Recommended drogue sizing

[IComeFromM31]

Hey all,

In the design phase for a rocket launch next June. It will likely be between 35 and 45 pounds. I need some insight as to how to size my drogue. What do you guys think will be appropriate for a drogue for a 45lb rocket projected to reach 10,000 feet?

 

[3stoogesrocketry]

I always go with a 10 to one ratio . Your rocket will be about 45 pounds , so your drouge should be rated at 4.5 pounds. A 4 foot drouge should get you about 45/50 fps decent rate .

Eric

 

[Bat-mite]

best bet is to look at the descent rate charts for whatever brand of chute you are going with. Skyangle, Rocketman, and Top Flight all have excellent charts. Make sure the shroud lines are capable of handling the weight.

Personally, I think's Eric's recommendation is good at 4', but I think his descent rate is off. You'll get more like 110 fps, which is excellent.

 

[Onebadhawk]

I look for at least 80 fps decent rate for a drogue..
The drogue should be just large enough to stop the booster and payload section from flailing around uncontrolled..
Your looking for an inverted "V"
with the drogue at the top of the "V" and the booster and payload section each one leg of the "V"...
The legs of the "V" should be spread wide apart,,
so that the payload section is pointed as close to horizontal as possible..
Then it will fire out into open clean unobstructed airspace...
Too big of a drogue and the legs of the "V" will be too close together,,
the payload section will be pointed down and will fire the main directly into the path of the falling payload section and booster...

Teddy

 

[thomas]

I have almost the same configuration as you for my L3 project so I have done the math.

For a 4ft rocketman I get a decent rate of 21m/s (69 feet/s) for a 22kg (48lbs) rocket, that correlates very well with the rocketman drouge decent chart.
For the decent velocity you get (following Knacke)

With c_w the drag coefficient, A the parachute surface, ρ the air density at deployment altitude, M the rocket mass and g the acceleration of gravity g= 9.81m/s^2.

You should also calculate the opening shock both for your drouge chute and the main chute.
In the infinte mass model this gives:

c_x is the opening shock coefficent, its about 1.1 for the rocketman chute.
For the main the infinite mass model highly overestimates the force.

For deployment speeds of 200km/h (180 feet/s) you get something like 1600 N and for 300 km/h (270 feet/s) 3700N for a 4ft rocketman and a 48 lbs rocket.
The shock of the main under drouge is around 6000 N but this is in a region where the infinite mass model is wrong, so in reality it will be much lower up to a factor of 10. That is all for controlled deployment.

You can find almost anything on this in the Parachute Recovery Design Manual by Knacke:
https://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA247666

With the freebag method and serial deployment you will highly reduce the change of entanglement.

The drouge should reduce the speed enough to restrict the main shock to a value your recovery gear can handle.

There is also a nice article by Jürg Thüring but it is in German:
https://www.argoshpr.ch/j3/index.php/knowhow/recovery/123-richtiges-design-von-bergungssystemen

 

[blackjack2564]

Drogue size hinges on how the fin can will fall...vertical or horizontal.

Fin cans with split fins/large fins will tend to fall horizontal on their own, sometimes spinning around their axis also, twisting shock cords.

The purpose of the drogue is to create just enough drag to make all the parts fall vertical and keep the main/payload ABOVE the fincan, so it deploys in free air [can't tangle with any other parts of rocket] and you want to attach the drogue close to payload for that reason.

This is the tough part requiring bit a black art. Just basing Drogue size on weight only will not always work. I've had rockets [2]both weighing the same using same size drogue 24in... worked great one, but not big enough to prevent spinning horizontal on the other, and fincan would float above the payload/ main. Had to upsize to a 36in, on the one with large fins & longer fin canto work properly.

I am simply pointing out."one size does NOT fit all" based on weight alone. Surface area of fins and tubes play a major role.

 

[Onebadhawk]

Drogue size hinges on how the fin can will fall...vertical or horizontal.

Fin cans with split fins/large fins will tend to fall horizontal on their own, sometimes spinning around their axis also, twisting shock cords.

