Andy Waddell builds a BIG 7.62" Saturn V!

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Home / Features / Andy Waddell builds a BIG 7.62" Saturn V!

Finishing, Stability and Flight

Project Review by Andrew Waddell

Wednesday, September 23, 1998

Article Index
Andy Waddell builds a BIG 7.62" Saturn V!
Recovery and Electronics
Finishing, Stability and Flight

Page 3 of 3

Exterior Detail Parts

All of the exterior detail parts were created using either dowel rod or basswood in various thicknesses. The measurements for the parts were taken from an article in the December 1989 issue of American Spacemodeling. I believe these drawings are also available through the NAR’s NARTS program. The larger pieces were sanded to the contour of the airframe tubing they attached to. This was done by taping a piece of sandpaper to the airframe tube and sanding the part until the matching contour was achieved on the back of the part.

I have no particular hints to give you on the exterior detail parts...it’s purely a hand-shaping project for each piece. This model has 34 various exterior detail pieces, made of 67 individual parts.

If you have built the 1/100 model, you know that the directions show to put pieces of masking tape on the tubing before painting. When done painting, you are to remove the tape pieces to expose unpainted areas underneath to attach the details. This was difficult for me to do, basically because I was unsure exactly how many details I was going to attach, and therefore where they’d be for masking. I attached the long half-round cable runs to the airframes before painting, but everything else was added after painting. The detail parts were attached to cardstock with a spray mount product available at art stores before mounting them to the airframe, then painted.

Launch Lugs

Figure 24 - Launch Lugs and Generic Lug Attachment Method

The launch lugs were ¾" copper tubing from PML. They were epoxied to ¼" basswood standoffs to ensure the launch rod would not scrape the airframe tubing. Shown in Figure 24 are the lugs and a method I use to ensure the lugs are attached perfectly in line with each other. I have various sizes of dowel rod I keep in my work area, and I slip the lugs over the correct dowel rod before gluing. Position the lugs properly on the dowel for their position on the rocket, then put a small piece of masking tape on the lugs to the dowel to hold them in place. Mark the front and rear of each lug on the rocket; apply the epoxy between those marks. Put the dowel with lugs on the rocket, then tape the dowel to the airframe until the epoxy sets. In this way you ensure that the lugs are perfectly aligned with each other. This is also useful because the dowel shows whether the lugs are positioned exactly parallel with the centerline of the airframe for a perfectly straight boost off the rod.

Painting

I used Krylon products for all aspects of this project. I find Krylon to be the best canned spray paint product in terms of color coverage, protection against runs and sags, repeatability of spray pattern out of the can, resistance to spray nozzle clogging, etc. Everyone has their favorite paints, but I’d recommend Krylon if you haven’t yet developed a favorite.

I used gloss Krylon for the entire project, then overcoated it when completely done with Krylon Matte Clearcoat. I find a matte (satin) finish to be more realistic on a scale model than a gloss clearcoat. If you want a truly flat finish, Testor’s Dullcote works very well, but can be quite expensive for a project with this much area to coat.

If you want a flat finish you should paint with a gloss paint, then attach the decals, then overcoat with a matte or flat clearcoat. If you must use a flat paint due to a special color that’s only available in flat, use a gloss clear coat over the flat, then attach the decals, then use a flat overcoat. Any decal will not attach well to a flat paint surface, and will have a "silvery" effect due to it not sticking everywhere underneath.

Make sure to clean the rocket thoroughly with a spray cleaner such as Windex or Fantastik before painting, and allow the cleaner to dry thoroughly as well. Especially important is to lightly scuff all the styrene parts before painting to give the paint something to "bite" on, as well as to clean the styrene to ensure any mold release compound from manufacture of the sheet is gone.

Decals

The decals were done by downloading a copy of the original 1/100 scale decal sheet from the "Jim Z." website (www.dars.org/jimz/estes.htm). That decal sheet was then sized by "eye". Basically I looked at the 1/100 decals and sized the downloaded one to fit the scaling needed. By measuring a certain area on my model, then looking at how a decal fit into that area, I could fairly accurately guess the size the decal needed to be. I printed some test sizes on paper, then when I was happy with the size of the test decal I reduced the entire decal set to that scale.

