Aspiring L2 Looking for Some Guidance

[Matt Pritchard]

I've been away from rocketry for a couple of years and looking to jump back in. I'm certified L1 and have a few H-power flights under my belt, though the last one ended with a spectacular shred.

I'm working on my next rocket, which I hope to use as an introduction to dual deployment and eventually L2 certification. I'm at the point in the project where I need to make some design decisions before I go too much further. I was wondering if there was an experienced L2+ flier out there who'd be willing to provide a little guidance and let me bounce some ideas/options off them. Most of my questions have to do with the redundancy in the recovery system and some construction techniques. I think I could get most of my questions answered over the course of a few email exchanges.

If you're willing to help, please let me know best to get in touch with you. Thanks.

Matt Pritchard

[Anthony Cooper]

This is the group for that. Fire away. There are plenty of lurkers who may need to learn the same things. 

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[James Marino]

This is the place for it. Whole big ol' bunch of L2+ fliers on here. :-D

J

[Matt Pritchard]

Alright...sounds like it's best to post them here and let folks chime in. Hopefully it's helpful to others as well - good point, Tony. Here it goes:

1. I’m trying to determine how much redundancy to build into the recovery system. The rocket (Binder Excel) is designed for dual deployment, and I have a Raven 2 altimeter.
   
• Should I be running dual altimeters, or is one enough?
• Should I set a back-up charge for both chute deployments? I have a total of four channels to work with.
• Can I just use motor deployment (full delay) as a back-up for my drogue chute and call it good?
2. Where is the best place to buy e-matches? Do I need a LEUP to get them?
   
3. Should I use shear pins at the separation points, or is a good friction fit good enough?
4. Any recommendations on using shear pins with a cardboard airframe?
   
5. What’s the best way to run a test of the deployment charge to ensure it’s sufficient? 
   

[Steve Kendall]

Hi Matt,

Great questions.  Lots of solutions.  Here's my take:

Redundant.  Not required for L2, always a good idea regardless. I flew an L2 bird at October skies last weekend.  Decided to use both altimeters. Good thing the Raven failed to fire its e-matches.  Use two, and if possible, two from different manufacturers.

Motor delay: great idea for back up, but beware late deployments... You can get heavy zippers.

BAR sells non-leup e-matches

Sheer pins are good, but not required unless you're going really high or can't get a tight fit. A tight fit is usually good enough.  Make sure you drill a vent hole.

Cardboard and sheer pins:  soak the hole with CA and re-drill. It will last for several flights.  Or, you can try things like brass tubing inserts, etc.. I don't bother any more. If the holes get sloppy, I just drill a new set.  But I rarely pin cardboard anyway.

Testing:  place your charges as usual with all the junk inside (parachutes, shock cords,etc.) All your pins, if you used them, and ruin the match writes out your vent hole. Prop it up, point it away from things you care about, and touch the leads to a 9V. If the pins don't sheer or the laundry doesn't come out, repack and try again until it all works well.  (This is a great way to meet your neighbors)

Good luck with your build!

Oh, and I'm sure there will be many other solutions presented. Pick what feels good for you and see if you can make it work.

Steve

Sent from my Verizon Wireless 4G LTE DROID

--

[Sam Fineberg]

Great answers from Steve. but I have a couple of things to add.

Motor backup for Drogue works well as long as you can get the right delay.  One thing to remember is that breaking an airframe apart alone will slow descent from 100's of ft per second (i.e. ballistic) to something much more reasonable (often <50 ft/sec).  That alone will suffice on many fiberglass rockets (though I don't recommend it, you should really be <25 ft/sec at landing).  So, a backup on the drogue is much more critical than the main.

Sheer pins are much more critical for the main, since it needs to stay intact through the drogue deployment, so I have often used sheer pins for the main and not the drogue.  Of course, I have also used them for both.  The main advantage of sheer pins is consistency.  They always sheer with a certain amount of force.  Friction can vary based on judgement when you wrap the joint with tape, wear on the tape, even weather.  Of course, none of this is precision engineering, so friction usually works.   Again my main concern is getting the airframe separated, to prevent a ballistic recovery.  If you can do that, the chances are you will get your rocket back.

