Drogue descent rate / chute sizing for L3

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

bclark989

Well-Known Member
TRF Supporter
Joined
Dec 27, 2014
Messages
217
Reaction score
23
I am working on my L3 rocket and have seen quite the spectrum of opinions on drogue descent rate and parachute sizing / style. I see a thread that touched on it from May, but figured I would get a new one going since that thread started out talking about mains.

So here are the facts:
  • Rocket weighs ~21 lbs at burnout.
  • Lower air frame and fin can section is 58" long
  • AV bay to the nose cone will total about 55" long.
  • Mass is a pretty even split, don't have exact numbers yet until basic construction is complete.
Until now, I was planning to use a 20" Top Flight X-form drogue. The simulation shows the descent rate stabilizing just after apogee at about 135 ft/sec. When the main deploys, it will be at about 100 ft/sec. The latter seems reasonable overall, but I don't think I have heard people say 135 ft/sec is safe. I also wonder if 100 ft/sec is too fast for a 72" main to pop out and handle. And all of that being said, on the other end of the spectrum, I have seen people fly rockets this size totally drogue-less, provided the mass is balanced enough that the separated air frame will tumble.

So I guess here are my questions:
  • What guidelines do you follow for drogue descent rate? Are they published somewhere, or from your own experience?
  • What calculations go into drogue placement on the harness? Or do people wing it based on anecdotal evidence or their own experience?
  • How do people justify flying drogue-less? Are their weight limitations? What is a typical drogue-less descent rate?
Thanks!
 
  • What guidelines do you follow for drogue descent rate? Are they published somewhere, or from your own experience?
  • What calculations go into drogue placement on the harness? Or do people wing it based on anecdotal evidence or their own experience?
  • How do people justify flying drogue-less? Are their weight limitations? What is a typical drogue-less descent rate?
  1. The purpose of the drogue, IMHO, is not to slow down the rocket, but rather to get the halves dangling and pointing in the right direction. This is my opinion.
  2. I put the drogue close to the AV bay to get a nice inverted V shape as the rocket descends. I don't think it really matters where you put it. Again, my opinion.
  3. With drogueless, you are at the mercy of chance as to where things will be pointing when the main comes out. If the NC happens to be pointing back at the booster, you may get a clash, or you may get entanglement.
So, in my opinion, make the drogue as small as possible, but beefy enough to not rip the shroud lines when it deploys. The Rocketman has some high-speed drogues for this purpose.
 
I am working on my L3 rocket and have seen quite the spectrum of opinions on drogue descent rate and parachute sizing / style. I see a thread that touched on it from May, but figured I would get a new one going since that thread started out talking about mains.

So here are the facts:
  • Rocket weighs ~21 lbs at burnout.
  • Lower air frame and fin can section is 58" long
  • AV bay to the nose cone will total about 55" long.
  • Mass is a pretty even split, don't have exact numbers yet until basic construction is complete.
Until now, I was planning to use a 20" Top Flight X-form drogue. The simulation shows the descent rate stabilizing just after apogee at about 135 ft/sec. When the main deploys, it will be at about 100 ft/sec. The latter seems reasonable overall, but I don't think I have heard people say 135 ft/sec is safe. I also wonder if 100 ft/sec is too fast for a 72" main to pop out and handle. And all of that being said, on the other end of the spectrum, I have seen people fly rockets this size totally drogue-less, provided the mass is balanced enough that the separated air frame will tumble.

So I guess here are my questions:
  • What guidelines do you follow for drogue descent rate? Are they published somewhere, or from your own experience?
  • What calculations go into drogue placement on the harness? Or do people wing it based on anecdotal evidence or their own experience?
  • How do people justify flying drogue-less? Are their weight limitations? What is a typical drogue-less descent rate?
Thanks!

Brian-

See what your L3 TAP recommends for the cert flight, but IMHO there really is no target descent rate for drogues as you will get a variety of answers on what people use or don't use on certain flight profiles. I attach my drogues 1/4 to 1/3 the length of the shock cord down from the attachment point of AV-bay.

From personal experience, I've flown heavier rockets than the 21lbs your rocket will be to 30k+ using smaller drogues or drogue-less. The Ultra X-Type drogue chutes will have no issues performance wise, it will just depend on how fast you feel comfortable bringing your rocket down and the advice from your L3 TAP.

Preston
 
Last edited:
Preston,

Thanks for the email back, and for the reply here =)

Looking at this X-form chute, I am definitely not worried about it holding up, but worried a bit about deploying the main at 100 ft/sec. Based on what you and John have said here, it seems like I probably don't need to worry.

Thanks again!
 
