John,
If the FSI model in question used a 27mm mount, I always build my clones with 29mm mounts. The 9" long RT-10 that is used for the central motor tube in the Intrepid is identical in size to Semroc ST-11, but its inner diameter is one tick too small for 29mm motors to fit. (My 29/40-120 RMS motor will go into ST-11, but no 29mm SU motors will do so. The tubing is a bit thin-walled for use with 29mm motors, anyway.) If you are building the kit stock, then you will need to create a 24mm adapter for it. (Easy to do with parts from Semroc.) If or when Quest ever releases those 28mm diameter black powder E's and F's that they announced last year, they will fit in that motor tube. Those Quest motors could in some ways be the reincarnation of FSI's mid-power motors; at least they will fill the same niche in the market. If you want to fly your Intrepid on 29mm motors, you will need to substitute an equivalent length of 29mm motor tube (such as Semroc LT-115). Semroc makes centering rings that will center LT-115 into the Intrepid's RT-17 airframe (which is equivalent to Semroc ST-18). I have yet to actually fly any of my 29mm-equipped FSI clones on a 29mm motor yet, though. (I haven't built up enough courage for that yet.
) It's a simple task to make a 29mm to 24mm adapter using Semroc components.
For 21mm FSI mounts, I install a standard 18mm mount. I don't know of any plans in the works for creating new motors in 21mm diameter. Quest's 20mm D5 would fit, but it only comes in a plugged version, and I don't know of any plans by Quest to release a version of it that has a delay and an ejection charge. The Quest D5's could be used in the Intrepid's outboards, but the motor has such low thrust that I don't know if using a pair of them would be worth it.
One other tip -- The instructions for most FSI kits with 27mm motor mounts call for gluing a thrust block into the motor tube. I never do that. The thrust blocks were meant for FSI's motors, and the spacing listed in the instructions were only for them. Since you won't be using FSI motors (I don't think), I wouldn't bother with installing the thrust block.
I have never laid eyes on an actual FSI kit; all that I know about them comes from the plans at YORP, the catalogs at Ninfinger, and some occasional posts on the forums from people who had built them. So I'm really interested in reading your build thread for this kit. I'm sure that it will fill in some of the gaps in my knowledge as well as answering a few questions.
One thing that I'm curious about is if the RT-8 tubes used for the outboards on the Intrepid really do have an inner diameter of 0.903". FSI catalogs list this tubing as having an ID of 0.903" and an OD of 0.921". I have questioned those dimensions, because they would result in a wall thickness of 0.009", which would be much thinner than any other paper tubing ever used in a production kit from any manufacturer in model rocket history. It would be about half the thickness of standard spiral-wound kraft paper tubing in that approximate diameter, and would be equivalent to a single wrap of index cardstock. An ID of 0.903" is approx. 22.94mm. Semroc's version of this tubing, ST-8F, has an ID of 0.885" which yields a much more believable 0.018" wall thickness. It's inner diameter is equivalent to about 22.47mm. RT-8 was the tubing that FSI used for its 21mm motor tubes, but those motors obviously did not have anything like a close fit in those tubes. FSI always recommended wrapping the motor with plenty of tape to achieve a firm friction fit. (FSI kits never used motor hooks.) If anyone ever produces 22.5mm diameter motors, then these would make great motor tubes.
I'm not asking you to go and mic the tubes, but if you happen to have any ST-8F on hand, would you be kind enough to compare the diameters of Semroc tubing and the FSI tubing? (Eyeballing them and feeling for any difference would be good enough.) Thanks in advance.
MarkII
