January Program

January Meeting Program

Rocket Design

This was the first part in our continuing project to get club members up to speed on building a High Power Rocket. If you missed the meeting here is the basic outline of what was discussed. Don't miss next month - Fiberglassing.

Objectives: Review the design considerations encountered when building a typical high power rocket.; Provide information on "best practices" based on collective experience of Club members.; Provide sources of rocketry parts, kits and information.

Suggested Project: 4-inch, 54mm, dual-deployment rocket that will be suitable for Level 1 and Level 2 certification.

General design parameters:: 4-inch diameter (or larger); Single stage; Approximately 75-80 inches long (20-to-1); Payload section suitable for housing dual-deployment electronics; Capable of single stage (motor) or dual-stage (electronic) recovery deployment; Single 54mm motor mount (38mm minimum); 3 (or 4) fins; Target weight = 5 pounds

HPR design and construction considerations:

Scratch-building vs. Kits
Kits are good if you are just starting out:
- Parachutes and recovery harness (usually) included
- Instructions (usually) included
- CP/CG relationship established by the manufacturer
- Represents a slight discount relative to purchasing all parts separately
Kits are not necessarily the best choice if:
- You already have some basic parts (tubes, chutes, nose cones, etc.)
- You intend to significantly modify the kit
Kits usually contain the minimum components to get the job done
- Construction methods used may not be the best
- Materials used may not be the best but are economical for the supplier
- Some additional items may need to be added (motor retention)
Scratch building is good if:
- You already have some experience building high-power rockets
- You already have some basic parts on hand
- You are designing and building your own rockets
- You are building a design for which no kit is available
Good kits for this project
LOC/Precision
- IRIS - $120
- IQSY Tomahawk - $130
- Sandhawk - $130
- View them at LOC/Precision
PML
- AMRAAM 4 - $140
- Endeavor - $135
- Andromeda - $220
- View them at PML
Binder Design
- Raptor - $109 - view
- Stealth 54 - $100 - view
- Excel Plus (38mm) - $75 - view
Basic project parts list for scratch-builders:
- 3.9-inch airframe tubes (2)
- 3.9-inch nose cone
- 3.9-inch tube couplers (3)
- 3.9-to-54mm centering rings (3)
- 54mm motor tube
- 3.9-inch bulk plate (4)
- 20-ft x ½-inch nylon shock cord
- 10ft x ½-inch nylon shock cord
- 48-inch parachute
- ½-inch launch lug
- Miscellaneous hardware (quick links, eye bolts, etc.)
- Miscellaneous 3/16-inch plywood (for fins)
Overall stability
All rockets must have stability equal to at least 1 caliber (body diameter). This is measured with the rocket in flight configuration (engine, chutes, etc. installed).
Center of gravity (balance point) must be at least 1 caliber ahead of the center of pressure
- Rockets with less than 1 caliber of stability are considered "marginally stable"
- Over 2 calibers, rocket is "overstable" and will easily turn into the wind after leaving the rod. Underpowered, overly stable rockets may fly horizontally resulting in a bad day.
Adjusting the CP
- To move the CP aft, make the fins larger, remove forward fins, add more fins (the last item becomes less effective after 4-5 fins)
- To move the CP forward, make the fins smaller, add small forward fins, remove fins (below 3 fins is not recommended)
Adjusting the CG
- To move the CG forward, add weight to the nose, lighten the aft end, or lengthen the rocket (excessive lengthening of the rocket can result in rigidity problems).
- To move the CG aft, remove weight from the nose, add weight to the aft end, or shorten the rocket.
Determining stability
- Very difficult without a computer.
- Good (free) software is available online.
- VCP - Visual Center of Pressure - download
- RockSim - Apogee Components - download demo
A good discussion of CG/CP relationship is on InfoCentral
Predicting altitude
Altitude achieved by a rocket depends on several factors:
- Weight
- Drag
- Motor impulse
- Launch angle (should be vertical)
Altitude is difficult to predict without a computer
- Good software is available on-line
- wRASP - windowed Rocket Altitude Simulation Program - download