Preston,

Thanks for the email back, and for the reply here =)

Looking at this X-form chute, I am definitely not worried about it holding up, but worried a bit about deploying the main at 100 ft/sec. Based on what you and John have said here, it seems like I probably don't need to worry.

Thanks again!

Brian-

You're welcome.....just make sure you consult with your L3 TAP as they will be the ones that are going to review/approve your plans for the L3 flight.

Preston
 
As another data point, my L3 flight was with an Extreme Wildman with a 24-inch SkyAngle CERT-3 drogue, rocket weighed 18.7 lbs after burnout, and the descent rate was about 72 FPS.

I've flown this rocket many times since and been happy with the drogue. I really wouldn't use anything smaller, but YMMV.
 
Completely agree with "go with TAP recommendation", the drogue is primarily there to stabilize the rocket and stop ballistic recovery.
When I use a drogue, placement is always so that the two halves cannot (should not) bash into each other on descent.
When I don't use drogue, it is always to get the rocket down faster.
Here is another thought for you.... I have started putting a chute release on the drogue, releasing at 1000ft, with the main at 600ft. This way, I have rapid descent "Drogueless" to 1000ft, then open a drogue to slow the descent enough to allow safe deployment of the main without chance of shredding that main chute.

Good luck, looking forward to seeing the flight :)
 
If you are concerned about high speed opening of the main chute, you can add a ring to the shroud lines as a slider. Start with it around all the shroud lines near( but not at ) the shroud to chute connections. It will reef the chute at first, then slide down toward the shock cord connection. As it does the chute opens slowly decelerating the rocket over a longer time.
Note you MUST allow extra fall distance to open and slow decent speed this way, so set you deployment altitude higher.
(I would recommend "playing" with this a bunch of times on smaller rockets, before a certification flight. I find it neat to watch a big chute slowly open.)
Mike
 
FYI I generally aim for around 70-80fps give or take for drogue descent. I am ok up to 100fps, but above that I get nervous. YMMV.

Consider whether you want your airframe to come in flat, or dangling under the chute. That will guide you in your selection also. Faster velocities generally have the airframe coming in flat and can result in clanging of sections.
 
Rocket weighs ~21 lbs at burnout.
I also wonder if 100 ft/sec is too fast for a 72" main to pop out and handle.

Your bigger concern is the Descent Rate on the Main chute.

https://descentratecalculator.onlinetesting.net

A 21 pound rocket on a 72" chute descends at 28 ft/sec . . .

15 ft/sec is the "target" max descent rate, especially on a Cert flight !

Dave F.
 
Completely agree with "go with TAP recommendation", the drogue is primarily there to stabilize the rocket and stop ballistic recovery.
When I use a drogue, placement is always so that the two halves cannot (should not) bash into each other on descent.
When I don't use drogue, it is always to get the rocket down faster.
Here is another thought for you.... I have started putting a chute release on the drogue, releasing at 1000ft, with the main at 600ft. This way, I have rapid descent "Drogueless" to 1000ft, then open a drogue to slow the descent enough to allow safe deployment of the main without chance of shredding that main chute.

Good luck, looking forward to seeing the flight :)
Cool ideaI I might have to try that next time I go really high.
 
Your bigger concern is the Descent Rate on the Main chute.

https://descentratecalculator.onlinetesting.net

A 21 pound rocket on a 72" chute descends at 28 ft/sec . . .

15 ft/sec is the "target" max descent rate, especially on a Cert flight !

Dave F.

My main is a Fruity Chutes Iris Ultra, which has an advertised Cd of 2.2. The simulation has a ground hit velocity of 17.6 ft/sec. I think even if it doesn't perform optimally, I should come in under 20 ft/sec.

I have also thought about shedding weight by cutting down the lower airframe section, but was hoping to avoid that initially to intentionally keep my apogee lower for the cert flight.
 
FYI I generally aim for around 70-80fps give or take for drogue descent. I am ok up to 100fps, but above that I get nervous. YMMV.

Consider whether you want your airframe to come in flat, or dangling under the chute. That will guide you in your selection also. Faster velocities generally have the airframe coming in flat and can result in clanging of sections.

It has always made me nervous too, but I don't actually know why, hence the thread. I think I just inherited those benchmarks from the rocketry hivemind. We'll see what TAPs say!
 
If you are concerned about high speed opening of the main chute, you can add a ring to the shroud lines as a slider. Start with it around all the shroud lines near( but not at ) the shroud to chute connections. It will reef the chute at first, then slide down toward the shock cord connection. As it does the chute opens slowly decelerating the rocket over a longer time.
Note you MUST allow extra fall distance to open and slow decent speed this way, so set you deployment altitude higher.
(I would recommend "playing" with this a bunch of times on smaller rockets, before a certification flight. I find it neat to watch a big chute slowly open.)
Mike

This is really neat idea, but I probably won't try it on the cert flight, hehe. I have a Hyperloc 835 with a good sized chute, maybe I will experiment with it on that. Thanks!
 
