Bomarc (stomp) Missile with optional display detail

[Retired_for_now]

Well, I decided that the amount of fiddling I'm doing to add some detail to the Bomarc surface to air missile probably warrants a separate thread. I'll start with a quick recap for those who don't cruise PASA. I'll also include information and tips graciously supplied by you all in the first posts.

My idea was to build a demonstration rocket that is a little more interesting that your basic crimped-tube stomp rocket.





The Bomarc missile is a striking, and still futuristic looking design while being simple enough to render as a stomp rocket.



Stomp rockets get launched up to 100 ft vertically and so are vulnerable to trees or landing in a really wet patch of grass (not even Krylon can save it), rocks, the odd wall or fence, roof, etc. That means they get beat up and require replaceable nose cones (as a minimum) or must be quick and easy to build so you can make another. For the design, this means simple and few parts as robustly connected as possible.

The Bomarc fits the bill, with small flying surfaces and a cylindrical fuselage.



The ramjets were initially modelled as simple cylinders also, with a bit of gratuitous detail to add a cone for the inlet spike and an internal band to locate the spike. For strength, the engines are glued to the fuselage along their entire length.



As a first effort, it worked out pretty well. It's recognizable (if a bit of a caricature) and flew well from the start. This version is already posted for download in the Aviation section.

Next up - basic rigging for stomp-launched aircraft.

Yogi

[Darwin]

Yogi, it looks as if many of the existing airplane kits would lend themselves to the stomp bottle launcher. The Nobi Mig-21 for one....and I've been speculating about the old Modelart F7U gutless. The X3 stiletto is another good posibility. I'm stalled out at the moment on the Colorado build...maybe a little stomp-bottle excursion might be the thing to get the juices flowing again. Has anyone considered merging spud-canon technology to stomp rocket launchers? Or would it just be an instant confetti factory?

[Retired_for_now]

Originally Posted by Darwin

Yogi, it looks as if many of the existing airplane kits would lend themselves to the stomp bottle launcher. The Nobi Mig-21 for one....and I've been speculating about the old Modelart F7U gutless. The X3 stiletto is another good posibility. I'm stalled out at the moment on the Colorado build...maybe a little stomp-bottle excursion might be the thing to get the juices flowing again. Has anyone considered merging spud-canon technology to stomp rocket launchers? Or would it just be an instant confetti factory?

Darwin - just about anything you can put a tube into can be launched. The limits are the weight - for the stomp rocket launcher - and stability. The more technical version of the stomp launcher uses a 3-4 inch PVC pipe reservior, pressurized by a bicycle pump, and a valve to launch the rockets. One example is here:
http://cindispace.utdallas.edu/educa...r_complete.pdf
It operates at up to half the rated PSI of the pipe you're using, so for 3 inch, schedule 80 that would give you 110psi (225psi operating pressure) to work with. Making a pressure tube is quick and easy, so it would be a simple matter to make several and test them up to the pressure limit.
A propane/hair spray/lighter fluid powered spud gun might work. I'd worry about the peak pressure bursting the paper since it's less controllable than the air pressure versions.

Any flying model you can put a tube into could be made into a "stomp rocket." An aircraft with a cylindrical fuselage allows the lightest model but has it's own challenges in rigidly attaching the wings and other surfaces. The problem gets worse as the size of the wings increases. A Mig-21 should work, and mbauer has posted pics and video of his stomp launched F-104.





Pictured is a test glider made before I started the Bomarc design. It's a pretty good flier when tossed. however, the wings lack a carry-through spar or any reinforcement, so tend to flop around when it blasts off of the launcher. The relatively large and thick wings are also problematic - creates a lot of drag and aero force at launch speed. This was just a paper tube, the Nike nose, and some quickly drawn tabbed wings and tail surfaces.

The problem is not, generally, the launch. It's stability and control (coming up next).

Yogi

[Retired_for_now]

Most flying models get a first test launch with a quick toss downrange. Since the Bomarc was designed for the stomp launcher, I did the equivalent of giving it an easy toss - just put your lips together and blow ...

Using a piece of 1/2 pipe like a blow gun, the prototype was a pretty good flier. It entered a (somewhat random) glide and did an easy stall into the grass.

Note 1) Models and wet grass. Unless you're planning to discard the model after use, it needs to be sealed first. The damp plays hell with the graphics and paper both.

After making the usual adjustments - warping the wings to straighten it out and adjusting the tail surfaces to eliminate the stall - it flew very well. Nice long, stable glides of up to 50 feet from a standing, flat launch using the blowpipe. So, after a coat of Krylon and some time for the grass to dry it was off to the launching pad.

Note 2) Trim speed. All aircraft are optimized to fly at some specific airspeed. To go faster or slower requires changing the geometry (control surface forces, flaps, slats, etc.). Push an airplane configuration outside of that trim speed and it gets interesting.

The Bomarc came off the launcher and went completely random - fast, but random. The problem was trim speed. My wing warp that gave a nice straight glide generated a ferocious turn when launched at many times the natural glide speed. The little tail adjustment turned into a screaming pullup to a stall. On the plus side, all this culminated about 75 feet in the air and was followed by a stall into a reasonable looking glide (into a magnolia tree - but anyway). Subsequent launches were a little better, after backing out some of the aerodynamic adjustments (and straightening any bent bits). The problem was, making the prototype into a good rocket made it a not so good airplane.

