Airstart Timers

What are peoples opinions regarding brand of airstart timers? Perfectflite Timer4, Adept ST236 or Missle Works PET2+? Need something that will reliably give enough juice to airstart a three 54mm motor cluster. Most likely ematch ignitors with copper thermite or pryrodex pellets.

mdostby said:

What are peoples opinions regarding brand of airstart timers? Perfectflite Timer4, Adept ST236 or Missle Works PET2+? Need something that will reliably give enough juice to airstart a three 54mm motor cluster. Most likely ematch ignitors with copper thermite or pryrodex pellets.

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Similar to the discussions of two-stagers, if you are airstarting motors, you should use an altimeter that can provide an altitude check to inhibit ignition, and not just timers. There are a half dozen or so altimeters that can do this, and you'd have to figure out which of them could handle three ematches (I would guess most of them?). At the moment, this is just my advice, but it won't be long before it is the rule. It would be a shame for you to design around timers and then find you were prohibited from launching.

Jim

Thanks Jim. Good suggestion. I'm using dual Stratologgers for recovery. I'll look into a third altimeter rather than a stand alone timer.

Eggtimer https://eggtimerrocketry.com/page2.php

interesting. I've heard about the egg timer but never really looked into it. I like what I'm reading. thanks Connor!

mdostby said:

Thanks Jim. Good suggestion. I'm using dual Stratologgers for recovery. I'll look into a third altimeter rather than a stand alone timer.

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The altimeters that I'm aware of that can do an altitude check are the Raven 2 or Raven 3, the Eggtimer, the RRC3, the Telemetrum line, and the GWiz HCX.

Jim

JimJarvis50 said:

The altimeters that I'm aware of that can do an altitude check are the Raven 2 or Raven 3, the Eggtimer, the RRC3, the Telemetrum line, and the GWiz HCX.

Jim

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I think a notice went out on the TRA prefect mail as a notice was posted on our club site about using an altimeter with an inhibit function for I impulse or higher. A good idea if you ask me. Kurt

JimJarvis50 said:

The altimeters that I'm aware of that can do an altitude check are the Raven 2 or Raven 3, the Eggtimer, the RRC3, the Telemetrum line, and the GWiz HCX.

Jim

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and the Marsa54L

How does altimeter check improve safety, not sure how that works.
I have used Pefectflite and PML Accufire.
The Perfectflite sinks as many amps as you will ever need.
I make sure to factor in a delay from when you set the timer to fire until a particular motor takes to come up to pressure, so I like lighting airstiarts AND 2nd stages a little sooner.
You certaily do not want motors lighting when its 45 or 90 deg to the ground!

Dave A said:

How does altimeter check improve safety, not sure how that works.

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altitude vs time. If not Alt A at time X, then no fire.

To elaborate on what Dave said, if your altimeter can check the minimum altitude vs some pre-set time you can be reasonably sure that the rocket is rising near-vertical, which is the point of this rule. To set the time properly, you have to model your flight using Open Rocket or RocSim, which you should be doing anyway with a rocket this complex. You can also use a minimum-velocity rule to do essentially the same thing. A simple launch-initiated timer can't sense (directly or indirectly) that your rocket hasn't gone unstable, and could actually fire the sustainer straight downward, which would be bad.

It is altitude versus time. You need a good sim so you know that the rocket will have to have flown pretty much to plan.

I think this is a good idea having see a few interesting air starts and staging attempts.

I think what Jim Jarvis is working on might become a serious contender too.

I think timers places have become very very limited.

Altimeters have become the clear option for staging and airstarts.

They still have a place in certain applications as a single ejection device, however that still relies on optimal motor performance and a good sim. Also, with the cost and size of things like the RRC2+, the argument for using timers is really really small. The only time I'd use them is for an "on motion - fire" Cluster start to make sure the outboards only kick on if the central motor lights.

markkoelsch said:

It is altitude versus time. .

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True for basic altimeter, but the telemega as other tools

• 1-axis 105-g accelerometer for motor characterization
• 3-axis 16-g accelerometer for gyro calibration
• 3-axis 2000 deg/sec gyros
• 3-axis magnetic sensor

The altitude check can be a little more complicated than you might think. I use the Ravens and EasyMega's. For them, the easiest expression of the logic is "fire the igniter when the altitude exceeds X if the time is less than Y". A variation on the theme is "fire the igniter when the velocity drops below X if the time is less than Y". It would be easier if you could say fire at Time = X, but there is no Time = function on those altimeters - only T> and T<.

So, you do a simulation. At the point where you want the motor to light, you might be at 3000 feet and 7 seconds as an example. You allow a second for the motor, so you want the igniter to fire at 6 seconds, which might be 2500 feet. The logic statement, with one second of margin, is then fire at 2500 feet if time < 7 seconds. One trick is that you have to know when your altimeter knows it's at 2500 feet (i.e., how is it filtered). It pretty much takes a test flight to understand how a particular altimeter does the logic, so you just have to leave a little margin for those errors.

Jim