upside down finless rocket

[Andyd103]

i have built an upside down fin less rocket that will run on 2 canted c's (in such a way that it will spin rocket), these are mounted at the very top of the rocket and my nose cone is at the bottom attached to the chute, on the top is a rounded thing to block that end.

Here is how i think it will work, i built the motor mount so that when the ejection charge pops it will allow the pressure to bounce off the hemispherical top and flow through the mount which is made from 2 1/4 inch sheets of play wood to the diameter of a bt-40 , and baffled, this should pop the bottom and allow the parachute to pop out. I have never seen anything like this but i will post flying results with aftermath pics when i have the time to fly it.

IF ANYONE has anything i should be aware of please let me know before i make a fool out of myself when i fly this at my local club

[Zeus-cat]

It sounds interesting. I'm sure there are people here that will have a better undertanding of how this SHOULD work; assuming it works at all. I am not sure I would launnch an untested rocket with no idea of what it will do at a club launch. This sounds more like test it with no one else around type of thing. Just my two cents.

[Andyd103]

It sounds interesting. I'm sure theree are people here that will have a better undertanding of how this SHOULD work; assuming it works at all. I am not sure I would launnch an untested rocket with no idea of what it will do at a club launch. This sounds more like test it with no one else around type of thing. Just my two cents.

well thats probably a good idea, the rocket feels stable

[NjCo]

Maybe run it by your club's RSO prior to launch day. It certainly seems like it'll be a rocket that might raise a few eyebrows at the RSO table on launch day.

[Micromeister]

Andy:
Yours is an experimental model design. Of that we can all agree.
Because of this your model Should NOT be flown at a Club launch open to the public, or where there will or might be other people.
As an experimental model this project should be test flown in as complete isolation as you can make it. As far away from people and property as humanly possible. Your model seems Like a great candidate for experimentation, but NOT with others around and a first class example of the type model rocket that should only be flown in complete isolation.

Here are a few things to think about.

* You have not mentioned where the launch lugs are located on this model or if they are centered between the motors allowing the model to freely rotate around this axis as the model begins its flight, building rotional speed as it raises. or you've mounted them offset on the outer body which would cause binding on the launch rod retarding the gaining of spin and forward motion.

* How long a launch rod is required to reach safe RMP & air speed?

* What is the minimum RPM your model must attain to achive spin stabilization?

* Have you considered starting with a less energetic set of motors. Say 13mm A10's? or even a micro version to test the fesibility of the design?

* "Feels Stable" What exactly does that mean? have you run any simulations on the design? done an aerodynamic calculations on the various states that this model must preceed through to obtain a safe and stable flight?

To be perfectly honest, I seem to recall forward mounted motors being examined rather extensively in the early 1970's. Perhaps a little research into the old Model Rocketeer magazine might give you some helps rather then taking the approach..."lets stick some motors in it and see if it flys." That is not a very scientific approach.
This could be a worthy experiment; but MUST NOT be done in the presents of a crowd. YOU and you alone or with as minimal a launch/recovery crew as you can muster should view these first "TEST FLIGHTS". Only after you have a WORKING, PROVEN Flying prototype do you take it to a club or public event. SAFETY FIRST Always.

[Andyd103]

if rocksim could do these calculations i would use it but for now I've done my pencil and paper work, there is no reason this rocket should not fly stable, rpm's are a different story, i have no idea how to calculate those but im guess it should be fast enough, i have a LR going through the center of the rocket, i drilled a hole in the motor mount the bottom cone and the top cone(it has a rubber valve on the top 1) so that when the rod has left that area it will reseal and not pop the bottom off, but i plan to do a test flight next Friday will post pictures

[ScrapDaddy]

Recomend this article on Emrr :) http://www.rocketreviews.com/reviews/all/scratch_no_fins.shtml

[Andyd103]

Recomend this article on Emrr :) http://www.rocketreviews.com/reviews/all/scratch_no_fins.shtml

thanks provided some insight but i want to try and keep the engines in the top to keep it unique and a learning experience

[Micromeister]

thanks provided some insight but i want to try and keep the engines in the top to keep it unique and a learning experience

Andy:
Not all that new or unique, well the USD recovery phase may be. Unfortunately I can't get to my back issues of the Rocketeer from the 70's currently the Basement rocket lab is torn-up for some rewiring, I'm pretty sure the articles i'm thinking of were in 1970-72 era, with or without them your experiment it should be a neat individual learning experience. Please give ourself plenty of room away from cars and windows.

