Streamer ejection speed limit?

[SolarYellow]

Have searched, haven't found anything on-point, so another new-guy post this afternoon.

I've been playing a little in OpenRocket with a 24mm minimum diameter build while I wait for parts to arrive for my 18mm. Looking at the AeroTech F32-8T. I was liking the numbers with an E9-8, but the F32 numbers are interesting. A whole 'nother level, obviously. But there are some issues, mostly with recovery.

First off, assume launching on a dry lake so the thing will be findable. The mirage dries up when you get close. And I don't mind wandering around in the desert, for some reason. Dirt bikes are fun.

I'm getting a 21-22 m/sec velocity at deployment on the F32-8T. OpenRocket throws a warning flag on the sim when you exceed 20 m/sec, or about 45 mph. The range of Estes C/D/E motors I've simmed on this one give 1-12 m/s, so pretty reasonable.

I'm thinking that a streamer will roll out progressively and more gently ramp up the drag force, rather than popping out and snapping from zero to max drag all at once like a chute. I suppose I could toss a streamer out the window of my car on a shock cord at 50 mph and see what happens. Don't want to zipper the tube, obviously, and really don't want to end up with a lawn dart, partial streamer, etc.

I can obviously tweak the model this way and that to slow it down, but then that carves away at the performance and lighting $20 bills on fire just isn't as exciting.

The sim is also showing 18.6 m/sec descent speed on a streamer. Fins are G10 glassed to the tube and reasonably protected by the stickout of the engine for a nozzle-first landing. The AeroTech engine has a thrust collar built in, and with both that and an engine block glued into the tube, I reckon it should be OK. The calculator at rocketreviews targets 17 m/sec, so that's at least livable. I can always make the streamer a little bigger to cut down the descent speed, but that also increases the risk of problems due to premature ejectuation.

I guess I'm looking for experienced feedback on how bad 22 m/s streamer ejection speed is with a streamer. I'd be forgetting about this and moving on if I was only thinking parachute. Is the speed with a streamer just something to be cautious about and engineer around with a longish Kevlar shock cord and zippering protection measures, or is the 20 m/s a wall that shouldn't be passed?

I'm aware of things like controlled ejection independent of the motor, and obviously this illustrates why they are valuable, but that takes things into a level of complexity and expense that I'm not really interested in getting into any time soon.

 

[lakeroadster]

Have searched, haven't found anything on-point, so another new-guy post this afternoon.

I've been playing a little in OpenRocket with a 24mm minimum diameter build while I wait for parts to arrive for my 18mm. Looking at the AeroTech F32-8T. I was liking the numbers with an E9-8, but the F32 numbers are interesting. A whole 'nother level, obviously. But there are some issues, mostly with recovery.

First off, assume launching on a dry lake so the thing will be findable. The mirage dries up when you get close. And I don't mind wandering around in the desert, for some reason. Dirt bikes are fun.

I'm getting a 21-22 m/sec velocity at deployment on the F32-8T. OpenRocket throws a warning flag on the sim when you exceed 20 m/sec, or about 45 mph. The range of Estes C/D/E motors I've simmed on this one give 1-12 m/s, so pretty reasonable.

I'm thinking that a streamer will roll out progressively and more gently ramp up the drag force, rather than popping out and snapping from zero to max drag all at once like a chute. I suppose I could toss a streamer out the window of my car on a shock cord at 50 mph and see what happens. Don't want to zipper the tube, obviously, and really don't want to end up with a lawn dart, partial streamer, etc.

I can obviously tweak the model this way and that to slow it down, but then that carves away at the performance and lighting $20 bills on fire just isn't as exciting.

The sim is also showing 18.6 m/sec descent speed on a streamer. Fins are G10 glassed to the tube and reasonably protected by the stickout of the engine for a nozzle-first landing. The AeroTech engine has a thrust collar built in, and with both that and an engine block glued into the tube, I reckon it should be OK. The calculator at rocketreviews targets 17 m/sec, so that's at least livable. I can always make the streamer a little bigger to cut down the descent speed, but that also increases the risk of problems due to premature ejectuation.

