Monday, January 31, 2011

Mega Review: Four missile-aneous scratch builds

This post includes:
  • Scrap Rocket 2 - once of my oldest, no wadding required
  • One Shot - may not have broken Mach, but who could tell?
  • Mini-Toobah - my tiniest tube finned rocket
  • Spikor - one of my newest, so all my readers already know about it


Scrap Rocket 2


The SR-2 was built in the late 80's by my son (~7 years old at the time) and I. I am providing this review as an example of using scrap materials and to document an approach to parachute deployment that does not require wadding.

Construction:
Parts list:

1. L'Eggs pantyhose egg (large portion)
2. 2 ¾" wrapping paper tube (3" section)
3. coupler tube made from wrapping paper tube (5 1/2" section, 2" shoulder exposed)
4. bulkhead made from 3/16" balsa
5. BT-20 balsa coupler (mates with BT-20 stuffer tube), Estes
6. main airframe, 9 ¾" section of wrapping paper tube
7. launch lug, Estes
8. hardwood stringer (4), 1/16" square, 9 ¾" long
9. hardwood stringer (4), 1/16" square, 8" long
10. plastic fins (4), 1/16" thick (styrene?)
11. balsa spacers (4), 3/16" x 1" x 3"
12. BT-20 motor/stuffer tube, 8 ¾", extends from 1" below main airframe
13. elastic shock cord, 12"
14. parachute, 12" or larger
15. engine block made from ¾" section of used 18mm casing
16. 1" section of dowel
17. Snap Swivel tied to shock cord.

The rocket body consists of a BT-20 tube centered in the main airframe. The nose cone assembly includes a bulkhead which seats in the BT-20 tube. The chute is packed between the BT-20 tube and the airframe, and is not directly exposed to the ejection gasses. Some of the gasses may come in contact with the chute, but this design has worked several times without incident. I think I invented this method, but it has been a while, and the memory is the second thing to go (and I don't remember the first one!)

Make the coupler tube by cutting a section of the wrapping tube lengthwise and turning in on itself. Glue this into the forward tube so that 2" extends out the rear and ?" extends out the front. Cut the bulkhead from balsa stock and glue the BT-20 coupler on its center. The balsa may be too weak to hold a hook-eye. Instead, punch a hole in the bulkhead midway between the BT-20 coupler and the edge of the coupler.

Feed the shock cord though the hole, tie it to a small section of dowel and glue the dowel to the bulkhead. Glue the bulkhead in the rear and the nose cone to the exposed front end of the coupler (use CA for the latter).

Wood stringers were added because the tube that was used was very weak. These would not be required for, say, a mailing tube. Mark 8 evenly spaced parallel lines around the tube using your favorite method. Glue the stringers on, alternating short and long lengths.

Cut four pieces of balsa to the dimensions indicated. Mark the BT20 tube and glue the spacers on. When dry, test fit the assembly into the rear of the airframe, making sure it mates with the nose cone assembly. With the nose cone assembly installed, glue in the motor tube assembly (should extend ~1" from the rear of the airframe).

For the fins, I used scrap pieces of plastic but balsa can be substituted. Glue the fins to the long stringers and a lug to one of the short ones. Feed the shock cord through a snap swivel and connect it to the inside of the airframe. I used the Estes folded paper method.

I left the nose cone its natural baby blue and the fins their natural gray. The rest is a hodgepodge. Use what you got! Buy fresh paint for a more 'serious' rocket.

Flight:
This rocket made several flights on C6-3's and C6-5s but a shorter delay is probably preferable. Performance was similar to a 18mm Fat Boy.

I fold the chute and wrapped the shroud lines around it. It is placed between the stuffer tube and the main airframe. The shock cord then is laid in next to the chute. The chute attached ~2 inches from where the shock cord attaches to the nose cone assembly.

Summary:
The main Pros of this rocket are: it was cheap and fun to make; it requires no wadding and is easy to prep. The only Con I can think of is that the plastic fins may be prone to breakage. (I flew this rocket on soft grass.) If I were to start over, it would have FatBoy-style fins.

One Shot

After losing my RocketVision Machbuster on a G55, I decided that I wanted to build a similar rocket that would fly on 29mm motors. I scrounged through my parts bin and, being a 'glass-half-full' type, named the result 'One Shot'.

