Schurter switches for flight computers

[jderimig]

Its very popular these days that people are flying redundant altimeters from the get-go when they start doing dual-deploy. I used to encourage at my club that novices fly single altimeter alot before going to redundant systems. These would be for modest flights where the trajectory would be safe if there is a failure.

The reason if you fly redundant while learning, that will cover up a lot of bad practices. Fly a few rockets into the ground and you will learn proper electronics technique quickly. I understand why this may not be a popular view.

 

[Cameron Anderson]

That’s a terrible comparison. Two engine planes use both to load balance; they don’t reserve one as a backup in case the other fails, which is what true redundancy is about.

The analogy was made to illustrate fail rates and no so much function.

And you completely ignored the bulk of the argument (the main part!) that switches are the component least likely to fail in a power circuit/switch/altimeter/ejection charge system.

Switch redundancy doesn't increase fault tolerance since the switch itself is already the most tolerant component in the system we are discussing. Adding other switches is asking for higher failure rates due to possible issues with power wiring, soldering, connection wiring, et cetera.

 

[jderimig]

Switch redundancy doesn't increase fault tolerance since the switch itself is already the most tolerant component in the system we are discussing.

The weakest component in the system is the person flipping the switch. (Sorry).

 

[boatgeek]

How many good switches (not cheap e-bay Chinese crap, or twist and tuck wires) have you ever had fail? Have many have you heard of failing?

Personally, I've never had any. Nor have I heard any. Bad batteries, bad soldering, batteries coming disconnected...now those things happen all the time. Even schurter switches (which I use) are only rates for a certain number of turns (200 if memory serves); after that, failure is possible, and I have had a schurter that wouldn't hold it's position after lots of use. But a failure in that case would be due to exceeding design tolerances, not a switch build to spec failing spontaneously.

Focus on the right points of failure, switches aren't it.

I have personally seen Schurters fail several times, not by exceeding the number of switch cycles, but by having the leads get torn off/messed up when making connections. The leads are really fragile and are easy to screw up unless the switch stays on the sled so it doesn't need to be disconnected. Maybe I (and the students I mentor) are not careful enough, but that was a major issue in service. Fortunately, all of the failures were on the ground so they were only headaches and not safety problems.

 

[Cameron Anderson]

For the record, I fly a mix of stand-alone and redundant altimeters. Some identical brands, others are intentionally dissimilar. Some use a common switch for both altimeters, some use separate identical switches, some use different dissimilar switches.

Most of the time the size of the av bay and flight profile determine which electronics I fly (if I have room I use my Marsa33 because the LED screen is great, if it's tight on room I go for my Raven 4). I base all my flights and electronics set up based on flight location, motor, airframe, and recovery plan. And more often than not, I choose my set up based on my personal flight objective. Almost none of my av bays and sleds have common traits between them (and that is among my 15ish rockets I routinely fly). I like playing with new design ideas. I like trying new things. I like to push the limits of programming on a new altimeter (set as a back up) while using a tried and tested (primary) altimeter for peace of mind.

But whenever I use separate switches, it is a matter of functionality that drives that decision, and not a concern about failure or safety. Switches are testable and mountable and verifiable to the point that redundant switches simply adds to the chance of failure and doesn't necessarily make a system more fault tolerant.

 

[boatgeek]

Its very popular these days that people are flying redundant altimeters from the get-go when they start doing dual-deploy. I used to encourage at my club that novices fly single altimeter alot before going to redundant systems. These would be for modest flights where the trajectory would be safe if there is a failure.

The reason if you fly redundant while learning, that will cover up a lot of bad practices. Fly a few rockets into the ground and you will learn proper electronics technique quickly. I understand why this may not be a popular view.

Motor eject as backup would prevent flying into the ground. I would also think that most people would see the intact charge and say that was the benefit of redundancy. Not that they can depend on it, but that it saved a bunch of heartache.

 

[Cameron Anderson]

The weakest component in the system is the person flipping the switch. (Sorry).

100% agree John. I have seen more failures due to user error than I can count. People don't ground test, people don't very set up, people don't read the manual!

 

[Reinhard]

My point is redundant switches creates more points of failure than it does increase safety margin .

You mentioned above that you already had a Schurter switch fail. It sounds like it didn't happen in flight, but the next time it might. Personally I wouldn't use Schurter switches at all *), and certainly not without some redundancy. I understand their appeal though.

Reinhard

*) In case anybody is interested, I'm quite happy with Omron D2F, w/o lever, oriented horizontal. Up to 6 switches operated by a common RBF pin.

