April Program

April Meeting Program

Dual Deployment Payload

This was the fourth 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 - Night Launch Payload. Diagrams for this session are at the bottom of the page.

Initial Design Considerations
Several “vehicle-specific” factors must be considered in designing an effective Dual Deployment payload system:

Size of the airframe: The smaller the airframe the more difficult it is to design a good payload.
The larger the airframe the more room you have to work
Positioning of the electronics bay: Preferably at least 4 calibers aft of the nose
Not immediately aft of any body joints
Not in line with any protrusions
Not aft of a transition section
Easily accessible
Redundancy requirements: The larger the airframe, the more you need to consider redundant electronics
High-value payloads (and rockets) also imply redundancy
Type of altimeter: Large variation in size and weight between different brands of altimeters
Some have onboard batteries vs. external batteries
Some need static ports, some don’t
Some need multiple ports for arming, indicator lights, etc

Rules of Thumb
Most dual-deployment payloads follow the general design first proposed by Stu Barrett. See HPR, June ’94.

Here are some general guidelines:

Position the altimeter bay in a payload section between the drogue and the main
Design the apogee charges to fire aft to:: Separate the payload from the booster
Deploy a drogue or streamer if desired.
Design the main charges to fire forward to:: Separate the nose cone from the payload section
Deploy the main chute.
Provide access to the electronics and charges through removable bulkheads.
Keep wiring runs as short as possible.

Design Goals

Easy to prep in the field: Easier to prep means less chance of making a mistake
Complicated prep can lead to disaster
All prep external to rocket: Install altimeter(s)
Install batteries
Prepare and install charges
Power up and testing external to rocket: Verify proper operation of electronics
Verify continuity to all charges
Power down before insertion into airframe
Arming or disarming on the pad without disassembling the rocket: Avoid standing in line with “live” electronics
Switches should be readily accessible and marked
Ladders should not be required
Verification of payload status: Beeps, lights, etc must be visible/audible under field conditions
Easy post-flight cleanup: Inspect for damage
Remove BP residue
Replace terminals, connections, etc., if necessary

Payload Access

In-line Modules: Payload module(s) are inserted into the airframe via slide-in containers and secured using removable bulkheads
Hatches: Payload electronics are inserted through the side of the airframe using access hatches

Switches

Switches are desirable for safety: Minimize danger of accidents by using switches for arming electronics; Switches should be easily accessible; At larger launches, LCOs may require on-pad arming
Select the right type of switch: Slide switches, toggles and phono-plugs can be activated by G-forces; Twisted wire can vibrate apart; Push-button switches are positive and effective; Key switches
Consider dual switches for motors: One switch for arming electronics; Safety switch for arming motor/ignitor circuit

Matches

Electric matches are preferred for activating BP charges and motors: Oxral, DaveyFire; Will ignite BP un-aided; Will ignite motors if dipped in pyrogen
Flashbulbs will also work for BP if used correctly

BP Charges
Determine correct amount of BP necessary for intended application

Factors involved are:: Diameter of the airframe; Volume of chamber to be pressurized; How tightly the parts fit together; Leakiness/integrity of the airframe
Too little BP can be bad, too much can also be bad: Must have sufficient force to separate components; Too much force can tear components apart
The larger the surface area of the nose cone or bulkhead, the more force is applied: Larger airframe can use less pressure than small airframes; Solve for total pounds of force, not simple lbs/ sq. in.
Ground test to make sure you have correctly sized the BP charge!: Wrap nose cone / payload section with blankets; Lay airframe on the ground; Ignite charges remotely; Re-test if unsatisfactory
For more information Visit InfoCentral: http://www.info-central.org/index_recovery.html
then select “Black Powder Use”

Charge Holders

Charge holders can be constructed from paper, plastic, etc,: Good solution available for free from Programmin’ by Pete.
    Disposable Ejection Canister
    Holder template also available here as a PDF file.
    (Be sure to uncheck "Shrink to Fit" before printing.); Info Central charge holders
Commercial charge holders available
Robbies Rockets LES;         30539 North Shore Dr.
        Elkhart, IN. 46514
        Phone: (219) 679-4143
        Fax: (219) 679-4539; Available from Robbies or:; Hobby Connection,; Pratt Hobbies,; others
Charge “cannons” act as first stage of baffle system: Provide containment for soot from BP charge; Focus and direct expanding gasses
Cannons can be made from various materials: Paper or phenolic motor tubes; PVC; Aluminum

Ejection Baffles

Optional part payload assembly: Can achieve same effect with ejection blanket or wadding; Not necessary if piston is used
Effective way to remove heat from ejection gasses: Most use re-direction of gasses to remove burning particles; Can also be accomplished with steel mesh, ChoreBoy, etc.
See below for construction examples

Static Ports

Static ports are necessary for barometric altimeters: Not used for apogee deployment accelerometers; They are still needed if the accelerometer uses barometer for 2nd event.
Port sizing is critical: Too small will result in “post-apogee” deployment; Too large will result in “pre-apogee” deployment
Rule if thumb is 1/4-inch diameter hole for each 100 cubic inches of volume
Ports should be:: Smooth and free of burrs, etc.; Not downstream of external airframe joints, fins, transitions, etc.
If multiple ports are used, they should not be directly opposite each other
For more information, visit Vern’s Rocketry site:: http://www.vernk.com/AltimeterPortSizing.htm

Diagrams of various dual deployment payload bays:

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