L2 cert flight descent rates

My L2 went well this weekend on my ARR Predator on a Loki J350. Folks really liked how that motor screamed.

I downloaded the data file from the Telemetrum (I realized the file from the radio stream had some drop outs which is why it looked weird).

Overall it looks very good. I made it under the AGL, hit Mach .8 and got a nice acceleration curve. Descent rates are the only part that bother me a little. Drogue was average of -76/ft/s. I'm pretty much okay with that. But main which opened at about 800 feet was an average of -34 ft/s. I was expecting something closer to 25-30.

The rocket had really minor damage. Just cosmetic paint scrapes, etc. that I think most people would just ignore. I touched her up anyway.

There is nothing even close to structural damage.

I'd prefer to have a softer landing but there just isn't any room for a bigger main chute. I can barely fit the four foot Rocketman in there as it is. I could put a larger drogue. But I'm not too excited about the prospect of having it go much further (it landed .63 miles away from the launch site).

Thoughts on a -34/ft/s descent? Maybe fire the main at say 1100 feet so I get a bit more time (main was open for 30 seconds on this flight). Or maybe just call it close enough and leave her alone?

34 ft/s is way too fast for comfort. Firing the main at a higher altitude won't do much to slow the descent rate at landing. 30 seconds is more than enough time for a rocket to slow to its new, slower, terminal velocity.

How heavy is this rocket? How large is the parachute bay?

My suggestion is to try a different type of parachute. Spherachutes tend to pack very small and are a great buy for the worksmanship and durability you get. If you need the minimum descent rate in a given parachute bay, you can get a Fruity Chutes Iris Ultra, but that's a good deal more expensive.

EDIT: A Predator 10k supposedly weighs 4 pounds; at sea level a 48" Spherachute will lower a 4 pound rocket at around 15 ft/s, and I could pack my own 48" Spherachute into ~4 inches of 2.6" airframe.

I'm starting to think that the main chute may not have fully deployed or got tangled. There is no way I'd get that much difference with a 48" rocket man. Nobody had eyes on it so now way to tell.

Atlasboy said:

I'm starting to think that the main chute may not have fully deployed or got tangled. There is no way I'd get that much difference with a 48" rocket man. Nobody had eyes on it so now way to tell.

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What did it look like on the ground? I (almost) always take a landing photo for documentation.

Great question. I wish I knew. Someone did not follow club rules and picked it up and drove it back (nice of them, but as you point out, I lost some data). So, long story short, I've got no clue. Another option would be to increase the drogue to 24" instead of the 12" I used. Loaded with casing and motor, my rocket is about six and a half pounds.

I may have missed something obvious. The Rocketman chute has an adjustable descent rate using the little clip thing on the lines. I flat out missed that. So, pause for noob question. . Would putting that clip towards the chute decrease the descent rate? I assume that is right and would explain my issue (I put mine all the way to the bottom towards the connection to shock cord).

If you put the clip towards the bottom at the shock cord, then the chute can open fully and slows the rocket more. If you put the clip right where the shroud lines meet the canopy, the canopy can't open as much and the rocket falls faster.

Well there goes that theory. I just checked the clip. It was only about four inches off the connection to the shock cord. I must have got a tangle or something else I'm not thinking of. I checked the charts at Rocketman and I should be getting 25. Time to put the think cap on again. Or maybe just get a two foot drogue instead of the 1 foot I used.

A bigger drogue will do almost nothing once the main is out, but it will make your rocket drift farther.

76 feet per second is a good speed.

Gotcha. Thanks.

+1 for everything CarVac said. I would fly it again and not change anything except make sure the chute is packed neatly. I used a packing procedure very similar to this and I haven't, knock on wood, had a chute foul yet.

I'm trying to dig into the Telemetrum data a little deeper. I think what I'm looking at is an average over the 800 feet of main. So, I expect if I look at the discrete data points I'm going to see a higher fps initially and then it should balance out when terminal velocity is reached. What I really want to determine is what the actual data point for descent rate was just before landing. I don't think the 34 represents that, although I'm not sure yet. I'll let you know when I look into it. I guess I could try to get a bigger chute in there. I really like the Rockeman chute in there. I could put a 5 foot (I think) but it is gonna be tight.

Here is another example of parachute folding from Apogee Rockets.

https://www.apogeerockets.com/education/downloads/Newsletter184.pdf

@Dave: thanks. That is a nice one sheet on folding. I have about a 50% confidence I could fold and get a 5 foot Rockeman in there. It is only $45. If I can't do it I can use that chute on some other bird. But I still want to make sure this is not all about nothing. I'm hoping to get into the raw data tonight. I still suspect it was not going 34 fps when it hit. If it was I'd expect some real damage. What I had was paint scrapes that I suspect would not bother most (I am touching everything up because it bothers me for some reason).

What is the size of the parachute bay?

Body tube length is 45.7 cm (although about 12.7 is taken up by half of the bay).

OD is 6.82 cm.
ID is 6.5 cm
Wall thickness is .158 cm.

So basically you have ~30 cm = 12 inches in 2.5" tubing for a parachute?

I could fit not just one but two 54" Spherachutes in that space with some Kevlar shock cord too. Do check them out: https://spherachutes.com/. They look gorgeous in the air, too.

Personally, I'm not a fan of the 4-shroud-line parachutes in the smaller sizes. One of my friends bought a 48" TAC-1 from Giant Leap and it's basically useless as a main parachute: I could barely get it to fit in 18 inches of 3" tubing, so any rocket that fits it is practically too heavy for it already. It would be great on a large L3 project as a drogue, but none of us are planning on that. I'm sure the Rocketman is smaller, but from what it sounds like it's still much more bulky than necessary.

