FastCargo
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Figured I'd start a build thread.
Considering this was the missile my aircraft could carry back in the day, I was surprised that I hadn't found any reference here to anyone building a scratch version of it, though I can understand why now that I've worked on one.
Anyway, as of right now the missile isn't complete yet, but it's close enough to show its progress.
You can Google the missile, either "AGM-69" or "Short Range Attack Missile" or just click on the "SRAM" on my signature to get more information on it. In short, the missile was a solid fuel INS guided SEAD weapon to be used to blast through air defense sites on the way to strike targets in the event of a full scale nuclear conflict.
The design of the missile is fairly straightforward with a truncated parabolic cone at the aft end (after the fairing is dropped for flight on FB-111s) and an ogive nose nose cone. The 3 fins are simple trapezoids with a roughly diamond airfoil (real thing, not this model). Each fin has a small fairing it's attached to, with one of the fairings transitioning to a wiring 'canoe' that runs along the top of the missile.
In general body design, one is reminded of simply taking a V-2 body and putting it on a diet. The result is a very good looking body, but not a whole lot of body tube.
I decided to model this with a BT-60 because Semroc (love those guys) had a nice tail cone as well as a nose cone needed for this model. Also, it would serve as a proof of concept model if I decide to upscale.
While doing some evaluations on OpenRocket, I had entertained the idea of using a BT-50 'stuffer' tube all the way through, use 2 tail cones and a small BNC-50 sized nose cone that would match the curve of the tail cone. The problem of course, would be making sure the nose cone actually matches the larger part of the nose...not necessarily an easy proposition. In addition, a smaller nose cone means less room to add weight if I need to drill the nose cone. This was a very important consideration, because of the small fin area, and the relative size of the engine to the overall model size (ie weight fraction). In the end, I went with a large BNC-60 nose cone.
I also considered if this was going to be a 'standard' engine flier or could it use 23mm engines. Openrocket showed that a 'D' rocket wouldn't get much higher than a standard sized 'C' power rocket due to the additional weight of the engine requiring additional weight in the nose to keep the CG on the stable side of the CP. So I decided to keep it a standard power rocket.
I wondered if a 'through the tail cone' mounting would be stronger than gluing the fins directly to the balsa tail cone. Several folks reminded me that the joint will be stronger than the balsa, so the break will typically happen above the fairing line anyway. So I decided to build that way, with one slight change. Instead of carving a fairing, gluing the fin to the fairing, then the fin/fairing assembly to the tail cone, I made the fins longer to glue directly to the tail cone, and built up the fairing with small wedges on either side of the fin attached to the tail cone, and a small piece to fill the gap, with wood filler to smooth out the 'sandwich'. In theory, this should result in a stronger overall joint.
Onto the build:
Picture one shows the general overall parts needed for the main body. What you see is a spare Alpha III body tube, plus an Alpha III engine mount parts. Also, you see the nose and tail cones, plus the cut BT-60 and 50-60 centering ring. I wanted to use the centering ring to help center the BT-50 tube and protect the tail cone that will be in the BT-60.
Pictures 2 through 4 were taken after the initial assembly of the tail cone, BT-60, BT-50, and centering ring. You can see the tube just isn't very long...and in fact I had to trim it later.
Considering this was the missile my aircraft could carry back in the day, I was surprised that I hadn't found any reference here to anyone building a scratch version of it, though I can understand why now that I've worked on one.
Anyway, as of right now the missile isn't complete yet, but it's close enough to show its progress.
You can Google the missile, either "AGM-69" or "Short Range Attack Missile" or just click on the "SRAM" on my signature to get more information on it. In short, the missile was a solid fuel INS guided SEAD weapon to be used to blast through air defense sites on the way to strike targets in the event of a full scale nuclear conflict.
The design of the missile is fairly straightforward with a truncated parabolic cone at the aft end (after the fairing is dropped for flight on FB-111s) and an ogive nose nose cone. The 3 fins are simple trapezoids with a roughly diamond airfoil (real thing, not this model). Each fin has a small fairing it's attached to, with one of the fairings transitioning to a wiring 'canoe' that runs along the top of the missile.
In general body design, one is reminded of simply taking a V-2 body and putting it on a diet. The result is a very good looking body, but not a whole lot of body tube.
I decided to model this with a BT-60 because Semroc (love those guys) had a nice tail cone as well as a nose cone needed for this model. Also, it would serve as a proof of concept model if I decide to upscale.
While doing some evaluations on OpenRocket, I had entertained the idea of using a BT-50 'stuffer' tube all the way through, use 2 tail cones and a small BNC-50 sized nose cone that would match the curve of the tail cone. The problem of course, would be making sure the nose cone actually matches the larger part of the nose...not necessarily an easy proposition. In addition, a smaller nose cone means less room to add weight if I need to drill the nose cone. This was a very important consideration, because of the small fin area, and the relative size of the engine to the overall model size (ie weight fraction). In the end, I went with a large BNC-60 nose cone.
I also considered if this was going to be a 'standard' engine flier or could it use 23mm engines. Openrocket showed that a 'D' rocket wouldn't get much higher than a standard sized 'C' power rocket due to the additional weight of the engine requiring additional weight in the nose to keep the CG on the stable side of the CP. So I decided to keep it a standard power rocket.
I wondered if a 'through the tail cone' mounting would be stronger than gluing the fins directly to the balsa tail cone. Several folks reminded me that the joint will be stronger than the balsa, so the break will typically happen above the fairing line anyway. So I decided to build that way, with one slight change. Instead of carving a fairing, gluing the fin to the fairing, then the fin/fairing assembly to the tail cone, I made the fins longer to glue directly to the tail cone, and built up the fairing with small wedges on either side of the fin attached to the tail cone, and a small piece to fill the gap, with wood filler to smooth out the 'sandwich'. In theory, this should result in a stronger overall joint.
Onto the build:
Picture one shows the general overall parts needed for the main body. What you see is a spare Alpha III body tube, plus an Alpha III engine mount parts. Also, you see the nose and tail cones, plus the cut BT-60 and 50-60 centering ring. I wanted to use the centering ring to help center the BT-50 tube and protect the tail cone that will be in the BT-60.
Pictures 2 through 4 were taken after the initial assembly of the tail cone, BT-60, BT-50, and centering ring. You can see the tube just isn't very long...and in fact I had to trim it later.
