Altitude to deploy main parachute?

[BsSmith]

I'm setting up the electronics for my Talon 3. What is a good altitude for deploying the main parachute? I was thinking of deploying the main at 500ft and setting a backup charge for 450ft. Is 450ft (2nd to worse case scenario) enough altitude to safely deploy a main parachute?

 

[cjl]

I usually go with 800-1000 feet for my larger rockets (>25lbs), and 500 feet or so for smaller ones. 450 should work fine for a Talon 3.

 

[brianc]

What is a good altitude for deploying the main parachute?

What kind of main? How long does it take to deploy and inflate? How fast
will your T3 be falling under drogue?

 

[MarkM]

I'd go with 600 ft with backup at 500. A 50ft difference between main and backup is too close, IMO. There is slight variance in altimeter sensing of altitude (even with the same model/brand altimeter). Only a 50ft difference could actually translate to deployment of both main and backup at or very close to the same altitude. You do not want your main & backup charge to fire that close together. A "double charge" going off could result in some damage to the rocket because of the overexuberant charge. I've seen it happen (personal experience).

That said, 600 ft should be fine. I've deployed 52" SkyAngles (10-12lb rockets) at that altitude routinely and it gives plenty of time to for the chute to inflate. The Talon 3 and equivalent size chute should inflate. I fly my Thor (6lbs) with a 44" SkyAngle and deploy at 600 ft....no problem...easily inflates in time.

 

[BsSmith]

What kind of main? How long does it take to deploy and inflate? How fast
will your T3 be falling under drogue?

The main is a 48" spherachute and the drogue is a 24" spherachute. The rocket weighs somewhere around 6 pounds at liftoff. I'm not sure how long the parachute takes to inflate, I have never used it for a flight before. I don't know how fast it falls under drogue either, I lost my rocksim file for it (with accurate weights), but I'm guessing that it will fall somewhere around 50fps.

I'd go with 600 ft with backup at 500. A 50ft difference between main and backup is too close, IMO. There is slight variance in altimeter sensing of altitude (even with the same model/brand altimeter). Only a 50ft difference could actually translate to deployment of both main and backup at or very close to the same altitude. You do not want your main & backup charge to fire that close together. A "double charge" going off could result in some damage to the rocket because of the overexuberant charge. I've seen it happen (personal experience).

I'm not sure if this will help or not, but both charges will be fired by the same altimeter (a Parrot). I've never used DD before, so I don't know if there would still be a varience, but I don't think there should.

But, it's always better to be safe.

 

[MarkM]

I'm not sure if this will help or not, but both charges will be fired by the same altimeter (a Parrot). I've never used DD before, so I don't know if there would still be a varience, but I don't think there should.

Not sure what you mean. Are both the main and backup fired by the SAME Parrot altimeter or by two different Parrots? Can a single Parrot fire a drogue and two different altitude main charges because that's what you are stating you're doing.

 

[BsSmith]

Can a single Parrot fire a drogue and two different altitude main charges because that's what you are stating you're doing.

I am only using one.

Yes, the Parrot has three outputs, which is part of the reason why I chose it.

 

[blackjack2564]

I'm setting up the electronics for my Talon 3. What is a good altitude for deploying the main parachute? I was thinking of deploying the main at 500ft and setting a backup charge for 450ft. Is 450ft (2nd to worse case scenario) enough altitude to safely deploy a main parachute?

You will be fine. I would suggest the back up be moved to 400 though just for a little more wiggle room.

All my rockets weighing less than 20 lbs are set for that.

All over 20 are set for 800 and 500. Heavier rockets = larger chutes which take longer to inflate.

 

[MarkM]

I am only using one.

Yes, the Parrot has three outputs, which is part of the reason why I chose it.

That's what I thought...just checking. I'd still go with about a 100ft separation between the main and the backup. There isn't much difference in air pressure between 500 & 450 so both charges could still fire nearly simultaneously. You don't want that.

400ft is the absolute lowest I'd set an altimeter to fire. I've used 300ft before for a 6lb rocket and, while I used a smaller-than-normal chute and it inflated in time (very windy day), the rocket still hit a bit hard (on soft grass)....no damage, but I won't set a deployment that low again; it didn't have enough time under chute to really slow it down enough.

 

[Adrian A]

I am only using one.

Yes, the Parrot has three outputs, which is part of the reason why I chose it.

Although the Parrot has 3 outputs, there is just one user-selectable trigger altitude for firing an output when the altitude goes below the trigger value. (there is a separate user-selectable altitude that can trigger a deployment when the altitude goes above above the trigger value, as would be used to ignite a second stage). You could use your 3rd output to back up your apogee charge, by setting one to use accel-based apogee detection and another to use baro-based apogee detection. Generally the accel-based trigger happens right at apogee and the baro-based version happens about a second later.

