Experimental rocket motor design and construction

[Laszlo]

I'll try adding individual comments to each picture but some basic stats are:

1) Motor case is 2" T-304 seamless stainless steel thickness of 0.065" giving an internal diameter of 1.87". I chose this material because it will hold up to 1500 degrees fahrenheit and should not fail unless the pressure exceeds 2800+ psi. I have 3 lengths 18", 15", and 8" to accommodate multiple motor configurations. Also this case fits into a insulating cardboard tube which in turn fits perfectly into a 54mm engine mount tube.
View attachment 150619
2) The nozzles are from RCS, it is their 54mm model: Molded glass/phenolic nozzle for 54mm diameter motors. Dimensions: 1.985" O.D. 0.455" diameter throat 1.250" diameter exit. This gives a 7.55 expansion ratio. I'd prefer something closer to 9 or 10 expansion ratio and a divergence of 12 to 13 degree half angle but I don't have any means of machining my own graphite nozzle because it tends to gunk up a standard lathe. If anyone has means to machine a graphite nozzle please let me know and we can work something out. The stock nozzle had to be modified somewhat by removing some material on the lathe and turning a slot for an O-ring. The nozzle-retainer assembly has 3 o-rings for high redundancy.
3) I machined the nozzle retainer on a CNC mill and a lathe at VCU's machine shop (I've since graduated and no longer have access :-( to the facility). The nozzle retainer is made of 6061 aluminum and has 10 evenly spaced 8-32 tapped holes for machine screws. The o-ring slots are 0.081" deep, 0.121" in height and they are spaced 0.071" apart. I ran FEA simulations on solidworks and autodesk simulation software and the retainer can withstand many hundred pounds of thrust (I forget the exact number, I'll have to find the excel sheet where I did the calculation.)
4) The forward bulkhead plugs the motor and is similar in design to the nozzle retainer in terms of O-ring spacing and o-ring size (#134) My friend Ryan did an excellent job turning the piece to design specifications. The bulkhead is 6061 aluminum. It is 2" in height, 1" slides into the motor case and the other inch sits on its shoulder outside of the motor case. I still need to tap the bolt holes. Because the forward bulkhead absorbs the greatest load during a burn I will use ten evenly spaced 10-32 steel machine screws 0.375 inches long.
5) The finished propellant grain in one of the pictures is a magical formula. This grain has a specific impulse of 132 Nsec and burns at about 2748 degrees fahrenheit (1782 Kelvin). I have static tested configuration over a dozen times and it's well characterized. The burn rate at 1000 psi is 0.52 inch/second! very fast burn. Talk about Vmax.I have several videos of these static tests if anyone is interested in seeing them just let me know.
6) the cardboard sleeve that fits into the motor case is very dense and works extremely well in reducing heat conduction to the metal case. However it reduces the effective diameter of the propellant grain from 1.870" to 1.735". Case bonding with KNDX 65/35 creates too much thermal stress and would likely result in CATO so it is out of the question. However case bonding APCP is doable by coating the case with a thin layer of pure HTPB and allowing it to set after "spinning" Derek Deville has documented this in his Qu8k rocket report.
7) I have a purchased a kitchen aid 600 stand mixer for only $349 (as opposed to $549-$599 standard price) from a store called "Tuesday Morning" located at Stratford Hill shopping center in Richmond, VA. They have several in stock and they are all brand new and unopened. My friend Ryan purchased the the HTPB APCP experimental starter kit starter from Firefox enterprises. I do not however have any Aluminum powder/mesh to add to it as of yet.
Before I can test or fly my experimental motor at an official TRA launch event I must get level-2 certified. I hope to do that very soon.

 

[G_T]

Hate to say it, but you'll be unlikely to get to fly it at a TRA launch. We're not allowed to use Stainless, though I wish we could.

Gerald

 

[Laszlo]

interesting, I suppose I'll invest in an aluminum tube. Thanks for the heads up on that

 

[patelldp]

Get rid of the screws, as well. Use dowel pins.

 

[Laszlo]

could you offer some reasoning to this? I would like helpful feedback but I'm not going to just start changing the design without understanding the logic or reason behind it. Are these dowel pins threaded? removable? I've been buying parts from lowes.

