Multistage Engine Mounts?

[JCRL]

I recently purchased an Estes designer box, I'd like to start working on my first scratch build, and I'd like that to be a low power multistager (B or C motors). However, I haven't built a multistage rocket before and I have a question regarding mounting the engines. In the Handbook of Model Rocketry, Stine talks about the ability to lengthen the booster stage by up to 12" by venting the body tube, rather than taping the motors together. In this configuration, what is the best way to mount the motors in their respective stages? Do I go about it the same way as building a single stage motor mount, or does the mount in the second stage need to be specially constructed?

 

[neil_w]

This would a good question for the "Staging, Airstarts and Clusters" forum. Anyway:

In the case of a gap-staged rocket, the motor mounts can be pretty standard. Just make sure that (a) the burn-through gases from the booster are effectively directed at the nozzle of the sustainer motor, and (b) that duct is vented close to the sustainer motor.

Example of (a): let's say your booster airframe is BT50. Run your BT20 motor mount all the way to the sustainer motor, rather than just making it 2.75" long for the motor.

[paging @BABAR]

 

[Bruiser]

I am by no means an expert but here is what I have found to work.

So your sustainer can be built pretty much stock. One thing you might need to change is to make sure the lower centering ring is recessed about 5/8 of an inch.

The booster can probably be built like the sustainer, just a shorter version. Imagine building a duplicate of the sustainer and chopping it off at 4.5 to 5 inches. At the top you will glue in a 1 inch coupler 1/2 inch into the booster. The other 1/2 inch will slide into the sustainer. The motor mount tube in the booster needs to end about 1 inch or so from where the sustainer engine is. That will help direct the hot gases/particles to the sustainer engine. You might want to put a centering ring near the top of the motor tube also or just move the upper centering ring that is already there up a bit. You can still use a engine hook to hold the sustainer motor in. Just make sure the metal does not block the motor core. You will need to make some (2 or 3) 1/4 inch diameter holes near the top of the booster. That will allow the air that is in the gap to be pushed out when the booster engine ignites the sustainer engine. If the air can not escape it can prevent the hot gases/particles from reaching the sustainer engine.

You might want to download OpenRocket and learn to use that. It's pretty simple to get the basics down and it will tell you if your rocket design will fly. You can also find files of proven designs that you can study to help you figure out what does work. There is a whole forum for plans with some excellent threads with OR files.

The build of my Tempest is in the staging forum. I think I put the OR file in the thread. There's a lot of pictures there that might help you.

BTW, Babar is the Grand Puba of staging so maybe he'll chime in here too

Hope this helps you,
-Bob

 

[JCRL]

Thank you for your help Bob, this is exactly the information I was searching for! I saw your Tempest early yesterday after Neil pointed out the Staging forum; it's a really cool build. I particularly like the fin designs you went with.

 

[Bruiser]

Glad to be of help and thanks for the compliment on the Tempest. I have gotten the fin repaired: it only needs a little touch up if I dare. Sometimes things are better left alone

Oh, and I just realized I forgot to mention that if you make the booster short and keep it light you can let it tumble recover. If you make your booster long you will need some type of recovery system for it as it will probably lawn dart in without one. That can get complicated...

-Bob

 

[Rex R]

one might also want to read the chapter on 'multi=staging' in the 'Handbook Of Model Rockets by G.H. Stine' .

 

[Ez2cDave]

I would be remiss, if I failed to mention the CENTURI "Pass-Port" Staging Method . . .

Dave F.

 

[BABAR]

BTW, Babar is the Grand Puba of staging so maybe he'll chime in here too

-Bob

Wow, THANKS!

JCRL, jumping to multistage for your first scratch build is quite a leap (for me I built 10 or more single stages before I tried it) but we are here to help.

My ONLY experience with staging is black powder to black powder NON-ELECTRONIC staging. It is a great technique, and I have seen it work successfully up to over 50 inches of gap.



I recommend you start with minimum diameter staging. I like 18 mm engine to 18 mm engine, but 13 to 13 is fine. 24 to 24 is a little bit more than I think you want to chew at this stage.

Let's say 18 to 18, which will be BT-20 to BT-20.
Once you have decided not to tape your motors but have some separation, you are into "gap staging."