Q2G2 igniters can also be used instead of the unregulated e-matches.  BAR has both.

Sam

[Steve Kendall]

Hi Sam!  Nice to hear from you.

 

I agree.  You went a lot deeper than my “watch out for late deployments”.  A late deployment is preferable to no deployment, but may cost a certification.

 

Since I fly a lot of composite airframes, I do use sheer pins.  #2 nylon screws, usually three of them.  (sometimes four on bigger airframes, but I’m not completely convinced more than three are required up to 5” A/F).  And pins are only consistent while their holes remain the original size.  Once they get sloppy, I’ve found that the pins don’t always sheer cleanly, especially in reinforced cardboard.  Once again YMMV, test before you fly.

 

Lately I’ve seen (and heard about) people using masking tape across the seams on un-pinned rockets.  Usually, they add a small initial tear in the tape at the seam, and this becomes the sheer release.  I tried it; it seemed to work.  But if you check your friction fit each time you fly (as noted the weather alone and change the fit) you should be good to go.  But then again, I still tape in my motors (on minimum diameter birds).

 

So Matt, keep asking questions.  Everyone here on this list has experience: some of us more than others, but collectively we’re pretty strong.  I certainly can’t tell you everything that works, but I can definitely tell you what doesn’t.  J

 

-Steve

1.      I’m trying to determine how much redundancy to build into the recovery system. The rocket (Binder Excel) is designed for dual deployment, and I have a Raven 2 altimeter.

o    Should I be running dual altimeters, or is one enough?

o    Should I set a back-up charge for both chute deployments? I have a total of four channels to work with.

o    Can I just use motor deployment (full delay) as a back-up for my drogue chute and call it good?

2.      Where is the best place to buy e-matches? Do I need a LEUP to get them?

3.      Should I use shear pins at the separation points, or is a good friction fit good enough?

4.      Any recommendations on using shear pins with a cardboard airframe?

5.      What’s the best way to run a test of the deployment charge to ensure it’s sufficient? 

[Tim Robinson]

Agree with all said by Steve and Sam (except I refer to them as shear pins).

As one of those that Steve cites as using masking tape on the exterior of rocket as a shear pin alternative for my smaller (2.6") cardboard rockets, I'll go into a little more detail.

I have been using 3 strips of 7/8" blue tape for a couple of years (over 15 flights) over the seam of the nose / payload bay joint. I use the same length strips each time and firmly rub the tape after applying. Blue tape sticks less than masking tape, so it's more likely to peel off from your rocket than to peel off your paint after the flight. I don't nick the tape after assembly. Unlike having to adjust friction fit (these are plastic nose and cardboard rockets and those materials have quite different temperature coefficients in my experience) for weather conditions, I've found it to be very reliable. Come to think if it, the tape is pretty smooth, so maybe the term sheer pin works too ;)

I once had a high-speed deployment (delay started burning before motor came up to pressure and my motor "backup" went off 5 seconds early) and from the acceleration forces and weights of components, I estimated the force to rip the tape as at least 12-15 lbs. Typically #2 nylon shear pins break at twice this level or a little higher.

I've used nylon shear pins on larger cardboard rockets where the nose weighs more and deployment forces might exceed the strength of tape ( I guess you could add more strips... ). I drill cardboard, soak the hole in ca and use a small piece of 0.010" brass sheet epoxied to the nose to provide a sharp edge to cut the pin.

If you're in the area - I'll be bringing one of my rockets I fly with tape shear pins to the Moffett launch this weekend.

-Tim

[Steve Kendall]

Funky spell checker…you should see what I get when I Swype on my phone….

 

But…my pins are sheer.  How else do you describe a #2 nylon screw?  Things are so tiny I can barely feel (or see) them anymore!

 

-Steve

[William Ferry]

Hi Matt,
I'll start out by saying I'm not an L2 yet, but I have built a Binder Excel w/DD and it's the rocket I've tried to do my L2 with, I just keep running into issues that are (mostly) not the fault of the kit. But I will comment on my thoughts as I've worked towards my L2 with the same kit. Comments in-line below.

