Toaster-Oven Rcandy

For those not fortunate enough to have a conventional kitchen-type oven in a rocketry-safe spot, there is hope.

Modest-sized batches of rcandy can be made in an electric toaster-oven.  

(Flash forward... while this method works, the electric skillet method is much better, and the one I currently recommend.)
(But the skillet is not as good for keeping propellant at working temp, the toaster oven is excellent for that purpose.)

The Oven:  

Get one of the larger, "boxy" toaster ovens, capable of holding a pan 10 inches in diameter with some room to spare.  Mine has a space 11 inches wide by 10 inches deep.  The door is convex glass - it bows out in the middle, providing a bit more clearance to the round pie pan.


The oven is relatively inexpensive. $40 to $50.
It is easily relocated to a fire-safe spot
Cooking is quicker, often much quicker
Small batch is easily consolidated by hand


Uneven cooking requires more diligence
Poor diligence may result in dark, slow-burning propellant
Inaccurate temperature dial
Small batch size

The recipe below makes enough propellant for one F motor, or 5 to 6 model rocket sized "C" motors.


In addition to the equipment mentioned in the primary rcandy page, you will need:
  1. One "nine-inch" round glass pie pan* (actual diameter is 10 inches, including rim, bottom diameter is about 7 inches)
  2. Toaster-oven, large enough to hold this pie pan
  3. Oven thermometer(s)
  4. Long extension cord
  5. Camp stove, hotplate, or some other heating device
If you have not yet seen the main rcandy page, I recommend reading it before attempting to make this propellant - it offers much more detail, as well as safety instructions.
Why a "Pie Pan?" Because it fits the oven.  Also, it is what I have used to develop this recipe, so is known to work.  But an 8-inch square pan will also fit in this oven, and will have a larger surface area.  I will test that soon.

Why a glass pan?  Because glass doesn't warp.  These pans must be flat, and metal pans often warp or get bent.  Also because glass retains a lot of heat for the initial cooking that occurs when you dump the propellant liquid into the pan.

Problems encountered, and their solutions

Problem 1:  Greed

This means trying to make more propellant than your system can adequately handle.  It is very tempting to want to make more and more, but very naive to think one can "double up" a batch and it will still work.  

Recommendation:  DO NOT exceed the amounts listed below, at least on your first batch or two.  

Your oven just might handle more, but to assume that on the first batch borders upon the delusional.  You should err on the side of "too little" rather than "too much," as too little will still result in good propellant, just a few grams less.  Too much will result in a waste of time, materials, and perhaps use up all your patience.

Making this propellant is as much art as it is science.  It is challenging enough when things work well.  Your first batch may well fail.   It is better to start with a procedure that is known to work and follow it as closely as you can.  If it fails then, you can be assured that the difference is in your materials, equipment, or technique, not in the recipe itself.  Once it works for you, consider making your own improvements, and if they work please share them with me.  

Problem 2:  Inaccurate temperature dial

Every one of the toaster-ovens I have owned over the years have one feature in common:  the temperature dial is incorrect!  Some were pretty close, some were 'way off.  But all were wrong enough to make this process fail, perhaps dangerously so.  

For accurate temperature control, it is necessary to recalibrate the oven using a good thermometer, preferably two.

I recommend placing the thermometer in the middle of the oven, turning it on to a given dial temperature, say 300 degrees, and wait fifteen or twenty minutes.

Better yet, place one in the back and another in the front.  This will show you how (un)evenly your oven is heating.  At 300 degrees, mine is a good 25 degrees hotter in the back than in the front.  But that is part of Problem 3, described in detail below.

Once the temperature has stabilized, make note of the thermometer reading.  It will probably not be 300 degrees.  Ease the dial up or down to where you think 300 degrees will be, wait, observe, and keep adjusting until you get the oven to stabilize at 300 degrees.  Now make your own mark on the dial where 300 degrees actually is.  Do the same for 200 degrees and 250 degrees.  

