Gap Staging - Placement of Vent Holes and Motor Tube Length

I'm about to try my hand at my first gap staged design, and had a couple questions. My plan is for a BT56-sized rocket that stages D to E BP motors.

My gut tells me that my vent holes should be pretty far forward on the booster, near the aft of the sustainer motor, since the hot gasses from the booster motor would be surging forward, but I'm not sure it really matters. I know they should be evenly spaced on opposite sides of the rocket, but I don't know if I've read where they should be placed on the length of the airframe.

My gut also tells me that I should have a longer motor tube, rather than a standard length tube, to direct the blast of hot particles from the booster motor, rather than just letting them spray all over the interior of the booster. I have a feeling I'll have a better chance of successful sustainer ignition if I have a tube that extends to maybe an inch or less from the aft of the sustainer motor, leaving enough space for the gasses to vent through the body tube holes.

Are my gut feelings correct? I can't remember whether I got these ideas from something I already read or not.

Thanks for your help!

Edit: I meant to select the question mark icon for this thread, but you guys got the smiley face. Which is just as well - I'm a friendly guy.

I think far forward and in opposition is good. I goofed an early attempt as I was told 1 hole near the sustainer motor would work. It left the pad and then cart-wheeled tightly and THEN lit the sustainer which, went racing over our heads. It could have been bad. I got lucky. I then added another vent directly across from the single one and it worked very well. I don't know about the longer motor tube but longer seems like a good idea. Of course long enough and it wouldn't be gap staging...

I believe I read in Handbook of Model Rocketry that the vent holes should be as near the sustainer motor as possible. Apogee's How 2-Stage Rockets Work also states the same. I'm trying to design a sport scale rocket and booster, and I will have to gap stage them.

gna said:

I believe I read in Handbook of Model Rocketry that the vent holes should be as near the sustainer motor as possible. Apogee's How 2-Stage Rockets Work also states the same. I'm trying to design a sport scale rocket and booster, and I will have to gap stage them.

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Thats how my gap staged SA-3 works, the top of the vent holes just below the bottom of the upper stage motor. As you can see from the pictures below the 4 .25" vent holes are hidden under the interstage transition. Make sure you have enough flow to vent the pressure allowing the hot particles and gases to flow and not pressurize the cooler air between the motors (forcing them apart before ignition), the gap between the two D12's is about 8" in this model. On the 6 flights flown with it before the sustainer was lost there were exactly zero failures to stage.

Gap staging just means the two motors are not directly in contact with each other, iirc HoMR says that 10" or less gap is preferable and will usually stage. A few people have truly gap staged the Aerobee-Hi.

Can't add to the responses already given but I did want to point out a fascinating and ingenious study on BP motor staging done for the NARAM 55 R&D competition.

The gap staging part starts at 14:29.

[video=youtube;1Nhe5Y78PRQ]https://www.youtube.com/watch?v=1Nhe5Y78PRQ[/video]

NAR members can access the report here:

https://www.nar.org/members/rd-reports-in-chronological-order/

samb said:

Can't add to the responses already given but I did want to point out a fascinating and ingenious study on BP motor staging done for the NARAM 55 R&D competition.

The gap staging part starts at 14:29.

[video=youtube;1Nhe5Y78PRQ]https://www.youtube.com/watch?v=1Nhe5Y78PRQ[/video]

NAR members can access the report here:

https://www.nar.org/members/rd-reports-in-chronological-order/

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Yep, Emma did a great job with her limited resources.

Here's what one version of my design looks like (thanks to @adrian for the info):

gna said:

Here's what one version of my design looks like (thanks to @adrian for the info):

View attachment 273796

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Nice looking rocket, only concern might be the booster possibly won't tumble, you might put it in its own design file and see if it is unstable by its self, if so it should tumble recover. To me it just looks a bit long and with the fins at the rear, it seems aerodynamic enough that it will come in ballistic after separation.

rharshberger said:

Nice looking rocket, only concern might be the booster possibly won't tumble, you might put it in its own design file and see if it is unstable by its self, if so it should tumble recover. To me it just looks a bit long and with the fins at the rear, it seems aerodynamic enough that it will come in ballistic after separation.

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Thanks for the tip; I'll have check that.

I'm basing it off of this diagram:



I should probably start a thread on this design, but I have no idea where. Plans? Scale? Scratchbuilding?

rharshberger said:

Thats how my gap staged SA-3 works, the top of the vent holes just below the bottom of the upper stage motor. As you can see from the pictures below the 4 .25" vent holes are hidden under the interstage transition. Make sure you have enough flow to vent the pressure allowing the hot particles and gases to flow and not pressurize the cooler air between the motors (forcing them apart before ignition), the gap between the two D12's is about 8" in this model. On the 6 flights flown with it before the sustainer was lost there were exactly zero failures to stage.

Gap staging just means the two motors are not directly in contact with each other, iirc HoMR says that 10" or less gap is preferable and will usually stage. A few people have truly gap staged the Aerobee-Hi.


