Strap on boosters

[kandsrockets]

I am looking for some ideas on how to get some strap on booster to drop off in flight. A couple of us from our club are looking at making a upscale of my Heavy Hitter III but we want to have the strap ons drop off. Our plan is to have all three motor lite off the pad and 2 seconds into the flight have the strap ons drop off. The outside motors will have a 1.5 sec burn and the central motor will have a 6 sec burn. The central body tube is 5.5" dia and the strap ons are 4". Any ideas and pictures would be great.

Thanks

 

[WillMarchant]

https://www.dmf-rockets.com/boost_drop/index.html is one possible answer.

 

[troj]

Take a look at how we did the boosters on the Delta III.

I looked at a lot of what had been done, including the DMF boosters, and wasn't comfortable that they'd handle the load reliably.

-Kevin

 

[kandsrockets]

Take a look at how we did the boosters on the Delta III.

I looked at a lot of what had been done, including the DMF boosters, and wasn't comfortable that they'd handle the load reliably.

-Kevin

Kevin do you have more detail on how the entire system worked? What held the bottom of the boosters?

 

[troj]

Kevin do you have more detail on how the entire system worked? What held the bottom of the boosters?

Look at the first photo in this thread and you'll see the rear of the main portion of the rocket. We cut a piece of plywood into "scallops", which cradled the rear of each booster. Small blocks where the booster rests would work, as well.

We then used a strip of thin brass between the rear of the booster and the rear of the main portion of the rocket. The brass was cut in the field with a pair of scissors (yes, it's that thin) and held in place via a wood screw into each portion.

When the separation charge fired, it pushed the top of the booster out and away from the rocket. As it rotated, the brass would tear, allowing the booster to fall free.

One thing we didn't want was two charges, with the risk of one charge firing while the other doesn't, resulting in a booster that's hung up. We wanted a system where the booster either separated, or it didn't -- nothing in between.

In addition, the recovery timer within each booster was built such that it wouldn't begin its countdown unless it detected a separation from the main. That way, if a booster separation charge failed, we wouldn't have a booster pop its recovery system while still attached.

The brass strip may seem light, but all it did was hold the rear of the booster in place -- the booster pin (1" Al rod) bore all the weight. With the motors canted in towards the CG (to prevent stability problems in case booster didn't come up to pressure at the same time), the boosters had a tendency to hold themselves in place, anyway.

On the first flight, due to a motor ignition timing problem, we actually lifted the 400+ lb rocket with six of the boosters. That gives you an idea of the strength of the booster attachment.

I'm actually working on a "mini" version of the Delta III, which will use an almost identical booster attachment method. In this case, though, everything will be significantly scaled down, as my boosters are only going to be 54mm, so 1" Aluminum rod would be horrific overkill!

I do need to get more photos posted, and better descriptions.

It's a method that's flight-proven and has worked beautifully. Our boosters each held an L motor, and the central motor was a P, to give you an idea of the forces involved.

-Kevin

 

[kandsrockets]

Here is a rendering of what I am working on.

 

[DMcCauley]

Scott,

Dave Flynns booster droppers are fine for lower power HPR. I've been doing some heavy mechanical analysis with the particular design, and they are more than adequate for a 60 lb rocket design i'm working on which is utilizing a central M650W and (2) external K550's.

Unless your flying super serious AP as in the Delta III, the Delta III design seems a bit overkill. Although you probably could scale it down a bit to fit your particular needs. Plus, i'm still a bit confused at how his entire arrangement works and was implemented. Maybe some more detailed photos or explanation could clear things up a bit. Having one charge vs. two does seem more reliable.

 

[DMcCauley]

Here is a rendering of what I am working on.

Hmmm . . .do i detect rip off???

Sounds like a drag race may need to be in order to settle this one once and for all!

 

[georgegassaway]

There was a thread named "Strap-on Boosters" from a few weeks ago, in the Mid-Power section:

https://rocketryforum.com/showthread.php?t=416

Some of the ideas mentioned there may be of some use.

- George Gassaway

 

[kandsrockets]

Hmmm . . .do i detect rip off???

Sounds like a drag race may need to be in order to settle this one once and for all!

Well unless you designed your before 2 years ago I would say mine is original, plus this was a LPR kit we sold but is now oop.

