Dual-Event Recovery Information

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Dual-Event Recovery Information

WHY DUAL-EVENT RECOVERY

Dual-event recovery is electronic based apogee (drogue) and main recovery. It adds complexity to the rocket, increases the cost, and increases the rocket preparation time. However, these disadvantages are quickly overcome by the desire to decrease recovery time (and therefore drift) as the rocket altitude increases. The Shuttle Solid Rocket Boosters used similar altimeter based recovery and so there’s also the joy of taking another step into real rocket science.

ROCKET DESIGN

Using a kit made specifically for dual-event recovery is an excellent way to begin. Follow the manufacturer instructions and you are well on your way to your first successful dual-event recovery. I began with a PML Thunder -N- Lightning. Starting with the sustainer as a single stage I was able to become comfortable using altimeter recovery. When I was ready, I already had the booster for the next step, two-stage flights.

A common dual-event recovery design is to split a rocket into three sections, with the sections connected by shock cords. Selection of shock cord length and width is the same as for single-event recovery. A good rule-of-thumb for shock cord length is three times the length of the rocket. Ejection charges controlled by electronics are used to split the rocket into the appropriate sections to deploy the drogue recovery system at apogee and the main recovery system at a predetermined altitude.

The center section is further divided to provide access to the electronics and ejection charges during preflight. Once the electronics and ejection charges are installed, the two center sections are secured together for flight.

DROGUE RECOVERY SYSTEM

The main purpose of the drogue recovery system is to have a controlled and stable high-speed descent. The higher the descent rate, the less drift there will be. However, the descent rate must be low enough for a safe deployment of the main recovery system without any damage to the rocket. A good rule-of-thumb for a good drogue descent rate is 50 feet per second.

The drogue recovery system is normally located in the aft section of the rocket and is deployed when the drogue (apogee) ejection charge separates the motor/fin section from the center section. Since the nose cone is generally lighter than the motor/fin section, the nose cone (and thus the main recovery system) is less likely to separate prematurely due to the drogue opening loads. In the event there is still significant velocity at apogee (arcing flight), having the motor/fin section separate first limits the possibility of rocket damage (assuming a long enough harness to dissipate the energy). Having the drogue recovery system aft and the main recovery system forward also moves the CG forward, increasing stability.

Rocketry Online's INFOcentral Recovery section has an excellent discussion on drogue sizing.

MAIN RECOVERY SYSTEM

The purpose and design of the main recovery system does not change for dual-event recovery. The only additional concern may be the limited room available in smaller dual-event recovery rockets since some of the airframe is now being used for the drogue and electronic systems. The main recovery system is normally installed in the forward section of the rocket and is deployed when the main ejection charge separates the nose cone from the center section. If the airframe space allows, the main and drogue locations can be switched, as long as the section security and rocket CG are considered as discussed above. Using the same arrangement on all of your dual-event recovery rockets will keep rocket (and electronics) preparation consistent. This will help prevent switching the drogue and main connections on the altimeter and inadvertently deploying the main recovery system at apogee.

Rocketry Online's INFOcentral Recovery section has an excellent discussion on main parachute sizing.

ELECTRONICS

Electronics that can detect apogee for drogue deployment and a predetermined altitude for main recovery system deployment, such as an altimeter or accelerometer, are required.

A basic altimeter should have an apogee (drogue) and a main deployment circuit. The main circuit may have two or more selectable deployment altitudes. The lower main deployment altitudes reduce the recovery time and drift, but higher altitudes are required for rockets where the main recovery system takes longer to fully deploy. Some altimeters will have a supersonic inhibit delay. Air pressure rapidly changes as a rocket passes in and out of the transonic and supersonic regimes. An altimeter will improperly sense these pressure changes as apogee and deploy the drogue recovery system at very high speed. The supersonic inhibit delay prevents the altimeter from using pressure readings for a user selectable time after liftoff. A good rule-of-thumb is to use this delay anytime the rocket will exceed 750 feet per second. If the rocket reaches apogee before the delay time expires, the altimeter will not deploy the drogue recovery system so this inhibit should be used cautiously and only when required. Altimeters will vary on what information they record and how it is displayed. A few altimeters do not have any altitude reporting capability while most will report maximum altitude through light or audio signals. Some "logging" altimeters record altitude at set time intervals that can be downloaded to your computer. These options do not affect the operation of the altimeter and which one you choose will be based on personal preference and how much you are willing to spend.

An accelerometer does just as its name implies and senses rocket acceleration. The accelerometer will differentiate the acceleration to derive velocity and altitude and it deploys the drogue recovery system at apogee when it "senses" zero velocity. For accuracy an accelerometer should have a pressure sensor for main recovery system deployment, and like the altimeter, there may be two or more selectable deployment altitudes. This sensor is usually not activated until after apogee so a supersonic inhibit delay is not required on an accelerometer. The information available from an accelerometer allows extensive evaluation of the rocket's flight from liftoff through touchdown, which can be an excellent learning tool for the young, and old alike.

