Reason #934 why I hate friction fitting motors.

[Dugway]

I had a 3 x 24mm cluster booster staging to a 3 x 24mm cluster sustainer. There were no motor blocks or retainers and the two stages were held together by the sustainer motors intruding by an inch into the booster motor tubes. As you would expect, the design relies on each black powder booster motor lighting the black powder sustainer motor above it. I wrapped masking tape around the middle of each motor to the point that it was difficult to install each of them, and I was worried about ever getting them out again without destroying the model.

All three booster motors lit perfectly, all three sustainer motors lit perfectly, the booster motors were ALL ejected out the back of the booster which remained attached to the sustainer, the booster was incinerated by the sustainer motors as it arched over and slammed into the ground before the ejection charges had a chance to go off.

Yes, I had a tiny vent hole in the small amount of space between the top of the booster motor and the bottom of the sustainer motor in each booster motor tube to keep the booster from separating prematurely.

What is the proper way to design a rocket like this? I couldn't have had any kind of positive motor retention in the booster since I also had a third stage that I was planning on using on the next flight.

I REALLY hate friction fitting motors.

 

[prfesser]

I've not done many staged BP motors, but it sounds like you'd needed a significantly larger vent hole or three. And more space between booster and sustainer.

 

[cerving]

Estes changed most of their 2-stage designs awhile back to put an engine block in the BACK of the booster motors. This prevents the motor from getting kicked out the back.

 

[Dugway]

I thought of this, but then how do you do a third stage???

 

[BABAR]

Vent hole was NOT the problem here. Inadequate venting causes 1 and/or 2 problems.

1. Too small or improper location vent prevents hot gas from reaching the sustainer nozzle, so sustainer doesn’t light.

And/or

2. When the booster burns through if there is a significant gap, the hot gas has to push the cold gas out of the way to reach the sustainer nozzle. If this cold gas is not sufficiently vented, the pressure “blows off” the sustainer prematurely.

In your case, since the sustainer motors lit, venting was adequate.

Regarding booster motor retention, if you want to preserve option for another booster to go to three stages, a short motor block at the back will keep your motors in place. You can easily make one with a 3mm segment of BT50, just cut a segment, cut a little piece out and roll it up and glue with paper glue inside the tail end of your booster. This will leave the motor slightly recessed in the tube, but not enough for Krushnic problems

You can gap stage a lower booster easily to cross the 3 mm gap when you go to a three stager.

I am a bit surprised your booster didn’t separate easily at staging. Sounds like whatever coupling mechanism you used worked a bit TOO well, I.e., was too tight. I don’t know about electronics for staging, for black powder the sustainer should be held kind of like the nose cone, enough shoulder/coupler to prevent ANY lateral movement but should easily separate with minimal pressure along the axis. Just drag alone should be enough to separate the booster once the sustainer lights.

For mine, I have to be careful just carrying the stack out to the pad and loading it on the rod/rail or the two will come apart before I even launch. If I just hold the rocket by the sustainer the booster will nearly fall off from gravity.

At launch, there is enough coupling to prevent lateral movement, and since the boosters are firing their force holds the stack together until staging, and since there is no booster motor delay, staging is close to instantaneous so there is no time for drag separation BEFORE the sustainer lights.

Can you post pics of your design?

 

[prfesser]

Vent hole was NOT the problem here. Inadequate venting cause 1 and/or 2 problems.

1. Too small or improper location vent prevents hot gas from reaching the sustainer nozzle, so sustainer doesn’t light.

And/or

2. When the booster burns through if there is a significant gap, the hot gas has to push the cold gas out of the way to reach the sustainer nozzle. If this cold gas is not sufficiently vented, the pressure “blows off” the sustainer prematurely.

Someone can correct me on this, but my understanding is that it's not so much the hot gases that ignite upper stages; it's the bits of BP that are formed when the propellant burns thru. At least that's what I gather from "Handbook of Model Rocketry" on gap staging. Vents permit gases, hot or otherwise, from forcing the stages apart until the BP bits reach the upper stage nozzle. But I agree that the coupling between stages may have been too tight.

Best -- Terry

 

[BABAR]

Someone can correct me on this, but my understanding is that it's not so much the hot gases that ignite upper stages; it's the bits of BP that are formed when the propellant burns thru. At least that's what I gather from "Handbook of Model Rocketry" on gap staging. Vents permit gases, hot or otherwise, from forcing the stages apart until the BP bits reach the upper stage nozzle. But I agree that the coupling between stages may have been too tight.

Best -- Terry

Great video on gap staging that debunks “particle theory”

 

[Dugway]

The rocket in question was a US Rockets Hi-Test 2225: https://v-serv.com/usr/kits/hi-test2225.htm. I've built two of them now and gotten a grand total of three launches out of them. The first was destroyed on its first launch by an E9 CATO, and then this one had a single successful flight on 3 x D12-7s, and then was destroyed as described above.

So maybe the root of the problem was that the booster was held too tightly by the combination of one inch of three sustainer motors. I guess I could have cleaned up the aft ends of the sustainer motors and sanded the tops of the booster motor tubes to try and get it to fall off more readily.