 

[Cameron Anderson]

I have personally seen Schurters fail several times, not by exceeding the number of switch cycles, but by having the leads get torn off/messed up when making connections. The leads are really fragile and are easy to screw up unless the switch stays on the sled so it doesn't need to be disconnected. Maybe I (and the students I mentor) are not careful enough, but that was a major issue in service. Fortunately, all of the failures were on the ground so they were only headaches and not safety problems.

The leads can be touchy, but that could be considered a poor design, or like you said, the result of user error. The first schurter I saw had 18 gauge wires, heat shrinked on each lead, with a larger heat shrink over the whole thing - no casual yanking is making it come loose!

All things being equal, properly soldered leads, resistance and continuity tested, on a mechanically sound switch can tolerate standard use with prep and flight.

 

[Steve Shannon]

The analogy was made to illustrate fail rates and no so much function.

And you completely ignored the bulk of the argument (the main part!) that switches are the component least likely to fail in a power circuit/switch/altimeter/ejection charge system.

Switch redundancy doesn't increase fault tolerance since the switch itself is already the most tolerant component in the system we are discussing. Adding other switches is asking for higher failure rates due to possible issues with power wiring, soldering, connection wiring, et cetera.

I didn’t ignore your argument that switches are reliable; I completely agree with it. I’ve argued it numerous times in this forum when people have argued that adding switches significantly diminishes reliability. But it’s a simple fact that having a single switch that controls both your primary and backup system (of any kind) creates a single point of failure. No good engineer charged with creating redundant systems would do that, especially since the price of the switch is so slight in comparison to the cost of failure.

 

[jderimig]

I would also think that most people would see the intact charge and say that was the benefit of redundancy. Not that they can depend on it, but that it saved a bunch of heartache.

Not always, I have personal (me) stories on that one. (the second charge can ignite from the one that fires, ask me how I know). I flew an entire season with a non-functioning backup altimeter that I thought was functioning because all the charges blew.

 

[Cameron Anderson]

You mentioned above that you already had a Schurter switch fail. It sounds like it didn't happen in flight, but the next time it might. Personally I wouldn't use Schurter switches at all *), and certainly not without some redundancy. I understand their appeal though.

Reinhard

*) In case anybody is interested, I'm quite happy with Omron D2F, w/o lever, oriented horizontal. Up to 6 switches operated by a common RBF pin.

I run a fairly regimented build and test program with my components. Between launches I fiddle with circuitry and set ups. The switch that failed was a test switch (I have flight gear and ground test gear) and it far exceeded the rated mechanical turn limit; it probably had several hundred turns. Flight switches get bench tested at home before a launch, table tested at the launch, and then used, so at 6 turns per flight, I have 30 flights per schurter, and I do log all my data. I track inane things like flights on my Kevlar recovery line too and replace it regularly).

But one thing I do in design is try to make things fail open (switch orientation and pole selection). So while it did fail, I made it fail, and it still would have functioned.

I am using the fingertech 3/32 hex screw switches right now. And I really like the featherweight magnetic switches. When room is not an issue I will use an eggtimer wifi switch.

 

[Cameron Anderson]

I didn’t ignore your argument that switches are reliable; I completely agree with it. I’ve argued it numerous times in this forum when people have argued that adding switches significantly diminishes reliability. But it’s a simple fact that having a single switch that controls both your primary and backup system (of any kind) creates a single point of failure. No good engineer charged with creating redundant systems would do that, especially since the price of the switch is so slight in comparison to the cost of failure.

A switch, one, singular, absolutely adds complexity (very minimal) and additional points of failure (again, extremely minimal and almost zero when tested, secured properly, wires correctly). But the safety one switch brings to the overall system (with pyrotechnic ejection and sometimes staging) offsets any increased risk of failure with it's contribution to overall safety.

My main argument is that switches need not be a redundant component for using two altimeters if the only thing you are looking at is functionality and safety. One switch is more than mechanically sound and reliable.

If you want to make an argument about utility (safety/redundancy) versus cost of piece of mind argument, I agree with you to a point.

I usually fly two altimeters, not because I need to (statistically speaking in terms of required redundancy and safety), but because I have a lot of them and they have different options and I don't fly as much as I'd like (I doubt anyone does). I fly two because, just like you said, it is cheap for burn a second battery and a few e-matches compared to the cost of failure.

 

[jsdemar]

Screw switches are second best to twisting wires.

I've been flying HPR since 1991 and EX/Research motors since 2001. In that time, I've seen every type of failure that I thought was possible, until I see the next new one.