> On Oct 20, 2015, at 6:17 AM, Matt Pritchard <mgpri...@gmail.com> wrote:
>
> Alright...sounds like it's best to post them here and let folks chime in. Hopefully it's helpful to others as well - good point, Tony. Here it goes:
> • I’m trying to determine how much redundancy to build into the recovery system. The rocket (Binder Excel) is designed for dual deployment, and I have a Raven 2 altimeter.
> • Should I be running dual altimeters, or is one enough?
> • Should I set a back-up charge for both chute deployments? I have a total of four channels to work with.
> • Can I just use motor deployment (full delay) as a back-up for my drogue chute and call it good?

The av-bay in the Excel has a LOT of room, being 14" long and 4" diameter. So there's certainly space to fly more than one altimeter if you want to. My other DD rockets have been 6" long bays either 38mm or 54mm in diameter, much harder to put multiple electronics and batteries into. That doesn't mean you have to do it, but if you feel better with redundant electronics this rocket makes it easy.

I have no experience with the Raven, seems like some people have had issues with it from what I've seen on the forums, maybe its hard to configure right or finicky or something. I've used PerfectFlite StratoLoggers (SL100 and SLCF) as well as the Altus Metrum Easy Mini (Bay Area Rocketry, our local vendor, sells both of these brands) as well as Eggtimer Rocketry's Eggtimer TRS, which you have to solder yourself. I built my Excel's av-bay to support a SLCF and an Easy Mini. I've also flown AltimeterTwo and AltimeterThree's in the rocket as well as an Eggfinder GPS tracker (for the rockets that use other altimeter(s) for deployment), I'm an engineer so collecting flight data is something I'm very interested in.

When I initially built my Excel I was going to use dual electronics, but a single charge well on each end of the av-bay, with two e-matches in the well, one from each altimeter. I later changed it to two wells per-side for total redundancy.

Ultimately, my opinion is that the redundancy depends on what you trust / have had good/bad luck with in the past (as well as how much you're willing to invest in electronics and spend on e-matches and BP of course). I have 100% confidence in my altimeters, I've flown them a bunch of times now and they've always worked properly. I've had pretty terrible luck with e-matches, both homemade by an L3 I know and commercially-bought, so I wanted redundant electronics purely so that I could have redundancy on the e-matches. The black powder has worked fine for me, but I've used 'proper' Goex 4F black powder and I'm still dialing-in the proper amount per charge, I also have a can of Pyrodex P that I'm afraid to use for charges since I've heard so many bad things about it compared to BP.

I've also had terrible luck with AeroTech motor eject, so for me I decided it wasn't even an option since they frequently went off way too early, way too late, or not at all, causing serious damage if not total destruction on several of my rockets. I have had better luck with CTI motors, and after my last failed flight I should have left the CTI motor charge in place, it might have saved the rocket from damage and my L2 cert.


> • Where is the best place to buy e-matches? Do I need a LEUP to get them?

I'd love to hear the answer to the second part from someone in the know, the whole LEUP thing still confuses me here.

The self-claimed "only ATF non-regulated" e-matches come from electricmatch.com, the MJG Firewire Initiators. Bay Area Rocketry sells these as well, both by the box (80x) and individually. MJG had a bad batch early this year, and I happened to get a bunch from that batch, so it's what led to my initial issues with e-matches as most of them never fired, they simply blew open-circuit when current was applied. But later batches have been working better for me so I'm getting over my distrust of e-matches finally.


> • Should I use shear pins at the separation points, or is a good friction fit good enough?

> • Any recommendations on using shear pins with a cardboard airframe?

The design of the Excel is that separation occurs at both ends of the av-bay (more commentary on that below). If you do this then you need to do something to keep the nose cone attached to the upper body tube. I used 3 small removable rivets from Giant Leap Rocketry (<https://giantleaprocketry.com/products/components_recovery.aspx#Removable_Rivets>) to secure my NC during flight, I wanted something removable as I have electronics in the nose so I needed easy access to them before/after flight.

