CTI Discussion Thread

The Rocketry Forum

Help Support The Rocketry Forum:

T34zac

Well-Known Member
Joined
Jul 23, 2014
Messages
944
Reaction score
31
Location
Massachusetts
Good to hear there is still some old stock of 29mm and 24mm Vmax. I flew an F240 late last year and still have one on hand. But I would love to get my hands on more, as well as some E75s
 

T34zac

Well-Known Member
Joined
Jul 23, 2014
Messages
944
Reaction score
31
Location
Massachusetts
I'm glad you mentioned that the L395 can be used with the AMW case and the hub cap. I have that combination in the works for an upcoming flight, but won't have any fallback if this were not to be feasible. Relieved to see that this works.

By the way, I hope no one is using the forward closure as a push point. Seems like a bang just waiting to happen.

Jim
Any 75mm load should be compatible with equivalent AMW hardware. And if your case is too long, up to two spacers can be used.
 

Dave A

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 22, 2013
Messages
1,174
Reaction score
129
"When push comes to shove" I only use 3/8 all threads no longer than 5 inches and only cases with no thrust ring groove. I have a reinforced bulk plate to mount that rod if the need arises. I typically am burning AT or CTI reloads. Thrust pushes the rocket up, the rod and bulkplate take the load, that is transferred to the fwd closure against, the seal disk (I use the seal disk on every load I can) Then against that heavy-walled liner which rests on the nozzle. Not worried about crushing the grains.
 

djs

Well-Known Member
Joined
Jul 27, 2014
Messages
3,369
Reaction score
251
Likely won't be many new propellants in the near future, but we are planning on certifying some more existing propellants, the Pro29 IM and more mellow are on the list.
Any thoughts of making the C-star propellant in 29/38mm cases?
 

Cesaroni Technologies

Lifetime Supporter
TRF Lifetime Supporter
Joined
Sep 29, 2020
Messages
79
Reaction score
161
Location
Gormley, ON
We've had a lot of inquiries about smaller Cstar motors. I think the propellant burns too hot for the plasic liners we use in the P24, 29 and 38 lineup, but we may look into it in the future.
 

cerving

Owner, Eggtimer Rocketry
TRF Sponsor
TRF Supporter
Joined
Feb 3, 2012
Messages
4,314
Reaction score
1,658
I understand what you're saying. When the motor is under pressure there is enough force on the forward closure to withstand the push from the motor. From my point of view (not saying it's the right point of view 🙂), since the forward closure isn't fixed to the case mechanically I wouldn't use it to push on.
I beleive on AT cases the forward closure screws onto the case, those I wouldn't have an issue with.
So a thrust ring that contacts only the case front (a la Estes) would be OK then?
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,155
Reaction score
2,439
Location
Melbourne Australia
The physics are fine as long as the motor is burning normally. Are there any conditions or motor types where this would not be the case? I'd be curious to hear from CTI on this as well.

I would point out that the motor isn't designed to perform this function and that the method brings other factors into play. Just some things that occur to me are what length of threaded rod will this work for - if the rod is 3/8 or 5/16 for example. Can I push on a 3-foot length of all thread? If the thrust is transferred to a bulkplate at the top of the coupler, then that bulkhead would take all of the force. You wouldn't design a motor mount with all of the force transferred to a glue joint on one centering ring? Is the bulkhead itself strong enough? Are the treads on the closure and the point where thrust is applied up to the task?

I was reviewing a college project earlier this year where, as best as I could tell, they were proposing this technique. I asked about it but things got derailed before they answered. But the above were a few things I was asking about for their project.

Jim
What sort of motor burning would result in the forces not being applied in the appropriate directions and keeping the forward closure in place? I can't think of any.

Of course you wouldn't push a 3' length of 5/16" allthread. That would be plain silly. If you are extending the distance between the motor an mount the extension needs to be capable of withstanding the buckling loads by some means. This is also something I do on some flights.