I printed the decals on material I found at a Kinkos copy shop called "drafting film". This material is pretty much a thin version of overhead transparency material with an adhesive and a peel-off backing. The drafting film is designed to be run through a laser printer. It does not work with an inkjet printer; you’ll have to find another method if you have an inkjet. I am lucky in that I have a color laser printer available at my work, which is how I printed the red, white and blue flags and the red USA logos. I used my personal home printer to do the black decals.

The drafting film does have one disadvantage in that the adhesive is a bit "milky" or frosted looking. This was not an issue for the Saturn V since all the decals were to be used against a white background. This may be an issue if you wish to use the film for other projects. I would also recommend, as with any decal, that the surface to which it be attached has a glossy surface.

Stability Issues

As noted previously, one of my design requirements was that the fins be the correct scale size. I knew right away that there would be issues with CG/CP, due to the very small fin area. I designed in a nose weight capability from the beginning, and later on also designed clear add-on fins so it could be flown without nose weight.

CP Calculation & CG Considerations

Running the CP calculation using VCP 1.64 was rather interesting. VCP does not offer the capability to do complex fin shapes. Also, I had to find a way to deal with the fin shrouds themselves since they do indeed offer some stability/corrective force in and of themselves.

What I finally ended up doing was to use a "section view" through the fin and shroud as a 2-D shape. Granted the 3-D nature of the shroud would add more corrective action than I was doing with my estimation, but I just considered that safety factor on top of my final numbers. The section view of the fin and shroud assembly essentially ended up to be a triangle (the shroud) and a trapezoid (the fin) sticking out from it. I then calculated the area of the triangle and the trapezoid; this turned out to be about 11 square inches.

Since VCP couldn’t handle a triangle with a trapezoid sticking out, I created a triangle of 11 square inches as a fin. To add the extra area to the triangular shape in VCP, I increased the span of the triangle and kept the length the same.

Running VCP with this arrangement gave a CP location of 29.35" from the rear of the rocket. This turned out to be about midway up the black roll pattern in the middle of the booster section. Loading the rocket with the chutes, the motors, and everything else needed for flight, it turned out the rocket would need to have about 5 pounds of nose weight. Though I had the capability to add it, I was concerned about adding that much weight.

There were a few reasons for that concern. One, I was bothered about the loading of the spacecraft/LES interface to the upper airframe, and then the loading of the upper airframe section to the lower booster. Under a 5G boost, that nose weight would then weigh 25 pounds. I was comfortable that the model’s design could handle 25 pounds of weight, but wanted to avoid it. The nose weight is by definition very far forward and I didn’t want to allow that much weight to act as a lever arm against the joint of the upper section into the booster section. Two, I was concerned that having that much nose weight would dramatically increase the recovery weight of the upper section, requiring a much larger chute. This was not a big concern, since I had plenty of packing room for a larger chute and the recovery harness cable could easily handle the weight. Third, I didn’t want that much weight in the top section on the first flight for safety reasons in case the flight did not go well.

Add-on Clear Fins

Figure 25 - CP Locations With and Without Clear Fins

To get around the nose weight vs. stability concern, I decided to add clear fins for flight to the bottom of the rocket. After viewing the rocket on display at the 1998 NARCON, Peter Alway suggested a clear fin style that would not only provide the flight stability function but would also make a nice display stand. Using that concept, I created clear fins that attached to the bottom of the shrouds. The fins were made from 1/8" Plexiglas. Since any fin is more prone to flutter when they’re a symmetrical shape, I intentionally made the fins asymmetrical to keep the flutter potential down. The clear fins can be seen in Figure 25. The Plexiglas material is somewhat flexible, which can be both a concern and an advantage. The Plexiglas can take some minor flexing, which may absorb some of the loading that might break a stiffer material. Also, another design point of the clear fins is that they would hit the ground first on recovery of the booster, and I wanted them to break to help absorb some of the impact so the booster would be subjected to less landing stress. This was another reason to use Plexiglas over Lexan as the clear fin material. However, on the larger Saturn V, I intend to use Lexan due to the greater load they will be required to handle since they will be proportionately larger than the Plexiglas. I do not believe Plexiglas would be sufficient in a size any larger than that I’ve used.

