Erythritol is another of the polyols, or sugar alcohols. One of the more recent of the naturally occurring sugars to be processed and sold as a low calorie sucrose substitute.
Having one of the two production facilities in the world within two hours of me tweaked my interest. While it is expensive at nearly $5 a pound, I think a wholesale purchase in large quantities may be possible, making the cost within reason. The best price I have found is at http://www.store.honeyvillegrain.com .
Update August 24, 2005:
After a few calls and e-mails, I finally got an account set up with Cargill so I could buy my erythritol in bulk. The nice part is, the plant being so close to me, it's only one day shipping. I ordered a 50 pound bag on Monday afternoon, and it shipped out the next day, so I had it on Wednesday. At $3.50 a pound it's still not cheap, but I think well worth it.
Small amounts can be purchased from Honeyville Grain for about $4.75 a pound plus a minimal shipping charge. If someone would like larger amounts, say 10 pounds or more. I'd be willing to order it and send it out to you. (End Update)
Early tests showed the propellant was a bit difficult to get ignited and to bring up to pressure. Most of these early tests showed a very low Isp, but that was mainly due to the fact the motor would burn for extended periods of time before producing any recordable thrust. As I increased the Kn, the Isp's started climbing, but the motors still burned for some time before fully coming up to pressure. I tried several additives to increase the burn rate and sensitivity, namely; iron oxide, sulfur and charcoal. They all helped to a degree, but they also increased the burn rate, and I really wanted to keep the burn rates down for large motors.
In Static Test 103 I used a hot pyrogen mix painted on the grains burning surfaces to facilitate more rapid ignition. I was a little concerned about creating an initial high pressure spike in the motor from the added pyrogen, but that was not the case. In fact, I saw no pressure spike at all. What did happen, was the pyrogen simply got the grains burning faster, exactly what I wanted. The result was a nice burn and an Isp of nearly 127 seconds.
As for the mixing and casting properties, it's a lot like the xylitol was, easy to melt, the ER perhaps didn't have quite the stickiness of XY, but it will still require a heavier and/or coated casting tube liner. One noticeable difference was the set up time, the ER was fully hardened as soon as it cooled. It was brittle to a degree, but no more or less than sucrose. I'm seeing less shrinkage as it cools than with xylitol or sucrose too. I'm using a steel coring rod covered in aluminum foil for small motors, pulling the coring rod at 20 minutes after the pour. This is working very well as the coring rods slide right out, in many cases the aluminum foil stays intact and comes out with the rod. For larger cores, I'm using Delrin rod. The Delrin rod expands when the heated propellant comes in contact with the rod, the propellant sets up faster than the rod contracts as it cools, allowing the coring rod to slide out with no force at all. But you must wait for the grain/rod to cool completely before attempting to remove the Delrin rod. Frankly, if I had Delrin rod in smaller diameters I'd use it for all my casting, large and small motors. It works that well.
The burn rates seem to be in line with the XY propellant, actually a little slower burn rate it seems. If these early numbers hold up; KN/ER propellant looks to burn at less than half the speed of KN/SU, sets up as fast as KN/SU and has better melting characteristics than KN/SU. My early tests have confirmed what I read, erythritol is virtually non-hygroscopic. I've left propellant strands out in July (Iowa) humidity with no apparent absorption of moisture. Very interesting and it seems promising at this point!
Here is a link to Cerestar, which has some very informative pages on the properties of erythritol.
This is a document on casting KNER grains for the I class motor, in PDF format.
This is a document on KNER ignition, in PDF format.
Here is a rather long, low quality video on casting KNER grains for the I motor.
This is a short video of a small block of KNER burning in open air.
Molecular-Formula: C4H10O4
Molecular Weight: 122.12
Melting Point: 249.8 degrees (F)
CAS Registry Number: 149-32-6
I finally got some help finding the heat of formation of erythritol, thanks to buddy Luke M. we now know it's -1732 calories per gram. So to put erythritol into Propep, or in my case Chem 2, here's the line:
Erythritol 4C 10H 4O -1732 .0524
After running Chem 2 at 1,000 psi here are the theoretical values:
Density: 0.066 pounds/cubic inch
Temperature: 1933 degrees F.
Gamma: 1.1531
C*: 2745.78 f/s
Isp: 138 seconds (frozen)
From the Cerestar Web Site, the hyrgroscopicty of various sugars.
Here is a list of the launch/static tests performed with erythritol so far, starting with most recent first:
Launch Test 112: SBS 1350 "M" Class motor in the Prelude Rocket
Static Test 109: "O" Class SBS-6250 First Test
Static Tetst 108: KN/ER with 5% Mg
Static Test 106: KN/ER Seconc Test of the SBS-1350 "M" Class Motor
Static Test 105: KN/ER "M" Class Motor Isp 131 with over 6 Second Burn
Static Test 104: KN/ER "K" Class Motor; It Just Keeps Getting Better
Static Test 103: KN/ER with ignition aid, Great Results
Static Test 99: KN/ER with 3% Sulfur
Static Test 98: KN/ER with .2% Fe2O3
Static Test 89: A test of KN/ER with 1% Charcoal added.
Static Test 88: Third test of KN/ER Propellant.
Static Test 87: Second test of KN/ER Propellant
Static Test 86: First test of erythritol based propellant.
| Test# | Length | Start Pressure | End Pressure | Aver. Pressure | Time | Burn Rate | Density |
| ER-1 | 1" | 192.3 | 375.0 | 283.65 | 7.734 | .1293"/sec. | |
| ER-3 | 1" | 519.2 | 971.2 | 745.2 | 3.817 | .2619"/sec. | |
| ER-2 | 1" | 716 | 1139 | 932.5 | 3.384 | .2955"/sec. | |
After a half dozen or so static tests, I've come up with some new numbers that come pretty close to how the propellant behaves in a motor, a= 0.0037, n= 0.64 . Again, don't take these numbers as absoulte. So many things play a role in the burn charicteristics of a propellant, you'll have to determine burn rates on your own, using the chemicals and process that suits you.
First Test Batch:
My first test batch was only 10 grams. I wanted to see its melting characteristics and do a quick check of its set up time and burn characteristics. I used granular KNO3 and granular erythritol, melting the mixture at a setting of 275 degrees F. The mixture quickly melted much the same as the xylitol had. I spooned the hot mix onto my counter top where it quickly cooled to a pure white, brittle solid.
I tried burning a strand in the open air. It would start burning, then go out.
So I decided to try another larger batch using powdered KNO3 and granular ER (erythritol). I mixed up a batch of 600 grams (65% KNO3, 35% ER), enough for test grains in the T-4 motor and some strands for strand burn rate tests. The density seems to be fairly good, as I didn't even get two 5.5" long grains. I scraped up a couple of thin strands, and dropped them on wax paper to harden. They quickly hardened and I again tried to burn them in the open air. They would start to burn, then go out. I'm not overly concerned that they don't burn real well at one atmosphere, they may well burn fine in a motor under pressure, or a burn catalyst may be added to improve the burn.
KNO3 74%, Erythritol 25%, Iron Oxide 1%
I was curious how low I could go with erythritol content. This was a 50 gram batch and was mixed dry by shaking in a sealed container. The dry mix was added to a preheated melting pot set at 250 degrees. The mixture melted into a thick paste. It was so thick I had to pack it into the cpvc pipe sections used for burn strands. In the open air it barely burned at all, and left a lot of residue. Probably too low on the ER content, but I'll try burning a strand or two and see what happens.
| Test# | Length | Start Pressure | End Pressure | Aver. Pressure | Time | Burn Rate | Density |
| ER-X1a | |||||||
| ER-X1b | |||||||
| ER-X1c | |||||||
65% KNO3, 34% Erythritol, 1% Iron Oxide
This is a more or less standard batch, but adding 1% iron oxide to hopefully improve the burn just a little. I burned strand in the open air and it burned fairly well, with little residue. This batch was again dry mixed by shaking in a sealed container, then melted at a setting of 250 degrees in my cook pot.
| Test# | Length | Start Pressure | End Pressure | Aver. Pressure | Time | Burn Rate | Density |
| ER-X2a | 1" | 519.23 | 927.88 | 723.55 | 1.85 | .5404"/sec. | |
| ER-X2b | 1" | 783.65 | 1365.4 | 1074.52 | 2.6 | .3846 | |
| ER-X2c | 1" | 533.65 | 1033.7 | 783.67 | 2.766 | .3615 | |
| ER-X2d | 1" | 427.88 | 937.5 | 682.69 | 2.533 | .3948 | |
| ER-X2e |