The purpose of the drogue is to create just enough drag to make all the parts fall vertical and keep the main/payload ABOVE the fincan, so it deploys in free air [can't tangle with any other parts of rocket] and you want to attach the drogue close to payload for that reason.

This is the tough part requiring bit a black art. Just basing Drogue size on weight only will not always work. I've had rockets [2]both weighing the same using same size drogue 24in... worked great one, but not big enough to prevent spinning horizontal on the other, and fincan would float above the payload/ main. Had to upsize to a 36in, on the one with large fins & longer fin canto work properly.

I am simply pointing out."one size does NOT fit all" based on weight alone. Surface area of fins and tubes play a major role.

+1...
Well said...

Teddy

 

[JimJarvis50]

Teddy and Jim have it figured out. It is not much about the descent rate, but rather, how the pieces fall. Drogues work well on "conventional" rockets, and you just need to select a drogue that isn't too big. The problems come in when the upper airframe is heavier than normal for some reason, or the fin can is draggy for some reason, and the pieces don't fly well together. My solution in those cases is to delay the opening of the main, which I've posted on that topic elsewhere.

But Jim, you want the drogue closer to the fin can and not the upper airframe. When the drogue catches, the upper airframe just keeps going straight, but the fin can must change direction and pass the drogue. You want to keep the fin can line shorter that that it doesn't crash into the upper airframe.

Hey, this is sort of like a glue thread!

Jim

 

[Onebadhawk]

Teddy and Jim have it figured out. It is not much about the descent rate, but rather, how the pieces fall. Drogues work well on "conventional" rockets, and you just need to select a drogue that isn't too big. The problems come in when the upper airframe is heavier than normal for some reason, or the fin can is draggy for some reason, and the pieces don't fly well together. My solution in those cases is to delay the opening of the main, which I've posted on that topic elsewhere.

But Jim, you want the drogue closer to the fin can and not the upper airframe. When the drogue catches, the upper airframe just keeps going straight, but the fin can must change direction and pass the drogue. You want to keep the fin can line shorter that that it doesn't crash into the upper airframe.

Hey, this is sort of like a glue thread!

Jim

Yes,, this...

yeah,, hahahahah...

Teddy

 

[Titan II]

I have a 5" x 113" 3FNC that is 35.6 pounds after the burn. Using a SkyAngle Cert 3 drogue, the descent rates have always been between 68 and 76 fps.

 

[rharshberger]

Teddy and Jim have it figured out. It is not much about the descent rate, but rather, how the pieces fall. Drogues work well on "conventional" rockets, and you just need to select a drogue that isn't too big. The problems come in when the upper airframe is heavier than normal for some reason, or the fin can is draggy for some reason, and the pieces don't fly well together. My solution in those cases is to delay the opening of the main, which I've posted on that topic elsewhere.

But Jim, you want the drogue closer to the fin can and not the upper airframe. When the drogue catches, the upper airframe just keeps going straight, but the fin can must change direction and pass the drogue. You want to keep the fin can line shorter that that it doesn't crash into the upper airframe.

Hey, this is sort of like a glue thread!

Jim

Hence the reason many of us like OneBadHawk 3rd loop recovery harness for our drogue parachutes. If using a Deployment Bag on the main chute I also use a 3rd loop to put my chute attachment point just above the lip of the upper payload bay. I agree with CJ, Teddy, and JimJarvis, the drogue may take a couple of flights and observation to optimize. My L3 project weighed 35lbs and used a 60" drogue (its what I happened to have, and when tested on a check flight everything worked good so I stuck with it, the descent rate was about 26-30fps so pretty slow), a proper size drogue for my airframe is about 36-40" and future flights will be made with a smaller one.

 

[markkoelsch]

I would think something like a 30" Recon/Skyangle.

 

[Coop]

Think a 30"x30' streamer would be about right. That'd put you 75-80 FPS.


Later!