I'm updating the firmware now for the deployment logic for the Raven altimeter, and this post is more motivation to add a feature I've been thinking about, which is to have a selectable option to each output to add a 1 second delay from when it would normally fire. This would allow an easy way to use one of the outputs to provide a backup for the same basic event, but with 1 second spacing guaranteed.

 

[powderburner]

400ft is the absolute lowest I'd set an altimeter to fire.

Everyone has been talking about high-power (relatively heavy) rockets, right?

If I asked the same question (when to trigger the main deployment on a dual-deploy setup) on a LOW-power rocket, maybe around eight ounces of weight, assuming an 18 or 24 inch diam main chute/sheet, and a one or two second opening, would it be safe to trigger at a significantly lower altitude (say, 200 ft) or would the 400 ft recommendation still apply?

I really really really hate to watch my rockets drift away in the wind around here.

 

[Pantherjon]

Everyone has been talking about high-power (relatively heavy) rockets, right?

If I asked the same question (when to trigger the main deployment on a dual-deploy setup) on a LOW-power rocket, maybe around eight ounces of weight, assuming an 18 or 24 inch diam main chute/sheet, and a one or two second opening, would it be safe to trigger at a significantly lower altitude (say, 200 ft) or would the 400 ft recommendation still apply?

I really really really hate to watch my rockets drift away in the wind around here.

I think that with a LOW power dual deploy that 200 feet would be fine..

 

[Adrian A]

Everyone has been talking about high-power (relatively heavy) rockets, right?

If I asked the same question (when to trigger the main deployment on a dual-deploy setup) on a LOW-power rocket, maybe around eight ounces of weight, assuming an 18 or 24 inch diam main chute/sheet, and a one or two second opening, would it be safe to trigger at a significantly lower altitude (say, 200 ft) or would the 400 ft recommendation still apply?

I really really really hate to watch my rockets drift away in the wind around here.

I set the main deployment for my dual-deploy Blue Ninja (200 grams, 24" chute) to 300 feet, and although it deployed with sufficient time, it got a little nerve-wracking at the end. 200 feet would probably work, but I would personally prefer something a bit higher unless I'm flying from a really small field.

 

[Zeus-cat]

I just launched a rocket I designed myself and used a 300 foot main deployment. The particulars were:

Weight: 24 ounces
Motor: Aerotech F39
Altimeter: Perfectflite MAWD
Both shock cords/tethers were about 5 feet long

Apogee: 425 feet
Drogue parachute: 12 inch rip stop nylon (deployed at apogee)
Descent rate under drogue: 30 feet per second

Main deployment setting: 300 feet
Main parachute: 24 inch rip stop nylon
Descent rate under main: 18 feet per second
Effective altitude of main chute: approximately 250 feet

So what I took away from this was that the rocket descended at the drogue descent rate of 30 fps for nearly two seconds after the main chute was set to deploy. It takes some time for the altimeter to detect the proper altitude, fire the deployment charge, get the chute out and inflate it.

I think you could get away with a 200 foot deployment, but the first time or two that you try it will almost certainly be a bit nerve racking.

 

[bobkrech]

The above post hits most of the critical parameters which I have expanded a bit below.

1.) The descent rate prior to main deployment is important. If your rocket is descending at 60 fps on a drogue, and it takes 2 seconds for your altimeter to realize it went through 300', so your actual deployment might start at 300'-120'=180'. This is strictly altimeter dependent.

2.) The length of the shock chord. Your parachute will not begin to open until the shock chord and parachute lines are fully extended. For the case of a high power rocket this might be 60'. Your chute might begin to inflate as low as 180'-60'=120' feet. This is dependent on your shock chord and shroud line lengths.

3.) Time to open canopy fully and slow down. The bigger the rocket and chute the longer the time, but probably 0.25 to as long as 1 second if nothing hangs up. Your chute might not be fully inflated and functional until you are down to 120'-60'/2=90'. A lot depends on how you pack your chute and whether you employ a device to slow opening to reduce shock. Smaller chutes usually open quicker.

4.) The main chute should be sized so that the descent rate is 15 fps or less. From 90' you are on the ground in 6 seconds. The Descent rate depends on parachute size, but if your doing dual deployment, you want a big main chute to eliminate damage to your rocket. In WW2 military static line jumps were conducted from as low as 250' to minimize the time troops were esposed to small arms fire. No reserve chutes were use because if your chute didn't deploy, you hit the ground in a little over 4 seconds and amazingly had a reasonably good chance of surviving with some broken bones.

For a big rocket this would minimize wind drift, but you would need nerves of steel and faith in your hardware. Smaller rockets would probably be descending slower under a drogue so a 200' main altitude deployment is certainly possible if you can pack your chute to insure that it will deploy promptly and not hang up.

If you are launching significantly above sea level, you want to increase you deployment altitudes by the inverse density ratio so account for lower drag at higher altitudes.

Bob