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mccordmw

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Ok. Getting ready to launch tomorrow for L1. I've got everything worked out now and tested except for one detail which is chute selection. I'm launching a Binder Design Excel Plus 54 dual deploy.

The stock chutes in the kit are nice and durable looking with substantial shroud lines that look like the can stop a truck - good stuff. However, when putting in the specs on Open Rocket, I get the following:

Rocket weight (empty): 2.7 kg
Drogue: 9"
Drogue Drag Coefficient: 1.4 (I estimated here)
Drogue Descent Rate: 30 m/s
Main: 36"
Main Drag Coefficient: 1.4 (again, estimate)
Main Descent Rate: 7 m/s

Isn't the drogue descent rate too fast here? I thought drogue descent was optimally under 60 fps. This is falling at 100 fps. I suppose that's OK since the main is built like a tank and can take the deceleration when the main pops. If I up the Cd to 1.7 (seen that estimate in places as well), the drogue descent goes down to 26.8 m/s (88 fps). Still high, but closer to the 40-60 I've seen.

On to the main, the stock chute will put the rocket on the ground at 7 m/s (15.6 mph). I had read that under 6 m/s is best. This is based on my assumption of a Cd of 1.4. OR defaults to 0.8 which is way too low. Is a 7 m/s impact ok? I do have a larger chute on hand that is 60" and packs down to a similar size. However, the shroud lines aren't as substantial. It would put the rocket down at 5.6 m/s using the manufacturer's statement that their Cd is 0.8. I like that speed better, and popping the main at 1000' won't make it drift too far; especially since winds are predicted to be 2-4 mph. I'm just worried it can't take the hit of deploying at 88-100 fps.

The 60" chute I have is from Sunward:

http://www.sunward1.com/para.htm

Says a 7 lb rocket will fall at mid-range speed.

I seem to be overthinking everything a day before L1. :p
 

RocketFeller

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I think that 15 mph is a quite gentle descent, typically 15-20 mph is considered a good rate, I believe. If you oversize your chute you run the risk of it dragging the rocket along the ground if it is at all windy. Not to mention the obvious issue of drifting.
 

MCriscione

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The Drogue may be a bit small. I'm using a 14" on my Madcow 2.6" DX3 this weekend. But I'll be on a huge field. You're probably fine, and in my searching around, I found 90-100 fps mentioned as an acceptable target, although a bit higher than other references. If you had the option to up it a bit, you probably wouldn't be doing any harm. A higher deployment speed, but with a well made main, you're in fine shape. Don't forget that some (crazy) people fly drouge-less frequently and seem to be recovering just fine. :wink: It's apparently not required to use a drogue, but the way I understand it, it reduces stresses on the Main when opening by reducing air speed, and somewhat reduces the chances of your main hanging up on your booster section when it deploys.

The way I understand it, the drogue is just there to keep the payload/nosecone/main (plus avionics bay as it's probably attached) above the booster. So long as the payload (plus avionics probably) half is lighter than the booster, and the chute has the same or larger cross-section area as the fin profile on the booster, you should be plenty fine. (I'm looking at the website now and the fins look to be about 5"x6" so 5x6 x2=60 or 9^2*pi=63.6. You're in the right ball park so long as your payload (plus avionics) isn't super heavy.
 

bclark989

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Yeah, your stock setup is totally fine based on everything I have seen. I have the 4" Wildman V2 and it weighs 8 lbs on the pad (its a brick, its only 3 feet tall), and I flew it drogueless with no issues. Stratologger CF showed a descent rate of 95 ft/sec or so before popping the main.

The other reason I would say you're fine, setting all anecdotes and data aside, is that Mike Fisher is very thoughtful and intentional with his kits. He wouldn't ship it with that recovery gear if it weren't sufficient.

BC
 

rharshberger

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Ok. Getting ready to launch tomorrow for L1. I've got everything worked out now and tested except for one detail which is chute selection. I'm launching a Binder Design Excel Plus 54 dual deploy.

