Single stage reaches higher altitude than multistage

Discussion in 'Low Power Rocketry (LPR)' started by HStuart18, Apr 12, 2019.

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  1. Apr 12, 2019 #1

    HStuart18

    HStuart18

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    The two files below are a couple of designs I made in Open Rocket. What I don't quite understand, is that the single stage version reaches a higher altitude than the two stage version. I understand that the two stage version has extra drag due to the additional set of fins to maintain stability when the lower stage detaches. But, I would've thought that losing the mass of the motors would have well in truly increased the altitude... Am I missing something?
     

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  2. Apr 12, 2019 #2

    kuririn

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    Looking at the multi stage sim, it could be because I don't see a motor in your second stage. But I'm no OR guru.
     
  3. Apr 12, 2019 #3

    HStuart18

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    That's right, there isn't a motor in the second stage - intentionally. I wanted to see if ditching the motors/firststage immediately after burnout would result in higher altitude because there is no longer any thrust.
     
  4. Apr 12, 2019 #4

    Rex R

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    shame that, adding a D12-7 in the upper stage puts the apogee over 1km :).
    Rex
     
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  5. Apr 12, 2019 #5

    dhbarr

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    Lighter rockets don't always go higher. Think about throwing a grocery sack vs. throwing a baseball vs. throwing a cinder block.

    Intuitively we know our arm, which can reliably produce the same amount of acceleration, will send the baseball farthest. The bag is under optimal mass, the cinder block over.

    I suspect in this case kicking the booster takes you from a little underweight to a lot underweight.
     
  6. Apr 12, 2019 #6

    kuririn

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    Ah, I see. Since the mass is dropped after engine burnout when thrust is no longer being applied, the motorless sustainer will not further increase its' acceleration. It is, in fact, decelerating (coasting) at the same rate as the booster.
    Think of Galileo's leaning tower of Pisa experiment, where he dropped two spheres of different masses, and they both hit the ground at the same time. It's like that in reverse.
    Add to that the extra weight of the second set of fins and the second parachute, and that would explain the lower apogee. But like I said, I'm no rocket guru.
     
    Last edited: Apr 12, 2019
  7. Apr 12, 2019 #7

    ihbarddx

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    I haven't reviewed your design, but I understand that the second stage is not powered. If the upper stage by itself has a higher ballistic coefficient than both stages together have, then the upper stage will coast farther by itself. Otherwise, the rocket will coast farther with the stages attached.

    BallisticCoefficient = Mass/(Cd*XsectionalArea)

    (Note that drag acceleration is inversely proportional to this quantity at any altitude and velocity.)
     
    Last edited: Apr 12, 2019
  8. Apr 16, 2019 #8

    caveduck

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    I worked on this for NAR/FAI payload competition a long time ago. It's hard to do in LPR, though I did get some successful flights, and the benefits were not as good as a decent piston launcher.

    For a really great idea of how this works with a high thrust O class motor, check out the Super Loki Dart program: https://apps.dtic.mil/dtic/tr/fulltext/u2/750796.pdf
     
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