Rendering of the main parachute bay.
Rendering of the main parachute bay.

The main parachute bay consists of an outer fiberglass body tube, its physical connections to the fin can section subassembly and the parachute with its hardware.  The airframe tube is 6.17in (157mm) OD, 6.0in (152mm) ID, G10 fiberglass airframe tube from Madcow Rocketry.  The bottom of the main parachute bay rigidly connects to the fin can section subassembly via six 8-32 flat head screws.  Each of these screws thread into adhesive mount nuts inside the tube coupler of the fin can section.  The top connects to the electronics bay subassembly via six nylon shear pins from Apogee Components.  To help prevent premature separation due to pressure differences between the inside and outside of the airframe during flight, a small, 0.188in (4.78mm) diameter hole is drilled into the bottom section of the parachute bay.

Main Parachute

The main parachute is a 18ft (5.5m) parachute from Rocketman Parachutes.  The parachute is connected to two different sections of shock cord.  The lower section, going from the fin can section to the parachute, is a 30ft (9.1m) length of tubular nylon webbing.  This connects to the fin can section by looping through the U-bolt on the top of the fin can section.  The other end connects to the parachute with a 2.75in (70mm) long quick link and a 1500lb (6.7kN) rated swivel.  The upper section of shock cord is a 15ft (4.6m) length of tubular nylon webbing and also connects to the parachute quick link.  The other side of the upper shock cord connects via a quick link to the U-bolt at the bottom side of the electronics bay.

Ejection Charge Sizing

To determine the amount of black powder necessary to reliably eject the main parachute, the internal volume of the main parachute bay and the strength of the shear pins must both be known.  Since the fin can section and electronics bay couplers and bulkheads reduce the internal volume significantly, only the volume that needs to be actively pressurized is considered.  The fin can section projects 6.13in (155mm) and the electronics bay projects 5.13in (130mm) into the volume to be pressured.  The active total length can be found by subtracting the two lengths that the other components project into the volume from the full length of the main parachute bay body tube, 35.0in (889mm).  Therefore, the internal volume is a simple cylinder, 6.0in (152mm) in diameter and 23.74in (603mm) long.  The internal volume is therefore 671 in^3 (11L).

The shear pins are known to require 64.24lbs (285.75N) to shear a set of three in half.  Since the design employs six shear pins, the required force is doubled, or 128.48lbs (571.50N).  To calculate the internal pressure required to generate this force, the area of the internal bulkhead must be found.  Since the OD of the bulkhead is 6.0in (152mm), the area is then 28.27 in^2 (182.4 cm^2).  Dividing the required force by this area results in a minimum needed pressure of 4.54psi (6.89kPa).  To ensure that the charges can both fully shear the pins and separate the sections of the airframe, the primary ejection charge nominal pressure should generate 2.0 times the minimum force and the backup charge should generate 2.5 times the minimum force, or 9.08psi (13.78kPa) and 11.4psi (17.2kPa) respectively.

Entering the needed pressures and bay dimensions into the calculator at NASSA Ejection Charge Calculator, the amount of FFFF black powder necessary to produce these pressures is 3.14g and 3.95g for the primary and backup charges respectively.

To protect the parachute from these sizable charges, an 18.0in (457mm) square NOMEX protector will be used.

Bill of Materials

Click the part number in the table to either view the part’s drawing if it is a custom design, or to view the part on the supplier’s website if it is purchased.

PART NUMBERDESCRIPTIONQTY.
1-012MAIN PARACHUTE BAY BODY TUBE1
18ft ParachuteROCKETMAN PARACHUTE - STANDARD - 18FT1
30ft WebbingROCKETMAN SHOCK CORD - 30FT1
15ft WebbingROCKETMAN SHOCK CORD - 15FT1
SW-15001500LB SWIVEL1
29314SUNWARD 18IN ORANGE NOMEX PARACHUTE PROTECTOR - APOGEE 293141
29615NYLON SHEAR PINS6
8947T28OVAL SHAPED THREADED CONNECTING LINK, TYPE 316 STAINLESS STEEL, 3/8" THICKNESS, 1/2" OPENING, 3900LB CAPACITY1
92703A31018-8 STAINLESS STEEL TORX FLAT HEAD SCREWS, 8-32 THREAD SIZE, 3/4" LONG6
Final Design – Main Parachute Bay Subassembly

Leave a Reply

Your email address will not be published.