After following a discussion on ARocket for the last few weeks, I had a thought on building propellant tanks that is mostly a combination of other peoples’ ideas. John Carmack made a good case for spiral welded propellant tanks. Instead of sheets of material welded at all four edges, a roll of material is used to produce a tank out of one continuous piece of metal with a continuous spiral weld. You get the full hoop strength of the base material with the welds only potentially weakening the long direction. Others discussed methods of producing isogrid tanks, which are desirable for the relative stiffness for a given tank weight.
The only problem with spiral welded tanks seems to be that it produces a tank with uniform thickness walls. Not too bad for pressure fed at current sizes and pressures, possibly a problem in larger sizes or pump fed vehicles. The apparent problem with isogrid tanks seems to be enormous labor costs of milling in all the little pockets in the full thickness of material that must be removed. Well over half of the original metal is removed by machine tools or chemical milling with some mention of combinations.
I am thinking in terms of producing isogrid tank material from a continuous roll to produce a Carmack style spiral welded tank out of isogrid material. By creating the isogrid material in a continuous piece before it is formed into a tank, less labor intensive and wasteful machining operations become possible. I am thinking of hammer forging the isogrid recesses in an assembly line type operation. The aluminum roll is delivered and threaded through a hammer forge with a die to match the isogrid recesses. When the die hammers into the material, the aluminum is squeezed up into the grids similar to the way a cartridge case is formed. With the pocket material forced into the grid sections, no material has to be recycled or trashed as in the milling options.
I have seen video of industrial stamping with the hammer seeming to come down every second or less. If it took ten die hits to create a single set of grids of an inch in length, then the material would be moving through the process at six inches per minute or thirty feet an hour. Two hundred forty feet an eight hour shift of four foot wide material would be nine hundred sixty square feet. That would be about thirty feet of ten foot diameter tank. Depending on the height of the grids and the characteristics of the material being forged, it may be necessary to have multiple forging machines with annealing steps between them.
Pressing or stamping parts is frequently mentioned as one of the most economical of metal shaping operations. The machines are very old tech. The skill required after initial set up is reported to be minimal. The tooling dies should be considerably cheaper than the milling bits, especially over the life of a long production run. They are said to be low maintenance production machines. An appropriate machine could probably be found in a closed factory considering the economy and the off shoring of so much manufacturing.
Roll forming the material for the spiral welding process is also old tech that should probably be done as the stamped material exits the forging area. For tanks four meters diameter and under, it could be set right on the semi trailer as it exits the rolling operation even if it is not welded at that time by having supports built into the trailer to handle the incomplete tank to prevent deformation during transport.
A ten foot diameter tank one hundred feet long would require less than eight hundred linear feet of weld using the Carmack spiral concept. The dome ends would be a separate assembly of course. This would be ideal for an automatic welding process as the material would be fed past the welder at a constant rate dictated by the initial stamping operation. Extra material could be left on the edges to compensate for any loss of strength the welding caused. At the production rate of material I suggested, it would take just over a three shift day to do the ten by one hundred foot tube. Thirty feet of weld an hour is six inches per minute. This should be slow enough to allow continuous inspection with fixed x-ray and other devices as well as allow time for correction while the tank is very near the welding station.
This is not an area I have ever studied. I will be interesting to see if there is any merit at all in this thought post.
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