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 Many rocketeers are using altimeters for recovery system control. Some are also watching their rocket whistle down from apogee to go splat. What went wrong? The answer could be one or more of a number of possibilities. This article lists ways to prevent problems, describes possible failures and troubleshooting steps for when trouble pays a visit. Many of the recommendations also apply to accelerometer and timer recovery control systems.
 | | Don't let this happen to you! |
Use a Fresh Battery
Most altimeters can make several flights on the same battery. But batteries are cheap and your rocket is expensive. Check the expiration date on the battery. Just because you purchased a new battery recently doesn't mean it's fresh.
Bench Test Test your device before you fly it. The simplest method is to attach igniters to the output terminals, seal it in the electronics bay, cover all but one hole, suck out the air and wait for the igniters to fire in the right sequence, with the igniters positioned so that nobody will be injured and nothing will ignite. If you want to save your igniters use flashbulbs for the tests. Bench testing is a good use for flashbulbs. The best test requires a vacuum chamber with a pressure/altitude gauge so that the altitude at which the altimeter fires the charges can be compared to the altimeter's configuration. In other words, if the altimeter is set to fire the low altitude igniter at 500' above ground level did it really fire at about 500'? Usually only the manufacturer has equipment to perform that test.
Ejection Charge Igniters
Some devices used to ignite ejection charges were not designed to operate under the conditions of a rocket flight. Use a low current igniter like a Daveyfire, Oxral or similar igniter. Although photography flashbulbs have been used in rocketry for several years as ejection charge igniters they were not designed for rocket flight. The reason that Daveyfire and Oxral igniters are better than flashbulbs is that their internals are soldered. The internals of a flashbulb are not soldered and continuity can be lost during flight.
Daveyfire is a company that makes igniters or the blasting industry, automotive industry (air bag inflator igniters), military and others. The recommended Daveyfire igniters are models N28B, N28BR and N28F.
Oxral igniters were developed for the fireworks display industry. There is only one model.
If you use flashbulbs try to use two per ejection charge to lessen the risk of a failed bulb and no deployment.
Test the Igniters
 Use an Ohm meter to test igniters before use. A digital Ohm meter should be able to test a low current igniter without firing the igniter. If you use two igniters per charge use an Ohm meter to ensure that they have not shorted. Shorted igniters will report good continuity to the altimeter, but won't fire.
Daveyfire's specifications for the N28B igniter are under an Ohm meter and igniter in this photo.
Powerup on the Pad
An altimeter should only be turned on when the rocket is on the pad and otherwise ready to fly. An altimeter should never be turned on before rocket assembly is finished. A switch, preferably a key switch, installed in the rocket and accessible through the airframe can be used to complete the circuit from battery to altimeter.
Some rocket designs require the altimeter to be powered up and then the rocket assembled. During assembly the air pressure in the electronics bay can increase and then decrease, and the altimeter may fire the ejection charges thinking that the rocket has launched and reached apogee if the altimeter uses air pressure to determine if the rocket has launched. A G-switch activated altimeter will help prevent this problem. But a better solution is to not use that type of design and to only send power to the altimeter when the rocket is on the pad.
 A potential problem shared by some electronic rocketry devices is a battery holder that is attached to the circuit board, and power is in traces that cross the board. A misplaced sweaty finger or a drop of sweat could accidently cause the on/off switch to be bypassed and unexpectedly cause an igniter to fire. A partial solution is to carefully cover the circuit board with RTV silicone that is safe for electronics. Check the packaging for a statement that the silicone is safe for electroncis. Some silicones will corrode electronics. Do not cover the piezo buzzer or air pressure sensor, of course. If you buy an altimeter directly from the manufacturer ask if the manufacturer will seal it in silicone. The worst that can happen is for the manufacturer to refuse to do so. Another option is to use an external battery holder and wire the on/off switch inbetween the battery and device.
Use the Right Power Switch
 Many switches will fail during flight. They may work several times and then fail making the failure seem mysterious. A switch may make and break contact during flight, causing an altimeter to reset. If an altimeter resets during flight the probability of the ejection charges firing is very low. Sometimes a switch will cycle when the apogee charge fires, resulting in the low altitude charge will never firing. A good key switch, such as those used on ATVs, is designed to operate under hostile conditions. Phono jacks can cycle during ascent or descent. Slide switches can slide unless a method is used to lock them in place during flight.
Shown in this photo are two key switches mounted in an altimeter bay. (One switch per altimeter.) The keys can be removed in the on and off positions. A 1/2" hole through the airframe provides access to the switch.
