3D Printing Safety First - Motor Casing Temperature

[Ez2cDave]

From the NAR . . .

QUOTE :

Safety First - Motor Casing Temperature


Do you know that model rocket motors are permitted to reach a case temperature of 200C (392F) and high power rocket motors are permitted to reach 220C (428F) during or after operation? These limits are in NFPA 1125, Code for the Manufacture of Model Rocket and High Power Rocket Motors. Long-burn-time motors are the types that most often approach these limits.


Obviously, hot melt adhesives are not appropriate for rocket construction, especially adjacent to the motor mount. Second, many 3D printing materials will be severely weakened at maximum motor casing temperatures. Polylactic Acid, or PLA, is a common material for many uses because of its ease of use and low cost. PLA has a maximum service temperature of under 60C. Even the most robust 3D printing materials typically do not have service temperatures over 150C. Finally, epoxy adhesives have a characteristic known as the glass transition temperature or Tg. Epoxies transition from a hard and relatively brittle material to a viscous or rubbery state as temperatures are increased (and revert to the hard state as they cool). The Tg is often less than 75C for many epoxy adhesives.

What does this mean for structural safety in your rocket's design and construction? Try to avoid direct motor case contact with 3D printed structures or epoxy bonds. Use a non-metallic motor tube to provide some thermal insulation. Use epoxies rated for higher temperatures (e.g. Original JB Weld) on bonded metal motor retainers (e.g. Aeropac). And be careful removing casings from your rockets post-flight! Finally, keep in mind that the temperature limits are just that; not every motor casing will get that hot.

Stephen Lubliner,
Safety Committee Chairman

END QUOTE :

Dave F.

 

[UhClem]

NFPA 1125 is currently beginning its revision cycle and a proposal to increase the temperature limit to 600F (316C) for non-metallic expendable high power motors passed the first ballot.

 

[cwbullet]

I line mine with FG or paper. I do this not because a rock has ever failed form melting or adhesive failure. If the bond to FG is not deleterious, neither will the bond with PETG or ABS. You have to remember, none of the printed items are solid. They are insulated by pockets of air. I have flown quite a few without tubes. The problem I had was difficult removing the motor.

I think there is a limit to what size motor should be in contact with a plastic motor mount tube. I probably think it is probably 24-29 mm or an impulse limit, but I do not know. I have only flown an H with plastic parts.

 

[Ez2cDave]

NFPA 1125 is currently beginning its revision cycle and a proposal to increase the temperature limit to 600F (316C) for non-metallic expendable high power motors passed the first ballot.

That will create even more issues with the adhesives used on Motor Mounts.

Dave F.

 

[rharshberger]

That will create even more issues with the adhesives used on Motor Mounts.

Dave F.

Especially since the combustion temp of paper is 451°F.

 

[Ez2cDave]

Especially since the combustion temp of paper is 451°F.

So, then, we will have the potential for fire and structural failure, JB Weld notwithstanding.

Dave F.

 

[cwbullet]

JB Weld begins failing above 500 degrees and is advertised to withstand 600 for 10 minutes. That, of course, is with good build technique.

 

[dhbarr]

Above 220C the yield strength of aluminum goes to garbage anyway, so this sounds to me like:
- asking for a lot to compromise back during negotiation
- allowing small portions to briefly exceed the current limit
- single-use casings that get fairly hot at low pressure

 

[Charles_McG]

So, then, we will have the potential for fire and structural failure, JB Weld notwithstanding.

Dave F.

Yet the paper of Estes motor tubes never ignites.

I’ve found two references to corrugated cardboard having an ignition temp on the order of 870F. I had to work one reference backwards from irradiance.

 

[Ez2cDave]

Yet the paper of Estes motor tubes never ignites.

Charles,

They are densely-rolled and the layers are bonded together. Possibly, something like Sodium Silicate may be also used the treat the paper.

Dave F.

 

[manixFan]

So how many instances of failures have been reported due to heat from a motor case? It seems if it was an issue the NFPA would take that into consideration. In all my years of flying I can’t recall of ever hearing of an issue due to heat from a motor case, other than someone getting hot hands from a quick recovery. Is there a repository of reports somewhere that warrants concern over this issue?


Tony

 

[rharshberger]

So how many instances of failures have been reported due to heat from a motor case? It seems if it was an issue the NFPA would take that into consideration. In all my years of flying I can’t recall of ever hearing of an issue due to heat from a motor case, other than someone getting hot hands from a quick recovery. Is there a repository of reports somewhere that warrants concern over this issue?


Tony

I am wondering if its more a specific manufacturer pushing for higher temp limits to ease certification of motors.

 

[UhClem]

The casing temperature standard unnecessarily restricts certain long burn motors with non-metallic
casings from being certified that are otherwise perfectly safe for consumer use. Provided the rocket
motor does not fail, non-metallic casings do not have the thermal conductivity or thermal capacity to
cause a fire hazard in the rocket or on the ground.

So says Gary Rosenfield in his proposal to simply remove temperature limits. (The NFPA committee somehow ended up only raising the limit.)

 

[Steve Shannon]

NFPA 1125 is currently beginning its revision cycle and a proposal to increase the temperature limit to 600F (316C) for non-metallic expendable high power motors passed the first ballot.

It passed the first ballot, but there were reservations. At the meeting I recommended that some of these motors be tested at Tripoli Research Launches between now and the next ballot to see what the possible unintended consequences might be.
Of course the case temperature limit does not mean that the entire case reaches that temperature. Usually a small portion of the exterior of the case is hotter than the rest of the case. We want to find out what this will mean in practice.

 

[jbr]

I have had a fire started in tall dry grass by a graphite nozzle landing in it