Construction:
Parts List:

1. 29mm phenolic tube, 9.25" (Giant Leap)
2. 0.047" G-10 stock for the 3 fins (Giant leap)
3. 1/8" aluminum tubing for launch lug
4. one NC, BT-55 (Estes)
5. one spent 29mm casing for nose cone shoulder and motor block.
6. one eyebolt
7. 5' of 1000 lb Kevlar® cord (Pratt)
8. 3/4" x 25' plastic tape (Home Depot)
9. lead shot

I started with a leftover section of 29mm phenolic tubing. I decided to add a motor block so I cut a 1/2" section out of a spent 29mm casing with a razor saw. This was epoxied into the tube so that a G80 would protrude by 1/2". After playing around with Rocksim, I decided on a fin design that I found appealing. Since I wanted the fins to sit flush with the end of the rocket and I wanted to be able to wrap a layer of tape around the end of the rocket and the engine, the last 3/4" of the root edge of the fin was recessed by ~ 1/4" (see pic or Rocksim file).

Using a dremel cutting wheel, I slotted the tube so the fins would slip in. To install the fins, I inserted a 29mm casing, applied a small amount of epoxy to the slot, and pushed the fin in until it touched the motor casing. After all 3 fins were installed, the casing was removed. This was easily accomplished before the epoxy began to set. This method of through-the-wall construction really works well for small fins, especially with relatively thick tube (0.062"). The fins were straight and weren't going anywhere. When the epoxy dried, I applied fillets.

Since the NC that I had wouldn't fit in the body tube, I cut the shoulder-end off so it sat flush with the tube's OD. Another section of 29mm casing was cut to use as a shoulder. This fit snugly into the NC and was held in place with 2 small screws. I added nose weight (lead shot, epoxy and the eyebolt) so the rocket would be stable. Because the end of this little missile was now largely lead, I was conservative on setting the CG (see Rocksim file).

The recovery harness consisted of 5' of 1000 lb test Kevlar® line. One end was epoxied into the body tube and the other was tied to the eyebolt. In the end, I built a rocket that was probably heavier than it had to be and had a non-optimal fin design. Oh well, won't be able to see the difference at ~5000ft anyway! I didn't bother filling the grooves in the tube since I wasn't optimistic about getting this rocket back. I did, however, give it a layer of Krylon chrome paint. Maybe a shiny surface would be easy to spot???

Flight:
Because of the predicted altitude and weight, this is not a rocket I would want to fly at a model rocket field, so I waited for a HPR launch. The streamer was a 25' section of 3/4" bright orange plastic tape. When rolled tightly, this fit nicely into the 29mm tube. Estes-type wadding was used to protect the streamer. I was torn between using a G125-15 and a G80-10FWL. The 15 sec delay on the G125 was about right and the 10 sec delay was too short. Anyway, I ended up using the G125 in another rocket and hoped the 1000 lb Kevlar® would hold up. Well, One Shot exploded off the pad and, like my Machbuster, was never seen or heard from again.

Summary:
PRO's 1. High performance rocket built from leftovers in the parts bin. CON's 1. Another non-recoverable rocket (when flown on big engines)

Tip:
When surface mounting small fins on minimum diameter rockets, you can get added strength by slotting the tube and gluing the fins into the slots. An engine casing is inserted to keep the fin flush with the inside of the tube. You must make sure you work fast and don't glue the casing in. In my case I used epoxy and a phenolic motor casing (AT SU). The thicker the tube, the better this will work. (Idea 'borrowed' from the RocketVision Machbuster.)

Mini-Toobah

I received samples of the new Hartle Engineering solid resin nose cones from JonRocket.  Hartle offers three styles of BT-5 cones and one BT-20 cone. You can see all of them on JonRocket's nose cone page. This review will describe the cones and the rocket I built from the longer BT-5 cone, the Mini-Toobah. The name is a homage to Dave Weber's Toober, which has logged over 200 high power flights.

Construction:
The parts list:

* Hartle NC-5B cone
* BT-5, 6.25"
* Estes 1/8" launch lug
* Spent mini-motor case (could use a real motor block)
* Thin Kevlar®, ~ 12"
* Thin elastic, ~ 4"

The Hartle cones are solid cast and their outer surface is perfect. The cone-to-shoulder transition was clean on the longer cone but was rough on the shorter one. The later was easily cleaned up with a sharp hobby knife. The attachment point appears to be a paper clip...works as good as anything. I found them to be a loose fit in the Estes BT-5, which is easily fixed with a little tape.

The cones are, as expected, heavier than the typical hollow plastic or balsa. However, unless you're building a MicroMaxx model, the added weight shouldn't be an issue. I had kicked around building a micro but decided to go for a 13mm design instead. The shorter cone would make a great micro-Fat Boy or Big Bertha but I opted for a downscale of my El Tubo Loco.

This was a trivial build. From BT-5 stock, I cut one 2.5" body section and six 5/8" tube fins. These were glued together in pairs and then to the body tube. I used Aleene's Tacky glue throughout. A 5/8" long lug is mounted in one of the fins. I cut a motor block from a spent casing. This was notched and the Kevlar® was tied around it. There is not much room in the body minus the motor and the cone's shoulder. It was all I could do to pack the shock harness.

