HIGH POWER ROCKETRY: DUAL DEPLOYMENT
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.
Initial deployment at high speed (usually because the motor
delay is either chosen incorrectly or does not perform as expected and
the rocket still moves fast when the ejection charge fires)
High altitude parachute deployment that results in long drift
times and thus difficult recovery of a landed rocket.
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.
A typical dual deployment equipped rocket is shown on Figure
(click on the picture for a larger version). From the top to
the bottom, the rocket parts are:
Nosecone: does not really need much of introduction or explanation.
Note however, that the empty space in the nosecone can be used as an extension
of the payload bay, which is locsted right below the nosecone.
Payload bay: hosts a payload (most often electronics of some
kind). The payload usually does not have any other function than to collect
its own data, it is not crucial for the flight (in other words, the rocket
can fly, deploy and recover regardless whether the payload is present and
active or not).
Bulkhead: caps the bottom end of the payload bay and prevents the
hot gases from the drogue ejection charge from entering the payload
bay and damaging the payload. Also has an attachment (usually an I- or
U- bolt) for upper shockcord of the drogue parachute.
Drogue parachute: a small parachute that will be deployed at the
apogee and that will control fast descent of the rocket until the main
parachute is deployed. The drogue parachute is attached both to the to
the middle section and the top section.
Drogue charge: a charge of a black powder attached to an electric
match (an igniter) that is attached to the deployment trigger. Activated
at the apogee of the flight. When activated, the black powder burns fast,
generating lots of gas. The gas overpressurizes the compartment with the
drogue parachute and pushes the drogue parachute out (click
here to see how it works).
ELECTRONICS BAY (E-BAY)
Bulkhead: caps the top of the electronics bay. Has an attachment
for the drogue parachute lower shockcord and electric terminals for the
drogue charge electric match..
Electronics bay: has two functions. First: it houses the deployment
electronics, second: it serves as a coupler between the booster and middle
section of the rocket.
Deployment trigger: usually an altimeter or an accelerometer, capable
of firing deployment charges at apogee and set altitude.
Bulkhead: caps the bottom of of the electronics bay. Has an attachment
for upper shockcord of the main parachute and electric terminals for the
main charge electric match.
Main parachute: a large parachute that is deployed at preset altitude
(usually 300-900ft above the ground level (AGL)). Its function is to slow
down the rocket descent from fast drogue controlled descent (50-90fps)
to a speed that is safe for landing (15-20fps). By deploying the main parachute
at a low altitute, the overall drift is minimized and the rocket is easy
to recover. Many dual-deployment equipped rockets land very close to the
launch pad even though they reach apogees of several thousands feet.
Main charge: just like the drogue charge, main charge is a load
of black powder (FFF or FFFF rating) that is ignited electronically by
the deployment trigger. The main charge is ignited at low altitude and
deploys the main parachute. Click
here to see how it works.
Bulkhead: blocks the heat from the motor and provides a solid bottom
for the main parachute.
Motor: makes the rocket fly
Fins: make the rocket fly in a generally upward direction
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:
Dual Event Altimeter. Altimeter is a device that continuously
measures atmospheric pressure. The altitude of the rocket is immeditatelly
computed from the difference of pressure at the ground level (as sampled
during altimeter activation) and currently measured pressure. Dual Event
Altimeter is an altimeter with some additional functionality. First
of all, it is able to detect apogee and "throw-a-switch" when the apogee
is detected (this can be used to fire a drogue parachute ejection charge).
Second, the dual event altimeter will continue to monitor the altitude
even during the descent and can "throw-another-switch" when a predetermined
altitude is reached (and thus fire the main parachute ejection charge).
Most of the dual event altimeters also record the graph of altitude vs.
time for the whole duration of flight.
Dual Event Accelerometer. Accelerometer is a device that continuously
measures acceleration (G's). It also computes instant speeds of a moving
rocket and detects an apogee as a point of zero speed. Just like a dual
event altimeter, dual event accelerometer it is capable of
"throwing a switch" at both an apogee and a preset altitude.
Timers. Timer is a device that can "throw a switch" after certain
time after the liftoff. Timers are usually single event devices (can throw
only one switch). Timers cannot detect apogee so their use is generally
limited to a deployment of the main parachute only (or other functions
that do not require apogee detection).
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
Liftoff: an altimeter detects a liftoff by a sudden change
in the altitude, an accelerometer registers sustained acceleration over
a period of time (usually 2g over 0.25 or 0.5 seconds) and timers are usually
equipped by a G-switch that also registers sustained acceleration.When
the liftoff is registered, all devices start functioning and ejection charges
are fully armed.
Apogee (first event): when an apogee is detected, the drogue parachute
ejection charge is fired and drogue parachute deploys. Rocket descends
fast (at about 50-90fps) but in a controlled manner. Rocket does not drift
much. See Figure 2. (click on picture to see a larger version).
Main parachute preset altitude (second event): when altitude chosen
for main parachute deployment is detected, the main parachute ejection
charge is fired, the main parachute is deployed and it slows the rocket
descent to a speed safe for the landing (15-20fps). Rocket drifts with
the wind. See Figure 3. (click on the picture to see a larger version)
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.
Landing: the rocket hits the ground. It is
important to watch the flight and make sure that all ejection charges fired.
If there is a suspicion that rocket still contains a live and armed ejection
charge after it landed, do not approach the rocket.