HIGH POWER ROCKETRY: DUAL DEPLOYMENT



Introduction
Dual deployment is a deployment technique often used for high power rockets (H class and above; 160.01Ns and above). Dual deployment technique addresses two main problems that could otherwise lead to extensive damage or a complete loss of a rocket. Those two problems are: When dual deployment is used, the initial deployment occurs exactly at apogee (the highest point of the rocket trajectory). In ideal case, a rocket at an apogee of its flight does not move at all. Because a rocket at apogee does not move, the deployment of a parachute is not likely to cause any significant damage (just imagine the difference of getting out of a car that is zooming at 100 mph and a car that is parked). A small parachute (so called a drogue or a pilot parachute) is deployed at the apogee. The rocket then quickly descends to low altitude (usually 300-900ft). Because the rocket drops fast and there is no large area that wind can push into, the drift is low.

Once the quickly descending rocket reaches a predetermined altitude, a second, much larger parachute (a main parachute) is deployed. The main parachute slows the rocket down to a speed safe for landing. The drift rate during this second phase is significant but because the rocket is already at a very low altitude, the overall drift does not present a significant problem for the recovery crew. Click here to see the complete flight sequence.

Rocket Configuration
A typical dual deployment equipped rocket is shown on Figure 1. (click on the picture for a larger version). From the top to the bottom, the rocket parts are:


Deployment Triggers
Unlike in smaller rockets, where the ejection charge is built into a motor and fired at a set time interval after the motor burnout, high power motors usually do not have ejection charge built-in. Ejection charge for high power motors is made by a user himself, using FFF or FFFF black powder packed either in a piece of saran wrap or a paper tube. Such an ejection charge is ignited using a special low-resistance igniter, often called electrical match or e-match. E-match is connected to a deployment trigger, which fires the e-match at apogee or at preset altitude or at set time interval after liftoff.

SAFETY NOTE: Making an ejection charge requires you to handle a loose black powder. Only holders of a Low Explosive User Permit (LEUP) are allowed to handle loose black powder. No person under 18 years of age can obtain a LEUP, so black powder ejection charges must be prepared and installed by a properly licensed teacher/mentor. This restriction is not valid for ejection charges made out of Pyrodex.

Using electronics deployment trigger allows us to fire an ejection charge at precisely defined moment or event (such as an apogee). This is not the case with simple low power motors, where the ejection charge always fires at a set time interval after the motor burnout, regardless whether the rocket is still going up, directly at apogee, or already on its way down.

There are three basic types of deployment triggers:


SAFETY NOTE: all deployment triggers must be disabled during the rocket preparations (to prevent erroneous firing of black powder charges when people are around the rocket). This is usually done by adding an ON/OFF switch that is reachable from the outside. All deployment electronics is activated only after the rocket is safely installed on a launch pad and ready to fly.
 

Dual Deployment Flight Events

A complete flight sequence (with both deployment events) is shown on Figure 4. (click on the picture to see a larger version). Note the change in drift rate after the main parachute deployment.