Sugar Rocket Issues

[ryandgarrison]

Hey guys,

 

I'm very knew to rocketry. I recently got into making homemade rockets with rocket candy. But I have a few questions.

 

I made a successful rocket a couple weeks ago that maybe flew 1500 feet or so. I have used cardboard tubes as a casing and a 65/35 ratio of KNO3 to Sugar. I have been heating the mixture so that the sugar caramelizes and then packing it into the casing, coring it as it cools. Then I tape a stick to the outside.

 

But today, my rocket did not even lift off, it just sat their, blasting into the ground!

 

So my questions:

 

1. All the YouTube videos and online tutorials that I have seen show the people taping a stick to the rocket. Why is that? Balance? Drag? Stability?

 

2. How long and how heavy should the stick be in comparison to the rocket itself? Maybe mine was too heavy? Or too long?

 

 

I attached a picture of the launch failure.

 

Thanks for your help!

 

 

 

Edited February 22, 2015 by ryandgarrison

[stix]

The stick is there for stability.

 

What did you do different compared to the one that flew 1500ft? Are you using a nozzle?, Red Iron Oxide, etc... etc... etc...

 

There is lots of good info on this forum, you just need to use the search function.

 

Here's a start: http://www.amateurpyro.com/forums/topic/10413-best-place-to-buy-sticks/

and http://www.amateurpyro.com/forums/topic/10324-substitute-for-fuse/

 

btw. I wish I had a launch site like yours

 

Cheers.

 

[EDIT] There are lots of people willing to help. Do a little research first, then if you get stuck, ask away.

Edited February 22, 2015 by stix

[Oinikis]

The size of the stick should be so it would shift the center of mass to the back of the nozzle (so you could barelly balance it on your finger on the nozzle/stick intersection). This is to make sure "axis of thrust" goes through the center of mass, and doesn't cause torque. Also, since the stick is thin and long, it acts as an aerodynamic stabilizer too, and is very handy for launching and stable liftoff. However, on windy days due to aerodynamics of the stick it might start pointing towards the wind, so beware.

If your failed rocket was made at the same way as the successful one, i'd guess it's due to moisture. Sugar propellant is very hydroscopic, and tends to get wet and useless quite fast, and if air is moist and it's unsealed, even overnight. However you could try wrapping it in plastic wrapper, but after some time, moisture still seems to get it. Some people use desicator bags to keep it dry, but I haven't tried it myself.

[ryandgarrison]

Thanks for the advice, guys!

 

I looked into those other threads and learned a lot there by reading through them, stix, so thanks a lot for those links.

I wasn't using a nozzle really, just driving a stick into the propellant as it cooled down to create a core.

 

The difference between the first launch and the second was that the engine was about half as long as the first had been. The ingredients were the same (so no red iron oxide). However, the first rocket had about an inch or so of extra tube that came down past the end of the propellant. This second one had propellant almost up to the lip of the tube. Can that effect thrust?

 

The stick was the same stick both times.

 

It was cloudy though, pretty windy, and perhaps fairly moist so maybe that had an effect on the propellant.

 

I'll keep trying and see what works. Thanks guys!

[stix]

Regarding the balance and fuel being hygroscopic, Oinkis has given a great explanation, so re-read it and ask if it doesn't make sense.

 

. . . The difference between the first launch and the second was that the engine was about half as long as the first had been. . .

 

That's the issue right there! Longer length means more fuel burning, means more pressure, means more thrust. That's why the first motor worked and the second didn't take off. Alternatively, if you go too long, it's CATO!!

 

If you haven't done so already, start writing things down so you can "tune" your motors in. Try to be consistent in your fuel prep. and that will at least remove one variable.

 

It's been said many times on this forum that there's no "one size fits all" approach to any pyrotechnic pursuit. I have my methods and ingredients, Oinkis has his, other have theirs and you have yours. If you are willing to put the effort in, you will get there and have the pleasure of building your own reliable repeatable sugar rockets

 

Cheers.

 

 

[Oinikis]

One of the most important factor in the motor is it's Kn (the ration between fuel burning area and exit hole cross-section area), because it tells you the pressure inside the motor, so if it's too low it won't work, if it's too high, there will be too much pressure and it will CATO (Just as stix mentioned). Here is a great article explaining Kn and pressure, and it is very useful when designing rockets, especially when you will advance towards nozzles ones.

