Designing Carbon Fiber Motor Casing around CTI Pro 75-4G... Need Grain / Spacer Dim.

[Dogbert]

Hello out there!

I'm an Aero. Eng. Undergrad testing a reusable carbon fiber motor casing designed around a CTI Pro 75-4G 5015L1115-P reload. My team and I intend to use the propellant grains and spacer O-rings but, we will provide our own insulation and Nozzle. My question is: Does anyone out there happen to know the dimensions of the grain spacer O-rings Cesaroni uses in their Pro 75 reloads? ... specifically their width? I need to know this so that I can figure out exactly how long the case needs to be. At present, this is the only dimension totally unaccounted for. Although, if anyone could confirm the length of the grains themselves, that would also be extremely helpful. I currently have information that they are 5 5/32" long...

The forum has been extremely helpful to me in the past but this is my first time posting anything. I really appreciate any help anyone can provide. Thanks!

[bandman444]

Hopefully this helps.

http://pro38.com/pdfs/Pro75_dimensions.pdf

Bryce

[Dogbert]

Sorry, Bandman. No Joy. Everything I need to know is located inside of Dimension 'A'. The casing we've designed will be an entirely different external dimension that the CTI Pro 75 because of it's different closure config. (designing for composites) however, the combustion chamber will be the same size. Maybe the included image will help depict what I'm getting at. Unlike CTI, our nozzle is only partly submerged. I'm trying to figure out the size of the gap between grains and confirm the grain length. Thanks for your response though. I really appreciate it!

[Dogbert]

Well adding an image failed miserably... sorry still learning the ropes posting around here. Maybe this link does the job: https://www.facebook.com/photo.php?f...type=3&theater

[chadrog]

Have you asked Cesaroni directly?

[COrocket]

A grain spacer measures 5 11/32"

[Dogbert]

Chadrog, I've left them some messages but really need a response pronto so, I figured I'd try all avenues hence... Here I am. But yes, I do hope to hear back from CTI at some point. I like cross checking things.

[chadrog]

Chadrog, I've left them some messages but really need a response pronto so, I figured I'd try all avenues hence... Here I am. But yes, I do hope to hear back from CTI at some point. I like cross checking things.

Good deal. Very interesting project, hopefully CTI will be willing to help you along.

[Dogbert]

A grain spacer measures 5 11/32"

Hmmm, perhaps the missing 3/16" between the grain dim. and the grain spacer dim. is making up for the O-ring that's not there anymore. That is a common o-ring size. Thanks!

Still looking for responses though, if anyone has better info. Also, if anyone's tried making composite cases before and has advice, I'd love to hear it. We're flying on a lot of best engineering guesswork right now lol.

[Dogbert]

Good deal. Very interesting project, hopefully CTI will be willing to help you along.

Oh, everyone's willing to help for the right price lol. I've discovered things happen slowly when you don't have cash to offer. In this case I'm trying to save money by not buying the reload just yet. And of course no one running a business wants to release too much. There, are things I'd love to know about propellant formulas that would be very helpful but are also proprietary. That's cool though. I think small budgets and demanding timelines make us better engineers.

[xenon]

Hmmm, perhaps the missing 3/16" between the grain dim. and the grain spacer dim. is making up for the O-ring that's not there anymore. That is a common o-ring size. Thanks!

Still looking for responses though, if anyone has better info. Also, if anyone's tried making composite cases before and has advice, I'd love to hear it. We're flying on a lot of best engineering guesswork right now lol.

I was just previewing a post explaining that this would be the case when I saw this reply. You can confirm this from the drawing by noting that each additional grain adds 5.33 or 5.34" to the length of the case. Additionally, note that all motors aren't going to be exactly the same length: the instructions allow for the use or exclusion of the last grain spacer o-ring for this reason.

[mikec]

In this case I'm trying to save money by not buying the reload just yet.

This seems like a false economy to me. All the labor hours and material cost to make your case could go down the drain with one incorrect measurement assumption. Better to just buy the reload.

