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Amateur Rocket Launch Assist (ARLA)

Newsletters and Random Thoughts

This page will include progress reports, notes, random brain farts, and other ramblings.


May 18, 2003

Tubular Ramjets

Wow, a year has gone by since I last updated this. While physical work on amateur ramjets has been almost nonexistent the subject has always been on my mind.

A problem that has plagued me for as long as I've been working on amateur ramjets is attaching the ramjet section to the upper portions of the vehicle and keeping everything aligned and true. To say that I'll just put fins on a can and everything below is a gross misstatement of the difficulties that have arisen. My latest attempt was just another disaster. But then an idea that's been in the back of my mind for months became more clear.

The idea is to wrap tubes around a central tube. Six tubes wraps perfectly around a seventh of the same diameter. Such tubes can be assembled and aligned very easily. A shroud can then be wrapped around the assembly to improve external aerodynamics. Below is a picture of the thought process.

In this image you can see my attempt to "pinch" one can around another to give fins. The outer can would become the cone-type flame holder inside the combustion chamber. The second object is a pvc-rendered tubular assembly. All tubes are easily assembled and aligned. The third object is similar but with many tubes around a larger tube. The goal, now, is to get the cross sections correct. That is the intake area should be about 1/4 the combustion chamber area. A seven-tube assembly does not do this directly but a multi-tube assembly can.

The PVC used was Schedule 20, which is fairly thin but not exactly supersonic material. That said, I've spoken to pilots who've taken the USAF F-4 to Mach 2.7 with a fiberglass radome for short periods. For hot fire testing this could be done with steel conduit which is fairly thin. Because it's usually coated with cadmium this must be done with great care to avoid the poisonous fumes while the cadmium burns off. For flight weight construction thin-wall stainless steel tubes should suffice.

As for the combustion chamber the tubes can be partially cut away on the sides facing inward to provide an area of the right cross section.


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This Page created May 13, 2002

Last Updated May 13, 2002