Joe Turners L2 Build Thread

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Re: Joe Turners L2 Build Thread

Postby OverTheTop » Sun Feb 04, 2018 2:16 pm

Decided to put in 1.8 grams. But just wasn't enough... (I really did think it would probably be sufficient).


What is the size of your parachute compartment (empty)?
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Re: Joe Turners L2 Build Thread

Postby joeman » Sun Feb 04, 2018 2:22 pm

Hi OTT,
It is 16"

Thinking probably need 4 grams.

Looked at http://www.vernk.com/EjectionChargeSizing.htm

In there, he talks about having to use additional charge for the Scorpion Main parachute.

Granted his tube is a little larger, 20" vs my 16".
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Re: Joe Turners L2 Build Thread

Postby OverTheTop » Sun Feb 04, 2018 2:50 pm

Thanks. 4" diameter, 16" long. I would have thought 1.8g would be enough, but with a tight chute it might need more. Your ground testing will give the answers you need :) .
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Re: Joe Turners L2 Build Thread

Postby mgaertne » Sun Feb 04, 2018 4:24 pm

Hi,
just my 2c on this. A chute nearly never is pushed out by a ejection charge. It is just not tight enough to hold the gases behind it. If you pack very tight it might but then...
As for BP sizing. There is a formula for this.
It all depends on how much pressure you want inside your compartment. Usually 0.5 to 1 bar will be enough but that can be calculated as well.
You need enough force to shoot the nose cone out AND to have the nose cone pull the chute. A 4" nose cone has a surface area inside the chute chamber of approx. 0,0151 m². So, a pressure of 1 bar will result in 0.0151 m² * 100.000 N/m² = 1510 Newton. That will accelerate the nose cone (F = m * a) and eventually the parachute, the air and the shock cord will stop the nc... That seems excessive (that is 150 Kg!)
So, at the moment I will assume that this force is good enough.
In order to generate enough gas to increase the pressure to 2 bar in total (1 bar environment, 1 as calculated above) you need the following formula
Screenshot_20180204_170819.png
Screenshot_20180204_170819.png (4.48 KiB) Viewed 1122 times

R and T are as shown for black powder, the volume V is the length of the chamber times the area of the nose cone, thus V = 0,4064 m * 0.0151 m² = 0,00614 m³. dP is the desired pressure increase in Newton/m²
Put that into the formula and you get
mbp = (100.000 N * 0.00614 KgK)/(119.12 Nm * 1837 K m²) = 0,0028 Kg or 2,8 gramm of black powder...
Don't overdo the amount, it seems pretty much to me... If you have too much than the shock cord will have to stop the nose cone. Since shock cords will not lengthen very much under stress, this will result in a very high force on the cord and all attachment points...
Also, take into account that we made some erroneous assumptions in this calculation!
1. We treated the chamber as to be empty but it isn't. That means the volume V that needs to be filled is actually smaller
2. No chamber is really gas tight
I *assume* that both effects even out so that the amount of bp is quite correct. The rest is up to ground testing (as you did, well done).

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Re: Joe Turners L2 Build Thread

Postby joeman » Sun Feb 04, 2018 5:30 pm

Thanks for your insight.

Yes, I was very mindful of the fact that it is the nose cone that is the work-horse, dragging everything with it.

During my thinking time I did wonder how to work out how much extra ejection charge to put in....
I considered the two points below-

1. The parachute when packed in is about 0.3metres and was thinking that if I could work out the drag force and that work is done over that period of travel - W = F x d. So perhaps I could determine how much additional energy is required to overcome this and still have enough kinetic energy to take the whole parachute out.

2. Another thing that got me thinking was that the parachute is nearly 1kg and the nosecone/coupler/bulkhead is about 700 grams. i.e. it really needs to have quite a bit of kick to drag (accelerate) this heavier item behind it. Something I should be able to solve using conservation of momentum/conservation of energy.

The maths is interesting and I might explore this a little later.

Below is a very short video of the 4gram ejection test.

https://youtu.be/dLMBQVLfCvc

RE: Protection against hot gases
I can see that I will need to replace my sheath. It has done a good job, nevertheless.

For the second test I also put some of this material between the shock-cord z-folds and the ejection charge. Comparing each side of the fabric, I can see that it has taken some of the blow.

Sorry, no pictures yet. Have a tonne to upload and sort through.

Cheers

Joe
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Re: Joe Turners L2 Build Thread

Postby OverTheTop » Sun Feb 04, 2018 5:49 pm

I usually don't take into account any chute or cord in the volume. I just use the empty compartment as a starting point. The actual volume of a chute is not great.
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Re: Joe Turners L2 Build Thread

Postby joeman » Wed Feb 07, 2018 8:29 pm

Did a drogue ejection test today.

https://youtu.be/W06OoFlwtBw

RE: Ejection Sizing
Started with 3.5 grams of ejection charge - expecting it to be enough. (Calcs suggested 2.84 grams, Extreme DarkStar suggest starting with 2 to 3 grams). My experiences with the other parachute made me think that I probably needed a slightly large ejection sizing to start with.