Have you considered the action inertia may play on your model? I still think you'd be much better off starting with a smaller motor combo.

[Andyd103]

Andy:
Not all that new or unique, well the USD recovery phase may be. Unfortunately I can't get to my back issues of the Rocketeer from the 70's currently the Basement rocket lab is torn-up for some rewiring, I'm pretty sure the articles i'm thinking of were in 1970-72 era, with or without them your experiment it should be a neat individual learning experience. Please give ourself plenty of room away from cars and windows.

Have you considered the action inertia may play on your model? I still think you'd be much better off starting with a smaller motor combo.

like what motors should i start with?, the inertia shouldn't be a problem the rocket empty will be pretty light and the way i have the tubes set-up it should provide enough air resistance to slow it down on the 5 second ejection charge

[Micromeister]

like what motors should i start with?, the inertia shouldn't be a problem the rocket empty will be pretty light and the way i have the tubes set-up it should provide enough air resistance to slow it down on the 5 second ejection charge

I was thinking perhaps a pair of A10-3T 13mm motors would give you a very high initial thurst, get things moving and maybe spin-up, without the longer burn that you may not want on this first flight.

Actually I was thinking about the matter at rest phase of inertia. whats keeping your NC and recovery system in place? Please don't say friction.

[stickershock23]

This idea WILL WORK. but YOU NEED TO CHANGE IT AROUND A LITTLE BIT.

I have done this several times. I have a witches broom I fly at halloween.

3 motors at the top canted inward is better than 2. because it it making it stable along more than one axis.

You need weight at the BOTTOM of the rocket. down where the motors should be on a regular rocket.

as long as you do that it will fly fine.

One thing. it is a heads up flight. if a motor fails to fire it will make a nice arc to one side.

Make sure you put it out on a far pad, and let it rip.

I have a little gripe. then I am done. if you are an RSO at a launch. letting something fly that is untested is not a big NO NO. Use your brain think about what it should do and what it might do and make a decision accordingly. How else do you LEARN? a sim, is just, it will NOT tell you for sure what will happen. sometimes trying it is the best way to go. and if if fails. learn from it.

JUST DO IT SAFE. put it out far enough, call a heads up, get EVERYONE ON THEIR FEET you are good to go.

This does not mean let EVERYTHING fly, there are times an RSO MUST say no. but in this case as long as it is built fairly decent. built SQUARE, and it is not a missile (metal, sharp, way over powered, made of something dangerous) let it fly.

I know some will disagree with that. but that is my thoughts. I won't hate you if you think different. don't hate me because I do.

[Andyd103]

I was thinking perhaps a pair of A10-3T 13mm motors would give you a very high initial thurst, get things moving and maybe spin-up, without the longer burn that you may not want on this first flight.

Actually I was thinking about the matter at rest phase of inertia. whats keeping your NC and recovery system in place? Please don't say friction.

why is friction a bad thing? there is quite a big of force required to remove the nosecone, it has 1.5 inches of bite into the rocket, suggest any other methods?

[NjCo]

why is friction a bad thing? there is quite a big of force required to remove the nosecone, it has 1.5 inches of bite into the rocket, suggest any other methods?

You'll leave that nose/tail? cone sitting on the launch pad.

[cjl]

3 motors at the top canted inward is better than 2. because it it making it stable along more than one axis.

This shouldn't be the case. 2 vs 3 motors should not impact stability at all, so long as they are spaced evenly and balanced.

