The Utah Rocket Club's official schedule is set and is available on the website for your viewing pleasure. Among the changes from last year, the club has reduced the number of member meetings in favor of quality over quantity. While the club does have some very good presentations throughout the year at meetings, if a specifc presentation is not planned the meetings are very administrative and not very interesting to the general membership. So, some of the monthly meetings have been deemd Administrative and will be for the officers and board members to plan activities and address issues. Members are still more than welcome to attend but should not feel obligated to do so.
An additional launch date after HellFire has been added to the schedule and an additonal Sport Competition has been added to the first half of the year. There will be additional workshops, contests and other events added to the calendar as they are confirmed.
We hope that these changes to the club's activity schedule continue to improve the overall enjoyment of our members.
As with many aspects of rocket building, there are many ways to do things. This is how I make my fin fillets and can be used on both methods of fin attachment, direct body and through the tube attachment.
I use balsa wood for the fillet material in the profile shape of a triangle. Square material could be used, but it would require more sanding. I cut the balsa a little longer than the fin root chord length and sand the sides to make it fit. The balsa is cut at 90 degrees and the area for the fillets is more than that, so some sanding is needed.
I disregard the fact that the fin is flat and the tube is rounded at the glue joint area. This small difference is filled in with the adhesive and is less noticeable with the larger rockets. I use slow cure epoxy for the adhesive. I have tried to use wood glue to save money, but when it dries it shrinks and makes huge gaps in the fillets when even using pins to hold the balsa in place.
I force epoxy in the fin/tube joint for through the tube attachments, apply epoxy to the balsa and attach the fillets. Be careful not to get epoxy on the fin or the tube outside the bond area. While curing, examine the fillets to make sure they are staying in place. Pressing on the fillets during curing a few times makes the fit better.
After cure, sand the fillets using a sanding drum tube held between the fingers. I use a 1/2" X 2" drum on the smaller rockets less than 2", and 1" X 2" on the larger rockets. While sanding, and as the flat side is becoming concave, be careful not to cut into the fin or the tube. It will make a groove that will have to be filled later. Stop sanding just as the edges of the balsa meet the tube or fin. While sanding, compare all fillets to make sure they are coming out even in shape.
The forward part of the fin/tube attachment is the hardest to shape. I use a pencil wrapped with sand paper to make this transition smooth, again making sure that all joints are identical. If the tube extends aft past the fins this procedure must be used again. If the fins end with the tube, sand this area flat and fill any voids with filler. I always glass the fins and tube for strength.
Here is way to estimate or "guesstimate" descent impact loads. First, get an estimate of the descent rate. One way to do this is to go into Rocksim and choose a tube. Just use the "mass over-ride" option to assign the mass of your rocket. Then attach a parachute to the tube and look at your descent rate. This is how fast your rocket will fall with your chosen parachute diameter. Now, you can estimate the equivalent drop height (H) for your rocket without a parachute by using the following formula:
H = 0.5 x ( (velocity)^2 ) / 32.2
"H" is the drop height in feet
"velocity" has units ft/sec.
32.2 ft/sec^2 is the acceleration of gravity.
As an example, if your rocket has a descent rate of 20 feet per second, the impact will be the same as if the rocket was dropped from 6.2 feet. Imagine dropping your rocket from 6.2 feet on the salt flats or the Pony Express without a parachute! This will be the same impact load. If you can make your parachute large enough to slow the descent rate to 10 feet per second, your equivalent drop height will be only 1.55 feet.
After trying several ways to create the nosecone for the ASP (Atmospheric Sounding Projectile), I've decided I won't use any of those methods again. Here are some notes and photos of what I did, and a few suggestions for what I'd do differently. Any input would be welcome.
We (Luke, Ian and I) had earlier experimented with gluing up just the foam (styrofoam insulation), turning it to shape, splitting it and then gluing it to the wooden ribs that were to support it. Bad idea all around in my opinion. It was a pig to put (and keep) on the lathe and cutting the finished product into four equal pieces would be a pain. Maybe it'd be OK for a smallish cone but not for this one. By the way, gluing a piece of styrofoam to a wood disc and attaching that to a faceplate on the lathe works pretty good for pieces up to maybe a foot long and a foot in diameter.
