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As of 1/4/2018, the latest edition of the Spaceport America Cup's Design Test and Evaluation Guide has the following requirements for SRAD pressure vessels. These requirements can be read in more detail here (add link).
4.2.2 DESIGNED BURST PRESSURE FOR METALLIC PRESSURE VESSELS
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Of course, an additional constraint on piston radius is the allowable space inside the Avionics Bay Coupler. The Team previously found that 6491K254, which had a 1in radius, was large and provided little room for Avionics to house its hardware, especially the batteries. Thus, a logical conclusion is to restrict the new piston geometry to radii below 1 in, which will provide an even larger safety factor on premature separation due to a pressure differential.
Geometry
Given Based on the calculations above, we conclude that the allowable bore-radius range of is 0.092 in → 1 .51 in (and more accurately, a 1 in upper limit), we opt to initially select a radius of 0.50 in, which is roughly in the middle. This is also a logical decision in. This range can be further refined to 0.25 in → 1 in because 0.5 in is the minimum typical bore diameter for tie rod air cylinders.
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First, it is necessary to determine an appropriate COTS solution for the piston. Due to timeline constraints associated with the difficult task of engineering base plates (most notably, all of the required seals), it is logical to take an existing piston and modify it to meet our needs (i.e. changing the throw on the piston, making mass saving cuts, etc). We recognized from the outset that we might not be able to find a piston that meets the previously selected 0.5 in bore diameter. In this investigation, I looked at we examined pistons between 0.5in 5 in and ~1 in bore diameter, with 1in being what I previously mentioned as a good option.
Solution No. 1: 1691T104
The first solution is a 0.5 in diameter " diameter (0.25" radius) compact tie rod air cylinder with a 4" stroke. Because the coupling section is 4.5", we need a much larger throw than that to achieve an appropriate factor of safety. Thus, it is necessary to replace the bore with a longer one (and also the tie rods). This is also necessary because the 1691T104 piston has a composite bore, which adds safety complications.
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- Rod end is internally threaded. We'd be making a new rod anyways, but there's always a question of compatibility...
- Different port size than our current piston. Not a huge issue because we'd probably redesign our actuation system anyways.
- Only two holes for mounting on each base plate–poor load distribution
- Not actually a problem, but its half the size of the desired bore diameter. We still have a good factor of safety on this diameter.
This piston has 4 inches of throw... Assuming that everything but the bore and the tie rods are identical, it may make most sense to actually purchase a piston with much less throw, such as the 1691T69.
Solution No. 2: 4211K121
This piston has a 9/16" diameter with 304 Stainless Steel as the body material and a 4" stroke length. Some notable advantages of this option are:
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- Again, need to machine a new bore due to increased necessary throw length as well as bore material (we want to avoid having a steel pressure vessel).
- Correspondingly need to machine a new rod and purchase new tie rods.Not actually a problem, but its way smaller diameter than the desired 1".
Again, it may make sense to go with a similar piston with less stroke length.
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- The square end plates fit the form factor we have already used.
- Stroke length is exactly what we need, so it is a complete COTS solution that requires no adjustment. We could make mass saving cuts, change rod material (which is currently 303 stainless steel).
- It's port inlet is 1/8 NPT, which for what it's worth, we already have compatible fittings for 1/8 NPT. This shouldn't be a driving factor, just a perk.Saves a ton of mass due to being a reduced 3/4" bore diameter, which is pretty close to the "optimal" 1in that I previously mentioned.
Solution No. 5: 6453K153
This piston has a 1-1/8" bore diameter made from aluminum with a 5.5" throw length. Some notable advantages of this option are:
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- The square end plates fit the form factor we have already used.
- Stroke length is exactly what we need, so it is a complete COTS solution that requires no adjustment. We could make mass saving cuts, change rod material (which is currently 303 stainless steel).
Solution No. 6: 6556K416
This piston has a 1-3/4" bore diameter made from aluminum with a 5.5" throw length. Some notable advantages of this option are:
Selected Piston Dimensions and Improvements
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