DD with back-up altimeter ... question.

[Bat-mite]

Hi, all.

When doing DD with two altimeters, which I have never done, I have a question.

Say your rocket needs 1.3g of BP at each end to deploy main and drogue respectively. If you are using two altimeters, how much BP do you put in each charge well? Or, do you just use one charge well with two e-matches in it?

If I'm not making sense, try this--if both altimeters detect apogee at the same time and both ignite their matches at the same time, and if you have each match in its own charge well, and if you have 1.3g of BP in each well, then you end up with a charge that is twice what you need, no?

But if you put half in each and only one detonates, then you risk not getting separation, correct?

So ... do you put one charge well at each end with two matches in each one?

Thanks.

 

[wfcook]

Typically you would have two charge wells and two starters at each end, but you would set the charges to go off at different times. My Torrent uses a Raven3 as the primary altimeter and it is set to fire the drogue charge at apogee and the Main charge at approx 500 ft. I use a Stratologger as the backup (I figure that by using two different altimeters I lower the risk that I'll make the same configuration mistake twice) and I have it set to fire the drogue charge at Apogee-plus-one-second and it is set to fire the Main charge at about 300 ft (I think).

I tend to use a slightly higher amount of BP in the backup charges under the assumption that if the primary charge didn't do the job, the secondary charge might need a little more oomph. This allows me to back the primary charge down to a bit, stressing the rocket less.

 

[matthewdlaudato]

Presumably if you are fully redundant (2 altimeters, 4 charge wells/charges, 4 ematches) you would could set the backup altimeter to use a delay (say, apogee-100ft) so that the airframe doesn't get seriously over pressurized. If you were partially redundant (2 altimeters, 2 charge wells/charges, 4 ematches) you could safely set both altimeters to the same altitude for both events - whoever wins will light the charge.

 

[Bat-mite]

Thanks.

Curious if anyone has tried it with one e-match form each altimeter going into the same well....

 

[matthewdlaudato]

I have not tried that - and I suspect that I won't. My next big build will be for an L3 attempt and it will be fully redundant - don't want to risk a leaky charge canister causing a very dangerous lawn dart.

 

[mikec]

Curious if anyone has tried it with one e-match form each altimeter going into the same well....

Yes. I flew my L3 this way, works fine. The only thing you are protecting with using completely redundant changes is your BP leaking out or something.

 

[Wizard]

Yes. I flew my L3 this way, works fine. The only thing you are protecting with using completely redundant changes is your BP leaking out or something.

+1
Ground test thoroughly to ensure proper charge sizing, and secure the charges in the rocket. If these criteria are met, then backup charges really only stress the recovery system more than necessary.

David

 

[azzie]

At ejection event, pressurization lasts for a fraction of a second. If you are using two altimeters from different manufacturers, then their sensors, math and logic are very different, and it is highly unlikely they will fire at the same instant.

If your rocket requires 1.3g BP, and at least one altimeter is configurable, then safest setup would be:

1) At apogee, altimeter #1 fires 1.3g charge
2) At apogee + 1 second, altimeter #2 fires over-sized 1.6g or larger charge (blow it out or blow it up but prevent rocket from going ballistic)

Whatever you decide, I would go with two separate charges at apogee. If you invested enough in rocket, electronics, etc, to bother with redundancy, then don't compromise it.

 

[matthewdlaudato]

+1
Ground test thoroughly to ensure proper charge sizing, and secure the charges in the rocket. If these criteria are met, then backup charges really only stress the recovery system more than necessary.

David

Stress in what way? As in, risk of simultaneous firing and over-pressurization? Or complexity of setup risk? Or both? Quite a few differing opinions on this topic (which never happens on TRF!)

 

[Fdog]

If I am going the road of redundant altimeters, then why go only part way? Hence, I fly:

• 2 Altimeters
• 4 ematches
• 4 charges

I find the backup charge size by:

1. Ground test complete flight configuration (parachutes, blankets, etc)
2. Arrive at correct charge size
3. 3 trials at above
4. Back-calculate to determine pressure generated by correct charge size
5. add 20% to pressure, or an "extra shear pin" of force, whichever is larger
6. =backup charge size

More than once the larger backup charge size has saved me, as it clears out a jammed parachute or tilted shear pin.


All the best, James

 

[wfcook]

If I am going the road of redundant altimeters, then why go only part way? Hence, I fly:

• 2 Altimeters
• 4 ematches
• 4 charges

I find the backup charge size by:

1. Ground test complete flight configuration (parachutes, blankets, etc)
2. Arrive at correct charge size
3. 3 trials at above
4. Back-calculate to determine pressure generated by correct charge size
5. add 20% to pressure, or an "extra shear pin" of force, whichever is larger
6. =backup charge size

More than once the larger backup charge size has saved me, as it clears out a jammed parachute or tilted shear pin.


All the best, James

+1 on all this.

 

[Wizard]

Stress in what way? As in, risk of simultaneous firing and over-pressurization? Or complexity of setup risk? Or both? Quite a few differing opinions on this topic (which never happens on TRF!)

The additional hot particles from the charge are hard on the shock cord in particular (kevlar is heat-resistant, but not heat-proof) or its protective sleeve. In addition, there is that much more soot/residue in the system.

Now, in full disclosure, I equip all DD birds with dual charge wells, and for the first flight of most of my DD birds I will use a backup charge, usually +15% BP, just in case my ground tests are not representative of flight conditions. If the primary charge would fail to deploy properly (hasn't happened yet but it probably will eventually, most likely through my error), then I'll revisit the setup. Otherwise if deployment was fully successful, I'll switch to a single charge for subsequent flights.

Side note: when ground testing, if you have a rail button that protrudes slightly into the rocket (as some designs do), test in several orientations to ensure the parachute doesn't hang up on it in any of them.

There are trade-offs to any setup, and different conditions may lead to different choices.

David