Black powder booster motors (0 delay motors, RED ink on Estes Engines, do NOT use anything but a 0 delay motor for black powder staging) have a nozzle, propellant, and nothing else. No delay charge, no clay cap at the front. Propellant burns from back (nozzle) to front. When it burns all the way to the front, there is no cap, so the flame instead of going out the nozzle goes forward toward the nose.

Research has shown (I can't find the article, but the "particle theory" that burning propellant particles are heaved up the nozzle of the sustainer has been debunked) that it is the light/heat from the flame that "shines" through the sustainer nozzle and lights the motor. This kind of makes sense, because the nozzle is basically a "cup" shape, and there is no way to get "flow" of particles INTO the nozzle.

The first challenge is that when the booster motor blows forward, it creates a pressure wave. Probably NOT as high a wave as a standard engine ejection charge, but still has some force. The trick is to allow these hot gases to travel forward to your sustainer nozzle, literally "light" the powder at the apex of the nozzle, WITHOUT prematurely separating the booster and sustainer BEFORE the sustainer motor ignites.

I use a standard paper hole punch, and punch at least two and no more than three vent holes in the forward end of the booster tube. I go down as far as the punch will allow, which is about 1.5 centimeters. Whether two or three, holes are equidistant so the "venting" will not "push" the rocket to one side or the other.

I put an expended motor casing in the forward end of the booster until I JUST see the cardboard start to show up at the holes (so about 1.5 cm.) I mark the casing where it sticks out from the tube. This mark tells me how far up to put the engine block in the SUSTAINER. I use the casing to place the engine block in the sustainer, now when I load a motor in the sustainer it sticks out the back about 1.5 cm, EXACTLY where I want it so the motor is just forward of (and not blocking) the vent holes.

The sustainer motor is friction fit. You can set up the booster how ever you want (most of my rockets are friction fit, or don't use motor mounts at all, but that's another thread)

The other PROBLEM with gap staging is the length of the booster. Non-gap stage rockets have very SHORT boosters (basically about the length of the motor!). With the big fins, they tumble recover fairly well (better on grass than pavement, but you deal with what you have.) IF the booster is much longer, it tends to be stable AFTER separation. This is a BAD thing because then the booster orients nose down and comes in ballistic, aka lawn dart or core sample (or accordion, if you have pavement to land on.)

So to start with, I would go with a short gap, maybe two or three inches. If you want to go longer than that, you have come up with a way of putting a recovery device on the booster. It is do-able (lots of ways to do it) but it is tricky.

After you try a minimum diameter stager, if you want to go non-minimum diameter, PM me maybe I will do a thread on that.

Hope you get two straight trails!

 

[JCRL]

My original idea was to attempt to parallel stage a rocket, but I decided that was too challenging and to start with series staging XD It seems like assembly of the rocket won't be much harder than a single stage, but getting the stability and CG/CP right will be the challenge.

I'm planning on joining my local NAR chapter this week. With enough support, I believe that I can pull off this build.

 

[BABAR]

My original idea was to attempt to parallel stage a rocket, but I decided that was too challenging and to start with series staging XD It seems like assembly of the rocket won't be much harder than a single stage, but getting the stability and CG/CP right will be the challenge.

I'm planning on joining my local NAR chapter this week. With enough support, I believe that I can pull off this build.

Resist the urge to put a high thrust motor in the sustainer for your first flights. For 18 mm, go with an A8-5 (if you can find them, otherwise an A8-3.) The A motors have a big nozzle and are more reliable for staging ignition, and they have enough thrust to prove your flight worked with less chance of a long walk or lost rocket.

Streamer recovery on the sustainer is also good.

Resist the urge to launch with winds over 10 mph.

 

[neil_w]

I use a standard paper hole punch, and punch at least two and no more than three vent holes in the forward end of the booster tube. I go down as far as the punch will allow, which is about 1.5 centimeters. Whether two or three, holes are equidistant so the "venting" will not "push" the rocket to one side or the other.

