L3 experiences, advice, or just general ponderings…

Forums Knowledge Base L3 experiences, advice, or just general ponderings…

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  • #53479
    James Russell

    Wow, I need to get out more, this is a great thread!

    I have not seen this until today and replying to this having not read all of the thread but If you are getting ready to go for your L3 you have already learned good construction skills even if it is on the lighter end of L2 with 38mm rockets. There is a big difference from the 38mm to the 75mm that most attempt their L3 with but it really is just a bigger rocket with a lot more mass and a lot more power. Most of the people building their rockets,(even me) over build so construction is not usually an issue. Design, attitude, altitude and recovery are the the things that I see people have problems with.

    I like seeing different things, so many of us have different ideas and experience, as some of you have said, fly what you are planning to after you get where you want to be. Some say KISS, do what you know. Its all the right thing. I personally like people to do what they know. Out of the close to 60 L3 certs I have done the failures that I have seen have been when doing something new there was not as good of comfort level from the flier the new element that they brought to the project. If it is the use of a D-Bag then make sure you know all the ways it can be use, know its pro’s and con’s. If it is a new flight computer, know everything that there is to know about it, play with it test it, know its weaknesses.

    I like to see people use the skills that they have learned over their journey in rocketry when attempting their L3. We don’t expect you to know everything because we don’t. But the one thing I do look at is how do you salve problems, do you work through things or just go with the blow it up or blow it out mentality.

    Things have really changed over the years with the availably of fiberglass tubing, digger motors, better electronics and bigger waivers, 10 years ago it was not that common to see L3 flights over 2 miles. now most are close to 3 miles. The altitude as I had said is another factor that I have seem cause failure. A lot of the people attempting their L3 have not flown to that altitude with larger airframes. That is why it is so important to ground test over and over. I would also say that you should solicit the help of someone that has because you may not know what to expect for separation at ground level and then at altitude.

    #53480
    Bret Packard
    Participant

    Great post James, and it hits on one of my biggest concerns for this L3 project, ejection charges. How do you figure out how to compensate for altitude? I’ve always ground tested my stuff, but how valuable is ground level information when the real action is going to be going on somewhere around 14,000 ft., and hopefully 20K+ down the road? Is there a rule of thumb factor? Apogee seperation is obviously one of the most critical points in the flight, but also probably the hardest to predict/perfect at altitudes.

    #53481
    Chris LaPanse

    Well, if you can contain the black powder, it works pretty well at nearly any altitude. I’ve had good success with surgical rubber tubing and zip ties for a number of 12000-19000 foot flights (including my L3), and I’m planning to use the same method for a 25k+ flight at Mayhem. John Wilke has experience with this method at over 30k, so he’d be a great one to talk to. Also, even standard charges seem to work decently at 12k or so. My L2 went 12k a couple of times with standard charges, and had no trouble. It also flew to 17,250 once, and it only barely separated with regular charges. I’d definitely go to some sort of charge containment for anything over 12-14k or so.

    #53482
    James Russell

    Chris is right, the longer you can contain the bp the more will burn. When ground testing you want to have a fairly energetic separation but not quite like Ernie’s. With the drag on the ground you may not get fully extended and that is fine because in the air you will have no drag and you will find the end of your shock cord fairly easy. That is why you want a long shock cord on the drogue side. You also might want to bundle the shock cord in small parts and tape them together to help slow the separation to help release some of the energy before the two parts hit the end of the cord.

    Some people will tell you that once over ~10k you want to add a little more for good measure but then others will tell you that if you can contain the bp and get a more complete burn you will have more of an efficient charge and more reliable charge. I think the more consistent you are with your charge making and a little good measure with good confinement you will be fine up thought 30-40k.

    #53483
    Steve Jensen
    Participant

    I flew my Pork Arrow at Hartsel and had a partial as in NON-separation at Apogee. You could see an apogee event of sorts, but no separation. Someone said “it’s still traveling…” giving me a rather sickening feeling. Finally the main blew as it was balistically descending causing the booster section to finally separate too. The result was a blown chute, a destroyed nosecone with interior centering rings ripped out but my DC-30 survived! So did the big diameter Kevlar (thanks John W.) and my altimeter even survived the event. The ebay and fore air frame were damaged and destroyed; however, the booster section was almost untouched. It will fly again shortly!