The stock chutes in the kit are nice and durable looking with substantial shroud lines that look like the can stop a truck - good stuff. However, when putting in the specs on Open Rocket, I get the following:

Rocket weight (empty): 2.7 kg
Drogue: 9"
Drogue Drag Coefficient: 1.4 (I estimated here)
Drogue Descent Rate: 30 m/s
Main: 36"
Main Drag Coefficient: 1.4 (again, estimate)
Main Descent Rate: 7 m/s

Isn't the drogue descent rate too fast here? I thought drogue descent was optimally under 60 fps. This is falling at 100 fps. I suppose that's OK since the main is built like a tank and can take the deceleration when the main pops. If I up the Cd to 1.7 (seen that estimate in places as well), the drogue descent goes down to 26.8 m/s (88 fps). Still high, but closer to the 40-60 I've seen.

On to the main, the stock chute will put the rocket on the ground at 7 m/s (15.6 mph). I had read that under 6 m/s is best. This is based on my assumption of a Cd of 1.4. OR defaults to 0.8 which is way too low. Is a 7 m/s impact ok? I do have a larger chute on hand that is 60" and packs down to a similar size. However, the shroud lines aren't as substantial. It would put the rocket down at 5.6 m/s using the manufacturer's statement that their Cd is 0.8. I like that speed better, and popping the main at 1000' won't make it drift too far; especially since winds are predicted to be 2-4 mph. I'm just worried it can't take the hit of deploying at 88-100 fps.

The 60" chute I have is from Sunward:

http://www.sunward1.com/para.htm

Says a 7 lb rocket will fall at mid-range speed.

I seem to be overthinking everything a day before L1. :p
If you have the 60" chute already, use it , the last time you want to have damage to rocket is on a cert flight. I use a 60" custom semi-hemispherical chute (read as my wife makes them for me) on my 3" Frenzy XL and I get a descent velocity of 18fps under the main (20fps is the max recommended from the the NAR L3 reqs iirc as a safe speed) . As for the Drogue the most important thing is that the main chute payload bay stays above the fin can as the rocket drops from apogee, I usually target a 75fps descent rate under drogue, my 3" Frenzy XL use as 18" drogue which is really to large at 37fps descent rate (but I get reliable deployments). FWIW the Frenzy XL weighs almost exactly 6 pounds RTF about the same as the rocket you are building/going to fly.
 
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mccordmw

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If you have the 60" chute already, use it , the last time you want to have damage to rocket is on a cert flight. I use a 60" custom semi-hemispherical chute (read as my wife makes them for me) on my 3" Frenzy XL and I get a descent velocity of 18fps under the main. As for the Drogue the most important thing is that the main chute payload bay stays above the fin can as the rocket drops from apogee, I usually target a 75fps descent rate under drogue.
Since the winds will be almost non-existent, I'm inclined to use the larger chute. I'm just concerned that the shroud lines aren't beefy enough to take a near 100 mph main deployment. The shroud lines are braided nylon about 3 mm diameter sewn into the chute.
 

rharshberger

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Since the winds will be almost non-existent, I'm inclined to use the larger chute. I'm just concerned that the shroud lines aren't beefy enough to take a near 100 mph main deployment. The shroud lines are braided nylon about 3 mm diameter sewn into the chute.
Then I recommend up-sizing the drogue to a 12" or 14" (If you can get/have one thats suitable), and using the 60". There is also no reason not to go with the chutes provided with the rocket, as they are a tested configuration, I just like slower descent velocities.
 

mccordmw

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I launch tomorrow, so no time to get a larger drogue. I'll probably play it safe and go with the kit's chutes. I'd rather break a fin with a fast landing than rip the main off and crash the thing at an unsafe speed.