- RockSim - Apogee Components - download demo
Once projected altitude is known, consider dual-deployment
- Dual Deployment is recommended for altitudes over 2000 ft
Airframe pros & cons
Cardboard (LOC, Binder)
- Least expensive
- Light weight
- Minimum strength required
- Medium finishing effort (need to fill spiral groove around tube)
Quantum tube (PML)
- Slightly more expensive
- Heavy
- Very strong
- Very smooth, just prime and paint
- Special construction techniques required
- Melts around Mach .9
Phenolic (PML, Giant Leap)
- Slightly more expensive
- Slightly heavier
- Stronger in compression but more brittle
- Easier to finish
- Very precise fit
Fiberglass (Hawk Mountain)
- Much more expensive
- Heavier
- Very strong
- Easy to finish
Carbon (Performance Rocketry)
- Very expensive
- Extremely light
- Extremely strong
- Difficult to work
- Only necessary for "extreme" projects
- Shields RF radiation
Suggested approach:
- Buy cardboard or phenolic tube and apply one or two layers of fiberglass yourself.
Is fiberglassing really necessary?
There are pros and cons to fiberglassing airframe tubes:
Fiberglass adds strength to an airframe tube
- Greatest benefit is added protection from recovery damage
- Additional strength rarely necessary during boost
- Fiberglassed rockets will usually be flyable longer and need fewer repairs than non-fiberglassed rockets
Fiberglass requires more work
- Adds (some) weight to the rocket
- Requires additional filling and sanding prior to painting
- Requires (some) special equipment
Fiberglassing a tube is easy to do
- As with most things, it's easy once you've done it
- The right equipment and the right techniques make all the difference
- We will cover fiberglass equipment and techniques in a subsequent class
InfoCentral has some articles on fiberglassing
Booster & payload tube lengths
4-inch airframe tubes come in 34-inch lengths (typically)
Booster length considerations
- Length of largest motor to be flown
- Hybrid vs. composite - Hybrid motors require space for the oxidizer tank
- Space required for booster chute & recovery harness
- Space required for payload coupler
- Anti-zipper vs. traditional design - Anti-zipper approach allows shorter booster
- InfoCentral has some articles on anti-zipper design
Payload length considerations
- Space required for chute and recovery harness
- Space required for electronics
- Space required for nose cone shoulder
- Space required for booster coupler (if anti-zipper)
General considerations
- Transportation - can you get the rocket into your vehicle?
- Storage - where will you store this rocket when not flying it?
- Does the length compromise stability or integrity?
Fin materials
Plywood
- Inexpensive
- Lightweight
- Medium strength
- Easy to work
- Requires filling and sanding
G10
- Expensive
- Heavier
- Very strong
- Very smooth
- Harder to work
Composite board
- Expensive
- Very light
- Very strong
- Easy to work but requires "edging"
- Great for large projects
Fin Design
Sizing
- Fin semi-span (distance fin extends from the rocket)
  - About 1 caliber for 4 fins
  - About 1.5 calibers for 3 fins
- Fin chord (root length along the rocket
  - About 1.5 to 2 calibers
Planform (shape)
- Clipped delta is most effective
- Many shapes are possible if sizing rules (above) are kept in mind
- Fins that extend below the rear of the rocket are more prone to damage upon landing
Mounting
- Typical mounting is "through-the-wall" where fin tab extends through a slot in the airframe and is bonded to the motor mount tube
  - Much stronger than surface mounting
  - Requires cutting fin slots through the wall of the airframe tube
  - Most kits come with "pre-slotted" tubes
Number of fins
- Minimum of 3 fins are required for stable flight
- Military rockets often use 4 fins
- "Futuristic" models may use 5, 6, 7 or more fins
  - Fin effectiveness increases only slightly as more fins are added
  - Aerodynamic drag and weight increase much more
Placement
- Fins should be placed as far aft on the rocket body as possible
- Multiple sets of fins or fins along the upper airframe:
  - Increase overall drag
  - Move the CP forward
  - May require additional weight to be added to the nose
Nose cones
Types
- Nose cones are of three basic types
  - Conic
  - Ogive
  - Parabolic
