Rocket Design

Fin Design Guidelines

• D=body tube diameter
• Root Edge=2D
• Trailing Edge=2D
• Outside Edge=D
• Leading Edge=close perimeter
• Total Perimeter~7D
• Thrust to weight: 6:1 to 12:1

Websites

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• RocketReviews.com
  • Contests: Egg Payload
• Commonwealth Displays Rocket Store
• Parachute Design and Construction
• Performance Hobbies
• Public Missiles
• Firestar electric match
• Rocketflight igniters
• Tripoli rocket association
• National Association of Rocketry (NAR)
• Model Rocket Calculators
• Aeronautics and Astronautics course materials listing at MIT
• Great Lakes Hobbies
• A2Z Hobbies
• Wildman Rocketry
• MagnumRockets: Currently not shipping product
• rec.models.rockets Google group
  • Using a flycutter to make centering rings
• Discount Hobby Center
• Estes Rockets
• Eggloft Duration competition link at the NAR page.
• Info at MaxThrust
  • How to make ignitors
• RocketFlite
  • Make your own cluster eggnitor
• From Aerotech Engine instructions
  • …insert the black-coated end into the nozzle and probe for the slot in the propellant grain. Slowly rotating the motor while probing with the tip of the igniter may help in finding the slot. Once the COPPERHEAD� has entered the propellant grain slot, continue inserting it until contact is made with the delay element at the forward end of the motor. Failure to insert a COPPERHEAD igniter as described may result in lowthrust ignition of the motor.
  • Insert the AEROTECH composite model rocket motor into your rocket. If your rocket does not have a motor mount with a motor hook or motor block or has a motor block deeper than the motor length, simply wrap a layer of 1/2" wide masking tape tightly around the nozzle end of the motor to a thickness equal to that of the motor tube. Wrap a layer of masking tape around the motor tube/motor junction to secure the motor in the rocket and to prevent motor ejection when the ejection charge fires. NEVER friction fit an AEROTECH composite model rocket motor into the motor mount.
  • Model rockets powered by AEROTECH composite model rocket motor must be flown from a launch pad having a launch rod or rail at least 36 inches long or two-thirds the combined length of the rocket body and nose cone, whichever is greater.
• Rocketry Stability: Tests have been made where the stability of the model rocket was in question. If it was completely unstable it would loop around and around in the air, seldom reaching over 30 feet in height and never reaching a velocity in excess of 20 or 30 miles per hour. However, occasionally one of these rockets would make a couple of loops, suddenly become stable due to the lessening of the fuel load, and make a bee line straight into the ground. Had anyone been standing in the wrong place a serious injury could have resulted.
• Rockets for schools: Basic Rocket Stability.PDF
  • How to find the center of pressure
    • Cut out a profile of your rocket in cardboard. It doesn't have to be the same size, but it does need to be accurate in shape and scale.
    • Balance the cut-out on the edge of a ruler. Mark where it balances. This spot marks the Center of Pressure of your rocket.
  • The swing test is a basic test to give you a rough idea how your rocket will fly. There are basically three things that can happen when you do this test:
    • If you swing your rocket and it points in the direction you swing it, it is a good indication that your rocket will be a stable flyer.
    • Sometimes a rocket will just cartwheel as you swing it. It is usually a sign that the CP and CG are too close together. Try adding a little weight to the nose, and see if your rocket straightens out.
• Apogee Rockets: The "BT-70" (Apogee P/N 10160) is a great size body tube to make egglofter rockets
• Google Rocketry Directory
• Rec.Models.Rockets FAQ
  • Rocket classes
    • Engine designations: The first letter indicates the power range, as specified in the table below. The number to the left of the dash is the average thrust of the motor, in newtons. The number to the right of the dash is the approximate ejection delay in seconds starting at the time of motor burnout. The final letter is an optional manufacturer designation for motor or fuel type.
  • Model Rocket Construction and Finishing
    • 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 1-2" 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.
    • 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 matboard…) 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.
    • Recovery wadding: 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!
    • Glue: 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.
    • Slight spin to reduce arcing: 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. 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.
    • Reinforcing body tubes: Wrap the tube with fiberglass cloth. Two ounce cloth is good for use on 2.5 to 4 inch diamter tubes. Five ounce cloth might be used for larger tubes.
    • Ignitors:
      • Copperhead: used to ignite single composite motors; not good for clustering. They will light most black powder motors. Requires strong 12V current source. It is certain that Copperhead igniters are not a good choice for igniting clusters. However, some have found the Copperhead to be a reliable igniter for single-motor HPR rockets.
      • Thermalite: a type of fuse used extensively in pyrotechnic applications. May be ignited by nichrome wire or flash bulb. Plain thermalite ignited by nichrome wire is often used in black powder clusters
