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- From: wolf@netheaven.com (Wolfram v.Kiparski)
- Newsgroups: rec.models.rockets,rec.answers,news.answers
- Subject: rec.models.rockets FAQ Part 06 - Model Rocket Construction and
- Finishing
- Followup-To: rec.models.rockets
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- Reply-To: wolf@netheaven.com
- Summary: This posting contains a list of Frequently Asked Questions
- (and their answers) about Model and High Power Consumer Rocketry
- Approved: news-answers-request@MIT.EDU
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- Archive-name: model-rockets/construction
- Rec-models-rockets-archive-name: rockets-faq/part06
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- Last-modified: 1997 January 6
- URL: http://dtm-corp.com/~sven/rockets/rmrfaq.toc.html
-
- Rec.Models.Rockets Frequently Asked Questions: PART 06 OF 14
-
- MODEL ROCKET CONSTRUCTION AND FINISHING
-
- This section includes tips and suggestions on various topics having to do
- with construction and finishing techniques. These have been posted to
- r.m.r or mailed to the moderator by way of r.m.r request. Refer to the
- High Power Construction section for additional tips, oriented towards high
- power and advanced rocketry requirements. Even more construction tips can
- be found in the Scale Modeling section.
- [Note: This part of the FAQ is maintained by Robert C. Santore
- (santore@ibm.net)
- Any additions or corrections should be sent to that address]
-
- -----------------------------------------------------
- 6.1 Cutting, Sealing, Attaching Fins
-
- From: JCook@Epoch.COM (Jim Cook)
- Skip using glue W/ balsa dust, dope, or any other junk for filling the
- grain in balsa fins or nose cones. Use Elmer's "Fill 'n Finish" diluted
- with water to a thick paint (like white glue is) and paint it on.
- Non-toxic and a coat or two will do. Use Elmer's "Carpenter's Wood
- Filler" thinned similarly to fill the spiral in body tube. Both come
- in a white plastic tub with an orange lid. Note - the latter is
- harder to sand, so don't make the mistake of using it on balsa as it
- will require a lot of sanding.
-
- From: cdt@pdp.sw.stratus.com (C. D. Tavares)
- Fill your fins BEFORE attaching them. (Don't fill the root edge).
-
- From: utidjian@remarque.berkeley.edu (David M.V. Utidjian)
- [To hold fins in place and aligned while drying I bought an Estes
- fin alignment kit]. At 15-16 bucks it seems a bit
- expensive but is well worth the aggravation and time it saves. You
- can even make your own if you are handy. I just set up my body tube
- in the jig and then check the alignment of all of the fins to the body
- tube. Then I use a thin bead of 5-min. epoxy. and in ten minutes I am
- done. When I do the fillets I can do them all at once but don't have
- to worry about the softening the glued on fins so they droop. You
- still have to lay the model on its side though but only for 5 minutes.
-
- From: kaplowro@hccompare.com (Bob Kaplow)
- The Estes Fin Alignment Kit has now been discontinued for '96. You can
- use the newer Rocket Builder's Marking Guide ($5-8) to do one fin at
- a time. [Editor's note: This is a HIGHLY RECOMMENDED item to purchase.
- It provides everything needed to mark BT-5 through BT-60 body tubes
- for 2, 3, 4, 6, or 8 fins.]
- -----------------------------------------------------
- 6.2 Body Tubes (Cutting, Joining, Filling)
-
- From: cdt@pdp.sw.stratus.com (C. D. Tavares)
- [On cutting Estes-style body tubes]
- The simplest and best I ever used was Howard Kuhn's jig from the old CMR.
- It's a simple piece of wood L-angle molding, with a notch for a razor
- blade cut into one side (from the wing toward the elbow) at one end.
- The only other parts are a wood block and a large black spring clip (the
- kind you hold really thick reports together with). If you want, say, a
- 6" piece of tubing, you set the block 6" away from the razor notch and
- clamp it there with the spring clip. Now lay the tube down the L-angle,
- butting it up against the wood block. Insert the razor blade, press
- lightly, and turn the tube. (Put a dead engine into it if the tube is
- the right size to fit one.) Three to six turns, and you have an edge
- that looks factory-cut.
-
- ----
- ( ) <- spring clip | <- razor (edge on,
- / \ | sharp edge down)
- ____________---------___________________________|_______
- | /_| |__/| | |
- | | |_______| || angle molding | |
- | | wood block || |
- |----------|____________|/-------------------------------j
- / lay tube here and spin it /
- /________________________________________________________/
-
- From: soc1070@vx.acs.umn.edu (Tim Harincar)
- [On cutting Estes-style body tubes]
- When I cut tubes, I always wrap the tube with about two
- layers of drafting tape with the edge of the tape along
- the cut line. This accomplishes two things: First the
- thick tape edge providing a excellent knife guide. Second,
- you can assure a straight cut. If the tape wrinkles when
- you wrap the tube, you know it is not on straight; simply
- remove the tape and try again until you know its down flat.
