Model Rocketry

Reference / Paper · 2004

Peak of Flight #119: Simulation of Side Pods in RockSim v7.0

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Apogee Components' Peak of Flight newsletter, issue 119 (2004).

Author
Tim Van Milligan / Apogee Components
Year
2004
Type
Reference / Paper
Credit
Apogee Components, Inc. (Colorado Springs, CO) — Peak of Flight newsletter
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Peak of Flight #119: Simulation of Side Pods in RockSim v7.0

Apogee Components' Peak of Flight newsletter, issue 119 (2004).

N I S S U E 1 1 9 , E S W L J A N . E 2 7 , T 2 0 0 3 T E R Simulation of Side Pods Using RockSim version 7 1130 Elkton Drive, Suite A Colorado Springs, CO 80907 USA www.ApogeeRockets.com orders @ApogeeRockets.com phone 719-535-9335 fax 719-534-9050 Page 1 ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Simulation of Side Pods Using RockSim Version 7 By Bruce S. Levison Bruce S. Levison has asked us to share this article with other RockSim users. It describes a method that he feels will help to simulate side pods, ring tails and tube fins using the concept of a fin mount in RockSim. Bruce feels that the CP will be in the right location on the rocket. Please note: The user assumes all risk for the information obtained with this method. Download the RockSim design files at: http:// www.ApogeeRockets.com/Education/downloads/sidepod.zip. After downloading and unzipping, place these files in the Design folder within the RockSim Folder on your hard drive. These files cannot be viewed using the demo version of the RockSim software. The fin mount concept was introduced in my Simulation Fins on Fins article (see issue #113 of the Apogee Peak of Flight Newsletter, October 27, 2003 http:// w w w. a p o g e e r o c k e t s . c o m / e d u c a t i o n / d o w n l o a d s / Newsletter113.pdf ) it allows for the easy simulation of both open and closed tubular surfaces on model rockets. Please refer to my previous tube and ring fin simulation articles to find out how these objects had been simulated using earlier versions of RockSim (see: Simulating the effects of Tube Fins with RockSim http://www.apogeerockets.com/ simulating_tube_fins.asp and Simulation of Ring Fin Designs Using RockSim Ver 4.0 issue #27 of the Apogee's Model Rocketry E-Zine, 11/5/00 http://www.apogeerockets.com/education/ newsletter27.asp) The central idea of my ringtail and tube fin articles is to accurately take into account the surface area of the open tube exposed to the air stream, which amounts to the "wetted" surface of the tubes. For tube fins, I previously suggested that: tube fins can be simulated as three flat fins that have the same area as the lateral cross sectional area of the tube fin (figure/ Figure 2. simulation 1). For a ringtail design, I suggest simulating a ring fin as six flat fins that have a span equal to the radius of the ring fin and a height equivalent to that of the open tube (figure/simulation 2). If you calculate the wetted surface area (ignoring the small frontal area) of either a ringtail or tube fin you get: Aw = 2*pi* ro*h + 2*pi*ri*h = 2*pi*h*(ro + ri) where ro = outer radius of the tube fin or ring tail, ri = inner radius of the tube fin or ring tail and h = height of the tube fin or ring tail. You have to account for both the inner and outer surfaces of the tube fins or ringtail since air moves through them contacting or wetting both sides. For these simulations since the ringtail or tube fin walls are very thin then we can set ro = ri and estimate the wetted surface area A w to be: Aw = 2*pi*h*(ro + ro) = 4*pi*h*ro. For the tube fins, since ro = do/2, this gives Aw = 4*pi*h*do/ 2 = 2*pi*h*do if you round pi off to 3 then A w = 2*3*h*do = 6*h*d where d would be the diameter of the tube. For three flat fins that have the same shape as the lateral cross section of the tube fin the wetted area would be Aw = 2*(3*h*d) where the factor of two is for both sides of the flat fin. For the ringtail the six flat fins that have a span equal to the radius of the ring fin and a height equivalent to that of the open tube the wetted area would be Aw = 2*6*h*r o again the factor of two is for both sides of the flat fin. Since 2ro = do (or d) substituting d for 2ro gives Aw = 2*(3*h*d). Thus, both the tube fin and ring fin simulations account for most of the wetted surface area of the actual fins. The major error is due to rounding pi to 3, which is 0.14/3.14 * 100% or about 4.5 %. The rocket motors typically used in this hobby are manufactured to a 20% tolerance so this error in rounding pi to 3 shouldn't matter much. You might be wondering what this has to do with side pods or the new version of RockSim. Since I shown how you Figure 1. Continued on page 3 A P O G E E Page 2 R O C K E T S ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Side Pods Continued from page 2 can construct and use fin mounts to position fins off the central axis of the body tube, you can simulate ring or tube fins as six flat fins that have a span equal