Post Crash Help

The Rocketry Forum

Help Support The Rocketry Forum:

lovs3fly

Member
Joined
Jun 26, 2004
Messages
20
Reaction score
0
A little help on the post crash analysis if you will.

Background, a ScottGlass Talon (added 8 inches), weight 6.5 lbs, dual deployment, PML CPR retro, piston on droge, nomex bag main, deployment charge - black powder 1gr.

Flew this Saturday on a I211. Great lift, drouge at appogee, main did not pop at as set (600ft). The charge did fire however.

When crash site was inspected the nose cone was ejected but this may have been cause by contact with ground. There was no black powder risdue around the fracture in the body tube.

My guess is that the chute was too tight in the tube which then created an over pressure (wittness by the risdue trying to make it's way back into the altimeter bay). Can a chute be too tight? It was a skyangle wrapped in nomex.


thanks in advance
Dennis
 

Justin Horne

Well-Known Member
Joined
Jun 23, 2004
Messages
426
Reaction score
0
Yes, from what i've seen, a chute can be too tight in a few ways. First, it can be wrapped too tight in cord. I have seen a great flight, I211, chute right at apogee. Shock cord stretched out... Chute stays wrapped up.. Hits hard.. Second, and i think this is your case, a chute can be too tight in a tube. Building a Hawk mountain Talon 3 (same kit, just fiberglass tubing) I know what you mean. Our kit is a rocketman R9, that, if packed wrong, will not come out of the tube.

The trick is finding the area between fit in tube and wrapping tightness...

It almost sounds to me like you had both too much and too little powder. You had too much powder that it tried hard to get into the bay, but not enough to push the chute out. My solution would be to get the main chute to be looser in the tube and add more BP. oh, and

GROUND TEST!!!

Truer words have never been spoken here on TRF..:)

Well, good luck, the Talon is a great looking kit..:)
 

Ryan S.

Well-Known Member
Joined
Mar 24, 2003
Messages
3,553
Reaction score
0
use more BP, you can never have enough. I take the amount of BP I think is enough then double it. I would rather my rocket blow up then not have a chute come out ;)
 

g_boxwood

Well-Known Member
Joined
Jan 20, 2009
Messages
564
Reaction score
0
Yes it can happen... I'm sorry to hear it but ground-testing should be considered a must when expensive and awesome rockets are taken to the skies for the first time (and after every consistent change in the recovery system setup) ...

When I ground tested my Big Nuke in single deployment setup (before considering dual via ARRD) I packed a R12C in a XL bag (following Rocketman to the letter) which is 6.5" in diameter. Of course the nosecone went off but the chute didn't. I then considered making my own bags sizing down (in diameter) the Rocketman's I got (I couldn't find any nomex at that moment). Just reduce the bag dia so it fits loose in the tube and make it longer so the 'chute will still fit inside it completely. It works. The bag is better vs. the nomex protector 'cauz the 'chute won't expand much more than the bag dia.

Use the correct amount of BP, don't oversize the deployment charge. An unproper setup won't work better just because the charge is greater... and greater charges mean greater stresses on the airframe... Again experiment in a confined and simplified enviroment and then get the beauty airborne with that little extra that put you on the safe side.

Hope this helps,
 

rdbones

Well-Known Member
Joined
Nov 9, 2010
Messages
1,044
Reaction score
3
Since I am still fairly new to HPR - maybe this is an easy question. If you are building a rocket with motor ejection, how can you ground test it ? I have no electronics set up in this rocket.

I have heard that you should reduce the amount of the BP in the ejection charge when using a piston ejection system, and others have said use what the manufacturer puts in the reload kit.

The airframe is 4" diameter and I will be using a 60" military surplus chute and the piston for ejection. I am not planning on using the D-Bag because of the piston. How tight should I wrap the shroud lines and how can I ground test this?

After all it is my cert flight, and I would like everything to go right !
 

g_boxwood

Well-Known Member
Joined
Jan 20, 2009
Messages
564
Reaction score
0
Ground testing a motor based ejection is easy using an empty Aerotech/Dr. Rocket motor case (supposing to have one). Simply feed the e-match wire leads through the forward closure hole all the way out of the case and seal the hole with some high temp silicone (the same used on R/C glow engines instead of the supplied gaskets). BTW I personally use Arexon's Motorsil, it is red, smells horrible and set in about 20 minutes. I use it to 100%seal my charge containers (Safe Eject) as well. Fold the e-match head until it rests in the charge cavity, pour the required BP amount and seal everything down with masking tape as usual. Install the motor, retain it and fire the charge following all the usual precautions. I haven't any experience with pistons but they do require less BP than conventional setup. Follow mfg or consider reading PML FAQ about the matter. One last thing; BP sizing (non-piston) is well covered both on INFOCentral and on EMRR in the calculators page.

Hope this help, feel free to ask again,
 

lovs3fly

Member
Joined
Jun 26, 2004
Messages
20
Reaction score
0
Listening to all the comments I now wonder if I had the correct BP charge. I have always used 1gr but that has been with a piston. In this case I was going without the piston for the main chute. I am also conviced now that the nose cone did in fact come off when the assembly hit the gound and fractured. I wonder, if a larger charge would does the trip. Sounds like a ground test will be the only way to go.

Dennis
 

Missileman

Well-Known Member
Joined
Apr 21, 2011
Messages
786
Reaction score
0
This may sound strange but it is the way I test rockets 4" dia. and under and 38mm and under using motor ejection.
Pack up the chute, install nosecone, wrap your lips around the aft end of the motor tube and blow.
Simple but effective. If you can blow the laundry out with your lungs then standard size charges that come with the motor will work just fine.
 

daveyfire

Piled Higher and Deeper
TRF Lifetime Supporter
Joined
Jan 26, 2009
Messages
3,218
Reaction score
56
Location
thank u, next
On my Arcas (4" rocket with a 24" main chute bay), I use 2.5g as the main charge to blow out an R9C, a lot of Tubular Nylon, and pop two shear pins. I haven't failed to get the main out (yet... knock on wood)! If you're concerned about using too much charge, put the nose cone on a separate chute (perhaps the pilot for your d-bag if you're using one) and allow it to come down separate from the rest of the rocket. No stress is applied to the harnesses that way.
 
Top