BTW, I thought Embry-Riddle used to collaborate with Loki for this sort of thing?

[Dogbert]

This seems like a false economy to me. All the labor hours and material cost to make your case could go down the drain with one incorrect measurement assumption. Better to just buy the reload.

BTW, I thought Embry-Riddle used to collaborate with Loki for this sort of thing?

You're probably right, and the time is rapidly approaching when I'll have to make that decision. This constitutes forward work at the moment as we're still in the middle of hydrostatic testing (just blew up a bottle last week at 2100 psi! ).

In response to your last: We did, until Jeff Taylor left the business. We were in the middle of several L3 launch projects when that occurred and couldn't wait for the transition period to end. We made the decision to switch our ops over to CTI hardware and found in the process that the propellant was not only much higher quality but also was easier to get since a lot more local vendors stock it.

[Jeroen_at_CTI]

You can confirm this from the drawing by noting that each additional grain adds 5.33 or 5.34" to the length of the case.

5.33 x 4 would be the length occupied by 4 grains + 4 spacers (2.5 mm each).

Jeroen

[cbrarick]

I'd go with using a commercial case: you really won't gain much making your own and if you do you will be doing EX/research because your case hasn't been certified for the propellant. Not sure what you have for machines but a composite case could cost much more then a commercial (why not spend the difference on the vehicle)

just my 2 cents

[Wingarcher]

There's educational value in doing a project like this. True, it's EASIER to buy a commercial case, but you don't learn anything in the process.

They're learning high-end composite fabrication, testing procedures, and hopefully the analysis to know that their structure was going to blow at around 2100 psi.

N

[troj]

Something to consider is that such a case will likely be single-use. I can't imagine you'll find resins that will stand up to the temperature/pressure for multiple firings.

-Kevin

[CarVac]

Whatever the Space Shuttle had holds up: I believe they switched from metal to composite cases, and they were reusable.

[bobkrech]

Whatever the Space Shuttle had holds up: I believe they switched from metal to composite cases, and they were reusable.

The space shuttle usds 120" diameter high strength steel pipe, never composites. btw it was never economic to resuse the steel casings considering the amount of rehab and transportation required.

Bob

[mikec]

Whatever the Space Shuttle had holds up: I believe they switched from metal to composite cases, and they were reusable.

No, they contemplated using filament-wound SRBs for launches from VAFB which were more mass-limited, but never actually flew these. The Advanced Solid Rocket Motor, also never flown, was a metal design AFAIK but with some changes to the joints and insulation to reduce mass and increase safety.

[CarVac]

Ah. I may have been mixing up the SRB's and the GEM's (graphite-epoxy motors) used on Delta rockets, which are composite but are not reusable.

[Dogbert]

Since it's been brought up. My team and I understand the benefits of using aluminum casings and how money is better spent on making a lighter, more durable vehicle, built with quality materials. I've worked on and lead 3 complex L3 projects myself all of which could have benefited from larger budgets for the airframes. (That being said we've made some insane rockets for ~$800 ) These all used traditional HPR SRMs from Loki and CTI but, this project has a different aim entirely. Those vehicles were all run of the mill 10 lb to 10k ft missions. This design is intended to support ERAU's attempts to produce a student designed / built sounding rocket capable of reaching the edge of space, ~62 mi AGL. This vehicle will use two ~6" dia stages with filament wound composite cases. Various design teams have gone back and forth on the merits of aluminum vs carbon and came to the conclusion that carbon dramatically reduced thrust demands by effectively halving the weight of the vehicle. It also maintains the educational spirit of the project through innovation since aluminum cases are fairly well understood.

My team's goal in our current development is to validate a proof of concept motor system similar, in all but scale, to the intended flight hardware. We also understand the issue of reusability with composites. However, reusability is currently a part of our mandate soooo.... we're doing the best we can to design around what we're classifying as limited reusability... good for 3 uses. I've had many a spirited debate with the powers that be over the merits of both aluminum and carbon in this regard but there comes a time when you just have follow orders. You all know what I mean lol.

