SecondRow
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The link doesn’t work for me.
Help needed with unstable rocket
- Thread starter delta1999
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Help Support The Rocketry Forum:
Does for me. Anyway it is Tripoli's safety code on metal in rockets. To sum it up, it states metal may be used in rockets, aluminum and copper, in any amount the builder deems needed for safe flight and recovery.
David's link didn't work for me, either. But it's where I ended up after following the link below.
Try this one:
https://tripoli.org/docs.ashx?id=886403
Try this one:
https://tripoli.org/docs.ashx?id=886403
lakeroadster
When in doubt... build hell-for-stout!
.....
You definitely need a longer launch rod (as others have stated)! The rocket with the launch "lug" needs to slide along for about 36" before leaving the rod.
Of course, the other rules stated by others would be good, too,
Of course, the other rules stated by others would be good, too,
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Never rely on the CG calculated by the program. It's fine to get a rough idea, but there are things you can't weigh that can affect the CG, like paint. After my rocket is done and painted, I find the CG by balancing the rocket on the edge of a ruler. Measure that point from the front of the nose cone and put in an override for the CG. Weigh the final configuration (with recovery components, without a motor) and place that in the override as well. The CP will be calculated fair enough by the program, since that is dependent on dimensional data (airframe size, length, nose cone design and fin design and size). You can do a swing test to find the CP, but it really isn't necessary if you use OR or Rocksim and you don't have a wacky design.
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In addition to the comments on rod length and the motor's thrust curve, this rocket is pretty short and it has fairly large fins for it's size. How windy was it? This bird is likely to weathercock a lot in winds upwards of 10 mph (17 kph). Adding that to short rod and low thrust and wacky happens.
Sorry, don't know why it does not work for anyone but me. I hate fricking puters
One other thought -- if you have access to a 3D printer but other materials are too expensive you may want to 3D print the whole rocket including the body tube and fins. I am normally not a fan of this approach since it is heavier than cardboard tubes and plywood fins but 3D printed parts are likely lighter than the metal and PVC pipe components. This is not my forte but there are plenty of people who 3D print the whole rocket.
I have printed several rockets that are all 3D. I do not think that they are that much heavier. All the way up to a 9' Saturn V. If you can print them, I say do it. Good luck.
1) bendy metal fins
2) launch rod is way, way too short. There is absolutely no chance that the rocket is at a safe and stable speed when exiting the rod. The OpenRocket flight sim should reflect that.
2) launch rod is way, way too short. There is absolutely no chance that the rocket is at a safe and stable speed when exiting the rod. The OpenRocket flight sim should reflect that.
Most importantly you need to ensure that what you are doing is legal where you are. Get the support of your school and do this as a school project, engage the local police/ fire brigade to make sure they are happy. Someone accidentally seeing a rocket in the sky could draw a lot of wrong attention to you. There are many things to be compliant with, many different sets of rules. Local, legal,and government rules.
Doing everything on your own in a country that does not "recognise" this as a hobby is daunting. You should try to find someone like minded to get support from. Hopefully someone with experience.
That said
Your tube at nosecone is not cut square. Wrap a sheet of paper around your tube. Where it lines up once wrapped, gives you a straight line.
The mating outside diameter of your printed nosecone does not seem to match the OD of your pvc? bodytube.
The fins should not be metal. You could use balsa, but I'd be tempted to use thin plywood.
Your launch rod needs to be about 1m ish 3ft in banana measurments for the rocket to slide on. That "should" be enough for you to get to flying speed. Your rocket must reach that minimum speed to have a chance of being stable.
Open rocket will calculate your centre of pressure correctly but will only calculate the center of gravity based on what information you put into it.
Once you have your rocket built and with a motor or simulated motor weight, check the CG location and ensure it matches the OR calculation or is further forward( more stable ) Tie a piece of string around the model at the CG and perform a swing test. The rocket should swing test with the nose forward. This might not happen first time round until it swings at minimum flying speed.
