the air. I had a good opportunity to this weekend, as it was windy, and simply forgot to bring them along. Oh well, they weren't going to fly on the L1 attempt anyway.

Speaking of, the L1 attempt has been 'rescheduled' for this weekend. It's official, I'm on in Berwick, still flying the smaller than I had originally planned CTI I212-SS, even with the bigger field. If I have time, perhaps I'll buy a second motor off Jason and go bigger. If I'm really feeling lucky and not exhausted, perhaps just perhaps I'll see about going for the L2 that day too. Although it will depend on someone having a written exam laying around, and I'm not counting on that. I also haven't studied quite enough for it to be totally confident that I'd pass immediately either. And I'd need a loaner case. So probably not. :blush:

Well, I'm WAY overdue to update this thread. Looks like last time I posted I was about to head out for my L1 flight. As you may have noticed in my signature, the flight was a success and I got certified. Hooray! No additional flights HP flights were made that day; I closed it out with a few last low power flights instead. I’ve since made an L2 attempt, but more on that later.

I flew the Azure Blast on a CTI 3 grain 38mm I-212 Smokey Sam (using a 54-38 aeropack adaptor) to a height of 1957'. My OpenRocket file projected this motor to hit 2414' and I'm not entirely sure what happened to those missing 400’. I’m guessing it’s two factors, first the flight had a small bit of arc, and landed about 1,200’ away from the high power pads, and second perhaps my paint wasn’t ‘smooth’ enough (all those masking layers left small ridges at all the color boundaries that even clear coat couldn’t remove).

The Duel Deploy went flawlessly (ground testing payed off!) though there was some surprise/questioning of my sanity from a couple folks on the field that I was flying DD for an L1 cert. It was all well intended though, and as usual both club members and guests were really great.

The chutes that were used were TFR thin-mil chutes, a Par18 and a Par 54. These performed well, although the Par18 did end up getting a bit ripped up. This was entirely my fault, as I forgot that the screws holding the 80 camera shroud protruded into the airframe and were potential snag points. Luckily they did not cause the harness to hang up or prevent the drogue from ejecting, but they did cause two tears, each approx. 2” long.

The day was a bit windy, and the Par 54 was probably too large for this rocket, as I expected. My decent rate under main was a gentle 14 ft/s, but once on the ground, the wind gusts were catching the main chute and beginning to drag the rocket across the grass. By the time I ran out to it, it was already 700’ from where it had landed, and by the time I caught it (more running involved, it had probably gone another 500’ or more and was still moving in bursts as the wind caught it. No significant damage luckily, that’s one advantage to flying on a sod farm I suppose, the grass didn’t cause any rash to the paint job.

Altimeter data from the Eggfinder was left to rot until just recently, but when I finally checked it out last night I found it exhibits a fairly typical flight. Posted here for posterity along with a few photos from the day…



A huge thank you to my friend Jeff for all the great photos!

Congrats on the L1! I miss flying on that field - it's hard to beat the experience of recovering on a square mile of turf. And good job making it happen with dual deploy.

-brant

Congratulations on your L1 flight. I finally got mine up and figured out, too!