Contributed by Dean Roth
G-Wiz LC Product Review
G-Wiz Partners manufactures rocketry electronics that can be used to collect maximum altitude, control parachute deployment and ignite the motor of a two-stage rocket. With some creative thinking the devices can probably be put to other uses, too, like camera activation upon launch.
There actually are two partners that comprise G-Wiz Partners: Robert Briody and Larry Lynch-Freshner. Like most of the companies that make rocketry electronics, G-Wiz Partners is a part-time business. The partners decided that they could build a better device than what was already on the market.
One of G-Wiz Partners' doctrines is that an accelerometer is a better way to detect apogee than an air pressure sensor. Many rockets arc over apogee and fall, sometimes several hundred feet, before a barometric-only altimeter activates the apogee event. This can cause a rocket to gain significant speed, which can adversely affect recovery. A barometric sensor is used to measure maximum altitude and altitude during descent so that when used for two-stage recovery the main parachute can be deployed at a low altitude, such as 800' above the ground.
An accelerometer makes an altimeter immune to Mach transitions, which can fool barometric sensors into activating ejection charges at the wrong time. (Some barometric altimeters have a "Mach timeout" feature to avoid this problem.)
An accelerometer allows the device to be used for more than just determining apogee. It can be used to activate another device upon motor burnout, such as an igniter for staging.
G-Wiz Partners, at the time that this review was prepared, makes three products: G-Wiz LC, G-Wiz LC Deluxe 400 and the G-Wiz LC Deluxe 800. This review covers the G-Wiz LC Deluxe 800, which will hereafter be called the "800". The G-Wiz LC Deluxe 400 is exactly the same as the 800 except that it deploys the main parachute at 400' above the ground rather than at 800'. The suggested retail price is $134.95. (The "LC," by the way, means "Low Cost".)
The most common usage of an 800 may be as a recovery system controller for two-stage recovery. Two-stage recovery means making a rocket non-aerodynamic at apogee, usually by separating a rocket into two sections so that it falls rapidly but not as quickly as when in a ballistic nosedive, and then deploying the main parachute at a low altitude, such as 800' above the ground. Two-stage recovery can result in nearby recovery even when a rocket flies to a high altitude.
The 800 has three electrical outputs. When used for two-stage recovery two outputs are used. One is used to fire a separation/deployment charge at apogee. The other output fires an igniter to deploy the main parachute at a low altitude. The device is configured at the factory for a low altitude of 800' above the ground, which cannot be altered by the user.
The 800's third electrical output can be used to ignite additional motors after launch or to start a two-stage rocket's sustainer's motor upon booster motor burnout. When used for recovery system control this output is not used.
LED's (Light Emitting Diodes) are used to report device status. Each electrical output has an LED to indicate if there is continuity through an igniter. A fourth LED reports pre-launch status and post-flight maximum altitude.
The manufacturer recommends the use of two batteries when the device is used for anything other than collecting and reporting maximum altitude. One battery powers the computer. The other battery is to fire the igniters. When only one battery is used the potential exists for a power brownout when igniters are fired, which could cause the computer to reset and cause a recovery system failure.
The recommended power source is a standard 9 volt alkaline battery. These batteries weigh nearly 2 ounces each, and two can be a lot of additional weight for a small rocket. G-Wiz says that 12 volt A23 batteries can also be used, though they have a shorter life than 9 volt batteries.
The G-Wiz LC Deluxe 800 consists of a narrow circuit board. Like most rocketry electronics it does not have a cover. An accidental electrical short could cause ejection charges to fire. The board, with the exception of the air pressure sensor, can be covered with RTV silicone that is safe for electronics. Upon request, the manufacturer will encase the board in epoxy for an extra $15.00, which also adds strength.
The manual consists of four pages. A person already familiar with rocketry electronics should not have a problem using a G-Wiz product. A person new to rocketry electronics, however, may be left perplexed. Although it is not the responsibility of G-Wiz Parters to provide plans for adding an electronics bay to a rocket, or to identify where appropriate separation/ejection pyrotechnics igniters can be purchased, some pointers would be helpful. A beginner should seek advice from an experienced person. (Pratt Hobbies' web site contains a G-Wiz frequently asked questions (FAQ) document that can help. The document is not available from the G-Wiz Partners' web site. An email mailing list - G-Wiz@egroups.com - is also available.)
