Little Joe II A-003 Mission Build

Discussion in 'Low Power Rocketry (LPR)' started by srfrich, Apr 21, 2019.

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  1. Jul 6, 2019 #61

    mikec

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    Most electronics are insensitive to magnetic fields from these magnets, which are pretty localized. For example, R/C planes often use magnets to hold hatches on right next to their receivers and ESCs. Exceptions would be anything that senses magnetic fields deliberately, like magnetic apogee detectors.

    I'm not seeing how moving the magnet would cause the BPC to come off, though, at least not during boost, since it would still be held on by aero forces.

    I've long thought about a staged Little Joe II but I wouldn't try it on a model this small.
     
  2. Jul 6, 2019 #62

    georgegassaway

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    Nearly 30 years ago, I modified my old beat-up Centuri 1/45 Little Joe-II for "Abort staging". I modified the BPC cone to hold an engine mount in the base of it, plus a rear eject system to deploy a parachute to recover it. And added a mercury switch system into a 1" long ejectable piston that fit in the upper part of the SM, just under the BPC cone, to both house the electronics and to keep the recovery system of the Main Body from being roasted by the A3 engine inside the BPC. It flew and worked out just great. Later I did a similar BPC Abort test with a 4" model, and C6 engine in the base of the BPC. It worked, but I decided not to pursue it for Contest models due to the reliability risks.
    [​IMG]

    I've never done an air-start staging, though. IIRC, Tom Beach did. I think it was an A-003 model, taking off on three C6's and then igniting 3 more C6's when the first set burned out.

    Here's something I've done a couple of times. A slightly smaller than scale Command Module, that fit inside of the BPC cone of my 4" (1/39.5) scale models, that came out after ejection. Using three parachutes, a tracking beacon, altimeter, and a live video camera looking thru the hatch window. Did it for extra scale mission points. [​IMG]
     
    Last edited: Jul 6, 2019
  3. Jul 8, 2019 #63

    srfrich

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    That's actually very encouraging about the insensitivity to magnetic fields.

    Yeah, but the flight plan idea was to jettison the LES after main ejection, not during boost, so was really not a 'staged' LES like you were thinking of, but an unpowered separation.
     
  4. Jul 8, 2019 #64

    srfrich

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    Impressive. And some inspiration for future builds. :cool:
     
  5. Jul 8, 2019 #65

    srfrich

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    Thanks for that. First, I should see if the basic air-start idea will fly; then branch out into more difficult add-ons, like the LES jettison. After all, a LJII kit is only $40, less than the cost of the 6 D motors!
     
  6. Jul 9, 2019 #66

    srfrich

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  7. Jul 14, 2019 #67

    srfrich

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    I took your word on that, and tested the electronics with two large neodymium magnets taped inside the base of the BPC. I heard the arming tone when I armed the system with the RC controller.:cool: IMG_1072.JPG After verifying that the magnets don't affect the electronics, I encased the magnets in 5/16" styrene tubing. IMG_1077.JPG IMG_1073.JPG
     
    Last edited: Jul 14, 2019
  8. Jul 14, 2019 #68

    srfrich

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    Spent the rest of the 'work day' tediously improving the fit of AVbay components with a Dremel. Had to sand down the corners of the PCB some more. It's a mess (nothing precision here), but fits comfortably now, and won't be frustrating for me at the launch site. Here are channel A and B outputs protruding through the base of the Apollo capsule. IMG_1078.JPG 2mm Gold Bullet female connectors soldered to the pins of the terminal block. IMG_1081.JPG IMG_1080.JPG Channel (A) is for the air-starts. Tweaked the male connectors for (A) so that they pull out with just a bit of force, freeing the capsule from the ignition leads that will run the length of the booster to the motors.
    The (B) channel connectors will be factory tight, for an e-match pair that runs to the base of the laundry room. The (B) channel is for an 'abort' command, or for ejection at apogee (visual). I don't mind a burned e-match hanging from the capsule during descent.
     
    Last edited: Jul 14, 2019
  9. Oct 10, 2019 #69

    srfrich

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    1st test flight in 4 days!!!
    Built a second CM/LES from a second Estes kit, using the stock kit LES tower legs and no BPC. This decision will let me concentrate on a first test flight without thinking about LES jettison. The new assembly on is on the right; same tower reinforcement design as the previous one, but it is one solid assembly with LES core tubes permanently anchored to capsule nose (1/2” and 7/16” telescoping).
    8899BE8F-0FAA-4CE3-A010-83F41E2BF300.jpeg

    6 oz of pine derby tungsten weights slip nicely into the 7/16” hole in the capsule nose.
    D8994CC1-8B5E-49DB-81CE-AA362346796F.jpeg

    Here is the CG balance test with full payload, and 6 D10s in the motor can. The CG is 8 1/8” from base of airframe, 1cm above the corrugated wrap, well above the “N” in the UNITED STATES decal.
    FEF9228F-86F1-4D12-8675-6BF9CFEFB130.jpeg

    Had to find the CG first, before finishing the motor can. For the CG test, it was dry-fitted with loose motor tubes.
    30072C4A-4973-4D0F-8080-27E707658117.jpeg
    77F7DB45-F0D8-41B8-8D37-A67D98E690C2.jpeg
    6 D10s look sweet...❤️
    ADE1CD31-5CB0-47AA-828D-75D02F322F92.jpeg
     

    Attached Files:

    Last edited: Oct 10, 2019
  10. Oct 10, 2019 #70

    srfrich

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    Laser bore-sighting the motor tubes to thrust through the CG (Build parameter 5).
    With CG known from previous step (8 1/8" from airframe base) the motor tube angle can be set.