I used only friction-fit on the av-bay to upper tube joint and have never had a premature separation. My fittings are pretty tight though as I did a "CA soak" on the body tube ends so that reduces the ID a bit (I had to do a lot of sanding to get things to fit at all after that, but I left it a pretty tight fit).


> • What’s the best way to run a test of the deployment charge to ensure it’s sufficient?

I strongly recommend a ground test, I should have done more of it myself. I was nervous about setting off charges in my back yard since I live in a pretty dense neighborhood and was concerned about the noise. The one test I finally did wasn't as loud as I expected (though I did start small with the charge), and I upsized from there without any further ground testing, and only ground-tested the drogue separation not the main, all of which I regret now. After I make my repairs I plan to do a lot more ground testing and I don't really care how much noise it makes, of course I might try to line it up with when a neighbor is mowing their lawn or something to help mask the sound.

I don't know if the Raven does anything to make ground testing easier, the SLCF and EasyMini both require connections that would require a big hole to fish out a pretty big cable plug. But I also have an Eggtimer TRS which can do remote firing via an RF link, so I did my ground test using that even though it's not an altimeter I fly in this rocket. As I said above I didn't see any reason to test the actual electronics, they've all been tested in the field already, I was just trying to ensure I had good e-matches and work on charge sizing.

I've flown with an on-board camera, and my first flight using the charge wells instead of motor eject I had filled the 1.5g well to the top, and that was definitely way too much. The camera showed that I threw the lower section away at about 40 mph, and it really jerked the cord at the end so I'm lucky I didn't damage anything. I did have each set of 5 Z-folds of my 25' kevlar shock cord (I didn't use the nylon that comes with the kit) blue-taped together though at that speed I'm not sure it helped any. So for my ground test I went down to 0.5g which was enough to separate the rocket but not fully-stretch the cord, I felt it was a bit light so I went up to 0.6g for the flights I did last weekend (October Skies), and for the drogue this was probably fine since its so loose in the tube, but I should have gone larger on the main charges, as in two separate flights the main failed to come out of the tube both times.

So all-told I've done 5 flights on my Excel, 3 were unsuccessful L2 cert attempts:

1) First flight was on an AT I280DM, I wanted to use something disposable for the first flight so I wasn't putting a casing in jeopardy and just make sure the rocket flew well (happens to be my favorite motor so far also =). It was motor-eject only (single deploy, so I put the main in the lower tube and the drogue in the upper), the electronics just came along for the ride to collect data as I had no functioning e-matches at that time. I got one too many questions wrong on the TRA L2 exam this launch, so I didn't get to fly an L2 motor at this event.

2) Second flight was a month later (after passing the exam) on an AT J350W, motor-eject single-deploy again as I had only received replacement e-matches the night before and had no opportunity to ground-test them so I still had no faith in them. Well, the motor eject never fired, so the rocket landed as it went up, at about 300mph. Everything was destroyed though I did manage to salvage two of the altimeter EEPROMs to get flight data at least. In hindsight I should have loaded-up the wells even without trusting the e-matches, as it very likely would have saved the flight/rocket. The only theory anybody could come up with for the BP in the forward closure not firing was grease at the opening, even though I had tried very hard NOT to get grease in that area so I'm still baffled. I had built the motor the month before and it sat on its side for a month, so I don't know if grease could have moved around inside over that time.

So I decided to re-build this rocket rather than try a different kit.

3) First flight of the re-built rocket was on another AT I280DM last month at XPRS, this time with no motor eject and all 4 charges loaded to their tops (the wells claim to be 1.5g, I didn't measure how much volume there really is once the Firewire Initiator is in place). This flight was successful (and all 4 charges went off) though the video showed the events were pretty violent and the chutes did get a bit tangled, but it landed without damage. I also noted during this flight that the provided 12" drogue seemed a bit small, things really didn't tumble in an orderly fashion so I decided to swap that chute out for an 18" drogue (Bay Area Rocketry yet again).

