Question on 2 stage - Rocket Motors butting each other?

[pcride]

I built a custom rocket, the first stage will be a D12-0 and second stage is E9. The rocket overall is about 34 inches and should go to the moon!!

My question is:
How close do you think the rocket motors have to be in order for the first stage to ignite the second?

Currently I have a thrust ring in between which is about 1/4 in thick. Do you think I will have a problem igniting the second stage with this gap?

I have heard you should tape them together or at least butt them up against. IN looking at the gap I couldn't imaging that a 1/4 gap would interfere with a mad fury of flames igniting the second stage?

I am launching tomorrow so any quick responses would be great

[Peter Olivola]

As described, your design will most likely fail to ignite the second stage.

There are two designs that work reliably with multi-stage black powder motors. The first is used by Estes in their designs and consists of taping the two engines together using a thrust ring on the aft end of the first stage to ensure the motor stays with the airframe.

The second is gap staging where some distance, generally less than 12 inches for 24MM motors, between stages includes venting in the first stage airframe to relieve burn through pressure so the second stage has a chance to ignite. The much greater pressure of second stage ignition separates the stages.

[bradycros]

I built a custom rocket, the first stage will be a D12-0 and second stage is E9. The rocket overall is about 34 inches and should go to the moon!!

My question is:
How close do you think the rocket motors have to be in order for the first stage to ignite the second?

Currently I have a thrust ring in between which is about 1/4 in thick. Do you think I will have a problem igniting the second stage with this gap?

I have heard you should tape them together or at least butt them up against. IN looking at the gap I couldn't imaging that a 1/4 gap would interfere with a mad fury of flames igniting the second stage?

I am launching tomorrow so any quick responses would be great

Read these:

"How Multi Stage Rockets Work, Parts I & II".

http://www.apogeerockets.com/educati...wsletter98.pdf

http://www.apogeerockets.com/educati...wsletter99.pdf

[[POW]Eagle159]

I say more or less yes, 1/4'' is nothing of a gap. Vent holes is a good idea though.

I've made 2 scratch build 2 stage rockets, one 9'' gap one 3'' gap both work perfect.

[pcride]

Can you expand on the vents? I will check out those links. Also thanks for the fast response.

I just attached my openrocket details on this post.
http://www.rocketryforum.com/showthread.php?t=26054

[pcride]

This was really my first attempt on a scratch rocket made from 2 kits, so I probably didn't do it the best way.. however I remember on a previous rocket kit I had, there was a thrust ring on the first stage separating the 2, it launched great. But that was a C motor.








What do you think? Any tips would be awesome

[pcride]

"The vent hole should be about 6.35 mm (0.25 inches) in
diameter, and even smaller if you use more than one hole.
Place the hole as close to the bottom of the upper stage as
possible. This will allow the hot gases to push all the cool air
out of the inter-stage tube."

From that PDF..

Does this mean I need to drill some holes in the first stage up through the ring motor mounts to the second stage?

[pcride]

I would think my design would work, its no different than the direct staging in the PDF.

Thoughts?

[dpower]

Does this mean I need to drill some holes in the first stage up through the ring motor mounts to the second stage?

No, simply drill the vent hole through the side, into the cavity between the two motors, to relieve pressure before the stages separate.

[pcride]

great! I'll do that. I'll post the results after launch!

[MarkII]

"The vent hole should be about 6.35 mm (0.25 inches) in
diameter, and even smaller if you use more than one hole.
Place the hole as close to the bottom of the upper stage as
possible. This will allow the hot gases to push all the cool air
out of the inter-stage tube."

From that PDF..

Does this mean I need to drill some holes in the first stage up through the ring motor mounts to the second stage?

That's the way I usually do it, but it's not the only way that works. As dpower said, a port in the side of the rocket also works and is quite commonly done.

[JStarStar]

Don't you always want to position staging vent ports in symmetrical patterns (i.e., 12-6, 12-4-8, 12-3-6-9 o'clock) because if you have a staging vent port on one side of the rocket without equivalent port(s) on the other side, the staging venting will act as a "attitude thruster" and knock the rocket off vertical just when the upper stage lights?