It made quite a "pompft" noise, but the drogue chute/parachute protector rotated a little and 'seized' up inside the tube. Would it have made the distance if it hadn't seized up? I suspect it wouldn't have. It might have gone another meter or two. Starting another debate on how much to add...but I suspect it will be more than 1 gram, probably 2 grams.

RE: Powder Well
The powder-well I have attached can only hold about 4 grams. And that is pretty full, about 3 or 4mm from top.
So will probably need to look at putting in a slightly longer powder-well.

RE: Protection of recovery systems
While my black wool/polyester clothe does offer some protection, it doesn't beat the Nomex blankets (not too surprising), so I decided to pull out the Nomex blanket for the main chute and put it into the booster compartment to protect the shock-cord a little more. [While I've ordered more shock-cord, I don't see the point in not trying to protect it a little more.] I also wanted to protect the bridle cord a little more.

Summary
I get the feeling that when we size up our charges on the ground, we have extra forces (friction on ground) and that this means we will always come up with a size that is above what is actually required..but atleast this means we over do it a little, rather than potentially under-do it and not get a proper ejection.
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Re: Joe Turners L2 Build Thread

Postby mgaertne » Wed Feb 07, 2018 8:39 pm

Summary
I get the feeling that when we size up our charges on the ground, we have extra forces (friction on ground) and that this means we will always come up with a size that is above what is actually required..but atleast this means we over do it a little, rather than potentially under-do it and not get a proper ejection.


I agree with that. That is the reason why my shock cords are usually longer than required. That will dampen the too much bp effect...
Have you thought about using a deployment bag? Helps the chute to slip out of the tube. it only adds the need for a pilot chute in most cases.
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Re: Joe Turners L2 Build Thread

Postby OverTheTop » Thu Feb 08, 2018 6:27 am

Have you thought about using a deployment bag?


I agree with this. Makes packing the chute a lot easier and I think it improves the reliability of the deployment. You just need to make sure you have enough drogue to pull it out, although it is surprisingly easy extract!
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Re: Joe Turners L2 Build Thread

Postby mgaertne » Thu Feb 08, 2018 7:33 am

With a "small" rocket like this I usually test that by deploying the drogue/pilot and then run fast. If it pulls out then it will in mid-air.
Unfortunately that does not work on larger rockets, too heavy.
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Re: Joe Turners L2 Build Thread

Postby joeman » Thu Feb 08, 2018 11:32 am

Hi,
The thought of a deployment bag has crossed my mind, though I haven't looked at it in detail.

Putting thought into how to stop things from rotating...reducing moments.

I've already made what adjustment to the packing, just by changing the place of the tape on the Z-folds.


-------_||___________||____
-||----------------||----- |
|-||-----------------||------------------------------- to booster

instead of

--------------------|
________||________
------------||---------- |
|------------||------------------------------------- to booster

Also changed the way I wrap up the shock-cord in this Nomex blanket...more of a sausage. Seems to come out VERY easy. Hard to imagine it having problems.


RE: Nomex blanket/parachute
Also the hole in the 12x12" nomex is on the edge of the fabric. If it was in the center with a knot behind it, it would probably less inclined to rotate.

I'll spend an hour or so on weekend doing some manual (run) tests. Sounds like a good idea to do before setting up ejection charges.

Cheers!
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Re: Joe Turners L2 Build Thread

Postby joeman » Thu Feb 08, 2018 11:34 am

I should probably have used the quote Tag in that ascii graphics in the previous post. Bit hard to visualze.

Essentially I'll just masking tape it differently...at end , rather than centre.
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Re: Joe Turners L2 Build Thread

Postby OverTheTop » Thu Feb 08, 2018 2:10 pm

Use masking tape with caution to bundle things up. It can become unusually strong when you are relying on it tearing sometimes. I have seen some people both surprised and embarrassed by this :)

I only use it to z-fold the harness.
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Re: Joe Turners L2 Build Thread

Postby mgaertne » Thu Feb 08, 2018 3:13 pm

Use a rubber band. It's even reusable
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Re: Joe Turners L2 Build Thread

Postby joeman » Thu Feb 08, 2018 4:37 pm

It's funny you mention that (Tape being strong)...your comment reminds me of photos that OTT has in another thread.
The tape in those photos is blue...painters tape perhaps? (painters tape has less "stick")

Cheers!
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