CNY Rocket Team Challenge Basics of Using RockSim 9 to Predict Altitude for the Central New York Rocket Team Challenge
RockSim 9 Basics 2 Table of Contents A. Introduction.p. 3 B. Designing Your Rocket.p. 3 C. Checking Your Rocket s Stability p. 21 D. Predicting Your Rocket s Altitude...p. 23 E. Determining Your Rocket s Actual Drag Coefficient...p. 27 F. Refining Your Altitude Prediction...p. 28
RockSim 9 Basics 3 A. Introduction RockSim 9 is a computer program from Apogee Components that helps you design model rockets, confirm their stability before you fly them, select the right motors for flying your designs, and predict your rocket s altitude and flight duration. It can also help you with some rocket construction steps by doing things like printing out patterns for fins you have designed and printing patterns to make your own paper adapter cones. This manual presents the basic information about RockSim needed to help you predict the altitude of the rocket you will build and fly for the Central New York Rocket Team Challenge (CNYRTC). B. Designing Your Rocket in RockSim RockSim is a great tool for creating new custom designs from scratch, but it can also help you perform calculations for manufactured. RockSim files are available on the Internet for the LOC Graduator and the LOC IV. Try the following locations: LOC Graduator: http://www.rocketreviews.com/unknown-magna-cum-louder.html?svg=y LOC IV: http://www.apogeerockets.com/rocket_kits/skill_level_3_kits/loc_iv (see Free RockSim design file near bottom of page) If you use a downloaded file, be sure to verify that the component dimensions match those in your kit. Modify them as necessary. Your CNYRTC kit contains a payload compartment that is not included in the standard LOC kit. You will have to add the payload compartment to the design that you download. Below is a description of how to create a RockSim Model of the LOC Graduator. The steps are the same for the LOC IV, but the component selections, dimensions and weights will be different.
RockSim 9 Basics 4 1. Open RockSim 9 and click on the Rocket design attributes tab. 2. Enter a name for your rocket. 3. Set the calculation method to Barrowman stability equation. 4. Set the number of stages to single stage. 5. Set the static margin reference to nose cone base diameter. 6. Click on the Rocket design components tab. 7. Click the nose cone button to add a nose cone to your rocket.
RockSim 9 Basics 5 8. Select the LOC PNC-2.56 nose cone from the database and click OK. 9. RockSim automatically fills in the design information (dimensions, shape, material, weight, etc.) in the nose cone window. Click OK to insert the nose cone in your design. Note: The component data in the RockSim database is not always accurate. When you select components from the database it is best to verify the dimensions and mass of your components. Adjust the dimensions and mass in the individual component windows as needed to match your actual components.
RockSim 9 Basics 6 10. Now is a good time to save your file by clicking File Save. Do that periodically as you add to the design, so you don t lose any work in the event of a program crash or power failure. 11. Next add the payload compartment tube by clicking the Body tube button. 12. Select the LOC BT-2.56 airframe tube from the database and click OK. 13. In the body tube window change the component name to payload compartment. 14. Change the length to 10 inches and click OK.
RockSim 9 Basics 7 15. Add a coupler by clicking the coupler button. This is the tube that connects the payload compartment to the main body tube. The coupler will be permanently glued to the payload compartment and will slide into the main body tube, but will not be glued to the body tube. 16. Select the LOC TC-2.56 Tube Coupler.
RockSim 9 Basics 8 17. Change the coupler length to 6 inches. 18. Change the Location to -3 inches from the base of the owning part and click OK.
RockSim 9 Basics 9 19. Click on the Payload compartment in the Rocket design components tab. 20. Add a bulkhead by clicking the Bulkhead button. This is the plywood disk that separates the payload compartment from the main body tube. The bulkhead will be permanently glued into the bottom of the coupler. 21. Change the location to -2.875 inches from the base of the owning part and click OK.
RockSim 9 Basics 10 22. Add the main body tube by clicking the Body Tube button. 23. Select the LOC BT-2.56 airframe tube from the database and click OK. 24. Confirm that the body tube length is set to 30 inches and click OK. 25. Click on the Body tube. 26. Then click the Inside tube button to add the motor tube.
RockSim 9 Basics 11 27. Select the LOC Precision MMT-1.14 motor mount tube. 28. Change the name to Motor Tube 29. Check the This is a motor mount checkbox. 30. Set the Inside Diameter to 1.141 inches. 31. Confirm that the motor diameter displays as 29 mm. 32. Set the length to 6 inches. 33. Set the location to 0 inches from the base of the owning part. 34. Click OK.
