Redundant Dual-Deployment Ejection Charges

[garlicsnapper]

I have a rocket that I will be recovering 100% on electronics with no motor deployment whatsoever. I do have both primary and secondary dual-deployment altimeters employed for this project. It has been ground tested, so the proper amount of black powder is known and confirmed. Now I’m positive I read somewhere—and for the life of me cannot find it—that secondary ejection charges are supposed to be larger than the primary ejection charges. Is that factual? And by how much should that ejection charge be increased?

 

[tim cubbedge]

It’s not factual. I have some rockets that share the same charge well with 2 altimeters. If I have space available, I’ll use two charge wells. Some of my set ups use same amount off BP. Opinions will vary when it comes to redundancy.

 

[spigalau]

On our 3" all electronic rocket - Personally we use 1.1g (Primary - Apogee) & 1.3g (Secondary - Apogee +1 second) on our drogue charges and 1.3g (Primary - 800 feet) & 1.5g (secondary - 500 feet) on main release. The reasoning for a larger secondary charge is it's a final 'all or nothing' or the 'hail mary'.
Larger charge is also more likely to be heard, so gets eyes looking in right direction too

 

[Igotnothing]

Blow it out or blow it up, 'cause at this point you have nothing to loose.

 

[DragonRocketry]

I up my back up charges 10 to 20% over the primary.

 

[les]

I also make the backup charge bigger. In the 20% range. The BP is relatively cheap.
If the primary worked, the backup is just discharging into open air - no issue. Just wasted an ignitor and some powder.

But if the primary failed - why? Did it just not go off or did something bind? If binding then the slightly larger charge may solve the problem. Again - if you are having the second charge (which I highly recommend) having it a bit bigger won't hurt and may just save your rocket

 

[OverTheTop]

I run with the backup charge 30-50% larger. I have never had a secondary charge the same size as primary.

 

[Bat-mite]

I'm with Les. The question is: what is more likely -- that your primary altimeter failed to light the charge, or that the chute/Nomex/harness got bound in the tube and needs a little more oomph to get it out? Granted, ematches can and do fail, but if you use redundant ematches for each charge, and since today's altimeters are really good, to me, it is more likely to have a bound chute. Thus, if the first one fails, make the second one bigger.

That being said, I once had an apogee ejection failure with redundant everything and even motor backup. Three shots at getting the drogue out -- no go. Main deployed while the rocket was coming in ballistic, which destroyed it. What was left core sampled. It was a really frigid day, like 15* F, so I guess the tubes shrunk enough to hold onto the drogue package. I did NOT make the second or the motor charge bigger. Wish I had.

 

[chilled]

I have always upped the second charge 20%-40% or so. on a related thought, if the rocket is large enough to fit it, i'll also run a backup altimeter on it's own power source.

 

[Titan II]

40-50%

 

[FredA]

if primary was calculated then ~33%.

If primary was a SWAG then 50%-100%.

 

[Johnly]

A separate altimeter and charges ~20% larger or 0.5 grams are required for teams participating in the NASA SLP. Over the many years that I have been part of the program, I have observed several airframes "saved" by the larger backup charge and never once seen a flight failure caused by one.

Fiberglass airframes are more robust that many give them credit for. Several years ago a team calculated that they needed an ejection charge 25 grains for a ground test but the person measured out 25 grams! Needless to say the separation was energetic, but the airframe was undamaged.

 

[spigalau]

This video -

Shows what happens when you wire both primary & secondary charges in reverse order on both flight computers.

Rather than blowing the nose cone free @ apogee and then cable cutting at 700'... it did the reverse, cable cut @ apogee and nose cone blown when coming in ballistic. Thankfully nil damage to airframe as his charge was sufficient enough to get some laundry out.

 

[TheTank]

I always add 10-20% to the backup in the event something binds up. That said, i have never had it occur.

This past weekend, I finally had an experience where my backup saved the rocket and separated it at apogee. I had a short somewhere in the wires to the terminal block for the primary drogue charge. Altimeter tested positive for continuity because of the short but charge never fired because of it. Backup separated the rocket and both charges fired for the main as they should. Extremely glad that 13lb rocket wasnt coming in ballistic from 4k ft and then try to blow the main at 700ft... that would have gotten ugly quick.

 

[FredA]

short somewhere in the wires to the terminal block

Not sure this applies in this case, but a good practice is to put some tape or other cover on top of terminal strips.
Murphy will put your quicklink right in the wrong spot and short terminals.

 

[firemanup]

Backup charge is always bigger on my birds as well.

Have a 3” bird that ground tested fine with 1 gram BP. Flew and recovered several times with a 1.5 gram backup charge.

