Strap-on Boosters

[Rocketlady]

How do strap-on boosters that fall away from the rocket work? I can visualize them on the rocket, but do not understand how they are released/jettisoned after their motors burn out. I realize this can be accommplished with the use of a timer.

I don't ever recall seeing build threads or articles anywhere on the web. Google searches turn up space shuttle sized rockets.

Any build articles, photos, detailed drawings would be wonderful!

[Len_Lekx]

Ray Dunakin talks a bit about this (parallel staging) in his Extreme Rocketry booklet ("How to Stage Model Rockets"). The way he does it is to have a retaining-bar at the aft end of the booster, and a PRM device at the forward end. At motor burnout, the PRM is triggered, and the booster falls away.

Check out the booklet, or Rays' website, for more info...

[Luv2launch]

I have done strap-on boosters in a semi scale Delta IV heavy that I built.I used dowels and the thrust of the booster motors to hold them on they fell off on burn out.

[Rocketlady]

Thanks, Andy. Those photos are just what I needed to see.

[Delta-IV]

I've been doing these now for over 3 years. For low power rocketry its fairly easy. Since the burn times and power ratings of the BP motors offer more possibilities.

For my rockets, I use the method of thrust/drag. The central sustainer uses a lower initial thrust but longer burn motor than the strap-ons. When the strap-ons burn out, their drag and the continuing thrust of the central sustainer allows the central to fly out from the boosters.

I use dowels and and tubes etc. for the separating mechanism.

Currently I am designing/building my HPR Delta IV Heavy and will have to use other means to accomplish the separation though.

[Delta-IV]

I have done strap-on boosters in a semi scale Delta IV heavy that I built.I used dowels and the thrust of the booster motors to hold them on they fell off on burn out.

Andy,

Any photos/video of your launches? Would like to see how it looked.

Thanks,
-Carl-

[blackjack2564]

What holds them on at rest? Snug fit?

[troj]

We did a different method of drop-away boosters, which allows for them to be attached when they're not firing. It was for a much larger project, but I'm planning to scale it down over the next year or so.

You can find more info on it on the website for the project.

-Kevin

[Delta-IV]

What holds them on at rest? Snug fit?

I built a special launch pad with posts sticking up to hold them. They need to be somewhat loosely fit to allow them to fall away easily.

Also, at one point I modified the setup to use thread to hold the base pieces together and routed it over the engine and taped onto the opposite side. This would allow the exhaust from the motor to burn through the thread which would then allow the boosters to fall away in flight.

That turned out to be harder to do during the pre-launch build up than the worth. Although I have had a couple of flights where the Estes igniter either shorted or was slightly late in ignition. The booster either stayed on the pad or flew it's own flight. The fins on the boosters allowed it to fly out balistically but recovered fine since I use short delays.

[georgegassaway]

I rigged my space shuttle model so that the SRB’s peel away, as is done by many boosters (particularly solids). The rear parts are hinged so that the boosters pivot about 20 to 30 degrees before they disconnect form the aft mount. The front end connects via a 3/32” brass rod perpendicular into the side of the ET, fitting into a hole in the ET. There is a notch in the rod so that a latch in the ET locks it in place, until a servo disconnects the latch.

A spring is used to push the SRB nose a few degrees away, to disengage the 3/32" brass rod, and then the airflow “grabs” the front of the SRB to make it pivot from that point on (pivoting on the aft attachents, then letting go). Actually even the Ariane rocket uses springs to push the nose of their solids away a similar manner to this, pivoting for a few degrees on the back then letting go. I think that the Delta-II uses springs in the nose as well, but cannot confirm.

I have heard of a couple of HPR birds that did something like that at the front without any mechanical device, but using what effectively was a homemade “explosive bolt”, but it was not really a bolt, just a homemade rod or other item that had a charge in it to disintegrate it when fired.

Video of a test of my 1/72 shuttle model boilerplate here on Youtube (background music is loud)

http://www.youtube.com/watch?v=OQbt2...eature=channel

Drawings below of details.