The motor you fit should have a nozzle that is in the middle and straight. If the nozzle is at an angle or offset and not thrusting through the centre of gravity. It won't go in a straight line up.....
Lastly. What you are doing comes under experimental rocketry. In Australia, legally, I could not make a sugar rocket. Fortunately, tested legally available motors can be purchased here. Expensive, but available.
Good luck. Don't get locked up.
Doing everything on your own in a country that does not "recognise" this as a hobby is daunting. You should try to find someone like minded to get support from. Hopefully someone with experience.
That said
Your tube at nosecone is not cut square. Wrap a sheet of paper around your tube. Where it lines up once wrapped, gives you a straight line.
The mating outside diameter of your printed nosecone does not seem to match the OD of your pvc? bodytube.
The fins should not be metal. You could use balsa, but I'd be tempted to use thin plywood.
Your launch rod needs to be about 1m ish 3ft in banana measurments for the rocket to slide on. That "should" be enough for you to get to flying speed. Your rocket must reach that minimum speed to have a chance of being stable.
Open rocket will calculate your centre of pressure correctly but will only calculate the center of gravity based on what information you put into it.
Once you have your rocket built and with a motor or simulated motor weight, check the CG location and ensure it matches the OR calculation or is further forward( more stable ) Tie a piece of string around the model at the CG and perform a swing test. The rocket should swing test with the nose forward. This might not happen first time round until it swings at minimum flying speed.
The motor you fit should have a nozzle that is in the middle and straight. If the nozzle is at an angle or offset and not thrusting through the centre of gravity. It won't go in a straight line up.....
Lastly. What you are doing comes under experimental rocketry. In Australia, legally, I could not make a sugar rocket. Fortunately, tested legally available motors can be purchased here. Expensive, but available.
Good luck. Don't get locked up.
I agree with this just by looking at the deposit in the nozzle. It probably produced asymmetric thrust causing it to become unstable.
What was your simulated launch velocity off the rod. That rod is way too short.
beeblebrox
8 C6-0, 12 D11-9, 20 D20-0, 20 E5-0, 3 Cinerocs
First thing I notice is that the fins are warped. The sugar motor is apparently about the diameter of the tube I.E. Minimum diameter rocket, therefore too tail heavy. Add 1oz (28g) to the nose, replace the fins with 1/6" basswood or aircraft plywood, make sure they are straight, lengthen the body tube by 2x. Liftoff seemed quick enough. (An Estes Alpha with any motor (1/2 A6-2, A8-3, B6-4, C6-5 etc.) will fly just fine if no wind just standing on its fins with no rod.) I.E. the rod issue is irrelevant. Yes, longer rod is better, but if the rocket is unstable the rod could be 20 feet long and the flight will still fail...
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Good Morning,
At first glance, and not having read everything,
1. The fins *look* rather thin, and may be prone to flutter. (Its hard to tell the rigidity in a photo)
2. Launch rod is too short
3. I dont know much about sugar/homemade motors, but any propellent must not have bubbles or contamination. (My apologies to the admin if I cant say that here...its not a formulae, but I dont want to cross a line! However since it sounds like he can not get commercial motors...)
4. Thust doesnt appear to be inline with Rocket.
5. Physiocally check your CG AFTER painting. Its not a huge factor in a small rocket, but even in Aircraft, the Aircraft empty weight has to be done after painting as paint adds that much weight to make a differance.
6 Follow the previous advice re. OpenRocket. Its a good program, but it only can only work with the info the user inputs! FWIW I use the CG and Mass override exclusivly. Build it, weigh it, CG it, OR it!
Cool looking rocket though! All the best in your efforts and Fly Safe!
By the way, if you dont mind me aslking, what country are you in?
At first glance, and not having read everything,
1. The fins *look* rather thin, and may be prone to flutter. (Its hard to tell the rigidity in a photo)
2. Launch rod is too short
3. I dont know much about sugar/homemade motors, but any propellent must not have bubbles or contamination. (My apologies to the admin if I cant say that here...its not a formulae, but I dont want to cross a line! However since it sounds like he can not get commercial motors...)