 

I have done some basic calculations for the nozzleless design and if you make the core 15-20 times longer than it is wide, you'll get initial Kn of 60-80, which will produce initial pressure of approximately 8-12 bar, which is fine for rockets like so and should produce enough thrust without any issues. Of course the Kn (so pressure too) will drop as the fuel burns, becouse crossection area increases as a square function of radius, while burning are increases almost linearly, and fuel tends to burn faster near the exit hole, but it will still make the very important initial kick, and later produce enough thrust to sustain the flight. Now of course this is over complication of a simple thing, but stuff seems to work out better when you know what you are doing and how it works. I suggest you reading on other peoples work, so you do not have more questions and make nice rockets.

[dagabu]

Hey guys,

 

I'm very knew to rocketry. I recently got into making homemade rockets with rocket candy. But I have a few questions.

 

I made a successful rocket a couple weeks ago that maybe flew 1500 feet or so. I have used cardboard tubes as a casing and a 65/35 ratio of KNO3 to Sugar. I have been heating the mixture so that the sugar caramelizes and then packing it into the casing, coring it as it cools. Then I tape a stick to the outside.

 

But today, my rocket did not even lift off, it just sat their, blasting into the ground!

 

So my questions:

 

1. All the YouTube videos and online tutorials that I have seen show the people taping a stick to the rocket. Why is that? Balance? Drag? Stability?

 

2. How long and how heavy should the stick be in comparison to the rocket itself? Maybe mine was too heavy? Or too long?

 

 

I attached a picture of the launch failure.

 

Thanks for your help!

 

 

 

 

Good question:

 

1.) Yes, all three are accurate and necessary for the proper guidance of your rocket. Oinikis speaks of traditional methods of determining the proper sizing of a stick, this is mostly true but you also want to be aware of the length of the stick as well. A 1" long hardwood block taped on the back of the rocket may balance it at the nozzle but it will not add any guidance to the motor. In the same way, adding a 36" long whip of a stick 1/16" wide may also balance at the nozzle but will give you 7 sq in of surface where a 1/8" wide square stick 12" long will will give you only 5 sq in of surface. While it may not seem a lot of difference, your motor may not be able to overcome the additional drag and may "windmill" into the wind losing much of its upward flight.

 

2.) The reality is that "tradition" will work for 90% of normal rockets (average power, flight and thrust) using a stick 1/4 to 1/3 the ID in width (square) and 5-6 times the motor length. Balsa, Poplar, Bass Wood are good candidates due to weight:strength ratios while the science says that a formula must be used to determine the stick(s), length, placement, width, density etc. Consider that most of the current advice about stick balance and such has its roots in model rocketry and thus NASA rockets in general, much of it is flawed considering the inertia we experience (most rockets and exceptionally with whistle propellants) v.s. the inertia you see in a model rocket.

 

After all, have you seen a NASA rocket with a big stick used for guidance? No, they use three equally spaced fins that if you perused the math, you would find that the surface of the fins would be pretty darn close to the perfect stick area on a scale rocket motor for pyro.

 

A.) Three things to carefully consider when adding stick(s):

1. What is the weight ratio of the stick? a 1:10 ratio is as good as it is likely to get, a 10g stick on a 100g rocket motor (not including a header, we will get into that later) is really hard to achieve in larger motors. Weight your sticks and see for yourself.

2. What is the length of your stick? If it is an antenna and can whip around, it may add too much drag.

3. How stiff is the stick? With a single stick, make sure it is stiff enough to resist the power (thrust) of the motor when it takes off. High speed video shows us that the motor pushes against the stick while in the tube causing the motor to move toward the stick BUT as it does so, it actually turns when the stick resists the push and the rocket "rolls over" changing the trajectory of the rocket.

 

Enough for now, I will add more later but keep in mind the old adage, "Ask 10 rocketeers what stick to use and you will get 11 answers."

[MrB]

Enough for now, I will add more later but keep in mind the old adage, "Ask 10 rocketeers what stick to use and you will get 11 answers."

All 10 of them, and their 11 answers, are wrong. You should grow your own bamboo, use it, and nothing else, for sticks. And, of course, harvesting, and drying it, just so it can be hand split once it's bone dry, is a perfect occupation for a spouse, or children... When they aren't preoccupied with pasting shells.

B!

[ryandgarrison]

Wow, you guys have been super helpful, thanks for the information!!

 

I'm trying again this Saturday once I get more potassium nitrate so I'll keep those things in mind and definitely start taking notes and keeping track of how I make the rocket in order to recreate it.

[ryandgarrison]

One of the most important factor in the motor is it's Kn (the ration between fuel burning area and exit hole cross-section area), because it tells you the pressure inside the motor, so if it's too low it won't work, if it's too high, there will be too much pressure and it will CATO (Just as stix mentioned). Here is a great article explaining Kn and pressure, and it is very useful when designing rockets, especially when you will advance towards nozzles ones.

