Peret Posted December 10, 2012 Author Posted December 10, 2012 You wouldn't need to instrument the shell. You could attach the mortar to a fast-sampling load cell rig and measure the recoil impulse, which gives you the momentum of the shell. Windage would make the figure slightly high, but as long as the shell was fairly massive the error would be small.
dagabu Posted December 10, 2012 Posted December 10, 2012 When kicking this arround, I had a number of thoughts. First off, Skylighter has a couple of charts: The amount of black powder lift needed for single break shells.http://www.skylighte...tables.asp#lift and Shell Burst Heights.http://www.skylighte...ables.asp#burst However, these would lony be ballpark figures and would change depending on the lift charge, the weight of the shell and shell diameter vs ID of launch tube, etc. To determine velocity, we used to sandwitch a frame with 2 pieces of AL foil seperated by a sheet of paper, and an identical one a foot away and measure the difference in time then the circuites were completed. This could also be done by breaking a light beam (laser or as in alarm systems) effecting a sensor that is normally on. Other approaches from rocketry are embetting some electronics in a shell with eithor GPS or an acellorometer(sp?). One method from, I believe, an older edition of the Handbook of Medle Rocketry involeves mounting an aiming stick attached to a protractor on a tripod, or similar and placing it a known distance from the launch point and recording the angle at apogee. Determining the size/height of things in images is part of the field of photogrammetry. An interesting paper on this subject can be found here:https://www.cia.gov/...i2a02p_0001.htm Does that suggest any good approaches to you? As to shipping things to Sweeden, it seems like most of the weight is in a single component that should be able to be obtained locally. thanks for the feedback! -t Yes, it can. 2" x 4" x 12" x 1/4" Aluminum is what we are currently using for the enclosures. -dag
Peret Posted December 11, 2012 Author Posted December 11, 2012 MrB, I made a mistake above. The accuracy of the load cell is not 2% as I stated, but 0.02%. I received a new batch today and checked the specs more carefully. All the other specifications that relate to error - creep, hysteresis, thermal drift and so on - are better than 0.02%.
dagabu Posted January 19, 2013 Posted January 19, 2013 As an update, the ACME Rocket Test Rigs are now shipping to interested parties. Please keep in mind when ordering that you will need to specify the rocket motor you want the custom holder to fit, the rig will come with one gratis and others will be available for a small fee. There are so many tubes out there that sending a sample (about 4" is plenty) to me will ensure you get the holder you are expecting. Dave
Peret Posted January 26, 2013 Author Posted January 26, 2013 Wow, that's a nice looking piece of gear. He's a craftsman, that Dagabu fellow.
tangent Posted January 27, 2013 Posted January 27, 2013 As an update, the ACME Rocket Test Rigs are now shipping to interested parties. Please keep in mind when ordering that you will need to specify the rocket motor you want the custom holder to fit, the rig will come with one gratis and others will be available for a small fee. There are so many tubes out there that sending a sample (about 4" is plenty) to me will ensure you get the holder you are expecting. Dave Is something more universal possible? Like the lathe chuck in post 12 of this thread? cost? thanks, -t
Peret Posted January 27, 2013 Author Posted January 27, 2013 Is something more universal possible? Like the lathe chuck in post 12 of this thread?It's not a matter of cost - this rig was always intended to come with a universal chuck. But we're having a quality problem. The chucks most recently purchased arrived already seized, too stiff to turn, and being as we're already three months late with deliveries we had to come up with an expedient solution until we get that sorted out.
dagabu Posted January 27, 2013 Posted January 27, 2013 (edited) Is something more universal possible? Like the lathe chuck in post 12 of this thread? cost? thanks, -t That is what was tried but only two chucks so far have been able to be opened and closed with no issues. They foul because of weather, BP, ash, sand, cold, etc. so they were rejected as a reliable way to hold motors. The goal of the ACME Test Stand is to provide the customer with a bullet proof way of testing rocket motors, the chuck was a possibility for holders. Not only that but the chuck only holds 1/4" of the motor wall, the caps hole about 1" of motor wall. -dag Edited January 27, 2013 by dagabu
MrB Posted January 28, 2013 Posted January 28, 2013 It would be a much larger mount, but a conical catcher, (Inside, like a very pointy funnel) and 3 wedges would allow for fairly safe mounting, while also being flexible in size, and ease of use. Preferably it would be made out of a lightweight material, know anyone that can shape plastic reliably? If i'm not making sense, let me know, i'll draw something, and try to explain my self. It's past 4 in the morning, i should be in bed, i blame that for any in-clarities.B!
MrB Posted January 28, 2013 Posted January 28, 2013 (edited) As i suck at drawing, i cheated, and only made some scribbles, leaving out any number of the wedges you'd like to add (i only stuck 2 in there, quarter offset, as is they wouldn't hold anything, you'd need at least one more) and i made the "rocket" a solid tube, giving the wedges & catcher/receiver transparency in different colors. It's just a concept, so please don't laugh to much. B! Edited January 28, 2013 by MrB
MrB Posted January 28, 2013 Posted January 28, 2013 The more i think about it, the wedges should probably be tweaked just a little. Instead of pulling them to a sharp point, make it a narrow tip, with a lip on it. The harder the rocket engine fires, the tighter the wedges will grip it. The drawback i see really is that this holder would have to be made of shatter proof plastic, and in the case of a CATO you'd still send the wedges flying, and being plastic, you'd probably need a new holder. On the other hand, make the holder out of metal, and it will pretty much just redirect the force from a CATO, the wedges should stay pretty much in place, but the whole thing would by necessity be fairly heavy... I'd prefer plastic, i think. But then, how would you get a new holder in the case yours blew up? You guys would have to "stock" them for spares.B!