 

[kandsrockets]

Scott,

Dave Flynns booster droppers are fine for lower power HPR. I've been doing some heavy mechanical analysis with the particular design, and they are more than adequate for a 60 lb rocket design i'm working on which is utilizing a central M650W and (2) external K550's.

Unless your flying super serious AP as in the Delta III, the Delta III design seems a bit overkill. Although you probably could scale it down a bit to fit your particular needs. Plus, i'm still a bit confused at how his entire arrangement works and was implemented. Maybe some more detailed photos or explanation could clear things up a bit. Having one charge vs. two does seem more reliable.

I am looking for something little more solid than the booster dropper. The booster motors will be 5 grain 54mm with a hard hitting propellant and the central motor will be a moonburner 5 grain 76mm. Boosters will be dropped off with in the first 3 seconds of the flight and the central motor had a burn time of 15 sec.

 

[DMcCauley]

Well unless you designed your before 2 years ago I would say mine is original, plus this was a LPR kit we sold but is now oop.

Dude. See the smiling face. It was a joke. Plus part of the joke was that as a 4FNC design, its not what you would call original. But you missed all that obviously.

Actually, i actually decided to do something along the lines of troj's design.

I did the design last night and have all my drawings ready to be machined. The fwd blocks are much smaller and will attach to the top centering ring in the rocket via (2) bolts which is easily
accessible (for removal, prepping, and cleaning).

 

[kandsrockets]

Dude. See the smiling face. It was a joke. Plus part of the joke was that as a 4FNC design, its not what you would call original. But you missed all that obviously.

Actually, i actually decided to do something along the lines of troj's design.

I did the design last night and have all my drawings ready to be machined. The fwd blocks are much smaller and will attach to the top centering ring in the rocket via (2) bolts which is easily
accessible (for removal, prepping, and cleaning), and the bottom will utilize a similar tearing scheme kroj used. Great idea kroj!

D, I did not mean anything by my comment, that is what stinks about forums, you can't show your emotion I was laughing as typing. My origanal design was to do a 2 stage with a booster with strap ons. I was going to fire all three motors in the booster off the pad drop the strap ons 3 seconds into flight and after 17 seconds into the flight the booster would be dropped and the substainer would fire for 15 seconds but that will have to be a project for out at balls.

I am going to do what Kevin did for the upper block only smaller version, but I did some drawings last night for a lower hinge unit. I am hoping to drop the plans off with a friend today to have a cheap steel prototype made and if it works I will drop of some alum blocks to be machined.

 

[NJnike]

My origanal design was to do a 2 stage with a booster with strap ons. I was going to fire all three motors in the booster off the pad drop the strap ons 3 seconds into flight and after 17 seconds into the flight the booster would be dropped and the substainer would fire

A few years ago I started working on a low-power model that does that, but never quite finished it. It started as just a little scratch-built two stager then I thought if I could get the motor burn times right I could try parallel staging boosters too. I ran out of extra tubes to build the boosters and just never remember to buy any because I have rocketry A.D.D., so it's still just a conventional two stager. But someday....

 

[troj]

Dave Flynns booster droppers are fine for lower power HPR. I've been doing some heavy mechanical analysis with the particular design, and they are more than adequate for a 60 lb rocket design i'm working on which is utilizing a central M650W and (2) external K550's.

Is your analysis based on the weight of the rocket itself, or the force from the thrust? My guess is you're doing the latter, but I thought I'd ask, to be sure.

My big concern on Dave's design is that it requires two matches to work properly. On smaller projects, I wouldn't be as worried. Even on something in the 60lb range, I start to get worried. (Not saying that even a 5lb rocket going awry isn't cause for concern)

I'll dig through the photos and get better documentation on how our system worked. I'm working on a scaled-down version anyway, so I need to do that. Plus, it's something folks ask about, a lot.

-Kevin

 

[kandsrockets]

Is your analysis based on the weight of the rocket itself, or the force from the thrust? My guess is you're doing the latter, but I thought I'd ask, to be sure.

My big concern on Dave's design is that it requires two matches to work properly. On smaller projects, I wouldn't be as worried. Even on something in the 60lb range, I start to get worried. (Not saying that even a 5lb rocket going awry isn't cause for concern)

I'll dig through the photos and get better documentation on how our system worked. I'm working on a scaled-down version anyway, so I need to do that. Plus, it's something folks ask about, a lot.

