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Thread: just how fast?

  1. #1
    Join Date
    27th December 2010
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    532

    just how fast?

    does anyone know how many mph these things are going? iam launching fairly light e engine rockets n they look fast. iam think if it goes 1200ft with a 3second burn then it really must be flying. cmon someone, give me a good answer!


  2. #2
    Join Date
    18th January 2009
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    How heavy are they, and what diameter? E engine rockets can definitely get to several hundred miles per hour.

    NAR #84281 L3
    TRA #11233 L3

  3. #3
    Join Date
    27th December 2010
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    532
    its a bt-50 tube with a fairly pointed nose. built with ca n wrapped with monokote so fairly light. sorry i dont have specifics but if it goes at least 1200 n 3 seconds then its really got to be moving faster than i realized!

  4. #4
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    347.7 mph
    Jeff Vegh
    TRA# 03011
    NAR# 92403

  5. #5
    Join Date
    24th April 2010
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    Quote Originally Posted by Rocket_Man View Post
    does anyone know how many mph these things are going? iam launching fairly light e engine rockets n they look fast. iam think if it goes 1200ft with a 3second burn then it really must be flying. cmon someone, give me a good answer!
    You can get an average speed with simple math.
    An Estes E9-4 Burns for 2.8 seconds and coasts for 4 seconds. If the rocket travels 1200 ft in 6.8 seconds...

    1200/6.8 = 176.47 feet per second
    176.47 fps * 0.6818 = 120 Miles per hour (average)

    But what about the top speed? The rocket starts at 0 mph on the pad accelerates to it's top speed and then slows back down to 0 mph at 1200 feet. It's easy to find top speed with simulation software like OpenRocket.

    OpenRocket simulates that an Estes Guardian on an E9-6 flies almost 1900 ft in about 9 seconds and hits a top speed of 325 MPH.
    -Scott Sager
    NAR 91621 L2
    TRA 15982 L2
    Woosh #558
    KC9WQK

  6. #6
    Join Date
    18th January 2009
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    Here's the old standby from Estes, Technical Report 10.

    http://www.oldrocketplans.com/pubs/E...R-10/TR-10.pdf

    Even though it only has charts up through D12, there are equations in the back that you can use to work with just about any size rocket or motor available, though you have to make two big assumptions:

    1. Constant thrust -- Of course, motors don't have constant thrust. These equations can only approximate the motors' effect.

    2. Subsonic speeds -- Different equations need to be used once the rocket starts flying at 80-90% of Mach.

    You also have to have some way of getting the hyperbolic trig functions to solve these equations. Fortunately most programming languages have a math package that has these functions.

    Still, just using the charts will give you a good idea of how a rocket behaves, and you can extrapolate what difference an E9 motor (@28ns) would make over a D12 motor (@17ns).

    Roy Green
    nar12605 L2
    Southern Area Rocketry

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