-
- Drafting tape is better than regular masking tape because it
- has almost the same thickness but is made to be removed.
-
- This method is in addition to reinforcing the inside with
- a stage coupler or spent motor. Also, always use a new
- x-acto blade for the best cut.
-
- From: Jim Bandy (NAR member not on net)
- Use a piece of aluminum 'angle iron' for joining body tubes. Place one
- tube in the angle, insert and glue the joiner, then insert and glue the
- other tube. It give very straight joins. The angle can also be used
- for marking fin lines on body tubes, etc.
- -----------------------------------------------------
- 6.3 I'm fed up with tangled plastic parachutes with broken shroud lines. How
- can I improve on the standard chutes, or make my own?
-
- From: soc1070@vx.acs.umn.edu (Tim Harincar)
- Making your own parachutes is pretty easy. Start with the desired
- material (usually mylar or a light plastic). Make a cutting pattern out
- of cardboard by first drawing a circle that will be the maximum size
- of the chute (i.e. 16"). Take a compass [or] something that will give
- you an accurate radius of the circle. Pick a point anywhere on the
- circle and using the radius as a length draw an arc that crosses the
- circle. At the point where the arc crosses, reposition the compass on
- that point and draw another arc. Keep doing that all the way around
- the circle - you will end up with six points including the starting
- point on the circle. Connect these points with a straight edge and
- Presto! a hexagon. Cut out the hex from the cardboard (I use artists
- matte board...) and this is your cutting template. Lay the template on
- the material and using an EXTREMELY SHARP XACTO KNIFE cut along the
- outside of the template. Make shrouds from a heavy gauge thread -
- cut three equal lengths twice as long as the diameter of the chute and
- connect the ends to corner points adjacent to each other.
-
- From: Roger.Wilfong@umich.edu
- I usually build 12-24 line round chutes out of Estes material (just cut
- around the outside of the red and white circle and attach at the red
- /white boundaries) because they look more like real parachutes. I use
- embroidery floss for shroud lines and separate the 6 strands (for 12
- lines - use two lengths for a 24 line). This makes a strong chute.
- With out crossing the lines over the top of the canopy, I've only had
- one failure of a 12 line chute (an EL that tipped off dramatically -
- i.e. cruise missile) and never had a 24 line fail. In the 10 years
- I've been back in the hobby and using this technique, my shroud lines
- have always come out the same length (within a couple of percent
- tolerance).
-
- From: hal@HQ.Ileaf.COM (Hal Wadleigh)
- 1. Use fisherman's snap swivels for your attachments. It lets you
- store 'chutes separate from rockets and helps prevent fouling due
- to spin at deployment.[Note...modelers have always reported mixed
- results with snap swivels; they have been known to fail...Buzz]
- 2. Use nylon coat thread for shroud lines on homemade 'chutes (and
- plastic bread wrappers are the best cheap 'chute material).
- 3. Pay special attention to the security of the attachment points.
- Those standard stickers often look secure, but are actually not
- attached. A small knot in the part of the shroud line under the
- sticker serves as a good anchor point (with the rest of that part
- looped around the knot, as per standard practice).
- 4. Very small 'chutes should be cross-form type. Cut about a 5" square,
- then take out about 1.25" squares from each corner. Attach 4 lobes
- of shroud across the flat ends and secure as above. Be careful to
- use small stickers for the corner attachments. These make good
- substitutes for streamers in .5" body tubes and can also be used as
- drogues to help in the deployment of large 'chutes [A note from
- cdt@sw.stratus.com (C. D. Tavares): Either round off the inside
- corner of that 1.25" square or reinforce the angle with something.
- Otherwise, it's a really handy place for the parachute to rip
- during a fast deployment.]
-
- From: smith@mrcnext.cso.uiuc.edu (Greg Smith)
- Nylon coat thread is very good for small, lightweight competition
- parachutes, but it's not real strong and does have a tendency to melt if
- it encounters a bit too much ejection charge heat. For sport and
- payload models with 12" - 24" plastic 'chutes, I use 15 lb. *braided*
- nylon fishing line. It's thicker than the coat thread, similar in
- diameter to the Estes cotton stuff, but tremendously stronger. In the
- last fifteen years, of the plastic parachutes I have built using this
- line (and always crossed over the top of the 'chute for reinforcement),
- I have had *zero* shroud line or attachment failures. The braided line
- has a hard, smooth surface that doesn't encourage tangling, and it
- doesn't unravel where cut.
