Scratch - 24mm Sprint {Scratch} Original Design / Scratch Built

Contributed by Tim Burger

Manufacturer: Scratch

(Contributed - by Tim Burger - 06/15/03)

Brief:

Two Sprints
Two Sprints: A sport version and a competition version
This is an upscale of the venerable Estes Sprint streamer competition model.. My NAR section recently selected E impulse streamer duration for one of the events for our Spring meet. An upscale of this classic kit to BT-60 size and sporting a 24mm motor mount would be perfect. By adding rear ejection to the picture it should be very competitive.

Construction:
Parts List:

  1. Nose Cone, parabolic, balsa, 5 7/8-inch long from the Balsa Machining Service
  2. BT-60, 16-inches long from Estes (local hobby shop) or Totally Tubular
  3. BT-50, 3-inches long from Estes (local hobby shop) or Totally Tubular
  4. BT-20, 12-inches long from Estes (local hobby shop) or Totally Tubular
  5. Tube coupler for BT-60 from Estes or Totally Tubular
  6. Balsa sheet 3-inches x 12-inches x 3/16-inches from the local hobby shop
  7. Centering rings, paper, CR2050, centers BT-20 in a BT-50, one required from Estes, Totally Tubular, or Apogee components.
  8. Centering rings, paper, CR2060, centers BT-20 in a BT-60, two required from Estes, Totally Tubular, or Balsa Machining Service
  9. Centering rings, paper, CR5060, centers motor mount, two required from Estes, Totally Tubular, or Balsa Machining Service
  10. 1/2-inch hardwood dowel, 1-inch long
  11. Screw eye
  12. 1-inch of 3/16-inch launch lug, Estes, or Totally Tubular
  13. Streamer, I use four 8-inch wide pieces of Japanese tissue taped together end to end, fan folded.
  14. 6-foot long Kevlar® string, 150#
  15. 1/8-inch elastic strap, 24-inches long, local fabric store

Start by gluing the CR2050 into the BT-50 engine mount 60mm from the aft end. This should allow about 10mm of the motor to hang out the end. This ring does double duty as both a motor block and centering ring for the BT-20 stuffer tube. Glue the CR5060 7/8-inch from the front end of the motor tube. Glue a ring onto the BT-20 2 3/8-inch from one end. This tube becomes the stuffer. Cut a four inch long piece of BT-60 from the 16-inch long section. Lightly sand the shine off of the outside of the tube with very fine sandpaper. Draw three lines equally spaced around the tube using a fin alignment guide and a door jam or marking guide on the four-inch tube. Draw a single line the length of the 12-inch tube. Because this is a competition model, we want to eliminate all unnecessary weight and in this case that includes extra centering rings. The problem with that is that it makes getting the parts aligned correctly difficult. When the glue is dry on the stuffer tube ring, test fit it in the 14-inch long section of BT-60. Don’t glue the forward ring but slip it into position in the BT-60 and on the front of the BT-20 centering the BT-20 in the 60. Add the other ring to the back of the BT-50 but do not glue it on. These rings, the coupler, and the long BT-60 are used to center the motor mount to produce a properly aligned mount and stuffer assembly. When satisfied that the fit is correct, glue the ring end of the BT-20 into the motor block ring. The aft edge of the stuffer should be flush with the aft edge of the block. Be careful to glue only the block and tube and not get glue on the other parts. Allow this assembly to dry completely before removing it from the BT-60. When dry, glue the motor unit into the 4-inch section of BT-60 with the after most ring 1/16-inch inside the BT-60. Use the forward section of airframe, the joiner, and the rings to hold the parts in alignment until dry. Remove the forward airframe and centering rings when dry. You should now have the motor tube sticking about 2-inches out of the aft end of the after-airframe and the BT-20 sticking out of the other end. Cut an 1/8-inch ring from the coupler and glue it into the rear end so that it is against the rear ring. This forms a lip for the boat tail to be made later. Glue the coupler half way into the front of the four-inch BT-60. Put glue fillets around all of the rings where they meet the tubes. Don’t overdo it though, weight in the aft end of the rocket isn’t good. Don’t use epoxy as it’s too heavy. Yellow glue works best here.

