Now that I'm back at school I've finally had a chance to mess around with those two excel spreadsheets (BatesCALC and U-Calc) that you had sent to me. They seem like they could be extremely useful but I just have a a couple of questions. To maximize thrust from your motor, do you necessarily have to maximize the pressure (and Kn max I think?) in your rocket motor? Say there are two motors that are each the same length but different inside diameters. If you mess around with the spreadsheet and set it up so each have the same Kn max, will both motors produce the same thrust even though one is bigger and contains more propelllant? And is it possible to make a motor with more thrust at lower Kn than at maximum Kn? Thanks for all the help!
Jimmy replies: Many things have been tried as substitutes for KNO3. Some can
be made to work, but have serious disadvantages - none are commonly used
as KNO3 is clearly the best oxidizer for sugar propellants.
Sodium nitrate can be used, and in theory could be even better. But that theory has not been realized in practice. It is more difficult to work, harder to make into propellant. The result is slow-burning, and very hygroscopic. So no one currently uses it, although I entertain the hope that it can be developed at some point. I've done a few tests with it, and some have worked OK, but not as good as with KNO3.
Ammonium nitrate has been tried. It is even worse than sodium nitrate, at least with sugar propellants.
The reason most chemicals won't work is because solid rocket propellant needs a good oxidizer. When nitrates and perchlorates are heated, they give off oxygen. Most substances do not. Nitrogen and chlorine bind loosely with oxygen, so it only takes a bit of heat to break the oxygen off, freeing it up to burn with the fuel.
That's how a solid propellant works. A fuel (like sugar) starts to burn, giving off heat. The oxidizer molecule next to it gets hot, releases it's oxygen, allowing more of the fuel to burn. That's how a propellant can pack a lot of self-contained fire in a tight casing. The oxygen is bound to other elements. I call it "powdered air."
Potassium chloride is the potassium equivalent of table salt, and is inert.
Magnesium oxide will not work. It contains oxygen, but tightly bound to the magnesium so the energy needed to break it free will be greater than the energy released by the fuel for a net loss of energy. Rocket propellant has to put out energy, not suck it up.
Perchlorates can be used in rocket propellants, but are not used with sugar. Ammonium perchlorate is used in professional rocketry, but it is bound with polybutadiene (essentially tire rubber) which serves as both fuel and binder.
Potassium perchlorate has been used with polybutadiene, but is not used much nowdays becasue ammonium perchlorate is more efficient and better-behaved.
Potassium perchlorate has been mixed with asphalt to make Galcit propellant,
not used much anymore because ammonium perchlorate/polybutadiene propellant
is much more effective.
Chlorates should be avoided at all cost! Potassium chlorate, sodium chlorate, and barium chlorate are very dangerous. Besides, they do not make good propellants. These are the chemicals people used to blow off their hands in years past, and where the term "basement bomber" came from.
Some pyrotechnists use them, but with extreme care. I hesitate to even mention chlorates because some fool might think I'm recommending them, try it, blow their eyes out, and claim that "Jimmy said chlorates are OK" when in fact I said over and over that they are BAD.
I will not converse with anyone I know or suspect
to be using chlorates, lest I be considered to blame when they lose body
I wrote a treatise on this last year: http://www.jamesyawn.com/kclo3/notes.html
OK, rant mode off. Just don't use chlorates. If you must, don't talk to me.
Potassium nitrate is the "right stuff" and well worth the trouble to obtain.
Back to the top
Question: How big a motor should I make?
Jimmy, how can I tell what a size a motor is that I may make or rather
would like to develop. Maybe this isn't a simple question as maybe I
need a way to measure thrust? I suppose nozzle diameter being one of
many variables? Maybe just the amount of propellant is how motors are
rated. I have been searching around for software to download to
calculate this. The "rasareo" software is pretty thorough but they go
by published motors and or historic motors, motors that are in production.
A couple of other questions must be answered before determining the size of the motor, mostly what kind of rocket do you want to launch and to what altitude?
By "kind of rocket" I mean it's mass, diameter and coefficient of drag. That will determine the altitude it will reach on a given amount of thrust.
Then you can consider that sugar propellant, in a good motor, will have an ISP of 130 or so. That means that each gram of propellant will generate 1.3 Newton-seconds of thrust.
With this input, you can reverse-engineer a flight using an altitude-prediction program like RocCAD, EZAlt, or RASAERO, to determine what total thrust will send such a rocket to the desired altitude. Divide the needed thrust (Newton-Seconds) by 1.3 to get the amount of sugar propellant needed (grams.)
Then you can start designing the motor so that it will include this much propellant and burn it in a timely manner.
There are many fine points to consider, but this should get you in the ballpark.
Example: Say we build a 3-inch diameter rocket weighing 4 lbs and we want to send to 2000 feet. Assume it has a COD of 0.5
I run the altitude prediction component of ROCCAD, input these values, and start with an arbitrary thrust of say, 200 Ns and a burn time of 1 second. I'll need 154grams of propellant to generate 200Ns, which is 5.43 oz, so I input that starting value.
Output tells me that this one will peak at 1420 feet, not quite high enough.
I re-run the sim and arbitrarily bump up the thrust to 250 Ns. That would require 192 grams of sugar propellant, which is fed to RocCAD as 6.7 oz. This puts apogee at 1966 feet.
So we are in the ballpark, and can start thinking about a motor size that will contain this much propellant and fit in this rocket.
I happen to know that a 38mm BATES grain contains about 50 grams of propellant. So four of them should give us 200 grams, which should give us 260Ns total thrust and send the rocket to 2080 feet.
I also happen to know that a 54mm grain contains about 195 grams of propellant. So a single-grain 54mm motor should do the trick.
I've taken a few shortcuts here, but this is just to get a first approximation so that we can tweak the design. It is also an inelegant way to determine the amount of propellant needed - perhaps I should modify EZAlt to work in this direction.
Let me know where this leaves you. No doubt I've missed a few bits and pieces, and will be happy to fill in the gaps.
Question: I see that APCP has been ruled out as an explosive and from what i have read that I could find (lol), it seems to be much easier to work with....My Q's are, What would be a good SIMPLE all around formula for APCP? Do u have to wrap the grains with an inhibitor for bates grain operation? I use scott' s bates grain calculator. Do u still need to keep the KN factor roughly under 200 with APCP such as with KNSU also?Answer: Folks are a little more protective of their APCP recipes than sugar propellant, so you might not be able to find everything for free on the web.
APCP certainly has its virtues, not the least of which is that it is declared "not explosive" at the moment. Sugar propellant is still iffy - I was hoping that the jury in the Megahed trial would declare sugar propellants "not explosive" and they sorta did, but not clearly enough to make me comfortable transporting the stuff w/o license. That one may have to go back to court, and I don't want to be the defendant!
AP also has its limititations and hazards, and should not be taken
on lightly. AP is toxic, for starters. So are many of the
additives. So you can't handle the propellant without
gloves. Fine metal powders must be handled carefully, especially
Mg. Face mask is usually required to prevent breathing of
toxic dusts, and a heavy-duty mixer is almost a necessity for mixing
the stuff up. AP propellants take time to cure... you can't dream
up an experiment in the morning and test it by noon like you can with
rcandy. Some require a hot box to cure, as they will not cure at
So when you say "It seems pretty easy..." place it in this context.
Best approaches for learning APCP are to get Terry McCreary's book
"Experimental Amateur Rocket Propellants" which describes the process in
detail. It's available from Loki at: www.lokiresearch.com
Or you could take a Thunderflame course from Tom Henderson (He's in SC - where are you in GA?) It costs a little bit, but Tom's course teaches how to make APCP, and after completing the course you can buy kits from him containing all the stuff you need to make propellant and to make it into rocket motor loads.
Alternatively, you could order one of the propellant-making kits from FireFox (www.firefox-fx.com) which includes instructions.
Kn ratios will differ. How they differ will depend on a lot of things - every type of motor you make will require a simulation or two to make sure it is in the "reasonable zone."
There are many recipes for APCP, some burn very fast, some very slow. So the Kn ratio for a given motor will depend on propellant burn rate and several other factors, including the motor size, casing strength, and propellant grain design.
You can get a copy of BurnSim off the web (www.burnsim.com) which lets you simulate thrust and pressure curves for motors. You can enter your own data, but it's kinda fun to play with the propellant parameters built into the program. You can try out different prospective motors, and see if they are likely to chuff, explode, or go like crazy.
For most motors, you will want to keep the inhibitor on the propellant grain(s). The BATES design requires an inhibitor. Moonburners do too. It is possible to use uninhibited grains too, but they are usually sub-optimal for flight motors.
The uninhibited grain makes for a strong burn, but of very short
duration. I use them for propellant tests and in model-rocket motors.
They are a good way to test new propellants or motor casings. But for
larger flight motors I stick to the BATES and moonburner designs.
Apologies for rattling on. Please let me know where this leaves you, I'll be happy to continue!
Answer: Please accept my apologies that I cannot offer an easy answer to this question. Rest assured that if I did, it would be all over my web page by now.Dear James.For several years I am following your experimental rocketry site with great interest.I want to have a 'sparking tail', but titanium is hard to get here (israel)do you think magnesium flakes would work? iron flakes? other metals?
I've tried Mg and a couple of other metals in sugar propellant, and some burn. But the effect is not dramatic. With Mg and Al dust, one gets a slightly brighter flame, but the difference is barely noticeable at night, not at all during daytime. Here's an example: www.jamesyawn.net/tests/1-06/index.html#5-21-06D1
I've tried Mg lathe turnings, and get a spark here and there. Again, if you were not looking for it, you would not notice it.
A friend of mine, Jonathan Carter, experimented a bit with iron and obtained a reddish flame. It's pretty, but it is not very bright.
Ti is the only thing I've found that makes folks go "Wow!"
I find that "flake" is the best form of Ti for sugar propellant.
The flake form of Ti is considered non-hazardous here in the US. Hopefully it is also considered non-hazardous there and thus might be obtainable if you can find someone who has it.
But a quick web search did not turn up any vendors in your country, I am sad to see.
There are many metals I have not tried. I have some copper powder and have not yet tried that... perhaps I will! A green sparkle would be nice, although it would probably not be very bright.
Problem is in the nature of KNO3. In reading Sutton and Biblarz, I searched for references to KNO3 in professional rocketry, and they are rare. The only mention was that it is used in some military missiles as a "flame suppressant." When burned with sugar, the major product of combustion is potassium carbonate. It's all that white smoke you see. And it is a military-grade flame suppressant. In fact, potassium carbonate is used in many dry-chemical fire extinguishers. It's really, really good at putting out fires. So a sugar motor is, strangely enough, an excellent fire extinguisher.
So you can put all kinds of color chemicals and flame effects in a sugar motor and get naught. The potassium carbonate snuffs it.
Ti is an exception only because the heavy particles are heated enough to burn and then thrown clear of the exhaust stream. In observing Ti/sugar burns, I note that the Ti burns outside the smoke stream, for the most part, with only a few burning inside of it. I believe this another advantage of Ti flake, as the flat flakes do not all fly in a straight line upon leaving the motor - many diverge from the ballistic path and encounter clean air outside of the fire extinguisher stream.
So my best recommendation is to stay the course and try to find a vendor for Ti over there. Failing that, one must enjoy the high thrust and beautiful smoke trail that sugar propellants love to provide.
I'll see if I can find my copper powder and give it a test. I'll let you know if I have any positive results.
Hope to hear from you!
Back to the top
Tight-Fitting Grain, Propellant Browning
Dear Mr. Yawn,
I have a number of comments and questions but I'd like to start with just two because I don't know how much time you have to answer questions from people you don't know.
I watched a series of videos you made showing how to make Rcandy using an electric skillet. In one of the episodes you put the finished grain into the cardboard tube (with the nozzle) and it seems to be a fairly snug fit. Since the grain was made from the same size tube along with the mold liner, wouldn't the grain fit loosely in the tube without a mold liner?
I made some Rcandy following your instructions and I noticed the batch started to go a bit darker - I guess it was 'carmelizing', so I quickly reduced the heat and continued on. Is this darkening something to avoid - does it have a large adverse effect on the Rcandy?
Thanks in advance for your response,
Dear Namewitheld: Because I hand-roll my own tubes, they vary somewhat in inside
diameter. The mold liner is thin, so it does not create a lot of leeway.
So I tend to choose a smallish tube to make the grain mold, but sometimes don't get it quite right. But no biggie - tight grains work fine as long as they will go into the casing. If a mold is too large, I can use another turn or two of mold liner to make it smaller.
I just looked at my movie again, and realize that I was using some commercially manufactured tubes at that time. They tend to be fairly consistent in size, but still vary a bit.
Browning slows the burn rate of the propellant. This is not necessarily a bad thing, as larger motors work better with slower propellant, and this stuff is pretty darned fast. But little motors need fast-burning propellant, so unless you are making large ones, browning should be avoided.
It is possible to rescue browned propellant by adding red iron oxide or some other burn rate enhancer. I use RIO routinely in my model rocket motors nowdays. It helps them ignite quickly and completely.
Thanks for writing... I'd love to know what kind of rocketry you are doing or plan to do.
Hope to hear from you soon,
A smoke grain is over cooked r-candy with a little RIO added is it not? I would like to fill the 54mm Loki with a smoke grain so I can follow it as you suggested. I have a few cardboard tubes that come with the Aerotech G motors that fit perfectly in the well. Would I need any O rings around this tube and would I need a fiber washer between the propellant and smoke charge?
Some kind of washer should be used to support the delay grain, to keep it from coming loose from its well and allowing gas blow-by. That said, in my 38mm Dr. Rocket motor tests I did not use a washer, and it worked fine... with a couple of exceptions, one of which was a premature ejection, likely caused by gas blow-by:
It sure made for a nice photo, don't you think?
Does your Loki head end closure have a threaded hole, closed with a brass screw and an O-ring on the "wrong" side? I've never quite figured out what that was for.
I use steel fender washers in my 38mm motors - they work well, and can be used many times. I'm sure a similar washer could be used in the 54mm casings.
I think the smoke grain is even more likely to cause problems with a solid header unless it is held in place. Several times I've had smoke grains get loose and cause trouble.
So I had Jonathan Carter turn me a 54mm head end closure with a 2 inch long smoke grain well and an internal snap ring groove. That way I can have positive retention of my loose-fitting smoke grain. It doesn't need O-rings because there is nowhere for the gases to escape forward. The grain is inhibited on the outside and the forward end, so a little gas leakage around it does not set other parts burning.
Hope this helps!
Dear Somone: I am sad to hear that KNO3 is "banned" in India, but suspect that it is more complex than that. There are a good number of fireworks manufacturers in India, most of which seem to be in the Sivakasi area. A complete ban would make the production of fireworks impossible. So I'm sure it is possible to obtain KNO3 there, somehow. The question is how difficult it would be. You may need a special license or permit to obtain or use it.
But sodium nitrate can be used, as you have observed. It has different
properties, which make it challenging to use in the recrystallization
process... it holds its water more closely, tales much longer to cook,
and makes for very slow-burning propellant.
I have successfully used NaNO3 with the sugar syrup process, described at:
Here is a motor I made using sugar syrup with NaNO3:
In sugar propellant, the ratio of sugar to oxidizer is not critical. For instance, my standard recipe is rather fuel-rich, and works very well. You could just substitute NaNO3 for the KNO3 and have a good propellant, perhaps even a little better as it would be less fuel-rich.
The molecular weight of KNO3 is 101.1, the weight for NaNO3 is 85.
So to do a direct replacement, you would need 84% as much NaNO3 in a given mixture as you would KNO3. In other words, if the formula calls for 100g of KNO3, you would only need 84g of NaNO3.
Using the "No Karo? No Problem!" formula, a good mix might be:
This would create the fuel-rich mixture I normally use, but may not be optimal for NaNO3 propellant since it burns so much more slowly. So I'd suggest using the original recipe of 100g NaNO3, 30 sucrose, 20 dextrose to balance the O/F ratio a little better.
A possible advantage of NaNO3 - it is a bit lighter for the amount of oxygen it supplies. So if the other issues can be worked out, it has the potential to make propellant with somewhat higher ISP than KNO3.
I hope this helps. Please keep me posted on your progress!
Hi there Jimmy.
First of all, I just love your site and have spent ages on in (my parents get really annoyed).
Secondly, I have just started making Rcandy and am having a problem with getting it to burn right.
When in the motor case it goes, phut--------phut-----phut--phut-Roar! Is it supposed to do this or am I doing something wrong?
And finally, with your 38mm Aerotech casing, Where did you get this from and is the nozzle reusable?
Keep up the great work
Tom: This behavior is normal with rcandy, but not desirable. And it
can be avoided, with attention to a few details.
It's called chuffing, and it happens when the propellant grain gets partially ignited but not fully. The motor oscillates between some pressure, no pressure, some pressure... then it goes like crazy.... most of the time. Sometimes it just chuffs itself out.
You can avoid this issue by any of several methods. I generally use more than one method in any given motor.
1. Head-end ignition. This is done with an electric igniter placed at the very head end of the propellant grain, as far from the nozzle as possible. This is to ensure that once any part of the grain gets lit, the rest ignitesvery shortly thereafter. It pretty much eliminates using fuse as an igniter. If you are lighting your motors with fuse... well that's the problem. They should be lit with an electric igniter placed at the head end.
That said, one can use fuse if certain precautions are taken to ensure that flame is carried forward to the head end of the grain immediately. I've done that on occasion by placing a length of black match in the core of the grain. Alternatively, to wrap the (uninhibited) grain in one turn of fuse paper often helps.
2. Catalyze the propellant. Using 1% red iron oxide makes the propellant much easier to ignite, and reduces chuffing greatly. I regularly use a little RIO in my small motors nowdays. Big motors don't need it, but little ones do.
Aerotech cases are available from a number of suppliers, including Aerotech, of all places!
.... but I had trouble just now finding anything except 29mm motors.
There are other places that carry these and compatible motors, like
Apogee is a very reliable supplier, I've used them many times.
My "Aerotech" motors were actually made by Dr. Rocket, since defunct. Dr. Rocket made Aerotech "clone" motor casings, which were every bit as good as the orignials. Rouse-Tech has taken up the torch and is making Aerotech-compatible "Monster" motors that are excellent. I'm now using Rouse-Tech motors for my propellant static tests since I blew up my Dr. Rocket 38/240 casing earlier this year.
Here is a link to Apogee's 38mm Rouse-Tech motors:
The nozzles can be obtained from RCS (www.rocketmotorparts.com) which is actually a part of Aerotech.
I suggest you download their order form, which lists all the parts and prices:
The nozzle to use for 38mm motors is # 01550, with a nozzle throat of 0.219 inch. This size will work with uninhibited grains in the 38-240 casing, with BATES grains in the 38/360 and 38/480 casings, and with moonburners in the 38/720 casing.
These are a heat-resistant plastic which can be used several times. They will erode a bit each time, but can be drilled out and used with a larger casing and/or a more aggressive-burning propellant configuration. Just be sure to calculate your Kn progressions and pressure levels before pushing the button. (I'll send you the software to do these sims if you need it.)
Please let me know where this leaves you. I'll be happy to pick up the ball at any point...
What nozzle sizes should I use with these motors? I'm not much for math anymore, and the software is confusing. Can you give me some hints?
Answer: I've made some assumptions, and generated a table that may
provide you a point of departure for these motors.
Assumptions are that you want the motor to ignite well, and the
pressure stay under 1000psi. Commercial aluminum motor cases can
handle 1000psi with a good safety margin.
That means an initial Kn ratio of 200 or more, and a maximum Kn of 250 or so.
Kn 250 results in a pressure of about 875 psi. I use Richard Nakka's "design table" for KNO3/sucrose which shows the relationship of Kn ratio to pressure for this propellant.
For the 38mm motors I use, the actual grain diameter ends up being 1.25 inches. That's because the case liner and inhibitor take up a bit of space in the case.
Grain diameter: 1.25 inches
Grain length: 2 inches
|Number of grains
||Core diameter||Nozzle throat diameter,
|Nozzle throat diameter,
|Initial Kn||Kn Max||Kn Final||Mass of each grain