Quantcast

Any special considerations for jam-packed payload tube?

The Rocketry Forum

Help Support The Rocketry Forum:

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
All,

My Formula 200 is just about complete. I am just waiting on my thrust plate from SCP. I have a 15' military chute in a Fruity D-bag, 30' of Kevlar harness, and a pilot chute.

With all that in there, I can just about get the NC in and pinned. It is very tight, with every bit of payload volume consumed.

I've never had a payload tube that stuffed before. I was wondering if there are any additional considerations for deployment under those conditions before I start ground-testing. For instance, if the BP calculate says 2g, should I double it? Halve it? Any other thoughts?

Thanks.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
4,883
Reaction score
2,123
Location
Melbourne Australia
My BP calculations always use the empty volume of the bay to work out the quantity. Primary charge is that number rounded up a little. Backup charge is usually about 50% more or so.
 

Handeman

Well-Known Member
TRF Supporter
Joined
Jan 18, 2009
Messages
7,776
Reaction score
364
Location
Stafford, VA
I've never had to test with a full tube, but just off the top of my head, I would start out with about half the recommended amount and work from there.

Others my have more insight.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
4,883
Reaction score
2,123
Location
Melbourne Australia
Forgot to mention: I use the formula out of Modern High Power Rocketry 2.

Diameter (in inches) x Diameter (in inches) x length (in inches) x 0.006 = Charge mass (in grams).

As an example, a 4" bay of length 18" calculates to 1.73g of BP. Actual flown charges would be 2g for primary, 3g for backup.
 

mpitfield

Moderator
Staff member
TRF Lifetime Supporter
Global Mod
Joined
Jul 7, 2013
Messages
4,903
Reaction score
431
Location
Toronto, Ontario
My 54mm MD carbon fiber Tomach is the same tight config. Every bit of space is used and I have to hold it together to put the shear pins in. Literally it is about 1/16 of an inch too short, just enough to pop open but not enough to pop open on it's own. When I ran my initial calculations I based them on the volume of an empty tube, then ground tested until I consistently got a full extension of the recovery gear, without snapping at the end, (1.1g). I learned a while back not to use a chute, but instead a packed substitute, however I do use my recovery harness. For flight I added a bit for margin (1.5) which was fine for the first 5 or so flights. However I ended up backing it off a bit (1.3g) due to a shear pin failure of my main @ apogee. I feel that this was the result of three issues. Compromised shear pins that were reused, an overly energetic deployment, and a recovery harness that was a bit on the short side. I adjusted all three and so far so good.
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
My 54mm MD carbon fiber Tomach is the same tight config. Every bit of space is used and I have to hold it together to put the shear pins in. Literally it is about 1/16 of an inch too short, just enough to pop open but not enough to pop open on it's own. When I ran my initial calculations I based them on the volume of an empty tube, then ground tested until I consistently got a full extension of the recovery gear, without snapping at the end, (1.1g). I learned a while back not to use a chute, but instead a packed substitute, however I do use my recovery harness. For flight I added a bit for margin (1.5) which was fine for the first 5 or so flights. However I ended up backing it off a bit (1.3g) due to a shear pin failure of my main @ apogee. I feel that this was the result of three issues. Compromised shear pins that were reused, an overly energetic deployment, and a recovery harness that was a bit on the short side. I adjusted all three and so far so good.
How did your final amount (1.1g) compare with what the calculator said for an empty tube?
 

mpitfield

Moderator
Staff member
TRF Lifetime Supporter
Global Mod
Joined
Jul 7, 2013
Messages
4,903
Reaction score
431
Location
Toronto, Ontario
How did your final amount (1.1g) compare with what the calculator said for an empty tube?
If I recall correctly the initial calculated amount was .8g or somewhere close to that. My first ground test was with the calculated amount. It succeeded in shearing the pins but not much else. The rocket just split but there was very little separation, a foot or so. I continued testing until it reliably extended the recovery harness fully.
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
If I recall correctly the initial calculated amount was .8g or somewhere close to that. My first ground test was with the calculated amount. It succeeded in shearing the pins but not much else. The rocket just split but there was very little separation, a foot or so. I continued testing until it reliably extended the recovery harness fully.
That's along the lines of what i was thinking. The BP calculator assumes an empty tube, and the idea is for the gas to pressurize the tube and push out the NC, and then the NC pulls out the harness, which pulls out the chute. But when the tube is essentially full, with the chute pressed directly up against the charge wells at one end, and the NC at the other, it seems more like the charge will need to push on the chute itself, and push it out of the tube. May be more diffilcult to break the pins, too (I'm using five #8-32 screws).

