arla-sab.htm
A sabot is a device that both centers the rocket in the launch tube and forms a pressure seal. These may take some design and experimentation but this is the approach I would try first.
I would try building a sabot from styrofoam similar to the one in the picture. It should be slightly larger than the tube to ensure a pressure seal. It should also be sturdy enough to take the full force of the gas pressure without collapsing.
This is an image of one type of sabot as it peels apart. This design uses a styrofoam rod that has been quartered and hollowed out. It was designed for the subsonic Sabot Cannon, intended to lob large numbers of paint balls.
This image is from Jesse Degraff's site at "http://www.vision-forge-graphics.com/" and can be linked to from Appendix D, Bibliography (so long as the images stays loaded).
This is a grapic simulation of a tank-penetrator rod separating from it's three-part sabot at supersonic speeds.
This image is from the site "http://www.msi.umn.edu/Projects/woodward/doe/" and can be linked to from Appendix D, Bibliography (so long as the images stays loaded).
A piston will form a pressure seal and transfer the force of the gas to whatever part(s) of the launch vehicle can withstand it. If the launch vehicle uses the annular inlet type of ramjet described in the section "Ramjet Construction" and uses fins on the rocket then a piston may be all that's required.
A piston can be as simple as a block of styrofoam. Or it may be built up from plywood. It must form a seal, maintain it's own position in the tube, and withstand all the forces on it.
The sabot or piston should be as light as possible, yet still do the job. In calculating launch vehicle acceleration the mass of both the launch vehicle and the sabot must be taken into account.
Gerald Bull, in his book _Paris_Kanonen-the_Paris_Guns_, describes some sabots he used in firing rockets from large guns. This is probably the best source I've found for describing sabots, pistons, and folding tail fins.
This Page Last Updated 22 Dec 98