Fixing a bent motor case.

[Gym_Class_Hero]

I found an old motor case (CTI 3G). :D Here's the catch, the threaded end of the case has a small dent in it (not very deep, but enough to keep the reload from screwing in. So I am trying to figure out a way to straighten it out. It's free, so I have nothing to lose, but I want to give it the ol college try. :D

What do you guys think would be the best way to try and straighten it out?

[Chrisn]

Sending us a photo will be the quick way :)

[ben_ullman]

I found an old motor case (CTI 3G) in Wildman's box of junk today, and he let me keep it for free. :D Here's the catch, the threaded end of the case has a small dent in it (not very deep, but enough to keep the reload from screwing in. So I am trying to figure out a way to straighten it out. It's free, so I have nothing to lose, but I want to give it the ol college try. :D

What do you guys think would be the best way to try and straighten it out?

if you have a flat head screwdriver small enough to fit between the threads put it in there and use a mallet to see if that will move it enough to be usable.

If not, send it to CTI and tell them its deffective ;) :p

Ben

[Chrisn]

Is it the thread which is damaged, or the casing has been dented and deforming the thread?

[ben_ullman]

Is it the thread which is damaged, or the casing has been dented and deforming the thread?

It sounds like the latter IMO. I.E. the dent is causing the threads to not be threadable.

Ben

[Binder Design]

Support the casing securely with a pipe vise being careful to use a shop rag around the casing as to not mar it.

Use a wooden dowel just a bit smaller than the casing ID, put it in about halfway then pry it towards the dent putting pressure directly on the dented side.

That should do the trick.

Mike Fisher

[troj]

If not, send it to CTI and tell them its deffective ;) :p

Sorry, Ben, but I don't find recommendations like this to be even vaguely humorous.

Now, ignoring that, don't expect CTI to provide any sort of warranty coverage on the case, should you repair it, unless they've specifically stated that the repair results in something they will cover.

-Kevin

[ben_ullman]

Sorry, Ben, but I don't find recommendations like this to be even vaguely humorous.

Now, ignoring that, don't expect CTI to provide any sort of warranty coverage on the case, should you repair it, unless they've specifically stated that the repair results in something they will cover.

-Kevin

Sorry Kevin. Didnt mean to step on anyones toes.

Ben

[Gym_Class_Hero]

Thanks for the input guys. I went ahead and did what Mike suggested, with a little help from a long nose pliers. It worked like a charm. The threads got a little roughed up, but they were straighted out by sliding a slotted screwdriver through while I heated the case over the stove.

[powderburner]

It's great that your DIY refurb worked out, but you should be extra careful of hidden damage.

I think troj sort of hinted at this (whether the manufacturer would be happy with your fix). You could have some subtle damage that you don't even know is there. With the "dented" motor case, and with the "un-dent" repair, the metal case could be weakened or cracked. (Some alloys are more forgiving than others for this sort of thing.) If you had any thread damage at all, it would be possible for the case to let go of an end closure.

I suggest that you spend at least one or two reloads on some ground proof tests. You need to find the propellant load that generates the highest internal pressure (regardless of the duration of that pressure) and use it. Inspect motor case and end closures very carefully afterward.

My two cents (which you didn't even ask for)

[barstoolmike]

I agree, a couple ground tests are cheeper & safer than trashing a rocket. Besides if the threads do fail it will be at ejection. Do you realy want it coming in balistic because you didn't test it first. (safety first & above all else)

Mike

Another 2 cents you didn't ask for.

[cjl]

Why would the threads fail at ejection? I'd guess that if they were to let go, it would be at ignition, which is often the point of maximum internal pressure.

[ben_ullman]

Why would the threads fail at ejection? I'd guess that if they were to let go, it would be at ignition, which is often the point of maximum internal pressure.

I agree. The threads are holding pressure of the motor not the motor at ejection.

Ben

[Binder Design]

Thanks for the input guys. I went ahead and did what Mike suggested, with a little help from a long nose pliers. It worked like a charm. The threads got a little roughed up, but they were straighted out by sliding a slotted screwdriver through while I heated the case over the stove.

Glad that worked for you. As long as the threads are mostly intact I wouldn't worry about flying it. Remember, most cases with commercial loads are rated very conservatively, the hardware is usually good to at least twice that pressure.

