Development of rocket engines is being carried out continually by PEADCO and the new information gained to this point is included here.
Solid propellant engines–even commercial model engines and large engines–are not as predictable as liquid fueled engines. Liquid fuel is pumped at a specific controlled rate. Once a solid propellant engine is ignited, there is no control over the consumption of fuel and any slight disturbance or inconsistency in the propellant grain or flame within the engine can totally change the thrust-time curve. Homemade engines with nearly identical propellant densities have been tested over and over and have yielded specific impulses as low as 40 seconds and as high as 70 seconds with about 90% falling between 50 and 60 seconds.
You might be surprised to learn that commercial core burning model rocket engines are not much more consistent than this and that your homemade engines will perform very satisfactory in your models.
A different adhesive has been found that makes rolling cases easier and the completed case is rock hard, strong and fire resistant. This chemical is sodium silicate commonly called water glass. Use the liquid as is with no thinning. Thinning with even a small amount of water has been found to decrease the strength of the case. Let the cases dry in a warm dry place for two or three days or dry them at a low setting in a dehydrator. Too much heat in the drying process often cause the cases to sag causing an oval cross section. To eliminate this, dry them standing on end. Dry the cases 100% before loading and keep them dry by storing them in a plastic bag. All ingredients used in the engine should be stored in airtight containers to keep them dry.
Since the cases are now loaded dry, they should be rolled with an outside diameter that will just slide down into the mold body after been bolted together. It should not be compressed onto the case as described in the manual.
The only reason for the mold body now is to maintain the piercer in the 'exact center of the case. The mold body can actually be shortened, made in one piece, made in two pieces and glued together or eliminated entirely if care is used to keep the case centered. The spacer should be shortened to be only thick enough to just fit inside the case to center it around the piercer. Well made loading dowels (drifts) will also help keep the piercer in the exact center of the case. Center the piercer tip in the center of the case as you load it. In the early stages of loading, if the piercer becomes off center, pushing on the case while compressing the charges will move it slightly. Once the case is about a quarter or third loaded, the case will stay in position.
Use a mixture of 75% potassium nitrate, 15% air-floated charcoal powder and 10% sulfur. The most inexpensive and best quality supplies can be obtained from the sources listed at the end. The 75-15-10 mixture is too fast burning by itself for the engine dimensions listed in Appendix I so it is used for a base and slowed by the addition of a small percentage of calcium carbonate. Dry the powder and then mix dry l½% calcium carbonate with 98½% of the dry propellant. Powder and mix them together thoroughly in a mortar and pestle or sift them through a fine mesh screen if there are no lumps. To more accurately measure the l½% calcium carbonate, first make a thorough mixture of 15% calcium carbonate and 85% powder. Then mix 10% of this with 90% powder. Whether this second procedure is used or not will depend on the accuracy of your balance and the total amount mixed. A large amount can be weighed more accurately than a small amount.
The reason the calcium carbonate is used instead of making the engine the proper dimensions to start with is so that if the engine-size/fuel-ratio is not the most efficient a different fuel mixture can be easily and quickly made rather than making an entirely new mold each time.
The propellant is loaded dry into the casing and the engine can be used immediately since none of the components need to be dried now.
COMPRESSING POWDER CHARGE .
It has been found that compressing the propellant more does not increase the specific impulse of the fuel. What does increase the specific impulse is increasing the internal pressure and exhaust velocity and so thrust for a given nozzle diameter. Compressing the propellant more does increase the efficiency of the engine in that more fuel is used in the same casing and so more total impulse is obtained for the same engine and the total impulse for a total given engine weight increases. The more the propellant is compressed the, slower it burns and so less calcium carbonate is added to obtain the desired thrust. For example, two half inch I.D. engines were tested. Both contained the exact same volume of propellant but one was compressed to contain 12.7 grams. It required 6% calcium carbonate to slow it and yielded a total impulse of 1.68 lb-sec. The second was compressed to hold 17.2 grams and required only l½% calcium carbonate to slow it and yielded a total impulse of 2.53 lb-sec. This corresponded to specific impulses of 61 and 67 seconds.
To obtain good results using the propellant described, the exact length of the propellant from nozzle to delay charge must be measured and the exact amount of propellant specified should be weighed out. When the exact height is reached, the amount of propellant left over should not be more than about 10% of the total or it becomes likely the engine will blow up. The engine casing I.D. must also be accurate. Use l½ wraps of 4 mil plastic around the rolling dowel for a spacer. This will give a .02 to .03 oversize I.D. (.012” in theory but in practice the paper cannot be wrapped by hand perfectly tight) for an easy loading dowel fit. This distance can be determined by marking a line even with the top of the case on the loading dowel resting on the nozzle, and then on the propellant charge and measuring between them.
CLAY NOZZLE AND TOP HEADING
Moist ceramic clay shrinks when dried and has never worked well. The problem with nozzles and top headings has been completely eliminated by using dry clay. There are many different types of clay and some work better than others. The dry clay that is used to make slip for poured ceramic pieces does not work well. 200 mesh bentonite clay is preferred by the author. Attapulgus fire clay also works well and there are probably many others that work well too. The clay should form a hard dense mass that does not crumble or flake easily once it is properly compressed in the casing. It is loaded dry and compressed in two or three charges exactly like the propellant. Whenever changing from one material to another in the course of loading the case, the excess loose powder should be shaken out before loading the next substance. This should be done between the nozzle and propellant, propellant and delay, delay and top heading and after the top heading is drilled before the ejection charge is added.
The top heading can be decreased to .50 I.D. if desired when the delay charge is 3/8" thick or more. When a booster engine is made with no time delay, the top heading should be increased to 1.0 I.D. To increase efficiency a 15º expansion cone should be formed at the exit of the nozzle as shown in the cross section below. This can be done simply with a knife. This cannot be done properly with a long section of casing beyond the end of the nozzle which is another good reason for shortening the spacer.
Instead of drilling the hole in the center of the top heading, drill it on the edge next to the case. This prevents premature ignition of the ejection charge and allows for a longer and more consistent time delay. Use smokeless powder to fill the vent hole so that the time delay will be provided entirely by the powder below the top heading. In ½" dia. and all larger engines, drill only a 13/16” dia. hole in the top heading. A larger hole is unnecessary and only requires excessive ejection charge to fill it.
The same technique used for slowing the propellant can be used to provide a slower burning time delay. Use one part calcium carbonate and six parts dry 75-15-10 mix (powder) prepared as described for the modified powder. Load propellant to a depth of exactly 3/8 I.D. above the top of the piercer. The delay mixture is loaded above this before the top heading. It burns at a rate of .072 inches per second so for a 5 second delay, the delay mix should be about 3/8" thick when compressed. If the top heading hole is bored into the delay charge instead of just to it or if the propellant thickness above the piercer is too short, the delay will be decreased.
One trick is to mark the exact location of the top of the powder charge (3/8 I.D. above the piercer tip) on the outside of the case and then the exact location of the desired delay thickness above this. Then fill the delay mixture 1/16" or more beyond this point. Load the top heading. Now position the drill bit for the top heading vent on the outside of the case with the tip even with the second mark on the case and put a mark or piece of tape on the drill bit even with the top of the case. Drill through the top heading into the delay charge and stop when it is just even with the top of the case.
The accuracy of the time delay will be directly related to the care used in loading the propellant and delay mixture but will also be affected by the fact that the delay charge must burn diagonally from the center to the outside where the top heading vent is located. Also the internal pressure and burning rate and characteristics may make a small difference in when the delay charge begins burning. As with the propellant, the density to which the delay is compacted (it should be the same as the propellant) will make a difference in the burn rate. Commercial engine delays have an accuracy of ±10% and using these techniques, your time delays will be well within this accuracy too.
STANDARD DIAMETER ENGINES TO REPLACE COMMERCIAL ENGINES
Standard series engines (A, B, C sizes) are 2.75" long and .69" dia. The ½” I.D. engine listed in the manual has a .75" O.D. but this can be reduced to .69" dia. with no problems as long as the cases are rolled tightly. A cardboard engine holder tube can be used to check the fit when the cases are rolled.
Nearly all standard series engines are port or end burning engines. All engines in this manual are core burning. There are severe problems involved in attempting homemade end burning engines (which we expect to have solved within the next year) and so it is not possible to duplicate the total power of a commercial engine in the 2.75" length. A 2.75" long home built engine would be equivalent to an A size but would require a smaller nozzle orifice. A longer engine can be used in a modified rocket by using a longer engine holder and moving the thrust ring forward. A shorter ½” I.D. engine can be made using no calcium carbonate by reducing the core length from 3" to 1-3/4". Specifications for this engine are:
case length 3-3/4"
propellant length 1-15/16"
propellant weight .37 oz (10.5 gram)
total impulse 1.17 lb-sec (5.23 n-sec)
maximum thrust 3 lb.
DIMENSION AND QUANTITY CHANGES
Due to the changes in procedure some of the values given in Appendix I will be changed. Top heading clay weight is not critical. The nozzle thickness is critical rather than the weight but the weight will be given as an aid. The top of the spacer to the tip of the piercer must exactly equal the core length plus nozzle length. This length will be 7.25 I.D. The total piercer length will decrease by the amount the spacer is reduced and can be found by using the formula C + 1-3/8" + spacer length. The changes in the formulas on page 30 will be: propellant length above core .375D, total case length 3.5D + L. Additional changes are given below.
ADDITIONAL CHEMICAL SUPPLIERS
Chempac Supply Co.
912 Crescent St.
Brocton, Mass. 02402
E. D. Cemco, Co.
64 Cottonwood lane
Westbury, N.Y. 11590
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