In 2000, I had a nominal flight to apogee on a J350, but the apogee event was so violent that the firmware was erased from the altimeter's flash memory! I've measured >+/-50 g's at apogee deployment on a 150 pound rocket with some considerable amount of thought put into the staged apogee deployment event.

Schurter switches are good for average HPR flights. Care must be taken when soldering to them. And the leads must be potted with epoxy or at least RTV to keep them from bending and breaking at the switch.

Rotating key switches are often made to not bounce (security requirement). Cheap ones may not hold up to the vibration and kickbacks of flight.

Toggle switches, slide switches, and cheap snap switches should not be used at all, IMO. No mechanical relays for the same reason.

The switch bounce "failures" you don't see are those that are saved by the capacitance onboard the electronics at their power input. Early Adept altimeter (and others) would reset at the slightest power glitch due to only having a small filter cap.

 

[jbr]

I will not use shurter switches, they have a 300 duty cycle and I have had 1 break after maybe 10

 

[Rob702Martinez]

All great info!

 

[Steven88]

It’s all rocket science. Thanks guys!

 

[cerving]

I think the "redundancy" issue here is for DEPLOYMENT SYSTEMS, not SWITCHES. For a deployment to fail, both systems would have to fail... the probability of which is the square of the probability of a single system failing. If the failure probability of a single system is 5%, redundant systems make that 0.25%. Sounds like a pretty good deal to me. If you're really worried about a single switch failure, you can put two of them in parallel, both would have to fail for the power to go dark.

 

[UhClem]

In 2000, I had a nominal flight to apogee on a J350, but the apogee event was so violent that the firmware was erased from the altimeter's flash memory! I've measured >+/-50 g's at apogee deployment on a 150 pound rocket with some considerable amount of thought put into the staged apogee deployment event.

Some data.

Toggle switches, slide switches, and cheap snap switches should not be used at all, IMO. No mechanical relays for the same reason.

There are some good MIL-SPEC relays that will stand up to all sorts of abuse. I recall shepherding a FTS system using magnetic latching relays through qualification. Shock, vibration, thermal vacuum, acoustic vibe, etc. At both hot and cold (-40C) temperatures of course. A few turned up at bargain basement prices at BG Micro recently.

 

[jsdemar]

There are some good MIL-SPEC relays that will stand up to all sorts of abuse. I recall shepherding a FTS system using magnetic latching relays through qualification. Shock, vibration, thermal vacuum, acoustic vibe, etc. At both hot and cold (-40C) temperatures of course. A few turned up at bargain basement prices at BG Micro recently.

I used some mil-spec relays in the squib firing system I designed when at NASA. They were around $1200 each. Space rated, extreme temperatures, 100g shock, etc. I couldn't use solid state relays due to radiation/single-event upsets. Nor could I twist wires to arm it. There were triple redundant radio links to arm and fire. Requirements spec'd two, but we added a third path. It was needed on one of the missions when TDRSS and Iridum links failed. The management didn't know it was there... a cheap UHF receiver at 300 baud. A good day.

 

[Steven88]

I will not use shurter switches, they have a 300 duty cycle and I have had 1 break after maybe 10

 

[Steven88]

Can you tell me which switches to use then? I was pointed towards the schurter switches on Apogee's website because it said that they are the preferred switch by some over a push switch. I thought I WAS getting a good switch.

 

[Rob702Martinez]

Fingertech robot or missile works screw switches.

 

[Steven88]

Hi Rob! Is the screw switch that like the one that you sent me?

 

[Steven88]

Where do you recommend that I buy them?

 

[Rob702Martinez]

Yes, the little PC board looking thing with a phillips screw. It just goes inline with the pos terminal of the battery. Missileworks is where I get mine but other people also make them.

 

[Speaknoevil]

Can you tell me which switches to use then? I was pointed towards the schurter switches on Apogee's website because it said that they are the preferred switch by some over a push switch. I thought I WAS getting a good switch.

I will purchase any of those "horrible" Schurter switches you'd like to get rid of.

 

[Handeman]

I will not use shurter switches, they have a 300 duty cycle and I have had 1 break after maybe 10

Yes, If you look a what they are used for, the expected use of the switch is 0 to 1 times in their lifetime. Most will be shipped to and are set to the correct power for the equipment they are shipped in. The ones that go where the power is different, make one switch to give correct power and that's it. These are not ON/OFF switches. Were never designed to be.

 

[Steven88]

I decided to go with screw switches, coming in the mail tomorrow from Chris!

 

[jbr]

with screw switches, be sure and pinch the treads a bit with pliers so that they have more friction and don't close with vibration
I had one close and one wiggle all the way out of the screw base