4" Ganymede

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Ironside
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4" Ganymede

Postby Ironside » Tue Oct 21, 2014 4:54 pm

I am scratchbuilding a fibreglass 4" rocket and am at the point of drilling the static port holes.

I have calculate the size of the holes (three radially along the sustainer and booster) but am unsure what the 'rule-of-thumb' is for locating them axially.

The drogue and main parachutes will be firmly jammed in along with the accompanying shock cords and hardware. Should the static port holes be located forward, centre or aft of the airframes? Does the packed parachute location influence the location? Or doesn't it matter where the holes are located?
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Re: 4" Ganymede

Postby matthew » Tue Oct 21, 2014 8:46 pm

I've had a really long day, Jeff, so forgive me if I grab the wrong end of the stick on this. :oops:

If you are making three static port holes, I'm assuming you are doing this into an avbay, so I can't imagine you'd have problems with the laundry getting in the way. I put mine in between the three rivets I use to hold the avbay in place in the payload tube (so, 6 locations around the tube - 3 rivets and 3 holes). Generally not in line with any turbulence creating items such as buttons, lugs, etc.

If you mean the pressure relief holes, I put mine on the ugly side of the rocket. Just below the upper rail button on the booster, and somewhere convenient on the payload tube.
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Re: 4" Ganymede

Postby Lamp » Tue Oct 21, 2014 10:18 pm

matthew wrote:I've had a really long day, Jeff, so forgive me if I grab the wrong end of the stick on this. :oops:

If you are making three static port holes, I'm assuming you are doing this into an avbay, so I can't imagine you'd have problems with the laundry getting in the way. I put mine in between the three rivets I use to hold the avbay in place in the payload tube (so, 6 locations around the tube - 3 rivets and 3 holes). Generally not in line with any turbulence creating items such as buttons, lugs, etc.

If you mean the pressure relief holes, I put mine on the ugly side of the rocket. Just below the upper rail button on the booster, and somewhere convenient on the payload tube.


+1 this is also what I do.
The pressure relief holes do not need to be very big, a single 1/8" hole in both the booster and payload bay should be enough.
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Re: 4" Ganymede

Postby SpaceManMat » Wed Oct 22, 2014 7:04 am

matthew wrote:the ugly side of the rocket.

Which side is that Matthew, you assume that we managed to get a decent coat of paint on it :lol:
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Re: 4" Ganymede

Postby cryoscum » Wed Oct 22, 2014 9:06 am

Lamp wrote:The pressure relief holes do not need to be very big, a single 1/8" hole in both the booster and payload bay should be enough.


Agreed, even for 6" and up, I just do a small hole in each affected "chamber".
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Re: 4" Ganymede

Postby Ironside » Wed Oct 22, 2014 12:39 pm

Thanks, fellas

The AV bay is all sorted. I was referring to the sustainer and booster tubes where the parachutes are located.

Based on the discussion, it appears that the pressure relief holes can be located anywhere along the tube.
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Re: 4" Ganymede

Postby CATO » Wed Oct 22, 2014 7:20 pm

Hi Ironside,

I was pondering the question "is a vent hole(s) really required if you were to utilise shear pins" (never utilised shear pins before), I spent 2 days doing the research, and I was surprised by the amount of pressure that builds up within the airframe(s) during accent. At a height of 6km, 1/2 of the atmosphere is below the rocket :shock: , that's an increase of @ 500mbar's (7.35 PSI), in a 7.5" dia rocket that translates to 325 lbs of force acting on the base of the NC / bulkplates, you would require a min of 7 x 4/40 shear pins (min shear strength of 50lbs) just to retain the sections, add to this the deceleration of the NC & frame sections, and drag on the fin frame and you are going to need 8 shear pins at a minimum (neglecting Lapse Rate), that's a lot of BP in my 33" frame if I do not utilise vent holes.

I will be utilising 5/32" vent holes, one in the upper and one in the lower airframe :)
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Ns 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: 1.14M

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Re: 4" Ganymede

Postby Lamp » Wed Oct 22, 2014 11:51 pm

Hi Cato, 5/32" vent holes are pretty big! you are talking lots of force because of the area but little actual volume that needs to get out to remove the pressure...on all my rockets I have never used anything bigger than 1/8"! Just make sure that the hole does not get covered by a chute....!

