Bomarc (stomp) Missile with optional display detail

[Retired_for_now]

Just to save a little wear and tear - cross referencing and indexing - most everything you need to know about sealing models is on this thread:
Spray sealing finished models

Yogi

[Retired_for_now]

So, we have a stomp rocket that is recognizable and works. Most importantly, it's 16 good sized parts on three pages that fit in a small (under 100 Kb) pdf file. You can print, score, cut, fold, bend, and glue one up in an hour or so - to replace the rocket that just got trashed.

But wait - there could be more!

We have a wealth of experience on the forum; people that are aviation/rocket experts and/or have lived and worked with the real thing. So when they ask, "How accurate is the model?" what are you gonna' say?

The simple Bomarc in the downloads is 1:40 scale; the scale determined by the fuselage diameter needed to accomodate the 1/2 inch PVC launch tube. Everything else is proportioned off of that using the photographs that are available. I scale the photos using a known dimension, then draw the parts over the photo, correcting for perspective (can you say projective geometry and optical distortion ... sure, I knew you could).





About 700 Bomarc missiles were made and stationed at 16 operational sites, the last one retiring in 1972. However, you don't see much detailed documentation on the missile - probably because it was extremely advanced for its day, highly classified and nuclear armed. Not too many vacation happy-snaps floating around the net.
You also don't see many on display, either. There are several Bomarc launch sites around, but very few display missiles. It turns out that thoriated magnesium is an excellent aerospace alloy for a high speed missile like Bomarc, but the thorium in the mix is radioactive (very low level alpha particle radiation that won't even pentrate your skin, but there you are). Thorium is commonly found in beach sand, but there's a lot of paranoia out there. The Bomarc missile displayed locally down here (Hurlburt/Eglin Aux 9 was the Bomarc test and operational training location) was removed from a local park many years ago for that reason (though I may have found where it went - Google Earth is some kind-o-spysat).

So, getting extreme or even moderate levels of detail can be difficult. That said, I took a look at everything I had to get an initial might-do list:
Primary focus areas:
1) The wing is flat without the knife-edge profile expected of a very high speed wing;
2) The ramjets are simple cylinders;
3) The ramjets attach directly to the fuselage rather than to pylons;
4) The horizontal stabilizers are attached to the sides of the tail rather than on top in line with the wing - a hold over from the pressure tube parts I recycled from previous efforts;
5) There is no prominent dorsal cable run (humpback) on the model, just a graphic stripe on the top;
6) The nosecone is a simple cone rather than an ogive built from either mulitple conics or petals;
7) The panel detailing is very sparse;
8) Insignia and placards are not complete.
With all that - I did get a good tail number for an A model Bomarc (sitting in the Hill AFB missile yard) and the major parts are in the right places.

So - to fix. The wing can be give some shape fairly quickly, and stiffened as well. A four inch wide, two inch tall isoceles triangle of thick (cereal box) card glued into the middle does the trick.



So, that was the easy one. Next up - motors.

Yogi

[jimkrauzlis]

Hi, Yogi!

I have to say I am intrigued by this thread...so much potential and information in one spot about stomp launchers.

As to the Bomarc, you probably already saw this link, but it contains over a dozen images of a Bomarc, that you might find useful:
Google Image Result for http://www.rocketryonline.com/jimball/jimball/scale-data/standard-arm1.gif
Look for the link for Tom Hilty photos of Bomarc.

I just hope that link works for you!

Cheers!
Jim

[Retired_for_now]

Thanks, Jim. There are some enthusiasts and veterans out there, just not many. Some nice photos there. Still a problem finding closeups, though I did find a walk around set that I think was taken in the Wright-Patt intensive care area - mostly aft & sides, not much on the top. Still hoping to sneak into the Eglin museum and find one.
Glad you like the concept - and just in time for the 4th. Stomp rockets are really useful for returning fire to the neighbors with the fireworks - or just keep the little humans busy during a cookout.
Yogi

[mbauer]

Originally Posted by 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)?

Hi Darwin,

Yes, there is an air tank made from PVC pipe. At the Challenger Learning Center here in Kenai, they use a utility cart. Mounted to the top is a small air compressor, a solenoid valve and an arming switch.

Will take pictures the next time I stop in.

Been doing plenty of testing,trying different things. was able to get a 4-oz model to about 15ft altitude and 20 ft distance. Most models fly fine if they are below 60grams (40+ft AGL). The lite weight ones easily go 100+ft.

These rockets are fun, cheap and easy to do. Got builder's block? Case of designers block? These will cure it all!

Mike

[Retired_for_now]

Concur on the fun, cheap and easy for stomp rockets, Mike. Of course, when you start adding detail the time sink really opens up. My hat's off (bald head glistening in the sun) to all you serious designers out there who are doing IPMS award quality work (the "P" stands for "paper," doesn't it?).
Yogi

[Retired_for_now]

The ramjet engines on the Bomarc are it's most striking and technologically advanced feature. While a ramjet is, in principle, the simplest jet engine it remains a challenging engineering proposition.
A ramjet is simply a tube with a constricted section in the middle. When moving (very) rapidly the air that enters the front of the engine gets compressed (at a cost in drag) as it tries to move through the smaller, constricted section. Compression heats the air (put your hand on a bicycle pump after pumping up several tires for a convincing demonstration). The faster the ramjet moves, the more compressed the air gets, and the hotter it gets. If the ramjet is moving fast enough the air gets hot enough to ignite fuel that's sprayed into the constriction - the fuel injectors are the second and final part of the ramjet. No moving parts inside the engine and only two pieces, what could be simpler?