 

I have done some basic calculations for the nozzleless design and if you make the core 15-20 times longer than it is wide, you'll get initial Kn of 60-80, which will produce initial pressure of approximately 8-12 bar, which is fine for rockets like so and should produce enough thrust without any issues. Of course the Kn (so pressure too) will drop as the fuel burns, becouse crossection area increases as a square function of radius, while burning are increases almost linearly, and fuel tends to burn faster near the exit hole, but it will still make the very important initial kick, and later produce enough thrust to sustain the flight. Now of course this is over complication of a simple thing, but stuff seems to work out better when you know what you are doing and how it works. I suggest you reading on other peoples work, so you do not have more questions and make nice rockets.

 

That all makes sense. So make the core 15-20 times deeper/longer than it is wide in diameter. But if my motor is 1 inch in diameter, I don't fully understand how to determine the ideal diameter of the core that I am creating.

 

I used a 3/8 inch stick. So if I make the core depth 15-20 times that, it would be 5.625 inches to 7.5 inches deep.

 

But is a 3/8 inch core too big or not big enough for a 1 inch motor?

Edited February 24, 2015 by ryandgarrison

[Oinikis]

3/8" core should work for 1" ID rocket I think. There isn't really an ideal diameter in this case but as a rule of thumb for these rockets make the core somewhere about 1/3 of the ID.

[dagabu]

Oinikis,

 

I haven't had any luck with larger nozzles on R-Candy motors but perhaps the longer core you mention would help the impulse. I have never done a nozzless R-Candy so this is all speculation on my part, 99% of my R-Candy motors have used graphite or phenolic nozzles and aluminum cases.

[Oinikis]

I have started with R-Candy making nozzleless motors, and later returned to make small casingless motors, which essentially are blob of fuel with core and fuse (they just fly around randomly, and disappear without a trace). it doesn't matter what initial core diameter is as long as it is correct length. What matters is how many times core diameter expands, becouse due to the way surface area and exit cross-section area increase (as described earlier), the Kn decreases, and how many times core gets wider, by that many times Kn gets smaller, so if core is 1/3 of ID, the initial Kn decreases 3 times(in this case to 20-30), and bellow that it isn't really effective, and also these are the ratios practice showed me they work. These motors actually can lift a decent amount of weigh and can be easily used as a cheap workhorse for small payloads, and once you get it, they are super quick to make.

Edited February 24, 2015 by Oinikis

[ryandgarrison]

Oinikis,

 

I haven't had any luck with larger nozzles on R-Candy motors but perhaps the longer core you mention would help the impulse. I have never done a nozzless R-Candy so this is all speculation on my part, 99% of my R-Candy motors have used graphite or phenolic nozzles and aluminum cases.

 

https://www.youtube.com/watch?v=hyPysthIRss

 

This is what I'm trying to do. It's a long video but you get the idea and you can skip forward to the end to see the launch.

Edited February 24, 2015 by ryandgarrison

[Oinikis]

First of, this has to be done outside, or in some sort of shelter, with fire safety present, using pan and utensils which are only being used for R-Candy, and I suggest using a pan, because it's much easier to mix it in the pan, and prevent it from burning. After all you are working with live pyrotechnic composition which has inedible things in it. Everything else is fine, but in the test of R-Candy they did it seemed to burn quite slow and not very energetic, suggesting they screwed up (my guess is bad cooking, due to using pot). Also in my opinion nozzleless R-Candy rocket are for smaller sizes, for this size i'd go with nozzled, BATES grain rocket, but that's only my opinion and a matter of preference. Also I work with bit cooler and more play-doh like R-Candy, and press it with a loose dowel, in order to get all the air out, because I wouldn't trust R-Candy itself to leave no air bubbles and gaps. But everyone has it's methods, and you'll find what works best for you as you gain experience, just be careful, and don't burn yourself and set stuff on fire.

Edited February 24, 2015 by Oinikis

[dagabu]

I have started with R-Candy making nozzleless motors, and later returned to make small casingless motors, which essentially are blob of fuel with core and fuse (they just fly around randomly, and disappear without a trace). it doesn't matter what initial core diameter is as long as it is correct length. What matters is how many times core diameter expands, becouse due to the way surface area and exit cross-section area increase (as described earlier), the Kn decreases, and how many times core gets wider, by that many times Kn gets smaller, so if core is 1/3 of ID, the initial Kn decreases 3 times(in this case to 20-30), and bellow that it isn't really effective, and also these are the ratios practice showed me they work. These motors actually can lift a decent amount of weigh and can be easily used as a cheap workhorse for small payloads, and once you get it, they are super quick to make.

 

 

 

Hmmm, really? Just make a bates grain with a bulkhead and launch? OK, that's pretty cool!

[stix]

I have started with R-Candy making nozzleless motors, and later returned to make small casingless motors, which essentially are blob of fuel with core and fuse (they just fly around randomly, and disappear without a trace). it doesn't matter what initial core diameter is as long as it is correct length. What matters is how many times core diameter expands, becouse due to the way surface area and exit cross-section area increase (as described earlier), the Kn decreases, and how many times core gets wider, by that many times Kn gets smaller, so if core is 1/3 of ID, the initial Kn decreases 3 times(in this case to 20-30), and bellow that it isn't really effective, and also these are the ratios practice showed me they work. These motors actually can lift a decent amount of weigh and can be easily used as a cheap workhorse for small payloads, and once you get it, they are super quick to make.

 

Well well well, thanks for than info Oinikis. I just learnt something. I was always under the impression that the Kn's of nozzleless rockets would increase over burn time, but not as much as they do with the nozzled types.

 

I don't do nozzleless except for small casingless one's like you describe. I use inserted bates grains with a nozzle.

 

I had to check the data and do some calculations myself and you are absolutely correct!. That now confirms my suspicions about nozzleless rockets. I prefer mine to increase in thrust over time. I'm not saying that nozzleless has no place, because they are used effectively all the time with pyro headers and having no nozzle is quicker to make - it's a matter of preference.

 

I'm looking at this from a "model rocket" point of view where altitude is the main purpose - so my views are a bit skewed in that direction.

 

Have you or anyone else ever done a test using the same amount of fuel using r-candy or black powder, with and without a nozzle. It would be interesting to see the results. I reckon with a nozzle you would get more altitude for the same fuel weight - a higher specific impulse and total impulse.

 

I have done some small nozzleless and casingless r-candy rockets (around 30grams). Very fun for a quick fix and easy to make. I melt my fuel (KNSU+1%RIO) and then form it around a cold metal rod. I then attached a thin balsa stick wrapped with a bit of aluminium tape. When ignited they take off like the "powers of piss". What I really like about them is that all that's left is a harmless stick of balsa and some aluminium fluttering to the ground.

 

Cheers.

[Oinikis]

I'm quite confident that thrust in nozzleless rockets still increases over time or at least stays neutral, because the surface area increases, just the Kn (and efficiency) drops.

I'm also all for efficiency and bates/nozzle combo too, I just started with nozzleless ones long time ago. However I prefer my rockets to either have neutral or bit regressive burn profile, for faster take off, and more consistent chamber pressure. So, due to that i'm currently making universal digital test stand for testing rockets of various sizes, so once i'll finish it, i'll test bunch of things, including comparison between nozzled and nozzleless ones, because i'm curious myself. Thanks for suggestion!

[stix]

I'm quite confident that thrust in nozzleless rockets still increases over time or at least stays neutral...

 

No it doesn't - you were correct in what you said before.

 

example:

 

15cm length core burner (nozzleless motor):

 

1cm core = 1cm nozzle = kn ratio of 60

as it burns out :

1.5cm core = 1.5cm nozzle = kn ratio of 40

2.0cm core = 2.0cm nozzle = kn ratio of 30

2.5cm core = 2.5cm nozzle = kn ratio of 24

 

The ratios above would be even more regressive as the fuel length is reduced - I have not taken that into account. The thrust to weight ratio over time should be taken into account, but for the purposes of this exercise it shouldn't have too much bearing on the result.

 

example 2 (fixed nozzle = 1cm):

 

15cm length core burner:

 

1cm core = 1cm nozzle = kn ratio of 60

as it burns out :

1.5cm core = 1cm nozzle = kn ratio of 90

2.0cm core = 1cm nozzle = kn ratio of 120

2.5cm core = 1cm nozzle = kn ratio of 150

 

The ratios above are "progressive" - as it will always be with a fixed nozzle and a core burner, unless you have a star shaped core or some other geometry.

 

I'm very confident in the math above - but someone else should verify. I believe it is correct, otherwise I'll go gay. Not that there's anything wrong with that!, but i'd prefer not to. Especially since I know a lot of nice beautiful women at the moment and it would be such a waste.

 

Cheers.

[Oinikis]

Lubricate your anus. Prepare to wear pink underwear.

Now, imagine having a nozzleless rocket with ID of 5cm, with a core 1cm by 30cm.

1cm -> Kn=120 -> surface area=94cm^2 -> nozzle area=0.785cm^2 -> chamber pressure~20bar -> thrust~157N (thrust = pressure * nozzle area)

3cm -> Kn=40 -> surface area=283cm^2 -> nozzle area=7.065cm^2 -> chamber pressure~7bar -> thrust~495N

Why is that? even thou Kn decreases, so effective exhaust velocity drops, but since the surface area gets bigger, more fuel burns, thus mass flowrate increases, and overweights the decreased effective exhaust velocity. You might be surprised by my statement that thrust equals pressure times nozzle crosssection area, but now think about it. If you have a rocket chamber, because there is a nozzle at the back, the area at the top is bigger than at the bottom, and the difference is nozzle crosssection area itself, no matter the ID of the chamber. and pressure is force/area. This is also why divergent part in de laval nozzle produces more thrust, becouse there is extra area after the neck for residual pressure to push against.

Edited February 25, 2015 by Oinikis

[dagabu]

Prepare to wear pink underwear.

 

Now, imagine having a nozzleless rocket with ID of 5cm, with and with a core 1cm by 30cm.

1cm -> Kn=120 -> surface area=94cm^2 -> nozzle area=0.785cm^2 -> chamber pressure~20bar -> thrust~157N (thrust = pressure * nozzle area)

3cm -> Kn=40 -> surface area=283cm^2 -> nozzle area=7.065cm^2 -> chamber pressure~7bar -> thrust~495N

Why is that? even thou Kn decreases, so effective exhaust velocity drops, but since the surface area gets bigger, more fuel burns, thus mass flowrate increases, and overweights the decreased effective exhaust velocity. You might be surprised by my statement that thrust equals pressure times nozzle crosssection area, but now think about it. If you have a rocket chamber, because there is a nozzle at the back, the area at the top is bigger than at the bottom, and the difference is nozzle crosssection area itself, no matter the ID of the chamber. and pressure is force/area. This is also why divergent part in de laval nozzle produces more thrust, becouse there is extra area after the neck for residual pressure to push against.

 

Nice math Oinikis! That explains a lot.

[stix]

Lubricate your anus. Prepare to wear pink underwear.

 

Now, imagine having a nozzleless rocket with ID of 5cm, with a core 1cm by 30cm.

1cm -> Kn=120 -> surface area=94cm^2 -> nozzle area=0.785cm^2 -> chamber pressure~20bar -> thrust~157N (thrust = pressure * nozzle area)

3cm -> Kn=40 -> surface area=283cm^2 -> nozzle area=7.065cm^2 -> chamber pressure~7bar -> thrust~495N

Why is that? even thou Kn decreases, so effective exhaust velocity drops, but since the surface area gets bigger, more fuel burns, thus mass flowrate increases, and overweights the decreased effective exhaust velocity. You might be surprised by my statement that thrust equals pressure times nozzle crosssection area, but now think about it. If you have a rocket chamber, because there is a nozzle at the back, the area at the top is bigger than at the bottom, and the difference is nozzle crosssection area itself, no matter the ID of the chamber. and pressure is force/area. This is also why divergent part in de laval nozzle produces more thrust, becouse there is extra area after the neck for residual pressure to push against.

 

Nah!.. I disagree... What a crock!! - perhaps you guys (oinikis and dag's) can jump around the tabletop with glee, swinging gayfully arm in arm, wearing your frilly pink and rainbow coloured underwear whilst drinking pimms and pink champagne ... ok... all fine, there's nothing wrong with that, but there is no way I'm going to agree with what has been put forward above.

 

I guess I'll have to prove otherwise, which will sidetrack me from what I'm doing now, but nevertheless needs proving.

 

Luv you guys Cheers.

[dagabu]

LOL! You might want to but a 12 pack of those pink panties there Stix!

[stix]

I was just "taking the piss" - gee I come up with some crapp sometimes!!

 

Yeah, you and oinikis are likely correct and I'll just have to take your word for it. So, back on topic:

 

I guess my assumptions were from a "fixed nozzle" point of view because that's 99% of the motors I've ever made. Where low Kn's means low thrust, and high Kn's means high thrust. That concept is embedded in my brain and I can't seem to grasp looking at it from the nozzless view.

 

Reviewing some of the previous posts I'm still a bit confused, but I think I'll leave it at that for the moment.

 

Oh well, I better have a look on ebay then - 12 pack you say?

Edited March 2, 2015 by stix

[dagabu]

http://editorial.designtaxi.com/news-lingerie2304/7.jpg