Dog barf in HPR?

So I'm finally planning my L1/2/3 flights and having a close look at what I need. My L2 flight will have a 4" airframe, and I realised I've got a big hole to fill or my laundry will be sitting at the bottom of a long tube. This would change my CG calculations in OR. In LPR/MPR, I'd fill the tube with dog barf, throw the laundry on top and away I went. I'm a 1000 miles from my nearest TRA club, so I can't wander over and look at what others are doing. So, do folks let the laundry sit way down there on top of the motor, or do they fill the tube to keep it forward and toward the top of the tube?

Used dog barf and put the chute on top as far forward as you can.

I always use nomex chute protectors for my HPR builds. They are put between the ejection charge and chute to protect the chute from melting. I really like the ones Top Flight sells https://topflightrecoveryllc.homestead.com/page3.html I realize that you can get a bale of dog barf for cheap, but I don't like the idea of littering the launch site, especially in the large amounts you would need to fill a 4" airframe. Given that the chute protectors are reusable, they are a good investment.

Until you get a parachute protector, go right ahead and use Dog Barf insulation. It's pretty environment friendly. Since ejection charges are something that LPR/MPR/HPR share and they are basically the same, other than size.

I used both a nomex chute protector and dog barf on my level 3. If was 10 inches in diameter.

Nomex only from now on for me!!!!

If I understand your original question, you are asking if you should fill the body tube with dog barf for the purpose of keeping the chute as far forward as possible. Are you are concerned about the CG being too far aft? Does your design require that weight to be just below the nose cone for stability?

Hi guys. Mainly concerned about my chute/shock cords etc tangling around the top of the motor and airframe, as well as everything sitting so far back. My L2 flight will likely be a J350W, so lot's of empty space in the airframe for an 18" drogue/cord/nomex to slip down the gaps. Interestingly, almost all the rocsim files I've looked at have the drogue up close to the avbay. So should my question really be: Should I alter my simulations to place the drogue back against the top of the motor?

I'm keenly aware I'm probably overthinking this. :grin:

Personally, I prefer designs that are volume filled so things cannot move around during handling or flight. I've always figured that if stuff can move, stuff may not end up the way you want it...

Is there an option to remove unneeded tube length?

Or...

When I made a Mad Dog dual deploy, I put an extra bulkhead between the motor region and the electronics bay. This consisted of a lower ring which would pass a 54mm motor, an upper solid bulkhead, and a couple inch segment of coupler to tie them together. This assembly holds the lower airframe forged eyebolt or U-bolt. This assembly is then bolted through the airframe to hold it in place solidly. But, it is also readily removable if desired. It worked well at removing the excess space. Deployment was electronic for drogue, not motor. Since the lower motor section was essentially sealed, any motor ejection charge had to be removed which was no big deal. The reduction in volume above the bulkhead meant less BP was needed for ejection.

Gerald

matthew said:

Hi guys. Mainly concerned about my chute/shock cords etc tangling around the top of the motor and airframe, as well as everything sitting so far back. My L2 flight will likely be a J350W, so lot's of empty space in the airframe for an 18" drogue/cord/nomex to slip down the gaps. Interestingly, almost all the rocsim files I've looked at have the drogue up close to the avbay. So should my question really be: Should I alter my simulations to place the drogue back against the top of the motor?

I'm keenly aware I'm probably overthinking this. :grin:

Click to expand...

Absolutely overthinking this!!!!! The weight of dog barf, condensed enough to prevent a chute from sliding toward the motor during thrust will weight as much or more then the recovery! The small difference in position of the recovery gear on a 8 lb rocket is not going to have a significant effect on CG. If the effect is enough to make or break the flight, you're way to close to the limits already!

I guess I assume L2 flights will be dual deploy, probably my bad, I started DD with my L1 cert flight and have never done a L2 without DD. I would put 4" - 6" of dog barf between the chute and the ejection charge in a 4" dia. tube and call it good. Or do a chute protector and just a little dog barf. I've got several chute protectors with nickle size holes because I didn't use a little dog barf.

If you're really worried, check the sim with the gear on the motor and at the top of the tube. I will expect there will not be a significant change in CG. Not enough to warrant jumping through any hoops to compensate for it.

Good luck and DON'T over think it!

Handeman is absolutely correct.

Just using my own experience as an example, my L1 rocket was topped with a long payload section, which was joined to the main airframe with a long(ish) coupler (which will be the e-bay when I finally get an altimeter). This left around 8"-10" of airframe immediately forward of the motor in which to house the shock cord and 36" parachute. So the recovery system was housed aft of the rocket's midpoint, in the space just forward of the motor. (Granted, my airframe was much narrower -- 2.25" as opposed to your 4".) The flight went off without a hitch, nice and straight with perfect deployment and recovery.

I used a Nomex protector and nothing else to shield the chute. I typically use Nomex in my mid-power rockets as well. If you have a decent length of Kevlar for the shock cord then you will normally insert that first so that it will sit in the rocket below the parachute and will help to take up some of the volume.

Handeman is right -- unless you are using a large, heavy-duty BFR-type parachute, the parachute in an H or I powered rocket won't have enough mass relative to the rest of the rocket to have any critical effect on the CG if it shifts rearward during acceleration. So this shouldn't be of any concern if your rocket has a normal margin of stability.

You can, of course, check this prior to your anticipated launch date by fully loading the rocket as it would be when it is on the pad, with the recovery device all the way back and sitting on top of the motor mount. Stuff a ziplock bag containing an equivalent amount of weight as the motor you plan to use into the mount to take the place of the motor, and then check the location of the balance point. If it is indeed too far back then you can trim the balance by adding a small amount of weight to the forward end of the rocket. Once you have it correctly trimmed in this worst case scenario you will no longer have to worry about this issue.