--Coop

 

[Steve Shannon]

Wow, I learned a bunch in this thread. Mostly what I have done is put a 12 - 24 inch TFR fluorescent chute on as a drogue and fly.
That's not to say I haven't tried other things. I have a Rocketman R24 drogue that I used for my L3 cert flight years ago, and a couple of larger Kevlar Rocketman drogue chutes I bought on the Rocket Planet auctions. They are both too big for use as a drogue for anything I do now, but would be great for a rocket that might result in a high speed deployment, such as a very high altitude flight.
I'm going to study and restudy the posts by Jim and Jim.


[emoji1010] Steve Shannon [emoji1010]

 

[b.wieting]

I have almost the same configuration as you for my L3 project so I have done the math.

For a 4ft rocketman I get a decent rate of 21m/s (69 feet/s) for a 22kg (48lbs) rocket, that correlates very well with the rocketman drouge decent chart.
For the decent velocity you get (following Knacke)
View attachment 295739
With c_w the drag coefficient, A the parachute surface, ρ the air density at deployment altitude, M the rocket mass and g the acceleration of gravity g= 9.81m/s^2.

You should also calculate the opening shock both for your drouge chute and the main chute.
In the infinte mass model this gives:
View attachment 295740
c_x is the opening shock coefficent, its about 1.1 for the rocketman chute.
For the main the infinite mass model highly overestimates the force.

For deployment speeds of 200km/h (180 feet/s) you get something like 1600 N and for 300 km/h (270 feet/s) 3700N for a 4ft rocketman and a 48 lbs rocket.
The shock of the main under drouge is around 6000 N but this is in a region where the infinite mass model is wrong, so in reality it will be much lower up to a factor of 10. That is all for controlled deployment.

You can find almost anything on this in the Parachute Recovery Design Manual by Knacke:
https://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA247666

With the freebag method and serial deployment you will highly reduce the change of entanglement.
View attachment 295750
The drouge should reduce the speed enough to restrict the main shock to a value your recovery gear can handle.

There is also a nice article by Jürg Thüring but it is in German:
https://www.argoshpr.ch/j3/index.php/knowhow/recovery/123-richtiges-design-von-bergungssystemen

AND it is in German!

 

[sharkbait]

Teddy and Jim have it figured out. It is not much about the descent rate, but rather, how the pieces fall. Drogues work well on "conventional" rockets, and you just need to select a drogue that isn't too big. The problems come in when the upper airframe is heavier than normal for some reason, or the fin can is draggy for some reason, and the pieces don't fly well together. My solution in those cases is to delay the opening of the main, which I've posted on that topic elsewhere.

But Jim, you want the drogue closer to the fin can and not the upper airframe. When the drogue catches, the upper airframe just keeps going straight, but the fin can must change direction and pass the drogue. You want to keep the fin can line shorter that that it doesn't crash into the upper airframe.

Hey, this is sort of like a glue thread!

Jim

Agree, it’s how the upper and lower dance together during the descent. I have a lot of birds that go without a drogue because they descend perfectly without it

 

[Chad]

Here's a thread that goes into the "surface area determines drogue size" concept. My two cents, on my 4" and 3" airframes i always use a 12" drogue unless the weight of the airframe puts the decent rate above 75 fps. For my L3 (4" airframe), i used a larger drogue to get the decent rate down a bit. The worry being the shock to the airframe connections when the main deploys.

https://www.rocketryforum.com/threads/drogue-sizing-thoughts.154776/

 

[sharkbait]

Here's a thread that goes into the "surface area determines drogue size" concept. My two cents, on my 4" and 3" airframes i always use a 12" drogue unless the weight of the airframe puts the decent rate above 75 fps. For my L3 (4" airframe), i used a larger drogue to get the decent rate down a bit. The worry being the shock to the airframe connections when the main deploys.

https://www.rocketryforum.com/threads/drogue-sizing-thoughts.154776/

For a lot of extreme altitude flights you have to bring em down pretty hot to stay in the cylinder, some east coast fields are surrounded with tree and a have a very small recoverable zone so same applies without extreme altitude, there are several techniques to deploy your main slowly and several techniques to absorb shock