4. Thust doesnt appear to be inline with Rocket.
5. Physiocally check your CG AFTER painting. Its not a huge factor in a small rocket, but even in Aircraft, the Aircraft empty weight has to be done after painting as paint adds that much weight to make a differance.
6 Follow the previous advice re. OpenRocket. Its a good program, but it only can only work with the info the user inputs! FWIW I use the CG and Mass override exclusivly. Build it, weigh it, CG it, OR it!
Cool looking rocket though! All the best in your efforts and Fly Safe!
By the way, if you dont mind me aslking, what country are you in?
This is NOT "fine" with the NAR ( "EX motor" ) . . . And, for HPR, one must be certified, whether NAR, TRA, or CAR.
Dave F.
As noted in the movie October Sky, the nozzle is critical in vertical flight.
I've been having issues with Quest motors in the tropical air causing the nozzles to give out. Just a slight asymmetry can cause the rocket to veer off. Quest motors starting 8/21 have a new clay formulation that's more resilient. Unfortunately, I still have a bunch of the old clay formulation motors.
In OR, you can indicate the length of the rod. Was the sim stable with the short rod? The length of the rod is a trade off between momentum, weight of the propellent, and velocity. The 15m/s is a sure fire velocity but have seen stable launches at 10m/s.
Tripoli has experimental motor launches. I've heard that half of the motors explode on the pad.
I've been having issues with Quest motors in the tropical air causing the nozzles to give out. Just a slight asymmetry can cause the rocket to veer off. Quest motors starting 8/21 have a new clay formulation that's more resilient. Unfortunately, I still have a bunch of the old clay formulation motors.
In OR, you can indicate the length of the rod. Was the sim stable with the short rod? The length of the rod is a trade off between momentum, weight of the propellent, and velocity. The 15m/s is a sure fire velocity but have seen stable launches at 10m/s.
Tripoli has experimental motor launches. I've heard that half of the motors explode on the pad.
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I never said it was ok with NAR. I am going to say this one last time. This website is not the NAR Facebook group or a NAR forum. Discussing metal fins is not forbidden on this forum.
Two simple things here.
Check the CG of the Rocket. If this was my rocket, I would add nose weight until the CG is at the 24cm mark. This is measured from the nosecone down. This should be checked with the rocket ready to fly with the motor and all recovery gear in the rocket.
Get a launch rod that is in the 48" range.
Check the CG of the Rocket. If this was my rocket, I would add nose weight until the CG is at the 24cm mark. This is measured from the nosecone down. This should be checked with the rocket ready to fly with the motor and all recovery gear in the rocket.
Get a launch rod that is in the 48" range.
Looks to me like the launch rod was much too short. However, just as important with experimental motors - how did you determine the thrust curve of your motor? Did you verify it with a test stand, or is the performance(thrust) assumed from your motor sim?
Dave
Dave
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Early on you asked what could cause the motor to put out thrust that is off centerline?
There are lots of things that could cause the problem. But looking at the picture of the "nozzle" that I assume you drilled, the problems seem pretty obvious.
Your nozzle looks more like a simple parallel hole drilled thru thru your aft epoxy closure than it looks like an actual nozzle.
The pic shows a lot of accumulation of "scale" (as in burned/melted on residue) on your motor's nozzle throat. You might want to look into getting a tapered drill to cut a simple 45 degree cone shape for the pouter part of your nozzle. It will work much better than a simple hole drilled thru the epoxy at the back of the motor.
Whatever you are using for you fuel mixture is leaving a great deal of unburned residue in the nozzle. Maybe go with a lower residue formula?
Or possibly, the material that you are using for your nozzle itself is eroding so fast that it is causing the build up in your nozzle.
You might want to look into a simple De Laval nozzle dimensions calculator to help you determine the right size and shape of your nozzle. I honestly haven't done that in a couple of decades as 99.9999% of my flights these days are commercial motors, but it might help you significantly.