[Andyd103]

You'll leave that nose/tail? cone sitting on the launch pad.

well why is that? explanations would help because i am experimenting and the point of an experiment is to formulate a conclusion, which i cannot do without data

[stickershock23]

This shouldn't be the case. 2 vs 3 motors should not impact stability at all, so long as they are spaced evenly and balanced.

I am thinking if they were pointed straight down ( but still angled out) as compared to pointed one direction ti cause a spin. If you spin it yes it should be stable. if you don't spin it you are basically building this.

[cjl]

Yes, but even then, 3 vs 2 doesn't make a difference. In either case, the net force vector is the same.

[Micromeister]

You'll leave that nose/tail? cone sitting on the launch pad.

Andy:
Come on think! If your relying on friction to keep your NC, chute and wadding in place your gonna be very unhappy at liftoff. Those parts have mass and that mass is a rest and wants to remain at rest. Making sure they are at the extreme rear will help a bit but your going to need some sort of launcher lashed pull pin that will keep everything together until they all get going. The highter the initial thrust your motors produce and more twist they impart the more likely your Friction fitting NC will be dislodged at or very shortly after ignition.

One more little side note:
While it's nice to seal off the hole for your launch rod, it's not necessary or even all that good idea as the self-sealing rubber stopper or whatever your using is causing friction between it and the launch rod retarding your launch acceleration and spin-up. That small 1/8" hole isn't enough to keep either of the motors ejection charges from working as planned unless you have one HUGE cavity of a core body.

Do you see now why I strongly suggest that your first few flights be done in isolation and with smaller motor combinations? It might also be helpful to vidoe tape the first few seconds of the flight if you have that options for review and possibly slow motion inspection.

[Micromeister]

This idea WILL WORK. but YOU NEED TO CHANGE IT AROUND A LITTLE BIT.

I have done this several times. I have a witches broom I fly at halloween.

3 motors at the top canted inward is better than 2. because it it making it stable along more than one axis.

You need weight at the BOTTOM of the rocket. down where the motors should be on a regular rocket.

as long as you do that it will fly fine.

One thing. it is a heads up flight. if a motor fails to fire it will make a nice arc to one side.

Make sure you put it out on a far pad, and let it rip.

I have a little gripe. then I am done. if you are an RSO at a launch. letting something fly that is untested is not a big NO NO. Use your brain think about what it should do and what it might do and make a decision accordingly. How else do you LEARN? a sim, is just, it will NOT tell you for sure what will happen. sometimes trying it is the best way to go. and if if fails. learn from it.

JUST DO IT SAFE. put it out far enough, call a heads up, get EVERYONE ON THEIR FEET you are good to go.

This does not mean let EVERYTHING fly, there are times an RSO MUST say no. but in this case as long as it is built fairly decent. built SQUARE, and it is not a missile (metal, sharp, way over powered, made of something dangerous) let it fly.

I know some will disagree with that. but that is my thoughts. I won't hate you if you think different. don't hate me because I do.

I totally disagree with your assertion that unproven models should be allowed at public events. By the Safety code we've all sworn to uphold ONLY proven models are to be flown at Public or club events.
Unproven models and ALL expermiental models should be flown IN ISOLATION. NOT for our safety but for the safety of the general public. NO-ONE knows What an unproven model will do just by looking at it period.

All that said why did you use forward motors on your witches broom? My full size Witches broom flys on a single D12 in the rear.

[cjl]

I disagree Micro. Given proper caution and correct motor choice, unproven models can absolutely be flown at public events.

[stickershock23]

I totally disagree with your assertion that unproven models should be allowed at public events. By the Safety code you've sworn to uphold ONLY proven models are to be flown at Public or club events.
Unproven models and ALL expermiental models should be flown IN ISOLATION. NOT for your safety but for the safety of the general public. NO-ONE knows What an unproven model will do just by looking at it period.

All that said why did you use forward motors on your witches broom? My full size Witches broom flys on a single D12 in the rear.