-
- From: whitman@nssdca.gsfc.nasa.gov (Rusty Whitman)
- I've tried about everything to keep shroud lines from pulling off of
- plastic or mylar parachutes. Those little tape disks are just about
- worthless. Tying knots and cyano'ing the ends helps but you still
- have problems. I don't know why I never thought of this before but I
- ran across a roll of duct tape in my closet and knew immediately that
- was the answer. I cut out some little squares of duct tape and
- attached some lines to a parachute and they won't pull free without
- ripping the plastic. I don't know who invented duct tape but they
- deserve some kind of statue, its got more uses than a paper clip.
-
- From: kaplowro@hccompare.com (Bob Kaplow)
- 1. Make shroudlines from Kevlar thread. This won't burn through. Tie
- a knot 1/2" from the end and fray the end. Attach that end to
- parachute.
- 2. Use the much stronger kite snap swivels instead of fishing swivels -
- make sure they lock, and don't just clip like a safety pin.
-
- -----------------------------------------------------
- 6.4 Alternatives to Recovery Wadding
-
- From jack@rml.com (Jack Hagerty):
- Just go down to your local building supply store and get a bale of
- cellulose wall insulation. This is just shredded newspaper treated in
- the same fire suppressant [as Estes recovery wadding]. A $5 bag will give
- you enough wadding to last years!
-
- From MASSEY@TRAVIS.llnl.gov (Warren Massey):
- I have found crepe paper to be a must more cost effective alternative.
- It comes in either sheets or rolls (I prefer the sheets) in a variety of
- colors and is every bit as flame retardant at a fraction of the price. I
- can even get several flights off a single ball of wadding. It is somewhat
- stiffer than the tissue but I've never found that to be a drawback.
-
- Unattributed:
- A piston ejection system works well on rockets of BT-60 size or greater.
- Pistons eliminate the need for recovery wadding of any type. Plans
- for a D powered rocket using piston ejection may be found on sunsite.unc.
- edu in the file 'pub/archives/rec.models.rockets/PLANS/dust-devil.ps'.
- The rocket was designed and drawn by pfeiffer@nmsu.edu (Joe Pfeiffer).
-
- From: kaplowro@hccompare.com (Bob Kaplow)
- Use a baffle system (I posted this to RMR a while back) to eliminate
- the need for wadding. This also provides the benefit of a third centering
- ring (see Pete Olivola article) as part of the baffle system. Stainless
- steel mesh can be used to trap hot particles, but be careful that
- ejection
- caps don't plug it up.
- -----------------------------------------------------
- 6.5 Are there any good tips when making my own nose cones?
-
- From Chris Jennison
- To keep nose cones from wobbling and coming out asymmetrical when using
- an electric hand drill as a lathe...
- Use a blank (dowel, broom stick or balsa block) 1/8 inch larger
- (diameter) than the nose cone that you need. Drill a 1/4 in. diameter
- hole
- as close to dead center as you can and push in a 1/4 in dowel. Dowel
- length should allow the nose cone end to seat against the face of the
- drill chuck. Find dead center by running the drill clamped in a vise at
- moderate speed & slowly move a soft pencil toward the end at what
- appears to be the center of rotation. After a couple of tries you will
- find the center because your misses will draw concentric circles like
- a bullseye. Now remove the dowel from the drill, clamp the shoulder end
- in the vise and rough shape the nose cone with a file or rasp using the
- marked center as a guide. Final contouring and finishing is done in the
- drill with progressively finer sand paper.
-
- -----------------------------------------------------
- 6.6 Getting Paint to Stick to LOC and Aerotech Nose Cones
-
- From: kwolfe@interramp.com (Ken Wolfe)
- VERY VERY important......WASH THE PLASTIC FIRST!
- I had this problem until I started to wash the plastic before even
- assembling anything. This solved most of the problems I was having.
-
- From: Roger.Wilfong@umich.edu (Roger Wilfong)
- I have had success painting nose cones from both companies using Krylon
- and Walmart paints. The technique I use is to wash the nose cone with
- a Brillo pad followed by a thorough rinse. Fill the mold parting mark
- with auto body putty and sand it smooth. I next use a coat of primer
- (I've used Krylon's gray sandable, Walmart's gray and Black Baron - the
- Black Baron was the best, but also the most expensive and took the
- longest to cure). This is followed by a light sanding and another
- coat of primer, followed by sanding. After the primer cures (a week, if
- I'm in the mood to paint, a year if I'm not), paint it with some paint
- that's compatible with the primer.
-
- This technique works fine on the LOC nose cones, the only problem I've
- had with the Aerotech nose cones is that the very tip tends to get
- chipped off.
-
- I have a LOC PNC-3.00 that has lawn darted into hard ground twice. The
- original paint is scratched, but it shows no signs of flaking off.
-
- From: smith@mrcnext.cso.uiuc.edu (Greg Smith)
- I rough up the surface of plastic nose cones with 60 grit paper, then
- use my basic epoxy painting regimen as I've described earlier. After
- the first coat of primer, the surface is *really* fuzzy; the paint
- reinforces and thickens all the little plastic strands that are raised
- by the sandpaper, and the surface feels like rough concrete. But a
- little sanding knocks off most of it, and after the third primer coat or
- so, the surface is as smooth as anything else on the model.
