When searching for my next altimeter, I wanted an altimeter with a few specific functions. First and foremost, I wanted an altimeter that had accelerometer-based deployment for the first or drogue stage. The reason I wanted an accelerometer and not a barometric based unit was because barometric units suffer from atmospheric effects. These effects can cause a late ejection (after apogee) or early ejection due to Mach effects when a rocket reaches the transonic region. It is true that most barometric altimeters have a “Mach delay” where you can tell the chip to ignore all the funky readings it may receive from the sensor for a specific time, but that does nothing for apogee problems. Also, I would also like to do some staging/clustering and finding an altimeter that could do these things would be a plus.

Enter the Pratt G-Wiz LC Deluxe Flight Computer.

I chose the LC Deluxe because it can do staging/clustering, deploy a drogue or main chute at apogee (accelerometer based), deploy a main chute at 400ft, and record peak altitude. Pratt also has the G-Wiz LC that does not have the barometric sensor and does not provide apogee/main deployment or peak altitude readings.

Some specifics of the LC Deluxe are:

• Maximum acceleration: +/- 50g
• Maximum barometric altitude: 35k feet MSL
• Number of Pyro channels: 3
• Pyro channel functions: 1) Stage/Cluster 2) Apogee deployment 3) Main deployment
• Maximum current per Pyro channel: 8 Amps
• Type of battery recommended: 9 VDC transistor (However, 12 VDC “N” type can be used)
• Pyro test current: (Using 9VDC) 3.5 mA
• Altitude readout: Flashing LED
• Main battery life: 20 hours
• Dimensions: 3.9” long and .70” wide

When configured for clustering (via a jumper) the LC Deluxe detects and confirms launch. 0.5 seconds after launch detection and confirmation, Pyro channel 1 fires. When configured for staging (again via that jumper) the LC Deluxe fires the second stage motors (Pyro channel 1) after booster burnout. This is great because you don’t have to guess when booster burnout will be. After the staging/clustering event, the LC Deluxe then senses apogee and fires Pyro channel 2. If you are doing dual deployment, the LC Deluxe will fire Pyro channel 3 at 400 feet.

If you are not going to stage/cluster, you can use the LC Deluxe for dual deployment or simple main ‘chute at apogee. When using only Pyro channel 2 for main deployment, the LC Deluxe does not need to be vented in the airframe as the main (Pyro channel 2) charge depends upon the accelerometer. However, you won’t get peak altitude readout.

I recommend using the dual battery set up recommended by the manufacturer. This way, you have separate power for the electronics and the charges. There has been some concern of power glitches when using one battery. Seems that the Pyro channels can draw quite a bit of power form the unit. For example, a DaveyFire N28F can pull up to 14.1 amps! Such and event may cause a power drain to the electronics that could lead to the LC Deluxe resetting itself, not a good thing. You can also use two “N” type 12-volt batteries instead of the 9-volt transistors. This would save some space and some weight.

The manufacturer recommends not using any type of arming switch. Rather, they recommend installing the batteries to power up the unit. I do not like this at all! As an RSO, if you hand me a rocket with an armed altimeter, I will not be too happy. Also, what happens if you must pull the rocket off the pad? How will you disarm the altimeter? I recommend a slide type switch mounted vertically (with the “On” position being down) or horizontally inside the airframe. If mounted vertically, make sure to trim the tab of the switch flush with the airframe. When mounted inside the airframe, use a small rod or screwdriver to move the switch. Don’t use phono jacks! Phono jacks do not hold up to g forces at all. They will fail…

The main fault I have with the LC Deluxe is continuity. The unit only has LED’s for continuity. To check if you have continuity (and if the unit is even powered up) you must be able to see into the airframe and look at the LED’s. I don’t like this and wish that it beeped out armed state and continuity.

One other problem is that the main deployment is set to only 400 feet. This is too low for some flights and can’t be changed by the user. I’ve heard that Pratt is going to have a unit available with a setting of 800ft which is better. On my L3 project, I am using the LC Deluxe and an Olsen unit. I will depend upon the LC Deluxe for apogee deployment (accelerometer based) and the Olsen for main (which is barometric based and I can set it to whatever I want).

The cost of the LC Deluxe is $134.95 (the LC is only $89.95). See: http://www.pratt-hobbies.com/ for more information. The people at Pratt Hobbies and Robert Briody are just great to deal with. Oh, and there is a new mail list for G-Wiz users. Go to www.egroups.com and look for the group G-Wiz.