Get rid of the screws, as well. Use dowel pins.

 

[patelldp]

could you offer some reasoning to this? I would like helpful feedback but I'm not going to just start changing the design without understanding the logic or reason behind it. Are these dowel pins threaded? removable? I've been buying parts from lowes.

Picture the screws that you have used. Effectively, they are as strong as the center cylinder, with the threads providing minimal strength. So, for the volume, and the hole size required to accept a screw, they are extremely inefficient and not designed for such an application. A dowel pin is much more resistant to shear loads than a screw.

 

[Area66]

This SS motor will be nice to make static test if you can't fly it.

 

[bobkrech]

I'll try adding individual comments to each picture but some basic stats are:

1) Motor case is 2" T-304 seamless stainless steel thickness of 0.065" giving an internal diameter of 1.87". I chose this material because it will hold up to 1500 degrees fahrenheit and should not fail unless the pressure exceeds 2800+ psi. I have 3 lengths 18", 15", and 8" to accommodate multiple motor configurations. Also this case fits into a insulating cardboard tube which in turn fits perfectly into a 54mm engine mount tube.

Hobby rocketry has standards for cases. Steel and Stainless Steel is not permitted. Hobby rocket cases are made from 6061T6 aluminum or equivalent only because if they fail, they fail in a known, predictable manner without making a lot of shrapnel. All cases must be designed to fail by spitting the forward closures and/or the nozzle as the area above and below the rocket is by definition cleared before ignition. The fore and aft closures should not fail below 2x MEOP, and the cylindrical casing should not fail below 4x MEOP.

2) The nozzles are from RCS, it is their 54mm model: Molded glass/phenolic nozzle for 54mm diameter motors. Dimensions: 1.985" O.D. 0.455" diameter throat 1.250" diameter exit. This gives a 7.55 expansion ratio. I'd prefer something closer to 9 or 10 expansion ratio and a divergence of 12 to 13 degree half angle but I don't have any means of machining my own graphite nozzle because it tends to gunk up a standard lathe. If anyone has means to machine a graphite nozzle please let me know and we can work something out. The stock nozzle had to be modified somewhat by removing some material on the lathe and turning a slot for an O-ring. The nozzle-retainer assembly has 3 o-rings for high redundancy.

Most research motors utilize Kosdon style casing as they are the simplest make and load. Loki Research, Gorilla, Tru Core and Animal all make casings, nozzles and closures which are built to the same dimensions so that parts are readily available and inexpensive. 2 internal C-rings are all that are needed for holding in the closures.

3) I machined the nozzle retainer on a CNC mill and a lathe at VCU's machine shop (I've since graduated and no longer have access :-( to the facility). The nozzle retainer is made of 6061 aluminum and has 10 evenly spaced 8-32 tapped holes for machine screws. The o-ring slots are 0.081" deep, 0.121" in height and they are spaced 0.071" apart. I ran FEA simulations on solidworks and autodesk simulation software and the retainer can withstand many hundred pounds of thrust (I forget the exact number, I'll have to find the excel sheet where I did the calculation.)

4) The forward bulkhead plugs the motor and is similar in design to the nozzle retainer in terms of O-ring spacing and o-ring size (#134) My friend Ryan did an excellent job turning the piece to design specifications. The bulkhead is 6061 aluminum. It is 2" in height, 1" slides into the motor case and the other inch sits on its shoulder outside of the motor case. I still need to tap the bolt holes. Because the forward bulkhead absorbs the greatest load during a burn I will use ten evenly spaced 10-32 steel machine screws 0.375 inches long.

This type of closure system is not typically used because it requires a lot of machining and creates stress concentration points.

5) The finished propellant grain in one of the pictures is a magical formula. This grain has a specific impulse of 132 Nsec and burns at about 2748 degrees fahrenheit (1782 Kelvin). I have static tested configuration over a dozen times and it's well characterized. The burn rate at 1000 psi is 0.52 inch/second! very fast burn. Talk about Vmax.I have several videos of these static tests if anyone is interested in seeing them just let me know.

??? Propellant manufacturing is a topic for the private research forum, not the propulsion forum. If you are high power certified, apply for membership. Until then please refrain from discussing formulations in the public propulsion forum.