I've got 7" of free space in the tube, and my calculator says 2.6g will get me 15 psi. I think I'll start with 4g and see what happens. I'm guessing I may end up at 5g to 6g.
 

rstaff3

Oddroc-eteer
Joined
Jan 18, 2009
Messages
11,759
Reaction score
14
How did your final amount (1.1g) compare with what the calculator said for an empty tube?
Dude, either blow it out or blow it up. Seems like with enough shock cord, a skosh more won't hurt.
 

Onebadhawk

Sponsor
TRF Sponsor
Joined
Jan 11, 2013
Messages
6,119
Reaction score
409
I have always liked the presence of a big main chute in the air,, beautiful...
I also like slow descents on the main..
With that said I am packed in the payload section more often then not...

You know quite well John,,,
Ground testing will tell all....

Teddy
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
I have always liked the presence of a big main chute in the air,, beautiful...
I also like slow descents on the main..
With that said I am packed in the payload section more often then not...

You know quite well John,,,
Ground testing will tell all....

Teddy
Yeah, I'm just trying to figure out where to start my ground tests.
 

Onebadhawk

Sponsor
TRF Sponsor
Joined
Jan 11, 2013
Messages
6,119
Reaction score
409
Yeah, I'm just trying to figure out where to start my ground tests.
I just take a good educated guess at the amount of 4F powder for the first ground test..
If you're doing the math and doing those calculations figuring volume
and you volume is full of stuff that's not accounted for in the calculations then knock the amount of powder back ...
I ground test a lot before I fly a rocket for the first time..
So start small,,, say half of your empty volume calculations...
Then go from there...

Teddy
 

mpitfield

Moderator
Staff member
TRF Lifetime Supporter
Global Mod
Joined
Jul 7, 2013
Messages
4,903
Reaction score
431
Location
Toronto, Ontario
I've got 7" of free space in the tube, and my calculator says 2.6g will get me 15 psi. I think I'll start with 4g and see what happens. I'm guessing I may end up at 5g to 6g.
Assuming the empty volume is 7" long and a 8" diameter airframe, my calculator shows roughly 350 cubic inches of volume. I do not know the forces involved to shear 5 x 8/32 nylon shear pins but below are the numbers I get in 1g increments starting at 4g, for solving pressure for a known qty. of 4F BP.

4g = 22 psi or 1106.9 lbf
5g = 27.5 psi or 1383.6 lbf
6g = 33 psi or 1660.3 lbf
7g = 38.5 psi or 1937 lbf

I have a Excel workbook that was put together by Chuck Pierce that provides these numbers, and I have found that the numbers yield a good starting point for ground tests.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
4,883
Reaction score
2,123
Location
Melbourne Australia
Calculate the cross section area of the rivet for your shear pin. If you are using a screw, use the minor diameter in the calculations rather than the nominal or major diameter. Find the shear strength of the material (nylon 66 specs are a good starting point if you don't know the exact plastic used). Then, based on the area calculated earlier, calculate the force needed to shear the pin. If you use more than one pin, just multiply by the number of pins for the total force.

For Nylon 66, the shear strength is about 58MPa (number varies according to whose data you look up). An M3 nylon screw has a minor diameter of 2.3mm (you can calculate on min or max tolerances if you like), gives 4.2mm2. Now 58MPa is the same as 58 N/mm2. 58 x 4.2 = 244N. Divide by 9.8 to get 24.9 kgf per pin.

I am not going to do the calcs in imperial :p
 

Nytrunner

Pop lugs, not drugs
TRF Supporter
Joined
Oct 15, 2016
Messages
7,761
Reaction score
3,389
Location
Huntsville AL
Divide by 9.8 to get 24.9 kgf per pin.
What is this madness of kilogram-force? Never in 4 years of Mech Engineering or 2 years of space and defense industry have I heard tell of such an animal.
Aww C'mon, Psi isn't that bad.

