Richard Nakka’s Experimental Rocketry Web Site
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Introduction
to Rocket Design
Appendix I
Delay Grain based Recovery Control and Deployment
System
Introduction
As mentioned in the Introduction to
Rocket Design – Recovery System webpage, a technically simple design approach for Recovery
Control is to use a Delay Grain in conjunction with a pyrotechnic based
Deployment System. This is the system used for commercial model rockets, as
well as for some commercial Hi-Power rockets. This method is also a valid
approach for EX rockets.
Overview
My SkyDart rocket was designed to be a very basic EX rocket utilizing a DED (Delay Ejection Device) system to deploy its parachute at (or
near) apogee. This webpage will detail this particular Recovery Control and
Deployment system. The DED, which screws into the bulkhead of the rocket motor,
is illustrated in Figure 1. The DED consists of a metal fitting with a Delay
Grain and Ejection Charge. The optional Pyrogen Charge helps ensure ignition of
the Delay Grain. The Delay Grain consists of a slow-burning pyrotechnic
material. For the SkyDart, I used a mixture of potassium nitrate, epoxy and red
iron oxide. It is necessary to characterize the burn rate for any particular
blend. Fine tuning of the delay period is done by drilling an appropriate
length touch-hole (see Fig.1).The Ejection Charge consists of granular Crimson Powder (CP). Black Powder can also be used if so
desired. The Recovery Deployment system utilizes a Non-fixed Bulkhead (NFB) which serves to isolate the heat of
ejection from the parachute compartment.
Figure 1: Delay Ejection Device (DED)
Design
and Operation
Figure 2 illustrates the SkyDart rocket. The body is fabricated
from lightweight 2˛ (51mm) PVC tubing. Three sheet aluminum fins
and a wooden nosecone make up this basic EX rocket. The Recovery Descent system
consists of a 24˛ parachute. The motor that powers the SkyDart is
the A-100M featuring either KNDX or KNSB propellant. The
bulkhead of the motor was drilled and tapped to accommodate the DED.
Figure 2: SkyDart
rocket (click for photo of disassembled SkyDart)
Figure 3 illustrates the details of the recovery system. The
parachute compartment is isolated from the deployment charge by the Non-fixed Bulkhead (NFB). The coupler (section of PVC tube) is
bonded to the forward body tube. The coupler joins the two body sections
together. Nylon screws that secure the joint also provide for containment when
the ejection charge fires. The NFB butts against the aft end of the coupler.
Note that the thrust bulkhead is rigidly attached to the aft body tube, as this
forms the aft end of the pressure compartment.
Figure 3: Detailed view of Recovery system
Following motor ignition
and liftoff, the delay grain in the DED begins to burn. Once the delay grain is
consumed (if timed correctly, this will occur near apogee), the ejection charge
fires. This pressurizes the compartment, and when the pressure level generates
sufficient force acting on the NFB (which butts against the coupler), the nylon
screws shear. This results in the joint forcibly separating, as illustrated in
Figure 4.
Figure 4: Ejection charge firing followed by
separation of joint
The resulting momentum
of the forward body section, combined with the restraining action of the chute
tether, leads to the parachute being extracted as shown in Figure 5.
Figure 5: Parachute extraction
Last updated December
19, 2024
Originally posted December
19, 2024