And three photos showing the aft ET and SRB attach fittings. The ring is a cast part, which had some “U” - shaped .025” wire pieces embedded into the half-post parts of the mold before casting to act like re-bar in concrete, since the resin alone was too weak and fragile for that purpose. Last one shows about how far the aft attach fittings stay engaged while pivoting, a few degrees more they are no longer attached.

I should note that for my shuttle, the SRB’s are along for the ride, they have no engines in them. If they had engines that produced some significant thrust, I might have had to revise the design to be able to handle “pushing up” thrust loads. In this case, all the thrust loads from the SRB’s would have been on that 3/32” diameter rod, so I might have needed to make it larger in diameter, and probably from brass to steel. As currently done, the thrust loads are from the one F engine in the ET, and the SRB’s are along for the ride being pushed at the bottom of the aft attach points, so that the 3/32” forward attachment rod has no fore-aft loads on it during boost, and only minimal loads on it during the brief time from deceleration to SRB sep (due to the thrust tail-off of the engine, and the drag, the model actually reaches maximum velocity halfway thru the burn, then starts to decelerate as the thrust continues to drop towards burnout. Very different from your typical C6 or D12 or E9 type rocket),

- George Gassaway

[troj]

I have heard of a couple of HPR birds that did something like that at the front without any mechanical device, but using what effectively was a homemade “explosive bolt”, but it was not really a bolt, just a homemade rod or other item that had a charge in it to disintegrate it when fired.

You're close to describing ours. What we did is have an aluminum rod that sticks out of the booster, into an aluminum block inside the main airframe. The fit is a bit loose, and there's an o-ring on the rod, to seal it. We drop a #10 nylon bolt down through the block and rod, to hold things in place.

At the bottom of the well that the rod fits into is a very small BP charge and an electric match, wired to the sequencer which handles igniting the boosters and firing their separation charges.

The bottom of each booster sits in a cradle, and is held in place with a thin brass strip.

When the separation charge fires, it pushes the top of the booster out and away, shearing the #10 nylon bolt in the process. When the rear of the booster falls away, the brass strip tears, releasing the rear of the booster.

One concern we had is what would happen if a booster's recovery system deployed when the booster was still connected to the main body of the rocket.

To prevent this, we have a separate timer inside of each booster. The timer is connected via wires to the sequencer, for a simple "I'm here!" circuit. The timer does not begin its countdown until the booster separates from the main body, at which point the connection breaks, and it loses communication with the sequencer.

One additional benefit to this is that when the rocket is on the pad, we can hook a laptop to the sequencer, and check the status of the entire system -- we can make sure the sequencer is talking with all of the timers, that the timers have continuity on their electric matches, that the separation matches have continuity, and that the booster igniters have continuity.

-Kevin

[Luv2launch]

Andy,

Any photos/video of your launches? Would like to see how it looked.

Thanks,
-Carl-

No no photos 1st launch i was alone second launch I forgot the camera.The second launch was its last one on the 1st it came down kinda on the fast side so I put a bigger chute on and then on the second launch it drifted off the field into the woods.It is going to be rebuilt very soon though.

[Luv2launch]

What holds them on at rest? Snug fit?

On mine I glued some spent casings on a board that propped up the boosters very simialer to whats its on in the pictures above without the case in the middle.

[georgegassaway]

Kevin wrote:

>>>>>
You're close to describing ours. What we did is have an aluminum rod that sticks out of the booster, into an aluminum block inside the main airframe. The fit is a bit loose, and there's an o-ring on the rod, to seal it. We drop a #10 nylon bolt down through the block and rod, to hold things in place.

At the bottom of the well that the rod fits into is a very small BP charge and an electric match, wired to the sequencer which handles igniting the boosters and firing their separation charges.
<<<<<

Yes, I think yours must be one of those I was thinking of, details were hazy. Plus, somewhere, a couple of years back, there was a guy in Germany working up a large shuttle, who did a pyro-type method too but I cannot recall exactly what he was doing for the mechanism.

>>>>>>
One concern we had is what would happen if a booster's recovery system deployed when the booster was still connected to the main body of the rocket.