Edit: Ah, now I see.
"Kilogram-force is a non-standard unit and does not comply with the SI Metric System."
 

rocket_troy

Well-Known Member
Joined
Sep 9, 2013
Messages
137
Reaction score
43
What is this madness of kilogram-force? Never in 4 years of Mech Engineering or 2 years of space and defense industry have I heard tell of such an animal.
Aww C'mon, Psi isn't that bad.

Edit: Ah, now I see.
"Kilogram-force is a non-standard unit and does not comply with the SI Metric System."
I often use KgF to help illustrate a force quantity in metric. If there's one thing the imperial system has over metric, it's that "pounds force" is much more illustrative than the metric equivalent "newton". Use newtons for the calculations and parameters, but I normally will do the KgF conversion to complement the N force result.
I suppose a similar argument can be made for pressure units too.

Troy
 

Buckeye

Well-Known Member
TRF Supporter
Joined
Sep 6, 2009
Messages
2,626
Reaction score
476
I often use KgF to help illustrate a force quantity in metric. If there's one thing the imperial system has over metric, it's that "pounds force" is much more illustrative than the metric equivalent "newton". Use newtons for the calculations and parameters, but I normally will do the KgF conversion to complement the N force result.
I suppose a similar argument can be made for pressure units too.

Troy
Ugh. Please stop. kg is mass, N is force, and Pa is pressure.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
4,883
Reaction score
2,123
Location
Melbourne Australia
I would have just used N or Pa myself, but it is difficult for me to know what makes things easy for you imperial guys :wink:

BTW, I do prefer psi to kPa myself. I was in primary school when the metric system kicked in here, but psi seems to have stuck with me as more intuitive.
 

NateLowrie

Well-Known Member
Joined
Jun 28, 2016
Messages
667
Reaction score
4
John, I am curious. When you installed the bulkhead on the nosecone did you set it back in the coupler? I ended up putting mine about 2in from the upper end of the NC coupler to give me a bunch of space back inside the coupler. Otherwise it would have been a stupidly small amount of space to try and fit everything in.

I was planning on starting with 5.5g in mine.
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
John, I am curious. When you installed the bulkhead on the nosecone did you set it back in the coupler? I ended up putting mine about 2in from the upper end of the NC coupler to give me a bunch of space back inside the coupler. Otherwise it would have been a stupidly small amount of space to try and fit everything in.

I was planning on starting with 5.5g in mine.
Man, I didn't think of that! My bulkhead is removable, because I'll be putting my tracker in the nose. So I actually bought an external bulkhead for it. I suppose I could use an internal bulkhead, with a CR epoxied in, but it is a little late now. Hmmm....

Nice to have you back, BTW.
 

NateLowrie

Well-Known Member
Joined
Jun 28, 2016
Messages
667
Reaction score
4
Man, I didn't think of that! My bulkhead is removable, because I'll be putting my tracker in the nose. So I actually bought an external bulkhead for it. I suppose I could use an internal bulkhead, with a CR epoxied in, but it is a little late now. Hmmm....

Nice to have you back, BTW.
Nice to be back.

What I did was make the coupler itself removable. Used PEM nuts and button head cap screws. I then epoxied the bulkhead above the PEM nuts touching them and did a nice fillet around the whole thing. That way I have the epoxy fillet holding it back but for the bulkhead to pull through it would also have to go through the PEM nuts. I'll post some pictures tonight.

Semi-related question: What size deployment bag did you use for the 15' military chute?
 

manixFan

Not a rocket scientist
Joined
Feb 15, 2009
Messages
1,970
Reaction score
952
Location
TX
That's along the lines of what i was thinking. The BP calculator assumes an empty tube, ...
I've got 7" of free space in the tube, and my calculator says 2.6g will get me 15 psi. I think I'll start with 4g and see what happens. I'm guessing I may end up at 5g to 6g.
Hmmm. Let me think out loud on this.

