Gap staging questions

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

STRMan

Well-Known Member
Joined
Nov 9, 2010
Messages
401
Reaction score
1
I've been searching through previous threads and I can't find an answer to these specific questions.

I am gap staging a couple of 24 mm engines. They will be a gap of about 1 inch from the end of one motor to the beginning of the next. I plan on drilling three vent holes 120 degrees apart around the tube, one dead center between each pair of fins.

What diameter should I drill these vent holes?

Should they upper edge of each hole be right at the lower edge of the upper engine, or set back some distance?
 
With a gap of only one inch, i would not worry too much about vent holes, the hot particles from the ejection charge will easily ingite the second motor. with a larger gap ,the vent holes are there so the cold air can be pushed out of the way and not act as a barrier.
but if you really need to vent (belt and braces) then 4mm will be more than enough.
 
FWIW, my experience has been with a pair of 1/4 inch (6.3 mm) vent holes when staging D motors. I have had good results with that "level" of venting. I don't have analytical notes to back it up (e.g., pressure-volume-time-etc behavior of the interstage gases) but it has worked reliably for gap distances of 1-2-3 inches.

Your setup (three holes, 4 mm) is about 60 percent of the same vent area. If anything, I would recommend that you use at least four, preferably five holes. Or, if you could switch to a 5 mm hole size, three vent holes looks like it would work well. (Based on my experience.) OTOH, your setup might work just fine. If you stick with your plan, please let us know about your flight results?

IMO, the vent holes need to be located (longitudinally) at or slightly above the upper nozzle. I am going to disagree with Gillard at this point, because I think it is indeed important to get rid of the cold air and make a ready path for the hot gas from the booster when using gap staging. I also disagree that particles (of any kind) have much to do with ignition of the upper motor; any burning particles that make it through the next nozzle are nice, but I believe the key ignition mechanism is intense radiant infra-red energy from the hot gas.

Think of an old-fashioned flash bulb going off right next to a sheet of paper and setting it on fire---there are no burning bits thrown onto the paper because the flash bulb is enclosed, but the heat of the flash is still enough to cause ignition. OTOH, maybe I am only giving away my age here (flash bulbs and all that...)

If radiated heat from the booster blow-through is what ignites the exposed black powder in the upper stage nozzle, then obviously you need to get that gas as close to the nozzle as possible. That is why I vote for plenty of venting. I also typically use a forward extension of the booster MMT to reach up within a 1/4 inch or so of the next nozzle, to help contain and focus the blow-through gases toward that nozzle.

I think if you must err, it would be better to go a bit toward the side of "too much" vent area. Definitely would be a bad thing to have "too little" vent area, as that would invite the rapid separation of stages....without ignition.
 
I disagree about not needing the vent holes if the motors are separated.
The vast majority of the time the motor burn through of the booster will just push the stages apart without igniting the sustainer unless you either PROPERLY gap-stage with vent holes or tape two butt-joined motors together with 1 layer of cello tape.
Very hard to butt-joint tape two motors together that are separated by a gap of 1"...that leaves you with properly gap-staging them.
Unless you actually like lawn-darts (they are GREAT if on someone ELSES rocket) of your rockets and the corresponding post-flight trash-bag recovery of a re-kitted rocket, drill the vent holes; I provided more detailed instructions to your post over on YORF.
 
Yep, you need the holes. Proximity doesn't matter, because the first hint of a pressure increase will cause a seperation if there are no holes.
 
Yep, you need the holes. Proximity doesn't matter, because the first hint of a pressure increase will cause a seperation if there are no holes.

I guess my Estes Omega never staged then, nor did all those other folks' with expensive (to us back then) Cinerocs.

The Omega has a short gap between motors and no vent holes.
 
I am gap staging a couple of 24 mm engines. They will be a gap of about 1 inch from the end of one motor to the beginning of the next. I plan on drilling three vent holes 120 degrees apart around the tube, one dead center between each pair of fins.

What diameter should I drill these vent holes?

Should they upper edge of each hole be right at the lower edge of the upper engine, or set back some distance?

I second the 1/8" to 3/16" for the three vent holes. At the top of the gap between the engines.

It may work without the vents, but they should improve your odds of success.

Check out the link under my signature. 8x Ds staged to 4x Ds staged to 4x Ds successfully. Had 2 inch gaps and four 1/4" vent holes at the top of each gap.
 