*hand wringing* :p
 

Banzai88

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I launch tomorrow, so no time to get a larger drogue. I'll probably play it safe and go with the kit's chutes. I'd rather break a fin with a fast landing than rip the main off and crash the thing at an unsafe speed.

*hand wringing* :p
Ask around at your field, never been on a rocket flight line where someone wouldn't loan me something simple like that......as long as I was willing to pay for it if I broke it! :wink:
 

markkoelsch

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I aim for 90-100 ft/sec under drogue. Why go slower? If the drogue cannot take that then I think you have a different problem.

I think you will be fine looking at your numbers. You might be a little high on the Cd of the chutes though.

If you have the 60" main handy go for it.
 

markjos

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I aim for 90-100 ft/sec under drogue. Why go slower? If the drogue cannot take that then I think you have a different problem.

I think you will be fine looking at your numbers. You might be a little high on the Cd of the chutes though.

If you have the 60" main handy go for it.
+1 to all of this. Slower than that on the drogue, and you begin to waste the advantage of minimizing drift that dual deployment provides. 90+fps on the drogue and 20fps on the main are good target numbers.

It's not clear where your parachute Cd is coming from, and I think those numbers are high, as well, unless the geometry is pretty special (though this doesn't help our case - it means that the descent rates will probably be higher than calculated).

Mark
 

mccordmw

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+1 to all of this. Slower than that on the drogue, and you begin to waste the advantage of minimizing drift that dual deployment provides. 90+fps on the drogue and 20fps on the main are good target numbers.

It's not clear where your parachute Cd is coming from, and I think those numbers are high, as well, unless the geometry is pretty special (though this doesn't help our case - it means that the descent rates will probably be higher than calculated).

Mark
The default Cd is 0.8, but forums here toss around numbers in the 1.4 range, so I tried that. If I revert to a default of 0.8, the stock kit looks too fast on landing.

Drogue desecent: 39 m/s (128 fps)
Stock 36" main descent: 9.3 m/s (30 fps)
-or-
Sunward 60" main descent: 5.7 m/s (19 fps)

I'd prefer the 60", but I'm not sure about it surviving the drogue descent. Or is Open Rocket not taking into account the airframe for desecent rate? If I take off the drogue entirely, the descent rate is predicted to be 29 m/s. That doesn't make sense to me. I think something is fishy on their calculations.
 

Banzai88

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OR's native CD for a flat sheet is something like .8.

The owner of TF recently posted this in another thread:

XT = .98
Standard = 1.34
Crossfire = 1.55
 

GregGleason

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Ok. Getting ready to launch tomorrow for L1. I've got everything worked out now and tested except for one detail which is chute selection. I'm launching a Binder Design Excel Plus 54 dual deploy.

The stock chutes in the kit are nice and durable looking with substantial shroud lines that look like the can stop a truck - good stuff. However, when putting in the specs on Open Rocket, I get the following:

Rocket weight (empty): 2.7 kg
Drogue: 9"
Drogue Drag Coefficient: 1.4 (I estimated here)
Drogue Descent Rate: 30 m/s
Main: 36"
Main Drag Coefficient: 1.4 (again, estimate)
Main Descent Rate: 7 m/s

Isn't the drogue descent rate too fast here? I thought drogue descent was optimally under 60 fps. This is falling at 100 fps. I suppose that's OK since the main is built like a tank and can take the deceleration when the main pops. If I up the Cd to 1.7 (seen that estimate in places as well), the drogue descent goes down to 26.8 m/s (88 fps). Still high, but closer to the 40-60 I've seen.

On to the main, the stock chute will put the rocket on the ground at 7 m/s (15.6 mph). I had read that under 6 m/s is best. This is based on my assumption of a Cd of 1.4. OR defaults to 0.8 which is way too low. Is a 7 m/s impact ok? I do have a larger chute on hand that is 60" and packs down to a similar size. However, the shroud lines aren't as substantial. It would put the rocket down at 5.6 m/s using the manufacturer's statement that their Cd is 0.8. I like that speed better, and popping the main at 1000' won't make it drift too far; especially since winds are predicted to be 2-4 mph. I'm just worried it can't take the hit of deploying at 88-100 fps.

