Minimum length for a Dual Deployment payload bay tube?

[Retrospace101]

Hi, I was designing some stuff and wanted to find out ( as I'm new to DD) the shortest you can make it and (successfully) fly and deploy a chute with it.

 

[Louis Wu]

0, if you use the nose cone. Otherwise, I would say the minimum coupler length for the body tube

 

[cwbullet]

The shortest length depends on the size of the altimeter and battery and the diameter of the tubing. It also depends on the volume of your payload.

 

[Antares JS]

No hard and fast rule here, it just needs to be big enough to hold your parachute, cord, and the shoulders of your e-bay and nose.

Edit: When you said "payload tube" I thought you meant the tube your main parachute is packed into. If you meant your e-bay tube, then what cwbullet said.

 

[Retrospace101]

Lets say I wanted to shorten a 21" tube thats 4" in Dia.

 

[Retrospace101]

No hard and fast rule here, it just needs to be big enough to hold your parachute, cord, and the shoulders of your e-bay and nose.

Agreed, But I havent tested any length yet

 

[Antares JS]

So test it. Do you have your parachute and cord? Pack all the stuff in there and see how much space it takes up.

Side note, you also need to make sure, using simulation software, that shortening the tube won't make the rocket unstable. It isn't likely but it's something to consider.

 

[Retrospace101]

So test it. Do you have your parachute and cord? Pack all the stuff in there and see how much space it takes up.

Side note, you also need to make sure, using simulation software, that shortening the tube won't make the rocket unstable. It isn't likely but it's something to consider.

Actually i havent bought any of them yet lol

 

[Antares JS]

Then I would recommend holding off on cutting the tube until you buy them and see how much space they take up. You can cut the tube shorter later easily, but it's harder to make it longer again later.

Don't forget to sim the rocket with the shorter tube to make sure you don't give yourself a stability problem too.

 

[Retrospace101]

Then I would recommend holding off on cutting the tube until you buy them and see how much space they take up. You can cut the tube shorter later easily, but it's harder to make it longer again later.

Don't forget to sim the rocket with the shorter tube to make sure you don't give yourself a stability problem too.

Agreed, did that

 

[Andrew_ASC]

I would seriously consider making each end of the av-bay past the airframe band at least 1.5* Outer Diameter of the tube joining it. This isn’t a hard rule. This is a dear rule I learned very hard on screwing up a multistage interstage design once. Short joints are prone to wobbling. Wobbling kills rockets. Put a side load on the thing on ground until it’s not shifting around. Especially if you experiment with shorter.

 

[Andrew_ASC]

The shortest length depends on the size of the altimeter and battery and the diameter of the tubing.

Very true. It scares me the shortest answer could possibly wobble in flight.

 

[Retrospace101]

I would seriously consider making each end of the av-bay past the airframe band at least 1.5* Outer Diameter of the tube joining it. This isn’t a hard rule. This is a dear rule I learned very hard on screwing up a multistage interstage design once. Short joints are prone to wobbling. Wobbling kills rockets. Put a side load on the thing on ground until it’s not shifting around. Especially if you experiment with shorter.

nice, but what do you mean by "Outer Diameter"? Is that like how far it goes up into the tube or something?

 

[Andrew_ASC]

nice, but what do you mean by "Outer Diameter"? Is that like how far it goes up into the tube or something?

Yes the length that goes into the next payload tube. 1.5* widest point across airframe tube. We used that rule on motor joints and nothing failed after that interation. We went from overhanging 29mm motors are 1cm to about 1.5” overhang into next stage. Basically if the connection wobbles in the slightest bit, kiss your rocket bye bye in about a microsecond. Murphy taught a good lesson.

 

[Andrew_ASC]

Just because you can make it shorter. Doesn’t mean you should when turbulence or a violent dynamic flight load hits it with a cross loading.

Theoretically, oh wow look at all this mass we save. Reality. Side load hits airframe. Short coupler wobbles. ** SH*T dies in flight while supersonic** Murphy lols and you cry. These guys got a lot more experience on here. If it can go wrong. They’ve found ten different ways wrong to do it. And doing it wrong cost lotta money.

Don’t get any theory ideas to sand wall thickness down to finger nail thin. Looks great in sim. Also dies in reality. Anyways good luck with project.

 

[JohnCoker]

3.9" is a good size, but it always seems that when the time comes to pack in the laundry there can always be more space.

