Closest to 320 N-sec

Discussion in 'Mid Power Rocketry (MPR)' started by trogdor, Mar 8, 2006.

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  1. Mar 8, 2006 #1

    trogdor

    trogdor

    trogdor

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    So I've been wondering about the various limitations placed on an uncertified (level 0) flyer at a NAR launch. What is the maximum total impulse possible while staying within all boundaries set by all rulemaking entities?

    Let's assume an adult flyer (age 18+) using commonly available and certified motors with any required FAA notification having taken place and a rocket within liftoff weight restrictions.

    As I understand it:

    By NFPA 1127, a single motor can have no more than 62.5g of propellant and must have an average thrust of 80N or less.

    This makes the Aerotech G64 and G80 pretty much the limits of these rules respectively.

    By the NAR model rocket safety code, the rocket will not contain more than 125 grams (4.4 ounces) of propellant or 320 N-sec (71.9 pound-seconds) of total impulse.

    So, what clustered combinations get you closest? Playing around a little, a G80 and a couple F50s are within 10 grams or so of the weight limit but only 260 N-sec. I'm not sure how much closer you can get to the impulse limit while staying within the propellant weight limit but blue thunder propellant seems to be the most efficient.

    Any ideas? Creative thoughts (512 1/4As?? :) )
     
  2. Mar 8, 2006 #2

    cjl

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    APCP is definitely the way to go.

    A pair of G80's gives you 200Ns and 95.8g, leaving you 29.2g to play around with. F42T has 27g propellant and 55Ns, putting you up to 255.

    I can't see any way to get above 255-260Ns within the propellant limits.
     
  3. Mar 8, 2006 #3

    ben

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    I can beat that....

    1,000,000 MMX motors :eek: :D :D

    thanx, Ben
     
  4. Mar 8, 2006 #4

    henry8minus1

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    The most efficient motors with regards to most impulse to propellant weight (which I think is refered to as specific impulse or Isp) is the D24T for the 18/20 casing and the E23T for the 29/40-120 casing. Both have an Isp of 2.30. The D24 has 20 N-sec and 8.7 grams of propellant, and the E23 has 40 N-sec and 17.4 grams of propellant. You could cluster 14 D24s for an equivalent of a H336, or 7 E23s for a H161. Both would be 280 N-sec and 121.8 g of propellant.

    If you really want to max it out, add an Estes A8-5 (2.5 N-sec and 3.12 g of propellant) for 124.92 grams of propellant and 282.5 N-sec.

    Edit: added the E23T
     
  5. Mar 8, 2006 #5

    doug_man_sams

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    You left out two other constraining metrics: maximum 160Ns per motor and 3.3 pounds max loaded rocket weight. When you start clustering a bunch of RMS cases together, you're gonna accumulate some mass, so you need to consider that.

    The 62.5g limit will keep you from getting to the 160Ns limit. The hottest fuels have about 2Ns per gram of propellant, so about the most woosh you're gonna get out of 62.5g is ~125Ns.

    Your G80+2xF50 example is a good case of efficient propellant, coming in at 2.03Ns per gram, but is in fact a tad over the propellant mass limit at 133.2g (57.4 + 2*37.9, using 2000 NAR spec sheets). (Is there a newer, slightly smaller G80 out now? I vaguely recall that, in which case you're likely back under the limit.)

    Anyway, given the ~2Ns per gram limit, you'd need about 160 grams of propellant to get 320Ns, so the limiting factor is the propellant mass.

    Doug
     
  6. Mar 8, 2006 #6

    bobkrech

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    6 SU E30s would cost ~$66 for 240 NS and several other combinations would be in the same ball park. ($0.275 per NS installed)

    13 SU D21s would cost ~$135 for 260 NS, or $3.25 per NS over 240 NS baseline above. ($0.519 per NS installed)

    14 D-24 reloads and casings would cost about ~$500 for 280 NS, or $18.25 per NS over the 260 NS example above. ($1.79 per NS installed)

    Using model rocket motors to get over 240 NS under LMR rules is very expensive.

    Bob
     
  7. Mar 8, 2006 #7

    trogdor

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    Looks like you're pretty close AND creative! Hmmm, could we also add a micromaxx... and maybe a couple parallel staged estes solar igniters?! :)
     
  8. Mar 8, 2006 #8

    trogdor

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    I got that data from the 2006 Aerotech catalog. I do remember the G80 and G40 having more impulse, didn't they shrink the cases a tad to fit in motor mounts better awhile back??

    I did mention the overall weight limit and implied the G motor max in the level 0 flyer statement but now that I think of it, where does the 120 N-sec limit in the Aerotech consumer motors come from? Just limits of the 62.5g propellant weight, or something else?? I guess the weight limit.

    Also, looking at the current data, a G33 had more than 70g of propellant (from 06 aerotech catalog also) so this is technically an HPR motor ?(assuming we're picking nits, which I just do for fun)

    And yes, I agree this exercise would be quite expensive but its interesting. I plan to cert level 1 (again) next month and then the limit is simply 320N-sec max in the rocket so no fun what if scenarios there...
     
  9. Mar 8, 2006 #9

    henry8minus1

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    Bob thanks for the financial reality check.

    To add the E23 option (that I added to my above post):

    7 E23 reloads and casings would cost about ~$370 for 280 Ns or $11.75 per Ns over your 260 Ns example. ($1.32 per Ns installed)
     
  10. Mar 9, 2006 #10

    bobkrech

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    I got that data from the 2006 Aerotech catalog. I do remember the G80 and G40 having more impulse, didn't they shrink the cases a tad to fit in motor mounts better awhile back??

    Yes. AT developed a new improved casing (cheaper and easier to assemble by using the same components as the AT limited use reloadables). The weight of the propellant that would fit into the new casing without changing the external dimensions dropped from 62.5 grams to 47.9 grams so the total impulse dropped from 120 NS to 100 NS but the thrust remained the same.

    I did mention the overall weight limit and implied the G motor max in the level 0 flyer statement but now that I think of it, where does the 120 N-sec limit in the Aerotech consumer motors come from? Just limits of the 62.5g propellant weight, or something else?? I guess the weight limit.

    Not exactly. The old G80 developed 120 NS with 62.5 grams of propellant for an efficiency of 1.92 NS/gram. The new G80 is more efficient: 100/47.9=2.09 NS/gram. The efficiency difference is probably due to a more efficient nozzle which operates the motor at a higher pressure and/or has a greater expansion ratio.

    Also, looking at the current data, a G33 had more than 70g of propellant (from 06 aerotech catalog also) so this is technically an HPR motor ?(assuming we're picking nits, which I just do for fun)

    Not just technically. The G33 is a high power rocket motor by NFPA 1127 definition because it contains 72.2 grams of propellant which is more than 62.5 gram maximum propellant weight permitted in a model rocket motor.

    And yes, I agree this exercise would be quite expensive but its interesting. I plan to cert level 1 (again) next month and then the limit is simply 320N-sec max in the rocket so no fun what if scenarios there...

    The L1 limit is 640 NS since you could then use H and/or I motors: 160 NS < H <= 320 NS, 320 NS < I <= 640 NS, however if the rocket contains more than 125 grams of propellant, your flight requires a formal written FAA waiver instead of the simple LMR notification.

    Bob
     
  11. Mar 9, 2006 #11

    trogdor

    trogdor

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    oops, you are right, didn't do that last x2 in my head...
     

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