Why wouldn't I design a motor mount to put all the forces to the outer airframe? Seems sensible to me. If moving the force through centering rings they generally share the load, with the ones having fin tabs (when doing TTW) being much stiffer and taking the majority of the loads. You just need to consider where the forces are being applied and how they are moved from the motor to the structure, and design appropriately.

Note that taking the thrust on the forward closure actually reduces stress on the forward closure threads, contributing to reducing stress on the motor assembly.
 

tbonerocketeer

Sponsor
TRF Sponsor
TRF Supporter
Joined
Jan 19, 2009
Messages
2,579
Reaction score
132
I use 75 and 98 CTI motors in AT cases regularly. Most CTI loads are certified in AT hardware. The reverse is not true. There are a few AT loads certified in CTI hardware but not many.
Almost all AT motors are indeed certified in CTI Hardware. They will have a crossload sticker on them and very few do not now days.
 

HyperSonic

Well-Known Member
TRF Supporter
Joined
Dec 31, 2016
Messages
222
Reaction score
120
Location
Loony Tunes Land
Here's a pic of the Gen2 Boat Tail. I'm sure it has been tested by CTI, so I'm not saying that it hasn't. I plan on screwing this on to the end of a 5800 for a Mach 3 attempt. There are only three threads holding this on. How much force can three threads take? Not only the thrust but the weight of the rocket pushing against those three threads. I don't know all the numbers here, but I do know that I will be giving the Gen 2 Boat Tail a helping hand on this flight by sharing some of the load with the front of the casing. What do you think?

20210420_180431.jpg
 

Cameron Anderson

Well-Known Member
Joined
Sep 14, 2018
Messages
761
Reaction score
329
Location
Reno, NV
Here's a pic of the Gen2 Boat Tail. I'm sure it has been tested by CTI, so I'm not saying that it hasn't. I plan on screwing this on to the end of a 5800 for a Mach 3 attempt. There are only three threads holding this on. How much force can three threads take? Not only the thrust but the weight of the rocket pushing against those three threads. I don't know all the numbers here, but I do know that I will be giving the Gen 2 Boat Tail a helping hand on this flight by sharing some of the load with the front of the casing. What do you think?

View attachment 460727
Three threads or 50, strength is the same. Three is magic number. An aircraft engineer told me that once, I don't know the mechanical engineering or physics behind it.
 

HyperSonic

Well-Known Member
TRF Supporter
Joined
Dec 31, 2016
Messages
222
Reaction score
120
Location
Loony Tunes Land
Three threads or 50, strength is the same. Three is magic number. An aircraft engineer told me that once, I don't know the mechanical engineering or physics behind it.
CTI must have known this! I most certainly did not. If three is magic number, why would you ever use more than three on anything with threads?
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,155
Reaction score
2,439
Location
Melbourne Australia
Three threads or 50, strength is the same. Three is magic number. An aircraft engineer told me that once, I don't know the mechanical engineering or physics behind it.
CTI must have known this! I most certainly did not. If three is magic number, why would you ever use more than three on anything with threads?
Three threads engagement is a rule of thumb which works in most situations most of the time. When you get to situations where the threads are significantly loaded or there are some other relevant factors (for example, perhaps you want reduced engagament for some reason) it is wise to do the proper calculations. These days it would likely be FEA on CAD-based designs.
 

rocket_troy

Well-Known Member
Joined
Sep 9, 2013
Messages
199
Reaction score
72
Three threads engagement is a rule of thumb which works in most situations most of the time
Perfectly phrased. There will always be situations where that rule of thumb is marginal like for applications that are constrained to non standard fine threads (maybe due to wall thinness), but for standard threads, it should hold pretty well.

TP
 

Reinhard

Well-Known Member
TRF Supporter
Joined
Jan 18, 2009
Messages
1,150
Reaction score
358
Location
Austria
Here's a pic of the Gen2 Boat Tail. I'm sure it has been tested by CTI, so I'm not saying that it hasn't. I plan on screwing this on to the end of a 5800 for a Mach 3 attempt. There are only three threads holding this on. How much force can three threads take? Not only the thrust but the weight of the rocket pushing against those three threads. I don't know all the numbers here, but I do know that I will be giving the Gen 2 Boat Tail a helping hand on this flight by sharing some of the load with the front of the casing. What do you think?