RockSim 9 Basics 12 35. Click on the Body tube. 36. Then click the Centering ring button to add the front motor tube centering ring. 37. Select the LOC Precision CR-2.56-1.14 centering ring.
RockSim 9 Basics 13 38. Change the name to Front Centering Ring 39. Set the location to 5.75 inches from the base of the owning part. 40. Click OK. 41. Repeat steps 35-40 for the rear centering ring. Set the location of the rear centering ring to 0.125 inch from the base of the owning part.
RockSim 9 Basics 14 42. Click on the Body tube. 43. Then click the Custom fins button to add the fins.
RockSim 9 Basics 15 44. Select the LOC Precision FS-16 fin set from the database. 45. Set the fin count to 3. 46. Set the location to 0 inches from the base of the owning part 47. Click OK.
RockSim 9 Basics 16 48. Click on the Body tube. 49. Click the Mass object button to add a shock cord. 50. Select the LOC Precision SC-375 3/8 Elastic Shock Cord from the database. 51. Set the mass to 0.6 oz. 52. Set the location to 6 inches from the front of the owning part
RockSim 9 Basics 17 53. Use the parachute button to add your parachute in the main body tube. The parachute included with the LOC Graduator is an 18-inch nylon parachute. 54. Use the mass object button to add the egg in the lower half of the payload compartment, and the altimeter in the upper half of the payload compartment. Your egg will weigh about 2 ounces plus the weight of the egg protection packing that you design. The altimeter will weigh approximately 0.4 ounces. 55. The camera will be fastened to the outside of the rocket. This can be added to the RockSim model as a Pod. Select the main body tube then click the Pod button.
RockSim 9 Basics 18 56. Name the pod Camera Pod. 57. Uncheck the two check boxes. 58. Set the location to 1 inch from the front of the owning part. 59. We ll simulate the cylindrical camera in the model by inserting a body tube in the pod we just created. 60. Select the camera pod, then click the Body tube button. 61. Rename the body tube to Camera. 62. Set the outside diameter to 1.1 inch. 63. Set the length to 3.9 inches.
RockSim 9 Basics 19 64. In the mass override tab, click the Use the mass and CG information below box. 65. Set the camera mass to 2.3 ounces. 66. To approximate the drag created by the camera, place a solid bulkhead at the front and back of the camera tube. First select the camera tube that is within the pod, then click the bulkhead button. 67. Set the Outside Diameter to 1.1 inch. 68. Set the Location to 0 inches from the front of the owning part.
RockSim 9 Basics 20 69. Repeat steps 66-68 to create a bulkhead at the rear of the camera. Set the location of the rear bulkhead at 0 inches from the base of the owning part. Your finished RockSim model can be viewed as a 2D or 3D drawing by selecting this button.
RockSim 9 Basics 21 C. Checking Your Rocket s Stability Before flying a rocket, it is important to confirm that the rocket is stable. A stable rocket will fly straight and true in a predictable manner. An unstable rocket may twist and turn in all directions, causing an unsafe situation for spectators. For a rocket to be stable, the Center of Gravity (CG) must be forward of the Center of Pressure (CP) by at least one full body tube diameter. This condition must exist with the rocket fully prepped and ready to launch with the motor inserted. A good rule to remember is that adding weight to the nose of the rocket makes it more stable. The CG is easy to determine as it is the point at which the fully loaded rocket (including motor) balances. You can determine the CG by balancing the rocket on your finger. The Center of Pressure is determined by the shape of the rocket. A wind blowing perpendicular to the main axis of the rocket will exert an equal force ahead and behind the Center of Pressure. Increasing the fin area at the back of the rocket makes it more stable. RockSim calculates the CP for us. Use the following steps to confirm your rocket s stability before you fly it: 1. Complete your model in RockSim and observe the reported CG and CP values in the model. RockSim reports the distance from the tip of the nose cone to the CG and CP. In the example below the CG is 25.29 inches from the nose. The CP is 40.53 inches from the nose. This puts the CG more than 5 body tube diameters ahead of the CP, making the rocket very stable. An over-stable rocket will tend to arc into the wind more than a rocket with a one body tube diameter stability margin, but this is a minor concern in comparison to the dangers of flying an unstable rocket.
RockSim 9 Basics 22 2. After your rocket is built and painted, you must measure the actual mass and Center of Gravity of the rocket. Prepare the rocket as if you were going to fly it, including installation of wadding and parachute. Measure the total weight of the prepped rocket (without the motor). Balance the prepped rocket on your finger and measure the distance from the nose to the balance point. 3. Go to the Mass override tab in RockSim. 4. Check the Use the values shown below for all simulations box 5. Enter the measured rocket weight. 6. Enter the measured CG. 7. If you make modifications to the rocket re-check the total weight and CG and update them in RockSim. 8. Always be sure the CG is at least one body tube diameter ahead of the CP before you fly your rocket.