6) the cardboard sleeve that fits into the motor case is very dense and works extremely well in reducing heat conduction to the metal case. However it reduces the effective diameter of the propellant grain from 1.870" to 1.735". Case bonding with KNDX 65/35 creates too much thermal stress and would likely result in CATO so it is out of the question. However case bonding APCP is doable by coating the case with a thin layer of pure HTPB and allowing it to set after "spinning" Derek Deville has documented this in his Qu8k rocket report.

Hobbyists normally use bates grains and in larger motors bond the grains to the liner. Case bonding to the actual motor casing requires a lot of knowledge, thermal calculations and is usually only done with single use motors.

7) I have a purchased a kitchen aid 600 stand mixer for only $349 (as opposed to $549-$599 standard price) from a store called "Tuesday Morning" located at Stratford Hill shopping center in Richmond, VA. They have several in stock and they are all brand new and unopened. My friend Ryan purchased the the HTPB APCP experimental starter kit starter from Firefox enterprises. I do not however have any Aluminum powder/mesh to add to it as of yet.
Before I can test or fly my experimental motor at an official TRA launch event I must get level-2 certified. I hope to do that very soon.

At present you don't know what you don't know. You need to work with a mentor who is well versed with research motors. If you friend is Ryan from MDRA, use him as a mentor after you get your L2.

Bob

 

[JMZawodny]

This SS motor will be nice to make static test if you can't fly it.

Your design is unconventional to say the least. I do agree with the possibility of static test application (which has been the sum of your experience to date), but am still concerned about the shear strength of the bolts. I concur with Bob that it would be much simpler just to use the commercial HW being made for EX work and it is not that expensive. I'm sure it is great fun to roll your own, but think about why you need to willingly take on unnecessary liability. This path has been well traveled and lessons learned along the way. Build upon the experience of others.

Cheers,

 

[JMZawodny]

As a follow on, we had an excited engineering student contact us wanting to fly his custom built rocket. He was very proud of the 10 pound all stainless steel rocket he machined and welded - including the custom 54mm integral motor casing. It was excellent workmanship. We tried hard not to completely dampen his enthusiasm when we told him there was no place or club that would allow that to fly. In his case it was an engineer student designing without any real knowledge of the requirements. Requirements and restrictions come first - always. Then you design within those. Only after a viable design has emerged and demonstrated that it meets the requirements - with margin - do you get to build hardware (or software for that matter). This is the way it is.

 

[DeeRoc29]

I'll add my two cents.

Your machining looks nice, and your design is not unconventional at all. Bolts have been used on rocket motors since the early days. Like Dan said, dowel pins are good, but they can be much harder to remove than bolts if not done right. If I were you, I'd stick with what you've got.

You can collect used Aerotech nozzles and reuse them. Cheaper than buying new from RCS. I've got one 54mm nozzle I've used in 4 research motors since its original K550 application, and it could probably be used again. The downside of your design is you have to machine your o-ring groove on the nozzle every time. I would cut a groove on the ID of your sleeve instead. Just like a 75mm CTI sleeve. Of course, the nozzle throat will open with each fire, so check your geometry.

Not sure where you are located, but you can fly your steel case at a place here in southern California legally. Not TRA or NAR affiliated. PM me if you'd like more info.

Good luck!

 

[BLKKROW]

https://www.mcmaster.com/#standard-dowel-pins/=p2ireu

These dowel pins are made to extract easily. Just something I found after reading this thread.

 

[Andre]

All this talk about steel not being an appropriate casing material is a complete nonsense. Steel is excelent material, especially for sugar motors where you can get away without any thermal insulation, due to relatively low combustion temperature. Propellant mass fraction is not very good in small motors, since density of steel compared to aluminum is higher, but that is usually not a concern in small amateur rockets. Also this myth about steel casing fragmenting into thousand shrapnels is just not true. It just does not happen. Shrapnels would form if you would detonate a high explosive inside the casing. Propellant burning is also called deflagration, which is something completely different (much slower) than detonation. When the case gets over pressurized it always fails at it's weakest point. That is usually at the seam if you are using seam welded tube, or the retaing pins shear, or the case fails in the middle. Anyway, I have yet to see a sugar motor with steel case behave like a hand grenade.
Screws are OK as well. There are hundreds of professional motors that use screws. If you size them correctly they work fine. For some applications pins work better and they are easier to use. You don't have to make threaded holes. My favourite are spring roll pins like these:

