# tips for a backup altimeter

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#### ericm541

##### Well-Known Member
I've generally just used one altimeter for dual deploy and somtimes motor ejection as a backup. My current project i will be using another device for backup,just need some tips, my first thought is i'm pretty sure i don't want 2 ejection wells going off together. I have 2 perfect flight MAWDS, missle works PE2 and RRc2 mini.

Eric M
NAR 85115

#### OverTheTop

##### Well-Known Member
TRF Supporter
I usually run one second delay between primary and secondary charges.

#### mpitfield

##### Moderator
Staff member
Global Mod
Because altimeters are typically not calibrated identically I use a 2 second delay @ apogee and a 200' difference on main.

#### jrkennedy2

##### Well-Known Member
I like Altus Metrum's Easy Mini as a backup and run it a second after apogee and ~200' lower on main...

#### dshmel

##### Well-Known Member
Because altimeters are typically not calibrated identically I use a 2 second delay @ apogee and a 200' difference on main.
+1.

#### FredA

##### Well-Known Member
Here's my tip for your backup altimeter: Put the backup unit in a different rocket.

#### ksaves2

Here's my tip for your backup altimeter: Put the backup unit in a different rocket.
Very valid point Fred as long as the OP is flying a modest sport rocket (as I suspect he is). Unless he's trying to gain
some practice for the future. I will admit that cramming two deployment devices in a small rocket can be an absolute
witch! Ematches are pretty "punctual", dependable and testing with a proper ohm meter helps. Single unit flying can be safe and reliable. I'm sure you would agree once a project becomes larger and heavier, hence able to do more damage, you would concur that a backup device be prudent to assure a soft landing whatever the rocket alighted upon on touchdown.
Smaller stuff? I agree with Fred, put it in second rocket so everything can be pre-prepped
and one has something else "interesting" to fly once they get the first rocket back!

Save a two unit dual deploy for a larger project where there is room to work in. Unless one wants to see how much they can cram into a small space. Then again, it's not a fun prospect as I and I suspect Fred already knows. Especially, two deployment devices AND motor backup for apogee
would be somewhat extreme for a small project. Kurt

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#### Buckeye

##### Well-Known Member
TRF Supporter
Here's my tip for your backup altimeter: Put the backup unit in a different rocket.
:clap:

#### markkoelsch

##### Well-Known Member
Because altimeters are typically not calibrated identically I use a 2 second delay @ apogee and a 200' difference on main.
That is what I do as well- when flying two altimeters.

Smaller rockets are still flying on a single altimeter. I have been doing that for a long time, and have not had an issue.

#### ericm541

##### Well-Known Member
Yes, all of my smaller rockets i've just used one altimeter, My current project is bigger, 5" performance kit which i could use just one altimeter and save the other lol. Thanks everyone, ill go with 2 second delay @ apogee and a 200' difference on main to start with.

Eric M
NAR 85115 L2

#### mpitfield

##### Moderator
Staff member
Global Mod
Here's my tip for your backup altimeter: Put the backup unit in a different rocket.
I put a back-up altimiter in every HP rocket that it will fit into, why...why not? I deal with redundant systems and high availability for my day job and one thing I have realized is that they are all a waste of time and money, until they are not.

#### OverTheTop

##### Well-Known Member
TRF Supporter
they are all a waste of time and money, until they are not.
Totally agree. It is a bit like insurance. Most people don't need to insure their rockets as they can cover the loss, but with a little outlay (time and money) the chance of that loss being a write-off is greatly reduced.

It is a matter of safety also. A failed deployment might ruin a rocket as it augers into the ground, but it would make for a really bad day if someone was standing there at the time.

It is a choice, and one of many we make as we build and fly, but I try to make it a conscious decision up front. If I chose to have no secondary system and the primary fails, I should not spend any time or effort lamenting the loss.

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#### mpitfield

##### Moderator
Staff member
Global Mod
Totally agree. It is a bit like insurance. Most people don't need to insure their rockets as they can cover the loss, but with a little outlay (time and money) the chance of that loss being a write-off is greatly reduced.

It is a matter of safety also. A failed deployment might ruin a rocket as it augers into the ground, but it would make for a really bad day if someone was standing there at the time.

It is a choice, and one of many we make as we build and fly, but I try to make it a conscious decision up front. If I chose to have no secondary system and the primary fails, I should not spend any time or effort lamenting the loss.
Good point about the safety factor.

I cannot envision a zero risk factor, both as a flier or spectator, in this hobby, and it is something that I am very aware of.

That being said. As you put it, "with a little outlay (time and money)" I personally want to take what I feel are reasonable precautions, in an effort to mitigate what could be a regrettable situation.

#### ericm541

##### Well-Known Member
Well said, i'm sure there's room for more potential error with another device. I've seen total loss simply because the flyer used one battery for 2 altimeters. My altimeters are old too, that could be an issue for me, another reason for backup IMO.