I'd say go for it, but you might want to make sure it's not in a one of your favorite rockets just in case. :)

Mike Fisher

[ben_ullman]

Remember, most cases with commercial loads are rated very conservatively, the hardware is usually good to at least twice that pressure.

I'd say go for it, but you might want to make sure it's not in a one of your favorite rockets just in case. :)

Mike Fisher

How would you know? What kind of experience do you have with motor pressures and design?

Ben























Disclaimer, Mike Fisher is a kick @ss motor maker ;) This was meant to push any and all of his buttons

[Gym_Class_Hero]

I'm pretty sure it would hold, the dent was very minor. Let me explain...

The dent only affected 1/8 or so of the circumfrence of the case. It was also only 3 thread layers deep. I feel that the re-bent threads are still around 70-80% their original strength. It should hold. :)

What do you guys think would be a good test load? The Vmax?

[Binder Design]

How would you know? What kind of experience do you have with motor pressures and design?

Ben

Disclaimer, Mike Fisher is a kick @ss motor maker ;) This was meant to push any and all of his buttons

Nah, I'm an easy going guy (most of the time).:) Thanks for the comment!

Mike

[ben_ullman]

Nah, I'm an easy going guy (most of the time).:) Thanks for the comment!

Mike

Mike you don't give yourself enough credit. You are on my top 5 "need to pick their brain" list in rocketry. a 38mm L5000 ? I can't even get a simple 98-13k N4000 to work.

Ben

[cjl]

I'm pretty sure it would hold, the dent was very minor. Let me explain...

The dent only affected 1/8 or so of the circumfrence of the case. It was also only 3 thread layers deep. I feel that the re-bent threads are still around 70-80% their original strength. It should hold. :)

What do you guys think would be a good test load? The Vmax?

I'd try either the vmax or the white thunder to start.

[ben_ullman]

I'd try either the vmax or the white thunder to start.

yeah that or the blue streak. The white is pretty tame. Its got a super rediculous KN (alot of whites do) Vmax and blue are pretty kicking.

Ben

[cjl]

The Blue Streak is pretty tame, as blues are concerned. Actually, I like the longer burn that it has compared to blue thunder. It gives a good alternative to all of the fast blues on the market.

[ben_ullman]

The Blue Streak is pretty tame, as blues are concerned. Actually, I like the longer burn that it has compared to blue thunder. It gives a good alternative to all of the fast blues on the market.

That suprises me. Any blue will (should) hit harder than a white. It may not burn like it but the pressures from a blue should be more than white. Just from my experience.

Ben

[cjl]

Honestly, just from having seen it, I can almost guarantee you that White Thunder hits harder than Blue Streak (both of which hit harder than, say, AT White Lightning or CTI standard).

Oh, and would you consider this to be a white propellant?
http://www.pro38.com/images/NATS2.jpg

[ben_ullman]

Honestly, just from having seen it, I can almost guarantee you that White Thunder hits harder than Blue Streak (both of which hit harder than, say, AT White Lightning or CTI standard).

Oh, and would you consider this to be a white propellant?
http://www.pro38.com/images/NATS2.jpg

I wish I could see the thrust/pressure data. Just because the MOTOR looks like it has a bigger flame doesn't mean the pressures higher. I246blue I315 white for the 38-3G. The blue has a higher total thrust.

Ben

[cjl]

Very true (I'm a student in aerospace engineering, and I've already taken a class involving nozzle design - give me some credit here). As for total impulse (often incorrectly called total thrust), it is completely irrelevant. Actually, what would be useful would be the nozzle specs. From nozzle throat diameter and initial thrust, a decent pressure estimate could be obtained.

Oh, and I repeat my question. Do you consider this to be a white propellant?
http://www.pro38.com/images/NATS2.jpg

[Chrisn]

Forbidden
You don't have permission to access /images/NATS2.jpg on this server.

Link us to the page it is on.

[ben_ullman]

Forbidden
You don't have permission to access /images/NATS2.jpg on this server.

Link us to the page it is on.

does that to me too, then hit refresh it will show up.

Ben

[ben_ullman]

Very true (I'm a student in aerospace engineering, and I've already taken a class involving nozzle design - give me some credit here). As for total impulse (often incorrectly called total thrust), it is completely irrelevant. Actually, what would be useful would be the nozzle specs. From nozzle throat diameter and initial thrust, a decent pressure estimate could be obtained.