-
- The only time I've ever damaged the finish on one of these nose cones
- happened when a model fell off the workbench and onto the concrete floor
- in my basement, which chipped the tip of the cone a bit. Normal flying
- (including one or two landings on concrete) hasn't affected them at all.
-
- From: jsvrc@rc.rit.edu (J A Stephen Viggiano)
- As I have said repeatedly, the most effective way to paint on these
- plastics is to introduce carboxyl groups at their surfaces. This will
- give the paints something onto which they can grab.
-
- A carboxyl group, also known as a fatty acid group, consists of a
- carbon atom, to which an atom of oxygen is doubly bonded, and also
- a hydroxyl group is bonded. In order to convert the end of a polymer
- chain into a carboxyl group, you need to provide oxygen and some
- energy. The oxygen may, of course, come from the atmosphere.
-
- In the packaging industry, when polypropylene and polyethylene must be
- printed, they are given either a "corona discharge treatment," in which
- the surface is passed beneath a high-potential device called a coratron,
- or a "flame treatment," in which a gas flame is allowed to impinge
- on the surface for an instant. For historic reasons, the second treatment
- may be referred to as a "corona treatment," even though no corona
- discharge is involved.
-
- I've used the gas flame from my kitchen range with excellent results.
- Don't overdo it, for obvious reasons. Only an instantaneous contact
- with the flame is needed.
-
- Since using this treatment, I have had virtually no problems with paint
- flaking from my polypropylene nose cones.
-
- From: M Preddy <preddym@ucs.orst.edu>
- I've had good luck with Rustoleum primer on LOC nose cones. Krylon
- sticks to it fine.
-
- From: kaplowro@hccompare.com (Bob Kaplow)
- Consider covering nose cones with econo-kote.
-
- -----------------------------------------------------
- 6.7 Is it possible to get a high gloss, durable finish on a model rocket?
-
- From: p_hamilton@usa.pipeline.com (Paul Hamilton)
- I put on a clear coat of dope or Krylon, sand with #600 wet-or-dry
- sandpaper, and then buff with "Rotten Stone" or similar rubbing compound.
- This is a fine abrasive that on mixes with water. I have used toothpaste
- or silver polish as a substitute for rubbing compound when there has been
- no hardware store around.
- -----------------------------------------------------
- 6.8 Are there any good paints for silver details on scale models?
-
- From: Orville ????
- The Krylon silver paint looks very much like real silver, that's
- because it's made of a clear coat plus real metal flakes.
- The only problem is when you touch it the finish becomes dull.
-
- The solution to this problem is:
- Finish your surface as usually.
- Spray a final coat of Krylon on and let dry 2 days, but don't
- even think about touching it. After 2 days spray Krylon clear coat
- over the silver with very light coats ,letting each coat dry 5 minutes
- between sprayings. This will give you a very nice finish.
-
- From: Peter "wondered if they had silver spray paint for the Tin Woodman's
- axe in the Wizard of Oz" Alway (alway@pooh.physics.lsa.umich.edu)
- I'm starting to get serious about silver paints, now that I am working
- on 1930's rockets. I think it's worth getting a sense of several
- metallic colors, all for different purposes. As near as I can tell,
- most are either simply glossy (Testors Chrome, Dutch Boy silver) or
- matte (Testors silver, and apparently Krylon.
-
- As an experiment, I tried buffing some Testors siver on a nose cone
- (it was still chucked up on the drill press wher I turned it) last
- weekend. I found I needed to use dome #600 sandpaper to get a truly
- smooth surface, then I just buffed it with ordinary paper. The result
- was pretty convincing, but a darker shade than the original
- paint. (unfortunately, on Goddard's rockets, the nose was usually
- a lighter shade of Aluminum) I was impressed that the result looked
- like real metal to me.
-
- Testors has some buffable paints in their Metalizer line. I tried their
- "Titanium" on a Glencoe 3-stage rocket ship (a von Braun design), but
- when polished, I thought it looked more like hematite than actual
- metal. I'm starting to believe that with silver paint, it's more
- important that you represent the differences between silver shades
- than get the siver just right. So for instance, on a Goddard
- rocket, I would use the glossy Dutch Boy silver for the nose cone,
- buffed silver paint for the nickel-steel propellant tanks, and
- testors silver for the duralumin body. This should at least
- suggest the differences between the materials that you can see
- in the photos.
