Aerobee

I wanted to build a completely scratch rocket and started looking through Peter Alway's Rockets of the World for inspiration. The "Standard Aerobee" rocket is a great looking one and is two-stage, but what really turned me on was the fact that both stages fired at liftoff with the stages remaining together during booster burn.

In order to handle this, I had to design a special interstage. which could handle the heat of a motor firing against it. Of course, it had to look somewhat prototypical as well. This also means that the selection of motors is critical.

The design and construction of this rocket is being serialized as an Rocketry Online How-to Classroom forum series. Come check it out!

The Pictures

The Design

See Rockets of the World, pages 95-102 for detailed plans and a discussion of the prototype rocket. If you don't have this book, you're missing out on the most valuable prototype resource in rocketry. You can order this from Saturn Press.


		Above: Aerobee USAF-2 displays its paint job before launch. See larger picture. Left: Peter Alway's excellent plans from Rockets of the World, page 96. See larger drawing.

The first thing to do with a scale rocket is to pick a scale factor, which determines the size, or pick the size which determines the scale factor. For me, the size is dependent on what standard body tubes are available, so There are two main body diameters, 15.0" and 12.75", and the ratio doesn't match any two standard tube sizes exactly. So I tried to find the closest tubes which would work, based around the interstage.

Since the interstage on this rocket is so special, I worked from there out. In order to preserve a good motor selection, I wanted the upper stage to use a 54mm motor mount. Because the struts on the interstage need to go around the motor mount, a minimum practical diameter for the interstage is about 3". (The interstage is at the bottom of a boat tail and the body of the sustainer is actually somewhat wider.) Making the inside of the interstage a standard tube size, 3" (3.2" O.D.), resulted in a 29% scale model. Based on this, the table below gives the outer dimensions.

	dimension	prototype	29% scale
diameter
	nozzle at aft	9.55"	2.8"
	nozzle minimum	5.0"	1.5"
	nozzle at booster	10.5"	3.0"
	booster body	12.75"	3.7"
	upper interstage	11.0"	3.2"
	sustainer body	15.0"	4.4"
length
	booster nozzle	15.18"	4.4"
	booster body	47.83"	13.9"
	interstage	13.03"	3.8"
	booster overall	76.04"	22.1"

	sustainer boat tail	17.5"	5"
	sustainer body	121.43"	35.2"
	payload section	15.0"	4.4"
	nose cone	87.87"	25.5"
	sustainer overall	241.8"	70.1"
fins
	booster root len.	27.7"	8.0"
	booster span	24.63"	7.1"
	sustainer root len.	27.0"	7.8"
	sustainer span (at aft)	25.5"	7.4"
	leading edge sweep	30°	30°
	root thickness	2.215"	0.64"
	tip thickness	0.16"	0.05"

3.7" and 4.4" aren't standard tubing sizes, but the tubing can be built up. The 3.7" booster can be built up based on the 3" tubing (3.2" O.D.) and the sustainer body can be based on standard 3.9" tubing (4.1" O.D.). To build up the tubes, I will laminate them with a honeycomb cell, which is 1/8" thick. This is very light and strong. Two .125" wraps on the booster body and one on the sustainer body bring the diameters to exactly the right amount! See my Honeycomb Tubes article for more on this technique.

See also my diagrams of booster and sustainer construction details. The nose cone and nozzle were turned by Missile Works (see my diagram of the nozzle detail).

The Interstage

The interstage is two parts, the upper and lower. In order to be true to the prototype, the interstage is open and the sustainer motor fires at the same time as the booster motor. This means that the interstage and booster airframe need to withstand high temperatures.

In order for this to work, I need to use metal parts for the lower interstage, perhaps even an exotic metal such as titanium. The good news is that they are only subject to this temperature for a couple of seconds (until the booster burns out and the stages separate). Metal is a good material for strength as well, so I decided to build the upper interstage out of metal too, although aluminum will be sufficient here. See the plans for the upper interstage and the lower interstage (in PDF, Adobe Acrobat, format).

The upper interstage houses the sustainer motor mount (single 54mm) and fits into a 3" airframe tube (3.2" O.D.) The sustainer boat tail starts at the lip of the upper interstage and grows out to the sustainer airframe width of 4.4" O.D. These parts are bonded together and make a more-or-less conventional rocket fin can. Sustainer recovery is standard using a break forward of the fins.

The lower interstage fits into the top of the booster airframe and forms the cap of the booster. This section has a built-in shoulder which slips into the inner motor mount tube. For booster recovery, the cap pops off just like a normal rocket's nose cone. For simplicity and space considerations, the booster uses motor ejection to eject the 'chute

I have been working with a local metallurgist to come up with an interstage which can withstand the heat and apparently 300 stainless steel is more than strong enough to resist the heat and corrosion for the few seconds required. So the lower interstage will be built out of 300 stainless and the upper interstage out of aluminum (to reduce weight and cost).

Motor Selection

The other challenge for properly staging this rocket is to make sure the booster motor fires with enough thrust to lift it, and the upper stage faster than the sustainer motor is lifting just the sustainer. Since the sustainer is being lifted in both cases, the booster motor needs to apply more thrust, minus the booster weight, than the sustainer motor produces throughout its burn. In addition, I want the sustainer to be stable by itself in case its motor ignites and the booster motor does not.

For prototypical operation, the booster motor should be a short burn and the sustainer a long burn. This also fits in well with the thrust requirements. The sustainer uses a 54mm motor mount and two good long burning motor are the AeroTech I65 (3% J, max. thrust 34#) and J125 (89% J, max. thrust 73#). If the sustainer comes out weighing around 4#, I will use the I65 for maximum burn time (18# avg. thrust). Otherwise I will use the J125 (34# avg. thrust), which can easily handle a 6# rocket.

Since the booster is mainly a 3" MMT, one could even image using a 76mm motor. However, the total booster length is only 15.8". Reserving 3" for the lower interstage shoulder plus 3" for a small 'chute leaves about 10" for the motor itself. (The lower interstage shoulder is hollow so the 'chute can also tuck into it.) In addition, to simulate the large nozzle at the bottom of the booster, I need to keep the motor diameter narrow, say a 38mm motor. One of the few motors which fits the bill is the AeroTech I435 (88% I, avg. thrust 109#).

For more information on motor selection in general, see the INFOcentral Motor Selection article.