They are very easy to instal, very light and very strong. I use them on all my newest motors, like here:

[video=youtube;Q2YJtmsqKSU]https://www.youtube.com/watch?v=Q2YJtmsqKSU[/video]

 

[CarVac]

All this talk about steel not being an appropriate casing material is a complete nonsense. Steel is excelent material, especially for sugar motors where you can get away without any thermal insulation, due to relatively low combustion temperature. Propellant mass fraction is not very good in small motors, since density of steel compared to aluminum is higher, but that is usually not a concern in small amateur rockets. Also this myth about steel casing fragmenting into thousand shrapnels is just not true. It just does not happen. Shrapnels would form if you would detonate a high explosive inside the casing. Propellant burning is also called deflagration, which is something completely different (much slower) than detonation. When the case gets over pressurized it always fails at it's weakest point. That is usually at the seam if you are using seam welded tube, or the retaing pins shear, or the case fails in the middle. Anyway, I have yet to see a sugar motor with steel case behave like a hand grenade.
Screws are OK as well. There are hundreds of professional motors that use screws. If you size them correctly they work fine. For some applications pins work better and they are easier to use. You don't have to make threaded holes. My favourite are spring roll pins like these:
View attachment 150703
They are very easy to instal, very light and very strong. I use them on all my newest motors, like here:

[video=youtube;Q2YJtmsqKSU]https://www.youtube.com/watch?v=Q2YJtmsqKSU[/video]

You can argue one way or the other, but that won't make any headway when Tripoli's insurance won't allow them to allow steel motor cases.

 

[Andre]

You can argue one way or the other, but that won't make any headway when Tripoli's insurance won't allow them to allow steel motor cases.

I agree. I was looking from technical perspective. Rules and regulations are another. As in everyday life there are rules and regulation that have no solid ground, but you have to live by them if you don't wan't to have troubles. Same case here.

 

[Laszlo]

As a follow on, we had an excited engineering student contact us wanting to fly his custom built rocket. He was very proud of the 10 pound all stainless steel rocket he machined and welded - including the custom 54mm integral motor casing. It was excellent workmanship. We tried hard not to completely dampen his enthusiasm when we told him there was no place or club that would allow that to fly. In his case it was an engineer student designing without any real knowledge of the requirements. Requirements and restrictions come first - always. Then you design within those. Only after a viable design has emerged and demonstrated that it meets the requirements - with margin - do you get to build hardware (or software for that matter). This is the way it is.

The engineering student you speak of is.... me I'm the same guy.

 

[bobkrech]

I'll add my two cents.

Your machining looks nice, and your design is not unconventional at all. Bolts have been used on rocket motors since the early days. Like Dan said, dowel pins are good, but they can be much harder to remove than bolts if not done right. If I were you, I'd stick with what you've got.

You can collect used Aerotech nozzles and reuse them. Cheaper than buying new from RCS. I've got one 54mm nozzle I've used in 4 research motors since its original K550 application, and it could probably be used again. The downside of your design is you have to machine your o-ring groove on the nozzle every time. I would cut a groove on the ID of your sleeve instead. Just like a 75mm CTI sleeve. Of course, the nozzle throat will open with each fire, so check your geometry.

Not sure where you are located, but you can fly your steel case at a place here in southern California legally. Not TRA or NAR affiliated. PM me if you'd like more info.

Good luck!

Dee

You are correct that steel bolted casing designs are not uncommon in amateur rocketry, but it is not allowed in hobby rocketry, the definition being limited to launches conducted under TRA and NAR safety codes. Steel motor casings are used in many professional SRM designs, however most applications are in single use motors. Launches of these motors are conducted under range rules that hobby rocketry can't comply with. As you implied, you can go to RRS or FAR and launch amateur steel motor casings however it's a rather expensive proposition to go the middle of the Mojave desert and launch unless you live there.

Bob

 

[DeeRoc29]

Bob,

I agree with everything you say. I don't think anybody here said that it was okay to fire a steel-cased motor at a hobby rocket launch, and I think the OP is clear on that.

