Isogrid Spiral Welded Tanks

 

 

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.

 

 Isogrid Spiral Welded Tanks

 

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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12 Responses to Isogrid Spiral Welded Tanks

  1. ken anthony says:

    Imagine a single machine starting with liquid metal. Instead of rolling our flat sheets it rolls out already pressed to form sheets (like a ravioli roller) so no stamping is required (although it may still stamp for final dimensions before the curving operation. Only one adjustment would be required, tank diameter. Trim to length on both ends and recycle the material back to the start. It could make any length or diameter of tank.

    Plus it would be tremendous fun to watch. I’ve only worked with ceramic presses.

  2. john hare says:

    I agree it would be one better. I was trying to think of ways to do it within expected capabilities of new space.

    If it were to be done as you suggest, why not extrude a pipe section with all grids, stringers, and other supports as part of the single piece. With a bit of effort, the dome ends could be completed at the same time with a variable diameter extrusion. :-)

  3. ken anthony says:

    I’m having some difficulty imagining that, but I trust your imagination. I’ve may have been watching too many episodes of “How it’s made.”

  4. Paul451 says:

    John,
    It seems logical to induce the roll in the material as part of the stamp tool shape instead of trying to bend it after you’ve created the isogrid. And it seems equally logical to weld that spiralling strip into a cylinder just outside the stamp press, while also joining the outer skin as you go. Ie, you feed two rolls of sheet alum into one end and you get a continuous cylinder of tank barrel coming out the other. Cut for length, weld on temporary end-caps for transport, and you have a complete tank barrel ready to be taken to the next step.

  5. johnhare says:

    I don’t see why there would be two rolls of aluminum. I agree about rolling and welding it all in one operation. That way there would be quality control focused in one place.

  6. Paul451 says:

    Second roll is for the skin. (Or is the isogrid in tanks open?)

  7. ken anthony says:

    My understanding was the grid and skin were all of a piece?

  8. ken anthony says:

    Also, bending is a simple operation using a roller that is adjusted for tank diameter.

  9. George Turner says:

    Odd. I’ve been thinking about how to do this for a while, having giving up on the geometry of a moving extrusion die, especially as the face is too thin for extrusion to be a good production method.

    I’ve think a multi-part shoe (the hammer face) based on a collet that makes a first impact as a small triangle, then is expanded by a tapered ram that first pushes out the tips, forming the flanged section around the node, followed by dropping in the center section dies that then get rammed sideways by a second tapered ram (that’s coaxial with the first).

    It’s hard to explain it in words, but essentially the forging operation makes the small triangle, then expands the tips, then expands the middles. While one such hammer is down, one next to it drops, using the first die as the side-anvil for finishing out the rib and underside of the flange. So multiple hammers would move along the aluminum plate, much like someone stomping their feet. Any flash or excess on the top of the flange is easily machined off with a horizontal mill in a finishing operation.

    One other potential benefit is that the ribs would no longer have to have a constant profile. Since they’re being stamped from the side you could go crazy and even stamp in a truss-structure on their faces.

  10. George Turner says:

    Okay, I see another method to make an isogrid using solid triangular dies (undersized so they clear the flange when retracting) that move down and then towards each corner, sliding underneath a large die that sets the top of the flange) with groups of six dies moving in concert toward nodes, and the nodes in a sequenced pattern.

    To simplify thinking about it, first try it with a regular orthagonal grid with square dies. A set of four square dies will push down (typically halfway through the original workpiece, as isogrids tend to have equal mass in the plate and the ribs & flanges) and then toward their shared corner. Each square die is welded to a rod that extends vertically, able to push the die down and then sideways.

    So the dies can be grouped as A, B, C, and D, and the groups will be moved in X, Y by four steel plates (or eight so each die’s shaft is held at two points for greater rigidity). All the ‘A’ dies are welded to plate A, ‘B’ to plate B, etc, and the plates are moved in X, Y by hydraulic cylinders around the rim of the forging machine. All the plates have large open holes around any dies they’re not supposed to control, so plate ‘A’ has narrow holes for welding to the shafts coming up from the ‘A’ dies, and big holes around the shafts for B, C, and D dies to prevent contact. Another set of cylinders over the top of the dies pushes them down a fixed amount prior to the sideways motions.
    Between the dies and the plates is a large die that defines the top of all the flanges, so the dies are going to squeeze together sideways and try to push the aluminum into a rib and flange shape.

    If the aluminum can flow up and around successfully, then a triangular version should be able to forge isogrid.

    The other option is to use full-size dies to make an overheight rib, then a set of inner collapsing dies so the top of the overheight rib can be forged into a flange and the die pulled apart and removed. In that kind of operation the full-sized dies could possible be part of a large diameter roller forge, where tapered rib impressions are cut into the surface of the roller.

  11. john hare says:

    I can see a hammerhead die as you describe that does the initial smack straight down and then oscillates toward other dies in pairs to create the three dimensional I beam construction. Certainly worth investigating. It could be triangle, square, or even hex pattern with a little thought, and a lot of sweat.

  12. George Turner says:

    My concerns are either tearing the aluminum face sheet on the sideways moves, and that the move into a corner wil also pile up aluminum toward the sides of the die, and that the flange material will have to be flowing counter to the die motion. The excess on the sides would get flattened back down in subsequent moves till the entire rib is formed, so perhaps that’s not an issue, but forming the flange consistently might be pretty dependent on lubrication, if it works at all. I’ve been going through ASM and other books on forging and stil can’t say whether it will work or not.

    If I get time I might machine a test rig to try the simple sideways move into a corner and just see what happens, although I’ve never tried to forge aluminum, just steel (blacksmithing). We do have an electric kiln that would get the aluminum to a precise temperature, and machining is no problem (My friend started a non-profit engineering education company and people are constantly donating mills, CNC machines, plasma cutters, grinders, EDM machines, etc.)

    A few weeks ago, out of boredom, I started welding up a steel isogrid out of T-channel. Other than making a better, lighter, more advanced welding table, we’re a bit stumped for the final application for it.

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