Well, in engineering simple and elegant don't necessarily mean easy. You need to design the shape of the ramjet's tube to provide enough compression to heat the air without getting it too hot (melts the engine) or making too much drag (missile no go). You need to inject the right amount of fuel in just the right place so its burning further heats the air to make thrust as it expands out the back of the engine while not making too much heat (melts the engine, again), burning in the wrong place (makes big tail of fire instead of thrust or moves too far forward and - did I say - melts the engine) or wasting fuel (tank runs dry, missile drops from the sky) while ensuring the fuel sytem works properly (fire gets loose, melts the whole missile and it blows up).
The final model of the Bomarc missile could go over 400 miles cruising at Mach 3 and was tested at Mach 4 - pretty good engineering for any day.

The distinctive shape of the engines can be seen in the previous photos. Carefully decomposing the shape reveals a round inlet with a conical spike, an expanding conic shape at the front, a narrowing conic in the middle, and a short expanding conic at the rear.
To model that we'll need to trade the 3 pieces in the simple model's cylindrical engines (1-roll cylinder, 2-glue band inside front to make a shelf to locate the spike, 3-roll cone for spike) for a much more complex arrangement.
Starting from the back end, we'll need to:
1-roll a conic for the exhaust nozzle;
2-roll a conic for the middle section;
3-roll a conic for the forward section;
4-modify the glue band that makes a shelf inside the front lip to locate the inlet spike into a curved conic since it will fit inside a conic and not a cylinder;
5-roll a cone for the spike - which will need to be inserted either through the front (hole is smaller than the part ...) or from the back (still has to pass the "shelf" where the hole is again slightly smaller than the part.
6-add detail, the inlet spike closes up the front satisfactorily but the exhaust nozzle is an open hole. We can take the opportunity to make a couple of round formers to both hold the engine's shape, close up the back end, and even provide a bit of detail for the burner section.

Lot's more parts and a bit of a challenge to fit together. With some thought, caffeine, and fiddling I ended up with -
1) assemble the three conical sections;
2) glue the band into the front section;
3) insert the inlet spike from the front by slipping it in at angle and compressing slightly, then fiddle until it sits correctly on the shelf band;
4) insert formers into the front and mid sections using the flat end of a dowel to keep them aligned as you push them into place;
5) glue the front and middle sections together - I used a tabbed connection with the tabs on the middle section since the geometry tends to hide the joint and I didn't want to make one of those little double-sided tabbed bands. As always, there's a tab/no-tab debate with some informative tips for doing this type of connection here edge joining cylindrical parts ;
6) glue on the aft section - I extended the aft conic so it nests up inside the back of the mid section to make the joint self-jigging.

When done:


Of course, we can't glue these directly to the fuselage any more since the shape has changed.


So, another part to make and connection to figure out: the pylons.

Yogi

[Retired_for_now]

The Bomarc's engines mount on stub pylons. The shape is straightforward since we're joining a cylinder and a conic. Just (angled) straight lines. Two layers of thin card wouldn't have much rigidity - or edge gluing surface - so I added a layer of thick card (cereal box, again) between the printed parts. This stiffens things up and gives you a lot more gluing surface to work with. The flat surface also helps keep the pylons aligned so they don't sag or flop around.
Since the whole point is to add detail I didn't use a full sized inner core. As you can see, the core stops about 1/4 inch short of the ends. This allows the leading edge fold and trailing edges to form a sharp edge - a more correct shape than just a square brick.


The pylons are edge glued to the ramjets. Then after the glue sets, to the fuselage. Before gluing I spent a lot of time staring and fiddling to make sure the cut surface of the edge to be glued was square. Lots of discussion on how to do this with the most recent thread here: Treating the edges . I also used a 1/2 inch PVC pipe inside the fuselage to stabilize that surface while I positioned the assembly. Seemed to come out OK - symmetric and at the proper angle.




Of course, this is still a stomp rocket. So I had to try it out. After completion it went for four good flights before the shock of landing started to separate that thin glue line (well, actually the very top layer of card started to separate - way to go Titebond!). So, for a display detail this works well. For a stomp rocket, not so well since the pylon has less glued area than the simple cylindrical engine and is hanging out on a lever arm.
Solution? I added thin tabs to the top and bottom of the printed pylon parts. Cut them off for a display model and edge glue in place. Use them for a much more robust connection for a flying model.

Yogi

[avro202]

This is very kool to see!

We had a couple of wings of those up here in Canada too. Here is a pic of one I took last year at the Canadian Aviation Museum in Ottawa. You tend to forget how massive these things were until you get up close!

[Retired_for_now]

Nice shot, Tim! I'll defer to Bomarc (the master modeller, not the missile) but it looks like a B-model. Wings are forward of the A-model location and the ramjet tailpipe looks different.

The tailpipes are a bit of a worry at this point. There are shots where they look like solid converging/diverging conics (several test launch pics) and others that seem to have a variable nozzle aft of a cylindrical section (like yours). There were a lot of technical changes on fairly short time frames during this era - likely that's what was happening. At this point, I'll leave that issue alone pending further research or my normal forgetfulness.
Yogi (what were we talking about?)