The 60" chute I have is from Sunward:

http://www.sunward1.com/para.htm

Says a 7 lb rocket will fall at mid-range speed.

I seem to be overthinking everything a day before L1. :p
I don't know where you got the value 1.4 for the CD. I back engineered the CD for the 60" chute on the Sunward site that you referenced.

The CD I get from Sunward's published data for the 60" chute is 0.89 to give a 6.75 lb (average of 5.5 lbs and 8 lbs from Sunward's site) rocket a descent rate of 18.0 fps (what Sunward refers to as the mid-range descent rate).

If I do the same exercise with the 36" chute, the CD is 0.82 for a descent rate of 18.1 fps.

If I plug in these values, your descent rate is 29.4 fps (~20 mph) for the 36" chute and 16.9 fps (~12 mph) for the 60" chute.

There is no published data for the 9" chute, but doing a similar exercise for the 12" chute and the CD is 0.82 for a descent rate of 18.1 fps.

So if we plug in an estimate an 0.82 CD for the 9" chute for your rocket's empty weight of 2.7 kg, the descent rate is 117.6 fps (~80 mph) for the 9" chute.




Sooooooooo, you may want to check my math to see if it is in the ballpark. I'd hate to be right and you be wrong on this.

I wish you the best of success.

Greg
 

Coop

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There's no way the Sunward flat chutes have a 1.4 Cd.... Greg's numbers are sound.


Later!

--Coop
 

mccordmw

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Ok. L1 cert went perfectly. Here's the specs and my sim data if anyone wants to calculate the Cd. The real data was quite a bit slower in descent than the sim.

Rocket weight on pad: 3.25 kg
Rocket burnout weight: 2.95 kg
Sim apogee: 2510 ft
Real apogee: 2540 ft
Drogue: 12"
Sim descent: 29 m/s
Real descent: 17 m/s
Main: 36"
Sim descent: 9.1 m/s
Real descent: 7.6 m/s

I'm thinking that OR doesn't factor in the drag from the shock cord and airframe tumbling as well?

Raw data attached comma separated if anyone is interested.

View attachment dtl2.txt
 

K'Tesh

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Congrats on your Cert!!! Thanks for sharing the data on the simmed and actual descents.
 

GregGleason

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Congrats!

BTW, the revised Cd for your rocket/chute for 12" is 1.47, and for the 36" chute is 1.24.

The Cd 36" is not all that surprising since you have the added drag from the drogue. The Cd value for the 12" drogue is surprising in that the rocket does contribute a substantial drag, and would likely have a lot lower numerical value if the chute payload was a lead ball of equivalent mass.

Greg
 

mccordmw

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Congrats!

BTW, the revised Cd for your rocket/chute for 12" is 1.47, and for the 36" chute is 1.24.

The Cd 36" is not all that surprising since you have the added drag from the drogue. The Cd value for the 12" drogue is surprising in that the rocket does contribute a substantial drag, and would likely have a lot lower numerical value if the chute payload was a lead ball of equivalent mass.

Greg
Thanks!

It would be interesting to see if everyone else has a much higher observed aggregate Cd on their rockets vs just the default for a flat sheet of 0.8. It seems that OR and Rocksim don't take the drag of the whole body into account?
 

rharshberger

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Thanks!

It would be interesting to see if everyone else has a much higher observed aggregate Cd on their rockets vs just the default for a flat sheet of 0.8. It seems that OR and Rocksim don't take the drag of the whole body into account?
Neither program would take the body drag into consideration as it would be very difficult to sim, and OR has a place to change the Cd for the chute so that if you know the chutes Cd you can put the correct one in.
 