Electronics will be designed with varying power supplies. Some will have built-in, rechargeable batteries while others will have a battery holder, or power connection, for a battery. Electronics designed for 9-volt batteries are very convenient. The 9-volt batteries are readily available and a fresh battery can easily be used on each flight (highly recommended!). A fresh 9-volt battery will have ample power to run the electronics, even with a reasonable delay at the pad, and still fire two or more electric matches.

There are many commercial systems available for mounting electronics. Some are as simple as an electronics bay that slides into a coupler connecting the two center sections. Others, like PML's CPR system, are more complex but offer some advantages. Using the PML CPR system allows electronics to be easily and quickly moved from one rocket to another no matter what the diameter (2 inches and above). The CPR system also makes connecting the two center sections very easy. Whatever installation you choose, make sure that it will hold any electronics that you may obtain taking into consideration the length, width and depth of the electronics with batteries installed. The installation must also protect the electronics against the hot gases from the ejection charges. Even if the electronics are not directly damaged, these gases are corrosive and will damage the electronics over time.

EJECTION CHARGES

Unlike using engine ejection, dual-event recovery is going to require extra effort in preparing the ejection charges. The required ejection charge size will vary based on the airframe space available and they could be very different for the drogue and main sections. Using a "standard" amount of black powder could leave one section under-pressurized, which could fail to deploy the recovery system, while the other is over-pressurized damaging the rocket. Take time to determine the correct ejection charge size for each application.

Rocketry Online's INFOcentral Recovery section has an excellent discussion on, and a calculator for, ejection charge sizing.

The ignition source for the ejection charge needs to be compatible with the power source for the electronics. At best, this is typically a 9-volt battery and electric matches are an excellent choice. USE CAUTION - Because electric matches require so little current, they can be ignited by static electricity. Use caution when handling the ejection charges and electronics with ejection charges installed. Follow the manufacturer instructions for the electronics.

The acceleration of liftoff and drogue opening loads will try to force the black powder out of the ejection charge holders. It is a good habit to build all ejection charges so that the black powder remains securely in contact with the electric match (or whatever ignition source is used). To hold the black powder in contact with the electric match this ejection charge has; a self-adhesive disk over the black powder; fire-resistant wadding to fill in the remaining space; and masking tape to hold the entire assembly together.

BUILDING CONFIDENCE IN THE ELECTRONICS

So what’s holding you back? For most people, unfamiliarity with the electronics and the added complexity breeds mistrust. Using a slight modification in the rocket design provides a way for you to get to know your electronics without having to depend on them. The drawback, you will need to modify an existing kit or design with a payload section and electronics bay (but read on because it’s not difficult).

ALTERNATE ROCKET DESIGN

This design is a simple addition of an electronics bay to an existing motor ejection based kit or design with a payload section. The payload section needs to be long enough for the electronics and a main parachute. My experience has been that this section needs to be at least 18 inches long, but that will vary based on the type electronics used and main parachute size.

The two required modifications are: 1) a shock cord between the nose cone and upper section, and 2) adding an electronics bay to the payload section coupler. Two possible electronics bay modifications are shown below.

This design can be used for motor ejection only, combined motor and electronic ejection or electronic ejection only. The motor ejection only option is nice when recovery time (drift) is not a consideration and you are not in the mood for a lot of preparation. When you’re comfortable with the electronic ejection only option, then you can comfortably move to rocket designs discussed above. The combined motor and electronic ejection option is where you can gain that confidence.

To gain confidence in your electronics, prepare the alternate design rocket with motor ejection, the main parachute loaded in the motor section and the electronics set up with either simulated or actual ejection charges. The simulated charges would be the resistors (or whatever the manufacturer supplies) installed in the deployment circuits. Do not, ever, directly short a deployment circuit by using a wire. When the electronics activate, the direct connection will damage the electronics. You will know if the electronics worked in-flight if the electronics are providing the proper signals per the manufacturer instructions. By using actual deployment charges, you can add visual feedback in-flight if the electronics are performing as expected. In this case, since the main parachute is loaded in the motor section, the electronic apogee function will act as a backup to the motor ejection. The electronic main circuit will simply deploy the nose cone (and anything you decide to pack in the upper section). When you gain confidence and using actual deployment charges, you can move the main parachute to the upper section and load a drogue recovery system in the motor section. The motor ejection will provide redundancy for the drogue (apogee) deployment. If for some reason the electronics do not work, and the main parachute does not deploy, chances are the rocket will not be a total loss because the motor ejection has deployed the drogue. You can even use a larger drogue initially and decrease its size as you gain confidence.