Secure the Battery
Rocket flight can cause a battery to fall out of its holder, or to break and make contact. Either situation spells doom for your rocket. A typical rocket flight may experience 5 to 15 Gs or more. A battery that normally weighs 2 ounces will weigh 20 ounces (1.25 pounds) at 10 Gs.
If your altimeter does not provide an absolutely secure battery holder, like a metal retainer that is held in place by a bolt, then use something like a nylon tie or several wraps of electrical tape so the battery cannot disconnect during flight. A $0.10 part might save a $300 or $2,000 rocket.
Not only do you need a good battery holder, but whatever is holding the altimeter must be sturdy, too. If the combined altimeter and battery weigh 4 ounces at rest, they'll weigh 2.5 pounds at 10 Gs.
If the battery connector or a wire disconnects during flight and the rocket you'll probably assume the broken connector or wire occurred upon impact and the real cause will not be discovered.
Redundancy Dual controllers and/or dual igniters per ejection charge improve the probability of a successful recovery. Two low current igniters can be used per ejection charge with a single controller. Or two controllers can be used with one igniter connected to each. Heavy and costly rockets should use dual controllers to not only improve the odds of a successful recovery, but also to decrease the odds of property damage or personal injury.
If you use dual igniters per charge ensure that the heads don't touch and short. No altimeter presently on the market will report a short during the continuity test, but it will report continuity to the igniter. Shorted igniters won't fire.
Most electric matches are shipped with a plastic hood over the head. Leave the hood in place if your ejection charge design will work with the hood in place. If two igniters are used in a charge the hood will prevent them from touching and possibly shorting. If you cannot leave the hoods in place then put some type of insulator over the solder area such as hotglue, silicone, etc.
Redundant controllers won't help, however, if other factors are ignored, such as soldered connections and batteries attached so that they cannot disconnect during ascent or descent.
Electrical Connections
 Don't twist wires together or tape them together. Solder them or use a terminal block. Cover connections with heat shrink tubing or RTV silicone that is safe for electronics to prevent shorts during flight.
An Adept ALTS25 has been mounted on perfboard and connected to a terminal block in this photo. The igniters and on/off switch will be attached to the terminal block. The perfboard and terminal block are available from Radio Shack.
 Use screw-down terminal blocks for connecting igniter wires for a connection that won't separate during flight. This photo shows a terminal block for ejection charge igniters attached to an electronic bay's bulkhead. A wire through the bulkhead runs to the altimeter. The hole is sealed with epoxy to prevent gases from the ejection charge from leaking into the bay.
Use Black Powder
Black powder burns fast with and without confinement. As long as it gets a spark it'll burn. Do not use a similar sounding but ineffective product like Pyrodex unless the rocketry community has determined that the substance can replace black powder.
Some people do successfully use Pyrodex, and it can work under certain conditions, like with a piston ejection system or some small diameter airframes. But most of the time Pyrodex will not burn fast enough to sufficiently pressurize the airframe and eject the parachute. Pyrodex will usually burn so slowly the gas will escape from the airframe without ejecting the parachute, or some will burn and blow the rest into the airframe unburned. If you have any thoughts of using Pyrodex or other black powder substitute perform a ground test to determine if the charge will eject the recovery system before trying a flight test.
Ground Test the Ejection Charge
Equations are available to determine the amount of black powder to use based on the airframe's volume. The equations should be considered a starting point, not the absolute truth. Ground test the ejection charge before a rocket's first flight to ensure that is sufficient to eject the parachute. The larger and more costly the rocket the greater the need for a ground test before the rocket's first flight. A large charge may ensure deployment, but it also might be energetic enough to cause the recovery system to rip apart and fail.
Once you've determine the right amount of powder use a permanent ink marker to write the information on the rocket or in a notebook. You won't remember it two months from now.
Use a Big Ribbon or Checklist
 The rocket is on the pad. The LCO is counting down. Did you remember to turn the altimeter on? Attach a large ribbon to the rocket that you only remove after the electronics have been activated. This ribbon shown here is attached to a key. Or use a paper checklist, not a checklist in your memory, and check off each step as it is completed. For a large, costly system have another person double check the rocket.
The author doesn't want to reveal the number of times that he started to walk away from the pad and then realized that the altimeter had not been activated. Each error was caught before the rocket was launched. He now attaches a large ribbon to the rocket that slaps him in the face before he can get away from the pad. Should that fail as a reminder, the ribbon is large enough to be seen by the LCO, who will hopefully not launch the rocket.