Brad
There are lots of things that could cause the problem. But looking at the picture of the "nozzle" that I assume you drilled, the problems seem pretty obvious.
Your nozzle looks more like a simple parallel hole drilled thru thru your aft epoxy closure than it looks like an actual nozzle.
The pic shows a lot of accumulation of "scale" (as in burned/melted on residue) on your motor's nozzle throat. You might want to look into getting a tapered drill to cut a simple 45 degree cone shape for the pouter part of your nozzle. It will work much better than a simple hole drilled thru the epoxy at the back of the motor.
Whatever you are using for you fuel mixture is leaving a great deal of unburned residue in the nozzle. Maybe go with a lower residue formula?
Or possibly, the material that you are using for your nozzle itself is eroding so fast that it is causing the build up in your nozzle.
You might want to look into a simple De Laval nozzle dimensions calculator to help you determine the right size and shape of your nozzle. I honestly haven't done that in a couple of decades as 99.9999% of my flights these days are commercial motors, but it might help you significantly.
Brad
Troy3003
Well-Known Member
Looks like the launch rod is fiberglass. They tend to have alot of flex. As far as whats allowed or not depends on local rules. If he is not a member of an association like NAR/Tripoli then those specific rules wouldn't apply to him, would they? Not saying he should skirt around safety either way.
Andrewcrowl1
Member
Yes, you say correctly, it has to do with one another, but on the other hand - who should take care of it?
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Little late to this party.
I really like the design. People have correctly beaten to death that your launch rod is way too short. But you also are wasting several inches of rod with your lug placed that far forward. The length of USEFUL rod is only that ahead of the tail end of your lug when it is on the rod. My tendency is to put the lug as close to the tail of the rocket as I reasonably can (things like a boat tail heinie might limit that.). For long rockets I may add a smaller second lug around the CG. not sure of length for MicroMaxx, but for standard rockets you like to have at least 30 inches or more of USEFUL lug.
as for NAR rules, yes, you aren’t required to follow them if you aren’t flying under their jurisdiction. But these rules weren’t just put on place by government pencil pushers who seem to make up stuff only to justify their existence. For the most part, they are pretty good general safety rules (well, maybe not the plugged motor rules, but I digress.). As a beginner you especially don’t have the experience to know which rules you might safely bend, and the rules are good guidelines for the safety of yourself, the people around you, the property around you, AND generally good rules for successful flights. I don’t want to rain on your parade, but you will have mores successful flights and greater likelihood of keeping all 10 fingers and both eyes if you follow them.
also, please put a blast plate under your launch rod, so you don’t catch the ground on fire when your rocket launches.
best wishes for many good and safe flights!
I really like the design. People have correctly beaten to death that your launch rod is way too short. But you also are wasting several inches of rod with your lug placed that far forward. The length of USEFUL rod is only that ahead of the tail end of your lug when it is on the rod. My tendency is to put the lug as close to the tail of the rocket as I reasonably can (things like a boat tail heinie might limit that.). For long rockets I may add a smaller second lug around the CG. not sure of length for MicroMaxx, but for standard rockets you like to have at least 30 inches or more of USEFUL lug.
as for NAR rules, yes, you aren’t required to follow them if you aren’t flying under their jurisdiction. But these rules weren’t just put on place by government pencil pushers who seem to make up stuff only to justify their existence. For the most part, they are pretty good general safety rules (well, maybe not the plugged motor rules, but I digress.). As a beginner you especially don’t have the experience to know which rules you might safely bend, and the rules are good guidelines for the safety of yourself, the people around you, the property around you, AND generally good rules for successful flights. I don’t want to rain on your parade, but you will have mores successful flights and greater likelihood of keeping all 10 fingers and both eyes if you follow them.
also, please put a blast plate under your launch rod, so you don’t catch the ground on fire when your rocket launches.
best wishes for many good and safe flights!
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