-
- From: Bob Craddock (craddock1@aol.com)
- After building and re-building about a half-dozen Saturn V's, I have a
- couple of recommendations to make:
- For the Service Module color, I suggest getting a bottle of Micro Metal
- Foil Adhesive and some Reynold's Wrap. Spread the adhesive on the
- **shiny** side of the foil. The silver on the SM was somewhat dull, and
- just about ANY silver paint sucks badly! The foil will give you the
- look
- you need, and the adhesive is extremely easy to work with. The white
- raditor details can be added by using shroud line or Evergreen styrene
- strips. White glue works well with the shroud line, but I would
- recommend
- fast epoxy for the strips. Mask of the radiator areas so you can paint
- them white when you paint the entire rocket.
- -----------------------------------------------------
- 6.9 How can I prevent a rocket painted white from yellowing?
-
- From: curcio@telerama.lm.com (LarryC)
- Future floor polish (it isn't wax) doesn't yellow. I find it useful for
- rockets but...
-
- 1) It must be used over some kind of paint or over acrylic gesso. If it
- seeps into cardboard or wood, the material will become brittle;
-
- 2) It will cause dope and (even permanent) Magic Marker color to run.
- This effect can be used to advantage, but it can also ruin a model if
- it's unexpected. Future does not cause enamel paint to run, and may
- certainly be used to cover decals. Don't know if the decal will yellow
- beneath the coating.
-
- 3) A glossy finish is not always desirable, and Future yields only a
- glossy finish. Art stores sell equivalent mixtures that come in gloss
- or matte finishes. The are called "Acrylic Gloss Medium," and "Acrylic
- Matte Medium," respectiely. They are thicker than Future, but they can
- be thinned with water.
-
- 4) Acrylic colors, available in art stores, can be used on rockets, as
- long as acrylic gesso is laid down first. The colors should be thinned
- with water or Future. Acrylic paints are normally labeled to tell you
- how opaque they are and how toxic they are. Both qualities vary from
- color to color. Even though the tubes seem expensive, the opaque colors
- have excellent covering power and they turn out to be very good buys.
-
- 5) From my own experience, commercial acrylic model paints are to be
- avoided. They are over-thinned, and they yeild undesirable, cracked
- finishes.
-
- From: kaplow_r@eisner.decus.org (Bob Kaplow)
- Try adding a small amount of BLUE to the white paint (a few drops per
- bottle). The faint blue tint hides the yellowing, much like bluing in
- the white laundry.
-
- -----------------------------------------------------
- 6.10 Which is better, white or yellow glue? Epoxy? Cyano?
-
- From: buzzman@netcom.com (Buzz McDermott)
- Yellow wood glue, such as Elmers Carpenter's Glue or Titebond, is far
- superior to regular Elmer's white glue for building wood and paper model
- rockets. Built carefully, with proper fin fillets, yellow aliphatic resin
- will hold together on rockets with up to G power. Yellow glue also dries
- faster.
-
- Three and five minute epoxy is often used for quick repairs. This quick
- drying time does not allow the epoxy to soak into the wood and/or paper
- very well, though. If epoxy is to be used, then use one with at least
- a 15 minute, and preferably a 30 minute, listed 'drying' time. Thirty
- minute epoxy will give a much stronger bond than yellow glue. However,
- 5 minute epoxy often yields a weaker bond than yellow glue.
-
- Cyano is often used for quick building. It bonds strong, dries extremely
- fast (especially when using an accelerator), and is relatively easy to
- use. Regular cyano can also be used to bond plastic to wood or paper.
-
- No matter which glue is used, the most important factor is to have a
- properly prepared surface. Glassine coated Estes-stype body tubes should
- be lightly sanded where the glue is to be applied. All bonding surfaces
- should be clean and unpainted. You want whatever bonding agent is used to
- be able to soak into the paper and/or wood.
-
- From: kingrat@sisko.dnaco.net (kingrat)
- I've been using CA to bond fins to the tube and then a fillet of
- epoxy. If you go this route I recommend thin CA and a fin alignment
- guide. Make sure the fins are aligned properly before you CA and check
- again before applying the epoxy. Unlike white glue, if you mess up it's
- REALLY hard to fix. It's also REALLY hard to pop the fin off too. :)
-
- This works just fine with balsa or plywood fins from what I've seen. CA
- is good for tacking the lugs in place too, however I would never use CA
- on a vital part of the rocket (ie. motor mount) CA just isn't strong
- enough because it turns brittle. Epoxy will flex somewhat without
- breaking
- and you can add microballoons to allow it to flex even more. In
- short, use
- CA to tack and epoxy to bond.
-
- From: santore@ibm.net (Bob Santore)
- There is an easy test to tell if the glue you are using is strong enough.
- A glue joint only needs to be as strong or stronger than the material it
- is gluing together. To test for glue strength you need to test the
- strength of the glue joint relative to the strength of the material you
- are bonding. This test is very easy and can be used for any type of
- glue.
- Glue some scrap pieces of whatever you are gluing together in a
- configuration similar to how you intend to use it (i.e., don't test a
- surface mounted fin if you intend to use a TTW fin). When cured,
- pull the
- two parts apart. Did the glue bond fail? If it did, don't use that
- brand
- of glue. If the pieces you glued didn't fail, then this glue is fine.