I completely understand the need to promote our safety code and to clarify where hobby rocketry stops and amateur rocketry begins. But as an educator, I have a very low tolerance for misinformation. When threads like this one pop up, I hate to see the OP scared off by discouraging-toned comments from HPR folks who may or may not have amateur rocketry knowledge or experience. We should be helpful to folks who show an interest in rocketry of any sort, shouldn't we? I think that "The Rocketry Forum" as it appears at the top of a Google search for rocketry information implies that we should be helpful. We can't expect everyone to know up front about NAR or Tripoli or the finer point of NFPA 1127. All I'm saying is that we should educate people, not scare them away!

 

[bobkrech]

As a moderator I try to educate and to prevent anyone for getting hurt. That's also why I accepted the role as Safety Lead for Sugar Shot. That' why I'm on S&T and teach rocketry and aerospace in CAP. Etc.....

I'm not trying to discourage Laszlo, nor do I think any of the posters in this thread were either. What we are trying to do is to point him in the right direction. He obviously has made sugar propellant and conducted static tests of his motors, and I suspect may have read Richard's website in some detail based on his casing design. In a University environment, there is oversight of students by others with knowledge and experience. After graduating not so much, so most folks recommend using a mentor with experience if you want to get into research rocketry. This also involves joining TRA and getting L2 certified. Not that the L2 cert means you become an expert in rocketry overnight, but at a minimum it means you have learned the rules of the hobby rocketry association and the legal aspects of rocketry such as obtaining a waiver for high power launching, and how to be safe.

We also suggested that he get high power certified so he can join the research forum were we can discuss research rocketry. I think this is encouraging, not discouraging, a young engineer to pursue the hobby he obviously enjoys, but to do so in a safe manner.

Bob

 

[Laszlo]

This SS motor will be nice to make static test if you can't fly it.

You sir will be notified instantly

 

[Laszlo]

Okay the forward closure has been finished. It is held in place by ten #10-32 s.s. socket cap screw. the drilling and hand tapping took hours but with both o-rings on it fits in perfectly. The nozzle end is assembled as well. Total length of the case is 15.5" and there is 12.5" inside available for four 3" long bates grains The total mass of the propellant is 1.47lbs (668 grams).

Maximum Kn (At/Ab) is 274. Maximum pressure is 794 psi. It should produce a maximum of 202 lbf thrust and an average of 165 lbf. I expect the burn time to be 1.16 seconds. The nozzle expansion ratio is 7.5 though I wish it were closer to 9.5 to 10.5. I am assuming 85% nozzle efficiency. The total Impulse is 848 N-sec and specific impulse is 129.5 Sec. Pretty low, but that's what you get with this type of propellant. This motor would be classified as a J732.

Of course this is all theoretical and I imagine actual results will vary by up to 75%-125%. I wish I had a load cell to collect actual data but I'm not there yet. Maybe I will place this puppy on a digital scale for a crude estimate of thrust lol. I will take video from a couple of different angles. I plan to post the static test video by Sunday night. Wish me luck.

 

[kramer714]

couple of things,

Waay back you mentioned that the tube would be able to hold something like 2,800 psi. Um, might want to check this, I think you will be around 1,500 at yield.

Steel vs Aluminum, did some work at the day job, if the case were to rupture, steel fragments have higher higher ballistic coefficient and higher penetration potential than aluminum. Not to say that steel dosen't make a good motor case, just saying if something goes bad, steel 'has the potential' to do more damage. This could be one of the reasons why hobby cases are required to be aluminum.

As far as a load cell goes, one quck and dirty way of measuring the force is with a pressure gauge. Attach the end to a cylinder filled with water, attach a pressure gauge to the cylinder. If you know the area of the cylinder, P(pressure) X A(area of the piston)= force. Have a camera set up on the gauge.

Cylinders are pretty cheep to pick up less than 20$ for a 2 inch cylinder on E-bay. A 2 inch cylinder with a 100 psi pressure gauge on it (most of the air cylinders are rated to 100+ psi) will read over 300 lbs of force.

