Is motor backup deployment viable for high-fliers?

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tibbe

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I'm looking into building my L2 rocket and one requirement for our launch sites is that when electronic deployment is used a completely redundant deployment mechanism is required. This second mechanism could be a second electronic system or motor backup deployment. I was looking into the second option because I perceived it to be more reliable. However, looking at my OpenRocket simulations many motor configuration has an "optimal delay" time that's higher than the longest motor delay (e.g. 14.6s optimal delay but motor delay charge is 14s), which wouldn't work.

In practice, for high flying rockets where the optimal delay is larger than the maximal purchasable delay, what type of backup recovery deployment mechanisms do people use? Timers?
 
People (myself included) typically fly a second altimeter of a different brand or model in the same avionics bay as their primary. I also like to use motor backup as I have yet to fly a motor without an ejection charge, but this option is obviously not always possible. Use whatever you are comfortable with, but know that if altimeters are used correctly they are very reliable. Many people fly just one with good results.
 
I'm looking into building my L2 rocket and one requirement for our launch sites is that when electronic deployment is used a completely redundant deployment mechanism is required. This second mechanism could be a second electronic system or motor backup deployment. I was looking into the second option because I perceived it to be more reliable. However, looking at my OpenRocket simulations many motor configuration has an "optimal delay" time that's higher than the longest motor delay (e.g. 14.6s optimal delay but motor delay charge is 14s), which wouldn't work.

In practice, for high flying rockets where the optimal delay is larger than the maximal purchasable delay, what type of backup recovery deployment mechanisms do people use? Timers?

Something you ought to consider is that neither your simulation nor the delay grain is really likely to be correct within 0.6 seconds. Your motor delay could easily be up to 2 seconds long or short and your simulation could be off for any number of reasons. If your rocket is truly draggier or heavier than your simulation the actual coast time to apogee may be less than 14.6 seconds. If you’re space or budget constrained, I would go ahead and try motor ejection.
When used correctly altimeters are usually much more accurate at judging apogee. Two separate altimeters would be the ideal.



Steve Shannon
 
So, presumably, you can launch an L2 rocket with nothing but motor deploy, and this would be acceptable to your club. However, my L2 rocket with one reliable altimeter (and multiple charges) is not good enough. So, your club puts more faith in motor ejection and error-prone electronic complexity. Nonsense. Where is your club so I can avoid it?

Yes, 0.6 sec is nothing to worry about. Do it.
 
So, presumably, you can launch an L2 rocket with nothing but motor deploy, and this would be acceptable to your club. However, my L2 rocket with one reliable altimeter (and multiple charges) is not good enough. So, your club puts more faith in motor ejection and error-prone electronic complexity. Nonsense. Where is your club so I can avoid it?

Where are you coming up with this? I can't read it into what the OP wrote. He didn't say anything about only using motor ejection. He said that when you use electronic ejection, they require a back-up. :confused2:

Back to the OP, I would not use the motor back-up in this case. You will likely get the motor charge going off before the altimeter charge, and possibly at the same time, which is even worse. The back-up is a fail-safe. You do not want both charges going off at the same time, or else you will get a way-over-energetic deployment. You're essentially using twice the powder you need.

Ideally you want your back-up charge to blow about two seconds late, so that if the primary charge does not separate the rocket, the back-up will. So I set my back-up altimeter to +2 seconds after apogee. That would mean your ideal back-up deployment time would be 16.6 seconds.

You could use your motor as the primary and set your altimeter to +2, but I would highly recommend using a second altimeter and removing the motor charge.
 
Motor ejection backup doesn't sound feasible in this case. I'd run a second altimeter.
 
...

Back to the OP, I would not use the motor back-up in this case. You will likely get the motor charge going off before the altimeter charge, and possibly at the same time, which is even worse. The back-up is a fail-safe. You do not want both charges going off at the same time, or else you will get a way-over-energetic deployment. You're essentially using twice the powder you need.

Ideally you want your back-up charge to blow about two seconds late, so that if the primary charge does not separate the rocket, the back-up will. So I set my back-up altimeter to +2 seconds after apogee. That would mean your ideal back-up deployment time would be 16.6 seconds.

You could use your motor as the primary and set your altimeter to +2, but I would highly recommend using a second altimeter and removing the motor charge.

The other option would be to use the motor as "primary" undrilled and set the altimeter for 1s or 2s after apogee. For an L2 flight I would probably use a second altimeter, but altimeter backup would work. Also, I don't read "redundant altimeter" as meaning a different brand, although that's commonly recommended. The main thing I would worry about is that it have a different power source and be set to fire at a different time than the primary.
 
Just my 2 cents, but dual everything. Separate power, arming switches, deployment charges, and altimeters. Tried to save weight once and payed for it with "Z" shaped 1/4" all-thread.
 
An Adept22 is $30 and a DDC22 is $25. Shipped you're probably talking $65 max for two DD altimeters. That's less than one RRC3.
 
The original question:
Is motor backup deployment viable for high-fliers?

Answer:
No the delay for this type of flight is too short. [even longest delay available]


Q..In practice, for high flying rockets where the optimal delay is larger than the maximal purchasable delay, what type of backup recovery deployment mechanisms do people use?

Normally another altimeter.
 
