OpenRocket (http://openrocket.sourceforge.net/) is a FREE rocket design and 6 DOF flight simulator. The tool is written by Sampo Niskanen as a part of his Master’s Dissertation in 2009 at the Helsinki University of Technology: “Development of an Open Source Model Rocket Simulation Software” The software is very easy to use. In terms of user friendliness its
interface beats RockSim by miles. Starting from version 1.1.0 OpenRocket support loading RockSim
rocket design files (.rkt). Unfortunately, not all RockSim features are supported, but it does not make the tool less valuable. Initially it was not possible to print out anything from OpenRocket. However, the Print option was added later and now it is not only possible to send a design to a printer, but the software can export it directly into the PDF format. Kerry Quinn created a very useful presentation describing OpenRocket, its features and what can be done with its help. Also Open Rocket Design site is very useful and has a library of OR designs. Adding New EngineNote: The below tip is irrelevant for the later versions. Starting from the release 1.1.9 custom engine data can be loaded through Edit/Preferences/Options/User-defined trust curves. It is possible to add a custom engine data file to Open Rocket, though it is not a straightforward procedure. Some advice can be found on the Open Rocket forum. One of the ways to do it is with a help of WinZip software. You have to open the .jar file through WinZip or any other compatible application. Then go to datafiles\thrustcurves\ folder. There you will see files representing motors available in OR. There are two different types .eng and .rsp. Some motors may include additional engline files in other, non compatible formats. Both can be viewed in a text editor. The .eng format is easier to work with and I usually use it if I want a custom made thrust curve. Elsewhere on http://www.thrustcurve.org/motorstats.shtml , there is a description of both formats, but basically it's just a list of time-thrust values with some additional information in the first row, such as a motor weight, delay etc. Otherwise I suggest you compare what OR shows and context of the corresponding file. Here are the steps again:
Engine File FormatLINK explaining details of the thrust curve file format. LINK to a little tool helping with the thrust file creation.
Simulating Water RocketUtilisation of OR for purposes of a water rocket design was discussed in length on the Water Rocket Forum. Simulating Boosted DartsA boosted dart rocket is a vehicle consisting of a rocket booster and an unpowered upper stage called " a dart". The dart separates from the booster due to a difference in drag and
coasts to apogee. Here are several examples of boosted darts:
Excellent NARAM -31's report by Spaceman Spiff Team provides very useful theoretical inside and experimental data related to the boosted dart type model rockets.
OpenRocket can be used to simulate these types of rockets. The trick is to represent the dart as a powered sustainer using a motor with an insignificantly small impulse. A "dummy dart motor" file is attached at the bottom of this page. After adding it to the OpenRocket thrustcurves folder as described above, the dummy motor appears in the motor list under TheSkyDart name (see the screen-shot below). The manufacturer can be changed to any name by editing the .eng file in a text editor. ; The motor provides several delay options, but again they can be easily modified to any desired value. The sustainer motor ignition should be specified as "First burnout of previous stage". Dynamic Stability CharacteristicsThis section is added to reflect upon the six-part series of articles in Apogee's news letter, Peak of Flight. These are: Issue #192 - (09/11/07) Basics Of Flight Analysis - Moment Of Inertia The formulae presented in the articles can be found in 'Advanced Topics In Model Rocketry' by Mandell, Caporaso, and Bengen. The book is quite rare and thus quite expensive. However, the basis of the Dynamic Stability section of the book was published as a series of articles in Volume 1 of Model Rocketry magazine (in 1968; #10 and #11 and in 1969; #1, #2, #3 and corrections in #4). There are several parameters important to a rocket's dynamic stability characteristics. The above literature provides the relevant formulae and suggests some design criteria to be adhered to to get required performance of a model. These characteristics are:
Of the above list only the Longitudinal Moment of Inertia is explicitly calculated in OR and it is available for plotting and exporting. The rest are not available in the current release of the tool. However, with some additional Excel processing it is still possible to get these parameters. NOTE: Staring from OR version 12.09 you are not limited to using just the built-in simulation variables in
your plots and analysis.
With the custom expression feature, you can write expressions to
calculate other values of interest during the simulation. These can then
be plotted or exported just like the built in variables. Practically all characteristics discussed in sub-sections below can be found in the List of useful custom expressions, therefore removing a need for any further Excel processing.