- If your rocket falls apart anyway, then you need stronger rocket parts!
- The only exception I can see to this very simple test would be near the
- motor mount where heat can weaken the glue. You could always heat the
- pieces before destructive testing to see if heat changes the
- properties of
- the glue joint.
- -----------------------------------------------------
- 6.11 Improving on the Estes Shock Cord Mount
-
- There are a number of ways to improve on the old paper-and-rubber-band
- shock cord mount used by Estes for the past 35 or so years. The
- following suggestions have been repeatedly made in r.m.r.
-
- 1. Replace the rubber band or short elastic in the kit with sewing elastic
- at least twice the length of the model. This will help to avoid
- tube zippering or ripped out shock cord mounts when you have a 'hard'
- ejection.
-
- 2. Epoxy the shock cord mount to the inside of the body tube rather than
- using white or yellow glue. Thirty minute epoxy works best for this.
- Be sure and lightly sand the inside of the tube where the shock cord
- mount is to be placed. Also be sure and 'cover' the entire mount with a
- very thin layer of epoxy. This method works best for models with BT-60
- or larger body tubes.
-
- 3. Use a 'Quest-style' kevlar-and-elastic shock cord mount. This type
- of mount uses a length of 50 to 150 pound test Kevlar (such as Stren
- Kevlar fishing line or Kevlar kite string). The Kevlar is tied and
- glued to the motor mount (motor block, centering ring, or around the
- motor tube). It is sized to end just shy of the front end of the
- body tube when a length of elastic shock cord is tied onto the free
- end of the Kevlar. This method eliminates any shock cord mount on the
- inside surface of the body tube. Estes-style shock cord mounts have
- been known to interfere with parachute deployment. You can used this
- method on any sized rocket. Size the Kevlar appropriately.
-
- 4. You can also use a 'LOC-style' shock mount for body tubes in the
- 1.5" and up range. With this you take a short length of Kevlar line,
- fold it in half, and make a !-2" loop in the closed end of the folded
- line. You then epoxy the loop to the inside of the body tube in such
- a manner as to have the end of the loop extend a little past the open
- end of the body tube. A shock cord is tied to the loop. The advantage
- of this technique is that is allows damaged shock cords to be easily
- replaced. It tends to work better on body tubes greater than 1.5"
- in diameter.
- -----------------------------------------------------
- 6.12 I've built several BT-80 based models. How can I strengthen future
- models to take larger motors, such as Aerotech 24mm or 29mm reloads?
-
- Estes recommends using just D motors in their BT-80 based kits, such as
- the Broadsword (Super Big Bertha) and Shadow (Optima). There are a number
- of steps that posters to r.m.r have done to strengthen these rockets to
- fly with larger motors. Remember in doing this that you are modifying and
- using the rocket kit in a manner NOT recommended by the manufacturer.
-
- From: Buzz McDermott (buzzman@netcom.com)
- Here's a summary of what I've done to strengthen my Shadow and Broadsword
- kits to fly on 24mm reloads and up to 24mm G42 motors. You will need
- a LOC-style 2.56 inch tube coupler, 3/4 oz fiberglass, 15-30 minute
- epoxy, and 20 minute 'coating' or 'finishing' epoxy:
- 1. If the kit comes with the 'plain' BT-50 type motor tube, replace
- that with LOC-type, thick-walled 24mm motor tube. You will only
- need about 6".
- 2. Reinforce each body tube section with 3/4 oz fiberglass. Apply it
- using
- one of the methods discussed in the section on High Power Construction
- Techniques. Be sure to completely sand off the glassine coating of
- the
- body tubes prior to applying the fiberglass.
- 3. Also fiberglass-reinforce the thick paper centering rings supplied
- with the kit. Glass both sides of the rings. If you want to build a
- payload section in the Shadow, then leave the center cut-out in one
- of the rings and glass over it.
- 4. If you plan to use only 24x70 disposable motors (including Aerotech
- E's) and/or 24mm reloads, then yellow glue or epoxy a motor block
- 2.5" into one end of the 6" motor tube. If you install the motor hook,
- file down the part that sticks into the tube. This will let you
- fit in 24mm reloads. Be sure and lightly sand the motor tube prior
- to installing motor hook. [NOTE: If you plan to use non-standard 24mm
- Aerotech motors than skip this step.]
- 5. Epoxy one fiberglass-and-epoxy-reinforced centering ring 1/2" from the
- rear end of the motor tube. Be sure there is a notch in the ring to
- allow some movement of the motor hook. Epoxy a second centering ring
- in the middle of the motor tube. Epoxy a third ring 1/8" from the
- front of the motor tube. Install the motor tube into the main
- body tube with the motor tube flush with the bottom of the main body
- tube.