To prevent this, we have a separate timer inside of each booster. The timer is connected via wires to the sequencer, for a simple "I'm here!" circuit. The timer does not begin its countdown until the booster separates from the main body, at which point the connection breaks, and it loses communication with the sequencer.
<<<<<<<

Sort of what I have with the shuttle SRB’s. For limited mass purposes, I went with converted wind-up Tomy Toy type timers, and a mechanical system so when a retaining disc rotated to open up a slot, it would allow a rubber-band driven dowel to push the nose off and let the chutes out.

As with your “I’m here” sensors, the wind-up timers have a rod pushed in by the ET to keep the timers from running. When the SRB seps, the stop-rod is no longer pressed, so that lets the wind-up timers start to rotate their discs. About 2 seconds later, they pop the chutes out.

This was about 10.5 years ago, there was not an electrical timer and battery system light enough (in the 5-6 gram range) that I would have trusted, though in 2000 I came across an ideal timer that could run off a 1.5v watch battery and fire a very low current e-match, but it was too late by then.

If I was doing any similar project like that now, I would definitely use an electronic timer, since there are few light and reliable enough now.

Although, I’d be really careful on the “I’m here” sensor switches since a false trigger can be so bad (at least with the wind-up timer, you could hear it start to run and then at worst I’d have to repack the chutes. No "Bang" of accidental pyrotechnic ejection). Add an arming switch for each, of course, with a remove before flight type of pin. But I’d probably use two switches, in parallel. When I did the shuttle model, I had an “I'm here” switch for the orbiter, on the right aft ET to orbiter attach point. But the micro switch was so tiny, and its level motion so limited I was concerned it might false trigger open. So, I added another switch to the left ET to orbiter attach point too. So, in parallel, both would have to detect the orbiter gone before the flight computer (in the ET nose) would run the “Orbiter has sepped” part of the program (the Orbiter was sepped by me manually by R/C, usually waiting till after the SRB’s were auto-sepped by the computer, but on a couple of bad boosts I sepped the orbiter first to save the orbiter and start an “abort” over-ride sequence in the flight computer to sep the SRB’s and fire ejection). That was important since sensing orbiter sep caused a 1 second countdown to firing the ejection charge in the ET (There were two other sensors switches in the ET, an on-the-pad sensor, to avoid false triggering in case for example the orbiter came off by accident, and a “thrust” switch pressed only when the engine was thrusting, to detect a truly valid liftoff.

>>>>>
One additional benefit to this is that when the rocket is on the pad, we can hook a laptop to the sequencer, and check the status of the entire system -- we can make sure the sequencer is talking with all of the timers, that the timers have continuity on their electric matches, that the separation matches have continuity, and that the booster igniters have continuity.
<<<<<<

Also some parallels with what I was doing with the shuttle. The “Flight Computer” was by Jay Marsh, made of two Basic Stamp PIC’s. He had developed this for his FAI Saturn-IB models, which eventually was 3-staged, and had R/C override to either fire stages early if it got into trouble early while pointing up, or stop the firing of stages if it get into too much trouble, otherwise the computer ran staging on auto. For my shuttle, he made a physically similar version, but with a lot of different code to do what was needed. If the computer was “Happy” it would put out one single beep every 2 or 3 seconds. If something went wrong it would do a lot of quick beeps as an alert, then settle into beeping out a number which was a code for the problem. Such as liftoff sensed without thrust sensed. Or orbiter sep sensed without liftoff. Or continuity bad for the two ejection charges. Any show-stopper condition sensed. Things like that. Of course, it had no idea of the status of the “dumb SRB’s” the shuttle model used, since it did not need to. Once it commanded the servo to sep the SRB's, they sepped, or if not, the comptuer coudl do no more about it anyway (they never failed to sep once bench-tests de-bugged the latch system of the boilerplate).

- George Gassaway

[troj]

As with your “I’m here” sensors, the wind-up timers have a rod pushed in by the ET to keep the timers from running. When the SRB seps, the stop-rod is no longer pressed, so that lets the wind-up timers start to rotate their discs. About 2 seconds later, they pop the chutes out.