- Conic (true cone shaped) nose cones are most efficient at supersonic speeds
- Parabolic nose cones are most efficient at subsonic speeds
- Ogive nose cones are a compromise between supersonic and subsonic efficiency
Materials
- HPR nose cones are typically made of plastic or fiberglass
  - Plastic cones are lighter and usually cheaper
  - Fiberglass cones are heavier but can withstand more stress
Motor Mounts & Centering Rings
Sizing
- Motor mounts must accommodate the largest motor you would ever fly in the rocket
  - Adapters can be used to fly smaller motors when desired
  - Larger mounts can also be used as "host mounts" to accept cluster motor adapters
Construction
- Mounts usually consist of a motor tube centered within the airframe tube by a series of centering rings
  - Centering rings can be plywood, G10, or composite materials
  - At least three rings are required for thru-the-wall fin mounts
Integral Motor Mount/Fin Assembly
- When possible build motor mount and fins as one assembly and then install the completed assembly into the booster airframe
  - Allows fin/motor tube joints to be heavily reinforced
  - Requires that fin slots extend all the way to the rear of the booster tube
Bulkheads
Sizing
- Bulkheads must withstand the extreme force of recovery events
  - Must be strong enough for expected stress
  - 3/16-inch plywood bulk plates are sufficient for this project
Materials
- Can be plywood, G10, or composite materials
Recovery Harness
Materials
- HPR harnesses are typically nylon or Kevlar
- Bungee-cord, elastic or other stretchy/springy materials are not desirable due to recoil
Sizing
- Tensile rating for recovery materials should be at least 50 times the static weight of the rocket
- For tubular nylon
  - ½-inch for rockets under 15 pounds
  - ¾-inch for rockets under 30 pounds
  - 1-inch for rockets under 50 pounds
Lengths
- Recovery harnesses should be a minimum of 3 times the overall length of the rocket
  - Longer harnesses minimize the chances of recoil damage
Connections
- Connections between harness and rocket are usually accomplished with quick-links on the harness connected U-bolts or eyebolts on the rocket
Recovery System Protection
Methods available to protect recovery systems from ejection charges:
- Recovery wadding
  - Large volume of wadding required for large diameter rockets
  - Generally not used for HPR due to many negative aspects, including littering the launch site, effect on livestock, etc.
- Pistons
  - Can be very effective when used correctly
  - Require maintenance to remove residue after each flight
  - Sensitive to temperature extremes
- Baffles
  - Very effective
  - No maintenance
  - No moving parts
  - Adds weigh to rocket
- Chute protector blankets
  - Typically Kevlar or Nomex
  - Can be very effective when used correctly
  - Nomex loses effectiveness with repeated use
  - Very light weight
Launch Lugs vs. Rail Guides
General Usage
- Either will work when sized correctly
  - ¼-inch rod OK for rockets up to 5 pounds
  - ½-inch rod OK for rockets up to 15 pounds
  - Rail preferred for rockets over 15 pounds
- For larger motors (J and above), long rockets, etc. use a rail
Placement
- Lugs should be placed at the CG, or equal distances fore and aft of the CG
- Rail guides should always be used in (at least) pairs with:
  - One at the CG
  - One close to the aft end of the rocket
  - One about 1/3 down from the nose, for longer models

Good Sources for Rocket Kits, Parts and Motors:

Impulse Aerospace
Motors, Aerotech kits, tubular nylon, fiberglass, composite board
Giant Leap Rocketry
Motors, parts
Magnum Rockets, Hobbies and More
Motors, parts, kits
LOC/Precision
Kits, Parts, chutes
Public Missiles, Ltd.
Kits, Parts, chutes
Binder Design
Kits, parts, chutes
Performance Rocketry
Custom nose cones, custom tubes

Sources of information:

Rocketry Online
Online portal to all things rocketry
Rocketry Online - InfoCentral
General technical information
NARTS
General technical info and scale data
Rockets of the World
Peter Alway's excellent book of scale data
Jim Ball's Scale Library
Scale data online

Sources of rocketry software

VCP
Center of Pressure calculation
wRASP
Flight simulation
RockSim
Flight simulation and Center of Pressure calculation

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