The solution - weight and balance, coming up next.

Yogi (patience, more pictures coming up later)

[Retired_for_now]

Darwin,





If you can get it on the launch tube...

Yogi (had a flying Einstein hako too, no pix though. Gave it to a science teacher)

[Darwin]

Yogi, as someone who cut his teeth on HLGs, you are 100 percent on regarding the stability/control issues. Looking forward to seeing the next section you put out on this. Your reply verifies some of the other speculation I was having regarding a pressure tank approach to the launch system....I was thinking about using a 3/4-inch ball valve as the launch control mechanism. Mounting a pressure gauge on the air receiver wouldn't be too difficult, but the air inlet valve might be a bit trickier. Just a tire valve stem, or something more sophisticated? Is there any info around regarding an "optimal" tank pressure? I'm just guessing, but I don't think the model will stand more than a three atmosphere impulse. My first instinct for receiver dimensions is 4-in diameter by a foot long....overkill? Has anyone tried using a twin-tube launcher for twin-engined models (like the F7U)?

[Darwin]

The description of your powered launch sounds like the trim target for a HLG. I knew I had it right if I could get the glider to do an Immelmann on a level launch and settle out into it's glide at about 100' altitude.

[Retired_for_now]

Darwin, I don't have any personal experience with the air tank launchers. From looking at the various plans, you assemble the reservior (two caps and a pipe between), drill a hole to insert a standard tire valve stem to input the air, and fit an appropriate PVC ball valve for the outflow (weakest part of the design if you ask me, they should hold pressure but I don't know how fast they actuate/dump the pressure). The valve stem on your car's wheels just inserts into an appropriately sized hole, sealing with an inner wide rubber section, groove where it penetrates the wheel, and an outer wide rubber section - just pops right in.
No idea what an optimal pressure is - see above comments on the working pressure for the pipe used. A glued paper tube, top closed with a shotgun shell crimp and glued disk, will take a lot of pressure (see glider pictures above). The paper tube/rocket also clears the launch tube very quickly so you're not building up a static pressure load like a steam boiler. Likely, the rocket will be long gone before you even reach peak pressure - they are very light and accelerate like a home-sick angel.

[Retired_for_now]

Originally Posted by Darwin

The description of your powered launch sounds like the trim target for a HLG. I knew I had it right if I could get the glider to do an Immelmann on a level launch and settle out into it's glide at about 100' altitude.

Makes sense. The problem with the Bomarc, though, is there really isn't enough wing surface - especially when compared to a really good glider. If you check out mbauer's threads, he's launched an F-104 from the stomp launcher that looks a lot like what you're describing - except it was his first launch and so wasn't, as you say, trimmed quite right.

NF104 Starfighter

Yogi

[Retired_for_now]

So, the problem is that to get the missile to fly/glide you need to adjust the surfaces to provide the right aerodynamic forces - which are the wrong forces when launched hard.

I guess we need to zero out those forces, and the way to do that is to adjust the center of gravity (balance point) of the model so that it flies/glides without any tweaks to the wing or tail.

Finding neutral on the surfaces is as simple as adjusting them all flat, launching fast with a firm stomp, and watching. If it goes straight initially, great. If not, adjust and try again (did I mention replaceable nose cones?). Of course, once I got a nice, straight launch I also got a rocket that quickly stalled/tumbled and drifted back down tail-end-first. So, we need to add some weight - but how much and where?

You could go with trial and error - add a bit to one end, see what happens, and adjust (the fact that it stalled and dropped tail first says add weight to the nose, but how much?). The problem is wear and tear on the paper. I wanted to get one working right before building more. So, time for some cipherin'.

As a general rule, the center of gravity for flying things should be somewhere forward of the midline (based on area) of the main wing. To find the center of gravity put a dowel on your workbench and set the model crosswise on the dowel (here I started under the ramjet engines). Slowly roll the dowel toward the high end until the model tips down - note this point.

Now, roll the dowel back until the model tips again - your center of gravity is halfway between these two points.

Check it by balancing the model on the edge of a thin steel ruler - the flat surface provides just enough support for stability while being thin enough to precisely locate the CG.

To move the center of gravity forward I added a few disks of thick card under the nosecone.

Then it's back to the blowgun for some easy launches and glide tests. Remember, we fixed the launch stability by zeroing out the aero surfaces with the stomp launcher. Now we're working on getting the model to glide...stably...nose-end-first. I launched the model and added weight until I got a nice, stable glide again. What we're doing is adjusting the model until the aerodynamic forces needed for flight stability are the same as the forces for launch stability. The large aero forces from a fast launch are dominant in the latter case, while a glide relies on a mix of forces to convert the downward pull of gravity into a forward glide (yes all you engineers, that's over simplified - don't make me get out the Reynolds numbers!).

We're aiming for a stable glide, but we're not going to get the best possible gliding performance. The longest glide requires a little bit of downforce from the tail, to lift the nose and provide a slightly positive angle of attack for the wing, so the wing develops optimum lift as is slides through the air transforming the downward pull of gravity into forward motion ... but I digress.

The model seemed to fly best with the CG in the middle of the markings on the top of the wings (USAF and winged star) and a quick launch or three gave me a straight lift off and stable glide - though I haven't tried a really hard stomp yet since I don't want to fish the missile out of the bay, off the neighbor's roof, or run out into traffic. An easy 100 foot glide from a 10 degree launch is the best effort so far.

Yogi