Eric M
NAR 85115 L2

#### OverTheTop

##### Well-Known Member
TRF Supporter
I've seen total loss simply because the flyer used one battery for 2 altimeters.
That is why I run one battery for the altimeter and one for the eMatch. Sometimes I run the eMatch and the alt off the same battery, but only if it is the last expected action. For example I run the Raven and the backup eMatch on one battery, and the main eMatch on a separate battery. If the secondary deployment shorts the battery or kills the altimeter it should have done its job reliably up to that point so the failure is inconsequential (and I have saved carting an extra battery along).

#### cerving

##### Owner, Eggtimer Rocketry
TRF Supporter
Redundancy is TWO of EVERYTHING. Anything shared becomes a single point of failure. Now in practice, if your charge well is big enough you can use one well with one match from each altimeter... BP doesn't fail.

TRF Supporter

#### mpitfield

##### Moderator
Staff member
Global Mod
Redundancy is TWO of EVERYTHING. Anything shared becomes a single point of failure. Now in practice, if your charge well is big enough you can use one well with one match from each altimeter... BP doesn't fail.
Or unless the initial charge was not enough to get the job done.

Yes, I always ground test and add a bit for margin, but I always add a bit more on my backup charge.

As an example, my 54mm Tomach, the apogee charge only needs to be .9g to get the job done, according to my ground tests. However 1.1g reliably separates both halves of the rocket to the full extension of the 25' harness, without snap-back So I go with 1.1g on my primary, but for my backup I use 1.3g.

1.3g in ground tests, results in a good bit of snap-back when the harness gets to the end of 25', but not so much that there is any chance of damage.

Having said that I do have a potential for shearing my nosecone shear-pins and deploying my main at apogee with 1.3g, however I will take that over a ballistic return. It already happened once, when I used 1.3g (primary) and 1.5 (backup), with a 20' harness and the rocket was @ 16,ish thousand feet AGL, resulting in a 3.46 mile recovery.

That event resulted in more ground testing, rolling back the charges by .2g, a slightly longer recovery harness by 5', as well as taping the recovery harness in an effort to take out some of the energy and mitigate the snap-back.

#### OverTheTop

##### Well-Known Member
TRF Supporter
Now in practice, if your charge well is big enough you can use one well with one match from each altimeter..
I have been known to use this method, but I wasn't overly happy. Two separate altimeters, individual batteries for each alt and pyro. Main chute release was an AARD, so I didn't have the luxury of two charges. Worked as planned.

#### Voyager1

##### Well-Known Member
TRF Supporter
I'm not sure whether this has already been mentioned, but there is a good argument for using different altimeters for primary and backup. This practice might avoid an altimeter-specific failure, quite apart from user created problems, such as installation or programming.

Most altimeters that I have used have been very reliable, but some may be more reliable than others.

Just a thought!

#### Buckeye

##### Well-Known Member
TRF Supporter
Well said, i'm sure there's room for more potential error with another device.
I started dual deploy in 1999. I never had a barometric altimeter fail to do its job for deployment. I have 3 MAWDs still going strong, even after a couple water dunkings. Also Missileworks, MARSA, and Raven. I ground test. Any failure was due to me, and I can count the incidents on one hand - loose battery connection, crappy homemade ematch, bad parachute wrap. I worry more about ematches these days, so I occasionally using 2 per charge or 4-channel alts if I am feeling up to it.

So, I am curious as to what started the multiple altimeter craze. Was there a spate of rockets augering into the ground with one altimeter in 1999? I don't think so. My guess is that price, not need, is when the redundancy started. Below $100, altimeters started to look like a bargain for most folks, and the usual over-building mindset sunk in. I don't have stat data, but the baro altimeters seem uber-reliable. Add another altimeter, and the build complexity goes up along with human error. So, to you redundanites, what are you protecting against - the altimeter or yourself? #### Banzai88 ##### Lvl 1,Wallet....Destroyed TRF Supporter NO matter what you do, color code your wires and double check. If you use doghouse/binder type connectors to be able to take off your bulkheads, make SURE that you key them differently so that there is NO WAY that you can mix up main and drogue/primary and back up. Ask me how I know! Another thing to think about is how an altimeter senses and processes "Apogee". Some, by design are A+1 as a STANDARD, so setting a back up as A+1, you get a double pop. You would have to set the back up as A+2 to get what you were looking for. Also, as I have found recently doing an RRC3/RRC2 primary/secondary combo, regardless of your settings = component tolerance stack and software 'give' may also be enough to give you primary and secondary charges close enough together to be overpressure. I have since invested in a cheap vac chamber where I can test any 2 units together to make sure the settings are what I want them to be. So, I am curious as to what started the multiple altimeter craze. Was there a spate of rockets augering into the ground.............? My guess is that price, not need, is when the redundancy started. Below$100, altimeters started to look like a bargain for most folks, and the usual over-building mindset sunk in.
This. Every 6 months or so a "Dual redundant dual deploy" thread comes about in some form or another, and every time it seems to come down to this.

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#### jderimig

Add another altimeter, and the build complexity goes up along with human error. So, to you redundanites, what are you protecting against - the altimeter or yourself?
Yourself mostly. Errors in deployment are largely external to the altimeter, wiring, power, connectors etc.

