by Brian Best
The Sprint ABM was supposed to be the worlds fastest missile and got so hot the casing had to be ablative.The accelleration was 100+g.
The model from Shadow Composites (kit #21 and the first in the UK according to the manufacturer) is 1:12 scale and is constructed from two shrouds over a simple motor tube and ring assembly. The inner sheath is Nomex paper the outer is a fine, glossy, printed card. The construction depends mainly on forming the two shrouds into cones by curling and taping them. This was very hard as the Nomex shroud was thick and sprung back even from masking tape. I eventually curled this shroud very tightly and used rubber bands, when I released it after about 3 weeks it stayed curled and I was able to work it. The designers had got their maths right as, when glued along its edge 'tab', the nomex shroud fitted the motor mount centering discs perfectly. To add the outer shroud you use spray glue (3M spray-mount type stuff) and fold the shroud onto the inner. This requires a steady hand and nerves, cos you don't get 2 chances!
The shock cord is plaited Kevlar which looks like a shoe lace and has some elasticity. There is an inner nomex reverse cone that goes into the very tail end. I used CA for this and it went in easily. The nose cone is solid bass wood, turned needle sharp. Extra weight in the form of 4 washers is added behind the screw eye, instead of this I weighed the washers etc. then drilled out the nose, filling it with lead shot and epoxy which also anchored a kevlar cord loop to connect to the main recovery system. I had seen newsgroup postings suggesting the model was unstable, this mod added about 1oz to the nose, about 1" further forward. To finish the model off as near scale you need to construct the 4 tiny control fins which live half way along the body. I can't get the balsa bases to look smooth enough and curved to fit the body, so I left them off for the time being while I have a think about making fresh ones.
I flew the Sprint on the only motor stated in the instructions (a D12-3) on a calm day . It seemed to accellerate noticeably as the flight progressed as opposed to the woosh and coast of a normal model. The motor note is hollow and most unusual to fit this very unusual kit. All who have seen it fly (4 times now) are very impressed. This is not a kit for inexperienced modellers.