First, the tube length you mention seems too short. According to what I see about that kit, the payload bay is 26" long. The coupler is 12" long so I'm assuming it extends 6" into the payload bay. That leaves 20" of 8" tube between the bulkhead and the end of the tube before the bottom of the parachute is clear.

The payload bay may be full when you ignite the charge, but as the nosecone and chute start to move you are creating an empty space. So you still need to pressurize that entire space regardless of whether or not it is full in the beginning. And you need to take the entire tube length into account, not just the empty space between the bulkhead and bottom of the nosecone, which is what I am guessing you are calling free space? You'll realize the fallacy of thinking about how much 'free space' you have when you have in fact said there is no free space, the entire area is filled. You need to think of the entire tube length from the bulkhead until the chute leaves the tube. You don't want just the shoulder of the nosecone out, you want the entire package out in free air. If you move the bottom of the parachute 7 inches forward is it free of the payload bay?

An online reference for shear strength for a 8-32 nylon screw showed 164 lbs. So 5 screws requires 820 lbs of force. That seems like a lot. My calculator says to generate 820 lbs of force in a 8" x 20" tube you need about 8.5 grams of BP. Of course you have to add in the margin required to move the weight of the chute and overcome friction, etc.

Re-run the calculations using the actual total length of the body tube and if they are as I assume, you'll see a much lower pressure - 4 grams in an 8" x 20" tube produces only about 390 lbf.

But of course I could be missing something. As others have mentioned, your ground tests will tell.


Tony
 

Bat-mite

Rocketeer in MD
Joined
Dec 5, 2013
Messages
10,986
Reaction score
1,738
Location
Maryland
Measurements on MadCow's site are wrong. Payload tube is 24". Coupler is more like 16", I can't remember for sure. Add in the U-bolts and charge wells sitting on top of the coupler, and I come up with about 8" of free space. And I have a 10" D-bag! Plus 30' tubuklar Kevlar harness, plus 36" chute protector. :facepalm:
 

NateLowrie

Well-Known Member
Joined
Jun 28, 2016
Messages
667
Reaction score
4
Hmmm. Let me think out loud on this.

First, the tube length you mention seems too short. According to what I see about that kit, the payload bay is 26" long. The coupler is 12" long so I'm assuming it extends 6" into the payload bay. That leaves 20" of 8" tube between the bulkhead and the end of the tube before the bottom of the parachute is clear.

The payload bay may be full when you ignite the charge, but as the nosecone and chute start to move you are creating an empty space. So you still need to pressurize that entire space regardless of whether or not it is full in the beginning. And you need to take the entire tube length into account, not just the empty space between the bulkhead and bottom of the nosecone, which is what I am guessing you are calling free space? You'll realize the fallacy of thinking about how much 'free space' you have when you have in fact said there is no free space, the entire area is filled. You need to think of the entire tube length from the bulkhead until the chute leaves the tube. You don't want just the shoulder of the nosecone out, you want the entire package out in free air. If you move the bottom of the parachute 7 inches forward is it free of the payload bay?

An online reference for shear strength for a 8-32 nylon screw showed 164 lbs. So 5 screws requires 820 lbs of force. That seems like a lot. My calculator says to generate 820 lbs of force in a 8" x 20" tube you need about 8.5 grams of BP. Of course you have to add in the margin required to move the weight of the chute and overcome friction, etc.

Re-run the calculations using the actual total length of the body tube and if they are as I assume, you'll see a much lower pressure - 4 grams in an 8" x 20" tube produces only about 390 lbf.

But of course I could be missing something. As others have mentioned, your ground tests will tell.


Tony
The payload section is 26in long. You have 8in of coupler from each direction which leaves about 10in. Once you add the bulkheads charge cups, etc. in I can see why there is only 7in of space. It's why I put my nosecone bulkhead 2 in from the top side of the NC coupler so I could gain another 12in of space.
 

NateLowrie

Well-Known Member
Joined
Jun 28, 2016
Messages
667
Reaction score
4
Hmm, they couldn't do a 7.5 bag in like 6in? I am ordering my DBag this week. I may just do the 7.5x10 because the rocketman 16' chute feels like it's going to pack bigger than a normal chute. Have an inquiry out to Fruity Chutes at the moment.
 
Top