View attachment 273762View attachment 273763

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That's an interesting-looking fin can. Do you have more pictures of the build?

lcorinth said:

That's an interesting-looking fin can. Do you have more pictures of the build?

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Yep, the build and the rebuild, there is an entire thread devoted to this particular model.

https://www.rocketryforum.com/showthread.php?70827-SA-3-Goa-Semi-Scale-2-Stage-with-Cluster-Booster

I can only tell you what has worked for me.
I put two holes 180 degrees apart in the booster just below where the sustainer engine will recess into the booster. If it is minimum diameter you just need two holes. They are on opposite sides to avoid the theoretical possibility of the ejection force asymmetrically displacing the flight path.
If you have a smaller engine mount you need holes on the inside tube and the outside tube, OR you can vent the outside back through the tail of the booster by using incomplete centering rings, just need an open pathway so gas can get out of the inside tube. The latter solution is more aesthetic and aerodynamic.

Long boosters have a natural tendency to come back to earth ballistic ally rather than tumble. One solution is to fire the booster as a cluster, with a second "pod" fixed to the booster with its own engine, a nose cone, and a chute. The extra oomph off the pad isn't the point (although it doesn't hurt!) but using a short (3 second or less) delay engine allows the zero delay engine to do its job in igniting the upper stage and the pod engine to deploy a chute or streamer a few seconds later.

I have successfully gap staged and recover intact both sustainer and booster using a 36" gap with this technique

https://www.rocketryforum.com/showt...taged-2-successful-flights!&highlight=36+inch

As others have already posted your vent holes need to be 180 degrees apart and large enough to vent the cooler air column ahead of the burst through gas and burning bits.
I generally make my vent holes approximately 1/2 of the interior diameter of the booster motor I'm using.

Length of the body between boost and sustainer can be as long as 14". Personally the longest motor separation I've flown is 11". DO keep in mind the LONGER the booster body the more important it is to create some form of active recovery system as these booster WILL return ballistic rather than tumble. By active recovery I meant some form of streamer or small chute that can be sustainer drag out with snap string or other types of booster deployment.

BABAR said:

I can only tell you what has worked for me.


Long boosters have a natural tendency to come back to earth ballistic ally rather than tumble. One solution is to fire the booster as a cluster, with a second "pod" fixed to the booster with its own engine, a nose cone, and a chute. The extra oomph off the pad isn't the point (although it doesn't hurt!) but using a short (3 second or less) delay engine allows the zero delay engine to do its job in igniting the upper stage and the pod engine to deploy a chute or streamer a few seconds later.


https://www.rocketryforum.com/showt...taged-2-successful-flights!&highlight=36+inch

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This was part of the reason for my SA-3s complex fincan, it used a 24mm booster engine and 2x18mm booster/ejection engines and then had 2 parachute ejection tubes for rear ejection.

rharshberger said:

This was part of the reason for my SA-3s complex fincan, it used a 24mm booster engine and 2x18mm booster/ejection engines and then had 2 parachute ejection tubes for rear ejection.

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Ooooh, I LIKE the idea of rear ejection if your outer tube diameter is sufficient to hold the additional tubes. Wouldn't take much. I wonder if the micromax engines would be sufficient?

BABAR said:

Ooooh, I LIKE the idea of rear ejection if your outer tube diameter is sufficient to hold the additional tubes. Wouldn't take much. I wonder if the micromax engines would be sufficient?

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Micromeister would be the person to ask about the ejection charges on MMX engines.

rharshberger said:

Micromeister would be the person to ask about the ejection charges on MMX engines.

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Great minds think alike
https://www.rocketryforum.com/showthread.php?129302-Potentially-novel-use-for-micromax-engines

I've deployed 10" diameter plastic chutes from tubes as small as T3 (.375"). 4, 5 & 6" .010" mylar (competition type) chutes from minimum diameter T2+ (.281"OD) Tubes.

I use all kinds of different width and length Teflon Streamers from 1/4" x 9" up to 3/4" x 15" in T2+ & T3 tubes on just about all my Micro models. In T4 (.448"OD) and BT-5 (.544"OD), 2" wide by 30" long Military Grade Teflon Streamers fold/roll with room to spare.

Dual rear ejection of a couple streamers or very small (3" or 4" diameter)chutes will slow the decent of most any size BP powered booster.
Hope some of this info helps.

samb said:

Can't add to the responses already given but I did want to point out a fascinating and ingenious study on BP motor staging done for the NARAM 55 R&D competition.

The gap staging part starts at 14:29.

[video=youtube;1Nhe5Y78PRQ]https://www.youtube.com/watch?v=1Nhe5Y78PRQ[/video]

NAR members can access the report here:

https://www.nar.org/members/rd-reports-in-chronological-order/

Click to expand...

That is really fascinating. She mentions angular staging at the end. What is that? I've never heard of it. I saw the motor mount she showed, but couldn't tell what it was for, except that one of the motor tubes looks like it's at an angle. Why would you do that? Is it for staging a cluster to a single sustainer motor?

lcorinth said:

That is really fascinating. She mentions angular staging at the end. What is that? I've never heard of it. I saw the motor mount she showed, but couldn't tell what it was for, except that one of the motor tubes looks like it's at an angle. Why would you do that? Is it for staging a cluster to a single sustainer motor?