So it would seem to me you wouldn't want just one vent port.

[MarkII]

Don't you always want to position staging vent ports in symmetrical patterns (i.e., 12-6, 12-4-8, 12-3-6-9 o'clock) because if you have a staging vent port on one side of the rocket without equivalent port(s) on the other side, the staging venting will act as a "attitude thruster" and knock the rocket off vertical just when the upper stage lights?

So it would seem to me you wouldn't want just one vent port.

I don't think that the thrust from the venting would be strong enough to overcome the corrective action of the fins. Remember that the rocket is traveling quite fast when this happens, and the staging only takes at most a second or so. Centuri's early gap-staging kits that had staging couplers mounted on the outside of the rocket had only one vent port, I believe.

[Marc_G]

I'm looking forward to hearing a launch report, or even better, video...

Hope you have a BIG field!

[pcride]

We are launching tomorrow and will be going to the desert. I was hoping to get the key chain camera I ordered to film it, I'll bring a camera and try to get some decent video.

I hope all goes well!

[pcride]

Well... didn't go so hot. The CG was not correct. Probably needed more weight in the Nose Cone.

Back to the drawing board!




http://www.youtube.com/watch?v=bR-sDEb566E

[Marc_G]

Epic skywriting fail!

Looks like it got off the rod OK... did the instability start upon staging?

[pcride]

It appeared to take off great before the staging, the second stage ignited perfectly but unfortunately it was already on the whimsy path of destruction.

[gdjsky01]

Your open rocket PDF showed only a Stability Margin of: 0.79 calibers. That's fine for short fat rockets, but really way to close for comfort for a 39mm wide body tube. Too many variables for you to trust OpenRocket that much (or RockSim for that matter). I think it nails the CP close, but the CG is for you to verify.

To me all that was needed to have it go unstable was for the shock cord and chute to slide down under thrust and who knows?

Also the PDF does not show the stability of sustainer alone.

[mwtoelle]

Definitely needed some noseweight on this one. I realize that you are kitbashing two different kits, but the fin area of each stage should increase as you go lower in the stack. IOW, the booster fins should have about 1.5 time the area of the sustainer fins. More info about designing multistage rockets can be found at http://www2.estesrockets.com/pdf/284...tion_TR-TN.pdf . Most of the reports in this date back to the 1960s, but are still applicable today. Better luck next time.

[Launch Lug]

Thanks for the link. Perfect timing as I was looking for info on how multi-stage rockets are built.

[pcride]

Hi, I am starting a new rocket and will be doing a dual stage like this one. The problem I had (as you can see from the video in this post) it spirled out of control, I think because the CG was too far back and it was too long and not enough weight in the nose.

I will be adding about another 6-8 inches to my new rocket and designing it in open rocket, where do I want the CG on the rocket? I would guess more towards the center.

[tbzep]

I don't know where you ended up cutting your vent holes on the original, but it reads like you used side vents. This time cut the holes through the booster's centering rings before assembly so that the pressure is vented out the back. That way nobody sees the holes and there is no side thrust. It's easy to do and the finished rocket looks clean, the best of both worlds.

CG needs to be toward the nose cone compared to the CP. A couple body tube diameters is usually plenty, so if your rocket uses BT-60 tubes, it would be around 3.5" ahead of the CP, give or take a little. If you move the CG way toward the nose, it tends to aggravate weathercocking issues.

[pcride]

Good info and to know. Thanks I'll keep that in mind.

[JStarStar]

I don't think that the thrust from the venting would be strong enough to overcome the corrective action of the fins. Remember that the rocket is traveling quite fast when this happens, and the staging only takes at most a second or so. Centuri's early gap-staging kits that had staging couplers mounted on the outside of the rocket had only one vent port, I believe.

Well, GHS in the Handbook cautioned that vent holes should be "equalized" to prevent the "thruster effect."

The sudden puff of gas from the vent port, at least in theory, might be enough to get the tail-end of the upper stage swinging in a lateral direction, just as the upper stage lights, which I would guess could be a recipe for bad things happening after that.