 

[Binder Design]

Forget the load cell. I've found pressure transducers to be much more valuable. Any time I've used both a load cell and pressure transducer, the output curves look almost exactly the same. So if you just use the pressure transducer by itself, you can get a close enough estimate of the thrust by just scaling the curve properly. Knowing the pressure is much more important than knowing thrust IMO.

 

[Area66]

You sir will be notified instantly

thanks for this tread, it's now fix in my mind, next spring I start my motor test bunker.

 

[CCotner]

I also would check that pressure calculation... I just ran through the simple thin-walled cylindrical pressure vessel calculation for those dimensions of T304SS, and I get a max internal pressure of only ~1250 PSI using von Mises failure criterion...

Also, in general, stainless steel is not a good structural material. It isn't as strong as 6061-T6 (depending on the specific alloy of SS in question), and 6061-T6 is a third the weight.

 

[Laszlo]

View attachment MY chamber pressure calculations.xls

couple of things,

Waay back you mentioned that the tube would be able to hold something like 2,800 psi. Um, might want to check this, I think you will be around 1,500 at yield.

Steel vs Aluminum, did some work at the day job, if the case were to rupture, steel fragments have higher higher ballistic coefficient and higher penetration potential than aluminum. Not to say that steel dosen't make a good motor case, just saying if something goes bad, steel 'has the potential' to do more damage. This could be one of the reasons why hobby cases are required to be aluminum.

As far as a load cell goes, one quck and dirty way of measuring the force is with a pressure gauge. Attach the end to a cylinder filled with water, attach a pressure gauge to the cylinder. If you know the area of the cylinder, P(pressure) X A(area of the piston)= force. Have a camera set up on the
gauge.

Cylinders are pretty cheep to pick up less than 20$ for a 2 inch cylinder on E-bay. A 2 inch cylinder with a 100 psi pressure gauge on it (most of the air cylinders are rated to 100+ psi) will read over 300 lbs of force.

Thank you for the info. Here is how I calculate the chamber pressure this case can endure... View attachment MY chamber pressure calculations.xls



Am I doing this incorrectly?

 

[CCotner]

View attachment 151137

Thank you for the info. Here is how I calculate the chamber pressure this case can endure... View attachment 151137

View attachment 151138

Am I doing this incorrectly?

There are two mistakes with this analysis. The first is that you have not applied a failure theory; you correctly solved for the pressure as a function of the tangential stress (the hoop stress), but you incorrectly assumed that you could let that stress approach the yield stress of the material. Whenever you have more than a simple unidirectional stress, you have to do more than simply make sure the principle stresses do not exceed the yield stress; you must apply a failure criterion. This takes into account that failure under multiple principle stresses is a function if internal energy, not simply unidirectional stress.

Second, I do not believe your yield strength is correct. ASM lists T304 stainless as having a yield strength of 31.2ksi, and i also linked a datasheet that shows a yield strength of 35ksi, both of which are significantly less than the 75ksi you assumed in your calculation. The 3XX stainless steels are not high-strength alloys.

https://en.wikipedia.org/wiki/Von_Mises_yield_criterion
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A
https://www.alro.com/datacatalog/009-stainless.pdf

 

[Laszlo]

Ahh, you're right I missed that yield strength value. When I plug in 35ksi it still keeps me pretty safe in terms of hoops stress. I need to make sure not to allow for any cracks or grain separation from its liner because either of these scenarios would consume the propellant way too fast and the stress would likely be too much.

Another point I'd like to make is to acknowledge that I did not do a full von-misses stress calculation, also the analysis assumes this is an adiabatic event (haha far from it). I was mainly concerned with hoop stress only. I'm certain the forward closure will not blow out and that is where we have the stagnation point and the most stress. The nozzle end will get considerably hotter (I think) which could cause failure if any combustion gasses make their way to the actual casing. I have applied RTV to these most vulnerable spots.

Also I want to state that this analysis is only theoretically accurate at room temperature. The entire motor will be subjected to temperatures well over 2400 Fahrenheit, almost instantaneously. So it is safe to assume that the motor case only becomes weaker then predicted in the excel sheet. I just don't have the time to dive this far into analysis or I would never get around to testing this thing. I feel "comfortable enough" with this design to test it. The insulation liners I'm using are relatively thick "coupler" tube and they have done a wonderful job in the past.

