Ejection charges for 7" diameter rockets

[Chad]

I acquired a large 7" rocket from a DARS.org meetup, I believe the rocket was originally owned by a member of DARS who had passed away and then given to the person I got it from. I would really like to fly it at a DARS launch this year for the club. I plan to outfit it with my standard dual stratologger setup but the volume inside the rocket is huge (i'm use to 3 or 4" rockets). For larger rockets, is it standard practice to add a bulkhead near the couplers to reduce the amount of space an ejection charge needs to pressurize and separate the airframe pieces?

 

[chilled]

In my limited experience, I can't say I've seen this so I'd assume this isn't standard practice. I'm also not sure I'm following how this would work in practice. Then again I have only built one "larger" rocket. For my 6" project I went about BP charges as per usual.. Estimating using an online calculator and then ground tested. Since the amount of BP was reasonable (about 5g), I wasn't too worried and just went with that.

 

[chilled]

I should add, sorry I can't be more help. Perhaps you can share a bit more detail? Also, are you trying to save on BP or is there some other reason for this approach? I'm curious

 

[rocketace]

I don't used a bulkhead to reduce space. In my 6-8" diameter rockets I use around 4-8g of powder depending on volume and shear pin set up. Just run some BP calculations, ground test, and adjust from there. Now on rockets this size, I do prefer to use a deployment bag with a pilot chute for the main.

 

[Bat-mite]

Six of one, half-dozen of the other. Use a good calculator to get a starting BP for your chamber size. If it is HUGE and you don't want to burn that much BP per flight, consider a chute shelf. My 7.5" and 8" rockets use anywhere from 4.x CC to 6.x CC, not ridiculous.

 

[Bat-mite]

In your ground testing, be sure your charge can break your shear pins. Don't skimp on pins! You really don't want your NC out at apogee. I use three #8 screws in each tube.

 

[PSLimo]

Hi Chad,

I fly 7.5" rockets and have never needed more than 3 grams of 4F to shear 256 or 440 pins. More can't hurt unless it blows up, lol.

Here's what 5 grams of 4F does in an 11.5" diameter build. My counts off in the video but that's a 10lb, 11.5" x 48" nosecone.
View attachment 361116-fdc377c5038ef52d925f973b83d6fabb.mp4

 

[SDramstad]

I do try to minimize the volume that needs to be pressurized with a bulkhead as far forward as is practical. It also helps keep the chute from sliding aft and changing the CG in flight.

 

[manixFan]

I do try to minimize the volume that needs to be pressurized with a bulkhead as far forward as is practical. It also helps keep the chute from sliding aft and changing the CG in flight.

I think this is often overlooked in big rockets. The same happens when the apogee charge fires. If the chute has slid all the way to the back of the compartment during acceleration, when the apogee charge fires the chute can then slide forward with considerable force and act like a piston on the nosecone, increasing the likelihood the main may deploy at top. When you consider how heavy a big chute is, it just seems prudent to limit its motion as much as possible in a big rocket.


Tony

 

[mtnmanak]

I have not seen a separate bulkhead used as a technique, but the two techniques I have seen (and used) for larger diameter rockets are "chute canons" and pistons. It may be difficult to retro-fit a rocket for either of those and I have never tried to do that. Personally, I am a fan of pistons to reduce the amount of BP needed, but a lot of people will tell you pistons can be problematic. I think they work well, but they put a premium on required maintenance to ensure they continue to work right. Also, your payload bay needs to be clear of obstacles like protruding screws, bolts, etc.

Having said that, as people have noted, the amount of BP needed for a 7" tube is not egregious.

 

[cwbullet]

I recommend you use a calculator and always ground test.

 

[bschultz32]

One gram of FFFFG per 200 cubic inches of volume. The volume of a cylinder is its radius squared, times 3.14, times its length, all dimensions in inches. This simple rule of thumb has never failed me.
Bob Schultz