Dual-Deployment: How often is one deployment charge used with motor ejection? (H-J)

As the title states: In dual-deployment, how often is one deployment charge used in combination with motor ejection when using H-J motors, particularly the lower-midrange of that spectrum?

It just seems to me that adding a "backup" charge to the very reliable motor ejection system seems overtly complicated, and just another potential failure point (premature drogue ejection, etc.)


Ideas?



Thanks!

I always use both charges in my DD capable rockets.

I setup my dual deploy electronics to fire on apogee.

The motor ejection is a backup in case the electronics fail to separate the rocket, or the altimeter shuts down.

Redundancy is cheap insurance.

It's the other way around. I dispose of my motor ejection and use electronic ejection 95% of the time. It's more reliable and predictable.

CAVEAT: I have yet to fly my first DD rocket! So caveat emptor!

It seems to me that the advantage of using the altimeter to deloy the drogue is that you get an ejection at true apogee, rather than after a specific number of seconds, which may, in some conditions, not be exactly at apogee.

When I do launch my first DD, I plan to leave the motor ejection delay grain at maximum delay, use the altimeter for apogee deployment (and hopefully prevent zipper), and then have the motor charge as a backup.

But if you are extrememly confident of getting your drogue charge timing to apogee in all conditions, then yes, the motor ejection is cheaper, easier, and quicker to use.

dixontj93060 said:

It's the other way around. I dispose of my motor ejection and use electronic ejection 95% of the time. It's more reliable and predictable.

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I use the motor ejection BP for my ejection charges.

But that's because I don't have any other BP source.

I use it 100% of the time.

It is cheap insurance.
I have seen far more lawn darts than premature drogue separations.
It keeps me in practice setting up charges and programming the altimeters.
It allows me to monitor the different components of the altimeters on a constant basis.
As another mentioned, it is more accurate.

I use the motor ejection as backup 100% of the time. I use Aerotech motors and I swap out the delay grain that comes with the motor (M) for the proper delay grain for a (L)ong delay.

CarVac said:

I use the motor ejection BP for my ejection charges.

But that's because I don't have any other BP source.

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Yeah, I shouldn't have said dispose of... Really it's disassemble and store for future use.

DuctTapeandRocketFuel said:

I always use both charges in my DD capable rockets.

I setup my dual deploy electronics to fire on apogee.

The motor ejection is a backup in case the electronics fail to separate the rocket, or the altimeter shuts down.

Redundancy is cheap insurance.

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This!

RocketDude77 said:

As the title states: In dual-deployment, how often is one deployment charge used in combination with motor ejection when using H-J motors, particularly the lower-midrange of that spectrum?

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I fly most of my smaller DD H-I rockets using motor eject for apogee and only one deployment charge for the main. Works fine as long as the motor delay is accurate.

Motor delays vary even if you drill them right...and even then you're just taking an educated guess at when it's going to hit apogee. If there's an altimeter in there, I think it's best to let that handle the apogee separation, and leave the motor with a long delay as a backup.

Many times electronics are the only way. For some flights the motor delay may not be long enough like with a long burn. If you leave the motor charge in, you'll risk a healthy zipper. Best altimeter charge only. Timers are an options too if you are certain about apogee.


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dixontj93060 said:

It's the other way around. I dispose of my motor ejection and use electronic ejection 95% of the time. It's more reliable and predictable.

Click to expand...

+1

I have seen both long and short delays over the years. It's the short ones that ruin your day when using as a back up. After a while you get to trust your electronics. If you don't, use a back up altimeter. I trust that more than the motor delay. I used to use motor delay for back up but hardly ever do any more. I do tend to fly bigger stuff though.

It's a good idea to keep it if: you aren't confident your altimeter is set up correctly or using it for the first time.

If you are going to use in combination with the motor ejection charge: leave the delay long or only shorten it slightly.
Having 2 charges going off at the same time is not always a good thing.
You can also gain valuable info in the timing needed for a given rocket by doing so.

JD

I never use motor delay charges on my DD rockets. The altimeter charges are just so much more reliable then the motor delays, not only in timing, but in likelihood of actually going off. I've had a lot more motor deploy rockets never deploy then I ever had DD rocket have problems.

I only had two flights where the drogue charge failed. The first was a battery issue I'll never repeat, it lawndarted. The second one was an ematch issue and the main charge failed also. Both for the same reason, the ematch bridge wires burned through and never fired off the matches. They read about 1 ohm before flight and open after. Besides the fact that two commercial matches from different manufactures failed on the same flight, the most amazing thing is the rocket fell flat from over 4000 ft and was undamaged.