- 6. Fiberglass-reinforce all of the fins. Apply the glass to both sides
- of the fins. Be sure that the fins are completely sanded (and any
- airfoiling/rounding completed) before applying the cloth. An optional
- step is to apply some 1/2" wide strips of glass along all of the
- fin edges EXCEPT the root edge.
- 7. Rough up the epoxy on the main body tube along the lines where the
- fins will attach. Use 220 or coarser sandpaper. You really want the
- smooth epoxy coating roughed up. You can also drill a few 'rivet'
- holes along the fin attachment lines.
- 8. Apply epoxy where the fins will attach and attach the fins. Do not
- fillet at this time.
- 9. When the fins are dry, apply 1/2" strips of fiberglass cloth along
- each fin root edge, with 1/4" on the body tube and the other half of
- the width along the fin side. Coat this will coating epoxy. When
- dry, YOUR FINS WILL NOT COME OFF.
- 10. Couple the bottom two body tube sections together. An option step
- is to make a payload section out of the third body tube section that
- comes with the Shadow. Use the LOC coupler to make a payload section.
- You can sand down the solid centering ring to slide inside the LOC
- coupler to form a bulk-head. Epoxy a 2"x 2" piece of scrap 1/16"
- plywood or 1/8" balsa to the inside surface of the bulkhead to add
- strength. Install either a large screw-eye or small eye-bolt to the
- center of the bulkhead, to be used to shock cord and parachute
- attachment.
- The end result of the above is a model which is really too heavy to
- fly on a D12. My modified Shadow came out to about 14 oz (I built it
- VERY heavy and added the payload section). My modified Broadsword came
- out to about 12 oz. Both have been flown on motors as small as composite
- D's (D21-4, D13-4R). The Shadow has flown on E15-4 (perfect), E30-4
- (a -5 is really needed), F24-7R, F39-7R, and G42-8. The Broadsword
- has just been test-flown on the D21-4 and E15-4. It WILL be flown on
- G42's, though.
-
- If you want to use 29mm motors in BT-80 based models, I would recommend
- either switching to plywood centering rings or sticking to low-thrust
- motors, such as the F14. In the end, if you want to fly 29mm, you would
- really be better off getting a kit designed as a Large Model Rocket from
- the start.
- ---------------------------------------
- 6.13 How can I prevent balsa fins from breaking off on landing (especially
- for models with swept fins)?
-
- From: Bob Craddock (craddock1@aol.com)
- Take your fin pattern, reduce it by ~90% on a xerox machine, and make
- as many copies as you need to glue one pattern on both sides of each
- fin. Put about two coats of sanding sealer on the new paper surface,
- sand, and then paint the fins all over again. A friend of mine was
- having the exact same trouble on his Super Big Bertha, and the paper
- reinforcement was his solution. It worked great, but next time I
- say use
- bass wood on everything.
-
- From: The Silent Observer (silent1@ix.netcom.com)
- There's a variation on this technique, that needs to be applied during
- building, that can make balsa fins stronger than bass (and still
- lighter).
- What you need to do is simply to cover the fins before painting.
- I used silk tissue (like model airplane tissue made from silk
- fibers) on my
- Big Bertha, and in a dozen flights (before it lodged high in a tree)
- never
- had so much as a crack, even when flown on a D21 (and including one
- "plastic wad" recovery when the rocket hit the ground fairly hard).
- You
- could use ordinary Japanese tissue, or Silkspan (R), or you could
- even use
- something like nylon cloth or very light fiberglass (attached with
- epoxy or
- CA in this case).
- With tissue, you need to cover the entire surface -- I simply
- wrapped it
- over the rounded leading edge, and trimmed it off at the tapered
- trailing
- edge, leaving the square "bottom" edge and the root uncovered. You can
- attach Silkspan with almost any glue, but silk tissue (as I found)
- "fuzzes"
- if you get it damp and handle it, so something like Testor's model
- airplane
- glue or thick, clear nitrate dope might be a better choice; it won't
- soften
- the binder that holds the fibers in the tissue together.
- Any of these, done after sanding (and filling, in the tissue cases)
- will add
- significantly to the strength of the fin, while adding very little
- weight.
- Making fins out of basswood or ply is probably okay with a Bertha
- derivative
- -- they tend to be overstable in any case -- but may lead to an
- unstable
- model if you have a design with less margin.
-
- From: David Bucher (dbucher@mcn.org)
- There are two things you can do, both of which lower the rocket
- in a "fin up" attitude. The first works by making a "yoke" or
- harness for
- lowering the rocket body horizontally ( if you choose). Install an
- anchor
- (screw eye, inch worm shaped brass wire clip etc.) through the body
- tube
- wall between the fins at the rear end of the body. Attach a squid
- line or
- kevlar thread to the anchor and run it up the outside of the body
- (tightly)
- and attach to the nose cone or 'chute. Configure it to lower rocket as
- above.
