Aerotech RMS Safety

Hi all,

I had an Aerotech RMS 38/720 survive coming in ballistic from 6000 feet. (It was in a 54 mm rocket, so it hit pretty hard.) The case is slightly out of round and there is a small crease near the closures on one end. I don't know what the factor of safety is on these things, but I understand that some 38 mm motors can get to 1400 psi. I'm wondering if it is safe to fly. I'm not really worried about the fact that it is out of round, but that crease is worrying me.

What do you all think?

Nope Nope Nope.. don't fly it. It'll likely fail and go bang.

If you do, it might make this flight or the next, or one after that.

or it might just be a spectacular CATO...


I wouldn't..

LOX said:

What do you all think?

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rfjustin said:

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raaaaadiooo.... raaaaaadio.

The only authoritative opinion is Aerotech’s.

Does a liner even fit into it at this point? The specs on these are pretty tight, so my guess is not.

In any case, no, I would not fly it.

Steve Shannon said:

The only authoritative opinion is Aerotech’s.

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oh yea? I just happened to stay at a holiday inn Express last night....

The case is toast and needs to be replaced. Examine the closures for any damage to the threads.

Just a note- sometimes the threading looks ok on the closures, but isn't. Did either of the closures pop loose when you hit the ground, or otherwise get banged up?

LOX said:

Hi all,

I had an Aerotech RMS 38/720 survive coming in ballistic from 6000 feet. (It was in a 54 mm rocket, so it hit pretty hard.) The case is slightly out of round and there is a small crease near the closures on one end. I don't know what the factor of safety is on these things, but I understand that some 38 mm motors can get to 1400 psi. I'm wondering if it is safe to fly. I'm not really worried about the fact that it is out of round, but that crease is worrying me.





What do you all think?

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I don't believe Aerotech would condone flying an out of round creased case, and nor do I, throw it in the trash, but thanks for showing the good sense to ask.
A second note is, do you know why you came in ballistic from 6,000'? If not then do the best forensic evaluation you can to try and find out and prevent it from happening again.

If you don not want it send it to me!. I'll cut the bad out, thread and try it. I have done that with some long 54/2560's (that had bulged, of course it did not hit the ground at 500+ feet a second) and made 54/1280's out of them. I do run them on the test stand first before flying, so far so good but one never knows.

I'd hang it on the wall as a reminder.

Out of round it may burn through and toast your favorite rocket. Kurt

ksaves2 said:

Out of round it may burn through and toast your favorite rocket. Kurt

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It can’t be too far out of round if the closures screw in and out.

Didn't the space shutter explode over an O-ring that shrank an almost immeasurable percent of normal?

Yeah but that's the space shuttle. I got reading they put a thermal coating on the 7075T6 aluminum to survive Mach 25 re entry and yet they didn't figure in what a paint scratch would do. Mi-15, MA-25S ,and the Li-1500 stuff protected mere aluminum from total temperatures at high Mach that would exceed melting point of materials. The doors were solid Berrylium rates for that temp. Couldn't take a scratch or aluminum melts lol. These are hobby rockets. If it screws in light it and MESS file it if it goes bang. There's no cracking on the casing. A crack would be a point of failure. The aluminum could deform under pressure back to factory round specs. Now they are all gonna Yabber how wreck less I am lol... And NASA put the shuttle thermally at risk by using aluminum to save a few million it simply requires the ablative to work or it melts, they could've built one third the airframes of other materials.

grouch said:

Didn't the space shutter explode over an O-ring that shrank an almost immeasurable percent of normal?

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No. Because of the cold the o-rings became too stiff to easily deform and maintain their seal.

Andrew_ASC said:

Yeah but that's the space shuttle. I got reading they put a thermal coating on the 7075T6 aluminum to survive Mach 25 re entry and yet they didn't figure in what a paint scratch would do. MA-25S and the Li-1500 stuff. Couldn't take a scratch lol. These are hobby rockets.. If it screws in light it and MESS file it if it goes bang. There's no cracking on the casing. A crack would be a point of failure. The aluminum could deform under pressure back to factory round specs. Now they are all gonna Yabber how wreck less I am lol...