The next experiment was to create a wooden support structure with a 1” dowel at the center and 1/4” plywood ribs for support. The dowel had four 1/8” deep dadoes cut into it to hold the plywood.
I then cut the ribs so that, when glued to the dowel, they would make a structure defining the size and shape of the finished nosecone. Stacking the rib blanks and taping them together made it fairly easy to cut them the same size and shape.
I drilled and cut foam circles just a little bigger than the wooden structure and quartered them.
A 3/4” center hole worked great and careful quartering produced a pretty good fit when I began gluing the pieces to the ribs.
The first attempt I made at this procedure can be seen on the lathe in the background of the picture above. I thought I might be able to do it cheaply by using liquid nails to hold the foam together. - -Wrong.- - The liquid nails ate the foam from the inside out and the whole thing blew apart under stress on the lathe.The assembly in the foreground has wood glue for the wood joints and Devcon epoxy for sticking the foam. I used 5-minute epoxy but I had extra hands there to put the cable ties on and help mix the epoxy. Working alone would definitely require slower epoxy.
This attempt went well. I planned on using a skew (angled knife) to turn the foam down until I hit wood and figured I'd get the right shape and a nice smooth finish that way. The first time I nicked the wood I could tell I'd have to change plans. Even with a razor sharp skew, the difference in density between the foam and the wood made the knife catch and pull. I ended up using a drywall sanding block and paper to get the finished shape while the lathe was running. One downside to sanding instead of cutting was that any amount of pressure applied to the sanding block wore the foam more than the wood resulting in a dip along one edge of the rib. Another downside to using the sanding block was that I couldn't get the nice crisp corners I wanted on the angles in the lower portion of the ASP's cone.
The Finished Product - Next time I'd make the wood ribs smaller than the finished cone size and fill in with 1/4” strips of foam. It would take some careful work with a skew and calipers to get the exact shape but I'd prefer that to sanding and I think I'd be happier with the results. Turning the foam on the lathe is easy and that's nice, but the stuff is so soft I ended up looking for a filler to take care of some nicks. Lightweight vinyl spackle seemed to do the trick.
Something else I'd like to try would be to turn a wooden form and create the cone out of fiberglass. Maybe glue a wooden support structure in later after the cone was removed from the form. That kind of glass work is beyond me right now but I'd love to learn. More to come, Evan
P.S. You can find out more about this project on Jim Yehle's pages at http://www.xmission.com/~jry/rocketry/projects/asp/asp-group-project.html.
Holding centering rings in place while you glue them can be trying. A properly cut centering ring (wood) will slide onto the tube with a slight friction fit and should stay by themselves. However, sometimes the ring is too loose to stay in place when you make the fillet.
A neat trick is to mark the position of the centering ring and then wrap several layers of tape at the mark. The tape holds up the centering ring so that you don't have to worry about it slipping when you make the fillet or as it dries!
First, please read the fine print... There are many different solutions to the rocket design challenge. Rules of Thumb simply provide a solid starting point that many have found useful in the past, and that will, in many cases, provide a suitable solution for your design problem today. Rules of Thumb are guidelines. They're not laws. They are nominal solutions that usually, in many cases, most of the time, get the designer in the right ballpark. Once a rocket designer's judgement has been formed by lots of experience, some Rules of Thumb can be stretched, bent, stood on their head, or ignored completely.
Using Rules of Thumb certainly does not take the place of stability tests, or attention to safety. Proof of stability and a constant focus on safety are the most fundamental and unchangeable Rules of Thumb I know. If you know Rules of Thumb that are not mentioned here, e-mail them to Tom Savoie and they could appear in a future update with your name as the contributor. Comments are always welcome.
Motor Mount Size
Build your rocket for the largest motor you might want to fly in it. You can always adapt down, you can never adapt never up.
Whatever your choice, use a primer, finish and clear coats that are compatible. Many times this means sticking to the same brands-e.g., Krylon primer, Krylon finish coat, and Krylon clear coat.
Diameter And Length Of The Rocket
The ratio of rocket length to diameter, sometimes referred to the aspect ratio, should be from 10 - 20:1. For example, a six inch diameter rocket would mean a length of 60 -120 inches.