If this is a new oven, please break it in first.  I made the mistake of marking the dial on a new oven the first time it was turned on.  Did a real fine job of it, too, getting the thermometer to the exact temperature then making an ugly black mark on the pristine silvery dial facing.  On the second heating, the temperature was different!  On third heating, still different, but the same as the second.  It finally occurred to me that some components tend to "burn in" and stabilize with a little usage.  This could be why the temperature dial is always wrong.  I recommend toasting a bagel or two before calibrating your oven.  

Problem 3:  Uneven Heating

Small electric ovens are notorious for heating unevenly.  The smaller the oven, the less even the heat.  

They will usually be hotter in the back or in the middle than in the front.  

And beware the heating element right underneath the pan - it will scorch things that are directly over it.

Solution:  Put things in the way.  My oven has a baking pan that can be used as a drip-tray.  It slides into runners right above the heating elements, blocking the radiant heat and diffusing the convected heat.  This improves the heat distribution dramatically.

A previous oven did not have this drip-tray feature.  I found a couple of ceramic tiles that fit nicely, and placed the candy pan on top of them.  That helped a lot.  Even a sheet of aluminum foil on the tray should help, perhaps to place foil in a hot area and leave the cooler areas unfoiled.

If you use this oven outside, you may need to arrange for a wind-screen on windy days.  The doors generally do not seal tightly, there are vents at the bottom and probably leaks at the joints, such that on gusty days the poor little oven may never get up to temp.  A sheet of aluminum flashing might be wrapped around it to keep the wind from blowing straight through.

Oven Settings

Notice the top heating element in your oven.  It is dangerous, and should not be turned on when cooking rcandy.  Most ovens will have a Bake/Roast setting where only the lower elements are turned on.  "Broil" means to turn the top element on.  So does "Toast."  "Preheat" turns on all the elements.  So does "Clean"

Some ovens turn on the top heating element regardless of the setting.  If your oven does, then you might need to remove that element while cooking rcandy.  If in doubt, turn on the oven and observe the heating elements.  Most will glow dull red when they have been on for a minute or more.  It should not be difficult to tell when the upper elements are on.  Hopefully yours will have a setting in which they never come on.

To determine if the top element is on at a given setting, I recommend the following test:

Start with a cool, (empty!) oven.  DO NOT do this while there is propellant in the oven.
Set the dial to whatever you want to test, like "Bake/Roast" or "Keep Warm"
Turn the oven on to 300 degrees (your mark)
Wait about 1 minute
Take a long, thin stick, like a bamboo skewer, and hold it briefly to one of the upper heating elements.

If the skewer starts smoking and turns black at the point of contact, that element is on!  And to think it took a rocket scientist to figure that one out.

Using this technique, I discovered that on my oven the "Bake/Roast" setting turns the top elements on as well as the lower ones.  I did not think so, because they do not get quite as hot as the lower elements, and do not glow red.  

Level the oven

Another problem occurs when the oven is not exactly level.  Fortunately, these little ovens can be leveled-up rather easily by placing something under the lower corners.  What counts, though, is the levelness of the rack upon which the rcandy pan sits, and it may not be square in the oven.  You might try placing a nearly-full pan of water in the oven and noting to which direction it "leans."  Shim the oven's feet until it the water is level.

The reason this is a problem is that the propellant solution will dry more quickly on the shallow end, and may turn brown before the deep end is even close to dry.  

So have some leveling devices available.  I found a stack of  3x5 index cards on my desk, which work fine.  This will suffice until a more permanent leveling setup can be installed.

Rotate the Pan

This is a good trick for either of the two uneven-heating problems.  No matter how diligent you are about leveling the oven and distributing the heat, the pan will invariably cook a little faster on one side than on the other.  Wait until it is solid bubbles all over - don't do this while there is liquid in the pan, or else the formation of bubbles will be disturbed and you won't get flakes and it will never dry properly.

Keep the door shut!

<mom mode on>
A sure sign of a novice cook:  Opens the door, stares for a minute or two, goes and gets a knife and takes a sample, rolls it into a ball, presses it on the counter, whistles Dixie, and then thinks to shut the oven door.  Duh!  Now the oven has to heat itself up again, and the prospect of a good batch is diminished.  When you must open the oven door, do it right:  figure out what you are going to do, open the door, do it, and close it quickly and gently.

Remember the old adage:  "Hot air rises"?  I thought you might.  When the oven is closed and cooking, the elements are cranking out heat and the heated air circulates up and around the substance being cooked.  That is how our propellant liquid gets heated in this process.  Yes, radiant heat is used in some forms of cooking, where it is allowed to transfer directly to the food.  But not here.  Remember that we went to some trouble to block the radiant heat, as it is likely to scorch the mixture.

When you open the door, the hot air starts to move.  Slowly at first, but as time passes the momentum builds and a draft forms.  The draft draws cool air in at the bottom of the oven as the hot air rushes out the top.  It takes about 15 seconds for all of the heated air to leave a kitchen-sized oven.  Then it has to start over with a new load of room-temperature air, and the stuff you are cooking cools down.  Your toaster oven will clear even more quickly, so keep the door closed a much as possible.

Oh, and don't slam the door either.  Hard on the nerves, hard on the hardware, and it can collapse the little bubbles needed to get the propellant to dry.

The Setup:

Here is the setup, on the picnic table in my screen house

Setup on Picnic Table

The pie pan is heating in the oven, and the Coleman stove is getting fired up... literally!

Don't worry... the propellant mix is non-flammable at this point, and the leaping flame will settle into a nice blue ring before I put the kettle on.

The toaster-oven is already turned on and heating the baking pan that is inside it.  I will level it up before putting the propellant liquid in.

Preheating the oven and the baking pan speeds the baking and reduces the failure rate.

The Recipe:

30 grams potassium nitrate
15 grams sucrose (cane sugar, beet sugar)
6 grams corn syrup
4 teaspoons water

Place the glass pie pan in the toaster-oven, and pre-heat it to 300 degrees while performing the following steps.

Add Corn Syrup  Add Water

Weigh-out the KNO3, sugar, and corn syrup, combine with the water in a saucepan.  

Bring ingredients to a boil   Out to the picnic table!  

Place the pan over medium heat and bring to a boil.  All solids should have dissolved by the time a boil is reached.  If not, stir it some.  If still not dissolved, add another teaspoon of water.  

Pour syrup into hot pan   Place pan in oven    Note the "leveling device" on the oven.

Remove pie pan from the oven, pour the boiling liquid into it.  
(Close the door while you do the pour - keep the oven hot!)

Rotate the pan 1/2 turn before putting it back into the oven - that way if one side is hotter than the other (which is likely) it will tend to even things out somewhat.

Return the pan to the oven as quickly as possible.  Don't let it cool down any more than necessary.

Now watch.... don't wander off.  This process requires your attention, especially the first few times.  

     Note the two thermometers.  My oven runs about 20 degrees hotter in the back than in the front.

This pan is ready to turn down!

After X minutes, a lot of bubbling and some drying will have occurred.  If one side starts to turn yellow or brown before the other, turn the pan halfway around to even the cooking.

At XX minutes, the whole surface will be white in color, and the "wet spots" will be gone.  Reduce the oven temperature to 200 degrees.  (Your own 200 degree mark, set by using the oven thermometer.)

Cook the propellant another XX minutes at 200 degrees.  This is about how long it will take to dry the propellant.

Time to take a sample and do the "snap test"  (see the Rcandy full recipe for photos)

Snap Test:  Take a tiny bit of propellant from the lighter side of the pan.  Roll it into a pea* and press it down on a cool, dry surface.  Give it 30 seconds to cool, and bend it in half.  If it snaps cleanly, the propellant is done.  If it bends a bit but is firm, that is OK too.  But if it forms a mushy ball or makes a floppy flake, then throw the sample back and let the propellant cook some more.

*If it won't roll into a pea, it is either too soft and turns to slime, or too dry where it turns to crumbs.  If too wet, cook it some more.  If too dry, remove immediately and process, adding more corn syrup if necessary.

When the propellant passes the snap test, put on your protective equipment (at least a face shield) and take the pan out.  Scrape up some of the flakes and press them down with the back of a tablespoon.  Scrape them up, press them down.  Do this until they consolidate into a cohesive mass with a clay-like or putty-like consistency.  Hopefully that will happen easily.

 If they refuse to consolidate, reheat to 200 degrees and try again.  If the flakes still refuse then the batch may be too dry.  Try adding 1/4 tsp. corn syrup, heating to 200 degrees and consolidating it again.  
Hey!  What's all this "XX" time stuff?  

It's my way to make sure you know that any numbers I give are going to be wrong. Your oven will cook differently from mine. You will be at a different elevation, at a different humidity, using different brands of ingredients....  

It is much better to observe, look for cues, and develop your own time frame.

Even then, you will find that the cooking times vary.  I have had batches that were overdone in 20 minutes, others that took over an hour.  They average about 45 minutes but remember these are ballpark figures.   My mileage varies and so will yours.  Cook until done.

That said, I can tell you that my oven takes about 10 minutes to cook this small batch dry, but that varies from batch to batch.  There is no substitute for experience here!

This batch is done!  

The little scrape you see in the pan is where I removed a bit to do the snap-test, and it snapped crisply showing that the propellant is dry enogh to be solid.

Now I will do the scrape-and-mash test, which will demonstrate whether or not it is moist enough to adhere to itself and make a "putty."  If too dry, add a few drops of corn syrup, reheat at 200 degrees and knead again.  That will do it.

I recommend using a non-metallic scraper, such as this wooden spatula, to avoid the possibility of creating sparks.



After the first mixing and partial cooling to 200 degrees, this propellant often assumes a "grainy" texture, somewhat stiff and crumbly, sort of like a praline.

All it needs it to be stirred again.  Knead with the spoon and the pleasant putty-like texture returns.  

After this second stirring, the grainy texture does not usually recur.

Test 2:  burn rate

I take out a little glob, about 1/4 teaspoon, and return the pan to the oven, now set at 200 degrees.  Let the glob cool briefly.  Touch it lightly to see if it is sticky.  If so, let it sit another 15 seconds or so.  When it is cool enough to handle without sticking to your fingers, roll it into a little rod one inch long.  It should be roughly 1/4 inch in diameter, but that is not critical.

Test for burn rate by igniting your test strand and timing the burn.  Lighter colored batches will burn continuously from one end to the other.  The batch illustrated on this pae burned at 8 seconds per inch, which is about as fast as it gets with uncatalyzed propellant.

If the propellant is overcooked and turns dark, it may not burn continuously from one end to the other.  I cal this "not continuous. "
I rate non-continuous batches two ways:

1.  If the propellant burns for a second or two or three and then goes out, I call it "Not quite continuous."

2.  If it goes out the instant the heat source is removed, I call it "Not at all continuous."

Slow-burning or non-continuous propellant not necessarily a failure - it is fine for larger rocket motors, requiring a little more effort to get it ignited.  But slow propellant does not work well in small low-KN motors such at the Model Rocket kind.  Even so, if you have some red iron oxide available, slow propellant can be salvaged and made usable for the smallest of motors.  

I find that 1 percent (0.5 gram) of Red Iron Oxide (Fe2O3) speeds up the burn rate dramatically.  Iron oxide and other burn rate enhancers should be added to the propellant after it is cooked, kneaded in thoroughly while it is warm and soft. This is preferably done after it has been consolidated into a cake, to reduce the risk in case of accidental ignition.  

Jimmy Yawn
rev 11/3/06
Recrystallized Rocketry