As long as your rocket isn't too heavy, I'd leave a minimum of 4" for the drogue, 6" for the bay and 10" for the main. You can probably squeeze that ebay down to 4" if you chose small electronics. (That's for traditional dual-deployment.)

 

[Joe Bruce]

Another word of caution: don't pack the laundry too tightly. Had to do surgery on my LOC Deployer NC to avoid blowing holes in my nomex / main chute. Shear pins didn't shear, either.

Too tightly packed = very bad.

 

[TheTank]

Another word of caution: don't pack the laundry too tightly. Had to do surgery on my LOC Deployer NC to avoid blowing holes in my nomex / main chute. Shear pins didn't shear, either.

Too tightly packed = very bad.

Im hoping to test fly my deployer this week and the couple charge test firings I have down are beating the heck out of my nomex... and I purposely left more coupler in the sustainer than in the payload bay to give me more room for the main. It's still tight though. I had to seal the cone well as well as that front bulkhead as it failed to deploy on my first two tests due to leakage.

 

[Joe Bruce]

Im hoping to test fly my deployer this week and the couple charge test firings I have down are beating the heck out of my nomex... and I purposely left more coupler in the sustainer than in the payload bay to give me more room for the main. It's still tight though. I had to seal the cone well as well as that front bulkhead as it failed to deploy on my first two tests due to leakage.

I had similar issues (TRF thread somewhere, on my phone right now). I cut the bottom off the NC and screwed / epoxied a bulkhead w/ eye bolt at the forward end of the NC coupler to get another 3". Ground tests were much better after that, and NC doesn't "need" shear pins to stay on (it was THAT tight previously). Still haven't flown it, but had 3 successful ground tests.

Good luck! LOC, if you're listening, consider adding a few inches to the Deployer payload tube.

 

[TheTank]

I had similar issues (TRF thread somewhere, on my phone right now). I cut the bottom off the NC and screwed / epoxied a bulkhead w/ eye bolt at the forward end of the NC coupler to get another 3". Ground tests were much better after that, and NC doesn't "need" shear pins to stay on (it was THAT tight previously). Still haven't flown it, but had 3 successful ground tests.

Good luck! LOC, if you're listening, consider adding a few inches to the Deployer payload tube.

thanks! You too! My nose cone was not like most of my LOC products. It was sloppy loose fit. I had to tape it to snug it up. Had three pins at first but went down to one and sealed the ends better. Those two unsuccessful tests made a mess in that payload bay. Drogue deployed fine right from the start.

 

[Retrospace101]

3.9" is a good size, but it always seems that when the time comes to pack in the laundry there can always be more space.

As long as your rocket isn't too heavy, I'd leave a minimum of 4" for the drogue, 6" for the bay and 10" for the main. You can probably squeeze that ebay down to 4" if you chose small electronics. (That's for traditional dual-deployment.)

Thanks!!

 

[manixFan]

If you are looking for a chute that will consume the least amount of space, the Fruity Chute Iris Light has the smallest packing volume of any chute I could find. The compact version actually takes more space (by about 10-15%). You also need a thin Nomex blanket rather than the standard thicker versions. You can use a streamer as your drogue, or go drougueless based on how your parts balance. The shorter you go the more likely you will need nose weight to maintain stability, so you need to take that into account.

But you need to ask yourself what is your goal? In my experience the smaller the payload bay the more difficult and time consuming (and expensive) things become.


Tony

 

[Retrospace101]

If you are looking for a chute that will consume the least amount of space, the Fruity Chute Iris Light has the smallest packing volume of any chute I could find. The compact version actually takes more space (by about 10-15%). You also need a thin Nomex blanket rather than the standard thicker versions. You can use a streamer as your drogue, or go drougueless based on how your parts balance. The shorter you go the more likely you will need nose weight to maintain stability, so you need to take that into account.

But you need to ask yourself what is your goal? In my experience the smaller the payload bay the more difficult and time consuming things become.There also seems to be an increased chance of failure.


Tony

Its just that I wanted to conserve a little weight for an flight on a H45 or I65, possibly a I49

 

[manixFan]

Its just that I wanted to conserve a little weight for an flight on a H45 or I65

Then the easiest way is to select a diameter and materials that are suitable for your goal. A 38mm rocket will be easy to keep to the wight you want, a 4" version, not so much. If you're new to DD, 38mm is a challenge, so try 54mm. Maybe even buy a CF tube from the vendor here that has them available for a decent price. Compare weights of G10 to plywood for fins, same with centering rings. Make it minimum diameter to reduce the amount of material needed for fins and motor mount and use a lightweight aluminum motor adapter.

