Ejection Charge for 3" Spitfire

[Wiley]

My 1.5X upscale ACME Spitfire is complete, and my Topflight 36" thin mil chute came in the mail today. The chute is great quality and very thin, but it still wouldn't pack into the nose cone with the short piece of shock cord I had planned on using. The only way it will fit is to tie the shroud lines directly to the eyebolt in the top centering ring (where you would put the clay weight in the kit), fold the chute tightly, and wrap it firmly with the shroud lines. These measures, combined with literally cramming the chute into the NC, allowed the NC to fully seat. In this design, the NC recovers seperately, so this 36" chute is only recovering the 35 ounce airframe. I'll post a pic in a few hours. Have any of you ever had a rocket fly successfully where they used no shock cord at all?

 

[cbrarick]

Smaller chutes are in order. I bet the airframe weighs less then 1.75 pounds, so topflight gives it a 24 inch chute. Even if you have the fulll 2.2 pounds, you only need a 30 incher. I'd use their "thin mil" chutes as they pack smaller. There are other chutes that are even smaller for the weight. Karen's chutes at CATO-Chutes.com are true hemispherical chutes and I'd not be afraid to use a 21 because they really have a lot of loft and are tough. Also, might want to have someone watch how you fold and pack your chutes to see if there is a better method.

good luck

 

[Wiley]

Thanks, but my question was more specifically about the lack of a shock cord. The airframe weighs 35 ounces, so it needs the 36" for a comfortable descent. The only thing I can think of that would make this possible is the fact that the parachute is wrapped with the shroud lines. Please note that a shock cord of normal length for this size rocket simply will not fit. Take a look at this link: https://www.nerocketry.org/newsletters/2006/Summer2006.pdf. On p. 7 there's a pic of Dennis Watkins' Spitfire which uses the same recovery mechanism as mine. Note the short length of the shock cord in the inset. It's only a couple feet long, so would it be detrimental to eliminate it entirely?

 

[Handeman]

I agree with cbrarick, a smaller chute is in order, unless you're flying out west on concrete like grounds. I fly @ Battlepark, and I usually use 24" chutes on 36 - 46 oz rockets. My BD Thug came with a 24" chute that has a 2" spill hole. It's always worked great. I cracked a fin on the Thug once and that was a calm day when I used a 30" chute.

A 24" hemispherical or elliptical chute will work just fine.

Remember, the descent calculators usually use 0.75 for a Cd which is fine for chutes made from flat sheets. The hemi or elliptical chute can have a Cd as high as 1.5. The same slow decent with smaller chutes. The hemi and ellipticals also use less material for the same diameters so they pack smaller.

 

[Wiley]

Note: I added pics in post #3

 

[xenon]

Your setup looks pretty reasonable as long as the parachute won't get stuck in the nose cone and not deploy. It seems pretty unlikely to increase the chances of tangling and despite the name, a short shock cord doesn't really do much for shock absorption unless you are using elastic or something.

Source: I had a 5.5" diameter two stager a few years back that had a Rocketman R12 in a bag in the booster. I used a small pilot that pulled the bag and held the interstage above the canopy, but nothing but a quick link between the parachute and recovery eye nut. I've also flown many projects with similar setups (1 parachute on 1 piece) with just enough shock cord to get the parachute risers out of the tube.

 

[Wiley]

Xenon,

Thanks, that's exactly what I needed! The setup is designed so that, when the ejection charge fires, the NC (which weighs 590g!!!) gets blown off with enough force that it yanks the chute out. Note that once the chute is rolled up and wrapped with the shroud lines, the NC can only move about 4" before it hits the end of the shroud lines and starts to pull the chute out. I'll post vids of the upcoming ejection charge test. Thanks again!

 

[ckreef]

Watching this one.

 

[tstone]

Thanks, but my question was more specifically about the lack of a shock cord. The airframe weighs 35 ounces, so it needs the 36" for a comfortable descent. The only thing I can think of that would make this possible is the fact that the parachute is wrapped with the shroud lines. Please note that a shock cord of normal length for this size rocket simply will not fit. Take a look at this link: https://www.nerocketry.org/newsletters/2006/Summer2006.pdf. On p. 7 there's a pic of Dennis Watkins' Spitfire which uses the same recovery mechanism as mine. Note the short length of the shock cord in the inset. It's only a couple feet long, so would it be detrimental to eliminate it entirely?

View attachment 119616 View attachment 119617 View attachment 119618

Did a tractor run over that thing? I think the ground will simply repel it.

 

[kjohnson]

DOH. Looked closer at the space you have for the chutes, so you might not have space to do what I put in below- which I have used on many two part recovery models.
--
I think you'll be fine without the shock cord. Pack the parachute for the nose in first, then the chute for the booster, so that it pulls the booster chute out on
separation. You shouldn't see shock loads very high in this setup.

kj

 

[davdue]

]You say you are using a 36" top flight chute but there descent tables show that for 35 oz = 2.1875 lb. it should be a 30" parachute (see link below). I have found that top flight chutes perform better than the tables say. I would give them a call. They are very helpful and he will give you the cd for the chute so you can put it into OR or RocSim and see what the descent rate would be. As long as it is in the 15-20 ft/s range I wouldn't be worried at all unless you are going to have a chance of landing on concrete or something hard like that.

https://topflightrecoveryllc.homestead.com/descent_rates.html

 

[Wiley]

DOH. Looked closer at the space you have for the chutes, so you might not have space to do what I put in below- which I have used on many two part recovery models.
--
I think you'll be fine without the shock cord. Pack the parachute for the nose in first, then the chute for the booster, so that it pulls the booster chute out on
separation. You shouldn't see shock loads very high in this setup.

kj

Its tight, but it does fit. I left the whole setup packed overnight, took it out on the lawn this morning, turned it upside down and gave it a good shake. The NC fell off and pulled the chute out easily with its own weight. I then started off at a fast walk with the airframe, and the chute, which had been crammed into the NC all night, unrolled and unfurled very neatly. I'm pretty confident that this will work.

Here's a pic of it all put together.

 

[ckreef]

Looking forward to the final finishing scheme

 

[Wiley]

What size ejection charge would be a good place to start? I know that there are ejection charge calculators out there, but this rocket is a little bit different. It has a 29mm tube 25" long that needs to be pressurized hard enough to blow of a 590 gram nose cone. Oh, and it has a piston machined from solid oak in the 29mm tube. My 4" Patriot used a full gram to deploy a payload section/NC weighing over 1#, so would .7g be about right for the first test?

 

[Coop]

Typically, I start with the stock charge for the motor I plan on using, divide it by two, and use that as my starting point, adjusting accordingly from observed results. 0.7g sounds like it'd get you in the right ballpark --and if not, it's a good place to start the ground testing...


Later!

--Coop