My gut tells me that this static test will be 75% likely to succeed. If a CATO event occurs I will shed a few tears and lob some objects as far as I can toddler-style. Grain casting begins in 45 minutes. Static test scheduled for 6pm est, Sunday. Thank you for your most excellent feedback CCotner.

 

[Laszlo]

As a moderator I try to educate and to prevent anyone for getting hurt. That's also why I accepted the role as Safety Lead for Sugar Shot. That' why I'm on S&T and teach rocketry and aerospace in CAP. Etc.....

I'm not trying to discourage Laszlo, nor do I think any of the posters in this thread were either. What we are trying to do is to point him in the right direction. He obviously has made sugar propellant and conducted static tests of his motors, and I suspect may have read Richard's website in some detail based on his casing design. In a University environment, there is oversight of students by others with knowledge and experience. After graduating not so much, so most folks recommend using a mentor with experience if you want to get into research rocketry. This also involves joining TRA and getting L2 certified. Not that the L2 cert means you become an expert in rocketry overnight, but at a minimum it means you have learned the rules of the hobby rocketry association and the legal aspects of rocketry such as obtaining a waiver for high power launching, and how to be safe.

We also suggested that he get high power certified so he can join the research forum were we can discuss research rocketry. I think this is encouraging, not discouraging, a young engineer to pursue the hobby he obviously enjoys, but to do so in a safe manner.


Bob

Thank you Bob, you point out some good points, the safety and oversight of my university is no longer present, even though I was the only mech-E student there doing anything with propulsion, my advisor was always nice enough to make grains in her lab. You are right about Mr. Nakka, (Richard) I have followed his work for a couple of years and have read literally every page on his website including his thesis ( several times) he is one of my heros. I would love to work with him one day

I am TRA (#14521) member but not level 2 (I hate building air frames and such) but I am working my way toward it simply because I want to get involved with the HPR community and so that they will in turn look at me legitimately.

Laszlo

 

[bobkrech]

There are two mistakes with this analysis. The first is that you have not applied a failure theory; you correctly solved for the pressure as a function of the tangential stress (the hoop stress), but you incorrectly assumed that you could let that stress approach the yield stress of the material. Whenever you have more than a simple unidirectional stress, you have to do more than simply make sure the principle stresses do not exceed the yield stress; you must apply a failure criterion. This takes into account that failure under multiple principle stresses is a function if internal energy, not simply unidirectional stress.

Second, I do not believe your yield strength is correct. ASM lists T304 stainless as having a yield strength of 31.2ksi, and i also linked a datasheet that shows a yield strength of 35ksi, both of which are significantly less than the 75ksi you assumed in your calculation. The 3XX stainless steels are not high-strength alloys.

https://en.wikipedia.org/wiki/Von_Mises_yield_criterion
https://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A
https://www.alro.com/datacatalog/009-stainless.pdf

Just a few cautionary notes.

Global failure theories are fine in concept, but they only apply to ideal, homogeneous system. I'm not sure they are applicable without extensive modification to jointed systems with designed failure points. Rocket motor casing systems are designed to fail longitudinally, so they have designed with weak points or stress concentrators where failure will occur before the cylinder ruptures, so in most cases shear and tensile failure calculations of the joints should be sufficient to predict failure stresses. AT and CTI casings are designed to fail by releasing the thread closures before the casing ruptures or by tensile failure of the thinned casing at the threads. Kosdon style casings are designed to have the c-ring release before the casing ruptures or by tensile failure of the thinned casing at the C-ring groove. Bolted casings have stress concentration points at the fastening points which can be hard to model.

Additionally the strengths of stainless steels and aluminums vary greatly with heat treating, annealing and/or cold-working. Factors of 2 variations are not uncommon between datasheets. The strength values also decrease as a function of time and temperature, and it is not necessarily instantaneous. Failure testing is almost always done at room temperature which may not be applicable in operation where the time/temperature history depends primarily on the quality of the insulation and only secondarily on the temperature of combustion. In the professional aerospace world, room temperature hydrostatic test results are extrapolated to operational conditions by detailed thermo-mechanical modeling and then instrumented testing is conducted to verify the design. This is well beyond the capabilities of the hobbyist.

Bob