to the radius of the tubular fin and a height equivalent to that of the open tube, these fins are placed evenly in a radial fashion (like spokes on a wagon wheel) about a fin mount through the central core diameter of the tube see figure/simulation 3 for the tube fin case and figure/simulation 4 for the ring fin case. Note that nothing has changed for the ring fin case since the fins for this equivalent construct are already placed in a radial fashion about the central axis of the main body tube. This new method for estimating the CP of a tube fin gives a more conservative estimate (more forward location) for the Figure 5. Figure 6. Figure 3. Figure 4. CP value. This is due to having more fin area closer to the main body tube (notice that there are only six fin tips at the full radial distance of the tube fin) hence less restoring force where as the old simulation method placed more fin area farther from the main airframe body tube (notice that there are 18 fin tips at the full radial distance of the tube fin) which translates into more restoring force and a CP located more aft. In this new simulation technique for tube fins the wetted sur- face of the simulation construct more closely matches that of an actual tube fin and thus should be more accurate. But if you look at my original argument for using three flat fins to represent one tube fin one thing becomes immediately obvious; this new method for estimating the CP of tube fins is much closer to the simulation using one fin of the same lateral cross sectional area to represent the tube fin than three flat fins, see figure/simulations 5 and 6. The difference in the predicted CP vales using the old and new techniques amounts to about 14 mm or about 9/16 of an inch on this example. However, the fins within these new side pod tube constructs were created at the same fixed radial angles. Spacing each subsequent fin set in each successive tube a constant radial angle apart shows that the CP value reaches a maximum when the fins for successive tubes are evenly spaced apart; at a radial angle of 10 degrees for six tube fins, see graph 1. Since these flat fins are representing tubular structures, the particular radial angle of the flat fins should not matter. I suggest using the radial angle obtained from incrementing the radial angle between the tube fin sets until the CP reaches its maximum see figure/simulation 7. Even at this maximum the new CP value is still about 1/2 inch from the three flat fins Continued on page 4 About this Newsletter You can subscribe “FREE” to receive this e-zine at the Apogee Components web site (www.ApogeeRockets.com), or sending an email to: [email protected] with "SUBSCRIBE" as the subject line of the message. A P O G E E R O C K E T S PAGE 2 Page 3 ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Side Pods Continued from page 3 Figure 8a. (mm) Graph 1. construct and more closer to the one flat fins case figure/ simulation 5 than the two flat fin equivalent figure/simulation 6. All that can be said is, a half-inch is not much difference in the CP on a 12-inch rocket and these two different techniques will have to wait for future experimental confirmation. Figure 7. The outer tube fins, which were originally inside tubes, serve to locate the CG of the tube fin. The thickness of the fins that are used for simulating the CP for tube fins is the same as the thickness of the tube fins. This is equivalent to the difference between the inside and outside radii or (ro - ri ). Since the radial fins within the tube fin have the same wetted surface and thickness (and hence frontal surface) of the actual side pod tube they should also predict both the correct CP and coefficient of drag or Cd. A mass override of 0.001 oz is chosen for these fins so they won't affect the CG value see simulation 3. This leads us into the case of side pods. Since air can't flow through side pods these structures only have half the wetted surface area of a tube or ring fin or just A w = 2*pi* ro*h. Rounding pi off to 3 gives A w = 2*3* ro*h which is exactly half of the estimated wetted surface of the tube or ring A P O G E E Page 4 Figure 8b. fin. This means you could simulate side pods as three flat fins (as opposed to six for open tubular fins) that have a span equal to the radius of the side pod with a height equivalent to that of side pod tube with the fins placed evenly in a radial fashion about a fin mount through the central core diameter of the side pod tube, see figure/simulation 8. These three fins take into account most of the wetted surface area of the side pod tube so RockSim can calculate a fairly accurate CP. However, the fins within these new side pod tube constructs were created at the same fixed radial angles. Spacing each subsequent fin set in each successive side pod a constant Continued on page 5 Graph 2. R O C K E T S ISSUE 119 - JANUARY 27, 2004 P E A K O F Side Pods Continued from page 4 radial angle apart shows that the CP value reaches a maximum when the fins for successive tubes are evenly