I guess I'm looking for experienced feedback on how bad 22 m/s streamer ejection speed is with a streamer. I'd be forgetting about this and moving on if I was only thinking parachute. Is the speed with a streamer just something to be cautious about and engineer around with a longish Kevlar shock cord and zippering protection measures, or is the 20 m/s a wall that shouldn't be passed?

I'm aware of things like controlled ejection independent of the motor, and obviously this illustrates why they are valuable, but that takes things into a level of complexity and expense that I'm not really interested in getting into any time soon.

For sure, a high deployment speed for a streamer won't be as big a deal as it would with a chute.

As for ground hit speed, the design of the rocket, fins and motor stick out, can help to mitigate some of the issues. If the motor sticks out the back past the fins, there's a good chance that will hit the ground first.

And something I just saw today here on TRF that may help also, this post by @AeroTech. Pretty slick design for getting strong fin attachment to the body tube on a minimum diameter rocket. It's also a neat way of mounting the shock cord, running the Kevlar thread under a fin fillet.

 

[SolarYellow]

And something I just saw today here on TRF that may help also, this post by @AeroTech. Pretty slick design for getting strong fin attachment to the body tube on a minimum diameter rocket. It's also a neat way of mounting the shock cord, running the Kevlar thread under a fin fillet.

View attachment 532257View attachment 532256

I saw that in the thread where he posted it yesterday. Only thing that bugs me about it is it's not replaceable. There are some other good removable options posted for MD rockets if one searches.

 

[lakeroadster]

I saw that in the thread where he posted it yesterday.

Aug 2020?

Only thing that bugs me about it is it's not replaceable. There are some other good removable options posted for MD rockets if one searches.

You could do the fin attachment as shown above, and the shock chord like this....

 

[SolarYellow]

Aug 2020?

I read it yesterday.

I think the method in this Peak of Flight issue takes the prize for simplest inspectable/replaceable setup on a small-diameter MD rocket I've seen yet. Not that I've been around very long yet, but I OCD pretty hard.

https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

 

[teepot]

IIRC the recommended landing speed is 4 to 5 meters per second. I have also seen 15 FPS as a recommended landing speed. Some chute vendors recommend 20FPS. A lot depends on what you are landing on. Grass would be nice. We fly off a dry lake bed. It is as hard and unforgiving as concrete. I try to land at 11' to 12' per second. And still might pop a fin off.
If you deploy at apogee the rocket is going it's slowest. Something like a few feet per second. But, you probably already know this.

 

[SolarYellow]

Been doing more searching. There are a bunch of threads suggesting OpenRocket gives weird results for ground hit velocity with streamers.

The rocket described above has a mass with spent motor of 80.8g. The "Streamer Calculator" at https://www.rocketreviews.com/streamer-calculator.html tells me an 8cm-wide streamer should be 86cm long. OR gives a ground hit velocity of 19.2 m/s, or 63 ft/s. Seems likely to break something. I've checked another report where someone cited a streamer size for a rocket I could figure out weight on, and the rocketreviews calculator was right on what he was doing successfully. Not going to go build that in OR, though.

Does anyone know what the target descent speed is for the rocketreviews model? They say they're using the calculation from "Timothy Van Milligan in his book Model Rocket Design and Construction." This old post https://www.rocketryforum.com/threads/streamer-descent-rate.71455/#post-784125 says it's based on, "a streamer should have at least 8.5 cm2 of single side surface area per gram of returned model mass (the mass of the rocket plus the mass of the expended rocket engine case.)"

So is the OR streamer descent model out to lunch?

 

[lakeroadster]

I read it yesterday.

I think the method in this Peak of Flight issue takes the prize for simplest inspectable/replaceable setup on a small-diameter MD rocket I've seen yet. Not that I've been around very long yet, but I OCD pretty hard.

https://www.apogeerockets.com/education/downloads/Newsletter343.pdf

Something to consider:

Avoiding direct contact with the flame and heat of the ejection charge is ideal. That method places the Kevlar right in the flame path.
The other methods previously discussed will allow a piece of heat shrink tubing to be slid over the Kevlar to protect it.

 

[SolarYellow]

True, but it's lighter at the back of the rocket, can't interact with the aerodynamics, and still allows for reasonably easy inspection and replacement as often as one finds appropriate.

 

[T-Rex]

going to ask a potentially stupid question. Is the rocket still traveling up with the 8 second delay, or has it arced over and on it's way back down? What is OR giving for a recommended delay?

 

[SolarYellow]

Still traveling up. Optimum delay is called out as 10.1 sec.

9.81 m/sec2 * 2.1 sec = 20.6 m/sec

 

[dhbarr]

Consider switching to the highly similar F35 white reload, which comes in a -11 ?

 

[SolarYellow]

Did some more analysis, pulling formulas and the general approach from the paper, "How to Calculate Streamer Size" by Bill Cooke.
https://www.sunwardhobbies.ca/images-tarc/Streamer Calculations (Cooke) (Jun 09).pdf

Set up a Mathcad file (awesome free program if you are happy with the "Express" version), because it's easier to keep track of what I'm doing that way than in Excel. Far more "human readable."

Wanted to explore the basis behind the "8.5 cm2 of single side surface area per gram of returned model mass" rule. Cooke's paper notes Cd for streamers ranging from 0.15 to 0.4, so I used those to bracket the solution. With 1 gm of descending mass, 8.5 cm2 area leads to the following:

Cd = 0.15 > v = 37 ft/s (11.3 m/s)
Cd = 0.4 > v = 22.7 ft/s (6.9 m/s)

I've also seen numbers as low as 0.9 cited in other places.

Note this is not really a proper use of Cd, as the formula for Cd is frontal area, not streamer area (which would be a lateral cross section). But hey, I'm not the one who started misusing the formula this way.

To develop the "8.5 cm2 of single side surface area per gram of returned model mass" rule requires that someone has both assumed an effective Cd and chosen an acceptable descent speed. (They might be in Tim's book, but I haven't bought that.) If based on empirical field data, that speed may not necessarily lie within the bookends noted above, for reasons to be discussed below.

The Cook paper explicitly ignores the area/Cd of the falling payload section due to it being so much smaller than the streamer, so I did the same, reasoning for the sake of the calculations that the rocket falling motor-first will likely be more or less straight down and have minimal area. That may not be a good assumption, as there will always be some amount of swing, causing the fins and body tube to have more drag, which will slow the rocket. The greater the disturbance, the greater the drag and lateral pressure, leading to more disturbance. The difference this makes will of course depend on fin area, etc. Note that the smaller the streamer, the greater the descent speed, and the greater the developed effect of any disturbance on the rocket. It will look different than a rocket hanging more or less straight down from a parachute.

At the limit, you'd get into a situation where the rocket descends with the body tube more or less horizontal. In this condition, the drag of the fins and the drag of the nose cone and recovery gear would be in equilibrium and the body tube would be maximizing its drag as well. With the motor retained, that would take some big fins. However, it's worth noting that this does lead to significant retardation. Just in searching through threads on this topic, I've seen many reports of rockets coming in sideways with a poorly-deployed chute acting more like a streamer and the rocket not sustaining significant damage. There are even some rockets that use horizontal fall/glide/float as a recovery strategy with no chute or streamer. Something as simplified as the "8.5 cm2" rule can't be fully accounting for variations in swing response between rocket designs.

For the rockets I'm playing with in OR, the disturbance angles will probably be small and swingy, but not zero. It could also be a relatively stable angle, dealing with aerodynamic disturbances by spinning or swinging rotationally. Most likely, all of the above. Try to model it - I dare you...

