Adept Rocketry - Common Questions with Answers
Copyright © 1999-2005 All Rights Reserved
Q: I hear that there is a problem when using two independent altimeters or timers in a dual redundancy configuration. The problem is when both devices fire a deployment charge at exactly the same time; this may cause an overpressure in the airframe. A: There is no problem here. Firing at exactly the same time is not possible. Explosions are very fast, and they have already done their thing in no more than a few milliseconds. Two deployment charges firing within the same few milliseconds is extremely remote.
Q: What does "CPR" mean? A: "Close Proximity Recovery" or another way of saying "Dual Deployment™." The technique of CPR was invented by Adept in 1990, and the term Dual Deployment™was copyrighted and trade marked by Adept. Then several years later, the folks at Public Missiles, Ltd. started using our Dual Deployment™ altimeters in some of their rockets. They coined the term "CPR." The technique allows a rocket to eject a small drogue chute or streamer at or near apogee (maximum altitude) for a quick controlled descent. Then a large main parachute is ejected nearer the ground for a soft landing, and the rocketeer doesn't have to walk so far to retrieve his/her rocket on windy days. "CPR" is Public Missile's substitute term for Adept's trademarked term Dual Deployment™. The technique also can be implemented in other ways. For instance, with a dual timer, but with less precision. Or an Adept APD1 or ALTS1 or DCS1 could blow a main chute at 500 feet above the ground, after the rocket motor's regular ejection charge deploys a drogue chute near apogee. Additional substitute terms that have been coined lately to avoid using Adept's trade mark "Dual Deployment™" are "Minimal Drift Recovery" and "Smart Recovery."
Q: When is the best time to ignite the second stage of a two stage rocket? I read in the Rocketry Handbook or somewhere that the second stage of a rocket will climb higher if it is ignited when the first stage is at maximum speed (at the instant of motor burnout). A: What you read assumes that the rocket is launched where there is no air such as on the moon. But most of us don't get the opportunity to launch rockets on the moon - we have to launch through air. In air the rule is different. At maximum speed there is maximum air drag that will limit the maximum altitude. It turns out that allowing the second stage to coast for a short while before ignition will allow it to reach a higher altitude overall. The coast time is usually just a few seconds, and calculating the correct amount is complicated. A very technical article on the subject, by Tom Kuechler, was published in the January 1973 "Model Rocketeer" magazine. And besides...the coast time is really cool to watch.
Q: Do your devices require wiring harnesses? A: No, but some of them plug into simple cables. The definition of a harness is a long, heavy, big, complex cluster of various size wires that are laced together with string or tape, and in which the various wires break out of the main cluster at various points along the length. Automobiles always have a harness or two under the hood, and you hope you never have to mess with one of them. A simple cable with a connector and five loose wires is not a harness.
Q: Can an accelerometer really detect a rocket's apogee? A: Nope.
Q: Why not? A: Although it is easy to double integrate (add up) all the readings from an accelerometer to determine velocity and altitude attained during part of a straight-up rocket flight, it is not possible to continue all the way to apogee. During a typical rocket flight, by the time the rocket reaches apogee it has already started nosing over and is already horizontal to the ground. After about two-thirds the way up (or maybe even as much as three-quarters the way up) all the remaining data from a single-axis accelerometer is garbage. It has absolutely no idea of what's going on after this point. If it tries to determine apogee, based on garbage data, it will be in error by a large amount. It is possible to measure for about two-thirds of a straight-up flight then estimate where apogee will occur based on what has happened up to that point, but the error would still be high. And the accelerometer would need to know things such as the weight of the rocket with and without fuel, air temperature, ground altitude, drag ratio, etc.
Q: Can apogee be measured with a three-axis accelerometer? A: Yep - if it is properly calibrated and has extremely intelligent on-board software.
Q: What else about accelerometers should I be aware of? A: If an accelerometer does not sample data at a rate of 100 samples per second or more, the data will be loaded with error. In order to catch the initial impulse at liftoff, the sample rate must be fast. 16 or 32 samples a second is nowhere near fast enough to capture the initial impulse information. With the initial information (the all important initial integration seed data) in error, all other data will be in error as the integration process continues. You may be able to plot the data, and it may even be very pretty and scientific looking. But your very pretty data will be wrong. There is no science here. The other problem with such devices on the market today is the simple fact that they sample date with poor resolution. Measuring data at one part in ten (or even worse) is very poor and nearly useless.
Q: What is the best way to store and protect my Altimeter or other electronic device. A: The precision amplifier circuitry and continuity sensing circuitry on an Altimeter may be sensitive to noise and static when being held. Always handle any electronic device by the edges when testing or installing to avoid touching any of the circuitry. Avoid carpeted floors and other sources of static electricity when handling and testing an electronic device. Never store in a clear plastic bag; however, pink-colored or smoke-colored antistatic bags are ideal. Storage in a small cardboard box, or wrapped in a paper towel inside a plastic bag is acceptable. Do not use Velcro to secure the device. Use care to keep the device clean and dry.