Image of the shroud pattern (Click the image for full size.) Now for the boattail. These are always tricky to get right, but it’s possible to achieve near perfection with a little patience. The arc is drawn on the card stock (a 3 x 5 card works well) using a compass with the inside arc being 71mm, the outside arc being 119mm, and the radius being 62 degrees. Cut carefully, and cut outside the lines so that it starts out a little too large. Use a sharp knife. Now using the edge of the desk or a metal rule, draw it across the edge to get it to curl. Carefully curl it on itself until it is so curly that the edges nearly meet. Now fit it on the end of the rocket and use a piece of tape across the edges to hold it. Trim it a sliver at a time on the small end until it just fits snuggly. Do the same to the front until it will just ride on the lip created earlier, but is tall enough to be level with the outside edge of the airframe. Trim the edges if needed until they just butt together. Glue the edges together using a strip cut from scrap as a joiner and yellow glue. I will normally put it on the airframe and use scotch tape to hold and keep it as round as possible while the glue dries. Draw a line on the motor tube at the end of the shroud. Remove the shroud, then run a stingy amount of glue around the airframe lip and around the line you just drew on the motor tube. Now slide it on and get it in the correct position, then wipe away any excess glue.

The best way to cut fins is to glue a stack of three slabs of wood together with a single small dot of slow CA between the pieces. Then draw the outline on the top piece and cut it out using a band or jigsaw. Then use a disk or belt sander to even out the edges and irregularities in the outline. Pop the three fins apart and grind them to a nice tapering airfoil using a sanding block with 220 grit paper. Finish sanding with 400 grit paper. Sand the first fin, then apply a thin layer of yellow glue to the root edge and work it in with your finger. While it is drying sand the second fin and do the same with the glue. Repeat for the third. Now lightly touch the root edge of the first fin (which should be dry by now) with sandpaper to make it smooth. Apply another thin layer of yellow glue to the root and glue it to the aft airframe alongside one line. Take care to get it on parallel with the line and standing straight out from the tube. It will take a few seconds to grab. Stand the assembly upright, and repeat the procedure with the other two fins. When dry, add fin fillets to the root edges of the fins. You may use epoxy for this if you are sparing. Epoxy will create glass smooth fillets, but it’s very heavy so take care. Yellow glue also works OK so don’t hesitate to use it instead.

Cut two 1/4-inch long lengths of 3/16-inch launch lug. The fore and aft ends should be cut at about a 20-degree angle. Glue these along the line drawn on the 12-inch long BT-60 fat side down, locate one six inches down from the top of the airframe, and the other an inch above bottom. Take care that these are aligned well.

Drill a three inch deep, 1/2-inch diameter hole into the base of the nose cone. It’s important to get the hole centered in the cone. Screw the screw eye into one end of the 1/2-inch dowel. Again, get this centered well or it will cause trouble later. Tie one end of the Kevlar®) cord to the screw eye and tie a loop into the cord about 15-inches below the eye. Cut four or five quarter inch long parallel slots in the top of the stuffer. The slots should be crosswise to the length of the tube with an half-inch between them. Thread the free end of the Kevlar® through the slots just cut. Tie a knot in the end and coat the parts with glue fixing the Kevlar® firmly to the stuffer. Tie another small loop in the cord just above the stuffer tube. Tie the other end of the elastic cord to this loop. Now thread cord/eye/block through the long BT-60 and plug it into the nose cone. Don’t glue it yet.. Put the nose on the airframe, but don’t glue it in yet, either. Slide the stuffer into the other end - the screw eye should not cause problems if it is centered in the nose. Shove the cords into the tube and finally slide the coupler in. You should have a fully functional rocket at this point, it just isn’t all glued together. The balance needs checked now. Place an unused motor into the motor mount (it must be an unused single use motor, or a loaded reload casing to get the proper weight). Locate the balance point. It must balance at or in front of a point 14 7/8-inches from the tip of the nose. Check this balance before flying with a new motor. If it balances aft of this point it will be unstable. Pour BBs or lead shot into the hole drilled in the nose and reassemble. Add a little at a time and check the balance. Glue the plug into the cone with epoxy when you arrive at the correct balance point. You may wish to pour a small amount of epoxy in with the BBs or shot so they won’t rattle around. Glue the nose to the rocket and sand it until is flush and even all around the tube. This is the only downside to this design; you must add ballast that could easily put you over the optimum mass causing loss of performance - this could be offset by lengthening the design. You’re on your own for balance if you do that, though.

Rocket halves; rear ejection employed
Seems like a lot of shock cord, but in this case, more is better! Note that the final version has no ring at the top of the stuffer tube. The cannon effect is abated, but more wadding is required.

The suggested recovery device for this rocket is a streamer. I prefer to make mine from Japanese tissue (tracing paper works very well too) that’s 8-inches wide, and about 80 or 90-inches long. Simply cut some 8-inch wide strips from two sheets of tissue. I like to use two different contrasting colors. Then, using scotch tape, join four of these strips together. Attach a short piece of Kevlar® or elastic using scotch tape. Tie the other end of the cord to the stuffer tube. Use a piece of tape or a dot of glue on the knot. Now fanfold the streamer in one-inch folds. A sheet or two of wadding in the stuffer seems to help with wear and tear on the shock cords. Place the stuffer in airframe and slide it in a little. Stuff some wadding around the stuffer tube in the airframe. Coil the shock cord and place it into the rocket just under the wadding - do not wrap the cord around the stuffer! Place the streamer in the rocket spreading the folds around the stuffer tube as you slide it into the airframe.. Make sure that the streamer is not caught or pinched in the coupler. Do not wrap the streamer around the tube. The stuffer should not bind as it slides in. At ejection the nose and forward airframe will be driven off the stuffer very forcefully so never wrap the cord around the stuffer or it will not be able to do it’s job. Also, check the shock cord before every flight. For sport flying a 12 or 18-inch ’chute may also be used.

