Harpoon

This is a description of the building of the Harpoon, and eventually flying.

The Quest Harpoon is a sport scale model of the U.S. anti-ship weapon, which can be air-launched, ship-launched or submarine launched. I say sport scale because it generally appears like the real thing, but it is not exact. Changes have to be made to let this rocket fly well without a sophisticated guidance system like the real one employs. Usually this means larger rear fins and smaller forward fins and/or a slightly longer body.  This model has enlarged rear fins. Another important distinction is that this model is made up to look like the two-stage variant that is used for ship and sub launch, but this model only has one stage which does not seperate.  The air-to-surface variant doesn't have a booster rocket stage. The real missile uses a turbojet engine for the upper stage, so it is actually more like a cruise missile.

Here is a picture of the two stage being launched from a ship. The fins/wings are on hinges and pop out after launch.

This second picture shows the upper stage turbojet being used during cruise to the target, the booster not being used since this was launched from aircraft.

Whatever you want to call it, it looks cool, so I am building one. All those fins sticking out of it and the blue/white paint job is what I like, and I like military missiles in general. I was thinking of converting it to a real two-stage rocket, but Quest makes another kit called the Navaho which is a two stage, so I'll leave this one stock.

I begin building this kit by deciding how I want to install an altimeter compartment.  Normally, I just add a length of payload to the top, but that makes the rocket longer and heavier. Also, it is difficult to get parts sized to Quest standards, which use millimeters, while almost every other brand and the parts suppliers use the standard sizes of Estes. I had a lot of trouble finding couplers for Quest, in many cases even Quest doesn't supply them!

So instead I hatched a plan to use the space in the hollow nosecone. Normally that wouldn't be so difficult, but I found out the hard way that most glues, including epoxy and CA glue does not stick to this plastic! I went ahead and made one only to find the CA glue just cracked and seperated right away. So while taking that failure apart, I happened to notice that the hot glue that I used to glue the padding in did stick to the plastic, and stick very well.  So now I use hot glue to glue the critical door hinge piece to the plastic nosecone. I say critical because if that joint fails, it is likely the altimeter will fall to its death and be lost in a giant field of tall grass.

So here is the problem, inserting the altimeter into the nosecone on the left and keeping it secure in there during the high Gs of flight, but have it easily inserted and removed.

I start by cutting the base of the shoulder open, leaving the loop to attach the parachute and shock cord.

I create a hinge part out of scrap plastic (in this case, from a panel of an old busted VCR). The bottom is rounded to the inside curve of the nose cone shoulder, and roughed up a bit to give a good surface for the glue. It's hard to see in this photo, but the inside shoulder of the nose cone is also scuffed up quite a bit.

The plastic hinge is hot-glued in to the side of the nose cone, protruding just a hair beyond the edge. I apply a liberal amount of glue and then some, as a failure here could mean the loss of the payload.  The glue hardens in only a few minutes.

Next, plastic foam padding which was cut to fit is inserted into the empty space, leaving just enough space for the altimeter to be inserted.  Since the attachment loop is in the way, it has to be done in smaller pieces assembled like a puzzle.  Fortunately you don't need to be very precise with the foam, as it is soft enough to squeeze and bend to your wishes.

You could stop there, but I like to form a small box out of cardboard to fit inside the space. It allows the altimeter to slip more easily in and out of the compartment.

With the foam padding inserted, verify that it is a good, snug fit, then dab a few drops of hot glue to the foam to hold it in place.  The cardboard box is now inserted into the space.

A few more dabs of glue holds everything in place.  OK, it doesn't often look very pretty, but at least it is not seen in the finished model.  At least make sure the tiny threads from the hot glue are cleaned up and cut off.

The last step is to rivet a door to the compartment. Before riveting the door in place, make sure that when it is open it completely clears the opening for the altimeter.  When closed, make sure that it covers enough so the altimeter can't be lost with violent shaking.  This may produce an odd-shaped door.  Make sure you have a tight rivet, the door should be difficult to move.  If it is loose, the altimeter may slide out.

Try to file, cut or sand smooth any hard edges on this compartment, or the shock cord or parachute lines may get caught and tangle on any protruding pieces.

Another thing to be aware of when making the door, do not try to make a tight seal. Remember that air must freely flow into and out of the compartment, or the pressure inside may be too high at apogee, resulting in the altimeter will reading too low. Small gaps around the outside are a good thing. If necessary, drill a few small holes in the door.  Remember also to add three or four evenly-spaced vent holes on the body tube, spaced just below the shoulder of the nose cone.

