how to determine droque size

[rvanler65]

Hello,

Going to be building a modified pml ultimate endeavour. Doing duel deploy without pistons. I will be glassing the airframe and was wandering how to figure droque size or should I go droque less.

[Handeman]

I don't recommend drogueless, just because you are depending on the aerodynamics of the two parts of your rocket to keep things stable. There's no real reason that on the 5th, 10th, or 20th, flight things couldn't get into a different mode with the upper part going ballistic and pulling the fin can behind it. " If that happens there is a much higher chance of zippers, shredded main, the fin can falling through the main, various other recovery failures that can cause a lot of damage. That's when you'll say/hear "It's never done that before".

Using a drogue is a positive method of ensuring the stability of the rocket as it falls. I don't know of any method of calculating the size of the drogue. You want the drogue attached to the half of the rocket that contains the main chute and you want that part to fall even with or above the other half. Calculating the size is very difficult because it depends on how fast the two halves fall in relation to each other. I have several drogues and keep going smaller if the fin can is well below the upper section. Once I get the fin can falling beside or slightly below the upper section, I stick with that size.

[Marsman]

If you're looking for a "rule of thumb," 50-70 fps I'd say is about ideal for drogue descent.

[JDcluster]

The Endeavor is a well balanced kit.

I rarely use a drogue.
If you over size a drogue: it can prevent the main from opening up properly.
Most rockets will descend horizontally, unless: you have a weighted nosecone, a short payload section, or a really short and fat rocket.

After 500+ flights, I have only seen that happen exactly once. Back about 15 years ago: a 12" dia. upscaled Loc Lil Nuke the nose cone had 18+lbs of nose weight. The rocket weighed in over 100lbs and the nose went through the main parachute.


I've flown drogueless many times, and have yet to witness the phenomenon you speak of...



JD


I don't recommend drogueless, just because you are depending on the aerodynamics of the two parts of your rocket to keep things stable. There's no real reason that on the 5th, 10th, or 20th, flight things couldn't get into a different mode with the upper part going ballistic and pulling the fin can behind it. " If that happens there is a much higher chance of zippers, shredded main, the fin can falling through the main, various other recovery failures that can cause a lot of damage. That's when you'll say/hear "It's never done that before".

Using a drogue is a positive method of ensuring the stability of the rocket as it falls. I don't know of any method of calculating the size of the drogue. You want the drogue attached to the half of the rocket that contains the main chute and you want that part to fall even with or above the other half. Calculating the size is very difficult because it depends on how fast the two halves fall in relation to each other. I have several drogues and keep going smaller if the fin can is well below the upper section. Once I get the fin can falling beside or slightly below the upper section, I stick with that size.

[blackjack2564]

I used a 36in. Skyangle Cert 3 drogue on my Ultimate Endeavor,but it really did not help much. Even tried a 45in. at that point it was a small main and drifted too much.
Due to the large finnage on the rocket, [6 fins] the fincan would spin,stabilize, and drop horizontal above the payload anyway.

I removed it and still fly drogue less till this day.

More importantly I turned the piston into a zipperless device. I wrapped the tubes with 2 layers of 6oz. but being phenolic they are still prone to cracking upon a hard hit. Repaired chips can be seen under my thumb. The glass holds the tube shape but the phenolic still chipped under it when flown with a full stack of 4 motors due to the weight after hard landings. I use a Cert 3xx chute rated to 75lbs. but still would crack edges of fincan.

2 eyebolts were placed in the top CR. with a piece of tubular nylon attached to each.
I sized a recovery harness such that it is a loop passing through the piston, then the ends tied together. The piston is sandwiched between 2 layers of tape placed above and below it on the harness.[wrapped around the shock cord]. The rest of the shockcord is attached to the loop. Piston is slid to bottom for flight and is pulled up the tube and stops with 1inch sticking out upon apogee. This creates a solid zipperless fincan that can take hard hits without damage.

By simply untying the loop, removing the masking tape stops, the piston can be removed for maintenance or cord replacement.

[Handeman]

The Endeavor is a well balanced kit.

I rarely use a drogue.
If you over size a drogue: it can prevent the main from opening up properly.
Most rockets will descend horizontally, unless: you have a weighted nosecone, a short payload section, or a really short and fat rocket.

After 500+ flights, I have only seen that happen exactly once. Back about 15 years ago: a 12" dia. upscaled Loc Lil Nuke the nose cone had 18+lbs of nose weight. The rocket weighed in over 100lbs and the nose went through the main parachute.


I've flown drogueless many times, and have yet to witness the phenomenon you speak of...



JD

I will say, I've never seen a nosecone drop through a chute, but I've seen the fin cans fall through several times, usually because of excessively long harnesses.

I don't understand why you say an oversized drogue would prevent the main from opening correctly. A drogue, attached to the section with the main will keep that section above the fin can and prevent the fin can from falling through the main. It also keeps the weight of the fin can on the harness as the main opens, preventing excessive shock and the extra weight of the fin can keeps the falling speed up so the main opens correctly. I suppose a very over sized drogue might slow everything down so there wasn't enough air flow to open the main, but if you're going that slow, you don't need the main anyway.

I see several rockets a season that comes in drogueless where the upper and lower sections are flying past each other and snapping back at the end of the harness. They rarely hit together and the main usually deploys correctly, and everyone considers it a sucessful flight, but once in a while there is some significant damage from mid air collisions between the two halves. This usually happens when the drogue harness is 5 or more times as long as the rocket.

I've seen 4 or 5 times where the upper section was nose cone down and the fin can was trailed behind it. The rocket was falling ballistic, just in two connected peices. Obviously it has a much slower terminal velocity in this configuration then if the rocket was still together, but much faster then if it were falling flat or with a drogue.

In one case the rocket also had an extra long drogue harnesses. When the main opened, the upper section reached the end of it's harness about the time the main was open and stopped in mid air. You could see the shock on the canopy when the upper section hit the end of it's harness. Since the main was sized for the whole rocket, it stayed pretty much in one place with just the upper half of the rocket under it as the fin can continued to fall to the end of it's harness. When it hit the end, it pulled the 3/8 U-bolt through the 3/4" aircraft plywood CR and the 60 lb fin can fell free while the lighter upper half drifted off into the woods. It was an expensive L3 cert failure that could have been much worse if the fin can had damaged something when landing or if the land owner hadn't found the upper section several days later.

I like flying DD and think it is the way to fly. I have come to realize that just because the drogue and main charges go off and the chutes come out, that doesn't mean it can't be a failed flight. There are a lot of things that can go wrong, but usually doesn't, with the recovery system and it's not always the electronics. Like most accidents, when you look at them, the conditions were right for failure for a long time before the actual failure. If you watch the DD flights at your launch with a critical eye, you will notice things about almost all of them that could be improved with slight changes in the system design.