Rocketry Planet

I began thinking of this project shortly after getting into high-power flying about 2 years ago. I have always been a Saturn V fanatic, and felt the Estes 1/100 scale was just not big enough. This project is 1/52 scale, basically because that scale factor matched PML 7.62" tubing that I used for the main booster airframe.

In the spring of 1997 I began getting the itch to do a high-power scratchbuild project. I decided to go all-out and build the "big Saturn V" I’d always wanted. In September of 1997 I began initial sketches of the project, using the 1/100 design as a baseline.

Andrew Waddell and his beautiful 7.62" Saturn V made from PML components.

Design Concepts and Implementation

Initial Design Thoughts

I wanted to have 5 motors in the project just like the real thing, and initially thought I would be able to fly on (5) G40 White Lightning motors. I ended up deciding on a PML Kwik-Switch 2000 system in the center, which would allow a 29, 38, or 54mm central motor; this turned out to be a very wise decision as the weight crept up during the build and the (5) G40s idea went out the window. I surrounded the KS2000 with (4) 29mm motor tubes. (By the way, anything not mentioned as entirely scratchbuilt or from another source was from PML: tubing, chutes, couplers, etc.).

This project was intended to test my design ideas and flight characteristics. I did not intend for the rocket to be a "competition scale" piece, accurate to the last thousandth of an inch; it was more intended as sport scale, to be quite accurate but also flown and enjoyed. This is one reason why I chose not to apply the corrugations that should be in place for a true scale Saturn V.

Now that this project has flown successfully, I intend to move on to the "real" project I had in mind all along, an 11.4"-based Saturn V. I already know some things I’ll do differently in the 12" version, and they will be discussed here. Please forward any ideas you have for improvements to my design to me at This email address is being protected from spam bots, you need Javascript enabled to view it  I’m very interested in making the 12" Saturn as good as it can possibly be!

Removable Motor Mount Assembly

Figure 1 - Motor Mount Assembly Components

Figure 1 shows the motor can components. Along the upper row are, L to R, the thrust ring to be epoxied into the airframe; the upper bulkhead (1/2" birch ply); the lower bulkhead (3/16" birch ply); another lower bulkhead that was to be used to mount scale engine nozzles (which was never used). Along the lower row are the (4) 29mm motor tubes, then the Kwik-Switch 2000 components.

Figure 2 - Upper Motor Mount Ring and Locking Tabs

Figure 3 shows the completed motor can assembly with the thrust ring. Motors were retained in the motor can via woodscrews into the lower motor can bulkhead through an L-bracket. One leg of the L provides correct height from the lower bulkhead; the other leg of the L reaches across to the motor casing. Simply looking at the L on this page shows how the bracket is installed; the screw goes through a hole in the short leg of the L through to the bulkhead. This system is sufficiently strong, but I will replace the woodscrews with T-nuts in the lower bulkhead and use machine screws to add more durability and retention strength to the system. This is another advantage to the removable motor can assembly, the ability to modify this area of the rocket as needed.

Figure 3 - Completed Motor "Can" and Thrust Ring

To accomplish this, I decided on a plunger switch arrangement. With this system, the spring-loaded plunger must extend almost 2 inches before the switch is closed. If the rocket (which weighs 17.5 pounds fully loaded with motors) has moved vertically 2 inches, I can be sure the J275W has ignited and is burning well.

Figure 4 - Plunger Ignition System for Outboard Motors

The plunger switch system is shown in Figure 4. The plunger itself is a piece of 3/8" dowel; I selected dowel since when the booster section returns under parachute, the plunger will be the first to impact the ground. With dowel, it breaks first, reducing some of the impact load into the structure of the rocket. I have made extra dowels to bring to launches so it can be easily replaced. As shown in Figure 5, there is a 1/2" hole in the upper motor can bulkhead so the plunger rod can be extracted through the top of the bulkhead. To replace the plunger, the cap over the 1/2" hole is removed, then the long woodscrew retaining the fender washer and the washer are removed. Then, the plunger dowel can be extracted and replaced. The cap for the hole is necessary to prevent ejection pressurization leakage into the bottom of the rocket.

Figure 5 - "Service Access" for Plunger Rod Replacement

When the rocket lifts off, the spring pushes the plunger down, with the fender washer activating the roller switch at the end of the plunger’s travel. The onboard batteries then provide current to the igniters. Figure 4 shows the system in the "ignite" position. The G40 igniters I use are HiRMI electric matches, which have a no-fire current of 200mA and an all-fire current of 400mA. Therefore, the (4) G40W igniters have a maximum current draw of 1.6A. A fresh 9v. alkaline battery is capable of providing about 4.0A of current, so it’s more than enough for the four igniters. However, as a somewhat redundant system and just to be sure, I have two 9v. batteries installed, wired in parallel. The battery retainers and connectors are shown in Figure 6.