4) Second flight was my next L2 cert attempt this past weekend, with a CTI J360SM (yes, I like the sparkies). The drogue came out just fine, and the main charge fired but failed to pull the main out of the tube, so it landed on the 18" drogue only. No damage to the rocket, but an unsuccessful flight since the main never deployed.

5) In the hopes of making the L2 cert "easier", I went back to single-deploy, swapping the main and drogue positions once again, though I stuck with electronic eject and no motor backup, and no charges in the upper (drogue) bay, with a CTI J145SM-LB this time. The apogee charges fired just fine, but the 0.6g once again failed to pull the chute out of the tube. So it came down separated but chuteless and hit hard, breaking one of the fin fillets on the outside (not sure how much damage the inside fillet took) and tore the tube a bit. Had I rigged the drogue to pop at 700' anyhow it *might* have slowed things down enough to at least avoid damage, or had I left a motor charge since it would have been behind the chute it might have helped push the chute out.

But that does seem to be one possible issue with Binder's design (I know it works for others, but I'm certainly having trouble with it), is that the main gets "pulled-out" instead of "pushed-out" by the charge. So I'm debating switching to making the upper BT to av-bay attachment more permanent and separating at the nose cone instead, so that the main charges can push the chute out the top. I'll also try ground-testing with larger main charges as more inertia might serve to pull the chute out as well, so if I can reliably do that on the ground I'd feel better. The stock main chute plus a chute protector (Sunward 18"x18" in my case) plus the 25' of kevlar shock cord doesn't make for THAT tight of a fit in the tube, and I even added some baby powder on that last flight to ensure things would slide more easily, yet the chute still didn't come out. Unfortunately I didn't get to inspect the rocket as it had landed as kids had moved the rocket and pulled the chute out before I could get to it, but the on-board camera video shows the bundled chute sitting at the very end of the tube but not coming out during the tumble.


FYI, another good resource I've used is The Rocketry Forum, I have a build thread for my Excel at <http://www.rocketryforum.com/showthread.php?121831>. I haven't updated it with this weekend's activity yet, still going through my pictures/video and the like (and catching-up on work that I missed last week because of the launch).

So I have some repairs to do on the fin, I may have to fiberglass it to get it sound again, will have to remove the cracked outer fillets and see just how damaged the tube is and if the internal fillets broke or not.


I will say that my Dad took a simpler approach and did his L2 last month at XPRS using a non-DD Excel. So he was pure motor-eject single-deploy, no electronics at all. He used a J250W DMS, the first one CATO'ed inside the rocket (top of the motor case blew apart) and did quite a bit of damage including tearing out the eye bolt, but we re-built it at the event and flew it again on a replacement J250W (AeroTech was on-site at XPRS so they replaced the motor for no charge) and his second flight was successful so he got his L2.


Good luck with your build and your L2! - Will

> On Monday, October 19, 2015 at 10:30:19 PM UTC-7, Matt Pritchard wrote:
> I've been away from rocketry for a couple of years and looking to jump back in. I'm certified L1 and have a few H-power flights under my belt, though the last one ended with a spectacular shred.
>
> I'm working on my next rocket, which I hope to use as an introduction to dual deployment and eventually L2 certification. I'm at the point in the project where I need to make some design decisions before I go too much further. I was wondering if there was an experienced L2+ flier out there who'd be willing to provide a little guidance and let me bounce some ideas/options off them. Most of my questions have to do with the redundancy in the recovery system and some construction techniques. I think I could get most of my questions answered over the course of a few email exchanges.
>
> If you're willing to help, please let me know best to get in touch with you. Thanks.
>
> Matt Pritchard
>

[Steve Kendall]

Here are my tried and true rules for rockets:

-->> ALWAYS GROUND TEST (Repeat: ALWAYS)

-->> There is a reason that for L3 you MUST use redundant electronics. Electronics FAIL. If you get BP smoke in your AV Bay, it will degrade your electronics. Trust me, this is true and well known by those that have flown for a long time. And typically, despite what you >believe< is a sealed AV Bay, unless you have machined edges with O-rings, your AV Bay is not sealed. Use redundant electronics whenever you can. It's just safer that way and the extra e-matches and BP are far cheaper than a new kit and new guts.