Simple, elegant, and it worked.

This was about 10.5 years ago, there was not an electrical timer and battery system light enough (in the 5-6 gram range) that I would have trusted, though in 2000 I came across an ideal timer that could run off a 1.5v watch battery and fire a very low current e-match, but it was too late by then.

We had the benefit of two very talented electronics types on the project, who were able to custom-design circuits to match our needs.

Although, I’d be really careful on the “I’m here” sensor switches since a false trigger can be so bad (at least with the wind-up timer, you could hear it start to run and then at worst I’d have to repack the chutes. No "Bang" of accidental pyrotechnic ejection). Add an arming switch for each, of course, with a remove before flight type of pin. But I’d probably use two switches, in parallel.

Our "I'm here" was actually simple communication between the timer and the sequencer. We verified that the sequencer was in communication with the timer, before it was armed.

There was indeed an additional arming switch on each timer, to prevent a timer starting its countdown because of a wiring or communication problem on the pad.

-Kevin

[Delta-IV]

I rigged my space shuttle model so that the SRB’s peel away, as is done by many boosters (particularly solids). The rear parts are hinged so that the boosters pivot about 20 to 30 degrees before they disconnect form the aft mount. The front end connects via a 1/8” rod perpendicular into the side of the ET, fitting into a hole in the ET. There is a notch in the rod so that a latch in the ET locks it in place, until a servo disconnects the latch.

A spring is used to push the SRB nose a few degrees away, to disengage the 1/8” rod, and then the airflow “grabs” the front of the SRB to make it pivot from that point on. Actually even the Ariane rocket uses springs to push the nose of their solids away a similar manner to this, pivoting for a few degrees on the back then letting go. I think that the Delta-II uses springs in the nose as well, but cannot confirm.

I have heard of a couple of HPR birds that did something like that at the front without any mechanical device, but using what effectively was a homemade “explosive bolt”, but it was not really a bolt, just a homemade rod or other item that had a charge in it to disintegrate it when fired.



- George Gassaway

George and Kevin,

Awesome, I was thinking of doing a servo type of latch on the forward end for my 5 foot tall HPR Delta IV Heavy. Now I have good ideas to really get started. I am keeping this thing in the H/I range so I don't have a lot of budget (weight and $$$) to work with but perhaps some home made bolt cutters with an e-match may work as well.

[MarkII]

I have never tried this, but couldn't you just retain the boosters with thrust and gravity? Hang the strap-ons onto downward angled dowels and have them so loose that they will just fall off. Then, while the rocket is on the pad, have the boosters supported so that the sustainer is hanging on them by gravity. During boost, the thrust of the motors in the boosters keeps them pushed up onto the dowel hooks, but then at burn-out, they drag-separate as described previously. No need for timers, complicated release mechanisms, etc. Wouldn't that work? I'm surprised that no one has described this method so far, so perhaps there's a problem with it that I just don't see yet?

Mark \\.

[Luv2launch]

I have never tried this, but couldn't you just retain the boosters with thrust and gravity? Hang the strap-ons onto downward angled dowels and have them so loose that they will just fall off. Then, while the rocket is on the pad, have the boosters supported so that the sustainer is hanging on them by gravity. During boost, the thrust of the motors in the boosters keeps them pushed up onto the dowel hooks, but then at burn-out, they drag-separate as described previously. No need for timers, complicated release mechanisms, etc. Wouldn't that work? I'm surprised that no one has described this method so far, so perhaps there's a problem with it that I just don't see yet?

Mark \\.

That works fine on low mid power like my pics above although I would be a bit tense trying it with AP motors, my semi scale delta IV heavy that I have pics of above has only flown on D12-3's in the boosters and an E9-4 in the sustainer.If I knew for sure I could get all 3 motors to light at the same time I would try it with AP.

[MarkII]

That works fine on low mid power like my pics above although I would be a bit tense trying it with AP motors, my semi scale delta IV heavy that I have pics of above has only flown on D12-3's in the boosters and an E9-4 in the sustainer.If I knew for sure I could get all 3 motors to light at the same time I would try it with AP.

