DarkStar Mini Build

[BobCox]

During Wildman's 2010 Black Saturday sale, I picked up a DarkStar 4DD to use for my Level 2 certification and a DarkStar Mini to practice my fiberglass construction techniques. This thread is for the Mini.

The DarkStar Mini comes in a large plastic bag with the components shown below. There are no printed instructions included with the kit; the builder is expected to get them from Wildman's web site.

The second photo shows all the parts dry-fitted together. The black band at the rear is an AeroPack retainer that I purchased seperately. The rear centering ring on the DS-Mini has very narrow web thickness, so any motor retention that relies on screws in the rear ring will be difficult to use. The weight of all the components (including the retainer) is 9.64 oz.

Before starting construction, I marked the rear of the motor mount tube to indicate the amount of overlap required for the retainer.

 

[BobCox]

The nose cone has fairly small mold lines that were easy to clean up with a hobby knife and a wet sanding sponge. I am impressed by the quality of the molding process. The outer surface looks a lot like resin casting material, but does not have the pinholes and voids that I have seen on other RC cones.

The nose cone shoulder was a tight fit into the body tube, so I did a lot of sanding on the shoulder to get a good fit that was snug but not tight.

 

[BobCox]

The instructions describe two possible methods of contruction -- the traditional method with epoxy and fillets, and the foam-and-fly method that uses CA and 2-part foam. Since I have never used the foam before, I decided to expand my skill base by using the foam-and-fly method.

I used a small file to make a notch in the front centering ring to allow the shock cord to pass through. I then used medium CA to glue the shock cord to the motor mount tube, which had been previously roughed up with 80 grit sandpaper. I also glued the front centering ring in place with the CA.

The rear centering ring is used to temporarily hold the motor mount tube in place when attaching the fins, but it must not be glued on. A loop of masking tape is attached to the ring so that it may be removed later. I had to sand the inside of the ring quite a bit to allow enough space for the tape loop.

After the CA dried, I inserted the motor mount into the slotted airframe, making sure that the shock cord is centered between the fin slots so that it does not interfere with the fin mounting. The ability to see through the tubing is one nice feature about fiberglass.

Note that neither of the centering rings gets glued to the airframe tube, so the motor mount assembly is free to slide around for now. Before attaching the fins, I lined up the mark that I had made earlier for the retainer ring was flush with the rear of the airframe tube. It is important to do this now while the motor mount assembly is still free to move.

 

[BobCox]

To attach each fin, I ran a thin line of medium CA along the fin root, then inserted it through the slot. An Estes fin alignment guide made it trivially easy to get a good alignment. The root chord of the fin is a littler smaller than the slots in the body tube, so I made sure that the fins were aligned with the read edge of the slot.

The 3 front fins went on almost as easily as the rear. The instructions show the use of a straight-edge to align the front and rear fins, but the Estes guide and an eyeball were all that I needed to line them up.

After all six fins were in place, I used the tape loop to remove the rear ring in preparation for the foam.

 

[BobCox]

As I already mentioned, this was my first experience with 2-part expanding foam. I learned a few valuable lessons:
1) Cover the entire work surface beforehand, and wear gloves. This stuff is very sticky, and once it hardens there is no solvent that will remove it.
2) Once the two parts are combined, stir them QUICKLY and get them poured into place. Not much happens for the first minute or two, but once the reaction takes off everything seems to happen at once and it is soon too late to do anything.

Per the directions, I mixed 30 ml of liquid (15 ml each part) and then poured the mixture into an epoxy syringe with the tip cut off. There was enough liquid to fill two syringes, but the foam got hot and started getting hard before I could get much out of the second syringe-full. Fortunately, I could see through the fiberglass that the liquid from the first syringe had expanded enough to fill the entire space, and then some. Based on the amount of excess foam left in my syringe and mixing cup and what flowed out of the gaps in the rocket, 20 ml of liquid probably would have been sufficient.

Fortunately, the rear of the motor tube was covered with masking tape, as was the exposed section of the motor tube that will eventually hold the retainer. That prevented the excess from flowing down inside the motor tube, and left a clean surface for bonding the retainer ring.

 

[BobCox]

I allowed the foam to dry overnight. The next morning I peeled off the excess that had overflowed out the back and between the gaps. Because no solvent would dissolve the foam, I had to use a scraper and a sanding sponge to clean up the residue. It turned out pretty good, but took about an hour and a half of scraping and sanding to get there.

At this point, the total weight (including the AeroPack retainer) is 9.70 oz. The foam and the CA did not add much weight.

 

[Lentamental]

Has anyone actually ever measured how much strength is added with the foam? I can't imagine it is all that helpful, as foam tends to be fairly soft and flexible, when compared to the rigid fiberglass and whatnot.

I feel like maybe, just maybe, it would be helpful if you were to plug the hole before it expanded, and then the expanding foam was under pressure, but that is just speculation.

 

[GRIFFIN]

Has anyone actually ever measured how much strength is added with the foam? I can't imagine it is all that helpful, as foam tends to be fairly soft and flexible, when compared to the rigid fiberglass and whatnot.

I feel like maybe, just maybe, it would be helpful if you were to plug the hole before it expanded, and then the expanding foam was under pressure, but that is just speculation.

This foam cures as hard as a rock. It is not the "foam" texture or feel as the hardware store "Great Stuff"

I have seen Tim Lehr fly a rocket (His LDRS drag race rocket) built with ONLY this type of foam on an "L" motor and the (G10) fin broke off at the root cord before the foam let loose.

I can't say I know of any way to "measure" the strength but if you have never used it before......try it, you'll like it!

 

[Lentamental]

I'm mainly just thinking about whether it would be much stronger than simply gluing the bottom of the tab to the motor mount tube, and leading and trailing ends of the tabs to the centering rings.

Anyways, where can I get some? I'd love to give it a try.

 

[GRIFFIN]

I'm mainly just thinking about whether it would be much stronger than simply gluing the bottom of the tab to the motor mount tube, and leading and trailing ends of the tabs to the centering rings.

Anyways, where can I get some? I'd love to give it a try.

It is not a replacement of glue. You DO still need to glue the fin to the motor mount.

Wildman sells the foam.

 

[BobCox]

Has anyone actually ever measured how much strength is added with the foam? I can't imagine it is all that helpful, as foam tends to be fairly soft and flexible, when compared to the rigid fiberglass and whatnot.

The foam is more rigid than styrofoam, and definitely not like foam rubber. It may not be as strong as epoxy or fiberglass, but the overal bond can be very strong because it covers such a large surface area.

I feel like maybe, just maybe, it would be helpful if you were to plug the hole before it expanded, and then the expanding foam was under pressure, but that is just speculation.

a) Even if you kept it contained, all that would do is make the air bubbles smaller and the overal mixture denser. That would defeat the purpose of foam over epoxy, which is fairly good strength with minimal weight.
b) When the foam wants to expand, it's going to expand. If you try to contain it, you will most likely blow out the plug or rupture the vessel.

Anyways, where can I get some? I'd love to give it a try.

I got mine from Wildman: https://www.wildmanrocketry.com/default.asp?groupid=910401138&groupid1=59163728