# Maximum impulse for an E motor?

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#### UPscaler

##### New Member
TRF Supporter
What is the maximum total impulse you can have to have a motor considered and E?

thanks!:confused2:

Here ya go, save it for future reference! Complete with pound thrust conversion.

Is there either a practical or a theoretical upper limit to the maximum thrust for motors of a given impulse class (and, I suppose, for a given propellant formulation)? Or is this yet another in my continuing series of bonehead questions? (Yes, I do recognize that this question is about a different thing than what UP asked about. Momentary applied force vs. total amount of power delivered.)

Regarding my question: for example, you rarely see an E motor with a peak thrust > 50 N or an F motor with a peak thrust > 100 N. Is there a physical or practical reason for that?

I have no particular reason for asking - I'm just curious about it.

MarkII

Is there either a practical or a theoretical upper limit to the maximum thrust for motors of a given impulse class (and, I suppose, for a given propellant formulation)? Or is this yet another in my continuing series of bonehead questions? (Yes, I do recognize that this question is about a different thing than what UP asked about. Momentary applied force vs. total amount of power delivered.)

Regarding my question: for example, you rarely see an E motor with a peak thrust > 50 N or an F motor with a peak thrust > 100 N. Is there a physical or practical reason for that?

I have no particular reason for asking - I'm just curious about it.

MarkII

Average thrust for a Model Rocket motor cannot exceed 80 Newtons. Nothing about maximum, just the average. This could ha e to do with 'weaponization" but it's been in the Federal definitions for many decades.

There are (were?) E motors with higher thrust but they are considered high power motors. Ask Blackjack2564 what an E110 does in a BlackHawk 29

There are (were?) E motors with higher thrust but they are considered high power motors. Ask Blackjack2564 what an E110 does in a BlackHawk 29
Yes, that's the kind of thing that I was wondering about. Wow, an E motor that required HP certification to purchase or use?1?

MarkII

There is another, seldom mentioned, rule regarding rocket motors. Its about a minimum burn time of 0.05s if I'm not mistaken. Consequently the maximum average thrust of an E motor would be 800N.

I don't now if it applies only to model rocket motors or to other classes (HPR, Amateur) too nor if it is part of NFPA 1125 or any other law/code. Does anybody know more about it?

Reinhard

There is another, seldom mentioned, rule regarding rocket motors. Its about a minimum burn time of 0.05s if I'm not mistaken. Consequently the maximum average thrust of an E motor would be 800N.

I don't now if it applies only to model rocket motors or to other classes (HPR, Amateur) too nor if it is part of NFPA 1125 or any other law/code. Does anybody know more about it?

Reinhard
It applies to model rocket motors. The only time it would come into effect would be for motors under 4 N-s. (4/.05=80) The 80N max average would be invoked first on larger motors. An A50 anyone?

Is there either a practical or a theoretical upper limit to the maximum thrust for motors of a given impulse class (and, I suppose, for a given propellant formulation)? Or is this yet another in my continuing series of bonehead questions? (Yes, I do recognize that this question is about a different thing than what UP asked about. Momentary applied force vs. total amount of power delivered.)

Regarding my question: for example, you rarely see an E motor with a peak thrust > 50 N or an F motor with a peak thrust > 100 N. Is there a physical or practical reason for that?

I have no particular reason for asking - I'm just curious about it.

MarkII
Mark

Propellant burn rate limitations. How fast you can burn the propellant determines the maximum theoretical thrust. Of course, the pressure generated during the burning must not exceed the casing burst pressure either, and the nozzle must also have a throat that is large enough to pass the generated gas.

An E-motor has a total impulse <20 Ns and up to 40 Ns. Model rocket motors are restricted to a maximum thrust of 80 N, so the shortest burn time of an E-impulse model motor will range from 20Ns/80N= 0.25s to 40Ns/80N= 0.5s.