Only after adding onboard camera did I find out the 1 gram wasn’t working at apogee and the backup was separating the rocket. So I’ve now adjusted the charges accordingly.

Luckily the backup was substantially larger and worked.

 

[G_T]

I'm of the school where if you want it to separate, hit it like you mean it! If it works on the ground nicely and smoothly, it might not work in the air when the rocket may not be stationary, may be vibrating, just hit temperature changes, air pressure changes, got a little debris in the joint, absorbed some moisture in the joint... I don't have a problem generally speaking of using one oversize charge, with recovery system designed to handle that charge, and multiple altimeters with ematches into that one hefty charge.

Be aware that a large charge for drogue deployment may generate enough shock to separate the main. Same can be the case with a larger than necessary drogue, or deployment of drogue with notable velocity. So size shear pins accordingly, and work on your harness so it progressively absorbs the momentum of the sections rather than slamming to the limit - and shearing the other shear pins or slamming all the segments together and then possibly tangling.

Most people worry most about getting the main out. Ballistic with a heavy rocket and then getting the main out tends to rain segments down at high speeds as it rips the recovery harness apart - which is generally better than coming in at high speeds all in one piece. But I'm more concerned with getting the drogue out. If the main doesn't deploy, the speed is at least managed by the drogue and you get to see it coming down and hopefully not be under it. If one of the deployments fails, it can be safer if that failure is the main not the drogue. This can depend on the details of course.

If you are going to 1000' and deploy main at 700', make sure to get the main out. Go to 15000' and deploy main at 700', be sure to get the drogue out.

BTW, I'm talking about BDRs. And of course you want to get both deployments to work and design so they won't tangle so it doesn't become a lottery how it comes down. You don't want that winning ticket.

Gerald

 

[TheTank]

Not sure this applies in this case, but a good practice is to put some tape or other cover on top of terminal strips.
Murphy will put your quicklink right in the wrong spot and short terminals.

I always tape the blocks as i can definitely see that happening.On this one, it seems the sled shifted ever so slightly and pinched the wires/silicone and put a slit in them from what I can tell.

 

[G_T]

I keep terminal blocks on the inside of ebays, have a hole through for wires, and a fender washer covering the hole exit. Works well particularly if you make some sort of channel for the wire to go out from under the washer. Admittedly it takes slightly longer to prep. But there is no exposed conductor outside the ebay.

 

[OverTheTop]

Not sure this applies in this case, but a good practice is to put some tape or other cover on top of terminal strips.

+1. It keeps the residue from the BP off them also.

 

[Dave A]

It’s not factual. I have some rockets that share the same charge well with 2 altimeters. If I have space available, I’ll use two charge wells. Some of my set ups use same amount off BP. Opinions will vary when it comes to redundancy.

This what I do on most of my rockets. Systems can fail but 1 well-prepped charge with 2 igniters are probably not going fail. I never had 4F fail me yet!

 

[Handeman]

I had a main charge and 20% larger back-up main charge both fail to pop the nose cone on a flight. I still have no idea why. The primary main charge worked perfectly on the first 6 flights. Lucky #7?
I'm considering 40-50% larger back-up charges, at least on the main.

 

[b.wieting]

With two altimeters, how do you know which is primary?

 

[Titan II]

With two altimeters, how do you know which is primary?

Which ever one you say is primary.

 

[TheTank]

With two altimeters, how do you know which is primary?

you set the altitudes for deployment so which ever you set higher (or less delay post apogee for drogue), will be primary.

 

[b.wieting]

Thanks! I'd skimmed over the primary/secondary mode instructions....

 

[OverTheTop]

With two altimeters, how do you know which is primary?

Relevant terminals and altimeters are marked "P" or "S".

 

[mtnmanak]

I like to color code my wiring so I have no questions. At some point years ago, I standardized all my wiring so that my primary computer always gets blue wires and my backup gets yellow wires. I also label the terminals (if I am using terminals) or etch the labels into bulkplate itself. In the photo below, the terminals I used for that rocket have very nice labeling plates.

 

[Orien]

The second charge is a backup? I thought it was the Range Safety Self-Destruct Charge.

 

[MidOH]

Which ever one you say is primary.

I'm just a noob, no L1. But on the Loc4 I'm building, everything on the left side of the bay is primary. Everything on the right is secondary. Two DD systems, completely separate, and symmetrical.

The connectors for the lower bulkhead charges are flipped from each other. One clear tap has a female pigtail, the other a male. So you can never get those mixed up. Only way to screw that up, would be to plug one altimeter into the other, and one clear tap into the other. And miss all of the error beeps.

I suppose I could switch types of connectors for primary and secondary. Maybe servo plugs for primary, JR plugs for secondary. Then I could connect the lower bulkhead with my eyes closed. I think I have an extra JR connector around.............