Question about when to use redundancy in DD.

I'm curious as to when a person should start using redundancy, i.e. two altimeters, etc., when doing DD. Is the decision based on the size of the rocket, the weight of the rocket, the motor thrust, projected altitude, or something else? I'm just looking for some basic guidelines that people use. Thanks.

In some respects, EVERY DD rocket should have redundancy/backups - for safety's sake.

But that's not always practical - due to size & available space usually, but also budget, complexity, extra weight/mass in high-performance rockets, etc. Also, sometimes you just want things to be simple and easy, and that's perfectly valid.

I myself have only used redundant systems in larger/heavier rockets (3" "full length" fiberglass and bigger) that are flying high enough that losing sight is a distinct possibility (say above 5000' or so). If I was really safety-minded, I'd use it much more often. But I often don't due to the issues noted above.

Everyone will have their own criteria, but it wouldn't be a bad thing to err on the side of "too much safety" I suppose.

s6

Redundancy in a dual deploy system is never a bad idea.

Let me say that again, redundancy in a dual deploy system is never a bad idea. :wink:

Depending on your motor and the flight characteristics, you can get cheap redundancy for the drogue by simply leaving the motor's ejection charge in, and in many cases you don't need to drill down the delay. That doesn't give you 100% redundancy, but most rockets will come down in more or less one piece just on the drogue, and it's a whole lot safer than coming in ballistic. I almost always leave the motor's ejection charge in, unless the motor has a short delay to begin with.

There are two "factions" here

One faction will argue to trust the electronics, that human error is a larger threat, and just keep things simple with single electronics

The other faction will argue that redundancy is a must.

Personally, I believe in redundancy. I even believe in two different types of electronics. And true redundancy - two electronics, 2 batteries, 2 switches. I even have two separate charges. I do set the electronics so there is a delay between them so both charges should not go at the same time.

The way I look at it, real aircraft have triple and quad redundancy. Why shouldn't we have at least dual? This is a safety issue. A "heavy" rocket coming in ballistic is an issue.

So again - I believe in redundancy
Redundancy is good
I would recommend redundancy

get my point? :wink:

My plans for DD always calls for redundancy. If not for the safety of the people/property that this may have a impact upon, then for the greater promise of a safe recovery of the rocket.

My personal opinion is to always use redundancy unless a design constraint prevents it. I don't have redundant batteries or switches as I don't see them likely to fail, though some people do, but eventually your primary altimeter will fail. At that point the outcome is either a potential loss of the rocket with all the onboard electronics, motor casing, recovery devices, and time and effort, or a successful recovery thanks to the backup. I'd rather pay now for a second altimeter then pay later to replace the whole rocket. The other advantage to redundancy it you can make your secondary charges a little larger or even place them differently in case your first charge fires but fails to deploy the chute.

Swissyhawk said:

I'm just looking for some basic guidelines that people use.

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FWIW, CAR requires redundancy for rockets of loaded mass over 5 kg (11 lbs). https://www.canadianrocketry.org/hpr_safety_code.php

Redundancy can be your enemy as well.
I had one flight where the backup killed my shock cords
Trashing the booster.

JD

JDcluster said:

Redundancy can be your enemy as well.
I had one flight where the backup killed my shock cords
Trashing the booster.

JD

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The old "both charges went off at the same time trick?" Technically not supposed to happen but I bet it can occur when a delay can't be programmed in to the backup.

Some degree of redundancy can be achieved with the newer multichannel altimeters though one has to assume the altimeter's integrity. If a bad ematch or undersized charge is the culprit, activating another charge from an unused channel might save the day. This would be a good idea as long as there is space on the bulkheads to carry all the connectors! A 38mm minimum diameter bird might not be amenable to this though. Kurt

Forget the connectors and wire the charges directly to the altimeter(s).

The biggest danger in HPR (in my opinion) is having a heavy rocket coming down ballistic. If you are a responsible flyer, you can not allow that to happen.

From that perspective, redundancy becomes a must. It can take many forms, but the goal is the same: having at least two methods of causing the rocket to separate.