spaced apart; at a radial angle of 30 degrees for two side pods in this case see graph 2. Since these flat fins are representing tubular structures the particular radial angle of the flat fins should not matter. I suggest using the radial angle obtained from incrementing the radial angle between the side pod fin sets until the CP reaches its maximum see figure/simulation 9. Figure 9a. But what about placing a nosecone on top of the side pods? RockSim can be tricked into doing this using a transition constructed as the equivalent for a nosecone. The way to do this is to build transitions assigned to the side pod tubes (don't worry about their location yet) that have very narrow forward diameters, 0.001 inch, with a lengths and diameters equivalent to that of the nosecones on the side pods. Shoulder lengths and diameters should also be entered for the transitions that are equivalent to the dimensions of the nosecones. The next step involves saving this RockSim file and changing the radial location, radial angle and location from the top of the tube the side pods are attached to (Xb) for the transitions to correspond to that of the actual side pod F L I G H T nosecones using a word processor. Since RockSim uses millimeters for the Xb values you need to determine the location of the tip of the side pod nose cone in millimeters from the top of the body tube the side pods are assigned to. One way to do this is under application settings on the units tab in the RockSim program set to Display lengths in: Millimeters. Open the main tube and write down its length in millimeters then subtract the length of the side pod and side pod transition nosecone (ignoring the shoulder) both in millimeters to give the value for Xb. For and example, use the word processor to change (edit) the line: <Xb>0.</Xb> to: <Xb>232.791</Xb> , <RadialLoc>0.</RadialLoc> to < R a d i a l L o c > 2 1 . 7 4 2 4 < / R a d i a l L o c > , and <RadialAngle>0.</RadialAngle> to <RadialAngle>0.785398185</RadialAngle> for each of the transitions. You will have to do some hunting for these program lines in the transition. You can use the find (or search) function of the word processor to look for the <Transition> and scroll down. The radial location and angle for the transition nosecone should be set to the same values for the side pod tube as shown below. Change: <Transition> <PartMfg>Custom</PartMfg> <KnownMass>0.</KnownMass> <Density>1049.2093265</Density> <Material>Polystyrene PS</Material> <Name>Side Pod Nosecone</Name> <KnownCG>0.</KnownCG> <UseKnownCG>0</UseKnownCG> <Xb>0.</Xb> <CalcMass>6.79150881426269</CalcMass> <CalcCG>33.4819543862979</CalcCG> <WettedSurface>2.8496627866633e-003</ WettedSurface> <PaintedSurface>2.8496627866633e-003</ PaintedSurface> <GlueJointLength>0.</GlueJointLength> <DensityType>0</DensityType> <PartNo></PartNo> <PartDesc></PartDesc> <RadialLoc>0.</RadialLoc> <RadialAngle>0.</RadialAngle> <Texture></Texture> to: <Transition> <PartMfg>Custom</PartMfg> <KnownMass>0.</KnownMass> <Density>1049.2093265</Density> <Material>Polystyrene PS</Material> Continued on page 6 Figure 9b. A P O G E E R O C K E T S Page 5 ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Side Pods Continued from page 5 <Name>Side Pod Nosecone</Name> <KnownCG>0.</KnownCG> <UseKnownCG>0</UseKnownCG> <Xb>232.791</Xb> <CalcMass>6.79150881426269</CalcMass> <CalcCG>33.4819543862979</CalcCG> <WettedSurface>2.8496627866633e-003</ WettedSurface> <PaintedSurface>2.8496627866633e-003</ PaintedSurface> <GlueJointLength>0.</GlueJointLength> <DensityType>0</DensityType> <PartNo></PartNo> <PartDesc></PartDesc> <RadialLoc>21.7424</RadialLoc> <RadialAngle>0.785398185</RadialAngle> <Texture></Texture> Figure 10a. Figure 10b. The Radial Location and Radial Angle values are the same as the side pods CG tube! <BodyTube> <PartMfg>Estes</PartMfg> <KnownMass>0.</KnownMass> <Density>1121.29241</Density> <Material>Paper</Material> <Name>Side Pod Body tube CG only</Name> <KnownCG>0.</KnownCG> <UseKnownCG>0</UseKnownCG> <Xb>304.8</Xb> <CalcMass>3.25538835061428</CalcMass> <CalcCG>76.2</CalcCG> <WettedSurface>0.</WettedSurface> <PaintedSurface>0.</PaintedSurface> <GlueJointLength>0.</GlueJointLength> <DensityType>0</DensityType> <PartNo>EST 3085</PartNo> <PartDesc>BT-20</PartDesc> <RadialLoc>21.7424</RadialLoc> <RadialAngle>0.785398185</RadialAngle> <Texture></Texture> Save the file in the same format, and close the word processor program. Re-open the modified file in RockSim and view the Rocket design components tab, see figure/simula- Figure 10c. tion 10. Also notice that these transitions as well as the outboard fins, within the side pod or tube fin, are located along the centerline in the 3D drawing even though the calculations of the CP use the radial angles and locations of the transitions location. This is because "The fin (or part) positioning code assumes that all fins (or parts) are radius lines from the center of the airframe." as per Paul Fossey, RockSim's creator and programmer. These transitions help locate a better CG and