I put an expended motor casing in the forward end of the booster until I JUST see the cardboard start to show up at the holes (so about 1.5 cm.) I mark the casing where it sticks out from the tube. This mark tells me how far up to put the engine block in the SUSTAINER. I use the casing to place the engine block in the sustainer, now when I load a motor in the sustainer it sticks out the back about 1.5 cm, EXACTLY where I want it so the motor is just forward of (and not blocking) the vent holes.

That's interesting. Why so far down? I normally have my motors sticking out 1/4" - 3/8" from the back of the motor mount. 1.5 cm is a pretty long way by comparison. Any particular reason?

 

[JCRL]

Thanks for sharing your knowledge, Babar!

 

[BABAR]

That's interesting. Why so far down? I normally have my motors sticking out 1/4" - 3/8" from the back of the motor mount. 1.5 cm is a pretty long way by comparison. Any particular reason?

Actually has less to do with the venting as it does for the “nesting” of the sustainer motor.
The motor itself is actually the “coupler” that connects the booster and sustainer when you do minimum diameter. So 1.5 cm gives me a comfortable overlap.

You may think that having the motor stick out so far would make the sustainer less stable. Technically true, but the flight dynamics are dramatically different. For single stage, the motor has to overcome the inertia and get the rocket up enough speed (airflow) for fins to be effective before the rocket leaves the rod or rail. I think most rockets are still accelerating a bit even after leaving the rod.

For SUSTAINER, the entire rocket (booster AND sustainer) is already moving and presumably stable at separation. So unless you want to fly the sustainer as a single stage, you can get away with smaller fins or CG shifts with the motor back a bit more, because the fins are going to have great air flow IMMEDIATELY at separation.

There are other ways of coupling where you could have less motor protrusion, but to me this is the best KISS principle coupling technique.

A major difference between your builds and mine is also the “style” factor. Recall your nozzle dilemma. On a hypercool Plasma Dart having a motor stick out the back detracts from the classy look of the rocket. Don’t get me wrong, it seems like your rockets FLY great too.

On my rockets I don’t worry much about how it looks, (heck most of my rockets aren’t even painted!) I am trying different flight modes (staging, clustering, both, heli, glider, horizontal spin, air brake, CATO recovery.). On Many of my rockets, I LOVE having the motor stick way out the back when I have chute or streamer recovery. Because then the first thing that hits the dirt or pavement is the rocket motor (it is also why most of my rockets have forward swept fins.). So damage to my rocket body and fins is minimal when my throw away single use low power motor casing takes the brunt of the impact. And in fact, regarding style points, for the sustainer, for display purposes and even on the pad through launch, you never even SEE the sustainer motor, you can’t even see it in flight. only time it is visible is when you pick it up off the ground, so to me doesn’t matter.

 

[BABAR]

Great video on gap staging that debunks “particle theory”

 

[neil_w]

All good points. I forgot that we were talking a minimum diameter stager, where the motor is the only coupler. In non-minimum diameter situations the rear centering ring on the sustainer can be recessed a bit, reducing the motor extension. But you're also correct that you don't really even see the sustainer motor, at least when you're flying two-staged.

Also thanks for that video, I don't think I realized the particle theory had been debunked!

 

[JCRL]

Yes, that's fascinating, I have the most current edition of Stine's book and it still explains particle theory.

Also, is there an altimeter small enough to fit in a minimum diameter body tube?

 

[DaveW6DPS]

Also, is there an altimeter small enough to fit in a minimum diameter body tube?

Look at the Perfectflite Firefly.

I also suggest building a few multi-stage kits before designing your own. A little experience helps a lot.

 

[JCRL]

Thanks everyone for your sharing your expertise and advice. Based on everything said here, I'm going to put off the multistage build for a bit and focus on getting more single stage builds under my belt. Once I'm ready to move on, I'll be trying to gap stage a rocket based on Bruiser and Babar's design suggestions above. Thanks again!

 

[BABAR]

Thanks everyone for your sharing your expertise and advice. Based on everything said here, I'm going to put off the multistage build for a bit and focus on getting more single stage builds under my belt. Once I'm ready to move on, I'll be trying to gap stage a rocket based on Bruiser and Babar's design suggestions above. Thanks again!

Good decision.
Also, on single stage rockets, for minimum diameter rockets, if you let the motor stick out about 1 to 1.5 cm, they will work as sustainers when you DO decide to go up to two stage rockets.