Click to expand...

Angled tubes could be used for just that reason, staging a clustered booster to a single sustainer.

rharshberger said:

Angled tubes could be used for just that reason, staging a clustered booster to a single sustainer.

Click to expand...

Or vice versa, single booster to multiple sustainers

BABAR said:

Or vice versa, single booster to multiple sustainers

Click to expand...

Good point, in the vein of the Estes MIRV, one central 18mm to 3x13mm motors (no tubes however).

Well, I haven't finished the design which prompted me to start this thread, but I did start another scratch build, and because of it I have more questions.

I started building a rocket for myself which I made months ago for a friend as a gift, and never got to see fly. I liked the design so much, I started my own the other night. It's called the Copperhead.



Little BT50-BT60 payloader I really like.

Anyway, as I started building, it occurred to me that it would be easy to build a booster. I wanted to use positive motor retention on this one, so I put motor hooks and thrust rings in both sustainer and booster (well, I'd already installed the sustainer MMT, so half the decision was made for me).

So, I came up with this:





I like the looks of this a lot.

But I have a couple of potential pitfalls. Here are the details:

There's a 1 1/4 inch gap between the two motors.



I decided to go with four vent holes rather than two, to be in line with the fins - you know, for symmetry and whatnot.

But I drilled 1/16 inch holes.



I know that The Handbook mentions two 1/4 inch holes on either side of a rocket. I assume four 1/8 inch holes would do as well. But I'm not sure about the 1/16 inch holes. I could bump them up to 1/8 inch, but I like the smaller holes. The way I read it, Bill and G. Harry don't say there must be two 1/4 inch holes, it's just the solution he/they came up with to the gap staging problem.

So, my question is, has anybody experimented with four smaller vent holes, and how did it go? It is a small gap between motors, but I don't know if that's gonna be a good thing or a bad thing - less distance for the blow through to travel to ignite the sustainer, but less space for the gasses to expand to if the vent holes are a little small, meaning the booster could just get blown off without igniting the sustainer.

Second potential pitfall: The coupler, a BT50 coupler from JonRocket, is a really great fit - for a tube coupler. Meaning it's not wobbly loose, but not so tight you have to sand it or chamfer anything or force it in. It slides in and out of the sustainer easily. So easily that I worry it might get blown off regardless of the vent holes.

Now, remember that the booster was kind of an afterthought with this rocket. I'm sure it can still work. But I only have 1/2 inch of coupler before it butts into the sustainer CR.





So, I'm wondering if I need to shim it up, just a titch. Not a lot - I'm thinking tape might be just a little too much shimming.

But here are some solutions I've considered, and I'd like your thoughts.

1) Put a little thin CA on the coupler, and allow to dry. When I've reinforced tube ends with CA, I've noticed that some components (nose cones, centering rings, couplers ) are a wee bit tighter, and sometimes might need a little sanding to get in there properly. Maybe for a slightly loose staging coupler, this would do the trick.

2) Launch it as a single stage rocket a few times, and wait for soot to build up on the sustainer. I noticed with my first two-stage design that after the first flight, the coupler, which was already pretty tight, needed a lot of sanding to fit back in properly, because of the soot. Maybe that would be a solution - test flight as a single stager, then do the booster, expecting things to be a little tighter.

3) Abandon this madness and just build a single stage rocket.

If I still lived in a small city with lots of open countryside, I might try to static test this booster/sustainer configuration and see what happens, but now that I live in the Big City, I'm not sure the Boston PD would take kindly to someone doing "rocket experiments" in the tiny front yard of his apartment.

Thanks, guys. I have a lot of hope for this little rocket. I appreciate any feedback you have.

not sure if those small holes will be enough to prevent pressurization separations prior to ignition. But your gap is really small, so you will probable be good.

There is another very cool venting solution when you are using NON-minimum diameter rockets. You can use incomplete centering rings on your motor mount for your booster. Run a "chimney" from your motor mount tube up to about 1/4" below the sustainer engine. The hot gases (must NOT say particles:bang::bang should run up the chimney to "bathe" the nozzle of the booster and will vent back DOWN the booster around the motor mount/chimney tube through the gaps in the centering rings out the rear of the booster . No side holes required.

As a couple of data points, the Quest (formerly MRC) Zenith II is gap staged with no vents, but the gap is a half inch or so. It uses motor hooks in both stages. Never had a failure to light the sustainer. It's 25mm/BT-50-sized. The booster is a fairly snug fit, though.

On the Estes ARFs with accessory boosters I wound up venting them with 3 1/4 inch holes to get reliable second stage ignition in spite of the nifty funnel for the burning bits that goes from the booster to the sustainer. On these, however, the booster fit is pretty loose. On the BT-55-based ARFs the three 1/4 inch holes may have been overkill....