Micro I don't want to argue. I think we just have different points of view.
I will say I wouldn't let every experiment fly. there should be some fact to back up stability. if a rocketeer can show me his thoughts on why it should fly correct, and show he's put some thought into it. and taken some good safety measures.. then let it fly. FROM THE AWAY PADS.. and again call heads up and everyone on their feet.

Personally if I were to tell someone like that NO you cant try it. I fell like I am stopping new thinking and learning. thats where I have a problem with it.

Of course if it does not seem safe or it built out of dangerous things. or the rocketeer can't even explain why he thinks it will be stable.. then the answer would be NO. Of course bottom line is it is up to the RSO on duty.


I guess for Halloween we are drag racing brooms... my has a cluster of 3 18mm's LOL

[Micromeister]

I must ammend my pervious post! as the Model Rocket safety code as been ammended removing the wording about unproven models, replacing it with far less safety minded verbage.

I do not however change my position on unproven designs. As we can not On the field, "Check stability before Flight", nor adaquately "Warning spectators and clearning them away to a safe distance" at public or Club launches.

Chris;
This is EXACTLY how accidents happen. examples like the X-wing, X-15, and on and on. Test flights for every type of flying vehicle in the world are done in Isolation, out in the middle of a dessert somewhere with minimal flight crews and observers. Why? to protect the unknowing by-standers. Our models deserve the same respect. Underestimation and disprespect will be the undoing of our hobby. We must always think SAFETY First. I hate to say it but honestly most RSO's these day are not qualified to make such decisions... myself included.

[Micromeister]

Micro I don't want to argue. I think we just have different points of view.
I will say I wouldn't let every experiment fly. there should be some fact to back up stability. if a rocketeer can show me his thoughts on why it should fly correct, and show he's put some thought into it. and taken some good safety measures.. then let it fly. FROM THE AWAY PADS.. and again call heads up and everyone on their feet.

Personally if I were to tell someone like that NO you cant try it. I fell like I am stopping new thinking and learning. thats where I have a problem with it.

Of course if it does not seem safe or it built out of dangerous things. or the rocketeer can't even explain why he thinks it will be stable.. then the answer would be NO. Of course bottom line is it is up to the RSO on duty.

I guess for Halloween we are drag racing brooms... my has a cluster of 3 18mm's LOL

I don't think its arguing at all:
We are not telling the modeler don't fly this widget. We are saying; "Look the Scientific Method says our experiments be done AWAY from crowds of unsuspecting distracted people.

You know as well as I do that even with PA announced HEAD-UP, Everyone on their feet calls. NOT everyone is paying attention to the flight. I see this all the time at launches all over. True it's mostly the very young and Teenagers but it's also the distracted MOMs dealing with the young kids. or the bunch of vets discussing this important flight detail from an hour ago. People NOT directly involved with a given flight just don't take them all that seriously...until something happens... then they are all " Well why didn't you folks do something".

We are not stopping any thinking or learning.. as a matter of fact we are encouraging the BETTER use of the Scientific method to presue the goal. AND encouraging and further driving home the NEED for SAFETY FIRST thinking. Flying unproven models in isolation can even be set up as a ClUB activity using limited launch crews and range provisions. Thats the problem with common sense.... it just seems it isn't so common anymore.

[Shade]

....dessert ...

??? :eyepop: I'm hungry!!!

[Micromeister]

??? :eyepop: I'm hungry!!!

:roll:
Dang! never said I could spell. I think i'm gonna leave it as is LOL!!!!

[luke strawwalker]

Andy:
Yours is an experimental model design. Of that we can all agree.
Because of this your model Should NOT be flown at a Club launch open to the public, or where there will or might be other people.
As an experimental model this project should be test flown in as complete isolation as you can make it. As far away from people and property as humanly possible. Your model seems Like a great candidate for experimentation, but NOT with others around and a first class example of the type model rocket that should only be flown in complete isolation.

Here are a few things to think about.