However we have a practice of flying altimeters that have survived hard landings or rough usage because they seem to work (power up) after the event. There could be weaknesses or damages in traces, pad solder joints or MEM's sensor damage that could be latent. If you are in that category, flying with a backup altimeter may not be a bad idea.

#### Voyager1

##### Well-Known Member
TRF Supporter
Yourself mostly. Errors in deployment are largely external to the altimeter, wiring, power, connectors etc.

However we have a practice of flying altimeters that have survived hard landings or rough usage because they seem to work (power up) after the event. There could be weaknesses or damages in traces, pad solder joints or MEM's sensor damage that could be latent. If you are in that category, flying with a backup altimeter may not be a bad idea.
I couldn't agree more! You might double the complexity and cost of your avionics, but the reduction in risk is worth the price.

#### ksaves2

I put a back-up altimiter in every HP rocket that it will fit into, why...why not? I deal with redundant systems and high availability for my day job and one thing I have realized is that they are all a waste of time and money, until they are not.
Again, if you are trying to see how much you can cram into small spaces or if you have a waiver altitude you want to stay under (Hafta add weight? Might as
well have it do something useful!) by all means. 2.75" diameter cardboard rocket doesn't need two. That said, I'm converting a 3" FG stretched Wildman rocket
to two devices after one successful flight with one. At the time it was built 12 years ago, I couldn't fit two devices, a GPS tracker into the 10" long ebay. I say
I "stretched it" mainly to cram more main chute into. Now I'm moving the tracker to a new nosecone and have plenty of room for two modern, smaller deployment devices. I just can't bear with a single unit failure with this much rocket coming in ballistic. Ahhhh, plus I stretched the sustainer when I built it
and unbeknownst to me the Loki 54mm M was going to come out so many years later. This rocket would be able to take it except I would have to go out
West to find a waiver high enough to handle it. Already went to 10k, Mach 1 with a Loki L-1400. I got the case and it fits with plenty of room. Kurt

#### Flyfalcons

##### Well-Known Member
Despite preflighting the avbay on my 4" BBIII at a recent launch, one of the altimeters failed. I haven't had a chance to post flight it yet (I currently suspect the screw switch wasn't tightened enough), but no matter the reason, having a fully redundant system resulted in an otherwise nominal flight. Well worth the slight cost and weight penalty, in my opinion, to add an extra layer of safety when possible.

#### mpitfield

##### Moderator
Staff member
Global Mod
I started dual deploy in 1999. I never had a barometric altimeter fail to do its job for deployment. I have 3 MAWDs still going strong, even after a couple water dunkings. Also Missileworks, MARSA, and Raven. I ground test. Any failure was due to me, and I can count the incidents on one hand - loose battery connection, crappy homemade ematch, bad parachute wrap. I worry more about ematches these days, so I occasionally using 2 per charge or 4-channel alts if I am feeling up to it.

So, I am curious as to what started the multiple altimeter craze. Was there a spate of rockets augering into the ground with one altimeter in 1999? I don't think so. My guess is that price, not need, is when the redundancy started. Below \$100, altimeters started to look like a bargain for most folks, and the usual over-building mindset sunk in.

I don't have stat data, but the baro altimeters seem uber-reliable. Add another altimeter, and the build complexity goes up along with human error. So, to you redundanites, what are you protecting against - the altimeter or yourself?
No disrespect, but my personal view is that it is not just a personal preference but it makes common sense. You said it yourself human error is the most likely cause of a failure not hardware. I suppose one could take the more is less argument, however I am less likely to make the same mistake twice while prepping. I had one rocket return with a live charge well (primary-drogue) and when I inspected it, I discovered that I had shorted the e-match.

As far as to why and when, I believe you are correct in that the entry price point is such that it becomes a viable option for many to consider.

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#### CzTeacherMan

##### Well-Known Member
Here's my redundant setup:
Main Altimeter:
-apogee charge, correct amount of BP
-main charge at X altitude, correct amount of BP
Backup Altimeter:
-Apogee + 1 second, BP+some (usually .5g extra)
-main charge at X-200ft, BP+some

#### jeff2space

##### Well-Known Member
Here's my redundant setup:
Main Altimeter:
-apogee charge, correct amount of BP
-main charge at X altitude, correct amount of BP
Backup Altimeter:
-Apogee + 1 second, BP+some (usually .5g extra)
-main charge at X-200ft, BP+some
+1, I concur.

#### Steve Shannon

##### Well-Known Member
TRF Supporter
I'm not sure whether this has already been mentioned, but there is a good argument for using different altimeters for primary and backup. This practice might avoid an altimeter-specific failure, quite apart from user created problems, such as installation or programming.

Most altimeters that I have used have been very reliable, but some may be more reliable than others.

Just a thought!
When Anthony Cesaroni flew his Hyperion rocket at RocLake in the early 2000s, it crashed because the two identical AltAcc altimeters both failed to integrate correctly. The AltAcc's sampling frequency was too low to accurately measure the acceleration at the frequency of the Hypertek motor.

Steve Shannon