It would probably take a lengthy series of tests to determine if this effect really exists (or to what extent), whether the "thruster effect" is really enough to knock a multistage rocket off course. It obviously can't be a major or consistent effect or everyone would have observed the Centuri single-port rockets always flying way off course.

But, I do know on the gap-staged rockets I have flown, I have always used "equalized" vent-holes and I have never had one take off cockeyed.


Although I do recall way back in my younger rocketry life, I DID have a lower stage that I butted the motors together with several layers of heavy plastic tape, refuse to separate on upper-stage ignition, it hung on (kind of flish-tailing) for about half the upper-stage burn before finally kicking off. The whole kaboodle veered off at about a 60-degree angle before the booster stage kicked off, then the upper stage straightened out and flew about a quarter-mile downrange.

The booster landed 100 feet or so from the pad, with a pretty thick charcoal-smoked effect on its front end.



Which just goes to show that when you use the direct-butting staging method, you want to use one turn (no more) of cellophane or mylar tape (not thick plastic) to butt the motors together. You want that sucker to light and then kick the heck off of there.

[MarkII]

Well, GHS in the Handbook cautioned that vent holes should be "equalized" to prevent the "thruster effect."

The sudden puff of gas from the vent port, at least in theory, might be enough to get the tail-end of the upper stage swinging in a lateral direction, just as the upper stage lights, which I would guess could be a recipe for bad things happening after that.

It would probably take a lengthy series of tests to determine if this effect really exists (or to what extent), whether the "thruster effect" is really enough to knock a multistage rocket off course. It obviously can't be a major or consistent effect or everyone would have observed the Centuri single-port rockets always flying way off course.

But, I do know on the gap-staged rockets I have flown, I have always used "equalized" vent-holes and I have never had one take off cockeyed.


Although I do recall way back in my younger rocketry life, I DID have a lower stage that I butted the motors together with several layers of heavy plastic tape, refuse to separate on upper-stage ignition, it hung on (kind of flish-tailing) for about half the upper-stage burn before finally kicking off. The whole kaboodle veered off at about a 60-degree angle before the booster stage kicked off, then the upper stage straightened out and flew about a quarter-mile downrange.

The booster landed 100 feet or so from the pad, with a pretty thick charcoal-smoked effect on its front end.



Which just goes to show that when you use the direct-butting staging method, you want to use one turn (no more) of cellophane or mylar tape (not thick plastic) to butt the motors together. You want that sucker to light and then kick the heck off of there.

You also want to make sure that the booster motor is securely held in the booster stage, so that they separate together. It's never happened to me, but I have seen several other folks report having the booster motor kick out of the booster stage while the stage itself remained attached. Having the sustainer motor thrusting down through it is not real good for the booster stage, I am told.

[JStarStar]

You also want to make sure that the booster motor is securely held in the booster stage, so that they separate together. It's never happened to me, but I have seen several other folks report having the booster motor kick out of the booster stage while the stage itself remained attached. Having the sustainer motor thrusting down through it is not real good for the booster stage, I am told.

Yeah, then you get a demonstration of the infamous Krushnic Effect (in which a motor firing deeply recessed inside a body tube has its effective thrust reduced to about zero), plus the booster stage itself gets crispy-fried from the inside.

[pcride]

Well just about a year later I am building another one using the same dual stage design. This time I will balance the rocket. Any suggestions as I get started?

[pcride]

Also, here is the link where my previous rocket schematics are.

http://www.rocketryforum.com/showthr...d-Calculations

I think the failure of this one was the center of grav and pressure was too far back, not enough weight in the nose.

[Rocketbuilder]

Yup, looks like more nose weight would help a lot. Just not too much- you don't want to exceed the liftoff weight of the booster motor, which is easier to do on multi-stagers due to the weight of the upper stage motor(s). Also, you might want to increase your fin are a good bit, to reduce the amount of dead weight in the nose.
Have you seen the two-stager E rocket build threads that have become a trend lately? In case you haven't heard, Estes recently came out with a new booster motor, the E12. Basically, it has a longer burn time and greater total impulse like the E9, but it has the same "kick" off the pad as a D12.
Good luck! I'm glad you came back to it instead of giving up after your first attempt.