Finish:
Fill the spirals, the nose cone base joint, and the joints of the shroud with Elmer’s Fill ’n’ Finish. Sand lightly and carefully until these areas are perfectly smooth. Apply a second layer of Fill ’n’ Finish as needed. Sand around the edges of the airframe tubes where they meet to remove any irregularities. Lightly sand the entire airframe with 400 grit paper to remove any shine and make the wood parts as smooth as possible. Now fill the balsa parts with Aerogloss sanding sealer. You will need at least three coats, possibly four, lightly sanding with 400 grit paper between layers. After the final layer switch to 600 grit paper. Wipe the rocket clean of dust and finger prints and then spray a light coat of sand-able white primer on the entire rocket. The primer will allow you to see fingerprints of glue, irregularities in the fillets, the spiral, gaps in the joints, etc. Work on these until they are as nearly perfect as possible. Sand the entire rocket with 400 or 600 grit paper until it is very smooth. Apply another thin coat of primer if needed to get a smooth and uniform white color.. Sand again with 600 or finer paper. The rocket should be as smooth as you can get it. Wipe it clean again and spray on finish coat of high gloss enamel or lacquer. In this application a lacquer finish may be preferable because it produces such a smooth and glossy finish but is very light. It does require a lot of coats with buffing between each coat to produce the best finish. Check that your paint is compatible with the undercoat before you start. When the paint is fully cured, you may use Future floor wax diluted 50/50 with water, or an automotive wax to both protect the paint and allow a buffed highly glossy, highly competitive surface.

Flying:

Stuffing the wadding followed by the shock cord.

The first flight of the prototype was on a C6-5 using an adapter. While it didn’t exactly set any new records, it did do fine on a smallish motor. Unfortunately, the recovery didn’t work out as well. The two halves of the rocket parted, leaving the top half in a tree. The lower half was recovered and inspected. The original version had an additional ring at the front of the stuffer tube and the shock cord tied around and glued to the top of the stuffer. The post-crunch inspection showed that this disk and the Kevlar® had let go of the stuffer. After some thought, it occurred to me that the top paper ring was causing too good a seal and creating a piston effect that we could do without. The second flight with a C11-5, this time without the uppermost ring, and with a better attachment of the shock cord was quite a bit better. The C11 performed admirably, carrying the rocket to a very respectable altitude, and the ejection charge deployed the streamer right at apogee. The third flight, on another C11, saw the ejection charge blow the nose off and the fin can out, but the streamer stayed inside the airframe. Guess I didn’t get the nose glued in so well the second time around. Despite this, I felt that the bugs are more or less ironed out.

Competition version in flight
The competition version with an Aerotech E15-7.
We tracked some flights at one of the club launches with the club’s theodolites and measured an altitude of 117M (380-feet) on a C11-5 (tracking at ejection), and 251M (815-feet) on a D15-7 (again tracking at ejection). It turns out to be too good a performer for the E-duration event because it went so high so fast that the timers lost sight of it; low score. The rocket was found, however, by another flier later in the day.

It should fly well on these motors: C11-5, D12-5 or 7, E15-7, and if you have the temerity to fly it with RMS hardware D9-4 or 7, D15-7, E18-7, E28-10, F24-10, and F39-9.

A sport version was also made at the same time, with a conventional recovery out the front rather than rearward and with a motor mount that will handle an E9. It also sported basswood fins, that are tough as nails so it can be recovered fast (read that hard) without tearing it up easily. That rocket flew quite nicely, and was flown a lot since it was easier to prep, but was lost to the trees.

Parting comments:
This was a fun rocket to build; more than the common three fins and a nose cone with some interesting permutations involved in getting a fully aligned motor mount with as few parts as possible. Less weight in the tail equates to less weight in the nose for ballast! Some fine tuning of the length could be done to produce a lighter model; tracking powder could then be used to bring it up to the optimum weight, but would be jettisoned at ejection making the rocket lighter for better hang time for duration events.

The rocket was test flown on a small field a few times with low power motors to work out the ejection system. The main event used a much more powerful motor; a little testing with that motor prior to the actual contest would have revealed the out of sight problem that could have been overcome had it been known in advance. Live and learn!

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