Very hard to butt-joint tape two motors together that are separated by a gap of 1"...
Au contraire, my friend :) I have two or three rockets that manage this.

What I do is glue an entension onto the booster motor in order to then tape it to the sustainer. I saw off a piece of spent motor case to make the extension.

I have at least two rockets set up for using either gap staging _or_ tape-together with a booster entension. While I've successfully gap staged many times, I still tend to prefer tape-together.

Also, AFAIK, this is legal since the extension is glued to the _outside_ of the motor.

Doug


.
 
I guess my Estes Omega never staged then, nor did all those other folks' with expensive (to us back then) Cinerocs.

The Omega has a short gap between motors and no vent holes.

Yes, you're right, it can indeed work with no vent holes.

I would point out that the Omega also has a (relatively) lot of internal volume in the interstage. The booster's forward CR is set well back, the upper stage MMT aft CR is set some two inches forward, and the whole bay is open and unobstructed all the way to the inside of the main BT (BT 60, with like 1.6 inches inside diam). There is room to push aside the cold air under the upper nozzle. The compartment size will allow this without vent holes. Most of the time, you will successfully get staging with an Omega.

But the point is: gap staging does work better with air vents. The further apart the motors are, the more important it becomes to have those vents.
 
Yes, you're right, it can indeed work with no vent holes.

I would point out that the Omega also has a (relatively) lot of internal volume in the interstage. The booster's forward CR is set well back, the upper stage MMT aft CR is set some two inches forward, and the whole bay is open and unobstructed all the way to the inside of the main BT (BT 60, with like 1.6 inches inside diam). There is room to push aside the cold air under the upper nozzle. The compartment size will allow this without vent holes. Most of the time, you will successfully get staging with an Omega.

But the point is: gap staging does work better with air vents. The further apart the motors are, the more important it becomes to have those vents.

All true, and pretty much a paraphrase of what I said on YORF earlier today, where the same question was asked. However, a couple of posts here seem to indicate that venting is necessary with all gap staging. ;)
 
Might be "less" necessary in some cases than others, but I would still recommend it in general for gap staging.

Don't you want the best chances for everything to function right?
 
But the point is: gap staging does work better with air vents. The further apart the motors are, the more important it becomes to have those vents.

+1
I have never seen a situation where someone used too much venting. I have often seen the opposite. If the pressure pops the stages apart before the sustainer lights, it's gonna lawndart.
 
I have not done much gap staging. My first try had a 1.5" gap and no vent holes. The motor mount in the booster was mounted on four equally spaced pieces of balsa instead of centering rings. This allowed venting out the back of the booster. It worked just fine.

Unfortunately a large RET got the sustainer. The booster was extended to a three inch gap to a new sustainer. I did add four 3/32" vent holes slightly above the sustainer motors lower edge. I don't know if these are really needed. If I have several successful flights with the vent holes, I might try to tape them up and see what happens.

As it was explained to me, you don't need the vent holes in any particular place, rear venting works fine, you just need to vent the tube to keep the booster from separating from the sustainer before it can be ignited.
 
Might be "less" necessary in some cases than others, but I would still recommend it in general for gap staging.

Don't you want the best chances for everything to function right?

Nah....there's no fun in 100% reliability. ;)
 
Nah....there's no fun in 100% reliability. ;)

I don't care so much if I crash an RTF or quick build, but after all the time I'm putting into this upscale scratch built model, failure would be a real drag.

I will add the vent holes. I'm leaning on putting them a bit closer to the bottom of the sustainer engine. I'm also leaning towards making 3 5-6 mm holes.
 
Ok, guess I learned something new. It sometimes does not require vents... Presonally, I've been 0-2 trying the no holes method. Before I even read anything about gap staging, I built a scratch stager with a small gap. It augered in, but I rebuilt it figuring I just got unlucky with the staging. The second flight was a carbon copy of the first.

Since the BAR phase, I've built 2 gap stagers with holes that have done great.
 
I built and painted my Rip Roar and it came out great. At the last minute before putting it the car to go and launch it for the first time I put a vent hole in it after reading here that someone lawndarted there's so I put one 1/4" hole in it and it performed flawlessly. Even though Apogee does not recommend one. If you think it's worth trying it without a vent hole then give it a shot, but I'm going to put a vent in all my gap staged rockets, it's to easy to do and finish it nicely before you paint them especialy.
 
Back
Top