* You have not mentioned where the launch lugs are located on this model or if they are centered between the motors allowing the model to freely rotate around this axis as the model begins its flight, building rotional speed as it raises. or you've mounted them offset on the outer body which would cause binding on the launch rod retarding the gaining of spin and forward motion.

* How long a launch rod is required to reach safe RMP & air speed?

* What is the minimum RPM your model must attain to achive spin stabilization?

* Have you considered starting with a less energetic set of motors. Say 13mm A10's? or even a micro version to test the fesibility of the design?

* "Feels Stable" What exactly does that mean? have you run any simulations on the design? done an aerodynamic calculations on the various states that this model must preceed through to obtain a safe and stable flight?

To be perfectly honest, I seem to recall forward mounted motors being examined rather extensively in the early 1970's. Perhaps a little research into the old Model Rocketeer magazine might give you some helps rather then taking the approach..."lets stick some motors in it and see if it flys." That is not a very scientific approach.
This could be a worthy experiment; but MUST NOT be done in the presents of a crowd. YOU and you alone or with as minimal a launch/recovery crew as you can muster should view these first "TEST FLIGHTS". Only after you have a WORKING, PROVEN Flying prototype do you take it to a club or public event. SAFETY FIRST Always.

"Presence" not "presents"... antonyms, -1...
LOL:) J/K... what I get for helping my English-teacher wife grade papers...

Seriously, "copy that" to everything you said.

ESPECIALLY about the 'try it on lower power motors first. 13mm adapters are easy enough to make to fly it on mini motors first, or barring that even A8-3's would be a better choice for a first flight.

Be sure you get REAL GOOD RELIABILITY in your ignition methods-- carefully inserted Quest Q2G2's would probably give the best reliability in this regard, IMHO...

Good luck! OL JR :)

[luke strawwalker]

I must ammend my pervious post! as the Model Rocket safety code as been ammended removing the wording about unproven models, replacing it with far less safety minded verbage.

I do not however change my position on unproven designs. As we can not On the field, "Check stability before Flight", nor adaquately "Warning spectators and clearning them away to a safe distance" at public or Club launches.

Chris;
This is EXACTLY how accidents happen. examples like the X-wing, X-15, and on and on. Test flights for every type of flying vehicle in the world are done in Isolation, out in the middle of a dessert somewhere with minimal flight crews and observers. Why? to protect the unknowing by-standers. Our models deserve the same respect. Underestimation and disprespect will be the undoing of our hobby. We must always think SAFETY First. I hate to say it but honestly most RSO's these day are not qualified to make such decisions... myself included.

"desert" (hot dry place) not "dessert" (tasty sweet after-supper treat)... antonyms -1... :D

OK, I'll shut up now before you tell me to go to a "hot dry place" (and I'm not talking about a desert... ) :D:y::dark:

Later! OL JR :)

[RangerStl]

Luke;

"Homophones", not antonyms... -1 :D

They sound the same (Homo-same; phone-sound) but have "different" spellings.

Antonyms are words with opposite meanings; not necessarily spelled or sounding the same.

[Andyd103]

great input... anyone else have any ideas on how to keep the nc on, i personally don't think the 25g of nosecone and streamer are going to create a big enough force to pop off the noce cone under the acceleration of liftoff but then again you probably know more than me so any ideas on how to keep it on will be great

[stickershock23]

I don't think its arguing at all:
We are not telling the modeler don't fly this widget. We are saying; "Look the Scientific Method says our experiments be done AWAY from crowds of unsuspecting distracted people.

You know as well as I do that even with PA announced HEAD-UP, Everyone on their feet calls. NOT everyone is paying attention to the flight. I see this all the time at launches all over. True it's mostly the very young and Teenagers but it's also the distracted MOMs dealing with the young kids. or the bunch of vets discussing this important flight detail from an hour ago. People NOT directly involved with a given flight just don't take them all that seriously...until something happens... then they are all " Well why didn't you folks do something".