Here is an example of the design change for a Tomahawk Cruise Missile. Notice the size (and weight) difference between the original nose cone, and a payload compartment previously added to the rocket.  The balsa bulkhead and body tube is all extra weight and length that is not needed if you can use the empty space in the nose cone.










With the payload task complete, I proceed to build the rest of the rocket according to the instructions. When building the fins, I break from "tradition".  Notice that the fins are laminated with a thin cardboard covering. This adds strength to the fins, and provides a naturally smooth surface for painting. No time-consuming layers of balsa fillercoat are needed, just a thin bit on the exposed edges.

After laminating, I shaped all of the fins for least wind resistance.  To do this, round the leading edge (top edge) and taper the trailing (bottom) edge.  This elongaged tear-drop shape is the most aerodynamic shape for the fins.  Be sure not to taper the trailing edge too thin, or it will be too flexible, easy to break and will not have an even edge.

Another technique I picked up on fins is to poke small holes in the root edge and the body tube before gluing. Although I haven't yet had an issue with fins popping off even after hundreds of flights in dozens of rockets, the fin to body tube joint can't be too strong.  These small holes allow glue to seep into the wood and cardboard, and provide more gluing surface. More gluing surface = more strength.  When dried, imagine the glue mass with many tiny fingers on it that acts like small rivets.  I say try it, it can't hurt!

With all the fins cut, laminated, shaped and poked, it is time for attaching them to the rocket body. This is the fun part, now that I have built a jig to hold the fins in perfect alignment while gluing them on.  I simply insert the body tube and hold it with bungees, pop the fin in and align the two pieces.  When satisfied, I pull the body tube down a bit and squirt in a line of glue. Then I release the body tube and it presses against the root edge, held fast until the glue dries.  Make sure you excersize patience here, Many rocket builders and kit directions tell you to glue all the fins at once, but it is better to do one fin at a time and let it harden without being disturbed at all.

Work has been slowly progressing on the Harpoon. It naturally slows down with the finishing, partly while waiting for good weather to spray outside, partly to give the many layers of primer and paint to dry, and mostly because all that sanding - necessary for a good finish - is also tedious and hand-numbing.  But slowly the rocket is getting finished...in fact almost done. All I need is one last sanding and a final coat.  After that the painting is done and the decals and final assembly will go quickly.  Here it is so far, I decided against pure white and went with a light gray, but the gray finish turned out much darker than I imagined.  I regret not using white, but at least it will photograph better in flight.


Well I just couldn't handle the gray, so I sanded and repainted gloss white. Much better!  It happened that the gray underneath was well covered by the white, and if it wasn't, I wouldn't mind.

The package shows the nosecone in a different shade of white, sort of off-white. I could have done that, but instead I chose again to make this one stand out a bit with a bright silver finish.

The first coat of silver (Rust-o-leum) was horrible, so I had to sand it off and try again. This time I used an old can of silver that always worked much better and the results were good enough (I believe the problem was the Rust-o-nauseum from underneath).


So here it is in the decal shop.  Starting to look like a real rocket now!

And finally...she's done!

Here are the "formal" portraits:

Of course we can't forget the imposing "big view":

Here's the tail end and motor mount tube, painted silver:

A look at the modified launch lug, set on a standoff. I no longer want the dirty launch lug to rub up along side of the rocket, especially this white one.

Here is a close view showing the static vent ports near the top of the body tube.  The shock cord and parachute will normally be behind these ports.


This is there to allow for the internal air pressure to match the outside air pressure at any altitude. Inside this area (housed in the nose cone just above) is the altimeter, which needs the ambient air pressure to make an altitude reading.



One last look, I wanted you to see the decal gap from the Quest supplied decals. Looks pretty stupid, huh?


OK, so I have it done after about 1-1/2 years since I started (life got in the way). Now it's time for its first test flights (which may take a while depending on the weather's cooperation)!

SPECIFICATIONS

Number of Stages: 1
Overall Length: 20.5" 520.7mm
Body Diameter: T40, 1.575", 40mm
Overall Fin Circular Span: 5.7", 144.76mm
Motor Mount: 18mm x 70mm
Motor Retention: Spring Clip
Recommended Motors: B6-4, C6-5
Stock Weight: 103.6 grams, 3.65 oz.
Altimeter Mod Weight: 3.5g, 0.123 oz.
Nomex Weight: 6g
Overall Weight (Less parachute, altimeter, motor): 114 grams, 3.8 oz.
Recovery Method: Parachute, 14" diameter
Recovery Protection Method: Nomex sheet, 6"x6"
Shock Cord Mount: Kevlar
Altimeter Capable: Yes
Altimeter Payload Volume: apx. 0.45 cubic inches
Date Completed: June 21, 2014

FLIGHT LOG

Has not been flown yet.