View attachment 460727
An N5800 will generate shear stresses of about 1,500psi in those threads. The shear strength of 6061-T6 is 30,000psi.

Controlled sharing of the load is quite hard, because of the stiffness of the motor and the tubing. You will end most likely with a flight where the vast majority of the load gets transferred via one end. It's better if you decide which end should transfer the loads.

Reinhard
 
Last edited:

cbrarick

Wildman CT
Joined
Jan 23, 2009
Messages
2,609
Reaction score
326
Three may be the magic number, but non-permanent thread locker is the magic salve.....

just say'in
 

HyperSonic

Well-Known Member
TRF Supporter
Joined
Dec 31, 2016
Messages
222
Reaction score
120
Location
Loony Tunes Land
Thanks everyone for telling me about the three thread rule of thumb. I feel better about the boat tail being up to the task. That being said, I still think I will push from the front because everything is set up already in the rocket for it. I won't go into detail here, but if you want to see what I'm talking about with the rocket, I will continue on a thread(3) called CF Fin Attachment. Once again, Thanks for the help.
 

Dave A

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 22, 2013
Messages
1,174
Reaction score
129
According to the AT Crossload chart it shows all 75mm combos but stops at the 4g reloads.
I know for a fact the M1850 and seal disk will not fit in the CTI 6g case. I tried it. Closure will not engage enough threads to make it reliable. I'm guessing the same goes for the AT 5g loads since it's not on the chart either.
 

ECayemberg

Well-Known Member
Joined
Jan 21, 2009
Messages
2,657
Reaction score
291
According to the AT Crossload chart it shows all 75mm combos but stops at the 4g reloads.
I know for a fact the M1850 and seal disk will not fit in the CTI 6g case. I tried it. Closure will not engage enough threads to make it reliable. I'm guessing the same goes for the AT 5g loads since it's not on the chart either.
Haven't tried the 6G setup, but an Aerotech M1315 works perfectly in CTI 5G hardware; verified last weekend.
 

Mike_Andrews

Member
TRF Supporter
Joined
Jun 15, 2020
Messages
19
Reaction score
2
Location
Wilsonville, OR
Not sure if this is where to ask, but here goes.
I had a CATO today on the second flight of my rocket.

First flight was with an I180 Skidmark that was perfect (with the exception of the altimeter not actually recording the flight - bluetooth woes...).

Second flight was an I236 Blue Streak. Rocket ignited and then just off the rails went unstable and smashed into the playa. When I took a look at the motor the eject charge had not even fired. It looks like hot gas went out the side at the joint on the bottom of the motor. I did take the delay section out to dial back the delay charge. But other than that I didn't mess with the motor.

While I was at the local vendor table someone mentioned that maybe I missed applying grease? I don't remember having to do that before and I have flown CTI engines in the past where I adjusted the delay and didn't have any problems. Have I just been lucky or is this something that you don't do on the one shot motors?

Thanks,
Mike
(I'll have pictures up later once I am done showing the rocket debris to one of my friends...) :-/
 

crossfire

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jan 16, 2011
Messages
4,714
Reaction score
593
I have flew many CTI motors and very few did I ever grease the rings.
 

Mike_Andrews

Member
TRF Supporter
Joined
Jun 15, 2020
Messages
19
Reaction score
2
Location
Wilsonville, OR
Had a chance to get some pics in good lighting and it looks like the nozzle was blown out. Here are the images.

I don't see a date of manufacture or batch code on the motor package. The QR code just gives the motor specs.
 

Attachments

Mike_Andrews

Member
TRF Supporter
Joined
Jun 15, 2020
Messages
19
Reaction score
2
Location
Wilsonville, OR
I did get a cellphone video of the launch with my Galaxy Note9. I'm trying to find a video editor that will allow me to slow down and maybe get to a frame by frame view. At least then we should be able to get a timeline of the failure.
 
Top