Quantcast
Short and Fat Rocket Stability
Missile Works Giveaway!
Results 1 to 12 of 12
  1. #1
    Join Date
    11th October 2010
    Location
    Connecticut - formerly The Constitution State
    Posts
    955

    Short and Fat Rocket Stability

    I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.
    Bernie

    L1: Madcow Black Brant II - AT H123W
    L2: Rocketry Warehouse Formula 98 - CTI J244-WH

  2. #2
    Join Date
    13th February 2012
    Location
    Central Jersey
    Posts
    5,249
    Quote Originally Posted by Tronman View Post
    I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.
    Hang it from a string in the breeze, or use a decent size fan.

    I have a rocket which is ~.7 calibers stable according to simulation programs and it flies fine: it's in the foreground here.

    Two rocket motors burned this year.

  3. #3
    Join Date
    11th October 2010
    Location
    Connecticut - formerly The Constitution State
    Posts
    955
    Quote Originally Posted by CarVac View Post
    Hang it from a string in the breeze, or use a decent size fan.

    I have a rocket which is ~.7 calibers stable according to simulation programs and it flies fine: it's in the foreground here.
    I think I'm going to need a pretty heavy duty string for an 8-pounder
    Bernie

    L1: Madcow Black Brant II - AT H123W
    L2: Rocketry Warehouse Formula 98 - CTI J244-WH

  4. #4
    Join Date
    13th February 2012
    Location
    Central Jersey
    Posts
    5,249
    Quote Originally Posted by Tronman View Post
    I think I'm going to need a pretty heavy duty string for an 8-pounder
    Shock cord?
    Two rocket motors burned this year.

  5. #5
    Join Date
    18th January 2009
    Location
    Oviedo, FL
    Posts
    3,197
    There's an article in the following issue of Apogee's newsletter on how to model short, fat rockets in Rocksim:

    http://www.apogeerockets.com/educati...sletter154.pdf

    -- Roger

  6. #6
    Join Date
    25th January 2009
    Location
    Out of my mind, please leave a message!
    Posts
    698
    Quote Originally Posted by jadebox View Post
    There's an article in the following issue of Apogee's newsletter on how to model short, fat rockets in Rocksim:

    http://www.apogeerockets.com/educati...sletter154.pdf

    -- Roger
    I have used this on my 4" and 7.5" Fat Boy up scale rockets with great success. It allowed me to remove most of the 1lb of nose weight I had put in the 4" version and stopped me from adding to much to the 7.5".

    Bill Richardson ENC USNR Ret.
    TRA 8703 L3

    It is my opinion that stupidity should be painful, very painful. If it was I think there would be less of it in the world today.

  7. #7
    Join Date
    6th February 2009
    Location
    Arlington, TX
    Posts
    306
    Quote Originally Posted by Tronman View Post
    I know that the 1+ caliber rule doesn't apply to short, fat rockets, but how does one determine if it's stable? Is there a formula? Can it be done in Rocksim? String testing is a little difficult with a short 8 pound rocket.
    The 1 caliber rule of thumb is just a simple way to capture all of the complicated bits of stability. The rocket doesn't have to have 1 caliber of static stability so long as the CG is always ahead of the CP. The problem is that both are mobile. The CG moves a little bit as the motor burns or the rocket contents shift under acceleration but these are usually so minor that they can be ignored. The CP on the other hand can move a lot. On long thin rockets it can move much more than 1 caliber while on short fat rockets it moves less. With Rocksim you can plot the CP location (use the Rocksim method) during flight. The biggest shift will be as the rocket comes off the rail in high winds. Check using the highest winds and lowest thrust motors you expect.

  8. #8
    Join Date
    7th August 2011
    Location
    Penfield, NY
    Posts
    653
    Open Rocket also allows you to calculate CG and CP at different speeds. I was able to gain a 1 cal. static stability by putting 12 oz. of weight in the nose of a Wildman Gizmo, in addition to the DD cannon I had there as well. Any software that factors in aft drag should be able to be effective in simming a short fact rocket, whose stability is gained through aft drag. In the case of my Gizmo, under weather conditions similar to our launch site and a launch configuration into the wind that we normally take, the difference in altitude between OR simulated and Stratologger reported was only 4%. That is an acceptable margin of error.

    Click image for larger version. 

Name:	Open Rocket Configuration.jpg 
Views:	46 
Size:	43.8 KB 
ID:	120610
    Endeavour.jpg

    Evan Brown
    NAR# 92851 L2
    TRA# 14596 L2

    History doesn't repeat itself, but it does have a tendency to rhyme. - Mark Twain

    Visit my YouTube Page...Google "EeeebeeeE rocket"

  9. #9
    Join Date
    13th February 2012
    Location
    Central Jersey
    Posts
    5,249
    Quote Originally Posted by EeebeeE View Post
    Open Rocket also allows you to calculate CG and CP at different speeds. I was able to gain a 1 cal. static stability by putting 12 oz. of weight in the nose of a Wildman Gizmo, in addition to the DD cannon I had there as well. Any software that factors in aft drag should be able to be effective in simming a short fact rocket, whose stability is gained through aft drag. In the case of my Gizmo, under weather conditions similar to our launch site and a launch configuration into the wind that we normally take, the difference in altitude between OR simulated and Stratologger reported was only 4%. That is an acceptable margin of error.

    Click image for larger version. 

Name:	Open Rocket Configuration.jpg 
Views:	46 
Size:	43.8 KB 
ID:	120610
    Base drag in drag calculations is completely separate from base drag in stability calculations.
    Two rocket motors burned this year.

  10. #10
    Join Date
    7th August 2011
    Location
    Penfield, NY
    Posts
    653
    @ CarVac, I stand corrected. However it is the base drag that increases stability. That's why pyramid and saucer rockets don't need fins.
    Endeavour.jpg

    Evan Brown
    NAR# 92851 L2
    TRA# 14596 L2

    History doesn't repeat itself, but it does have a tendency to rhyme. - Mark Twain

    Visit my YouTube Page...Google "EeeebeeeE rocket"

  11. #11
    Join Date
    11th October 2010
    Location
    Connecticut - formerly The Constitution State
    Posts
    955
    Thanks! This is just what I was looking for.
    Bernie

    L1: Madcow Black Brant II - AT H123W
    L2: Rocketry Warehouse Formula 98 - CTI J244-WH

  12. #12
    Join Date
    11th October 2010
    Location
    Connecticut - formerly The Constitution State
    Posts
    955
    Quote Originally Posted by UhClem View Post
    The 1 caliber rule of thumb is just a simple way to capture all of the complicated bits of stability. The rocket doesn't have to have 1 caliber of static stability so long as the CG is always ahead of the CP. The problem is that both are mobile. The CG moves a little bit as the motor burns or the rocket contents shift under acceleration but these are usually so minor that they can be ignored. The CP on the other hand can move a lot. On long thin rockets it can move much more than 1 caliber while on short fat rockets it moves less. With Rocksim you can plot the CP location (use the Rocksim method) during flight. The biggest shift will be as the rocket comes off the rail in high winds. Check using the highest winds and lowest thrust motors you expect.
    I plan on using high thrust motors only and avoiding launching in high winds. My concern is that Rocksim shows a static CG only slightly ahead if the CP, so I guess I better chart the in flight movement.
    Bernie

    L1: Madcow Black Brant II - AT H123W
    L2: Rocketry Warehouse Formula 98 - CTI J244-WH

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •