Recovery anchor to reduce separation charge volume

[jmasterj]

I just bought a Wildman Demon 5. It's a HED build (a first for me) with a boat tail that pretty much contains the entire fin and motor mount assembly, leaving the vast majority of the 48" airframe empty. The only things that have to fit are the drogue and drogue harness, which aren't going to need a ton of airframe length in a 5" diameter rocket. I have two objectives I'd like to accomplish as I build this (besides the usual):
1. Keep the bottom of the drogue harness accessible and replaceable.
2. Reduce the quantity of BP required (Vern K's rough estimate is 8g if I leave the whole airframe empty).

#2 was not initially on my mind, so my solution to #1 was to build the tail cone/fin can as its own assembly, attaching it to the main airframe with several removable screws through the airframe and the tail cone coupler. This would allow me to remove the airframe to check/repair/replace the recovery harness mounting point. That's still on the table, but when I thought of the volume of that empty airframe, I thought why not put a bulkhead 18" (or so) down from the nose end? This is dual deploy and there's no way I could fit enough BP in the motor well to be a useful backup, so blocking the motor section off from the separable parts doesn't seem like an issue. The airframe plus the tail cone is so long it's not going to prevent me from using a Pro75 6XL casing (which is probably not ever in my future anyway), so that's also not an issue. I could probably reach down 18" into a 5" airframe, but why not make this mount removable too?

My thought is to sandwich a bulk plate (with a coupler bulk plate on either side for strength and centering) between two short pieces of coupler, and mount a u-bolt to that. I would attach it to the airframe with several (3? 4?) screws (maybe 1/4", I have those lying around) into pem nuts mounted in the coupler. Here's the section from my OR sim (not including the hardware).

Does this make sense? Will it work to accomplish my objectives? Is it overkill? Do you have a better way to accomplish the two objectives above? I know I'm not the first person to try to accomplish them, so I'd love to learn from other people's experience.

Thanks!

 

[rfjustin]

On paper the concern is valid, but IMHO you are trying to solve for a problem that has not presented itself yet. Now if the Demon was already built, and you could not get the rocket ground tested successfully with a nominal amount of powder, I would understand making the modification to solve for that specific issue. Otherwise, KISS. For a 5" HED bird, I also like the idea of a replaceable/serviceable drogue recover cord. Here is how I did that on a similar 5" HED bird, pics below.

I would recommend a robust charge well to ensure full burn/containment of your BP mounted to the bulkheads. Goes without saying, but ground test for sure!

Best,

 

[rfjustin]

@StreuB1 ground test videos are below as well for consideration:

 

[jmasterj]

On paper the concern is valid, but IMHO you are trying to solve for a problem that has not presented itself yet. Now if the Demon was already built, and you could not get the rocket ground tested successfully with a nominal amount of powder, I would understand making the modification to solve for that specific issue. Otherwise, KISS. For a 5" HED bird, I also like the idea of a replaceable/serviceable drogue recover cord. Here is how I did that on a similar 5" HED bird, pics below.

Thanks for the reply, Justin. Just so I'm clear on what you're showing: top picture is looking up at the drogue end of the av-bay, middle picture is looking from the nose end toward the fin end of main airframe, and the bottom picture is showing that you've attached the bottom of the drogue harness to a plugged/tapped/eye-bolted forward closure on the motor case, right? I'd forgotten about the threaded closure option as an attachment point. Is that easier or stronger than a u-bolt in the fore centering ring?

Do I have it right that the sum of your thoughts is that #1 can be solved easily with a plugged/tapped forward closure and #2 is unnecessary?

Edit: what about 38mm CTI motors? No threaded forward closure there.

 

[StreuB1]

I would not suggest making the tail assy a bolt on affair. Part of the structural component of the lower airframe is the bonding of the coupler. I am not sure of your ideas for motors but mine, I built to flog the daylights out of it. If you mount your recovery bridle like Wildman suggests (1"W kevlar strap laminated to the MMT), then that should last you the life of the rocket. If it ever fails for some reason, you can also pivot to a plugged end closure and mount your recovery to the motor with an eyebolt. I actually have a safety lanyard (3/8" tubular kevlar) that I connect to the motor which then connects to the delta link which connects the bridle to the riser (shock cord). This is a backup in case something does happen to the kevlar bridle. At least the motor connection will keep the entire stack connected to the chute.

I used a Onebadhawk Ultimate harness kit (7/16" kevlar) for my cords. PLENTY strong.

8g for the booster is an insane amount of powder, and thats coming from someone who firmly believes in the saying "Blow it out or blow it up!!" lol

Check my videos that Justin posted. I also have TONS of build pics and other pics that I took to share with another guy who is also building one.

I am currently finishing a Demon 150 as well and pretty much mimicking the Demon 5 build. If its not broke, don't fix it!

Flew the Demon 5 for the first time last November on an L1520T and it was AWESOME! Video on my YT channel so check it out. Full onboard video and ground video.

 

[rfjustin]

Thanks for the reply, Justin. Just so I'm clear on what you're showing: top picture is looking up at the drogue end of the av-bay, middle picture is looking from the nose end toward the fin end of main airframe, and the bottom picture is showing that you've attached the bottom of the drogue harness to a plugged/tapped/eye-bolted forward closure on the motor case, right? I'd forgotten about the threaded closure option as an attachment point. Is that easier or stronger than a u-bolt in the fore centering ring?

Do I have it right that the sum of your thoughts is that #1 can be solved easily with a plugged/tapped forward closure and #2 is unnecessary?

Edit: what about 38mm CTI motors? No threaded forward closure there.

Yep, summary of my thoughts is spot on. Using 54mm and 75mm plugged and tapped closures as anchor points not only saves time, a bit of weight, and serviceability.

Enjoy.

 

[StreuB1]

 

[StreuB1]

I am also using a 20" SkyAngle drogue on a 6 foot 1/4" kevlar riser at apogee to keep the stack oriented properly.

 

[jmasterj]

I would not suggest making the tail assy a bolt on affair. Part of the structural component of the lower airframe is the bonding of the coupler.

This is interesting to me - can you explain a little more? The lower coupler is only ~5.5" only, so there's only maybe 3" of overlap with the airframe. The fins overlap maybe 5". Does gluing that vs bolting it really make a huge strength difference? I don't understand the coupler part, but I do see how bonding the fins to both parts would increase the strength.

I am not sure of your ideas for motors but mine, I built to flog the daylights out of it.

I'm definitely starting smaller; I need to work my way up to an L in steps. Won't have space to fly it on more than a J until December when MDRA goes back to Higgs Farm anyway.

 

[StreuB1]

Part of the affect of getting the laundry out and the recovery system stretched out is the impulse the ejection charge produces. Yes, pressure does something, but it is the pressure vs time curve that really does the work. Because of that, I am an enormous advocate of using charge wells (mortars) to generate your ejection impulse. I use PVC caps mounted to my bulk plates, the match goes in over the edge and to the bottom side of the well. Powder goes in and it gets vibrated with a percussive device so settle it. Then I use layer by layer of [flat] folded up paper towel to compress the powder. I fill the ENTIRE well with paper towel, compressing between each addition until it is even with the top. I then apply (3) crossing layers of 1" masking tape over the top of the well and then compress it again. Then I put a radial wind of tape around the OD near the top to assure the cross pattern does not peel up and come off. All of that compressed towel and tape allows time for the match flame front to propagate through the powder and to generate a little bit of pressure. The pressure curve of BP is so steep that it doesn't take much compression to generate a substantial shockwave in the body tube. That is what will shear those pins and impart momentum to the NC assembly. The pressure pulse after that shock is what will help keep that momentum going. Thats how I can get a full stretch on 3.6g of powder, and that was 3F, not 4F.

 

[Titan II]

I'd forgotten about the threaded closure option as an attachment point. Is that easier or stronger than a u-bolt in the fore centering ring?

An eyebolt in the forward closure is likely the strongest attachment point you are going to find and it makes checking and replacing the harness easy.

May I ask the reason you want to lower the powder charge? Why complicate the matter? I have a 5" bird with a 60" booster section. I use 5g primary and 7g secondary. I really doubt you are going to need 8g primary. Power is cheap.

 

[StreuB1]

This is interesting to me - can you explain a little more? The lower coupler is only ~5.5" only, so there's only maybe 3" of overlap with the airframe. The fins overlap maybe 5". Does gluing that vs bolting it really make a huge strength difference? I don't understand the coupler part, but I do see how bonding the fins to both parts would increase the strength.

The coupler is what the entire MMT pulls on. It also effectively doubles the thickness of the airframe at the joint and all joints are potential flex points. Flex points, especially at the point where the motor is (this is where it transmits forward force) are areas of stress focus since there is a structural discontinuity. By epoxying the airframe to the coupler, you help mitigate the structural discontinuity. A mechanical connection will not mitigate a structural discontinuity and the forces will still be focused through the edges of the airframe against the boattail and not circumferentially via the bonded connection. This rigid connection makes the chance for a bending moment to be all but impossible.

I'm definitely starting smaller; I need to work my way up to an L in steps. Won't have space to fly it on more than a J until December when MDRA goes back to Higgs Farm anyway.

Not sure how big your field is but the Demon 5 gets pretty heavy once its fully built out.

K1100 sims to around 4k

If you want a J, you'll need a high thrust J like a J1799 and that flies really low. Barely 2k.

I have an OR file for it if you need one. Don't have it in my OneDrive so I'd have to post it up when I get home tonight.

 

[jmasterj]

@Titan II I don't have a great reason for wanting to use less BP. Maybe just a leftover from flying motor eject? To my brain 5" diameter x 4' long seemed like a lot to pressurize.

The coupler is what the entire MMT pulls on.

Not sure what you mean there. How does the motor mount "pull?" For the lengths in my kit, the entire motor mount is behind that coupler. I haven't done a whole dry fit yet but I'm planning on butting the coupler up against the fore centering ring.

Not sure how big your field is but the Demon 5 gets pretty heavy once its fully built out.

I have an OR file for it if you need one. Don't have it in my OneDrive so I'd have to post it up when I get home tonight.

At the MDRA summer field, 2K' is where I want to be. NOVAAR's field is limited to J impulse and I'd like to stay under 3K'. How heavy is yours? I'd certainly be interested in comparing your sim to what I have so far. Thanks for the help.

 

[jmasterj]

It looks like plugged tapped closures are going to be my answer to my accessibility objective and "don't do it, not worth it" is the answer to the less BP objective.

 

[jmasterj]

What is the best way to secure the eyebolt to the plugged tapped forward closure? A lock nut at the top of the eyebolt threads? Do you use loc-tite?

 

[mtnmanak]

Pistons are an option to reduce the amount of BP needed. Piston deployment uses roughly half (or less) BP.

Piston deployment is not without it's downsides, however. It requires a different approach to construction, the piston and tube need to be cleaned very well before each flight, and the weather is a factor in expansion/contraction of the tubes.

Not saying you should or should not use a piston (I have used them many times on larger rocket tubes), but in answering the question "is there a way to reduce BP charges?", pistons are a viable solution.

Having said that, I have plenty of 5" rockets and larger and have not had a problem with larger BP charges, especially in fiberglass tubes.

 

[StreuB1]

Full hex nut as a jam nut. I would not suggest using loc-tite. You can get plenty of jam with a full nut on a 5/16-18 thread (54mm closure) or 3/8-16 thread (75/98mm closure).

 

[StreuB1]

Pistons are an option to reduce the amount of BP needed. Piston deployment uses roughly half (or less) BP.

Piston deployment is not without it's downsides, however. It requires a different approach to construction, the piston and tube need to be cleaned very well before each flight, and the weather is a factor in expansion/contraction of the tubes.

Not saying you should or should not use a piston (I have used them many times on larger rocket tubes), but in answering the question "is there a way to reduce BP charges?", pistons are a viable solution.

Having said that, I have plenty of 5" rockets and larger and have not had a problem with larger BP charges, especially in fiberglass tubes.

Agreed on the large charges in FG tubes. You can be EXTREMELY aggressive with charges and be completely safe. Though there absolutely is a point of diminishing returns when it comes to charge size. It is my belief that you want to always settle on the smallest charge, with additional margin, that achieves the intended result. At times, it takes experience and paying attention to your rocket during its recovery profile to see how it reacts to figure out what that margin is. When a rocket is in freefall, what looks to be an energetic event in ground tests will be far more energetic in freefall. In freefall, the rocket assembly is experiencing relatively close to zero G's where in ground test, it is experiencing 1 G. That actually has an effect on the overall efficacy of the ejection event. The sleeve joint that is the airframe and coupler intersection is in bending during ground testing which increases the friction component in the joint. Where in freefall, that component is usually not there and if it is, its very low. That added frictional component is additive with the rest of the retention components (shear pins, tape, etc.) Without that additive affect, your charge has less work to do and thus allows you to do the same work with less material.


I have 0% experience with piston systems so cannot comment on them at all.

 

[mtnmanak]

...paying attention to your rocket during its recovery profile to see how it reacts to figure out what that margin is.

For this reason, I try to install two onboard cameras on my larger rockets - one pointed aft and one pointed towards the nose. It gives me a 4K first person view of exactly what is happening to my rockets during flight. I have adjusted many parameters based on studying these videos.

 

[StreuB1]

For this reason, I try to install two onboard cameras on my larger rockets - one pointed aft and one pointed towards the nose. It gives me a 4K first person view of exactly what is happening to my rockets during flight. I have adjusted many parameters based on studying these videos.

I am finishing the build on my Demon 150; first 6" 98mm FG build and I honestly have been thinking about doing the same! I always put a downward shooting camera on my rockets but I am really considering installing a forward looking one for the exact same reason.

What camera do you use? I have a Mobius HD but they're harder to find now and I've heard of issues with the RunCam 4K.

 

[mtnmanak]

I am finishing the build on my Demon 150; first 6" 98mm FG build and I honestly have been thinking about doing the same! I always put a downward shooting camera on my rockets but I am really considering installing a forward looking one for the exact same reason.

What camera do you use? I have a Mobius HD but they're harder to find now and I've heard of issues with the RunCam 4K.

I use Runcam 2 4K cameras. I have tried other cameras, including Mobius, but the Runcams give you about the best video you are going to get right now in a package that small and fit for purpose. I usually use the Additive Aerospace 3D printed camera housings. I don't have a 3D printer, so happy to pay them for a great product.

The Runcams are definitely finicky and problematic. I have written about the significant issues with them in other threads. I would go with something else if there was a better product, but I have not found one. If you get them, a couple recommendations. First, don't use the wifi app. It will only cause you to want to chuck the little bastards off a cliff. I have found them much more reliable if you just use the hardware button on the cameras themselves. Second, when you power them on and then press the button to start recording, watch the indicator light for a solid 15 count. If it keeps blinking after that, you are most likely good. The failures to record usually happen within the first 10-15 seconds. Third, use the Samsung EVO SD cards they recommend. Other cards will give you problems, regardless of how fast they are. Last, the batteries they come with outright suck. I feel lucky to get a flight or two out of a charge. I highly recommend getting at least a couple extras if you are going to have multiple flights in a day. Nothing sucks more than to find out the battery died right when the LCO gets to "one" on the five-count. However, if you get them work, the video they take is beautiful.

Here are some example Nose and Aft videos from this past weekend at Red Glare. They are from the AGM-33 flight on an N2220. They provide excellent examples of why the forensic analysis of onboard video is critical.

On the aft cam, you can see the drogue shock cord stick to the drogue shroud lines for about 3 seconds. Not sure exactly what happened there, but it did finally shake loose. You can see the drogue collapse and then re-inflate when the shock cord breaks loose. This tells me I need to pack that shock cord way from the drogue a bit.

The nose cam is more troubling. I have flown this rocket 5 times now and all the other times, the main deployment was flawless. I changed nothing in this flight (same charges, same parachute, same shock cord, etc), however, in this flight, you can clearly see that the primary charge did not eject the nosecone. I had the backup charge set for a couple hundred feet lower than the primary and you can see it kick the nosecone out. Not sure what environmental variable caused this, but I will definitely be upping the charges for the main in future flights.

Both issues are difficult to see in real time. I slowed down the video of the two incidents to a few frames per second to illustrate. Usually, when I review these files, I watch them through in real time, then go back and review the relevant events frame-by-frame and often pick up issues that need to be addressed that aren't obvious from the ground or normal speed video.

 

[jmasterj]

@rfjustin I'm going to be flying this rocket with adapters. Are there motor adapters you would trust or not trust when using a plugged tapped forward closure as the harness attachment point? Adapting from 75 to 54 I have a slimline adapter, which has an aluminum business end. The 75 to 38 adapter I have, though, is a LOC model that's mostly cardboard. The cardboard thrust ring being the ultimate anchor doesn't make me feel great. I could nest slimline adapters or put my 54/38 Aeropack inside the 75/54 slimline, I suppose.

 

[rfjustin]

@rfjustin I'm going to be flying this rocket with adapters. Are there motor adapters you would trust or not trust when using a plugged tapped forward closure as the harness attachment point? Adapting from 75 to 54 I have a slimline adapter, which has an aluminum business end. The 75 to 38 adapter I have, though, is a LOC model that's mostly cardboard. The cardboard thrust ring being the ultimate anchor doesn't make me feel great. I could nest slimline adapters or put my 54/38 Aeropack inside the 75/54 slimline, I suppose.

I would have no concerns about the Slimline 75 to 54, but I would not endorse a cardboard option for 54mm. All of my successes have been with using Aeropack 75 to 54, YMMV. Oh, and for the recovery attachment point on the motor, I like solid rod ends. They are not weight rated though, be advised.

https://www.mcmaster.com/2434K23/https://www.mcmaster.com/2434K22/

 

[jmasterj]

I would have no concerns about the Slimline 75 to 54, but I would not endorse a cardboard option for 54mm. All of my successes have been with using Aeropack 75 to 54, YMMV. Oh, and for the recovery attachment point on the motor, I like solid rod ends. They are not weight rated though, be advised.

https://www.mcmaster.com/2434K23/https://www.mcmaster.com/2434K22/

What do you like about he solid rod ends vs a standard forged eyebolt?

 

[rfjustin]

What do you like about he solid rod ends vs a standard forged eyebolt?

Weight, size, and form factor. IMHO, your recovery system cordage would break before one of these would.

 

[mtnmanak]

The "yield strength" ( amount of pounds you can apply longitudinally before the bolt yields) on a rod end bolt should be much higher than the "load limit" on a forged eye bolt.

If you look at the forged eye bolts McMaster-Carr sells, they have a load limit vs. a tensile strength. For example, the load limit on their 5/16" threaded forged eye bolts is 900 pounds for lifting. This indicates the eye will break before the threads strip out.

The rod ends, however, are listed by tensile strength because the eye in the rod end is much stronger, so the threads will strip out before the eye breaks. All their high strength rod ends are rated for a 125,000 psi tensile strength. To find the "yield strength" of the rod end, you multiply the tensile strength x the cross sectional area of the bolt. Thus, for a 5/16" McMaster-Carr rod end you get:

y = 125,000 x 3.14 x (0.3125 / 2)^2 = 3051 pounds.

So, while their 5/16" eye bolts may break at a 900 pound load, the rod ends should be able to handle about 3000 pounds. Obviously, this is a theoretical limit and also depends on having the bolt well seated in a socket (a deeper socket engages more threads), but even if you have a shallow socket like a forward closure and cut the yield strength in half, it is still far stronger than the eye bolt.

In all likelihood, if you use a rod end, the weakest part of the connection would be the threads in the forward closure.

One of the trade-offs with a rod end is a smaller eye, which could make a difference if you try to put a quick link on the bolt. If you thread strong kevlar through the eye instead, you have a very strong anchor.

 

[jmasterj]

Weight, size, and form factor. IMHO, your recovery system cordage would break before one of these would.

What size quicklink do you get through that 3/8" rod end? I think I see one in that picture.

 

[rfjustin]

What size quicklink do you get through that 3/8" rod end? I think I see one in that picture.

I'll check this afternoon/evening and circle back.

 

[jmasterj]

I'll check this afternoon/evening and circle back.

Thanks!

McMaster's rod ends have hole IDs the same or smaller than the threaded bolt part, so a 5/16" rod end has a 5/16" ID. That probably limits me to a 1/4" quicklink, which is significantly weaker than the rod end. A tubular kevlar loop threaded through might be the stronger choice, but I don't know how negatively the looping would affect the kevlar strength.

 

[rfjustin]

Thanks!

McMaster's rod ends have hole IDs the same or smaller than the threaded bolt part, so a 5/16" rod end has a 5/16" ID. That probably limits me to a 1/4" quicklink, which is significantly weaker than the rod end. A tubular kevlar loop threaded through might be the stronger choice, but I don't know how negatively the looping would affect the kevlar strength.

Near as I can tell the 3/8" Solid Rod End will take a 1/4" quick link and the 5/16" SRE will take a 3/16" no problem. To your point, if the quick link is weaker than the SRE, and you end up breaking or stretching a quick link, you have other problems in play. I agree with @mtnmanak that the threads in the forward closures of the motors are likely the weak link in the whole stack anyway.

3/8" SRE with 1/4" wide quick link pictured below.