    It’s a Performance Rocketry Arrow — 4 inch diameter air frame with a fair to large amount of space in the booster section. I ground tested twice with 2.5 grams of BP in the surgical tubing set up and did use shear pins. It’s a moderately heavy rocket — about 12 pounds loaded, so the K-630 took it to 5800 feet but over 14000 feet ASL. I learned to add a bit more BP for altitude. I’ll shoot for 3.0 grams next time and maybe use one of my Ravens with a back up charge. I also plan on sanding, priming, and wet sanding the coupler junction to ensure it’s as smooth as silk. Somehow it bound up and/or needed a higher charge at altitude. I will still use shear pins, just more black powder.

    Still learning. I’ve now overcharged and undercharged ejection. It’s a bit of an art. I’m still glad the main blew on the Pork Arrow. A ballistic deploy of the main is better than a ballistic landing, a.k.a. trash bag recovery. I retrieved my altimeter, DC-30, booster section, motor casing, and harness. In essence, I got most of my investment back. I got the chute repaired (thanks Fruity Chutes for a nominal repair fee) bought a new fore AF and Nosecone, and am rebuilding the e-bay. Off we go again!

    #53484
    John A. Wilke
    Participant

    I’ve really never worried about black powder under about 20K MSL. Above that, I do three things…

    First, I follow the technique here http://www.wimpyrockets.com/page16.html Note that the author there is none other than Tony2. I’ve used this with clean recovery to over 35K and Tony has used it to about twice that. Our Q project years ago had charges that worked at about 59K. Unfortunately, the rocket was exiting mach at the time.

    The 2nd thing I do is I add a bit more BP than I needed at the ground test, just in case the rocket is binding on itself for whatever reason. Improbable, but possible. You may also have some headwind to deal with. Throw in the temperature changes and stuff moving around inside under gees and gremlins and the low cost of BP, and I think an extra 1.0-1.5g is pretty cheap.

    The 3rd thing I do is I use L-O-N-G shock cords, mostly to the paragraph just above this one. No sense having a great flight and then having rebound damage.

    #53485
    Bruce R. Schaefer

    Well, JW, I agree with you on LONG shock cords, my L3 100 foot can attest to that. Although this guy, JW, doesn’t use masking tape to reef the Kevlar, he uses electrical tape! I have yet to use latex glove fingers–as many in this club do, but that seems reasonable for highest of altitudes, and lower.

    I use 2+ grams for a 3″ airframe, depending on the volume, so 2.8 g on a 4″ PR kit seems a little low. Ever since Conway used 12 grams on his secondary charge on his 14′, 6″ L3, I would go higher. It’s all about pressurizing the volume you’re dealing with. I switched to fiberglass when I blew up my beloved GLR VA both during a ground test and in the air.

    You know what’s so good about this thread is that it deals with all of us, at ANY level. If you’re L1, you want to learn how to go stronger. If you’re L2, you want to know what’s next. If you’re L3 you want to pay back and help, as well as LEARN.

    #53486
    Steve Jensen
    Participant

    are a must. Heftier ones than you think you need seem to reap benefits.

    On the other hand, if failure deterred us, we’d have all quit!

    Nothing is guaranteed, only death, taxes, and if your rocket goes up, it will hit the ground eventually. …

    #53487
    Bruce R. Schaefer

    On the other hand, if failure deterred us, we’d have all quit!

    My friend , failure is NOT an option. ūüėČ

    #53488
    Chris LaPanse

    I use 2+ grams for a 3″ airframe, depending on the volume, so 2.8 g on a 4″ PR kit seems a little low. Ever since Conway used 12 grams on his secondary charge on his 14′, 6″ L3, I would go higher. It’s all about pressurizing the volume you’re dealing with. I switched to fiberglass when I blew up my beloved GLR VA both during a ground test and in the air.

    Honestly, 12 grams on a 6 inch is a bit excessive. Similarly, for the vast majority of 4 inch rockets, 2-3 grams should be plenty. Obviously, this should always be verified through ground testing, but in general, there shouldn’t be a need for anything over about 2-3 grams on a 4 inch, or around 6-8 grams on a 6 inch. Of course, it is also rocket-dependent, and some rockets will take more than usual, but I’m assuming fairly standard size parachute bays with a non-excessive quantity of shear pins.

    (Of course, if there’s one thing that can’t be said too often, it’s that every untested configuration should be ground tested. If you aren’t sure, try it out before risking it on a flight)

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