Ejection charge pressure

[Drt_trkr]

It looks like to me that there are a lot of experienced Scratch Builders. So me being a newbie to scratch building I come up with questions all the time.

That being said here we go....

I have built a 4' tall 3" diameter rocket that weighs 2.5lbs and is powered by four D12 engines. I came up with the idea of building a funnel on the inside of the 3" tube that goes over the ejection side of all four engines. I have it funneled down to a 1' tube that runs up inside to just below my wadding and parachute. The idea was to create more ejection charge pressure too ensure it will push out the recovery system. I had thought that with such a large diameter and 3' of tube to travel up that the ejection charge would be too light and my rocket would become a lawn dart.

Was this a smart idea? I have concern that I am right at the edge of not having a safe flight so I want to shave some weight off of the rocket. Removing the inner tube would help tremendously.

Any advice????? Thank you all for your help...

 

[dedleytedley]

It looks like to me that there are a lot of experienced Scratch Builders. So me being a newbie to scratch building I come up with questions all the time.

That being said here we go....

I have built a 4' tall 3" diameter rocket that weighs 2.5lbs and is powered by four D12 engines. I came up with the idea of building a funnel on the inside of the 3" tube that goes over the ejection side of all four engines. I have it funneled down to a 1' tube that runs up inside to just below my wadding and parachute. The idea was to create more ejection charge pressure too ensure it will push out the recovery system. I had thought that with such a large diameter and 3' of tube to travel up that the ejection charge would be too light and my rocket would become a lawn dart.

Was this a smart idea? I have concern that I am right at the edge of not having a safe flight so I want to shave some weight off of the rocket. Removing the inner tube would help tremendously.

Any advice????? Thank you all for your help...

Four D-12 should provide plenty of pressure to eject the nosecone and parachute. I'd suggest omitting the cone and stuffer tube and adding a ring near the top to form a parachute compartment. I'm curious as to why your rocket is so heavy.. Did you use a heavy wall tube or heavy nosecone? If your fins are plywood you could carefully drill holes in them and skin them with paper or vinyl to lose a little weight. What size chute are you planning to use? Are your launch lugs at least 1/4 inch? A longer launch rod will allow the rocket more time to get to a safe speed for stability. My Nike Ajax 2-stage was unstable on it's first flight due to wind and a short launchrod. The second flight was good with a six foot rod and no wind.
CG location is critical for stability on slow rockets. Too far to the rear and you get a dangerous flight, too far to the front and it will weathercock badly. How are you figuring CP? Rocksim or cardboard cutout? Ted

 

[Drt_trkr]

To be honest with you I haven't figured my CP. I am not able to purchase Rocksim at this time and have not done the "cardboard Cutout" method.

I do have a 6' X 1/4" launch rod for this rocket. Also the fins are made out of pressed cardboard. (This cardboard came out of old 3 ring binders that you cut the plastic off of.)

Here is an unfinished picture if the rocket:

 

[The EGE]

Four D12s equal one F41 (avg thrust = 10.7 N each) with 67 Ns of total impulse. That's an average thrust of 9.2 pounds, for a thrust-to-weight ratio of 3.7.

That's a very marginal thrust-to-weight ratio; wind over 5 mph or one motor failing to ignite could very easily cause your rocket to prang.

You'll definitely want to reduce weight, and 4 D12 ejection charges will have plenty of oomph even without the stuffer tube. We don't call them 'shotgun' charges for nothing

 

[Drt_trkr]

Four D12s equal one F41 (avg thrust = 10.7 N each) with 67 Ns of total impulse. That's an average thrust of 9.2 pounds, for a thrust-to-weight ratio of 3.7.

That's a very marginal thrust-to-weight ratio; wind over 5 mph or one motor failing to ignite could very easily cause your rocket to prang.

You'll definitely want to reduce weight, and 4 D12 ejection charges will have plenty of oomph even without the stuffer tube. We don't call them 'shotgun' charges for nothing

Thank you for this Info.

What is the "ideal" rocket weight that i should try to get too?

 

[rcktnut]

If you give me some dimensions for the fins, body tube, nose cone, I'll Sim it for you. Will be able to get the CP. What is the nose cone made of ??

 

[quickburst]

It looks like to me that there are a lot of experienced Scratch Builders. So me being a newbie to scratch building I come up with questions all the time.

That being said here we go....

I have built a 4' tall 3" diameter rocket that weighs 2.5lbs and is powered by four D12 engines. I came up with the idea of building a funnel on the inside of the 3" tube that goes over the ejection side of all four engines. I have it funneled down to a 1' tube that runs up inside to just below my wadding and parachute. The idea was to create more ejection charge pressure too ensure it will push out the recovery system. I had thought that with such a large diameter and 3' of tube to travel up that the ejection charge would be too light and my rocket would become a lawn dart.

Was this a smart idea? I have concern that I am right at the edge of not having
a safe flight so I want to shave some weight off of the rocket. Removing the inner tube would help tremendously.

Any advice????? Thank you all for your help...

No RocSim ???????

You don't need it.

Use this:
https://v-serv.com/vcp/

I have been using it for years, it's free and it works very well. Use the cash you save to buy more motors.

 

[dedleytedley]

I like the look of your rocket. The cardboard you used is fairly heavy for it's strength and the fins may be prone to flutter. That type of cardboard does tend to delaminate as well. I think removing some weight from the fins is your only option to lighten it. Drilling holes or removing pieces from the fins and then gluing paper skins on with CA will save some weight. The thin CA should soak into the cardboard stiffening the fins. Less mass near the outer edge of the fins will reduce flutter as will the skinning. Reducing weight from the rear gives you the most advantage for stability and reduces the need for noseweight, something you can't afford with your thrust limitations. Ted

 

[MarkII]

This simulation program has been generating quite a bit of interest and positive buzz in the past few months. And best of all, it's open source and free. One of the neat things about rocket design and simulation programs is that they can help you learn more about rocketry as you use them to create designs.

OpenRocket.

MarkII

 

[rcktnut]

Drt_trkr,

I have it back!!!
Here's a little info for you. I couldn't attach anything in PM. It flys good, not high but good!!! Notice 5 ft. launch rod needed. 36 in. chute used, weighed in at 42 oz. with motors. MMT is 3 in. long -2 centering rings. PM or what ever if you need more info.

 

[ScrapDaddy]

This simulation program has been generating quite a bit of interest and positive buzz in the past few months. And best of all, it's open source and free. One of the neat things about rocket design and simulation programs is that they can help you learn more about rocketry as you use them to create designs.

OpenRocket.

MarkII

A buzz? Once my rocksim demo ends (tommorrow) I'm switching back to this thing when I jump back and forth between These softwares I can bealy tell the difference. And 2.5 pounds on a quad of D12's? It sound shaky to me and I have been known to take risks. I would switch to a single 29mm mount and fly it on somthing like a G64 or a G80T

 

[dave carver]

You can also Google VCP and WRASP, both free or share ware.

VCP(virtual center of pressure) has good tools to work with like fin pattern and fin marking guides. It's for designing rockets and WRASP(Windows Altitude Simulation Program) for altitude prediction.

Both programs give you the basic tools to work with for free. VCP is flexable enough I've modeled Mercury Redstone rockets with it and it was written by a member of this forum.