Reinforcing the Airframe
The larger the rocket, the more important reinforcement becomes. Two layers of a lighter fiberglass fabric work better than a single heavy layer. Two layers of 4oz fiberglass works well for 3-4 inch rockets, 2-3 layers of 6oz for 5-7.5 inch rockets. A final wrap of 2 oz glass provides a good sanding veil. Glass a rocket measuring 2.56" or greater that will reach equal or greater than 0.85 Mach.
A fin that is 2 diameters of the airframe in root length and span and a chord length of about 1 diameter will be effective.
Fin Shape or Planform
The shape you see more than any other is called the clipped delta, and is known for its effectiveness. The clipped delta resembles a parallelogram, with the fin swept somewhat to the rear. The root and chord lines are near parallel, and the leading and trailing edges are near parallel. There are many, many shapes that will get the job done. Some look cooler to me than others. One of the most efficient fin designs looks like a simple rectangle attached to the tube.
Shaping the Fin
The leading edge of the fin should be rounded, the trailing edge shaped like a V. The chord edge should remain square.
Number of Fins
Three fins will almost always do the job. Four fins work too, but only marginally better as far as improving CP. Some have said that four fins reduce wind-induced spin.
Black Powder Ejection
Use enough BP to yield a 15 psi pressure within the airframe. See article on Ejection Charges for a detailed discussion.
Sizing The Parachute
You want your rocket to descend at about 15 feet per second under nominal conditions. Slow it up over playa and concrete. Use 3.5 square feet of chute per pound of recovered rocket weight. Determine chute size by doubling the square root of the weight of the rocket. For example, a 16 pound rocket would use a 2X4=8' chute. A 49 # rocket would use a 2X7=14' chute.
Streamers should be 10 times as long as they are wide.
Drogue recovery descent should be about 50 ft/sec.
A full-hemispherical canopy has very little performance gain over the more efficient and less bulky quarter-spherical--the top-half of a full-hemispherical chute.
Recovery Harness Strength
Tensile rating for recovery materials should be at least 50 times the static weight of the rocket.
Sizing Tubular Nylon
9/16" serves well in rockets up to 15 pounds. Go with 3/4 up to 30 pounds. 1" up to 50 pounds.
Length of model rocket shock cord
Make shock cords for model rockets a minimum of 2 to 3 times the overall length of the rocket. Middle or high power rockets should use tubular nylon at least 5 times the rocket length.
Use enough wadding to fill 2 x the diameter of your BT. Any more is probably overkill. Any less may allow hot particles through to hit your chute. Do not pack it tight.
Knots, Loops and Sharp Bends in Shock Cord or Bridle
Knots, sharp bends, including sewn loops, in the tubular nylon or flat webbing will weaken its load capacity by 50%.
How Tight is Tight?
Many people use masking tape to finesse the fit between an airframe and a coupler that must separate at deployment. A common question is: how tight do I want it to be? Use enough masking tape so that you can pick the rocket by the nose cone without the rocket coming apart. If you vigorously shake the rocket up and down, and don't see any movement off the coupler, you've probably got too much tape on, Jack.
Use 25% less Black Powder if your deployment system is piston driven.
Running a damp cloth through your airframe after flying will clean out powder residue and keep your piston moving freely.
Use shear pins on any rocket where you need a little extra piece of mind to know everything will stay in place until the proper time. Use 1/16" styrene rod or #2 nylon screws on almost any high performance rocket. For example two styrene shear pins each on a 2.6" phenolic airframe, 4 nylon screws on a 6" bird. See the article on Shear Pins in the CONSTRUCTION area for more detail.
Shortening Delay Elements
Note: Adjusting the delay as described below is considered a modification to the motor and is therefore against the rules in a TRA/NAR sanctioned launch. Delay grain burns at the rate of 1/32" per second. Shorten delay time by drilling a 1/16" bit to drill a hole into the ejection charge end of the delay. Drill to a depth of 1/32" for every second you want to shorten the delay. A piece of tape wrapped around the drill bit at the proper depth will help ensure an accurate depth. Don't drill more than 25% into the length of the delay.