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Not “wreck less”, perhaps not even reckless, but certainly ignorant of your own lack of knowledge and devoid of self control.

Well the OP could always take pictures and send to Aerotech. Or he could put the O-ring in the freezer and get a free casing replacement after the O rings get stiff and fail to seal. File a Mess. I guess I'm a horrible guy. Use a physical limitation of rubber to replace a bent out of round casing. Name the rocket heads up or use a test stand.

Andrew_ASC said:

Well the OP could always take pictures and send to Aerotech. Or he could put the O-ring in the freezer and get a free casing replacement after the O rings get stiff and fail to seal. File a Mess. I guess I'm a horrible guy. Use a physical limitation of rubber to replace a bent out of round casing. Name the rocket heads up or use a test stand.

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Or he could just heed the general opinion of this thread and not use it..

He could (and probably has) sought the opinion of eh manufacturer / supplier..

dr wogz said:

Or he could just heed the general opinion of this thread and not use it..

He could (and probably has) sought the opinion of eh manufacturer / supplier..

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Yeah that's the wiser scenario. Not worth getting hurt over.

Andrew_ASC said:

Well the OP could always take pictures and send to Aerotech. Or he could put the O-ring in the freezer and get a free casing replacement after the O rings get stiff and fail to seal. File a Mess. I guess I'm a horrible guy. Use a physical limitation of rubber to replace a bent out of round casing. Name the rocket heads up or use a test stand.

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Fan of insurance fraud too?

IMO intentionally launching something you have designed to fail is in CLEAR violation of Tripoli / NAR codes and should never be condoned.

No. I just got a bad sense of jokes that pisses everyone off. CCI blazer replaced a whole case of ammo once it was a manufacturer lot defect all I had to do was call once. They doubled filled the powder charge.

Andrew clearly you are a dishonest person who has no issue deceiving people for your own gain. I look forward to the day you get out of the hobby and decide to list your hardware for sale. If I am paying attention I will be sure to place a link to this thread so any potential buyers can gain some insight in to who they are considering buying from.

I was making a joke. And now you guys are all pissed. I can never make this forum happy. Ever.

Can you static fire the motor? My "guess" it will either pass/fail.

That said, the crease is more the worry than the out of round (though that's not that insignificant). The crease probably has some tiny fractures in it and is now going to be a weak point in the case. It may never fail on the first or second flight, but one thing is for sure: It will never fail if you don't fly it.

If you have to err, err on the side of caution and ground it.

Greg

Replacement casings are a lot cheaper than tissue, bone, or healthcare expenses for certain. Flying shrapnel is sharp and hot. All joking aside I'd replace before flight. And likely static test from a covered position far away. Use extra lube and don't stick o-rings in the freezer or extort Aerotech.

If it's permanently deformed it will always be weaker than when elastically deformed returning to original shape after loading. Unknown irregularities such as surface finishes and small micro cracks can cause points of failure that you can't see. The service life of the component long term is compromised when damage is severe enough. Areas exposed to plastic deformation exhibit stretching and necking in elongation which has more stresses on that local area on the next loading cycle. Think of each loading cycle as a rocket flight when the propellant Burns and the casing is rapidly briefly pressurized. These areas will be prone to higher failure rates. And there's no way that I know to simply hammer out a dent when a metal has yielded beyond ultimate tensile strength and permanently deformed. Even if it's smashed back to factory spec the material itself is compromised. In theory terms. Normally a factor of safety is implemented into design of pressure vessels to account for unknown material flaws during manufacturing. It may or may not fail depending on severity of damage. It's easier to determine the motor casing isn't safe to fly than to risk it, yourself, or others safety.

If you had a motor reload of higher pressure than average or a slightly liner defect, the factor of safety is there to protect you the end user from a material failure and casing rupture theoretically. When the casing is damaged to an unknown extent, the margins are lower of it surviving a higher than average reload pressure or some other kind of motor defect. Normally aluminum has millions of life cycles for use before metal fatigue occurs compared with integrals on paper. Sometimes in real world this doesn't happen due to material defects and other kinds of sustained damages weakening the part over a lifetime of stress accumulating. Corrosion on surface cracks long term is very bad and reducing the stresses the structure takes before failure.