The Design Space
Our redesigned piston must have the same form factor as a McMaster piston to allow for easy descope. Given the team's Fall 2017 semester experience with tie-rod pistons, we elect to continue using this style.
DTEG Requirements
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).
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You can find a complete list of DTEG requirements that affect the Recovery system on the Hermes Recovery System page.
Desired Performance
The piston must be able to supply enough force at its operating pressure to break the shear pins with a 2x safety factor, which is the safety-critical guideline for parachute components presented by NASA (Section 3.3.1.5). As of January 4, 2018, we are designing for 180lbs of shear pins, and thus the piston must supply 360lbs. The same source (Section 3.3.1.6) dictates a design burst pressure factor of 2x the maximum design pressure, which aligns with DTEG requirement 4.2.2. [5] Thus, we expect the piston to burst when it supplies 720 lbs. Here, we make use of thin-walled pressure vessel theory [2], paraphrased below:
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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 the allowable bore-radius range of 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 because 0.5 in is the minimum typical bore diameter for tie rod air cylinders.
Tolerances
To create the bore, we plan to take an existing piece of hollow aluminum tube and turn it down to the proper outer radius (the inner radius can be achieved by drill and then reamer). The chosen wall thickness of 0.0625 is achievable within the 0.5 thousandths diameter tolerance of lathes on campus.
COTS Solutions
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.
Solution No. 1: 1691T104
The first solution is a 0.5 in diameter 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.
Some notable challenges with this option are:
- 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
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
Some notable advantages of this option are:
- It is compatible with a sensor (which would cost an additional ~$60) that we may be able to use to tell Avionics if the piston hasn't fired. Not sure if this is any huge advantage though. If the piston doesn't fire, then we're sorta out of luck at that point.
Disadvantages include:
- 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.
Again, it may make sense to go with a similar piston with less stroke length.
Resources:
The following resources are useful materials for learning about pressure vessel and piston theory:
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