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Normally, when using motor backup, you would intentionally set the motor delay to be on the long side so that the altimeter would likely go first followed by the motor backup. In the scenario stated by the OP, the motor delay is 14 seconds versus a 14.6 second simulation. I would expect the actual time to apogee to be below the optimum time from the simulation for all of the obvious reasons. Therefore, a 14 second motor delay may indeed be on the long side. For this specific scenario, I would go electronic apogee no delay with undrilled motor delay.

A few other suggestions would be to consider the materials of construction for the air frame and the recovery equipment. A simultaneous event might be a problem for cardboard or a heavy cone but not for fiberglass or a light cone. It would also be possible to ground test the possibility of a simultaneous event by doing a test with the combined BP from both the motor and the electronic charge, and to also adjust the motor charge if it is larger than needed.

Having said the above, redundant electronics is the best option. Once you do that, you won't look back at motor ejection.

Jim
 
Where are you coming up with this? I can't read it into what the OP wrote. He didn't say anything about only using motor ejection. He said that when you use electronic ejection, they require a back-up. :confused2:.

The OP said "when electronic deployment is used", implying there are times when electronic deployment is not used, ie, motor deployment. I am testing the logic of the club, here. If they permit an L2 rocket to fly with motor only, then their extra rules for redundant electronics are silly and burdensome.

The construction of the rocket also comes into play. An FG airframe is more likely to withstand a non-ideal +/- 2 s motor ejection error than a cardboard one.
 
The OP said "when electronic deployment is used", implying there are times when electronic deployment is not used, ie, motor deployment. I am testing the logic of the club, here. If they permit an L2 rocket to fly with motor only, then their extra rules for redundant electronics are silly and burdensome.

There are two separate rules:

* Flights above 2000ft AGL need dual deployment.
* Electronic deployment requires backup (which could be redundant electronics, motor backup, or something else).

So for example the club doesn't say that you can go to 3000ft using either motor ejection or electronic ejection with motor backup.
 
Lots of feedback from very experienced rocket dudes on your question...

Not knowing where you are flying, I'd guess that the DD over 2K rule is because of a restricted recovery area...the club wants the rockets to come down fast.

And the "electronic deployment requires a backup" rule is because they have some experience with less than optimal DD set-ups. There are a lot of ways things can get screwed up, and a lot of us here have learned some of what we know the hard way.

Without knowing anything about your rocket, I would have said that the difference between 14.6 optimal and 14 rated on the delay is trivial. If it is fiberglass, a couple of seconds too early or too late is survivable. If you think the rocket can take a deployment when it is moving at 50fps or something in that range, you might be able to meet the club requirements with motor deployment for the apogee event, and a Jolly Logic Chute Release for the main event.
 
you might be able to meet the club requirements with motor deployment for the apogee event, and a Jolly Logic Chute Release for the main event.

Probably not, if the rule makers are consistent. The JLCR is an electronic device for main only, and since it has no backup, it cannot be used by itself.

Dumb, dumb, dumb
 
You could use two Jolly Logic Chute Releases in series ("daisy chain") if you need redundancy. If either opens, the chute inflates.
If you'd like to use motor ejection for your first event, maybe consider adding some weight to bring the optimum delay time down?
Usually if you're using motor ejection for a qualification flight, it's not a bad idea to choose a fatter rocket, so that delay times are within reason and the flight stays a little lower and there is more room for a chute that gives a nice soft landing. Remember, you have to find it and it has to be intact, so low and slow is not a bad cert strategy.
 
There are two separate rules:

* Flights above 2000ft AGL need dual deployment.
* Electronic deployment requires backup (which could be redundant electronics, motor backup, or something else).

So for example the club doesn't say that you can go to 3000ft using either motor ejection or electronic ejection with motor backup.

This is very interesting and at first blush, doesn't seem to jive with the OP's question or Dad Man Walking's analysis of the site. If you have to use electronic deployment above 2000 ft. but you can fly a rocket high enough to need more than 14 seconds of delay, what is the waiver? If you are going that high, it would seem that motor deploy at 3000 ft or so would bring you down within the same area.

Lots of feedback from very experienced rocket dudes on your question...

Not knowing where you are flying, I'd guess that the DD over 2K rule is because of a restricted recovery area...the club wants the rockets to come down fast.

And the "electronic deployment requires a backup" rule is because they have some experience with less than optimal DD set-ups. There are a lot of ways things can get screwed up, and a lot of us here have learned some of what we know the hard way.

Without knowing anything about your rocket, I would have said that the difference between 14.6 optimal and 14 rated on the delay is trivial. If it is fiberglass, a couple of seconds too early or too late is survivable. If you think the rocket can take a deployment when it is moving at 50fps or something in that range, you might be able to meet the club requirements with motor deployment for the apogee event, and a Jolly Logic Chute Release for the main event.

I would tend to agree with this analysis, but have to wonder what altitude the OP is intending to fly to if he needs more than a 14 second delay.

Probably not, if the rule makers are consistent. The JLCR is an electronic device for main only, and since it has no backup, it cannot be used by itself.

Dumb, dumb, dumb

I tend to agree with your analysis of the rule makers intent, but no sure about the "Dumb, dumb, dumb" part. There really isn't enough information available to make that opinion viable.

What is the waiver?
Why the altitude limit requiring electronic deployment? The availability of the JLCR makes a DD flight profile available for motor eject flights. How has that been incorporated into the rule, or has it?

Bottom line, to answer the OP question with the available information, use a second, totally independent altimeter. I'm familiar with Perfectflite Stratologger and know they can be programmed with an apogee delay and work very well as a secondary/backup altimeter.
 
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