CP for the rocket. RockSim will not recognize the effects these transitions and the side pod tubes have on the Cd of the rocket. The side pod tubes, which were originally inside tubes, serve only as constructs to locate the CG of the side pod. The thickness Continued on page 7 Advertising Space Is Available In Every Issue. Call Tim at: (719) 535-9335. A P O G E E Page 6 R O C K E T S ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Side Pods Continued from page 6 Figure 11a. Figure 12. the fin spans are already ro setting the fin thickness to ro gives ro2 . There are already three fins specified and since you could round pi to 3 then the full frontal area of the side pod is covered in the simulation to estimate its Cd see figure/simulation 11. The regular ogive 18mm nosecone alone (figure/simulation 12) has a Cd value of 0.11571. The RockSim predicted Cd of the model without side pods (figure/simulation 13) is 0.95427 adding the two nosecones would give 0.95427 + 2*0.11571 = 1.18569 with the fin thickness set to ro, the Cd is 1.15061. See figure/simulation 11 which should be close Continued on page 8 Figure 11b. Figure 13a. Figure 11c. of the fins that are used for simulating the CP for the side pods should be set to be the same as the radius of the side pod to estimate the Cd of the side pod and its nosecone. The frontal area of the side pod is Af = pi*ro2 (if the end was flat). Since A P O G E E Figure 13b. R O C K E T S Page 7 ISSUE 119 - JANUARY 27, 2004 P E A K O F F L I G H T Side Pods Continued from page 7 enough for the simulation of this rockets flight. If you vary the shape of the nosecone you will notice that is Cd value varies from 0.09 conical to 0.17 power series shape control = 0. The fin thickness could be adjusted up or down to match the Cd value for the nosecone and or side pods if this value is known. Another option if the Cd value of the added side pod is known would be to use a fixed Cd value on the Rocket Design Attributes menu tab. This article introduces the use of fins on fin mounts to simulate the CP and Cd of side pods, ring fins and tube fins. The versatile location of the fin mount construct allows it to be situated at the central diameter of these tubular structures, fins assigned to the fin mounts, six for open tubes and three for closed tubes, give estimates for the CP and Cd values of the ring or tube fins and side pods respectively. CG values can be simulated for these tubular structures can be simulated from inside tubes with a specified radial location and angle that are converted into outside tubes using a word processor. In closing I must again mention that this simulation work is only a theoretical approximation that has yet to be confirmed with real flight data; use this unproven technique at your own risk. I welcome any comments and criticism on this work. Bruce S. Levison, NAR #69055 Figure 13c. Send Mail to: [email protected] About the Author: Bruce S. Levison (NAR #69055, MTMA #606) is a rocketeer from Ohio, and a member of the National Association of Rocketry (NAR). He has published numerous articles on model rocketry, related to the many practical aspects of the hobby. Bruce enjoys tricking the RockSim software into performing simulations of non-standard rocket designs. You can usually find him helping other people with their simulation problems in "The Rocketry Forum" at: http://www.rocketryforum.com/ (be sure mention RockSim in the title of you post). Bruce earned an advanced degree in chemistry, and works as a research scientist at the Cleveland Clinic Foundation. Newsletter Archive On CD-ROM - FREE! Tech n 36051 $35.0 0 6 P/N s ion at Get the entire collection of Peak-of-Flight newsletters on CD-ROM in pdf format. It’s absolutely FREE when you buy the Apogee Technical Publications (#1-#35) set. Not only do you get the great technical reports, you get all of the first 112 Public issues of the Peak-of-Flight newsletter on a seperate CD-ROM for no extra l a icReports #1 through #2 charge. The Peak-of-Flight CD-ROM gives you much more than the web-site archives. You also get:  All issues are in PDF format. This allows you to print them out quickly so you can share them with your friends.  Images are high-resolution, so they print out great.  Many of the earlier articles have been re-written and updated with new information and illustrations.  QuickTime videos have been embedded into many issues to show action sequences (11 movies total).  Many extra reference articles included - So you don’t have to Newsletter #1 to #100 download them from the internet. Order Today, and also receive #101-#112  Shrox plans, decals, and RockSim design files included. http://www.ApogeeRockets.com/technical_publications.asp  Plus excel spreadsheets, programs, and much more... www.ApogeeRockets.com eq 20 03 - Ap ogee Comp on en ts er © R ne uir www.ApogeeRockets.com e s: or v4 Adob an e A cro b at r dA we e i p ple QuickTime V w P/N 01084 Re er qui ew res: or n and Adobe Acrobat v4 er Apple QuickTime View A P O G E E Page 8 R O C K E T S