Q: What is an Electric Match? A: An Electric Match is a low-current ignition device, or igniter, used by many rocketeers to ignite the motors of single or multistage rockets, or to deploy a parachute. The industry standard is the Daveyfire Electric Match which literally looks like a red-tipped book match with wires added. It is a low current device. Other manufacturers have started to appear who call their medium to high current igniters by the name "Electric Match" in order to avoid shipping problems, or for some other reason. Just because an igniter is called an Electric Match does not make it so. Be careful. The long time accepted industry standard definition of an Electric Match is a low-current device which is equivalent to the low-current Daveyfire brand in performance.
Q: What is the meaning of "Continuity Safe?" A: This term is usually in reference to the standard hobby version Estes launch controller. This small low-cost controller uses four AA alkaline batteries to ignite standard Estes igniters, which require an electric current of about 2 amps. A small 6-volt lightbulb is used to test the igniters for continuity. The current that lights the bulb also passes through the igniter, and the amount of current is much lower than is necessary to fire an Estes igniter. However, it is plenty enough to fire a low-current electric match or a flashbulb. The Estes launch controller is not "continuity safe" for low-cost electric matches or flashbulb based igniters. Many other launch controllers and other electronic devices use very small currents to sense igniter continuity and are continuity safe.
Q: How do you spell igniter? Or is it ignitor? A: Right the first time - igniter is preferred.
Q: Is it rocket motor, or is it rocket engine? A: Motor is correct. It is interesting that in the hobby of small model rockets they know how to correctly spell igniter, but they say engine instead of motor. In the hobby of high power rocketry they get the motor thing correct, but they can't spell igniter.
Q: Why should I "vent" my parachute deployment charges: A: Most deployment charges are sealed tubes or other containers of explosive materials such as black powder. When a rocket is launched to a high altitude (where air pressure is much lower) the air trapped inside a nonexpandable deployment charge can cause it to split open or pop open causing the contents to spill out where it can no longer be ignited. Many rocketeers have learned this the hard way. For high altitude flights, most rocketeers vent deployment charges slightly with the tip of a knife or other pointed object.
Q: I hear that a handy way to mount an electronic device in a rocket is with Velcro. Is this a good idea? A: Nope! Velcro is a very good insulator, and tremendous static voltages can build up on the device, especially where humidity is low. Also, when Velcro is pulled apart, arcing from tremendously high voltage static electricity occurs (try this in a dark room).
Q: How about mounting the device on insulated mounts. A: Still not a good idea. When a rocket moves through the air, especially at high speed, the rocket body can build up a significant static charge (this happens with clouds before they generate lightning). The insulated device still has the original charge it had on the ground. A static discharge arc from the inside of the rocket body to the electronic device can damage the device. Note: mounting the device with metal screws to wood or cardboard or metal will limit the buildup of static electricity.
Q: Are Adept altimeters sound barrier compatible? A: Absolutely! Rocketeers push these things through Mach all the time, some as fast as Mach 3. When a rocket penetrates the transonic region and into the supersonic region, a significant air pressure pulse usually occurs. A recording altimeter will capture the glitch, and it is quite interesting to see the phenomenon in the data. Adept altimeters with deployment switches ignore the glitches.
Q: How do Adept altimeters avoid the sound barrier problem? Do you use a "lockout timer?" A: Absolutely not! A "lockout timer" as it is called is a very poor way to solve the problem. A band-aid fix on a serious vulnerability problem is completely foolish. This method has many problems. For instance, if a "lockout" timer is set to ten seconds and the motor has a failure, the rocket may very well crash into the ground before the "lockout timer" allows the parachute to deploy and save everything. In Adept altimeters, altitude readings are constantly being differentiated in real time to determine the rate of ascent (velocity). The apogee charge is locked out as long as upward velocity is more than 500 feet per second. Problems with premature deployment due to sound barrier effects cannot occur in Adept altimeters as they can in others. And there is none of the problems associated with "lockout timers."
Q: How precise are the altitude readings from an Adept altimeter. A: Using a proprietary technique, Adept altimeters take readings in one-foot increments. This is one to two orders of magnitude more precise than anything else available. The Adept units use an absolute pressure device and a logarithmic amplifier to measure altitudes up to 60,000 feet above sea level. The 16-bit conversions are done in real time, and results are direct altitude readings in feet. Also, the reported readings are direct above-ground altitudes in feet. You do not need a silly look-up table or any other kind of conversion foolishness. The readings are final and direct in actual feet.