CarVac said:

So basically you have ~30 cm = 12 inches in 2.5" tubing for a parachute?

I could fit not just one but two 54" Spherachutes in that space with some Kevlar shock cord too. Do check them out: https://spherachutes.com/. They look gorgeous in the air, too.

Personally, I'm not a fan of the 4-shroud-line parachutes in the smaller sizes. One of my friends bought a 48" TAC-1 from Giant Leap and it's basically useless as a main parachute: I could barely get it to fit in 18 inches of 3" tubing, so any rocket that fits it is practically too heavy for it already. It would be great on a large L3 project as a drogue, but none of us are planning on that. I'm sure the Rocketman is smaller, but from what it sounds like it's still much more bulky than necessary.

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Honestly the 4 line design isn't the problem - the material used by many of the parachute companies is just too bulky. I have some very compact 4 line chutes that I love - but they are as big as the one discussed here and fit in the palm of my hand when folded - I could likely comfortably put these into an 8" long section of 2" or maybe even 1.6" tube.

UncleVanya said:

Honestly the 4 line design isn't the problem - the material used by many of the parachute companies is just too bulky. I have some very compact 4 line chutes that I love - but they are as big as the one discussed here and fit in the palm of my hand when folded - I could likely comfortably put these into an 8" long section of 2" or maybe even 1.6" tube.

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While my experience is limited to 20 pounds or less, in my experience many chutes are overbuilt in the wrong way. If you have few shroud lines, you have to have very strong fabric to prevent rips, or you have to have non-lift-generating triangles to reduce stress concentration. The Tac-1 does both of these, and also puts webbing over the top, which I feel is unnecessary in the 4-foot size of the one I've handled.

My opinion is this: Topflite is great when you want compact at great prices, Fruity Chutes is the best, but they cost a lot, and Spherachutes are a great compromise and look the prettiest in the air.

Keith over at Altus helped me analyze the raw data so I wasn't working with averages. It looks like the big fin section hit at about 29 ft/s and then of course everything slowed down and the remainder came down at about 19 ft/s.

Here's a thought. . . . what about adding a smaller second chute to the main? I know, I probably am just going to have to bite the bullet and get a new bigger chute. I just hate to have a beautiful (not cheap) chute not be used. . . .

Is there anyone here that would be okay with their 5-6 lb rocket coming to the ground at 29 ft/s? I know CarVac would be a no! Anyone else say they'd be okay with it?

I'm a sissy--I try to get every landing this side of 20 FPS.

Later!

--Coop

CarVac said:

While my experience is limited to 20 pounds or less, in my experience many chutes are overbuilt in the wrong way. If you have few shroud lines, you have to have very strong fabric to prevent rips, or you have to have non-lift-generating triangles to reduce stress concentration. The Tac-1 does both of these, and also puts webbing over the top, which I feel is unnecessary in the 4-foot size of the one I've handled.

My opinion is this: Topflite is great when you want compact at great prices, Fruity Chutes is the best, but they cost a lot, and Spherachutes are a great compromise and look the prettiest in the air.

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The older Rocketman chutes are much lighter weight and pack very nicely. I to have heard great things about Topflite and Spherachutes. I love my Fruity Chutes 36" Iris ultra because it packs small and has a CD over 2.

Atlasboy said:

Is there anyone here that would be okay with their 5-6 lb rocket coming to the ground at 29 ft/s?

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It's your rocket. I pretty regularly end up descending at 25 fps, but 29 seems a tad high. NAR doesn't have a specific requirement for descent rate, even for L3, 20 fps is only a recommendation.

I looked at the Sphereachutes that CarVac suggested. The 60 would pack pretty easily and looks like I'd get 15 to 20.

I've got topflite chutes in my 14 pound Dark Star, have a few flights on it, with no problems. Planning on using a Topflite main at least in my L3 project.

All of my cert rockets have Top Flight chutes in them. I have a 60" X-Type in my L1 rocket and it easily packs in a 3" tube. My L2 has a 24" drogue and 70" Main. The 70" fits a little tight in a 3" tube but If pack it really neat like the document I attached earlier it fits fine and deploys good as well. I do DD with that rocket and set the Adept22 at 600' for the main.

@CarVac: you mentioned Fruity as being the best but more expensive. It is about twice the cost. I'm not all that concerned about that. I'm just wondering why you think it is the best as opposed to the others you mentioned and also like.

Packs up tighter, has a higher Cd... Gene has recently announced (I forget where) that Kevlar shroud lines are now their standard, permitting an even smaller pack.




Later!

--Coop

Okay. So I think I'm going to go with the 48" Ultra Iris Fruity. From the stats it looks like I could go one size smaller but at this point I'd rather go a little bigger. I'm deploying at 800 feet so I'm not going to get that much more drift one size different.

Everyone think the 48" Fruity a good selection for my issue? I was planning on simply connecting with a quick link with no swivel unless you guys think a swivel makes sense under the circumstances.

Atlasboy said:

Okay. So I think I'm going to go with the 48" Ultra Iris Fruity. From the stats it looks like I could go one size smaller but at this point I'd rather go a little bigger. I'm deploying at 800 feet so I'm not going to get that much more drift one size different.

Everyone think the 48" Fruity a good selection for my issue? I was planning on simply connecting with a quick link with no swivel unless you guys think a swivel makes sense under the circumstances.

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Fruity Chutes come with swivels sewn in to the recovery harness.

They do have a few disadvantages: if the lines tangle you cannot just untie the knot at the bottom to free it, and iris chutes are difficult to fold alone without a helper.