-->> There is no requirement to certify L2 using electronics. I believe in the KISS principle when doing your cert flight. Don't add complexity that you don't need. (Electronics is fine, but maybe as the back up to motor ejection. This works well, and while there is no specific requirement as to how you get the laundry out, there is one that it has to deploy. Be safe and think safety at all times.) Redundancy has nothing to do with luck or past performance (see above regarding failures).

-->> ALWAYS USE FRESH BATTERIES ON EVERY FLIGHT. A battery is cheap; that altimeter is not, nor is the casing.

-->> You do not need a LEUP to store AP motors. You do need one to store e-matches and BP (there are some exceptions, but as a rule, this is the case). Having said that, you only need one for STORING these things. Buy them at the launch and burn them. No LEUP required. And trust me, you really don't want to go down that road. Ask anyone in Aeropac about inspections.

-->> Shear (spelling fixed) pins are important because:
> If you have a late deployment, your N/C is pointed down. When the drogue deploys, that whole section of airframe gets pushed towards the ground, aided by your friend Gravity. When it hits the end of its rope (the end of the shock cord) there is additional force (all that mass your N/C is holding back) pushing on the N/C. It very easily can come off, with a long walk ahead. Ask me how I know.
> If you don't want to drill vent holes, you must pin if you fly high. The air is thinner up there: the air inside the A/F is at a higher pressure than the outside air. Tubes will separate when you least want them to.

-->> To quote a very experienced L3: "You don't have too much shock cord if it still fits.". If you're using 25' and you're still bottoming out, if you can, add more S/C.

And a final note. There are BP calculators online that will help you estimate your BP charge size. Remember, too, that the airframe size matters, but maybe not as you think. If the A/F is 2" diameter, the surface area of the bulk head your charge is acting on is about 3 sq. inches. In a 4" airframe, the surface is ~12.5 sq. in. The calculators will tell you how much pressure you can generate in a specific volume. Let's say for grins that it's 10 psi. Multiple that by your surface area, and guess what? Yup, in the 2" airframe you're pressing on that bulk head with about 30 pounds of force. In a 4" A/F, it's more like 125 (for the same gas pressure - you need more BP for that) but... my point is, just because it's a larger A/F doesn't mean add more BP. It might really need less. But if it's small, and you use shear pins, beware: it might not shear as expected. ALWAYS GROUND TEST!!

I tell new fliers that I can't tell them everything that works, but I can tell them with confidence everything I've found that doesn't work. Pay attention to other people's failures. If you learn and understand why they failed, then you can avoid that failure yourself. This really is rocket science. Behave that way. If you're not sure about something, ask the List. Lots of real scientists and engineers out there (and just plain experience) and they are all willing to help you avoid a Bad Day Event.

-Steve

Oh, and I've had 2 Ravens fail so far. One just quit working. The other one seems to fail to light e-matches from time to time. Like Saturday at October Skies. Another one bites the dust. Sure glad I had a back-up altimeter in there. Carbon fiber tubes are not cheap.

[Matt Pritchard]

Steve, Sam, Tim, Will:

You guys are awesome! Thanks for all the great input - very helpful. I'm currently finishing up the fin can and I'll be moving on to the av bay next. I think I'll lay out the av bay with flexibility in mind. I'll give myself the option for redundant charges at both ends. And I think I'll pick up a second altimeter to back up the Raven 2, probably an EasyMini or StratoLogger. Thanks for the suggestions on e-matches and also shear pins vs. friction fit. Either way, I got the message: ground tests are critical to dial in the charge and test the system. As I continue to make progress on the build and get closer to a maiden flight, I'm sure I'll have additional questions about test flights, motor selection, etc. Thanks again for your help.

Matt Pritchard

On Monday, October 19, 2015 at 10:30:19 PM UTC-7, Matt Pritchard wrote: