'Old Rocketeer' Harry Stein, who authored the monthly "Rocket Trails" column in American Modeler magazine for many years, presents here a simple treatment of the technicalities of model rocket engines such as those produced by Estes, Centauri, and Model Missiles (of those, only Estes still makes motors), but Quest and Aerotech are other modern model rocket motor suppliers. Specific impulse, thrust, burn time, motor sizes, delay times, and ejection charges, etc. are all covered.
Rocket Trails: Basic Info on Propulsion Systems for Small "Birds"
by Harry (Old Rocketeer) Stine
The gizmo that makes model rocketry a hobby instead of a debacle is our model rocket engine, that simple-looking inexpensive package of power that comes ready to install in the tail of your latest bird. Us mod-roc-nuts have been using these paper tubes full of zip-stuff for five years, often with little knowledge as to what makes 'em tick. Things have changed, however. The National Association of Rocketry has released a set of reports giving all the poop on NAR-certified engines, including thrust-time curves.
First Things First. To avoid any confusion let's define the terms used in talking about model rocket engines.
Thrust is the amount of force or "push" produced by an operating engine. It's measured in ounces for little engines, pounds for big ones. The jet blast of high speed gas coming out of the aft end of the engine produces this thrust, or so sez Sir Isaac Newton. The thrust of an engine can change, by the way. So you can speak of max thrust as the highest amount of push given. Or you can talk average thrust - but let's wait a bit before tackling that one.
The length of time that thrust is produced is called duration. It's measured in seconds. Two seconds is a very long duration for a model rocket engine. In engines where thrust varies, the time from ignition to max thrust is called T-max.
To accurately determine thrust, max thrust, duration, T-max, and other operating parameters, we gotta conduct static tests. An engine is fastened down with recording instruments attached to it ... and it's fired. With luck, everything works and the instruments give data unless some idiot forgets to start the recording gadget. Data recorded from most static tests is a record of thrust as a function of time. This is called the thrust-time curve. It's a chart of the thrust during every instant of operation. Believe me, with most engines it takes quite a stack of gear to record this! It ain't easy, although Irving Wait of Rocket Development Corp. in Utah is going to bring out a recording-type static test stand for sale to model rocketeers.
If we take a thrust-time curve and calculate the area under the curve by laying a sheet of quadruled paper over it and counting the squares (or using a polar planimeter as I do), you get something known as the total impulse - pounds times seconds, or simply pound-seconds. If a rocket engine had steady thrust from the instant of ignition until the instant of burnout, you could get the total impulse by multiplying the thrust by the duration. Unhappily, no engine has this kind of simple thrust-time curve.
Cross-sections above are (A) Estes' Series 1 Type B. 8-4 and (B) Estes' Series 2 Type B3-5 rocket engines. Key: 1) Nozzle; 2) Propellant; 3) Paper Casing; 4) Delay Charge; 5) Ejection Charge; 6) Retainer Cap; 7) 0.406"; 8) 0.690"; 9) 2.75"; 10) 0.107" for "A" and 0.1875" for "B".
The physics book definition of total impulse sez it is the change in total momentum of a rocket produced by the operation of the engine. Momentum is mass times velocity. For a model of given weight, an engine of higher total impulse will produce a higher speed at burnout, thereby giving a higher peak altitude. Several issues ago, I said that thrust is only part of the story. This is why. Total impulse is the thing to think about. Which is why the NAR rates engines in classes based on total impulse.
Average thrust is always a computed value determined by dividing the total impulse by the duration. It gives you a figure that tells what the thrust of an engine would be if it were steady from ignition to burnout. Unless you know calculus, average thrust is what you have to use in figuring rocket performance.
After this 60-second background in rocket engine theory, let's look at some data.
Model Rocket Engine Performance Chart
Engine Line-up. All of the NAR Type Half-A, Type A, and Type B engines sold by Centuri Engineering, Estes Industries, and Model Missiles are almost identical in size, weight, total impulse, and performance. The performances of the various engines of the "point-8" series is shown. Note that the only significant difference between them is the duration and total impulse. A cutaway of a typical Type B engine is also shown. On all of these engines, thrust rises rapidly to 23 ounces about 0.180 seconds after ignition; this helps kick the model off the launcher and into the air fast so that its stabilizing fins will work. Thereafter, thrust settles down at about 10 ounces for accelerating the model in the air.
The tables shows the duration, total impulse, and weights of these various engines. Estes has all of them, so does Centuri Engineering. Model Missiles sells Type A.8-3 as Standard A-4, Type B.8-4 as Super B-6, B.8-0 as Lower Stage, and Type A.8-4 as Upper Stage.
These engine type numbers shouldn't scare you. They are in a standard code developed by the NAR and used by most engine manufacturers. Knowing the code will tell you a great deal about an engine. The first letter indicates total impulse range as follows:
Type Half-A = 0 to 0.35 lb-sec.
Type A = 0.35 to 0.7 lb-sec.
Type B = 0.701 to 1.20 lb-sec.
The first letter indicates the average thrust in pounds or fractions of pounds. The number following the dash tells you the number of seconds of time delay built into the engine before it pops the chute or recovery device.
All of the engines we've discussed are pretty much alike. Looking at the cross section, you can see that there is a slight indentation in the propellant grain next to the nozzle; this provides a little extra burning area right at the start and results in the high initial peak thrust. Following this, the propellant burns in a forward direction like a cigarette. When it's all gone, it lights off the time delay charge, which is a very slow burning propellant producing no thrust at all and giving the model time to coast upward on the momentum it's gotten during the powered portion of flight. Finally, the time delay burns through and ignites the ejection charge which produces a quick burst of gas pressure. This can be made to do mucho different things such as kick out a chute, slam the engine back, eject the engine, pop off the nose cone, and etc.
About a year ago, Estes introduced a new engine, now called the B3. It's exactly the same size as other type B engines. But what a difference in performance! Look at its cutaway diagram. There is a hole in the propellant charge. This gives more burning surface area, hence higher thrust. I had a terrible time trying to get test data on the B3 engines; took me a year to figure out how. When you look at the thrust-time curve, you'll see why. The Type B3 engine is a sledgehammer, a boot in the tail, an impulse engine. It burns for about 0.3 seconds and produces a maximum thrust of nine pounds! This will give a one-ounce model rocket an acceleration of about 143 gees! Ka-whoom, and it's up there. You don't even see it leave the launcher.
Estes and Centuri both sell the B3 engine now, and it comes in two styles:
B3-5 for single staged models, and B3-0 for lower stage boosting. Because the total impulse of the B3 engine is 1.1 pound-seconds, it doesn't heave a model any higher than a Type B.8-6 engine. In fact, it doesn't take it as high because burnout is lower with the B3 engine. It hits burn-out just a few feet off the launcher.
The B3 is a terrific engine for drag racing and for acceleration studies with single-staged models. For staged models, the B3-0 is the answer to a rocket-made maiden's prayer because it will boost a heavy three-stage model off the launcher in One Big Hurry so that the bird gets airspeed quickly and doesn't wallow around. For years I sweated to get a successful 3-stage model with only three engines. Now that the B3 is available for boosting, I've watched 13-year-old Steve Lewis of Norwalk, Conn. fly his 3-engined 3-stager several times in a row, perfectly straight up to better than 1500 feet on each shot. Undoubtedly, other guys have done it, too, but I mention this one because I vas dere, Charlie.
Tests of the Coaster Type F engines are nearing completion, and new Centuri Mini-Max engines are coming right along in development.
Next Month: More about boost gliders, the USAF meet, other mad ramblings. If you characters will write in with some good questions, I will stroke my long white beard and answer them. For data on NAR, or if you want to tell me what you think, drop me a post card care of this magazine. Ye Kindly Olde Editor, bless his propeller-flicking finger, will see to it that I get 'em.
And, honest, fellas, while my house does have a white front door, there's no hole in the roof!
Posted May 23, 2015