RockSim 9 Basics 23 D. Predicting Your Rocket s Altitude One of the objectives of the CNYRTC is to predict your rocket s altitude. You can use RockSim to predict the altitude that your rocket will reach using various rocket motors. To perform an initial altitude prediction, use the following steps: 1. The finish on your rocket can have a big impact on the drag created by air flowing over the rocket s surface. For each component exposed to the airflow (nose cone, payload compartment, body tube, fins, and camera) double click on the component in the Rocket design components tab. Select the Finish dropdown and pick unfinished, matt, gloss or polished, then click OK. 2. In the Cd Override tab, check the Calculate Cd at simulation time box
RockSim 9 Basics 24 3. After your design is completed in RockSim, click the Prepare for Launch icon, or select Simulation / Prepare for Launch from the top menu. 4. On the Engine selection tab click the Choose engine button.
RockSim 9 Basics 25 5. From the Engine Selection Database select the Aerotech F50T if you are flying the LOC Graduator or Aerotech G80T if you are flying the LOC IV. 6. Choose the 6 second ejection delay if you are flying the LOC Graduator or 4 second delay if you are flying the LOC IV. 7. Click OK 8. In the Flight Events tab, make sure the device is parachute, and the event description says Deploy at Max Ejection Delay. 9. In the Starting State Tab enter the length of your launch rod or rail and 0 degrees for the launch rod angle. 10. In the launch conditions tab enter the site altitude and latitude. The altitude of the Syracuse University Skytop launch area is 700 ft above sea level. The latitude of the Skytop launch area is 43.01 degrees. 11. Enter the relative humidity, temperature, barometric pressure, wind conditions, and cloud cover information. For your initial simulations just use your best estimate of conditions when you expect to do your first launch. When it s time for an actual launch, you ll adjust these parameters to match actual weather conditions at the site.
RockSim 9 Basics 26 12. Click Launch to run the simulation. The predicted altitude will display on the Flight simulations tab. 13. To see more detailed simulation results data, right click on the simulation results row, and select Display Details.
RockSim 9 Basics 27 E. Determining Your Rocket s Actual Drag Coefficient The RockSim simulation you have performed before your first flight provides a good estimate of your rocket s expected altitude, but accuracy of your future simulations can be improved by doing multiple test flights and using actual results to refine the drag coefficient, Cd, of your rocket. The flow chart below describes the procedure for determining the actual drag coefficient.
RockSim 9 Basics 28 F. Refining Your Altitude Prediction Performing multiple test flights will improve your probability of achieving an accurate altitude prediction on the day of the CNYRTC contest launch. Here are a few more tips to help you refine your altitude predictions: 1. Rocket engine performance will vary from one engine to the next. The more test flights you do to determine an actual drag coefficient, the more confidence you will have in your Cd value. 2. If several test flights produce a tight grouping of Cd values, and one flight produces a very different value, the outlier may be the result of non-representative engine, and that result should be disregarded. 3. Thrust and total impulse from rocket engines are generally fairly consistent. Ejection delay times are less consistent. It is not uncommon to see the delay times on engines with a nominal 6 second delay vary over a range of 4 to 8 seconds. That s why step 2 on the previous page suggests that a member of your team measure and record the time to ejection during your test flights. If you have a test flight with an early ejection, resulting in a lower than predicted altitude, that flight data should not be used in calculating a revised drag coefficient. 4. On the day of your competition launch you can bring a laptop computer to the launch to do a final simulation to predict the altitude of your contest flight. You will use the Cd value that you determined from your test flights, enter the actual weather conditions on launch day, and run a final simulation before the launch. 5. Another important factor to consider is wind and launch rod angle. On a very calm day, you will probably keep the launch rod vertical. On a windy day it may be necessary to angle the launch rod into the wind slightly. You can make a device with a string, a weight and a protractor to measure the angle of the launch rod, and enter the rod angle into RockSim for each flight. 6. In your final simulations make sure that you use your measured rocket weight, not the calculated rocket weight. To do this CHECK the Use the values shown below for all simulations box in the Mass Override tab. 7. In your final simulations make sure that you use your field-determined drag coefficient (unless your contest flight is your first flight). To do this UNCHECK the Calculate Cd at simulation time box in the Cd Override tab. Good luck with your Rocket Team Challenge contest flight!!