We are not stopping any thinking or learning.. as a matter of fact we are encouraging the BETTER use of the Scientific method to presue the goal. AND encouraging and further driving home the NEED for SAFETY FIRST thinking. Flying unproven models in isolation can even be set up as a ClUB activity using limited launch crews and range provisions. Thats the problem with common sense.... it just seems it isn't so common anymore.



All great points, I can agree with that.. and DEFINITELY with the SAFETY FIRST..

I wish we could hand out COMMON SENSE at every launch. to bad everyone does not have it or they loose it when they come to a launch!


OK back to the main topic.

I think your idea will work for the most part. a little more thought and you are well on your way to a cool and different rocket!

[ScrapDaddy]

I am on micro's side, but this is rather off-topic though. Anyway, back on topic, I know very little about spin stabibization, could someone please tell me how adding more tail weight would make it more stable? It sounds counter-intuitive....:confused:

[tbzep]

great input... anyone else have any ideas on how to keep the nc on, i personally don't think the 25g of nosecone and streamer are going to create a big enough force to pop off the noce cone under the acceleration of liftoff but then again you probably know more than me so any ideas on how to keep it on will be great

The old Centuri Buck Rogers rocket held its parachute in an open rear compartment just by friction. I never had a problem with it coming out. The nose cone will have more mass, but as long as there is a decently tight friction fit and the nose cone is made of light material, it shouldn't come off. Just don't make it too tight or you might pop a body tube like a can of biscuits. :eek: :D

What will be the diameter? If it's small enough, you could tape some scrap body tube on the side of a larger stable rocket like a Broadsword on an AT E30 or F50 and see what it takes to keep the nose cone in place. Just stick another nose cone on top or at least flatten and tape over the end so air won't help push it out.

[billspad]

I knew I'd seen something like this before. Take a look at the drawing on the right. It's got 3 motors and they're not canted but other than that it sounds a lot like your model.

If you want to come to a CMASS launch (You may even be a member for all I know.) I can promise you we'll consider letting you fly it. For unconventional and unproven designs that we think have a chance of working we send the flyer out to one of the high power pads. If it looks like it doesn't have a chance of making it the 100' back to where we're all standing if things go bad it we'll let you try.

[Andyd103]

I knew I'd seen something like this before. Take a look at the drawing on the right. It's got 3 motors and they're not canted but other than that it sounds a lot like your model.

If you want to come to a CMASS launch (You may even be a member for all I know.) I can promise you we'll consider letting you fly it. For unconventional and unproven designs that we think have a chance of working we send the flyer out to one of the high power pads. If it looks like it doesn't have a chance of making it the 100' back to where we're all standing if things go bad it we'll let you try.

i've been to CMASS before in amesbury, but i think im going to take the advice of the other members and test it in isolation first, and then after that you may see me down there this summer

[KennB]

Bill, it's good to see you found a device in which to use your Czech motors. At least the rocket's plans had some relation to the motor's origin.

We may have to set up Pad D in the middle of the swamp to fire those. :eyepop:

[billspad]

Bill, it's good to see you found a device in which to use your Czech motors. At least the rocket's plans had some relation to the motor's origin.

We may have to set up Pad D in the middle of the swamp to fire those. :eyepop:

That would be a better idea than the static test in my basement. It didn't smell so good.

Do the Czechs and the Russians get along?

[cjl]

Chris;
This is EXACTLY how accidents happen. examples like the X-wing, X-15, and on and on. Test flights for every type of flying vehicle in the world are done in Isolation, out in the middle of a dessert somewhere with minimal flight crews and observers. Why? to protect the unknowing by-standers. Our models deserve the same respect. Underestimation and disprespect will be the undoing of our hobby. We must always think SAFETY First. I hate to say it but honestly most RSO's these day are not qualified to make such decisions... myself included.