So to run the three calculation tools together:
My minimum-diameter F32-8T powered OR model has a recovery-phase mass of 72g. Put that into https://www.rocketreviews.com/streamer-calculator.html, specify an 8cm-wide streamer, and it tells you to make it 76.5cm long.
Put that into OR and it gives a ground hit velocity of 19.7 m/s. Put all that into Mathcad based on the Cooke paper, and the effective Cd for the rocket and streamer together is 0.049, about half the lowest value I've seen estimating streamer effectiveness.

Going back the other way, because the streamer dimensions were determined using the "8.5 cm2" rule, entering Cd = 0.15 or 0.4 (which implies combined swinging rocket and streamer) in the Mathcad sheet changes the velocity to 11.3 m/s and 6.9 m/s (as above). If you think maybe the swinging rocket adds as much as 0.1 on average, try Cd = 0.25 > v = 8.7 m/s, or ~29 ft/s. That's fast enough for a lot of people to be nervous.

Back to the original question: How worried do I need to be about the 21 m/s deployment speed? Rearranging to solve for the drag force, and dividing it by the weight of the rocket, we get:

Cd = 0.09 > 4.2g decel > .66 lb force on the recovery gear
Cd = 0.15 > 6.9g decel > 1.1 lb force on the recovery gear
Cd = 0.4 > 18.5g decel > 2.9 lb force on the recovery gear

Given that the rocket weighs ~2.5 oz., we can calculate force on the recovery gear. Don't know where we start zippering tubes. Figuring that out will require testing. But we can at least begin benchmarking and doing some real-world testing of attachment methods, shock cord strength, zippering and anti-zippering measures, etc.

Conclusions:
1. OpenRocket's streamer model probably is out to lunch. Lots of other people have seen really weird stuff with it in doing other sims. I'm not going to worry about its calculated ground hit velocity when specifying my streamer.

2. With effective "Cd" in the literature ranging from 0.09 to 0.4, you probably have no clue what effective Cd to use for the streamer. Compounding this is the unknown (and probably impossible for normal earth people to model) contribution of the swinging rocket, which will tend to be greater as the streamer gets smaller and the descent speed increases.

3. Given the tools that normal earth people who geek out on this stuff on model rocket forums have available, it's not going to be possible to pinpoint exact numbers and solutions for any of this stuff. By using a bunch of tools together, we can figure out when one of them seems to be pointing in a bad direction and learn to not worry about it. We can advance, and get closer to understanding where we are and where we need to be, and possibly building better rockets. Which we will ultimately just have to go fly and see what happens, decide whether we like it, and then probably build another, hopefully better one using what we learn.

4. Given that it's been in use for a long time and people seem OK with it, the "8.5 cm2" rule is probably as good a starting point as any. Just understand that it's not clear (at least without reading Tim's book) what assumptions are behind it for Cd and acceptable descent speed. Both of those vary significantly with your model and environmental factors. It is not a guarantee, and you should proceed with caution and collect field data.

5. People will continue to have different opinions about this stuff, with many of us thinking we know more than we really do, for the foreseeable future.

Sorry for the long post. Don't mean for this to be a dissertation, and it should be obvious I'm not a rocket guru. Just a guy with a reasonable amount of experience figuring stuff out, thinking it through in public and getting it all down to clarify it. Hopefully, it helps someone else as much as it's helped me.

 

[SolarYellow]

Consider switching to the highly similar F35 white reload, which comes in a -11 ?

I plugged that in. Apogee is awesome, the ejection time matches the delay very nicely at 0.7 m/s. Unfortunately, stability factor of the rocket gets a lot smaller than with the single-use motors due to the additional weight. Will require some additional mods to the rocket itself, and probably bigger recovery gear, etc. It's also a lot more cost, possibly the tipping point sending the whole thing down a spiral of increasing costs to go with other increasing costs to go with other increasing costs.

I think I've looked over the edge enough on this mid-power stuff for now. If I do build a the 24mm MD rocket, I'll probably just stick with a slightly larger version of the 18mm I'm building, which sims really nicely with a range of Estes motors from C to E, giving enough performance to be fun and interesting. That keeps things in a cheaper and cheerfuller zone for me. At least that's what I say now.