-Kevin

Kevin,
Send me a pm with your email address. I will send you a pdf file of how I am going to do the bottom connection. I am working on the drawings for the upper unit that is based off of what you did for the Delta III

 

[WillMarchant]

My big concern on Dave's design is that it requires two matches to work properly. On smaller projects, I wouldn't be as worried. Even on something in the 60lb range, I start to get worried. (Not saying that even a 5lb rocket going awry isn't cause for concern)

Hi Kevin:
Instead of using two of Dave's gizmos on each strap-on, what about using one forward and have the aft end of the booster retained with a latch that disengages when the booster rotates away from the top? Isn't that essentially what your design does? The other difference being the use of a frangible plastic bolt in Dave's, of course.
Best wishes,
Will

 

[troj]

Instead of using two of Dave's gizmos on each strap-on, what about using one forward and have the aft end of the booster retained with a latch that disengages when the booster rotates away from the top? Isn't that essentially what your design does? The other difference being the use of a frangible plastic bolt in Dave's, of course.

That's actually a method we considered. I'll have to draw up how we considered building it.

In the end we didn't do it, because of cost and labor. The plywood we already had, and we made ours out of scraps from cutting centering rings. Strips of wood would be even cheaper.

There's no reason I can think of why it wouldn't work. Until we hit on the brass strips, it's the direction we were going to go.

One thing to understand about us is we refer to what we do as Redneck Engineering -- don't make anything any more complicated than it has to be. Thus the reason why we had people refer to our last build as "This Old Rocket".

-Kevin

 

[DMcCauley]

Here is the approach i'm proposing using which is based on Troj's design. The rods will fit into the enclosure and then the rod (which is notched) will be clamped down using a nylon screw which will break when the charge initiates.

The bottom hinge is simply a simple ball type hinge. Basically, you have a spherical hole in the bottom of the booster rocket (not using end mill, but using drill), and then a small hook with a balled tip. In the upright position is provides a very positive fit, however, once the booster tips over, it quickly disengages and falls off with no chance for hang-up (i.e. slit hinge arrangement)

Here is a photo of the explosive carrier design. I'm going to do a quick machining of the carrier tonite and testing it this weekend.

 

[DMcCauley]

Just some additional updates . . .

Ideally, i'd like to put the centering rod through the middle of the centering ring, but it might get prove difficult to get good alignment that way vs. just attaching to the top of the centering ring. Either way, the load is on the proper side of the centering ring.

 

[troj]

Once I get my parts back, I'll take some pictures to show you what we did to dramatically simplify the installation.

You don't need the booster pins to run the span of the booster, but you do want a bulkhead there, instead of a centering ring. That's a high-stress area.

I need to head to Nate's and grab a full booster, and get you pictures of that.

-Kevin

 

[kandsrockets]

I have attached a pdf file of how I plan on doing mine.

View attachment Boosterdropassembly.pdf

 

[DMcCauley]

Excellent stuff!
And those photos will be great Troj!

Its fantastic that people can share ideas on a forum as opposed to being very secretive. Its great to see different approaches for a particular problem and then take all the available information and ideas to come up with an approach.

Thanks all!

 

[DMcCauley]

BTW, Troj.

How much BP did you end up using, and did you end up with that amount through calculation or trial and error?

Granted, you had a lot more space, but for my particular design, i can fit about 1g maximum (which is probably more than enough) due to the airframe / motor mount size.

 

[troj]

How much BP did you end up using, and did you end up with that amount through calculation or trial and error?

Granted, you had a lot more space, but for my particular design, i can fit about 1g maximum (which is probably more than enough) due to the airframe / motor mount size.

We kinda guessed what we would need, then increased it a bit.

Going from memory, our charges were significantly less than a gram. It's a very confined space, so you don't need much.

What we did was take a set of booster hardware, rig up the components on a piece of wood, then attached that to one of the supports on my deck, and let 'er rip. It separated quite energetically, so we weren't worried.

Keep in mind that all you really need to do is get the top of the booster moving away just a little bit, and physics will do the rest for you.

-Kevin