1) Altimeter at apogee (primary) with motor delay (backup). This works for both single-deployment and DD flights. The only assumption is that motor delay is long enough to occur after the apogee.

2) Altimeter at apogee (primary) with second altimeter at apogee (possibly plus delay) (backup).

Other benefits of redundancy are better protection of your investment (rocket and electronics), but I would still put the safety first.

Well, enough theory. From personal experience, out of my last 10 HPR flights, one was saved by redundancy. $50 backup altimeter saved $1k worth of gear (plus liability!) - that's just 5% premium. Yes, you have to write (and follow) checklists to operate it properly.

Titan II said:

Forget the connectors and wire the charges directly to the altimeter(s).

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You are correct. I have a couple of rockets like that. Have holders that press fit plastic holders in place with the wires going to the altimeter directly..........
But, bulkhead connectors are convenient and reasonable as long as there is stress relief on the leads.

Some of the commercial charge holders only require a nubbin of an ematch that is less likely to tangle and remain competent. Kurt

Actually; they didn't go off at the same time.....
The backup was a bit too powerful.
It was a good 2 sec later.

JD

ksaves2 said:

The old "both charges went off at the same time trick?" Technically not supposed to happen but I bet it can occur when a delay can't be programmed in to the backup.

Some degree of redundancy can be achieved with the newer multichannel altimeters though one has to assume the altimeter's integrity. If a bad ematch or undersized charge is the culprit, activating another charge from an unused channel might save the day. This would be a good idea as long as there is space on the bulkheads to carry all the connectors! A 38mm minimum diameter bird might not be amenable to this though. Kurt

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Starting off as a noob in electronics and av-bays..have learned a little bit over the last year.

If there is room for redundancy, add it. Kinda kills the 3d printed sled bubble happening now, even a 3 inch sled gets crowded quick, but is worth the effort.

The secondary, cheapie altimeter may not have additional settings like, 'apogee plus 1 second' or hook to the computer for data export..so make it your PRIMARY altimeter and set the other as SECONDARY altimeter with the 'apogee plus one' and a slightly lower main altitude deployment, or slightly higher ..just not the same.

If you can mount the switch(es) to the sled it eliminates another set of connectors and wires that have to be tucked in every time the bay is buttoned up. Having the lid wiring detachable is worth the effort - I reverse the connectors on the second lid so they are NOT interchangeable and with redundant altimeters route the main/main lid wires though one 4 pin connector and the same to the apogee/apogee for the other lid.

You don't have to nail it on the first go around. Have worked through avbay sled 1.0 with lotsa loose (not battened down) wires going everywhere and twist and tape wire as switch. Then avbay sled 2.0 with all but the lid wires lashed or epoxied down and mounted rotary switches to new and improved av sled 3.0 with the same but with nylon standoffs for alt mounting so I don't have to worry about dropping the little nuts or nylon spacers I was using.

One thing that has helped me is to try to use the same color wiring for the same purposes on different sleds..use green as or to switches, black for batt ground and red for batt positive. My best sled was 4 inch..could route all the wires in back and drill holes in sled to correspond to terminal blocks in altimeter and just poke through the wire and connect..one wire , one hole . I could pop the altimeters off and when reattaching never had to look at wire diagram..just attach the corresponding wire.

Kenny

Titan II said:

Forget the connectors and wire the charges directly to the altimeter(s).

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That is exactly what I do. Simplifying the signal path is a good idea.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

Having fully redundant dual deploy altimeters saved my rocket on it most recent flight. I have two identical altimeters but should have used two different units. I have separate batteries, switches and ejection charges. It all fits into a 2.6" diameter bird. On the flight in question the main altimeter failed to fire the charges. When it landed the altimeter was still beeping three beeps to indicate continuity for both drogue and main chutes. (Not sure what went wrong, still looking into it.) Had I not had the backup altimeter I would now own an expensive fence post.

I think I'm at three flights now where the second altimeter did the job that the primary did not. Two of these flights were with rockets in the 70 to 85 pound range. One was only about 20 pounds. The larger rocket on two flights had an altimeter shut down at apogee (1st flight) that I thought I identified the problem and fixed so that (2nd flight) it shut down at boost instead. Rebooted. Shutdown at apogee. Rebooted. Shut down at main and never came back. It didn't seem to like G's. That's a bit harder to test at the bench!