Oh, and I repeat my question. Do you consider this to be a white propellant?
http://www.pro38.com/images/NATS2.jpg

yeah yeah I know. You boasted about that at LDRS Mr engineer :neener:

I was looking at what was given to try and get an idea of what the differences are between the 2 with thrust and duration. Not 100% irrelevant with pressure (IMO)

Yes I do consider that white as it has a white flame? Am I missing a joke?

Ben

[cjl]

Actually, it was more to prove a point. If you consider it white, it kind of proves your point about blues hitting harder than a white wrong (well, at least I have yet to see a blue that hits harder than Vmax, which is the propellant in that photo).

Oh, and for anyone having trouble with the above link, try this one:
http://i27.photobucket.com/albums/c183/chris_lapanse/NATS2.jpg

[ben_ullman]

Actually, it was more to prove a point. If you consider it white, it kind of proves your point about blues hitting harder than a white wrong (well, at least I have yet to see a blue that hits harder than Vmax, which is the propellant in that photo).

Oh, and for anyone having trouble with the above link, try this one:
http://i27.photobucket.com/albums/c183/chris_lapanse/NATS2.jpg

Ah a trickery photo! I should have known it. Not only because its from you ;) But because the flame pattern doesn't match most othe whites. I wasn't sure since CTI does have a kicking white.

Ben

[MarkII]

I thought that one of the defining characteristics of a "white" propellant was the vast billowing clouds of white smoke that it generated during the burn. I didn't see that in your photo, so I would not have identified that motor as burning a white propellant.

Just my :2:. OK, I'll leave now. :o

MarkII

[butalane]

That suprises me. Any blue will (should) hit harder than a white. It may not burn like it but the pressures from a blue should be more than white. Just from my experience.

Ben

Just because a blue motor is generally faster than a white doesn't mean it does or should hit harder. The propellant formulation/operating pressure/motor configuration are all going to dictate the "hard hitting" characteristics of a motor not the flame color. The blue color is just generally the result of a catalyst such as Copper Oxide, but you could make a real hard hitting white motor with loads of Iron Oxide that is faster.

These motors were fast whites:

http://www.mdra-archive.org/photos/ESL107/BobUtley/Full/th-littlejohn.wmv

Ignore the misspelling of my name and swollen cheeks, this was years ago when I had just gotten my wisdom teeth removed.

However as you can see the motors are pretty fast and had no trouble lifting Todds 250# + rocket, it helped that they were 7600 loads too...though they don't have any catalyst in them.

* I guess you can get me on a technicality because they were "yellow" but they looked like the picture CJL posted. They ended up being more like M2200s lol.

[patelldp]

When I think of "hitting hard," that generally means this: If two motors have exactly the same total impulse delivered, the one with the higher average impulse "hits harder." For example, assume that every load for the 38-600 Aerotech case has the same total impulse, the I435 therefore hits harder than the I284. While this is a fun argument and everything, I think it's fairly inconsequential.

Generally speaking, to offset the increase in pressures exerted by my blue motors, I counteract by lowering the Kn. This in turn makes a fast, but relatively weaker motor. Sure it "hits harder," but it does not deliver the same impulse. For example, my 6 grain 29mm motor (29-450 homemade case), I have done a white load and a blue load. The blue load containted 15% less propellent in order to rework the Kn, and the motor burned for approximately 0.5 seconds. The white load contained that extra 15% of propellent, and burned for almost 1.5 seconds. Much more total impulse, but "hits like a girl" compared to the blue.

"Hits harder" is not a technical term. "Has a greater average impulse" is. For someone making N motors, these should be the terms used.

[cjl]

While we're talking technicalities, "average impulse" is a somewhat meaningless phrase when referring to a single motor (average thrust would be the correct term). As for hits harder - I agree, it is somewhat vague. It could refer to how quickly the motor pressurizes off the pad (which would be highly igniter dependent), how much thrust it has, or any number of other factors. Chamber pressure would be more relevant to this discussion, which is not really all that corellated to the thrust of the motor. A high thrust motor could run at a quite conservative chamber pressure and be using a very fast propellant with a large nozzle. Alternatively, it could be running a very high pressure on a smaller nozzle with a slower propellant and achieve almost exactly the same results. Without knowing the details of the motor, you can't simply say that (for example) because the I435 has more thrust than the I284, it necessarily has a higher pressure and would stress the casing more.

[shreadvector]

Get out the Jerkmeters.

http://en.wikipedia.org/wiki/Jerk_(physics)