I would typically open my mains at 90 ft/sec and never had a problem. My burnout weight was about 30 pounds and I had two home-made mains that were about 54" inflated diameter. It would pull about negative 8 g's when the mains opened. I would think that a top-notch chute like a Fruity Iris Ultra will handle opening loads significantly better than my home made ones.
 
My rocket was rear-ejection, so I was able to circumvent concerns of tangling and bashing (but there were other tradeoffs of course).
 
I have also thought about shedding weight by cutting down the lower airframe section, but was hoping to avoid that initially to intentionally keep my apogee lower for the cert flight.

If you want to decrease altitude, increase Drag . . . Make a removable "Drag Plate" for the base of the rocket.

Dave F.
 
Here is some advice I give my L3 candidates on drogue and main selection (whether they want it or not:)

Generally, people use a drogue that is too big and results in the nose section pointing straight down. If you are unsure of the correct drogue size, use the smaller option.

Anytime you fly a rocket, watch how the rocket falls under drogue. Most people don't watch for some reason, but I always do. You want to see if you have the "V" formation you want or if the nose is too straight down or if the fin can is "flying". You adjust the drogue size based on the observation of how the rocket falls.

You can't estimate the descent rate for a drogue using a calculator because if the parts aren't hanging straight down, they will slow the descent below the calculator result. The purpose of a drogue is to achieve a shape, not a speed.

A deployment bag should be used to organize the deployment of the main for most L3 flights. Better organization is needed as the chute gets bigger. It's likely the biggest reason that L3 flights fail.

For rockets with zipperless couplers, I recommend putting the drogue closer to the fin section. For non-zipperless couplers, closer to the av bay.

Avoid using excess hardware on the harnesses. Hardware provides places for the harness to tangle.

Jim
 
I also second the SkyAngle Cert 3 drogue chute. I also agree with Jim if you are using an anti-zipper coupler, you want the chute closer to the coupler. If not, have it closer to the altimeter bay.
 
I totally agree with Jim, "The purpose of a drogue is to achieve a shape, not a speed." It's there to control how the rocket falls. Watch how the rocket falls under drogue on every flight. Binoculars help. That is usually the longest part of the flight and has the highest potential of messing up.

I've been getting better with more experience, but I seldom get the drogue sized right on the first flight or two.
 
Completely agree with "go with TAP recommendation", the drogue is primarily there to stabilize the rocket and stop ballistic recovery.
When I use a drogue, placement is always so that the two halves cannot (should not) bash into each other on descent.
When I don't use drogue, it is always to get the rocket down faster.
Here is another thought for you.... I have started putting a chute release on the drogue, releasing at 1000ft, with the main at 600ft. This way, I have rapid descent "Drogueless" to 1000ft, then open a drogue to slow the descent enough to allow safe deployment of the main without chance of shredding that main chute.

Good luck, looking forward to seeing the flight :)

That is one of the neatest ideas I've read in a long time. Going to file it in my brain for future consideration.
Could possibly use a little larger drogue maybe but not too large in that scenario?
Kurt Savegnago
 
I totally agree with Jim, "The purpose of a drogue is to achieve a shape, not a speed." It's there to control how the rocket falls. Watch how the rocket falls under drogue on every flight. Binoculars help. That is usually the longest part of the flight and has the highest potential of messing up.

I've been getting better with more experience, but I seldom get the drogue sized right on the first flight or two.




Getting into a drogue achieving a specific shape, I was wondering how you would attach a three loop nylon tether line between the AV bay and Booster?

I've got a 25/ft tether with 3 Loops. The 3rd/Middle Loop is only 3/ft way from one end. So with one end connected to the AV Bay and the other to the booster, should the middle loop (only 3/ft away from an end) connecting the drogue be closer to the AV Bay or the Booster?

A: AV-Bay
B: Drogue Parachute
C: Booster


Config 1: (A)=====(B)===============(C)
Config 2: (A)===============(B)=====(C)



Thanks,
 
Getting into a drogue achieving a specific shape, I was wondering how you would attach a three loop nylon tether line between the AV bay and Booster?
The third loop typically goes toward the end closet to the AV-Bay. Not saying that is the only way but it is the only way I have ever seen it.
1626867215111.jpeg
 
The third loop typically goes toward the end closet to the AV-Bay. Not saying that is the only way but it is the only way I have ever seen it.



Thanks Tim. That's what I figured, but wanted to make sure I wasn't delusional. :)
 
Back
Top