Let's see, 6" diameter, 85# minus O propellant, ballistic from 18Kft... That would not have ended well! As it was, the flight was perfect to all outward appearances. Redundancy pays off. BTW, I never use two of the same type in a flight. That eliminates one source of systematic error.

When possible, it is a good idea to have at least one of the altimeters in a flight be one with some actual flight experience. That is, one that is known to work well. It can save the day when one has an un-flighted altimeter in the bird and it just doesn't do the job. In our group, we've been known to put additional altimeters in for the ride, to give them a test without as much risk.

I've seen light cardboard rockets come in ballistic due to deployment errors and land around cars and people. They come in very quickly and will do damage to whatever they hit. That's the LOC size and type rockets. These are still dangerous and warrant some thought towards redundancy and assuring that recovery works right each time every time. After all, the flight is optional. The recovery is not.

Gerald

Thanks for all the great replies. This will be my first DD rocket. It's only going to be 7-8 lbs, but that is twice as heavy as anything else I've launched and it just feels like it would be really dangerous if it came in ballistic. It's a Madcow 4" FG V2. I really want to avoid losing it in nearby trees doing an apogee deployment with the motor. Probably not the best choice for first DD since it will be HED, but I'm willing to give it a try. Part of the issue is simply getting all the pieces and parts together that one needs to do this, and then I have to figure out how to fit it all into a 4"x4" coupler.

Gregg

Swissyhawk said:

Thanks for all the great replies. This will be my first DD rocket. It's only going to be 7-8 lbs, but that is twice as heavy as anything else I've launched and it just feels like it would be really dangerous if it came in ballistic. It's a Madcow 4" FG V2. I really want to avoid losing it in nearby trees doing an apogee deployment with the motor. Probably not the best choice for first DD since it will be HED, but I'm willing to give it a try. Part of the issue is simply getting all the pieces and parts together that one needs to do this, and then I have to figure out how to fit it all into a 4"x4" coupler.

Gregg

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I'd absolutely go with redundant systems in a rocket like this. Or, as you say, motor ejection as a backup at the very least.
Also, a 4x4" coupler should give you plenty of room to work with.

s6

I typically use 1 altimeter if I have motor backup on lighter rockets, 2 altimeters on heavier rockets and when I don't have motor backup. Separate switches, batteries, and deployment charges. That way, if anything fails, then there is a full backup that should be operational. I have used a single charge with each altimeter's igniter in it before but it was pointed out to me by my L3 TAP that if the rocket fails to fully separate from the first charge, then the second charge should finish the job. I have been using 2 charges ever since.:bangbang:

Ravenex said:

My personal opinion is to always use redundancy unless a design constraint prevents it. I don't have redundant batteries or switches as I don't see them likely to fail, though some people do, but eventually your primary altimeter will fail. At that point the outcome is either a potential loss of the rocket with all the onboard electronics, motor casing, recovery devices, and time and effort, or a successful recovery thanks to the backup. I'd rather pay now for a second altimeter then pay later to replace the whole rocket. The other advantage to redundancy it you can make your secondary charges a little larger or even place them differently in case your first charge fires but fails to deploy the chute.

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Actually, I believe the most reliable piece in a DD setup, other then the BP, is the altimeter itself.

I've had batteries and ematches fail, no switched yet, but never the altimeter itself. If you use 9V batteries, never reuse one that had a hard impact. You can get a broken connection inside that will still allow enough current that everything looks good, but when the rocket launches, the battery fails. Commercial ematches do fail rarely. I happened to have two different manufacturer commercial matches fail on the same flight, super rare! What was even more amazing was the rocket fell flat the whole way from 4400 ft. and landed without damage. I have a HiAlt45 altimeter that was in a lawn darted rocket when the battery failed. It still works perfectly and has been used many times since.

As many things that can go wrong with DD, I still see more lawn darts and issues with motor deployed rockets then with single altimeter DD rockets.