The fastest burning hobby rocket propellant is AT's Warp-9 with a burn rate of ~1" per second. A single 17.5 g grain of Warp-9 could provide a total impulse of 40 Ns in a RMS 29/60 case. The resulting high power motor would have an approximate designation of 40E250N with a burn time of 0.16 seconds. Similarly (2) 17.5 g grains of Warp-9 could provide a total impulse of 80 Ns in a RMS 29/120 case. The resulting high power motor would have an approximate designation of 80F500N with a burn time of 0.16 seconds, and (2) 17.5 g grains of Warp-9 could provide a total impulse of 120 Ns in a RMS 29/180 case. The resulting high power motor would have an approximate designation of 120G750N with a burn time of 0.16 seconds.

There wouldn't be a large enough market to make these loads profitable, since the user would need high power certification and a huge field if used in a minimum diameter rocket which would be the only purpose for the motors.

Bob

Mark

Propellant burn rate limitations. How fast you can burn the propellant determines the maximum theoretical thrust. Of course, the pressure generated during the burning must not exceed the casing burst pressure either, and the nozzle must also have a throat that is large enough to pass the generated gas.

An E-motor has a total impulse <20 Ns and up to 40 Ns. Model rocket motors are restricted to a maximum thrust of 80 N, so the shortest burn time of an E-impulse model motor will range from 20Ns/80N= 0.25s to 40Ns/80N= 0.5s.

The fastest burning hobby rocket propellant is AT's Warp-9 with a burn rate of ~1" per second. A single 17.5 g grain of Warp-9 could provide a total impulse of 40 Ns in a RMS 29/60 case. The resulting high power motor would have an approximate designation of 40E250N with a burn time of 0.16 seconds. Similarly (2) 17.5 g grains of Warp-9 could provide a total impulse of 80 Ns in a RMS 29/120 case. The resulting high power motor would have an approximate designation of 80F500N with a burn time of 0.16 seconds, and (2) 17.5 g grains of Warp-9 could provide a total impulse of 120 Ns in a RMS 29/180 case. The resulting high power motor would have an approximate designation of 120G750N with a burn time of 0.16 seconds.

There wouldn't be a large enough market to make these loads profitable, since the user would need high power certification and a huge field if used in a minimum diameter rocket which would be the only purpose for the motors.

Bob

That is assuming that you go with a traditional Bates or C-slot core. What if it were one 17.5g grain in a 29/60 case or 35g endburning grain in a 29/120 case? I know the G69, I49, I59, and L339 have endburning grains.

That is assuming that you go with a traditional Bates or C-slot core. What if it were one 17.5g grain in a 29/60 case or 35g end-burning grain in a 29/120 case? I know the G69, I49, I59, and L339 have endburning grains.
Aviator

1.) What's the maximum total impulse of an E motor?

The answer is 40 Ns and this is independent of propellant type.

2.) The second question that was asked was what is the maximum thrust you can get from an E and an F motor?

My answer is that using available propellant (in a standard 29 mm casing) the maximum thrust possible from an E motor is ~250 N and from an F motor is ~ 500 N, and you can only get this performance from a Bates grain.

Thrust depends on the product of burning area and burn rate. A 29 mm end-burning Warp-9 motor would be boring as 40 N is all the thrust you would get out of it. Other cheaper and slower burning propellants can be used in a Bates grain configuration to get this performance and you can use motor ejection to boot instead of the electronic ejection required for Warp-9 motors.

Bob

Thank you very much for your explanation, Bob. My apologies to UPscaler for introducing another topic into his thread. My question was intended to stimulate a discussion of the hypothetical upper limits for Es and Fs, not for any practical reasons, but simply knowledge for knowledge's sake. Your answer, along with those by Rocketjunkie and Reinhard, have been fascinating. I suspected that the hypothetical maximum thrust of these motors would be higher than any actual motors that have ever been produced, but I never dreamed that it would be that high. I realize that motors with anything like those performance characteristics would never find much of a market and would certainly be impractical for us, but it was an interesting intellectual exercise. Thanks!

I have zero background in propellant chemistry or in motor design, but I thought that if I threw the question out there, someone who did have such a grounding might provide an answer. If I had thought of such things when I was younger, I might have devoted some serious study to them.

MarkII

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