 

[kuririn]

+1 to what Babar said on your staging coupler (if you have one) being too tight. According to JumpJet on this post, it should be very loose:
https://www.rocketryforum.com/threa...and-dirty-staging-design.136856/#post-1630500
And he should know. (In case you didn't know, JumpJet is John Boren, Chief Estes designer).
I have a rocket with a similar design, US Rockets Hi Test 3100. 4X29mm to 4X29mm. Back end of the sustainer motors couple to the booster motor tubes. Gap staged, slotted vents near the sustainer nozzles. This has no staging coupler.
So I'm wondering why your booster did not separate from the sustainer. Do you have a coupler in addition to the motor intrusion?

 

[kuririn]

EDIT: Just saw your post before mine. Yup, the fit sounds way too tight. Try sanding the ends of the sustainer motors down a bit for looser fit.
And what's this about a third stage? Details please.

 

[BABAR]

The rocket in question was a US Rockets Hi-Test 2225: https://v-serv.com/usr/kits/hi-test2225.htm. I've built two of them now and gotten a grand total of three launches out of them. The first was destroyed on its first launch by an E9 CATO, and then this one had a single successful flight on 3 x D12-7s, and then was destroyed as described above.

So maybe the root of the problem was that the booster was held too tightly by the combination of one inch of three sustainer motors. I guess I could have cleaned up the aft ends of the sustainer motors and sanded the tops of the booster motor tubes to try and get it to fall off more readily.

Sounds like a plan! about 1.5 cm "nesting" of the sustainer motors should be more than enough for lateral stability (especially with 3 motors!)
If you pick up the rocket stacked vertically by the sustainer, even slightly briskly, the booster stage should fall off.

I went to your site.

https://v-serv.com/usr/kits/hi-test2225.htm.

Seems to have mixed references, to me "direct" staging is where the motors are taped with cellophane tape, booster blows straight into the nozzle of sustainer, tape melts and the motors separate. Anything that has a "vent" to me is a gap stage, not direct stage.

Hope you get 6 straight trails and an untoasted booster. I will be interested to see how that booster recovers, tumbling with the weight of three 24 mm casings. Hope you have a grassy field.

 

[Dugway]

After destroying my first Hi-Test with a single stage CATO, I still had an unflown booster. After building the second Hi-Test (sustainer and booster), I decided to sim it in RockSim and found it was quite stable as a three stage rocket, with D12-0s in the first stage, E12-0s in the second stage, and E12-8s in the third stage. It was my intention to launch it next in this configuration if it had survived the two stage flight. Oh well.

 

[neil_w]

I admit I don't know much about cluster staging, in particular with regard to reliability. I'm impressed you got all three to light on the booster. Is it likely to be reliable that the timing of all three motors is sufficiently close together that they will all burn through and light the next stage at the same time? Is the burn-time consistency the same for all motor types? In a three stage scenario, will the accumulated timing error among the three motor columns be likely to inhibit simultaneous ignition of the sustainer? I confess to worrying about the safety of this, while not knowing if my concerns have any basis in fact.

Also, 3xD12 + 6xE12 is a lot of weight. What's the first-stage performance look like in your sim? Just curious.

 

[kuririn]

Here's an alternative to friction fitting your booster motors:

Loc 29SS clone.
Bolt or threaded rod epoxied into the valley between the motor tubes.
Washer and wing nut for retention.
Downside is you likely won't be able to stand your rocket upright without a custom made display stand.
I got lucky and found one that works.
After I did this I thought that I could have epoxied a threaded insert into the valley and just screw on a rod so that it would be removeable. Don't know if it's worth the added cost.
Laters.

 

[kuririn]

I will be interested to see how that booster recovers, tumbling with the weight of three 24 mm casings. Hope you have a grassy field.

Yeah, the length of the motor tube and weight of the casings preclude tumble recovery. The design does not include a booster recovery device so I'm thinking this must be some kind of violation of safety rules. Even worse with my 3100.

 

[BABAR]

Post deleted. I see you are Level 2 certified so propellant load shouldn’t be an issue

 

[BABAR]

i did a three-two-one rocket that worked,

Booster 1 had 2 C6-0s and an A8-3. The Cs ignited the second booster, the A deployed a chute.

Booster 2 had a C6-0 and A8-3, the C ignited a sustainer and the A deployed a chute.

Sustainer was on an A8-3

 

[mrwalsh85]

I think your problem is having the sustainer motors extend into the booster motor mount tubes...

Instead of doing this, connect each motor of the cluster to the next stage with cellophane tape. I think this is the typical Estes staging method. That way you know you have a good chance of separating.

Tape your two motors together, three times, then insert into booster (or sustainer?), then slide the opposite airframe over your motor assemblies. And go fly. Of course, check your motor fits. Don't be afraid to try a little friction fitting, unless you can get a motor block in... Just my 2 cents.

 

[BABAR]

I think your problem is having the sustainer motors extend into the booster motor mount tubes...

Instead of doing this, connect each motor of the cluster to the next stage with cellophane tape. I think this is the typical Estes staging method. That way you know you have a good chance of separating.
.

What you describe is standard NON-gap staging. It works great (I think you only need one wrap of cello tape, FWIW), but the rocket has to be designed with a very short Booster (basically about the length of the booster motor).