Then I started thinking about clustered motors. The problem with internal clustered motors is that the body tube needs to be much larger, resulting in a much greater aerodynamic drag on the rocket. So then I thought about true outboard motors, mounting them outside the rockets main body tube. There would still be a drag penalty, but not as much as with a very large diameter body tube. I convinced myself, outboards it will be.
One thing struck me as odd. Why do people refer to extra motors in the body tube as outboards? To me that's a cluster, whether they are ground or air started.
I need to keep the weight down, so I decided on aluminum for the motor casings. Something like my "T" series of steel motors would be about right. So I ordered some 6061 T-6 casing in 1.75" outside diameter. That is small enough that I can also make "all graphite" nozzles for the motors, again keeping the weight down. I'll use the LW-1 motor as the central motor, and the two smaller motors as the outboards.
To run software predictions on flight performance I needed to add the surface area of the outboards to the rocket airframe diameter. The actual diameter is about 3.6", adding the surface area of the outboards resulted in an area equal to a rocket with a diameter of 4.54".
The LW-1 motor as tested provided 199.2 pound seconds of total impulse. The outboards should hold just over 1 pound of propellant each, with an assumed Isp of 125 that gives the outboards a total impulse of 255 pounds seconds. So the rockets installed total impulse should be about 454 pound seconds, a strong "K" class designation.
The plan is to light both outboards on the pad at launch, and the central motor at some point after the outboards burn out. I'll have to do some calculating to determine the best time to light the central motor.
Here is the start of the construction. The lower body tube was laid up with 5 layers of 6 oz fiberglass cloth and polyester resin. The two motor mount tubes were laid up in similar fashion. In the picture above, you can see the first outboard tube being epoxied in place. The fin slots were cut in advance of the outboard tube mounting.
Here you can see the second outboard motor mount in place. Once the first batch of epoxy was cured. I made some filleting material from slow set epoxy and fiberglass strands. The joint ended up just over one inch wide. Hopefully the joint is strong enough to handle the impulse of the motor.
Here are the fins roughed out. They are longer than the fins on my first fiberglass rocket to offset the extra weight of the lower body section with the outboard motors. The leading edge has a greater sweep angle for higher speed flights.
Here is a capture from Aerolab. I had to add the outboards as Aerolab didn't have a provision for outboards. The center of pressure indicated does not include the outboards. Disregard the center of gravity, as I don't have all the weights calculated yet. Actually, I probably won't input all the weight parameters as the center of gravity can be checked once the rocket is built.
Here you can see the fins stacked and clamped together and the notch cut out for the fins to mount into the centering rings.
The fins have had their leading and trailing edges finished. I dry assembled everything to get an idea of how things fit and how it would look.
I really haven't been slacking off, the lay up of all the fiberglass and the bonding is slow going. But, as you can see in the picture I am making progress.
At this point the fins have all been glassed. The fins that don't have the outboard tubes between them were glassed tip to tip, the fins with the outboards between them were glassed from the tip of each fin to half way up on the outboard motor tube. That should help strengthen the motor tubes.
The first motor casing has been cut to length and the upper bulkhead is done (laying between the 1.75" tubing). Oh yes, that tube behind the rocket will be my next project, it's 3.5" 6061 aluminum, .125" wall and 24" long, stay tuned!
You can also see in the picture I made a couple of little nose cones for the outboard motor tubes. They are 2" diameter, and will be bonded into the motor tubes and to the rocket body tube. The nose cones will also serve as the thrust bulkhead for the motors, so I left them solid for strength.
Aside from a lot of sanding, the lower rocket is about done. I fear I won't get a very good finish on this rocket, there are just too many things to sand around that are hard to get at. This project is about like building three rockets, so it's going a little slower than expected.
The black circles show the locations of wood pins that were epoxied in place. The pins on the outboards go all the way through the solid wood nose cone into the rocket body tube. A lot more sanding, sigh... Then I'll start on the upper body.
Here are some early numbers:
Rocket weight: 7.6 pounds without motors or drogue, everything else installed.
Overall Length: 73" less motors, with 40" upper body tube.
Center of Gravity: 52.25" from nose tip with loaded central motor; 55.75" from nose tip with all motors loaded.
Barrowman Center of Pressure: 60.311" from nose tip.
Notes on the preceding numbers: It looks like I'm in pretty good shape stability wise. When the drogue chute and harness is added it will move the CG forward slightly, as will the electronics in the nose cone. I would like at least 1.5 caliber's of stability, and I should be close when done.
Here is the rocket painted.
Like I mentioned earlier, the finish isn't the greatest on this rocket. I guess I can always go back and spruce it up if I take it to "show and tell". The color isn't what I really wanted either, the cap on the spray can was a lot brighter red than it turned out. At some point It will need to be sanded down and repainted anyway I guess.
Well, the out board rocket is about done. I designed it to use the same electronics bay as the first fiberglass rocket, although I may build a second bay to have a spare. I still need the recovery components, or I could use them from another rocket but I think I would prefer to have them separate.
For the rockets first launch, I think I'll use only the outboards. I haven't decided on a launch date yet, we'll see about that after I get the parachutes made for it.
Update 5 July, 2005:
It's been some months now since the rocket has more or less been completed. I almost flew it back in May, I even had propellant cast for the motors, but I got going on the Cosmo 2 rocket and pretty much put the outboard rocket on the back burner. With my homemade altimeter performing well, I decided it was time to get the outboard rocket ready to go. I added some code to the altimeter to light the sustainer motor after launch, and added another firing circuit as well. The new plan is to use the altimeter to light the sustainer 3 seconds after liftoff +300'. I may even go liftoff +500' to increase the safety margin.
I rebuilt the electronics module to include the altimeter, a timer for apogee back up and an FRS radio for tracking. I'll also use a personal alarm as a sonic locator, deployed with the main parachute. I haven't decided on the propellant load yet, but I plan on a light load to keep the altitude reasonably low.