 

[shockie]

Did some more analysis, pulling formulas and the general approach from the paper, "How to Calculate Streamer Size" by Bill Cooke.
https://www.sunwardhobbies.ca/images-tarc/Streamer Calculations (Cooke) (Jun 09).pdf

Set up a Mathcad file (awesome free program if you are happy with the "Express" version), because it's easier to keep track of what I'm doing that way than in Excel. Far more "human readable."

Wanted to explore the basis behind the "8.5 cm2 of single side surface area per gram of returned model mass" rule. Cooke's paper notes Cd for streamers ranging from 0.15 to 0.4, so I used those to bracket the solution. With 1 gm of descending mass, 8.5 cm2 area leads to the following:

Cd = 0.15 > v = 37 ft/s (11.3 m/s)
Cd = 0.4 > v = 22.7 ft/s (6.9 m/s)

I've also seen numbers as low as 0.9 cited in other places.

Note this is not really a proper use of Cd, as the formula for Cd is frontal area, not streamer area (which would be a lateral cross section). But hey, I'm not the one who started misusing the formula this way.

To develop the "8.5 cm2 of single side surface area per gram of returned model mass" rule requires that someone has both assumed an effective Cd and chosen an acceptable descent speed. If based on empirical field data, that speed may not necessarily lie within the bookends noted above, for reasons to be discussed below.

The Cook paper explicitly ignores the area/Cd of the falling payload section due to it being so much smaller than the streamer, so I did the same, reasoning for the sake of the calculations that the rocket falling motor-first will likely be more or less straight down and have minimal area. That may not be a good assumption, as there will always be some amount of swing, causing the fins and body tube to have more drag, which will slow the rocket. The greater the disturbance, the greater the drag and lateral pressure, leading to more disturbance. The difference this makes will of course depend on fin area, etc. Note that the smaller the streamer, the greater the descent speed, and the greater the developed effect of any disturbance on the rocket. It will look different than a rocket hanging more or less straight down from a parachute.

At the limit, you'd get into a situation where the rocket descends with the body tube more or less horizontal. In this condition, the drag of the fins and the drag of the nose cone and recovery gear would be in equilibrium and the body tube would be maximizing its drag as well. With the motor retained, that would take some big fins. However, it's worth noting that this does lead to significant retardation. Just in searching through threads on this topic, I've seen many reports of rockets coming in sideways with a poorly-deployed chute acting more like a streamer and the rocket not sustaining significant damage. There are even some rockets that use horizontal fall/glide/float as a recovery strategy with no chute or streamer. Something as simplified as the "8.5 cm2" rule can't be fully accounting for variations in swing response between rocket designs.

For the rockets I'm playing with in OR, the disturbance angles will probably be small and swingy, but not zero. It could also be a relatively stable angle, dealing with aerodynamic disturbances by spinning or swinging rotationally. Most likely, all of the above. Try to model it - I dare you...

So to run the three calculation tools together:
My minimum-diameter F32-8T powered OR model has a recovery-phase mass of 72g. Put that into https://www.rocketreviews.com/streamer-calculator.html, specify an 8cm-wide streamer, and it tells you to make it 76.5cm long.
Put that into OR and it gives a ground hit velocity of 19.7 m/s. Put all that into Mathcad based on the Cooke paper, and the effective Cd for the rocket and streamer together is 0.049, about half the lowest value I've seen estimating streamer effectiveness.

Going back the other way, because the streamer dimensions were determined using the "8.5 cm2" rule, entering Cd = 0.15 or 0.4 (which implies combined swinging rocket and streamer) in the Mathcad sheet changes the velocity to 11.3 m/s and 6.9 m/s (as above). If you think maybe the swinging rocket adds as much as 0.1 on average, try Cd = 0.25 > v = 8.7 m/s, or ~29 ft/s. That's fast enough for a lot of people to be nervous.