I had a good practical example of the effects of decreased pressure at altitude last Sunday. I drove up to the Chilean Flamingo reserve near the Bolivian and Argentine borders which is at an altitude of around 4,000m. I purchased a plastic bottle of water (the non fizzy kind) at San Pedro which is at about 2,800m. I opened the water at the reserve and it went Fffft like it was the fizzy kind. I drank most of the bottle, put the cap on and driving back I heard the bottle pop a couple of times, when I had a look at it back in Calama (about 2,700m and warmer) it was somewhat crushed from the change in pressure, I estimate about half the volume it was at altitude. You would not think that 1200m or so would make such a huge difference!
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Re: 4" Ganymede

Postby CATO » Thu Oct 23, 2014 6:21 am

Lamp wrote:Hi Cato, 5/32" vent holes are pretty big! you are talking lots of force because of the area but little actual volume that needs to get out to remove the pressure...on all my rockets I have never used anything bigger than 1/8"! Just make sure that the hole does not get covered by a chute....!


Thanks Lamp, and sorry for the hijack Ironside, I have read articals stating between 1/8" and 1/4", given I still have to paint the airframes, I accomodated for the paint and didn't what the finished holes to be less then 1/8".

My vents are located at the tops of the airframes, 1" below any obstrastrutions (nc shoulder, AvBay, coupler, etc).
"In thrust we trust"

AMRS 21 L3

Impulse:
2017: 5,973 Ns (13.14% M)
Ns 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: 1.14M

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Re: 4" Ganymede

Postby jase » Thu Oct 23, 2014 8:06 am

Pete's advice is top-shelf as usual 8)

And, the larger the diameter of the airframe, the worse it gets - doesn't necessarily mean you need larger holes for larger airframes though because as David has alluded to, you still need the ejection charge to work.
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Re: 4" Ganymede

Postby cryoscum » Thu Oct 23, 2014 5:15 pm

Lamp wrote:I had a good practical example of the effects of decreased pressure at altitude last Sunday. I drove up to the Chilean Flamingo reserve near the Bolivian and Argentine borders which is at an altitude of around 4,000m. I purchased a plastic bottle of water (the non fizzy kind) at San Pedro which is at about 2,800m. I opened the water at the reserve and it went Fffft like it was the fizzy kind. I drank most of the bottle, put the cap on and driving back I heard the bottle pop a couple of times, when I had a look at it back in Calama (about 2,700m and warmer) it was somewhat crushed from the change in pressure, I estimate about half the volume it was at altitude. You would not think that 1200m or so would make such a huge difference!


Hi Pete. I bought a can of Red Bull in Calama and it exploded in my bike's topbox on our way to Ollague (Bolivian border, about 4000m). The one GoPro was never quite the same after that! No idea where it was canned, clearly not in the area - maybe Iquique or Santiago?
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Total impulse for 2016: 32,458 Ns (thus far)
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Total impulse for 2014: 40,757 Ns
Total impulse for 2013: 62,927 Ns

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Re: 4" Ganymede

Postby Ironside » Sat Oct 25, 2014 9:23 am

so, just to summarise:

One hole in the sustainer and one in the booster is appropriate
hole size somewhere between 1/8" and 1/4"
keep the parachute (or any other object e.g. piston) clear of the hole

anything else?

Thanks fellas
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Re: 4" Ganymede

Postby Lamp » Sat Oct 25, 2014 9:22 pm

Ironside wrote:so, just to summarise:

One hole in the sustainer and one in the booster is appropriate
hole size somewhere between 1/8" and 1/4"
keep the parachute (or any other object e.g. piston) clear of the hole...


That pretty much sums it up...personally for a 4" rocket I would only use a 1/8" hole. If you are worried about paint clogging the hole put the hole on the rail button/rail guide side of the rocket and gently re-drill the hole after painting. As Jase pointed out, if you use a hole that is too big you will potentially lose some of the ejection charge pressure.

Pity about the go-pro Nic, I would say the can would probably have been sealed in Santiago. It would be interesting if a can was sealed up at 4,000m altitude....would it be almost flat at sea level? I was really surprised how much the water bottle collapsed!

Sorry for the thread hijack Ironside, I'll shut up now! :wink:
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Re: 4" Ganymede

Postby Ironside » Sun Oct 26, 2014 5:30 pm

No worries. Lamp

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Re: 4" Ganymede

Postby CATO » Mon Apr 11, 2016 9:37 am

Westmar April 2016
jeff.jpg
"In thrust we trust"

AMRS 21 L3

Impulse:
2017: 5,973 Ns (13.14% M)
Ns 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
PB - Gorilla N2717WC, H: 10,260', S: 1.14M


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