nater Posted January 28, 2013 Posted January 28, 2013 I build build motors from 1/2" - 1" ID, which makes for a pretty big difference in the OD of the tubes. With the wedges, the larger tubes would not be resting all the way on the bottom of the holder. When the motor fires, some of the force could be used to further wedge the motor into holder and skew the results of the test. Even the New England Paper Tubes still flexible, especially on the end if there is nothing to support it. When I test motors, I don't test a full delay and I press a solid plug on top of the grain. This leaves some empty space in the tubes, which I would not trust to stay in place if the end of the tube was not flush with the holder. As much as I like the idea of the chuck for an adjustable holder, if it isn't reliable I would rather just have a spare holder for each size of tube I use and swap out as needed. It takes longer to prep a motor for testing or flight than it does to swap out the plungers on this rig, so time is not an issue.
MrB Posted January 28, 2013 Posted January 28, 2013 I build build motors from 1/2" - 1" ID, which makes for a pretty big difference in the OD of the tubes. With the wedges, the larger tubes would not be resting all the way on the bottom of the holder.No engine would rest at the bottom, thats pretty much the whole point. The limiting factor is how low you'd want the angle of the receiver / wedges to be, as a sharp point wedge needs a much larger receiver to fit the larger rocket engines. Size isn't the main concern, but it sort of gets silly when the smaller rocket engines are completely contained down inside the receiver. Even the New England Paper Tubes still flexible, especially on the end if there is nothing to support it. When I test motors, I don't test a full delay and I press a solid plug on top of the grain. This leaves some empty space in the tubes, which I would not trust to stay in place if the end of the tube was not flush with the holder.The above would provide a much larger "clamping" surface then the chuck first intended for use, (And more then the static tube holder) and force "wasted" on jamming the rocket further in to the holder was why i think the lip should be added to the wedges. If there is any give at that point, then your going to have a runaway rocket engine if your just using a shallow single size holder. You'd need something like a complete engine-sleeve to get more support. I don't think it's a problem. Ideally the wedges would grip a third of the rocket, to prevent them from crushing the tube / engine, but only a third of the wedges should need to fit down the receiver with a fairly pointy wedge. (low angle) You could get away with only a tube as the receiver, but then there will be a point where the rocket engine will push the wedges downwards while the force is applied far back on the wedges potential making them pivot, digging in at the rear, and opening up at the front. The rocket engine shouldn't be able to go anywhere but down at that point, but the test result would be invalid. The main drawback would be that it's not indestructible, and it might end up a bit large. Should still be lighter then the drill chuck, but shipping also has to calculate for size. B!
dagabu Posted January 28, 2013 Posted January 28, 2013 MrB, I think its a great design, I will have to look for some materials to play with and make a set. I do see your point about the wedges as well, keep up the good work. Nate, The plungers with a tapped hole are pretty easy to make, a cap for each size is easy.
tangent Posted February 26, 2013 Posted February 26, 2013 Any update on this? Was a solution ever found? What is the current availability of the units? thanks, -t
dagabu Posted February 26, 2013 Posted February 26, 2013 The cups work very well and are easy to make, no need to go any further unless you want an all-in-one solution. We are ready to go, just place an order with Peret. I have materials for a dozen more if there are orders for them.
Juiceh Posted February 27, 2013 Posted February 27, 2013 What are the cups made of? Have you CATO'd a motor in one? How well do they hold up? I'm gonna need 5 sizes!
dagabu Posted February 28, 2013 Posted February 28, 2013 Yes, I COTO'd a #1 benzo motor this fall in one of the holders and it didn't break or crack. The remaining tube just shredded leaving a 1" stump in the holder. I was trying to break it too. What are the OD sizes? I can make them up rather quickly or you can just use a few wraps of paper to bring them up to size. Motors are very stable in the rig when fired. the holders are made from PVC end caps, #1 run about $.50 each.
tangent Posted March 14, 2013 Posted March 14, 2013 The cups work very well and are easy to make, no need to go any further unless you want an all-in-one solution. We are ready to go, just place an order with Peret. I have materials for a dozen more if there are orders for them. How do you place an order w/ Paret? cost? -t
Peret Posted March 14, 2013 Author Posted March 14, 2013 Send me an email to petehand(at)gmail(dot)com. The cost is $400 but don't send money until it's in the box ready to ship.
tangent Posted April 8, 2013 Posted April 8, 2013 OK, just a query. I think it's been 3 weeks. You might have said a month. I've been watching my inbox. Still interested. -t
Peret Posted April 8, 2013 Author Posted April 8, 2013 OK, just a query. I think it's been 3 weeks. You might have said a month. I've been watching my inbox. Still interested. I think I said two weeks, which was too optimistic. Unforeseen circumstances intervened to make it late, but they'll be ready to ship this week. I'll accept your money now, check your email again. (I could have sworn I already replied to this, but I don't see my previous message.)
dagabu Posted April 8, 2013 Posted April 8, 2013 Like Pete said, there were some issues with an ATV that I had to deal with for the first batch but with all the kinks worked out we are at full operation now with 10 units ready by the weeks end. As you probably already know, starting up is the hardest part, there was lots of up front capitol that had to be amassed and then all the bugs had to be worked out. The plan is to keep them in stock (basic rig) and then to do customization (chucks etc) as the orders come in. As far as chucks go, I will provide a 100% ready to drop in chuck (2" miniature three jaw chuck) for $50.00 each plus shipping but will not come with any kind of warranty or guarantee for the chucks due to known issues. Cups will come standard and will only be a few bucks for extras when you order.
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