Might I point out that because of safe stand-off distances and proper heads up announcements, none of your claimed examples actually came anywhere close to hurting anyone? I agree that proper respect and care should be taken, but I disagree that this requires an isolated launch. Take the example of the rocket in the OP - if it were launched for the first time on A motors from a 50+ foot standoff distance, it would likely be unable to make it back to the flight line even if unstable and if it headed in the worst possible direction. I really don't see how that would pose any significant danger.

[KennB]

Do the Czechs and the Russians get along?

Not so much that they get along or don't these days but, when those motors were produced, the Russians had some influence over the Czechs. Now, at least, the Czechs have more choice of liking them or not.

[powderburner]

I am thinking if they were pointed straight down ( but still angled out) as compared to pointed one direction ti cause a spin. If you spin it yes it should be stable. if you don't spin it you are basically building this.

In the example you showed (the Mercury capsule escape tower), there are multiple nozzles canted outward to get the rocket exhaust to clear the capsule itself.

This works because (if you have to actually escape) there is ONE MOTOR firing through those multiple nozzles. There is an automatic lateral thrust balance (one side cancelling the other) because the nozzles all start at the same instant and end at the same instant, and the single motor automatically provides equal mass flow and pressure to each nozzle and generates equal thrust from all nozzles. The longitudinal (forward) vector component of each nozzle is aligned with the capsule centerline and pulls it clear.

If you have separate motors on the front end your design is immediately subject to the quirks and vagaries of getting matched motor performance and perfect simultaneous ignition timing. If one motor starts late (or not at all) there will be imbalanced lateral thrust components from each motor, and the net lateral thrust will be trying to turn the rocket's nose (in some random direction).

It is a common misconception that rockets in the nose will impart any degree of stability. They will not. You are probably thinking in the back of your mind of the case where the forward rockets are always pointed vertically, and any deflection that moves the following rocket toward one side will simply swing back under the motors. Your design does not have vectoring, vertically-pointed motors. When the forward motors are locked to the rocket, and the rocket tilts or turns, the net thrust vector also turns. In cases like this, there is no corrective force provided by the forward rockets.

Even the great Dr. Goddard made this mistake.

Your rocket will have the benefit (to some degree) of the effects of overall rocket pitch inertia, which will act to damp any tilt or turn motions. But once the rocket has turned, this inertia will also act to continue the turning motion. This is not a good plan either.

You can achieve some degree of stability by canting the motors (and the thrust lines) evenly around the rocket longitudinal axis to make it spin. This approach will be strongly influenced by how well you can match all motor ignition timing, and of course by using the same motors all around with the same thrust curves and net impulse. If there are launch rod provisions that allow the rocket to spin freely while still on the launcher (as your design seems to include) and if there is "enough" rotation before it leaves the launch rod, you could be OK.

I don't think I have ever seen hard numbers put to this sort of design to establish a safe minimum RPM. You may have to do some experimenting with motor cant angles and a few other things. As already mentioned, until this thing shows reliable, consistent, SAFE inflight behavior, you should be launching ALONE.

To improve your chances of getting simultaneous ignition you might want to try the new Quest Q2 igniters. They seem to help clusters get a better start.

May I suggest you read "A Problem in Stability" on page 18 of:
http://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v01n03_01-69.pdf

[shreadvector]

I suggest the newest Quest Q2G2 igniters as the semi-new Quest Q2 igniters are similar to Estes "solar" igniters (and similar potential to short or crack the bridge wire if mishandled as the Estes, but the Quest Q2 seem to be a little more robust).

Q2G2 are almost foolproof. The only user-error I've sen is when folks do not secure them in the nozzle firmly and they "pop" out loudly without igniting the motor. If you cram that protective straw/stirrer into the nozzle very firmly next to the igniter, it will hold it in perfectly.


snip....
To improve your chances of getting simultaneous ignition you might want to try the new Quest Q2 igniters. They seem to help clusters get a better start.