Ah, yes. I was thinking BP motors. I wouldn't want to try it with composite propellant motors. That's the factor I was overlooking.

Mark \\.

[georgegassaway]

I have never tried this, but couldn't you just retain the boosters with thrust and gravity? Hang the strap-ons onto downward angled dowels and have them so loose that they will just fall off. Then, while the rocket is on the pad, have the boosters supported so that the sustainer is hanging on them by gravity. During boost, the thrust of the motors in the boosters keeps them pushed up onto the dowel hooks, but then at burn-out, they drag-separate as described previously. No need for timers, complicated release mechanisms, etc. Wouldn't that work? I'm surprised that no one has described this method so far, so perhaps there's a problem with it that I just don't see yet?

Mark \\.

Here is the problem. On a given flight the cluster ignition is not dead-perfectly timed. One of the side boosters ignites a bit late. The rocket and other booster start to take off, moving enough for the late booster to fall off, and then the late booster ignites, going off by itself. While the rocket with one booster attached goes veering off in the direction of the missing booster.

May work a few times, but inevitably, that is what will happen. Unless the ignitors are dead-perfectly reliable (and fire exactly together) and you stick with black powder only (Composites are even harder to get to ignite right at the same moment as BP motors).

So at the least, something has to be done to prevent the boosters from falling off early. Does not to be as elaborate as the other ones, but the method you describe will inevitably have that problem.

- George Gassaway

[Delta-IV]

Here is the problem. On a given flight the cluster ignition is not dead-perfectly timed. One of the side boosters ignites a bit late. The rocket and other booster start to take off, moving enough for the late booster to fall off, and then the late booster ignites, going off by itself. While the rocket with one booster attached goes veering off in the direction of the missing booster.

May work a few times, but inevitably, that is what will happen. Unless the ignitors are dead-perfectly reliable and you stick with black powder only (Composites are even harder to get to ignite right at the same moment as BP motors).

So at the least, something has to be done to prevent the boosters from falling off early. Does not to be as elaborate as the other ones, but the method you describe will inevitably have that problem.

- George Gassaway

Perhaps I didn't write well in my above posts, but what Mark wrote is exactly what I do and as above I have had a couple of flights where I had one booster lite late and it flew on its own.

In addition to my HPR Delta IV Heavy build. I am also doing a new build of my 24mm BP Delta IV Heavy and am planning on incorporating a fail safe method of keeping any late or no ignition booster on.

20 Flights:

16 Good ones
1 Central core did not ignite, boosters separated and recovered, core lawn dart destroyed payload section only
2 One side booster was late in ignition, flew by itself, all componets recovered via parachute. No damage
1 One side booster didn't ignite stayed on pad, other componet flew and recovered via parachute.

Here is a video showing that it always doesn't work. I would never ever attempt this method with APCP.

http://www.delta-iv-rockets.com/joom...Flight_DVD.mpg

[Pem Tech]

Alrighty then!
This is a topic I have been looking at over the last couple of weeks now. For the EMRR Challenge the plan is to build a parallel staged rocket that ejects the boosters, using composite motors only.
Thanks to everyone for their contribution.

[Delta-IV]

Alrighty then!
This is a topic I have been looking at over the last couple of weeks now. For the EMRR Challenge the plan is to build a parallel staged rocket that ejects the boosters, using composite motors only.
Thanks to everyone for their contribution.

I'm in....thanks....

For my Black Powder version, here is how I built it (clear fins omitted):

[troj]

Alrighty then!
This is a topic I have been looking at over the last couple of weeks now. For the EMRR Challenge the plan is to build a parallel staged rocket that ejects the boosters, using composite motors only.
Thanks to everyone for their contribution.

Oh! Well....

In that case....

The Delta III technology is er...um....uh....patented...and trademarked...and....yeah. All that legal mumbo-jumbo.

....and I get half of the winnings of anyone who uses it.


In all reality, I hope the description of that and George's info helps someone on a project!

-Kevin