      • Flashbulb/thermalite: some types of camera flashbulbs ignite with very little current (typically as little as 50ma) and burn very hot. These are used to ignite a piece of thermalite fuse running into the motor. Used for igniting high power motors and all forms of clusters.
      • Composite motors are ignited at the top of the core (nearest the delay charge).
      • Flash in the pan ignition: is used to ignite clusters of small black powder rocket motors. It consists of a thin layer of black powder on a paper plate under the motor nozles. The powder is ignited via a regular model rocket igniter, such as an Estes Solar igniter. When the power ignites, the burning particles and hot gasses from the 'flash' ignite the motors. It is used to ignite clusters of 7-469 motors and reduces the number of igniters needed to one.
      • The bottom-line-consensus of the 'net' seems to favor parallel wiring for most clusters of 7 or fewer motors, using a 12V (or more) launch system capable of dumping plenty of amps to the igniters. This generally means a relay based system with the primary ignition power source close to the launch pad
    • Launchpad: For blast deflectors, we use are discarded grinding wheels. Fireproof, non-conductive, free, plentiful, large, and pre-drilled. Use clay flower pots for blast deflectors. Get Stainless steel for launch rods from welding or metal supply houses.
      • A 12V motorcycle battery can put out something like 15A for a short period of time.
• Rec.Models.Rockets archive site
  • Flashbulb Ignition
    • "Flashbulb ignition has long been recognized as the most reliable way to fire clustered models. Developed by John Langford in 1971, this method virtually guarentees successful firing of clusters up to 30 motors."
  • Tips #11
    • "Avoid clustered model power prangs! When using electronic ignition for clustered models, make sure you use at least a 12-volt motorcycle battery as your power source. The smaller, 6-volt systems simply do not provide enough juice to reliably ignite more than one motor."
• Rocket Equations
  • Equations quick reference
• Rocketry Online
  • CG/CP Relation
    • "the center of pressure must be a minimum of 1 body diameter BEHIND the center of gravity on a rocket fully prepped for launch to ensure stability"
    • "keeping the CP behind the CG makes sure the front of the rocket stays the front!"
    • "Up to two body diameters is usually even better, but don�t go much over two diameters or the rocket will be overstable. By the way, this is what�s meant by �one-caliber� or �two-caliber� stability"
  • Clustering Motors
    • "To make your small rocket go twice as high, use two stages. To launch a much larger rocket to single- stage altitude, use a cluster."
    • "It's common to put a long-burning, low thrust "sustainer" in the center and high-thrust, short burning "booster"* engines outboard."
    • "Two, three, or more engines at the rear end of the rocket concentrates weight where it is least wanted and shifts the CG rearward. Cluster models often need large fins to counteract this. Be sure to test the stability of a new design with the heaviest motors you intend to use in it"
    • "Engines are usually mounted in clustered motor mount tubes. The spaces between the tubes need to be filled so ejection charge pressure doesn't leak out there instead of doing its job. Fill the gaps with a putty made from (1) tissue paper and white glue, or (2) epoxy and a filler such as microballoons. You could also cut plywood or balsa plates and glue them in place."
    • "A car battery is best. Four fresh alkaline "D" cells or a fully charged 9.6v RC car battery pack will fire a cluster of three Solar ignitors or electric matches reliably, but 4 AA cells generally will not. In general, fully charged NiCad rechargeables are much better than alkalines."
    • "some have used Thermalite or a sprinkle of loose black powder to interconnect the upper-stage nozzles"
  • Rules of thumb
    • "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."
    • "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."
    • "Fin Dimensions: 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 Panform: 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."
    • 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.
    • "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."
    • "Sizing Tubular Nylon: 9/16" serves well in rockets up to 15 pounds. Go with � up to 30 pounds. 1" up to 50 pounds."
    • "Wadding: 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"
    • "An over stable rocket will tend to dramatically turn into the wind. A marginally-powered, over stable rocket can end up almost horizontal"
    • "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."
    • "To move the CP aft (more stable), increase the size of the fins. To move the CP forward, decrease fin size."
    • "In selecting a motor to power your rocket, you need to have at least a 5:1 thrust to weight ration. See a detailed discussion of this guideline Motor Selection in the PROPULSION area."
      • "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."
• Bowling Ball Lofting site
• Monroe Astronautical Rocket Society
  • Forum
  • Benefits of belonging to MARS: "Access to a large flying field, never lose your rocket again"
  • Dues are $12 per year for adults. If you wish to join, send an email to our membership officer: Ferenc Gy Roka at moc.liamtoh|akoRyGF#moc.liamtoh|akoRyGF.
• DesCon 7 - Easter Egg Rocket
• DesCon 11 - Hybrid Easter Egg

Weights

• 1 gram = 0.035274 oz
• 1 oz = 28.3495231 gram
• 1 lb = 16 oz
• 1 kg = 2.2046226 lb

Object	Weight
Egg	2.3oz (65g)
Egg carrier	0.35oz (10g)
6 Eggs w/ Carriers	~1lb
D12-0	1.5oz (42g)
D engine clip	0.04oz (1g)
D engine mount	0.04oz (1g)
D engine retain	0.04oz (1g)