Nonetheless, I think my team and I have come up with a worthy set of compromises flying in close formation. We've had past issues with porous laminates not sealing. This appears to have been solved by using Performance Rocketry Filament Wound Tubing. Even during failure cracks and delaminations seem limited to the outer plies (which is consistent with expectations the stress is highest on the skin of a pressure vessel) meaning that seal against the inside is maintained until burst. If sealing becomes an issue again, we may need to investigate a separate plastic sealing liner. and yes, no epoxy matrix will survive a burn without special care taken to thermal protection. The glass transition temperature (Tg) of an FRP composite material is typically 50 °F (~10° C) above the matrix material‟s cure temperature. Most modern aerospace grade epoxies are cured at 350 °F (~177° C). Thicker insulation (we're planning on 0.25" thick phenolic) should solve any heating issues. The attached pdf pic might help show the mostly current state of design. If anyone's reeeeeally interested I can send you our design review doc but I won't subject you to that tortuous read unless you ask for it lol Thanks again for all the support you guys! Sorry if this post was long or sounded frustrated at any point but this has kinda been my life for the last two months and it gets frazzling sometimes

[daveyfire]

We did carbon cases back when I was at SC. They're still doing them. The mass fraction benefit is quite useful, especially in 4" and above motors. The leakage issue was a sticky one for us as well. As for reusability, we built, hydroed, and fired a 2 grain 8" ID motor twice in the same case. The motor for the successful Mach 4.2 shot at Balls two years ago was basically a 114mm 22.5k case except out of carbon. I dare anyone to make an aluminum cased 114mm 22.5k motor go that fast.

Keep it up ERFSEDS dudes, it's a noble goal. Also, Brittany Fey says hi.

[Dogbert]

I dare anyone to make an aluminum cased 114mm 22.5k motor go that fast.

Keep it up ERFSEDS dudes, it's a noble goal. Also, Brittany Fey says hi.

Dude, that rocks!!! Yup, no one out there is goin Mach 4 with their hair on fire on an aluminum casing that's for sure! How did SC eventually solve their sealing issues? After getting positive results with higher quality filament wound tube we figured our problems were mostly caused by poor manufacturing capability. Since we can't afford / have had difficulty acquiring real nice custom tubes we're trying to build our own computer controlled filament winder to get away from buying airframe tubes like we are now and control the laminate properties in the future. We understand that composites are usually over-wrapped so that they just provide backbone for a thin metal. This way typical mode II and III composite failures are permitted without compromising sealing. However, lacking the means to produce this configuration we've been experimenting with the ability of the composite alone to seal up to a certain pressure over a limited number of uses. I wish we had more capability to proof test rather than just burst test. Also, how do you know Brittany? Tell her hi back for me lol. Do you work at Space X? I'm really hoping all this hands on work in practical applications can get me a job in commercial space.

[bobkrech]

Since this is not a high power rocket, but an amateur one, you mihgt find that a high strength maranging steel casing is the most efficient solution.

Also if you look up a few of my posts, you will see that 10" is the optimum diamete for flights to 100 km and for standard APCP, this can and has been done with a single stage rocket. The CSXT GoFast rocket reached 120 km and was 22' long and 10" diameter rocket.

Remember your propellants burn at about 0.5-1 cm per second. Unless you're planning and endburner, for a 10 second burn, the grain thickness would be 2"-4" thick. It does not pay to go very fast at low altitudes as the drag penalty goes at velocity cubed times the air density. The most economical rocket hit M=5.5 at ~40kft for this type of flight. Retained momentum is also important. Retained momentum is proportionl to sectional density, and although it may appear to be counter intuitive, a heavier, slower rocket wil go higher because of retained momentum. The motor and the rocket are not decoupled. and must be planned as a system rather than individual components.

Bob