The clear fins are attached via a bracket that is attached with 4 screws to the shroud lower of each fin. These brackets are made of basswood, with a carpenter’s glue joint of the verticals to the base of the bracket. I then reinforced this joint by predrilling and installing 2 small wood screws to each joint of the vertical, giving a total of four per bracket.

The fin itself slips into the verticals on the bracket, and are held in place by a screw that passes through the verticals and a hole in the fin. The fins are cut and shaped such that they touch the bottom of the fin, the bottom of the shroud, and the inner side of the shroud to ensure they do not rotate about the screw passing through the verticals. As well as providing the necessary flight stability, the clear fins do indeed provide a nice display stand function.

Motor Selection

When originally planning the project, I was going to fly on (5) G40 White Lightning Aerotech motors. As the project moved forward, it was quite clear that would not be sufficient. Since I had designed the center motor mount to be a KwikSwitch I then decided that an H123 or I161 might do the trick. As it finally turned out, I determined a J275 would be the best bet for a center motor since it could safely fly the project on its own. With the H and I configurations they were too dependent on at least 2 of the G motors firing properly.

On the first flight, I used slow thermalite (burn rate approx. 1ft/3 sec.) "X-ed" across the nozzle of the J275 to ignite the G motors. The slow thermalite was sheathed in Teflon tubing at the entrance point to the G motors and through the motors, with some thermalite sticking through the tubing and bent over it at the top of the motor for ignition. This scheme worked well, but due to a number of factors I wanted to do something different for subsequent flights (see Plunger Switch for 29mm Ignition earlier in this document for details of the change).

One: thermalite is no longer produced, so I had no way of ensuring I could get more later. Two: the thermalite burn rate made it such that the G motors didn’t ignite until about 2.5 seconds into the flight. This made for a longer overall burn, which was fun to watch, but also did not achieve the visual effect I wanted of 5 motors burning at once almost immediately off the pad. Third: the somewhat late burn also had an undesired flight profile effect. The J275 thrust drops off sharply in the last second of burn. The thrust was reducing as it both had to fight increasing air resistance due to speed as well as carry the dead weight of the non-thrusting G motors. Due to this, shortly before the G motors began to burn the rocket had started to arc slightly to one side (an estimated 5-10 degrees). Then, the G motors lit and drove the flight somewhat downrange due to the arc.

On the second flight at NARAM 40, the plunger switch arrangement worked well with G motor ignition very shortly after liftoff. However, one caveat to all considering clustering: make sure the igniters for outboard motors are very firmly attached so they cannot be pulled out either by thrust or other reasons. I would suggest using the red plastic caps provided with the motors, with the igniter leads running over the side of the nozzle and captured under the cap, as suggested in the motor instructions for the Copperhead igniters provided.

Launch Checklist

I created a launch checklist to ensure I missed no steps in the preparation of the vehicle for flight. The checklist came to 147 items, and was invaluable in prepping all the subsystems properly. Any time I have seen failures of major projects, it has seemed to be because of something simple the flier missed due to the complexity of the project. I am happy to say all systems worked very well in flight, and I attribute much of that success to a carefully designed and followed checklist.

Information Sources

Rockets of the World

Peter Alway’s book Rockets of the World was used for all of the major dimensions of the model. This book is an invaluable resource for the scale rocket project builder.

NARTS

Drawings from December 1989 issue of American Spacemodeling NAR magazine.

Internet Resources

Jim Z. Website: www.dars.org/jimz/estes.htm (decals)

Apollo Saturn V Reference Page: http://www.apollosaturn.com/ (detail photos)