Figure 6 - Plunger System Battery Mount and Safing Switch Interconnect

Airframes

The main booster airframe is 7.62" OD PML phenolic tubing, and the spacecraft section is 3.0" OD PML phenolic tubing. The center section is 5.4" OD phenolic tubing that has been modified to be 5.0" OD to meet scale requirements.

Figure 7 - 5.4" Tubing Reduction to 5.0"

Once the cuts were made, I got two nylon tiedown straps similar to those used to tie down motorcycles. I put the straps around the tube at two points and pulled them down until, holding a ruler against the end of the tube, it came to 5.0". When the measurement was correct, I slathered the strip I had cut from the outside of the tube with epoxy, and put in inside the tube. I clamped and weighted the strip to hold it tight until the epoxy set. Figures 7 and 8 show the tubing after the strip was glued inside; a piece of the 5.4 with the cutout part is also shown in Figure 7.

Figure 8 - Interior View of 5.0" Tube with Strip Epoxied In Place

Thrust-Loaded Components

Here I want to specifically discuss some sections of the rocket that may be able to be done better. None have failed or shown degradation, but they are areas you may be able to improve upon if you attempt such a project.

Upper Section to Main Airframe

Figure 9 - 3" Airframe and 5.0" Airframe Sections

There are two centering rings which were epoxied to each other "back to back" on the 5" airframe section. The two rings shown mounted on the 5" tubing in Figure 9 are exactly the ID of the 7.62" airframe tubing. These two centering rings form the "coupler" section of the upper stage assembly that fits into the 7.62" airframe. The ring shown in the center of Figure 9 above the bulkplate with the eyebolt is exactly the OUTER diameter of the 7.62" airframe. This ring is bonded to the upper centering ring on the 5.0" tubing. The upper ring is also beveled because it is the attachment point for the upper transition.

This arrangement means that all the weight of the upper stage section is supported by the interface between the 0.080" x 7.62" ring of the upper bonded centering ring and the wall thickness of the main 7.62" airframe tubing. Said another way, the whole upper section is lifted by contact around the wall of the body tube only. This gives about 0.61 square inches of support area. Due to the high compression strength of PML phenolic tubing, I felt this would be sufficient, as it has proven to be in flight. This is really no different than a tubing-to-tubing interface of any other rocket that separates in the middle for recovery. The only difference is this interface is also dependent on the bond of the rings to the 5.0" tubing instead of simple physical contact. If someone has a better way (that would not interfere with the deployment system), I’d like to improve upon this aspect of the design. Figure 10 shows the assembled upper section (without transitions).

Motor Thrust Ring

Figure 10 - Assembled Upper Section

A couple of things come to mind to strengthen this joint, which I will consider on the 12" version:

1. "L-brackets" could be mounted around the top of the ring and epoxied to the airframe, then overlaid with fiberglass. This would require a larger ID to the ring, or at least some tabs protruding inward to allow mounting the L brackets to the ring.
2. Screws could be driven through the airframe into the ring. There may be questionable overall strength improvement here due to the relative thinness of the airframe, especially considering the screws would likely need to be countersunk and filled over to preserve appearance.

Spacecraft/LES to Upper Airframe Interface

Figure 11 - Spacecraft, LES & Tower, and Nose Weight Provisions

As shown in Figure 11, the spacecraft/LES has been drilled and tapped for a threaded rod. The bronze discs are added to the threaded rod, then secured with a fender washer, lockwasher, and wingnut. After building the rocket, the CG/CP, figured very conservatively, required about 5 pounds of nose weight to fly with only the true scale fins. This is where the strength of the interface to the upper airframe is concerned. I have no concern with the ability of the tubing to handle the 5 pounds of weight, but upon ignition of the J275W and (4) G40’s, wRASP shows a G-loading of 4.83G with the nose weight added to the dry weight of the rocket. This means the spacecraft/LES assembly would be placing about a 25G load on the interface.

Again, this does not seem to be a problem, but it is an area of concern I’d like to do differently. I currently fly with add-on clear fins so as to not have to fly the weight, but I will eventually fly it with nose weight and no clear fins to see the effect. Again, CG/CP calculations show this to be a safe, stable flight, but flying that much non-functional weight seems quite a waste.

Fin Assemblies

Figure 12 - Fin Mounting Provisions (and Motor Can Tabs, Thrust Ring)

I then filleted the outer joint of the fin to the airframe with epoxy. At this point I created strengthening ribs which I attached to both sides of the fins to the airframe. They were as tall and long as they could practically be and still fit under the shrouds. The shrouds themselves are discussed elsewhere. A strengthening technique that could be used would be to drill and drive small woodscrews through the endcap, perpendicular to the fin. However, the fins are very strong as is, and the shrouds themselves take the impact upon landing, not the fins.