Shepard v2.0 Dev Log (Structure Concept) Started 06-19-13
Conceptual development and experimentation done on the Shepard structure for version 2.0.
There was a conversation about the structure concept that Aaron created on Google+. I wanted to make sure it got captured here.
Chris Sigman Yesterday 8:02 PM
Aaron Harper Yesterday 8:06 PM
The motor mount is going to be the PITA. This will take some fabrication, but the rest of the construction takes 5 min, not counting bolting it to the brick.
Chris Sigman Yesterday 9:01 PM
What where you thinking for the design of the motor mount? I've got a picture in my mind (one based upon your little paper model and another).
Aaron Harper Yesterday 9:22 PM
Basically the paper model done in steel. I was shooting for 18ga 1018 mild steel, since this will contain an explosion with almost zero chance of coming apart, but it will be a real challenge to bend.
The design also created an engineered failure point should the explosion exceed the rating of the steel. With a crimped edge, the crimp will undo and deform the mount. this will start at the front and rear (depending on location of the breach), opening the enclosed area in a cone shape.
This cone allows the pressure wave to escape to the front or back in a way that will reduce pressure by increasing volume rather than attack a specific point in the motor mount, causing shrapnel.
Finally, if the pressure is simply too much, or the explosion started in the exact center of the mount, the ear on the top will shear and the crimp will let go very shortly thereafter, allowing the partially uncurled mount to deflect much of the blast and debris. This part will remain attached to the load cell since it is still bolted in place.
In such an event, I would no longer trust the load cell, and the temp sensors are likely toast, but this will make the test stand safer than the models the motors are supposed to power.
Jaye Sudar Yesterday 11:31 PM
The pictures came out much better than I expected.
Aaron Harper Yesterday 11:58 PM
Yes they did. Using my halo lamp as flood fill on top of the flash really helped.
J. Simmons 12:30 AM
Aaron, I like the look of this design. It is very sexy, and compact (great for kits). But I do have a couple of design questions/concerns. (Note, all of this should really go on ODE, but I don't want to lose any of these thoughts.)
1. I want to have a pretty significant discussion about motor mounts before much more work happens. We discovered lots of concerns on the first design iteration and as the primary operator of v1.0 I have some additional feedback. I want to make sure we create an opportunity to discuss those points.
2. How is the load cell going to be calibrated in the current design? In the previous design we used a pulley to turn gravity horizontal. I am not sure that is an option in the current design. And my next thought (just turn the test stand on its end so the load cell is facing gravity) both includes the load cell's mass in the calibration load and puts the test stand in an unstable orientation with a weight hanging off-axis, creating a moment that could unbalance the test stand and knock it over.
3. I assume the screws holding the frame to the block will easily withstand the shear load from the motor thrust, but we should document (and test) that to cover our bases.
4. Without a rail system, will we have any issues with misaligned thrust during firings? While the rails were large and creates some problems, they did at least ensure the motor's thrust only went in the desired line of force.
5. Where are you thinking of mounting the electronics? I know we never got that far with v1.0, but the idea was always to place the electronics on the back of the test stand.
All of that being said, I love the simplification of the new design over the original. It is compact, light weight (I assume), and it removes issues like the rails being fouled. This is definitely a huge step forward in terms of design for Shepard. And, it clearly has kit users in mind for assembly.
Aaron Harper 1:45 AM
Hi +J. Simmons thanks for the feedback. I will be capturing all of this to ODE, but I want to make sure we hit the high points while the iron is hot. To be honest, you have not brought up anything I haven't thought about, but it's always good to make sure I am thinking straight.
1. Discussion on the motor mount design: I would have it no other way. I think I have a good design, but I want to open this part to the discussion process to a lot of discussion before we start to bend metal. The question that's bugging me is this: How do we engineer a failure to verify the failure mode of the design? I get the feeling that working with the metal I have (16ga) will be a colossal PITA, but I don't want to build it out of something that will shred and turn to shrapnel either. We have got to come up with a way to test this.
2. Calibration: On my unit, I can just set the brick on it's nose and set the weight on the arm after the mount is removed while I stabilize the brick. On more solidly mounted units, I would recommend the purchase of a spring type hanging scale to attach to the arm with the motor mount removed. The other end of the scale should be attached to another immovable object using a turnbuckle. As you tighten the turnbuckle, the spring changes both the reading on the hanging scale and changes the tension applied to the load cell. This technique is one which will become mandatory as the test stands become larger.
3. The weak link are the aluminum angle brackets which are rated for 686 N of force. The M5 hardware is all 304 stainless with a yield rating of at least 400N/mm^2. This means that for a 5mm bolt, de-rated by 1 mm for the cut threads, we have a cross sectional area of a little over 12.5mm, resulting in a yield strength of over 5kN. Another weak link is the plastic anchors... which is why I would not include these in the kit. Mine are in there good enough to where I can pick up the whole assembly, brick and all, by the frame and shake it with no slop. I can confidently say that removal of the frame from the brick will definitely require much more than the 40 newtons of force an Estes motor can muster.
4. Force alignment. I was worried about that too, and at the distance the mockup demonstrated as a torque arm (34mm), it would take 12.5 kg of thrust to deform the sensor in any meaningful (+/- 0.25 degree) amount. Using a torque wrench and laser boresight, I verified the amount of deflection was less than a quarter of a degree at the specified torque, and that the sensor returned true. I would not want to use such an offset for larger engines, but for the smaller versions it seems fine. I have a design which does not feed to the side, but it will be much more expensive. I will document that in ODE as well as time permits, but it is overkill for Shepard's task.