Avoid Black
Black may not be the best color for a rocket with an altimeter. Electronics fail at high temperatures. The temperature inside a rocket painted black that's sitting in the sun on a hot day may get hot enough to cause a failure, such as causing the altimeter to think that the rocket has launched when it's really sitting on the pad.
Ambient Temperature
Let your rocket reach the ambient temperature before flying it. If the temperature is 40 degrees and your car is 70 degrees let the rocket cool before flying, and vice versa. But remember that cold is bad for batteries. Be sure to use a fresh battery.
Venting
An altimeter bay must be vented so that the air pressure in the bay equalizes with the air pressure outside of the rocket. If venting is not provided the altimeter will not function correctly. Following the altimeter manufacturer's recomendations for venting.
The altimeter's bay must be sealed from the apogee ejection charge. If gases from the apogee charge leak into the bay the altimeter will sense the pressure rise in the bay and fire the low altitude ejection charge.
Oh, no! Your rocket crashed.
Diagnostic tools that you'll need are listed below. - Shovel
- Digital Ohm meter
- Altimeter manual
- Igniter specifications
- Notebook and pen to record information
Diagnostic steps are listed below.
Carefully examine the rocket pieces. Don't stomp on them in anger. Don't whack them with the shovel. Don't move them without carefully examining them or evidence may be lost.
If you were lucky the failure was that the low altitude charge fired early at a high altitude and the rocket landed safely, though you may have a long walk to recovery it. This problem can be caused by gases from the apogee charge leaking into and pressuring the altimeter bay, causing the altimeter to think that the rocket has reached a low altitude.
Did you activate the altimeter? It cannot work if it wasn't on.
Did an ejection charge fire during ascent? If so then the altimeter probably lost and regained power due to poor wiring, a poor battery connection or a battery with internal problems. Nine volt batteries contain multiple smaller cells. Some brands solder internal connections, and some don't. Duracell does. A cheap battery can sometimes ruin your day rather than make you happy that you saved some money.
Is the altimeter smashed to bits? If so then much of the evidence has been lost and you may never discover what failed. However, extract the ejection charges and determine if they fired. If the charges fired but deployment did not occur then the charges may have been too small, the airframe may have been too leaky, or the airframe parts may have been secured too well preventing separation. A situation has been observed where the airframe material shrank significantly when cold. The airframe-coupler fit was perfect at room temperature, but at 40 degrees F the airframe shrank and grasped the coupler with a death grip. The rocket sections did not separate even though the ejection charge fired. The ejection charge size was the same as had been used for other rockets made with a different airframe material.
Is the altimeter beeping the maximum altitude (if the device beeps)? If so this indicates that the altimeter probably did not lose power. The ejection charge igniters may have malfunctioned, shorted or the ejection charge may have come apart during flight separating power and igniter. Determine if the igniters fired. If not use an Ohm meter to test the igniters. (You did this while assembling the rocket, right?) Do they have the right amount of resistance? Did they short? If an igniter was shorted the crash may have undone the short. If an igniter is shorted the altimeter will report good continuity but the igniter will not fire. At this time no recovery system controller on the market will report a shorted igniter along with a continuity test.
Is the altimeter beeping the igniter continuity test rather than maxium altitude? If so then it probably lost and regained power during flight. If the apogee charge fired but the main charge failed then it probably lost power when the apogee charge fired or during descent as the rocket flailed about.
If the igniters fired do you see evidence that the black powder burned? If not was it possible for the igniter and powder to separate? Did the ejection charge container break and allow the power to escape?
Bench test the altimeter if it survived . Bench testing does not always reveal problems. A possibility is that whatever method has been used to mount the board in a rocket is stressing the board, opening a circuit, and this flaw will not appear during bench testing.
If the apogee charge fired but the main failed to fire there should be intact pieces to examine to determine if the wiring remained intact during flight. Do you see any broken or loose wires?
Was the electronics bay correctly vented? An altimeter needs to measure the outside air pressure. To do that air has to exit the electronics bay during ascent and enter during descent. If air cannot exit and enter fast enough the altimeter won't be able to accurately detect apogee and the low altitude at which you desire the main parachute to be deployed.
 About the Author
Dean Roth has used altimeter controlled recovery system deployment with every high power flight that he has launched, and several non-high power flights. He has never used motor based recovery system deployment with any high power motor. Dean has used six altimeter models from four manufacturers. |