The manual warns about reversing the battery's polarity multiple times but doesn't always state why. The reason for the warning is that any attached igniters will immediately fire if polarity is reversed. Reversing polarity won't harm the computer.
Because G-Wiz Partners is a small, part-time company, the owners expect the seller to be the first line of support. However, support directly from the manufacturer is available.
Contacting the G-Wiz Partners turned out to be a more interesting journey than anticipated. The manual does not include a street address, telephone number, email address or web site address. An attempt to locate the company through the web sites of Rocketry Online, Rocketopia and Pratt Hobbies all failed. The G-Wiz Partners' web site was eventually located through the use of a search engine, and an email address was found, but the address didn't work. A call for help on the Internet Usenet group rec.models.rockets quickly resulted in another email address to contact one of the partners, and another attempt was made. A response was quickly received, and Robert Briody was peppered with questions over a few days, all of which were professionally answered within 48 hours.
The 800 was flown in a rocket that contained an existing electronics bay. The bay has a small hole for venting, which has worked with other altimeters. The status LED's could not be seen. Because there was no audio status reporting one could only assume and hope the G-Wiz was ready for flight. Additional holes could have been drilled into the bay so the LED's were viewable, but that would have affected the bay's use with other altimeters.
Daveyfire N28B igniters were used to ignite the ejection charges. The apogee and low altitude black powder charges each contained two igniters.
The rocket rose to apogee, arced over and continued a ballistic descent to the ground. Neither the apogee or low altitude charges fired. The airframe was totally destroyed by the fall from 1400'. The motor, parachute, electronics bay and the 800 survived. The 800 passed post-crash bench tests. The cause of the crash has not been determined. However, it is believed that the 800 either lost power on the ground or during flight, or it failed to detect launch. The custom battery holder might have been the cause. The 800 was installed in the correct orientation required by the accelerometer. All wires were still firmly attached after the crash. If the problem occurred during flight preparation and if the LED's had been viewable, or if audio status reporting was available, the crash might have been prevented. A different 800 has been observed to perform correctly when it was used for staging and booster recovery.
Combining an accelerometer and barometer into a single altimeter is a good idea. An accelerometer should do a better job of detecting apogee than a barometer. The G-Wiz LC Deluxe 800 altimeter is such a device. It is also compact, lightweight (without batteries) and can be used for more than just recovery system control. However, it could use some improvements, and there is a cautions about its use.
The G-Wiz altimeters need audio status reporting. Without audio status reporting use an electronics bay that allows the status LED's to be viewable when the rocket is on the launch pad.
A user selectable low altitude is desired. The G-Wiz LC Deluxe 800 has only one low altitude setting - 800' above the ground. The desired main parachute deployment altitude can vary from day-to-day and flight-to-flight depending upon wind speed and direction and rocket weight. A higher main parachute deployment altitude can be desired with a heavy rocket or a rocket that uses a complex and slower opening parachute system, like a parachute deployment bag, to ensure enough time for the recovery system to correctly function. Sometimes a few extra seconds can be needed to shake out a problem like a tangle. However, a higher altitude can also mean more drift if the parachute opens quickly. Therefore a lower altitude, like 400' above the ground, may be desired for rockets with simple and fast opening recovery system.
When one battery is used to power the computer and another battery is used to power the igniters, current flows through attached igniters as soon as the battery that powers the igniters is attached, even if the computer is turned off, even if the battery the powers the computer is not connected. Therefore a rocket either needs two switches, one for the computer battery and one for the igniters' battery, or the rocket cannot be prepared for flight until shortly before it flies or the battery for the igniters will slowly drain. Also, if a rocket isn't flown after being prepared it must be torn down to remove the igniter battery. This may be true of any altimeter that uses separate batteries for the computer and igniters.
A means to avoid needing two batteries should be considered. Other rocketry altimeters function fine with a single battery.
Pros: Combines an accelerometer and barometer in a single altimeter
Cons: Lack of audio status reporting
Written and submitted by Dean Roth for Rocketry Online -- Copyright 1996-2000
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