I've always left the moter eject charge in, IMHO I think its a bad idea to leave the full delay. I still calculate the delay but add a couple seconds. That way if the electronics fail, the rocket isnt coming in ballistic for 5 or 6 (or longer depending on the delay) seconds leading to damage.

Lots of good info on this post, since I'm new to DD.

TRF is such a great forum!

I have never used motor ejection as backup for the reasons you mentioned.

I have flown a lot of dual deploys from g motors up.


Mark Koelsch
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Currently, the total is 7-6 ish, motor backup - pure electronic.

gerbs4me said:

Lots of good info on this post, since I'm new to DD.

TRF is such a great forum!

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Ditto on both counts- I learned from your question too!

pure electronics here AT or Loki loads , powder isn't added that's the way I go not long enough delay anyway for backup

dgreger said:

I've always left the moter eject charge in, IMHO I think its a bad idea to leave the full delay. I still calculate the delay but add a couple seconds. That way if the electronics fail, the rocket isnt coming in ballistic for 5 or 6 (or longer depending on the delay) seconds leading to damage.

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I also follow this school of thought, but more and more my rockets exceed the maximum delay available from motor manufacturers. I have had two "saves" when my altimeter malfunctioned, one came in drogueless and still landed undamaged. Gotta love fiberglass rockets! Most of my flights are DD, and in the last year I have had zero failures using Stratologgers and CJ's lighters. I also have an Eggtimer with one good DD flight, and use RRC2+ for redundant systems in two rockets. The two redundant systems are in bigger rockets that don't fly motors with delays.

I always run the motor eject on my DD, I run them a second or two long and mostly rely on the altimeter to eject at the proper altitude.


TA

I've never used motor ejection as a backup, mostly because I prefer the "backwards ejection" design which precludes it. There's nothing wrong with taking advantage of another source of redundancy, but I wouldn't distort the design to do so.

RocketDude77 said:

As the title states: In dual-deployment, how often is one deployment charge used in combination with motor ejection when using H-J motors, particularly the lower-midrange of that spectrum?

It just seems to me that adding a "backup" charge to the very reliable motor ejection system seems overtly complicated, and just another potential failure point (premature drogue ejection, etc.)


Ideas?



Thanks!

Click to expand...

My Dark star is set up using motor ejection as a back up for the drogue. This maybe saved my rocket from a main parachute shred on it's last flight, due to a broken wire on the drogue charge.

JohnCoker said:

I've never used motor ejection as a backup, mostly because I prefer the "backwards ejection" design which precludes it. There's nothing wrong with taking advantage of another source of redundancy, but I wouldn't distort the design to do so.

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By backwards do you mean drogue forward and main aft? I have been wanting to do this but have yet to try, one potential problem being the drogue drag helping premature separation.

watermelonman said:

By backwards do you mean drogue forward and main aft?

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No, I mean that the coupler is on the fincan section, so there is less chance of a zipper which means there is no path for the ejection charge to reach the drogue bay.

I use motor ejection for the drogue all the time with MPR DD builds, it makes things less complicated. You can put the DD altimeter in the nose cone (along with a tracker), and you only need to set it up for the main. You don't even need a "real" AV bay, just a coupler that's blocked off with bulkplates works fine.

JohnCoker said:

No, I mean that the coupler is on the fincan section, so there is less chance of a zipper which means there is no path for the ejection charge to reach the drogue bay.

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Motor ejection can be used in combination with a zipperless coupler. The holes in the coupler (to pass the ejection gases through) can be smaller than you might think. Here's the well-seasoned write-up of the concept.

https://www.info-central.org/?article=132

Jim

In rockets where the motor delay is longer than the simmed flight to apogee, I generally drill my delay to 1-2 seconds after apogee and use it as a backup to the DD. As many people stated above, motor delay isn't accurate. You may drill to 7 seconds, but it may take 9, depending on how well the delay charge ignited when the motor fired. Or it might only take 5. Then apogee charge of an altimeter will fire at apogee far more accurately than a motor delay. I've even gone to using an altimeter to fire a single-deployment ejection for rockets where significant damage could happen if the ejection fired late.

The other problem with motor ejection is that if you are firing a high performance rocket, you may not have enough of a motor delay. For instance, my Mad Cow Arcas, when flying on a CTI J530 will not reach apogee until 21 seconds after launch. The longest delay charge of the J530 is 15 seconds. If I use the motor ejection, the rocket will separate at about 150 MPH. Not a good thing. In those instances I remove the motor charge altogether and backup the altimeter with a timer, set to deploy at about 1 second after apogee.