...snip

[Peartree]

The old Centuri Buck Rogers rocket held its parachute in an open rear compartment just by friction. I never had a problem with it coming out. The nose cone will have more mass, but as long as there is a decently tight friction fit and the nose cone is made of light material, it shouldn't come off. Just don't make it too tight or you might pop a body tube like a can of biscuits. :eek: :D

What will be the diameter? If it's small enough, you could tape some scrap body tube on the side of a larger stable rocket like a Broadsword on an AT E30 or F50 and see what it takes to keep the nose cone in place. Just stick another nose cone on top or at least flatten and tape over the end so air won't help push it out.

It may be true that Centuri did this before with a parachute compartment, but upon any significant acceleration, everything gains weight in multiples. At 3g's a parachute weighing only a few grams still weighs only a few grams more. A nosecone, particularly one with added weight for stability that multiplies its weight several times at liftoff will likely exert more force downward than many of us would appreciate.

I'm not saying that it won't work, but just that this is a significant failure mode that needs to be seriously looked at prior to launch.

[stickershock23]

In the example you showed (the Mercury capsule escape tower), there are multiple nozzles canted outward to get the rocket exhaust to clear the capsule itself.

This works because (if you have to actually escape) there is ONE MOTOR firing through those multiple nozzles. There is an automatic lateral thrust balance (one side cancelling the other) because the nozzles all start at the same instant and end at the same instant, and the single motor automatically provides equal mass flow and pressure to each nozzle and generates equal thrust from all nozzles. The longitudinal (forward) vector component of each nozzle is aligned with the capsule centerline and pulls it clear.

If you have separate motors on the front end your design is immediately subject to the quirks and vagaries of getting matched motor performance and perfect simultaneous ignition timing. If one motor starts late (or not at all) there will be imbalanced lateral thrust components from each motor, and the net lateral thrust will be trying to turn the rocket's nose (in some random direction).

It is a common misconception that rockets in the nose will impart any degree of stability. They will not. You are probably thinking in the back of your mind of the case where the forward rockets are always pointed vertically, and any deflection that moves the following rocket toward one side will simply swing back under the motors. Your design does not have vectoring, vertically-pointed motors. When the forward motors are locked to the rocket, and the rocket tilts or turns, the net thrust vector also turns. In cases like this, there is no corrective force provided by the forward rockets.

Even the great Dr. Goddard made this mistake.

Your rocket will have the benefit (to some degree) of the effects of overall rocket pitch inertia, which will act to damp any tilt or turn motions. But once the rocket has turned, this inertia will also act to continue the turning motion. This is not a good plan either.

You can achieve some degree of stability by canting the motors (and the thrust lines) evenly around the rocket longitudinal axis to make it spin. This approach will be strongly influenced by how well you can match all motor ignition timing, and of course by using the same motors all around with the same thrust curves and net impulse. If there are launch rod provisions that allow the rocket to spin freely while still on the launcher (as your design seems to include) and if there is "enough" rotation before it leaves the launch rod, you could be OK.

I don't think I have ever seen hard numbers put to this sort of design to establish a safe minimum RPM. You may have to do some experimenting with motor cant angles and a few other things. As already mentioned, until this thing shows reliable, consistent, SAFE inflight behavior, you should be launching ALONE.

To improve your chances of getting simultaneous ignition you might want to try the new Quest Q2 igniters. They seem to help clusters get a better start.

May I suggest you read "A Problem in Stability" on page 18 of:
http://www.ninfinger.org/rockets/ModelRocketry/Model_Rocketry_v01n03_01-69.pdf



All great points and things to think about.

[tbzep]

I'm not saying that it won't work, but just that this is a significant failure mode that needs to be seriously looked at prior to launch.

As I mentioned in my last paragraph about testing it beforehand.

BTW, I've used rear ejection with a balsa bulkhead friction fitted as a plug to retain the recovery system before. It had more surface area than a nosecone shoulder and a little less mass, but it worked. ;)

[powderburner]

I suggest the newest Quest Q2G2 igniters as the semi-new Quest Q2 igniters are similar to Estes "solar" igniters

That's what I meant, just didn't get all the correct nomenclature in there. Definitely, look at these igniters for clusters.