Binder Design

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Late to the party here, but as you can imagine, this combination of chute sizes has been extensively tested. I'd say if you were landing on a very unforgiving surface to upgrade the main to a 45". The drogue size doesn't matter much, you can leave it completely off and it will still recover safely.
 

mccordmw

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Late to the party here, but as you can imagine, this combination of chute sizes has been extensively tested. I'd say if you were landing on a very unforgiving surface to upgrade the main to a 45". The drogue size doesn't matter much, you can leave it completely off and it will still recover safely.
Yeah. This was just the pre-L1 overthink about everything and freak out stage. The OR sim didn't take the body into account, so it kept alerting about fast descents, so angst kicked in. :p

Thanks for the advice and the beautiful model. I'll put it up on an L265 Mellow without much concern over its flightworthiness.
 

mccordmw

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I did a lot of reading up on drogue descent rates and the effect of tube tumble in fluid dynamics. I think I've hit of a good way to estimate descent. OR and Rocksim don't calculate the contribution the airframe has on drag and descsent. However, you can add that it and get a really close estimate to measured descents.

For the drogue:
- use the proper drag coefficient for the flat sheet of 0.8
- or use a Cd of 1.4-1.7 for hemispherical drogues (see vendor sites if they have the data)

Add in the airframe tumble:
- take measurements of the largest section of the airframe after deployment (since that contributes most of the drag)
- calculate the total surface area (ignore the fins)
- calculate the diameter of a circle with the matching total area to get an equivalent drogue size
- add another "drogue" to the build in OR/Rocksim set to deploy at apogee since that's when tumble starts
- set the Cd to 0.75 as that is the approximate drag coefficient of a cylinder in a fluid environment

The simulation will now show a more accurate descent.

For the main:
- use the proper drag coefficient for the flat sheet of 0.8
- or use a Cd of 1.4-1.7 for hemispherical mains or x-mains or whatever the shape (see vendor sites if they have the data)

The simulation still factors in drag from the drogue and airframe tumbling "drogue" in the new descent rates. It closely matches my real flight data.

Example:
My real drogue is 9" with a Cd of 0.8
OR descent was calculated to 39 m/s
-- now --
Calculate the airframe surface area as an equivalent drogue size circular area
- diameter = 4"
- length = 34"
- area of a cylinder = L x pi*D = 427.26"
- convert that to area of a matching circle = 427.26" = pi*r[SUP]2[/SUP] = 23.3"
Add a "drogue" to the rocket with a 23.3" diameter and a Cd of 0.75
Rerun simulation

The simulation showed my rocket with the following profiles:

Before new drogue:

descent on drogue = 39.1 m/s
descent on main = 9.3 m/s

After new drogue addition:
descent on drogue = 15 m/s
descent on main = 8 m/s

Real flight data:
descent on drogue = 16 m/s
descent on main = 7.9 m/s

Sim is now really really close to real life. At least in this example. Need someone to test theirs out to see if it extends. If so, it gives you a simple way to get a close estimate of what chute sizes you should add after factoring in the airframe's drag.
 

mccordmw

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I'll add that I just tested it on my single-deploy, scratch built rocket that has a tube of 3" x 48" for that main part. That was the equivalent of a 24" drogue with a Cd of 0.75. It adjusted the landing speed from 7.2 m/s to 5.6 m/s which more closely matched the measured rate of 5.8 m/s. Nice.

I'll make it my SOP to add the airframe as a drogue at apogee on future designs. Now if we can just have that as an auto-factored in option in OR or Rocksim.
 

Banzai88

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There's an issues of Apogee's Peak of Flight newsletter that spells all that out, just like that. https://www.apogeerockets.com/education/downloads/Newsletter361.pdf

The problem with it is that is assumes that the airframe will always lay sideways and tumble, which is not always the case.

BUT.....if your rocket booster section does lay flat, it works.

I use the computations just like you do and my sims get within 2%.
 
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