Twisting Failure

[mzhengxi]

My team and I are worried about a potential twisting failure where the connection between the NC and the rest of the body twists around the drogue or main and strangles the riser. Does anyone have some ideas about how to mitigate this issue? Also, we are using 2 tender descenders in parallel to release the main. Does anyone know if AARDs are more effective or may help with the twisting problem.

 

[Steve Shannon]

Please explain the use of tender descenders “in parallel”. If you really put them in parallel, if either fails to open, your main doesn’t open. They should be in series (daisy chained) so if either one works the main can open.

 

[mpitfield]

Based on the info provided I am not sure if an ARRD would mitigate the twisting risk. However I can tell you that getting the assembly of an ARRD correct, which is where some fail, is a degree or two more difficult than the Tender Descender.

 

[mzhengxi]

Please explain the use of tender descenders “in parallel”. If you really put them in parallel, if either fails to open, your main doesn’t open. They should be in series (daisy chained) so if either one works the main can open.

We have it set up for redundancy- either one going off will allow the main to open.

We've already tested and flew this setup before.

 

[Steve Shannon]

We have it set up for redundancy- either one going off will allow the main to open.

We've already tested and flew this setup before.

Ok, thanks. That diagram helps.

 

[mzhengxi]

For more info: we use a piston system to push out the drogue/ divide the body from the nose cone. The issue is that before or after we trigger the tender descenders to release the main, the cord connecting the body tube and the nose cone twists around the drogue or main risers, fully or partially preventing the main from opening. Any ideas about how to mitigate this? We also found that there is twisting just from us integrating the rocket/putting all the parts together.

 

[dhbarr]

Do you have a swivel inline?

 

[mzhengxi]

Do you have a swivel inline?

I believe we have swivels attached to both of the eyebolts. There is one eyebolt in the body tube section and then another in the nose cone section. The drogue and main are attached to the swivel in the nosecone section.

 

[Handeman]

First, are you asking about wrapping or twisting? Those are two different things. By body tube section are you talking about the fin can, or the payload section?

Your diagram shows the line from the fin can wrapping around the line holding the drogue. If that is what is happening, I assume the fin can is spinning in circles around the payload section and that is causing the wrapping. This is unusual because usually both will spin in a circle together and one won't wrap around the other. I would try a larger drogue to ensure more separation between the payload and the fincan. Keep the line between them taught and that should prevent the wrapping.

If the tangling is caused because the line between the fincan and payload is being twisted excessively and the other line is being caught up in the twists, then a swivel on the fin can is probably your best solution. My experience is that only ball bearing swivels work well, the non-ball bearing types just don't work. I have one rocket where the fincan will spin at near 100 rpm on the way down. With a good ball bearing swivel on the fincan, there is no twisting of the tubular nylon shock cord.

 

[mzhengxi]

First, are you asking about wrapping or twisting? Those are two different things. By body tube section are you talking about the fin can, or the payload section?

Your diagram shows the line from the fin can wrapping around the line holding the drogue. If that is what is happening, I assume the fin can is spinning in circles around the payload section and that is causing the wrapping. This is unusual because usually both will spin in a circle together and one won't wrap around the other. I would try a larger drogue to ensure more separation between the payload and the fincan. Keep the line between them taught and that should prevent the wrapping.

If the tangling is caused because the line between the fincan and payload is being twisted excessively and the other line is being caught up in the twists, then a swivel on the fin can is probably your best solution. My experience is that only ball bearing swivels work well, the non-ball bearing types just don't work. I have one rocket where the fincan will spin at near 100 rpm on the way down. With a good ball bearing swivel on the fincan, there is no twisting of the tubular nylon shock cord.

Sorry, I suppose it is more of a wrapping issue around the cord between the fin can and the nose cone. Thank you for your suggestions!

 

[mikec]

Making the drogue large enough so that the main body of the rocket is guaranteed to hang below the forward section during descent will help. In all honesty these sorts of failure modes are hard to mitigate with tether-like systems -- at least, if there's a way to do it I don't know it.