- The other (and better!) way for the rocket you describe is to use rear
- ejection. This will not help you with the present rocket, but any other
- rocket with sufficient body width will work just fine. When making the
- motor mount assembly. substitute a longer motor tube (29mm LOC
- tube for instance) and make up some ply or G10 centering rings
- including
- two with a fair spread between where you can wrap the 'chute around
- the motor tube. Install a solid bulkhead with cable lanyard to serve
- as a thrust ring and pressure block. Make sure the motor mount unit
- slides well in the body and attach elastic to the cable lanyard and now
- you've got a rocket that ejects to the rear. Just cut a small notch
- in the
- farthest forward centering rings to allow the shock cord to pass.
- This me-
- thod works great and if you're confused by what I just wrote (a not un-
- heard of possibility!) just think of the internal "power pod" in some
- BGs. It works the same way except you must make provision to conn-
- ect ALL parts together. There are two things you can do, both of which
- lower the rocket in a "fin up" attitude. The first works by making a
- "yoke" or harness for lowering the rocket body horizontally ( if you
- choose). Install an anchor (screw eye, inch worm shaped brass wire
- clip
- etc.) through the body tube wall between the fins at the rear end of
- the
- body. Attach a squid line or kevlar thread to the anchor and run it up
- the outside of the body (tightly) and attach to the nose cone or
- 'chute.
- Configure it to lower rocket as above.
- ---------------------------------------
- 6.14 I just lost my favorite rocket and the kit is discontinued. How can I
- make another one just like it?
-
- From: <silent1@ix.netcom.com> and <bmcdermo@ix.netcom.com>
- I don't know if everyone else already does this, but I've started
- saving the
- kit card, instructions, and a copy of the fin shape or shapes for
- every kit
- I build -- rockets have a way of getting lost or broken, and model
- rocket
- companies have a way of discontinuing my favorite kits. Getting a
- color copy
- of the decal sheet (or better yet, a color scan) is also a good idea.
- ---------------------------------------
- 6.15 How can I reduce damage to the booster stage of two stage models caused
- by the engine exhaust of the second stage?
-
- From: phunter@numill.com (Perry Hunter)
- Try scotch tape instead of masking tape. It should release fractionally
- faster and >might< reduce scorching of the lower stage.
-
- In some cases, it's possible to line the inside of the top of the lower
- stage with 20lb xerox paper, and it will take the damage rather than
- the exposed section of the stage. It's not possible to cover everything
- (slip fit couplers , etc. prevent lining all of it) but it can help.
- ---------------------------------------
- 6.16 Is there a way to increase the stability of a model with near
- neutral stability?
-
- From: Peter "My views are not to be confused with those
- of a rabbit librarian" Alway (alway@pooh.physics.lsa.umich.edu)
-
- Sounding rockets that are aerodynamically stable are often spun at a
- slower rate that insures that any off-axis thrust will cause the
- rocket to corkscrew, rather than follow an arc. The corkscrew may
- be subtle--but it beats an equally subtle arc. A sounding rocket
- that naturally describes an arc with a 20-mile radius due to its
- asymmetries cannot reach higher than 20 miles. But if the rocket
- is spun through 360 degrees every few hundred feet, the
- 20-mile-radius arc turns into a very suble corkscrew.
-
- Imageine the modeler puts a very slight misallignment between the
- forward and rear fins of a sidewinder. Suppose it's just one degree.
- also suppose the fins are 1 foot apart. the rocket will naturally
- arc in a circle with a 360-foot circumference and a 57-foot radius.
- That's instant doom! make the error half as bad and you are in
- trouble. But if the rocket spins every 10 feet, the path will be
- a generally upward corkscrew, less than ideal performance, but
- a safe flight.
-
- So with model rockets, a spin on ascent is a good way to make a
- marginal or asymmetrical model safe. Estes used to sell a space
- shuttle orbiter kit that had a spin tab for this reason, and the
- old Astron Space Plane had spin tabs as well.
- ---------------------------------------
- 6.17 How can I build a rocket with less wind resistance?
-
- From: John DeMar (smdemar@mailbox.syr.edu)
-
- The best thing you can do is to NOT use launch lugs. Use a
- launch tower instead. A polished, smooth finish makes a big
- difference too. If the design allows, use a boattail and make
- sure all transitions are smooth (from nosecones/payload sections,
- etc.). Fin shape is a minor affect if they are relatively thin,
- otherwise make sure the edges are at least rounded.
-
- Here are some numbers for comparison:
- Standard finish, no transitions, with lug: Cd = 0.88
- Standard finish, no lug: 0.68
- Polished finish, no lug: 0.61
- Standard finish, no lug, 2:1 boattail: 0.52
- ---------------------------------------
- Copyright (c) 1996 Wolfram von Kiparski, editor.
- Refer to Part 00 for the full copyright notice.
-
-
-