Can you set up two ejection charges with dual deploy?

[LivingThing]

Finally taking the step into dual deploy after my L1 and came across a plugged engine. Wondering if I could have two ejection charges (one on the head end of the AV bay and one on the bottom) and successfully fly and land without the engine having an ejection charge. Assuming I have the right electronics of course

 

[soupmanrmc]

Yes you can. Make sure to size the charges appropriately and TEST the system before flight. TRF and many others have good tutorials on dual deploy.

 

[cerving]

Normally that is what you do with dual-deploy. It's usually better NOT to use motor deploy with electronic deployment... if you want redundancy, use a second altimeter. The maximum delay for many HPR motors is often way too short anyway. I almost always leave out the BP on motors with dual deploy.

 

[kjhambrick]

One thing I can add is a ink to @cerving's EGGTIMER EZ-DD ROCKET/ALTIMETER SUPPORT page.

LOTS of good stuff there !!

-- kjh

 

[jahall4]

Normally that is what you do with dual-deploy. It's usually better NOT to use motor deploy with electronic deployment... if you want redundancy, use a second altimeter. The maximum delay for many HPR motors is often way too short anyway. I almost always leave out the BP on motors with dual deploy.

... or way too long. Anyone see something wrong in in this video, in regard to ejection charges?

 

[Titan II]

Whether I use single or dual deploy, I always leave the motor charge in if the maximum delay is at least a couple seconds longer than the optimal delay per simulations.

 

[drewnickel]

Finally taking the step into dual deploy after my L1 and came across a plugged engine. Wondering if I could have two ejection charges (one on the head end of the AV bay and one on the bottom) and successfully fly and land without the engine having an ejection charge. Assuming I have the right electronics of course

Dual deploy electronics don't require any motor ejection charge. For hybrid motors for example (basically all I fly), none of them have motor ejection charges built in, so all hybrids are flown with entirely electronic deployment.

For solid motors though, I like the redundancy of having motor eject as a backup. Drogue in the bay exposed to the motor ejection charge so that you can deploy at least the drogue if the electronics fail, also because typically the parachute in the lower bay would be pulled out by the upper half of the rocket upon ejection it helps for that chute to be smaller. Main in the front end so it gets blown out with the nosecone and also so that the heavier of the two chutes is up front (to help with cg).

In short, yes you can absolutely fly without the motor eject. If you have the space / funding for a second altimeter, redundancy is always good to have and is the only thing you really lose flying without the motor ejection charge.

 

[Jim Green]

If your electronics support it set the backup charges to go off a second or 2 after apogee and a lower altitude for main. That way they don't go off at the same time as the primary. Also the backup charges should be 25% larger than the primary electronics charges.

Jim

 

[Back_at_it]

I'm just getting into DD and I had a very similar question and answers I got were "it depends the rocket and motor".

Using my LOC Warlock with an added payload bay as an example with an I357-14. The recommendation was to use the altimeter to pop a drogue chute at apogee then set the second charge to deploy the main around 500ft which should occur around 14 to 15 total seconds into the flight. If for some reason the second charge failed, the motor ejection would fire around 16 total seconds into the flight as a back up.

If using the DD on something smaller you could use the motor ejection to deploy a drogue then have the altimeter to pop the main at a much lower altitude. My first ever DD flight was using an extended 3.0" BMS School Rocket. I flew it on a G74-9 to around 1500ft. The motor ejection split the rocket about a second after apogee deploying a small chute. The altimeter was set to fire the charge at 500ft. which deployed the main chute. This worked perfectly.

This second scenario is how I see myself using DD as I rarely fly large rockets.

 

[drewnickel]

I'm just getting into DD and I had a very similar question and answers I got were "it depends the rocket and motor".

Using my LOC Warlock with an added payload bay as an example with an I357-14. The recommendation was to use the altimeter to pop a drogue chute at apogee then set the second charge to deploy the main around 500ft which should occur around 14 to 15 total seconds into the flight. If for some reason the second charge failed, the motor ejection would fire around 16 total seconds into the flight as a back up.

If using the DD on something smaller you could use the motor ejection to deploy a drogue then have the altimeter to pop the main at a much lower altitude. My first ever DD flight was using an extended 3.0" BMS School Rocket. I flew it on a G74-9 to around 1500ft. The motor ejection split the rocket about a second after apogee deploying a small chute. The altimeter was set to fire the charge at 500ft. which deployed the main chute. This worked perfectly.

This second scenario is how I see myself using DD as I rarely fly large rockets.

One of the issues with using motor eject for the main backup deployment charge would be, if you didn't at least separate the rocket and it's coming in fast, when you deploy the main you could shred it or rip the recovery harness and then have pieces of the rocket in free fall. Using motor eject on the drogue, you can accommodate flights with up to ~14 or so second flights to apogee and you know the rocket will at least have a controlled descent under drogue if the main doesn't deploy. 50-100 ft/s ground impact with the rocket tumbling is significantly better than terminal velocity without a parachute and the rocket remaining stable.

 

[Back_at_it]

One of the issues with using motor eject for the main backup deployment charge would be, if you didn't at least separate the rocket and it's coming in fast, when you deploy the main you could shred it or rip the recovery harness and then have pieces of the rocket in free fall. Using motor eject on the drogue, you can accommodate flights with up to ~14 or so second flights to apogee and you know the rocket will at least have a controlled descent under drogue if the main doesn't deploy. 50-100 ft/s ground impact with the rocket tumbling is significantly better than terminal velocity without a parachute and the rocket remaining stable.

Totally agree but I feel like shredding or zippering the rocket is a better alternative to having it come in ballistic. I'd rather have a safe recovery but anything to slow it down is good.

I've seen a lot of very close calls with failed ejections including one that missed me by less than 10ft at mid-west power a couple of years ago. RSO lost track of it and never called for heads up. a 10ft fiberglass rocket buried itself in the dirt 10 feet from me.

 

[RokitFlyr]

Finally taking the step into dual deploy after my L1 and came across a plugged engine. Wondering if I could have two ejection charges (one on the head end of the AV bay and one on the bottom) and successfully fly and land without the engine having an ejection charge. Assuming I have the right electronics of course

That IS dual deployment.

 

[soupmanrmc]

When starting DD, I had motor eject about 2 seconds after expected apogee. This backed up the drogue, though drogue charges went off without a hitch. Probably will just plug w/ dog barf and tape it going forward.