Sugar rocket pressure problem

[dagabu]

BTW- I found that copy paper and 5 minute epoxy made for the best inhibitor in my R-candy trials. It did the BEST job at keeping the heat away from the casing over every material commonly available. The only better material was asbestos gasket material.

[MrB]

Sadly, B, this is not the case with BATES grains, they are progressive, the surface does not remain the same throughout the burn though 'rod and tube' and 'star' designs do offer relatively neutral thrust curves. This is why the BATES grains with a progressive burn is so desired in model rocketry but doesn't have the initial thrust to launch a ball shell on a pyro rocket.

 

Yeah, with the inside only, on fire, the surface area only increases, and the burn speeds up. But this is actually desired in amateur rocketry? In that case the design choice is self explanatory. More shit i didn't know. Here comes another stupid question. WHY is this a desired trait? A good kick to get of the launchpad, with enough thrust to instantly be stable in flight has a lot of advantages which i'd see translate in to amateur rocketry just as well. Structural reasons? I don't think so much about the materials needed to withstand the heat of an all round burning grain, but rather whats needed to counter the G-forces on launch.

 

Star, square rod, with and without hole i've seen used in improvised rocketry, but were not talking amateur kinds. Nasty devices.Most designs are intended to give a huge kick on initial firing, to get the darn thing going, and then just enough thrust to keep accelerating for a while after that.

 

Anyway, i suspect i'm screwing up the thread. Sorry. I'll try to stop.

B!

[dagabu]

The arrow wobble effect:

 

https://www.wired.com/2016/08/physics-archery/

 

When a rocket, arrow etc. are hit with a lot of energy from behind, the "push" distorts the object. As inertia is overcome, the whole item does not accelerate at the same time, the length of it "catches up" as the energy is pushing it up.

 

Using a progressive motor allows for a slow rise initially and speeds up as the rocket lifts off the pad.

 

Yes, if you are launching a propane tank full of HE, a "kick" is fine but to launch a 50:1 amateur rocket, a slow ramp up is preferred. I ended up with a star shape in my grains for a flat curve, the acceleration was balanced by the thrust of the motor. Balance the thrust with the lifted weight and you got gold!

[MrB]

So, if i understand you right... The bodyshell flexing and breaking apart is the reason hobby rockets launch "more gently" (weird choice of word for a rocket launch, but still) and it's been determined through trial and error i suppose, that the lighter body shell returns a higher altitude, then the increased efficiency, and stability of a harder launch, and a more rigid body?

I mean... I don't for a second think it's the cost that is prohibitive. I've seen a few of the rockets you "amateur" rocketry guys fly. Hardly on the cheap side.

 

Anyway, thank you for sharing. Doubt it's information i'll ever use, but learning something new is always nice.

B!

[dagabu]

"...that the lighter body shell returns a higher altitude, then the increased efficiency, and stability of a harder launch, and a more rigid body?"

 

Not sure what that means.

[stix]

"BAllistic Test and Evaluation System" - well you do learning something everyday Dave. Unless of course, perhaps I've just forgotten - it's been a while and a few many beers later.

 

It was my understanding that a "true" bates grain is inhibited only on the outer cylinder, leaving the core and both ends exposed. This configuration theoretically gives a reasonably even burn profile: ie. whilst the internal core gets wider, the length is also reduced. Kn ratios and pressure should remain relatively flat.

 

I've actually never tested a true bates grain myself simply for the reasons pointed out above. I prefer my rockets to take off more gracefully then increase in speed. My standard method was to inhibit both ends as well as the outer, therefore having an exponential thrust curve. You can see this in the attached graph.

 

http://i.imgur.com/Dm4HTyK.jpg

 

Looking at the graph above, you can assume that if the grain/s were wider in diameter then at some point CATO is inevitable. It's also worth pointing out that where you see the "dashed black line" the Cursor info at the top shows .5 secs and there is approx. 6 G's of force, which may be enough for stability on it's own but probably not with added rocket body weight.

 

This particular motor configuration has only been tested on a test stand - never in an actual rocket, but the data suggests that if the motor was inserted into an actual rocket, and with the added weight of the rocket body, there may not have been enough thrust and speed at launch for stability, therefore it probably would have slowly arced over, then with ever increasing acceleration, rapidly disappeared on its journey to its unplanned destination. Whoops!

 

So I'm thinking my next motor design will have the nozzle end of the grain un-inhibited. This should give more power at take-off with less of an aggressive profile curve as shown above.

 

-----

 

@MrB

 

"the lighter body shell returns a higher altitude"

 

Well, if I understand that correctly, the short answer is NO, if dealing with sub-sonic and low altitude rockets where wind drag has a big influence.

 

Look at it this way, an extreme example is to swing around a ping-pong ball on the end of a piece of string then let it go. How far will it travel?

Do the same with a lead ball of the same diameter and same length of string - it will go further. Sure, you will need more power to get the lead ball moving, but it will go further.

 

It's a trade-off. Wind resistance/drag, affects a lighter object more than a heavier one. Therefore after the thrust/power phase, the rocket is now only subjected to gravity and drag (unless we are going into orbit) then we have the "coasting" un-powered phase.

 

There are some software programs out there that can easily confirm this - I'm still working on that part (altitude calculations) with my own software - perhaps in Ver 2.0?.

 

There is a point where adding extra weight can actually be beneficial to the overall altitude reached. If altitude is your goal.

 

Cheers.

Edited March 20, 2017 by stix

[dagabu]

@Stix,

 

Agreed! Your thrust curve is progressive and if the overall rocket is light enough to be lifted (in your case, 500g or less) then this would work fine for your needs. The rockets we were lifting came in at 2.7 to 4 kilos so we needed around 8 kilos of thrust to get things moving. The flat curve we used gave us the OOMPH we needed of the pad then accelerated to coast height.

 

Many ways to skin a cat.

[MrB]

There is a point where adding extra weight can actually be beneficial to the overall altitude reached. If altitude is your goal.

 

That far i was already. What i haven't "figured out" is why amateur rocketry goes the lighter, "slow" launch route rather then the heavy handed approach. As i wrote above i guess trial and error has shown that a given amount of money gets a better result this way, even if it technically might be advantageous to go with a heavier, faster launching unit, given that someone else with unlimited funding pays for the whole experience.

 

Anyway, i've derailed the thread enough. I'm not going to get a deeper understanding, and it's ok.

B!

[dagabu]

Hmmm, "...the lighter "slow " launch route..." is actually a physical integrity and acceleration issue much more than it is a choice for rocketry. If you have a slim, steel cased shoulder fired rocket, it may be able to be pushed very hard out of the tube with no deformation and since they only weigh a few pounds, there is very little resisting the acceleration so the rocket can be moved from a stand still to its top velocity in a very short period of time.

 

If you have a hand laid tube with imperfect walls, pre-made thin wall plastic or aluminum pipe with slip-fittings, using a 100:1 thrust to weight ratio (http://wichitabuggywhip.com/fireworks/rockets/whistletests.html) will crumple the walls or set up a wobble that will break the tube in flight. Put that thrust to weight ratio on a multi million pound rocket and you get a very slow ascent.

[FlaMtnBkr]

I think Dag answered this above with his wobbling arrow effect reference.

 

A higher initial thrust and acceleration can cause forces and vibrations that can stress the structural integrity of the rocket which surely can effect flight path, cause damage, or even cause the rocket to come apart in flight.

 

I would imagine a more gentle and even acceleration is preferred for most things that want to be kept safe and whole, and when trying to maximize lifespan, except when trying to make advancements by pushing boundaries or when speed, competition, and pride get involved.

 

At least that is what I took away from his comment and what I would expect to be true.

[dagabu]

Pretty much...

 

What we found is that all things are NOT scalable. I found a tube that EXACTLY matched the bottom (main) stage in a 100:1 (396" to 3.9") in both the thickness and height, we then found the other tubes made from composites and discovered that the 1, 746,000 pounds of thrust the Saturn V use in the main booster would equate to 17,460 pounds of thrust for a 3.63 foot tall rocket.

 

ABSURD! Even 1000th of the thrust would be too much at 1746 pounds of thrust for a 10 pound rocket.

[NeighborJ]

I would think that to scale that rocket booster properly the operating pressure would also need to be included in the scaling down. Unfortunately fuel does not burn at a linear rate and has a specific, optimum pessure range it likes to burn.

[dagabu]

For sure!

[FlaMtnBkr]

Also, it sounds like there was an attempt at scaling of the linear dimensions which doesn't account for the exponential increase in volume and therefore an exponential increase in fuel weight.

 

I bet if instead the rocket was scaled down in relation to fuel weight, the corresponding scaled thrust would be much more feasible or realistic.

 

I would throw out the numbers but what I'm finding for data on Saturn V isn't jiving with the above numbers for the rocket.

[dagabu]

The numbers I got were from Wikipedia, you will have to argue the point with them...

[NeighborJ]

The Saturn V rocket having variable thrust liquid fueled engines would have very different results, being able to compensate it's thrust to achieve an optimal power to weight ratio at any size scale. Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit. My head is spinning trying to understand what this would mean. Basically I would expect there to be a lot shorter burn time so I couldn't expect it to go nearly as high.

[stix]

. . . Still time would still need to be scaled down along with the rocket otherwise one could expect a scaled down rocket to achieve orbit . . .

 

Yeah NJ that does seem to be the case.

 

The "scaling theory" was fleshed out a bit in this thread.

http://www.amateurpyro.com/forums/topic/11653-space-shuttle-srbs-fuel-burn-rate/

 

Although, that thread was more about scaling UP from a smaller motor, but the issue is the same as you suggest, as in time would need to be scaled as well. The only way of doing that (when scaling up) would be to add a burn rate modifier.

[NeighborJ]

I didn't read the whole thread but it all makes sense and I can see other issues. If the large rocket motors lift off with a minimal amount of g force to protect the passengers and payload then a scaled motor would lift even slower. Without any guidance system to stabilize it, then it would be almost impossible to achieve a stable flight. Essentially a smaller motor would require tuning to achieve an equal amount of g force as the big ones or have a scaled down amount of gravity and crosswind.

It is simply easier to design a motor for a specific application. I never had calculus so I wouldn't know where to start with the math but I do understand the concepts and theories which are all pointing to the impossibility of creating an exact scaled replica of a large rocket.

I enjoy these kinds of threads, they tend to open my mind to new ideas and brush the cobwebs out of my head.

[stix]

. . . It is simply easier to design a motor for a specific application . . .

 

Yes NJ, that is one way.

 

The other way/method is to design a motor, test it, evaluate the data, then extrapolate that data and apply it to your new design and then predict the outcome. "Outcome" being very subjective. When scaling up using small motors (approx. 20mm - 40mm ID) this method does seems to hold true for this size. As we go bigger then the fuel burn rate and pressure will come into play more.

 

What I'm talking about is "scaling up".

 

If you're into mucking about with various fuels, grain geometry, nozzle designs (lots of fun), then it makes sense from a cost and time point of view to start with smaller designs, then go bigger.

 

kramrocket has done some good tests and my effort will be to show the results in a way that can easily be understood.

 

Sorry that this thread has gone off too far a tangent. I'll try my best to get this together over the next week and post a new topic for those who may be interested.

 

Cheers.

[JMan]

Sorry I was away for spring break and came back to a lot of work I don't really mind the tangent it carries a lot of stuff to be learned and interesting topics but I tried to make a new batch today and over cooked it bummer.

 

But talking about the scaling the burn rate is measured by a linier length (radius) so it would scale down porportionally assuming the pressure stays the same (it won't for the following reasons) the pressure/thrust is measured in surface area of burning propellant so it will be scaled by an exponent of 2. This includes pressure and thrust which is why your Saturn v calculation was so much higher it's not 1/2 rather 1/4 (just 1/2 squared). Weight is measured by volume scaling it down would be by an exponent of 3 so although burn rate decreases directly and pressure decreases by a factor of x to the -2 your new weight now weight x to the -3 your 100:1 rocket will now weigh 1000000:1 (100 squared it 1000000 right?). Then you all talk about keeping it stable with the low thrust to weight well that is no longer a thing, in fact its thrust is much higher (i think at least I'm sitting in bed not doing math right now) but smaller objects are much stronger porportionally.

 

It's kind of hard to explain and I doubt you want to read much more but vsauce 3 has a good video on it (could Godzilla exist) it describes how getting larger makes things not as strong and vise versa

[JMan]

Also a new question but has anyone tried dehydrating their rocket fuel rather thank cooking it? I'm thinking that the water would be driven out much more throughly than cooking it and it not only would help the burn but also decrease the weight slightly.

 

i was thinking of trying to cook most of it off to where it's an apple sauce consistency then pour that into my rocket, dehydrate it and pour some more (assuming it shrunk) and repeat untill its full of perfectly dry and air bubble-less fuel perfectly conformed to my rocket case

[NeighborJ]

JMan I tried that same experiment with less than satisfactory results. You may have better luck but mine took weeks to dry over a space heater and the KNO3 was recrystalizing on the surface. The resulting fuel was very slow.

Ideally it can be made without adding water and with much stirring if you have a skillet with exceptional temperature control but it is very easy to ignite the whole batch if the skillet heats unevenly or overshoots it's setpoint. I've read a thread here somewhere of someone granulating cooked fuel then pressing it into a motor and it was claimed to have given good, hot, consistent results.

I think at some point anyone who has made sugar rockets looks for an easier casting procedure but there are few safe options without changing the type of sugar used or highly precise equipment.

[JMan]

Yah that's how I started by melting the sugar and mixing the kno3 but the dissolving method seems to work best and actually easiest I think but right now I have a simple radiater (water of course no flame or sparks) that my finished castings sit atop to stay dry I live in Ohio and weather is quite unpredictable (is was 80 wendsday and Thursday night Friday morning it snowed?!?) so that radatier keeps any moisture away rather than a zip lock bag

 

I tried zip lock bags and even the trunk of my car and, contrary to popular belief my hummad hot trunk stores the casting better than a bag sitting in the house? I'm seeing if maybe this radatier drives out even more moisture than cooking and sealing. I'll keep you updated but I just can't seem to get my batch down after break the test burns boil before ignition(too much water) yet when I cook it slightly more they crumble like chips ahoy cookies (surprisingly similar actually) so I packed it while it was in the playdough-ish phase maybe a little dryer (peanut butter like idk how to describe them in technicals terms) and packed and cored and left to radiate

[dagabu]

Also a new question but has anyone tried dehydrating their rocket fuel rather thank cooking it? I'm thinking that the water would be driven out much more throughly than cooking it and it not only would help the burn but also decrease the weight slightly...

 

JMan,

 

That is a great question but think about other answers. I see the use of heat to dehydrate the fuel but I offer this instead.

 

Vacuum. I have a couple vacuum pumps I could use to evacuate the mixed liquids to precipitate out the finished fuel. Heat would certainly make the process much faster and much safer IF the crystals dont drop out independently.

 

I theorize that the same result will happen as when drying it out over heat over time, I think its the stirring that keeps the KNO3 and Sucrose in a homogeneous mix.

[Redrocketman]

Couple of points. The core diameter MUST always be larger than the nozzle diameter, or erosive burn will occur, and the core itself will be acting as the nozzle, not the nozzle!! Sounds funny but it's true. Personally I strongly advise against threading PVC pipe when putting it under such extreme pressure and heat fluctuations- the 2 main enemies of PVC. It's only creating a weak spot in an already venerable motor design. I'm not saying it to take the piss, I soley make PVC motors, from a 3/4" G-160 to the rather scary K-550, soon to be tweaked to a K-1100!! But, all possible avenues to lower the risk of a failure are taken and still they sometimes happen, don't add another weak spot!!