Margin of Stability
The CG should be forward of the Center of Pressure by 1-2 calibers. A caliber is simply the diameter of the bird. One caliber of stability is also known as a margin of stability. In other words, in a four inch rocket, the CG must be ahead (closer to the nosecone) of the CP by 4 - 8 inches. More than .5 but less than 1 margin of stability (less than one caliber) and a rocket is "marginally stable'. More than two calibers of stability is known as "over stable". An over stable rocket will tend to dramatically turn into the wind. A marginally-powered, over stable rocket can end up almost horizontal.
Adjusting the Center of Gravity
To move the CG forward, add weight to the nose, lengthen the rocket, or lessen the weight in the aft end of the rocket. To move the CG aft, (for example, if your rocket is overstable), do the reverse.
Adjusting the Center of Pressure
To move the CP aft (more stable), increase the size of the fins. To move the CP forward, decrease fin size.
How Long is Too Long
A rocket must maintain its rigidity in flight. Any tendency to bend will be magnified in flight resulting in a kinked tube and likely a failed flight. If you hold a rocket horizontal by its tail section and notice any curvature in the rocket, your bird probably isn't stiff enough. Sorry, rocketeers, Viagra will not cure this problem.
Sizing the Motor
In selecting a motor to power your rocket, you need to have at least a 5:1 thrust to weight ratio. See a detailed discussion of this guideline Motor Selection in the PROPULSION area.
Launch Rod Diameter
Determine by motor size:
A,B,C - 1/8"
D,E - 3/16"
F,G,H and a body tube less than 2.6" - 1/4"
F,G,H,I w/ 2.6" to 4.0" body - 7/16"
I,J - 1/2"
Over J and body tube over should use rail buttons
Minimum Speed for Stable Flight
44 fps (30mph) is generally accepted as a minimum safe speed for stable flight. Faster speeds are necessary to achieve stability in windy conditions.
Mounting launch lug(s)/button/s
When mounting a single lug, cover the center of gravity with the lug. Always mount at least two rail buttons. When mounting two lugs or buttons, mount the lower piece at the rear of the airframe. The second should be on or just behind the center of gravity.
Submitted by Tom Savoie
I enjoy painting plastic models and pewter miniatures used in games. I've used many different kinds of paints, so when it came time to paint another rocket, I decided to try some of them, rather than another can of Krylon. I've settled on using artist grade acrylics for my miniatures, so I decided to start there.
I chose acrylics for a variety of reasons. Typically they are non-toxic, they clean up with soap and water, can be thinned with water, are permanent, durable, and are readily available.
I purchase most of them from either Robert's or Michael's craft stores (don't forget the 40% off coupons in the Sunday newspapers).
The brand I like best is Liquitex, but I have had good luck with Golden and Winsor & Newton. The Liquitex paints come in a little wider color range, and are a little cheaper. Experiment yourself and see what works best! The model acrylic paints (like Testor's Model Master Acryl line) do come in a wider range of colors and are already thinned, but they seem fragile-any little scratch seems to take them right off.
Most of the artist paints can be heat-treated to improve their durability. Usually baking them at 200 F for about 30 minutes works, however, my wife would not appreciate an Alpha in the oven, so I put them on the dash of my car, and park in the sun.
Acrylic paints consist of two parts: a pigment and a binder. Usually the pigment is a naturally occurring mineral that is finely ground, and the binder is a plastic resin. Most artist acrylic paints across brands are compatible, but not all. Some very interesting colors can be mixed together. Get a color wheel and experiment. Also, there are a wide variety of pearl colors, or interference colors. Pearl colors appear as metallics, and interference colors change color depending on the angle you look at them
Make sure to buy the artist-grade, and not the student-grade paint. The student-grade paints are about 2/3 the price, but have less pigment in them, and come in fewer colors.
These paints have different viscosities between brands, and even between lines in the brand. High-viscosity paints have almost a peanut butter type consistency, and need large amounts of thinning for our purposes. The lower viscosity paints typically have the same amount of pigment as their high-viscosity counterparts, but need less thinning, so they go further. I use the medium-viscosity Liquitex, or Golden's Most of these paints have 2 ratings on them: Lightfastness and Opaqueness. Lightfastness refers to the pigment's ability to withstand fading. The highest grade of Lightfastness means the paint will stay it's original color for 100 years when exposed to museum-quality lighting. Most paints fall into this category, however many of the fluorescent colors shouldn't be used-they will break down in a few days of outdoor light.