To make sure it is stable, I weighed the components and made a RockSim model. The cone alone is 0.24 oz and the entire rocket is 0.34 oz. It is quite stable.

Finishing:
I first scuffed the cone with fine sandpaper and shot a coat of PlastiCoat primer. I anticipated this will be hard to find in the grass, so I went for a bright finish using Rustoleum day-glo orange. I added two vinyl strips to snazz it up a bit.

Flight:
There wasn't much prep as it uses tumble recovery. It flew nicely on an A3-4 and I could even see it recover!

Summary:
The Hartle Engineering cones are nice to work with. In this case, their solid construction and weight helped with stability.

I like the design of this little rocket but, being small, it could easily disappear into rocket eating grass, shrubbery, or trees.



Spikor

This was a quick build featuring a mailing tube, recycled 29mm motor mount and fins, and a long plastic garden spike.

Construction:
When I spotted the yellow plastic garden spikes at Home Depot, I couldn’t resist getting one of each size. The spike on the bigger one, which was used here, is 15” long and the base matches a 2.125" mailing tube well. The smaller one is 10” and fits nicely in a 38mm tube. The latter will be the subject of a future review.

Please refer to the attached image to see how the spike was used. I made a coupler using the tried-and-true method of sectioning a piece of tube and joining it back together with the strip that was removed. This was then glued into a short piece of mailing tube. When I chopped off the hook that protruded from the spike's base, I left a slightly flat area. It turns out that the base of the spike fits well in the coupler as long as the joining strip sits on the spot where that hook was. So, I attached some 1/8" Kevlar® to an eyebolt that I installed in the spike's base, inserted it in the tube assembly and sealed the base, which is inset couple of inches, with Gorilla Glue. I had thought that I'd drill a small hole in the nose section's body and pour in some foam. However, once the Gorilla Glue set it seems sturdy and, since the hard-point is on the heavy spike, I left it it as-is.

The body is a 23.75" long mailing tube. One good thing about saving bit and pieces of old rockets is you have bits and pieces. I found a recycled 29mm mount with rings to match this tube. It also had an attachment point for the shock cord. I attached some more Kevlar® to the mount and wrapped the lower 6" or so with a strip of car airbag. I sure wish I had more of this, but all that's left is a few scraps. I affixed it with a few twists of bare wire.

I used the G-10 fins from the now defunct Stratoshpere. One of the bad parts about having an assembled mount is you can't install it sans rear ring, so inner fillets are tough. I marked the mount so the fins would avoid the shock cord mount and motor retention hardware and swabbed on some 5-minute epoxy. When it set just enough not to run, I swabbed some more in the tube and inserted the mount. I slid all the fins in and babied it as the epoxy set. This means that I set it on end, eyeballed the alignment, and adjusted the alignment as the glue dried. The results are not perfect but are pretty good. I then masked the fins and added some Super-Fil fillets. I also added some epoxy clay between the base of the body tube and the motor mount just to toughen it up a bit. This will likely take the force of landing since the fins are up-swept.

I was a little concerned how the ribs on the cone would effect stability so I entered the design in RockSIM. Although the ribs do seem to push the CP forward, the spike is heavy enough to make it very stable. It looks like this will get to around 1800’ on a G64-8.

I treated the ends of the mailing tube with thin CA and primed the body with Painters Touch. I used a couple of coats and waited a couple of days before painting. The body is Rustoleum green hammered metallic and will no doubt appeal to Packers fans. The green will not help with recovery but I've never let that get in my way.

The blue painters tape that I used to mask the fins lifted some of the finish from the holographic stickers on the fins. Luckily, I had some scraps that I could use to cover the blemishes. I also printed a couple of stickers, one to pay homage to the rocket's name and one that just seemed appropriate. I sealed these with a clear coat and covered the edges with clear finger nail polish. I have found that stickers made from the Avery white label stock tend to peel up over time. This is mostly the case when it is used on curved surfaces like the body tube. Hopefully, the thicker polish will keep them tacked down.


Flight:
I used both a little dog barf and a small ‘chute pad. I decided on an undersized 18” ‘chute to keep the rocket in the same county. The G64-10’s delay was drilled to -8. The boost was fast and high as would be expected. Ejection appeared to be at apogee, although it was hard to see. It recovered close by and with no damage.

Summary:
The main point to this build is the garden spike nose cone. At $3.25, it was inexpensive. I think it looks pretty cool and appears to fly nicely. Others at the launch seemed to share my opinion.

Someone commented that this seems pretty dangerous. While it is on the pointy side, I don't believe it is any more dangerous than many MPR and most HPR rockets. Certainly no worse than conical fiberglass cones, for example.