If you really are just starting, you're starting at the wrong end of the rocket. Shaving a few inches off the length of the rocket is the kind of thing you do last to meet your objective. It all starts with diameter and building materials.


Tony

 

[cerving]

Take the smallest space that you think you can fit the chute into, and add 50%. One of the primary modes of deployment failure is packing the chute too tight in the payload tube and having it jam in the tube on nose cone separation, it's especially a problem with cardboard rockets because you can't compensate for it by just using a bigger BP charge.

 

[RocketRev]

As a TAP and L3CC member, I can't name names, but I've seen dozens and dozens of payload bays that were WAY TOO SMALL. Folks desperately trying to fit their parachute in place when there just isn't enough space. I've seen shear pins shear as a result of the pressure from too much parachute and harness in the parachute bay. Then I've seen them shear 500 feet off the ground. and clearly 5 seconds later see and then hear the recovery charge go off after the laundry is already out and shreading. All because they did not have enough space in their parachute bay. I've only seen one that was bigger than needed and that was on my own L-3 cert rocket's drogue end of the dual deployment system. It is far FAR harder to add more space than it is to take it away. So be very very slow to cut tubing.

As far as weight is concerned, if your design to fly a 4" diameter rocket on an H-45 or an I-49 motor will be compromised because of the weight of a few inches of 4" LOC air-frame, then in my opinion you have a serious design flaw somewhere. You might want to look elsewhere to save on weight.

How about fin material/thickness? Here would be a good place to go with CF.
How about thickness of centering rings?
How about your recovery harness materials? tubular kevlar? What diameter? More often than not, one can go with a lot smaller than we usually use.

There are many places to save weight, but shortening the air-frame is not a good place to start.

Brad

 

[Retrospace101]

As a TAP and L3CC member, I can't name names, but I've seen dozens and dozens of payload bays that were WAY TOO SMALL. Folks desperately trying to fit their parachute in place when there just isn't enough space. I've seen shear pins shear as a result of the pressure from too much parachute and harness in the parachute bay. Then I've seen them shear 500 feet off the ground. and clearly 5 seconds later see and then hear the recovery charge go off after the laundry is already out and shreading. All because they did not have enough space in their parachute bay. I've only seen one that was bigger than needed and that was on my own L-3 cert rocket's drogue end of the dual deployment system. It is far FAR harder to add more space than it is to take it away. So be very very slow to cut tubing.

As far as weight is concerned, if your design to fly a 4" diameter rocket on an H-45 or an I-49 motor will be compromised because of the weight of a few inches of 4" LOC air-frame, then in my opinion you have a serious design flaw somewhere. You might want to look elsewhere to save on weight.

How about fin material/thickness? Here would be a good place to go with CF.
How about thickness of centering rings?
How about your recovery harness materials? tubular kevlar? What diameter? More often than not, one can go with a lot smaller than we usually use.

There are many places to save weight, but shortening the air-frame is not a good place to start.

Brad

Actually I did have an idea to take off the Drogue and shock cord and just have a main parachute ejected a little after apogee and have the main wrapped under a jolly logic....though that probably may not do too much.

 

[richP]

Actually I did have an idea to take off the Drogue and shock cord and just have a main parachute ejected a little after apogee and have the main wrapped under a jolly logic....though that probably may not do too much.

That's how a lot of folks do it, and nothing wrong with it, although it is not a traditional dual-deploy.
There are quite a few creative ways to save weight on a rocket without shortening it.
Use lighter hardware components, aluminum instead of steel, kevlar instead of nylon, high cd/lightweight chutes.
Drilling holes in the electronics sled, centering rings, etc.
Use less adhesive, smaller fillets, high-strength epoxies.
Less or even no paint.

 

[Dan Griffing]

Another word of caution: don't pack the laundry too tightly. Had to do surgery on my LOC Deployer NC to avoid blowing holes in my nomex / main chute. Shear pins didn't shear, either.

Too tightly packed = very bad.

I strongly agree with the caution against packing the laundry too tightly.

But its not restricted to dual deployment.

My Zephyr, which had made both L1 and an L2 certifications, including with dual deployment, crashed on its third motor ejection flight of the day in July — despite using the same laundry configuration. This same configuration had passed several ground ejection tests. Its not easy to determine how tight the margin of safety is until you cross it.

 

[Retrospace101]

I strongly agree with the caution against packing the laundry too tightly.

So "Laundry" is a slang? cool.