To thin these paints, a variety of items can be used. Water is the most common, but don't use too much. As the water dries, the paint shrinks, and can crack. Add a drop or two of dishwashing detergent to the water to lower the surface tension. Windshield wiper fluid is another thinning agent, however I have had the alcohol react with the plastic binder in a few brands. Do not use mineral spirits or turpentine! Unfortunately, the best thinner for acrylics is not the cheapest--mediums. Mediums are sold for different types of thinning, from brushing to airbrushing, to giving a very glossy or dull finish without any overcoat. Most mediums are the plastic resin in a nonvolatile suspension. One thing I have learned, is to use the same brand of medium for the paint - different resins can react with each other. You can use several mediums, like combining airbrush medium with gloss medium. As a rule of thumb, do not user more than 25% medium in the paint. Try thinning to different levels, but remember, it is easier to thin the paint more than thicken it back up. A little medium goes a long ways.
I have applied these paints using brushes, rollers, and airbrushes. The best results have been with an airbrush, although a roller or brush can be used equally well with a few cautions. For brushing, use the highest quality, softest, synthetic fiber bristle you can find. Natural bristles tend to absorb the moisture in the paint, so it doesn't dry naturally, and can crack. Softer bristles leave less of a brush mark. Don't skimp on a quality brush, you wil thank yourself later.
If you don't have access to an airbrush, a spray bottle (like the kind for misting plants) can work too. Find the finest spray pattern you can, the finer the spray, the more uniform the paint will be. Pump-style hairspray bottles can work, but very little paint will go down per pump. Even though the paints are labeled non-toxic, I would still recommend using a NIOSH approved paint mask for any airbrush work. To use this paint in an airbrush, thin the paint to the consistency of heavy cream. Set your air pressure to about 20-23 lbs., and hold the airbrush as perpendicular to the rocket as possible. Start the spray before the rocket, and use a smooth stroke, ending the spray after the rocket. I practice on an old piece of cardboard, so that I can get the color and consistency that I want.
If you have spirals in the body tube to fill, I've used a product called Gesso with some success. Gesso is used to prime canvas for acrylic or oil paints, so that the paint does not soak through the canvas. It looks like a thick, syrupy material. I thinned it using water, then applied it using a brush directly onto the spirals. I had to let it sit overnight for complete drying, though. After that, I sanded the Gesso smooth. You can apply masking tape about 1/16" from each side of the seam, to make a guide if you like. It may take more than one coat, try to apply thin coats. If you want to use a primer, I've used whatever I had handy-spray cans of Krylon, airbrushing model paint primer, or an acrylic primer made by Designs From the Heart, or thinned Gesso. Each seemed to work equally well, but I let the oil-based primers dry for about 48 hours before I tried to apply acrylics over them. The solvents in the primer should be completely outgassed, or the paint will bubble later.
As a top coat, I have used either Liquitex's Solvar gloss varnish, the Gloss medium, Future floor wax, or simply left it alone. Some of the paints leave a little rough surface, and do need topcoating. Future has worked well, and it is cheap. The varnishes are a little on the expensive side, but very durable. Solvar is also removable with turpentine, so you can repair the paint easily. Solvar also has some UV inhibitors in it, which helps the paint resist fading.
Liquitex has a website with good technical information at www.liquitex.com. Golden's is www.goldenpaints.com. Winsor & Newton's website is www.winsornewton.com, but it seemed to concentrate more on their oil paints. Each has technical contact email, which I have found to be fast, friendly and expert advice (Liquitex put me in contact with their chemical engineer when I had a question on heat curing!). Acrylics have been a good alternative for me. They are more versatile, cheaper, easier to use than oil based enamels, and I because of their non-toxic nature, I can paint inside my house with them. Experiment with them, and see if they work for you.
Have fun, and keep the pointy end up! This article originally appeared in the December, 2002 issue of the Ballistic Beehive.