    Initially squared up the motor can by using the work surface as a common reference plane for the front ends of the motor tubes and the front centering ring, which is 1/8” above work surface (checked level by sliding 1/8” scrap balsa several points around ring). The central tube is the only one that rests square on the surface, the 6 outer tubes are angled. I found that with judicious finger pressure, the assembly squared up nicely on the reference plane.
    61577E91-7B3A-46AF-AEC1-D26FA4C1E60A.jpeg
    The central tube was temporarily fitted with a special 18mm casing made from 2 expended casings - a nozzle on both ends.
    973A665D-F9E3-4362-8EA5-E6513CBAA085.jpeg
    In this shot, the second nozzle of the special casing is visible. 2F6FC79C-414D-429D-9117-F358D487A2A1.jpeg

    The motor can was then impaled upside down through the 2 nozzles, on 1/8” music wire which was held vertically in a vise. Used a clothes pin to keep the motor can from sliding all the way down. The CG, 8 1/8” from the tail end (up) of the motor can, was marked on the music wire. The laser bore-sight is shown here in one of the motor tubes. 9CF4402C-CE37-4078-A385-FFA87753D5C1.jpeg

    Then I slid the small plywood centering ring up and down to adjust the angle of the motor tubes until the laser dot hit the the CG marked on the music wire. (The large black cardboard centering ring is stationary and keeps the front ends of the motor tubes at a constant position, and the small moveable plywood centering ring adjusted the angle of all tubes equally at the same time).
    EA08E7B0-159C-4FBD-9D7E-6EF395EBFF31.jpeg
    DC3C9CBB-B40B-43DF-B42F-BF119CE5EBEB.jpeg
    With the angle set, measured the new distance between the centering rings (used a scrap of balsa). I made sure to measure all around. This balsa piece came in handy in the next step.
    D458E096-5E55-42FD-8698-50E35398EA1A.jpeg

    Carefully removed the motor can from the music wire and squared it up on the work surface again, measuring the centering rings again all around. I supposed I could have left the assembly on the music wire for the next step, but I wanted to true it one more time on the work surface.
    FAC86262-1570-4EED-A994-496A07FEE3DB.jpeg

    When confident that everything was true, drops of CA froze the centering rings and motor tubes in their proper angle. The motor can is ready to be handled and prepped for more serious glueing.
    8F9D5D4A-5792-45BD-B22C-22074A3AA5C1.jpeg 1EA1D127-2BBF-4B06-BA65-1539197FC5C6.jpeg
     
    Last edited: Oct 10, 2019
  11. Oct 10, 2019 #71

    srfrich

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    The aft centering ring, using kit parts, will need to be modified, ‘cause the holes aren’t wide enough apart to accommodate the new nozzle positions of the angled motor tubes. A Dremel with a small sanding drum should work.
    EA869101-0DFF-43E7-B5B6-6D169D4ABB29.jpeg
     
  12. Oct 12, 2019 #72

    srfrich

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    Finishing up the motor can.
    with CA, tacked the two layers of the kit-provided centering ring together, slightly offset. The offset was measured by 'squeezing' an 18mm motor tube, with casing inside for support, then counter-rotating the rings. The offset is slight.
    IMG_1260.JPG
    Then, tooled the outboard edges of the holes with a Dremel; elongating them to accept the angled motor tubes. Also, made new slots for motor retention hooks. Glued everything up with 2P-10.
    IMG_1262.JPG
    I wouldn't use 2P-10 near the nozzle-ends in future, because it is slower curing than advertised and it wicked/sagged into the motor retention hooks, causing problems.

    Finally, cut a small piece of the 24mm motor tube and used it with the 24mm thrust ring to secure the central hole of the centering ring to the central tube of the motor can.
    IMG_1261.JPG IMG_1263.JPG

    Then put the motor can on a bench sander and sanded the tops of the motor tubes flush with the top centering ring.
    IMG_1284.JPG
    The bi-layered aft centering ring is seen edge-on, as well as the angled nozzle-ends of the motor tubes. I decided against going for a 'finished look' by trimming the nozzle ends of the motor tubes flush with the aft surface of the aft centering ring in this iteration. May do that for a future build.
     