5. Electronics: The electronics consists of the beaglebone, a daughtercard (they call them capes... I am seeing Edna from The Incredibles... "No capes!"), and a separate small board to mount under the motor mount on the frame. The beaglebone and "cape" can be installed in an enclosure on the right side of the frame in an enclosure only slightly larger than an Altoids tin length and width, and double the height. This would allow the enclosure to contain the DAQ system and battery for use wirelessly or through Ethernet cables. We're still working on the best way to pull that off.
The really cool thing about this is how compactly it can ship. The entire thing can fit in a 4x3x2 mailer and would weigh under a pound and a half, including the DAQ. the thing that consistently blows my mind about working with the aluminum extrusions is how light they are, yet can support an ungodly amount of weight, especially across their long axis. Anyhow, enough for tonight. I'll talk with you in the morning. :)
Chris Sigman 8:01 AM
One more question... how is the motor affixed to the motor mount so that it doesn't move around in there?
Aaron Harper 8:45 AM
The traditional way is a friction (interference) fit with an end stop and a clip to hold it in place. This isn't modeled on the mockup because I was just working on the basic shape. :)
Chris Sigman 9:26 AM
OK, I've drawn up an idea that once this is in ODE it'll be easier to share, but the idea is this:
The motor mount will primarily consist of a piece of steel tubing (18ga if you'd like, but since you don't have to bend it you could go higher). The tubing would be attached to the load cell by using adjustable pipe fittings to hold 2 right-angle brackets to the side, with a bolt through the angle brackets attaching them to the load cell. To hold the motor (allowing for various diameters of motor) to the inside of the mount, several holes will be drilled through on the opposite side (perhaps at 45° from center both ways) for bolts. A bolt will be held in place using 2 nuts: one on the outside of the tube, and one on the inside. On the end of the threading of the bolt will be a rubber footing, keeping the bolt from damaging the motor casing. Finally, if needed, one end of the tube would be closed off for the non-business end of the motor to be placed against.
Of course, a picture's worth a thousand words, and I don't have nearly that many there.
Chris Sigman 9:32 AM
I also have another item of note not entirely related: why is Shepard fired horizontally? Why not have it oriented so that the thrust fires up, pushing down towards earth?
Aaron Harper 9:55 AM
The trouble I have with a steel tube is the failure mode. A steel tube with no seam will allow pressures to build and exit at random, potentially causing shrapnel.
Shepard 1.0/1.1 use a cardboard mount to limit the mass and pressure during such an event, and thus potential damage. My design holds for a bit more pressure, but is designed to yield in a specific way (burst along the crimp while the bolted center holds.
The reason Shepard fires sideways is because this way the instruments measure only the thrust component and don't register the reduction in mass as the fuel is burned. This variable mass issue is why most solid and hybrid fueled designs, including the SRBs made by ATK are tested on their side.
In a liquid fueled design, the mass of the engine is fixed, and when the rocket engine is bolted to the stand the weight added to the sensor can be tared, adding the weight necessary to overcome it's own mass to the recorded thrust.
Not all test stands follow this though. Many of the high power rocket folks drop the motor with the nozzle up down a tube supported by legs (looks like a tripod). A sensor in the bottom of the tube registers the weight as well as the downward force when the motor is fired. Once the engine is done firing, the empty is weighed again, and the sensor data is adjusted based upon the linear reduction of mass over the run.
The trouble is that the consumption of fuel and reduction of mass is not quite linear. This method is for "close enough" work. The method we are using for Shepard is very accurate, depending only on the linearity of the sensor and accuracy of the calibration.
Chris Sigman 10:09 AM
With a tube, you have a few options for handling failure. The first is you could rather easily go with a higher gauge steel, because there's no need to bend it. There's also engineering a failure point. Now, I don't know much about that, but I would think you could drill holes along one side (the 'top' for example) that don't go all the way through.
Aaron Harper 4:20 PM
Hmmm... that might work, as it would open like a zipper. I'd like to keep the machining to a minimum though. I wonder if we couldn't use something 3D printed...
Aaron Harper 6:00 PM1
Speaking of pipes, I wonder if a length of MIL-T 6736B spec 4130 steel tubing with a wall thickness of 2.1mm open on both ends (except the retention ring) wouldn't just hold the pressure and laugh it off. Time to dust off my books again.
For those who want to do the math, use a 4" section of this pipe: http://www.onlinemetals.com/merchant.cfm?pid=7551&step=4&showunits=inches&id=1&top_cat=0 and figure on 3/4" orifices on both sides.
The engine for our worst case scenario would be an Estes E9-6 with 35.8 grams of propellant. I can't find the specs on the propellant to do the analysis, but it can't be too high test... the casing is cardboard after all.
This brings me to another point. If the mount were snug and unyielding as well as tolerant of the temperatures of a burn-thru, would the motor rupture at all? This is starting to climb outside my engineering pay grade... does anyone else wanna take a stab at it?
Aaron Harper 9:50 PM
Youtube video to go with it: http://youtu.be/Lgkbm5svo50
Jeremy Wright 9:56 PM
I'm tempted to copy the text from this post (since it's shared privately) directly into the dev log on ODE. Thoughts?
Aaron Harper 9:58 PM
I was headed in that direction myself. Go for it!
Aaron Harper 9:59 PM
I can build the structure (if I'm not gabbing) in under 7 min. :)