Ground Testing... How far?

[danielhv]

How far do we go? Keep reducing charges until it doesn't deploy enough? The article I read in Top of Flight basically said you want the smallest charge to do the job. So is that measured by the smallest amount that will shear the pins? The smallest amount that will move the cone far enough to get all the shock cord out?

I started at .95g for Drouge and .55 for the main. Been reducing and now I'm at .80 for the drouge and .40 for the main. Thoughts? Should I keep reducing? Guess I didn't expect the charge loads to get this low.

I'm running a single #2 shear pins in the body tube and 3x #2 shear pins for the nose cone.

 

[NAR29996]

What I try to do in my ground testing is to make sure that the nose cone and laundry come out, but the shock cord is not pulled taut. YMMV

 

[Titan II]

Should I keep reducing?

No

 

[Viper3]

The charges in the video look good. Do not reduce them any more.

 

[kjhambrick]

Were both videos for the main ( nose cone ) ?

Or did I miss something ?

They both looked fine to me ...

Did your dog retrieve your nose cone for another try ?

-- kjh

 

[Art Upton]

Also remember that BP at 2000+ feet does not have as much bang as it does on the ground.

In our local club's mind it is better to have a little too much then not enough. Post #2 sums that up.

 

[danielhv]

Were both videos for the main ( nose cone ) ?

Or did I miss something ?

They both looked fine to me ...

Did your dog retrieve your nose cone for another try ?

-- kjh

Nope, first video is the drogue. The av bay went with the nose cone. 2nd is the main, separating from the av bay. I probably need to test with the nose cone and av bay sitting on the ground since they will be separated in flight before the main.

 

[danielhv]

Well, I just ran the test again and .40 didn't pop the nose off. Retested at .50 and nada. Guess it's NOT best to use the smallest charge possible as that doesn't give you much wiggle room if it doesn't pop. I'll probably take the drogue charge up to .9 or 1.0 and the main charge up to .80 or .90 just to make sure it's enough each and every time.

 

[Art Upton]

I used 1.5g in 3"-4" rockets back in the day, or it was likely 1.4 from the AT measuring vails; that was for the apogee charge which is the most important to prevent a ballistic recovery.

Blow the chute out or blow it least apart on the Kevlar cord was the moto

 

[Art Upton]

If you want some other data, my Avatar 4" body 75mm M rocket was going to be fired from 4000 feet above sea level. It would go 17K above that. Reading from a bunch of Black Rock Folks that the charges had to be boosted.

So for Apogee one charge well had 5 grams of BP with tapes and sealed in that well. The Backup charge was in surgical tubing sealed. I had no way to determine which charge made the apogee event.

 

[MikeT]

Blow it out or blow it up.

 

[DAllen]

Blow it out or blow it up.

Been there done that. Not always great results.

 

[Donnager]

Blow it out or blow it up.

From a safety standpoint, this is not unreasonable (to a point). A separated damaged mass falling is safer than a whole rocket returning point first.

I see no reason to ever use less than what is supplied in a DMS motor. Work from there. Trying to tweak a black powder titration to the minimum at ground level is a waste, and could subject you to ballistic returns.....

Some numbers, depending on the rocket, that have worked for me:

3" stuff --usually 1.8-2.0g on the bottom, 1.2-1.5g on the top
4" stuff --usually 1.9-2.4g on the bottom, 1.8-2.2g on the top
5.5" stuff --usually 2.0-2.8g on the bottom, 2.0-2.4g on the top
6"stuff -- 3.0g on the bottom, 2.8 on the top, but it completely depends on the rocket (standard, HED, etc...), and I cringe even posting this.
Anything larger--consult a calculator and talk to someone with experience.

For stronger rockets, I invariably go to larger charges (for instance, I will use standard 3g charges on 4" fiberglass). Also, realize I usually use extremely heavy recovery, that is tough to drag out at separation.

You need to have your shear pin situation well understood, or you can have a complete ejection at apogee.

Feel free to criticize these numbers.

 

[Steve Shannon]

I’ve seen a lot of failures to deploy that worked fine on the ground. The shock cord should stretch all the way out so it snaps the parachute out. The chute cannot be counted on to fall out on it’s own.

 

[SDramstad]

Never ever ever ever use the minimum amount!!!!!!!! From what you are showing us I wouldnt go less then 1 gram for both charges.

 

[Handeman]

Just consider what the larger charges do. They put more stress on the recovery harness and attachment points. You may have to use larger, heavier, quick links, u-bolts, and bulkheads. Of course the heavier parts increase the momentum of the parts which again increases the stress on the parts. The whole rocket is a system and changes to one thing will affect everything else.

The trick is to find the right balance. My drogue charges are almost always about 1/2 size of the main charges. I use fewer if any shear pins on the drogue side and you only have to get the drogue chute into the air flow and it will take care of pulling all the recovery harnesses out. With the main, there are more shear pins, you need to get the chute out, which may have to be pulled out by the momentum of the nose cone. Unless you are using a piston, don't count on the charge pushing the chute out.

Unlike the drogue charge, which if too large can cause the nose cone and main to deploy when they hit the end of the shock cord, the main charge only stresses the recovery harness and attachment points which is usually overbuild and can handle the abuse. The biggest issue is usually having the right size drogue so the rocket is in the best orientation to get a clean deployment of the main and not tangle or cause even more stress on the recovery system when sections free fall past the open main and hit the ends of the recovery harness with large shock loads. You don't want the main to be deployed straight down. Bad things can more easily happen.

 

[Titan II]

Been there done that. Not always great results.

Been there done that. Always great results.

For me, a key factor is effective z-folds on all shock cords. I use them 100% of the time.

 

[waltr]

Been there done that. Always great results.

For me, a key factor is effective z-folds on all shock cords. I use them 100% of the time.

I also always use either Z-folds (Kevlar) or braiding (nylon) to reduce force to attachment points.

As for BP amount, I have had failure in flight with an amount of BP that worked well in ground testing. I now increase the BP amount for flight.

 

[SDramstad]

Just consider what the larger charges do. They put more stress on the recovery harness and attachment points. You may have to use larger, heavier, quick links, u-bolts, and bulkheads. Of course the heavier parts increase the momentum of the parts which again increases the stress on the parts. The whole rocket is a system and changes to one thing will affect everything else.

The trick is to find the right balance. My drogue charges are almost always about 1/2 size of the main charges. I use fewer if any shear pins on the drogue side and you only have to get the drogue chute into the air flow and it will take care of pulling all the recovery harnesses out. With the main, there are more shear pins, you need to get the chute out, which may have to be pulled out by the momentum of the nose cone. Unless you are using a piston, don't count on the charge pushing the chute out.

Unlike the drogue charge, which if too large can cause the nose cone and main to deploy when they hit the end of the shock cord, the main charge only stresses the recovery harness and attachment points which is usually overbuild and can handle the abuse. The biggest issue is usually having the right size drogue so the rocket is in the best orientation to get a clean deployment of the main and not tangle or cause even more stress on the recovery system when sections free fall past the open main and hit the ends of the recovery harness with large shock loads. You don't want the main to be deployed straight down. Bad things can more easily happen.

Coming in ballistic puts a lot of stress on it too

 

[Handeman]

Coming in ballistic puts a lot of stress on it too

Sure does! A busted shock cord tends to do both. Ballistic recovery and drifting away on the same flight. Lots of failure modes available.

 

[SDramstad]

I have never had a shock cord break on me. Ever. I did have a string of ballistic recoveries back when I was trying to use the minimum amount of black powder so I didnt stress out my rockets.....

Correction, I did shred one shock cord. My minimum charge didnt let the drogue out but my main came out as it was coming in ballistic at 500 feet per second. Shredded that Kevlar and zippered the main body tube big time. There is a lot of room between blow it out and blow it up.

 

[Donnager]

I have never had a shock cord break on me. Ever. I did have a string of ballistic recoveries back when I was trying to use the minimum amount of black powder so I didnt stress out my rockets.....

Correction, I did shred one shock cord. My minimum charge didnt let the drogue out but my main came out as it was coming in ballistic at 500 feet per second. Shredded that Kevlar and zippered the main body tube big time. There is a lot of room between blow it out and blow it up.

While this is true, I don't believe in blow it out or blow it up. I simply don't believe in trying to minimize charges at ground level elevation to see the absolute minimum requirement.

I see no reason to baby the charges "just because" you "might" 'not' need more. If a 1.5-1.7g charge is adequate and reliable in a DMS, no reason not to use those as a basis.

Use 1.5-1.7 in the bottom (as a minimum) and work your way forward.

As I said, 3g charge at the apogee in a 4" fiberglass rocket isn't something to be concerned over. Haven't sheared the nose pins on the jerk when using this. Just takes three pins.....

 

[-Slick-Ice]

Coming in ballistic puts a lot of stress on it too

Even a few seconds late on the ejection charge will take a lot more force. What is interesting is early seems to have the opposite effect. That one the rocket flew apart.