    Last edited: Oct 12, 2019
  13. Oct 12, 2019 #73

    srfrich

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    Jettison the motor can? Yes.
    The idea is that the motor can will thrust against a bulkhead inside the LJ-II airframe. The motor ejection charge(s) will do the work of ejection, and there is space around the 6 motor tubes to roll a parachute. Reason for this craziness? With the mass of 6 spent D10 motors in the aft end of the ship, it would be a hard landing for the fins.
    Neodymium magnets will hold the motor can inside the ship. The magnets basically self-positioned and were set with CA on the centering ring and bulkhead.
    IMG_1286.JPG
    IMG_1285.JPG
     
  14. Oct 12, 2019 #74

    Scott_650

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    Nice use of those strong little magnets!
     
  15. Oct 12, 2019 #75

    srfrich

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    Hey thanks!!!
     
  16. Oct 12, 2019 #76

    srfrich

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    Using LEXEL sealant to set the fins on the airframe.
    It's been so long (4 months) since I attached the vacuform ABS corrugated body wrap to the airframe (see post #36), I forgot how fun it is to work with LEXEL. LEXEL spreads like soft butter, doesn't string, sag, or run, and remains flexible after cured. It is a 'gorilla glue' of sealants, so I successfully used it like glue between the ABS wrap and paper body tube, creating a flexible 100% surface bond.

    In this recent pic, look at how the fin mount tabs (furthest from the camera) actually got compressed toward the body tube as the LEXEL cured inside them. The tabs closest to the camera are on an unused corrugated roll from a second kit, held there for comparison. My theory is that if the LEXEL has filled all the air gaps between the ABS wrap and body tube, and I also glue the fins to the ABS wrap with LEXEL, filling all the air gaps, then there is a continuous 100% flexible surface bond between the body tube and a significant amount of surface area inside the fin roots, with a sandwiched layer of ABS. For the joint to fail, the LEXEL would have to fail.
    fullsizeoutput_f1.jpeg
    Gluing the fins was a snap with the LEXEL.
    First, trimmed some scrap styrene sheet to make little plugs for the big void inside each fin, as these spaces will be filled with LEXEL. Glued the plugs in with Plastuct Plastic Weld. In the pic below, the fin on the left was done, on right still waiting for a plug.
    IMG_1264.JPG
    The are 4 voids at each fin root. I filled the bottom and upper middle void with LEXEL, then flattened it with a putty knife, making sure the void was completely filled for a 100% surface bond.
    IMG_1272.JPG
    This is a bad photo taken of the third fin with bottom and mid-upper void filled with LEXEL, before the LEXEL was worked with a (messy) putty knife. The LEXEL is easy to work with. Lesson learned: next time clean the putty knife (with mineral spirits) after each fin.
    IMG_1273.JPG
    The fins attached like they were born to do; used a thin rubber band for peace of mind. No drips. I love it. We'll see if I still love it in 40 more hours, if the LEXEL cured enough to endure a test flight.
    IMG_1275.JPG IMG_1278.JPG
     
    Last edited: Oct 12, 2019
  17. Oct 12, 2019 #77

    srfrich

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    Securing tungsten nose weights inside LES tower.
    the weights slip fit inside the 7/16" inner LES core tube, which extends thru nose into the capsule. IMG_1280.JPG
    Made a hollow double-wall plug by gluing a short length of 5/16" tube into a 1 3/4" length of 3/8" tube.
    IMG_1281.JPG
    Jammed the hollow plug all the way in against the core of tungsten weights using glue.
    IMG_1282.JPG
    Filleted with epoxy.
    IMG_1283.JPG
    FWIW, I can still see the bottom tungsten weight through the hole. With this through-the-nose mounting of the LES core tube, the 6oz of nose weight doesn't stress the LES tower. (see post #39), and fulfills build parameter #6.
     
    Last edited: Oct 12, 2019
  18. Oct 14, 2019 at 4:38 AM #78

    srfrich

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    Maiden test flight a total fail. Things I had hoped to learn on this test, but didn't:
    a. performance of the angled motor can with 3 D10s for boost.
    b. descent rate of capsule+LES under 3 chutes
    c. the damage the rocket would sustain under a full 'chutes landing.

    I used the best igniters I could for a small nozzle. All 3 igniters fired, but only one D10 actually ignited. The bird barely cleared the tower, then pancaked fully loaded. (6 D10s were carried but only 3 had igniters, to test a full launch weight for a 3x3 boost/air-start configuration.)
    fullsizeoutput_10e.jpeg
    Lessons learned:
    a. The LES core tube is weak. It needs to be double-wall all the way thru the capsule nose, and possibly filled with micro-ballons+CA or epoxy. Or go with 1/2” OD aluminum tubing.
    b. The LEXEL has a 2-week cure time, and the fins got only 40 hours. That's not enough. Luckily, after the old LEXEL is scraped off, the fin that separated can be repositioned.
    c. Come up with a better way to light D10s. This looks promising; wish I thought of it earlier.

    Despite today's results, this was a successful test because of the lessons. :confused:
     

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    Last edited: Oct 14, 2019 at 7:19 PM
  19. Oct 15, 2019 at 12:44 AM #79

    srfrich

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    Last edited: Oct 15, 2019 at 2:36 AM

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