Back to the original question: How worried do I need to be about the 21 m/s deployment speed? Rearranging to solve for the drag force, and dividing it by the weight of the rocket, we get:

Cd = 0.09 > 4.2g decel > .66 lb force on the recovery gear
Cd = 0.15 > 6.9g decel > 1.1 lb force on the recovery gear
Cd = 0.4 > 18.5g decel > 2.9 lb force on the recovery gear

Given that the rocket weighs ~2.5 oz., we can calculate force on the recovery gear. Don't know where we start zippering tubes. Figuring that out will require testing. But we can at least begin benchmarking and doing some real-world testing of attachment methods, shock cord strength, zippering and anti-zippering measures, etc.

Conclusions:
1. OpenRocket's streamer model probably is out to lunch. Lots of other people have seen really weird stuff with it in doing other sims. I'm not going to worry about its calculated ground hit velocity when specifying my streamer.

2. With effective "Cd" in the literature ranging from 0.09 to 0.4, you probably have no clue what effective Cd to use for the streamer. Compounding this is the unknown (and probably impossible for normal earth people to model) contribution of the swinging rocket, which will tend to be greater as the streamer gets smaller and the descent speed increases.

3. Given the tools that normal earth people who geek out on this stuff on model rocket forums have available, it's not going to be possible to pinpoint exact numbers and solutions for any of this stuff. By using a bunch of tools together, we can figure out when one of them seems to be pointing in a bad direction and learn to not worry about it. We can advance, and get closer to understanding where we are and where we need to be, and possibly building better rockets. Which we will ultimately just have to go fly and see what happens, decide whether we like it, and then probably build another, hopefully better one using what we learn.

4. Given that it's been in use for a long time and people seem OK with it, the "8.5 cm2" rule is probably as good a starting point as any. Just understand that it's not clear what assumptions are behind it for Cd and acceptable descent speed. (They might be in Tim's book, but I haven't bought that.) Both of those vary significantly with your model and environmental factors. It is not a guarantee, and you should proceed with caution and collect field data.

5. People will continue to have different opinions about this stuff, with most of us thinking we know more than we really do, for the foreseeable future.

If you are a NAR member then you will have access to a number of streamer R& D reports that may provide additional info.

I can't remember if they include any CD values, but they do have descent rates so you could backtrack to get CD values as you've done in your analysis above. Quite good too!

I wonder where Bill Cooke got his CD range values?

Pleated mylar streamers ( or Beinfang paper, etc)
are know to have lower descent rates than just plain old straight material.

A series of pinholes in the streamer will also increase drag . Don't use a pin to just poke holes thru the mylar. Heat the pin and melt the hole. This seaks the hole so that the holesdont cause rips abd tears. As will having a sawtooth shape around the perimeter of the streamer.

Finally it has been known for quite some time ( in the NAR /FAI competition workd) if you attach a streamer at the rockets burnout cg and have it come down sideways, it provides much more frontal area along with the fins to produce more drag.

If you incorporated all of the above into your steamer you might be able to raise the overall drag by a great amount, increasing the Cd substantially. This would result in a significant lower descent rate.

 

[BABAR]

I don’t know HOW I do it, but many of my streamer rockets seem “balanced” so the streamer seems to support the nose cone (I rarely add nose weight) and the rocket fins and body come in horizontally. The shock cord attachment is internal, so it’s not the competition harness method.

It may be that I build mostly minimum diameter and light, so rear end of rocket once propellant is gone is light. I think it is essentially a streamer augmented backslide. Also may be that I am not using multifolded streamers, just marking tape, so maybe the combination of nose blow instability and fall rate does something, your mileage may DEFINITELY vary.

 

[SolarYellow]

That seems ideal.

 

[BEC]

Something to consider:

Avoiding direct contact with the flame and heat of the ejection charge is ideal. That method places the Kevlar right in the flame path.
The other methods previously discussed will allow a piece of heat shrink tubing to be slid over the Kevlar to protect it.

In my experience, kevlar shock cords that are this this close to the ejection charge can fail in as few as two flights. FYI and YMMV.