Corrective Moment CoefficientTo compute C1 the following formula is to be used:
p- density of air, approx 1.24 kg/m^3 There are two ways to obtain the Normal Force coefficient Cna. It can be calculated as follows: Alternatively, a value of Cna corresponding to a particular angle of attack is available on the Stability tab of Analyze/Component Analysis menu. The required value is the Cna column row Total (3.47 on the picture below): Note that the total Cna depends upon Angle of Attack and speed (Mach number). The Angle of Attack should be set to 0deg, but the Mach number can be set to any value between Vmax and 0 (I need to do some more characterisation here) Natural FrequencyOnce C1 coefficient is calculated the Natural Frequency ωn can be calculated as follows: ωn =sqrt(C1/IL) (3) Damping Moment Coefficient The Damping Moment Coefficient is calculated using the following formula: Damping RatioFinally, the Damping Ratio is calculated as follows:DR = C2/(2*SQRT(C1*IL)) (8) Where: C2 - Damping Moment Coefficient (4) C1 - Corrective Moment Coefficient (2) (see Calculation of Natural Frequency in OpenRocket) IL - Longitudinal Moment of Inertia, kg*m^2 (available in OR export) Designing a Back Glider Rocket using Open RocketReferences:[1] – Super Roc Rocket Gliders by Robert Alway and Peter Always, Research and Development Project for NARAM 42, LINK [2] - Open Rocket Technical Documentation For OpenRocket version 1.1.6 by Sampo Niskanen, LINK (pdf download, 1.3 MB) OpenRocket ModelBased on the description in [1] (Figure 2) an OR model of 40 ½ “ backslider was created: To verify correctness of the model its Barrowman Centre of Pressure (BCP) for different fin sizes was compared to the report (Figure 3). As expected no significant differences were found.Centre of Lateral Area (CLA) was calculated independently and compared to data in [1] (Figure 3). Again, no disagreement was found. Changing of the rocket CG was simulated by using OR’s option of the sustained CG override rather than by changing the nose cone/ tail weight as in [1]. Thus the model mass was kept nearly constant in all simulations. Static Stability Analysis
After the above initial preparation models with different fin sizes were
analysed using the Component Analysis power tool available in OR. The tool performs a static stability analysis of a rocket for a given
angle of attack and speed. It was found that the minimum CP distance (the
minimum distance from the nose tip to CP) corresponds to an angle of
attack less than 90 degrees. That minimum CP distance is referred to as
Min CP in Fig. A below. Further increase of the angle of attack
to 90 degrees leads to slight increase in CP distance (referred as 90
deg Angle of attack CP). Mathematical Background
Refer to the OR's technical documentation [2] (pdf download, 1.3 MB) for the software mathematical background. In context of a rocket behaviour in a large angel of attack condition section 3.2 ([2], page 21) provides necessary formulae. In particular, it is important to note that the effect of body lift must not be neglected, especially in a case of a long, slender body ([2], page 24). The normal force exerted on a cylindrical can be calculated using equation 3.26. From 3.26 and 3.19 it can be seen that the normal force coefficient derivatives, C_{Na}, used to calculate CP of cylindrical elements of the rocket depend upon the angle of attack a as sin^{2}a/a. Dynamic Simulation
OR simulation of all flights presented in Table 1, [1] was performed in
two wind conditions – 0 m/s and 2 m/s. Main results of the simulation
are presented in Table A below. Note that the results for 0 m/s wind speed
are not shown in the table due to the insignificant difference from
the presented 2 m/s wind speed data. Figure A As it can be seen from Table A, simulated behaviour of the model
for given fin size and CP location closely resembles the real flight
data. There are only two exceptions – flight #4 and #6. However, CG
for these flights is at the border of “backwards gliding” area (see
points #4 and #6 in Fig. A). Such borderline condition may explain inaccuracy of the simulations compared to the real flights. A completely different behaviour can be observed on Fig. C corresponding to flight #3 with “backwards glide” descent. After the apogee the model vertical orientation changes to approx. 7 deg and stabilises at that value. This indicates that the rocket does not turn down vertically, but remains in an approximately horizontal position. Note stabilisation of the vertical velocity after approximately 11 seconds. Fig. D presents an interesting simulation results for flight #9.
Non-dumped oscillation can be observed. This is due to the burnout CG
equal to 90 deg Angle of attack CP. Simulation plots for all test cases in Table A: ConclusionIt appears that it is possible to simulate behaviour of a backwards gliding rockets (Super Roc Rocket Gliders) using OpenRocket simulation software. A relationship between CP, CLP and CG of a model yielding the desired gliding behaviour of the model during the descent phase can be easily established using static analysis power tool. Dynamic simulation provided quantitative data for further analysis. Further developmentTo verify the above conclusions a back glider rocket was designed. Three successful flights demonstrated a behaviour predicted by OR's simulations. However, a lawn dart dive on the forth occasion destroyed the model. Post-flight OR analysis showed that for the given design (CP and CG locations) and the high wind turbulence flight condition experienced during the launch the lawn dart should occur in approximately 2% of flights.
Useful LinksOpenRocket Distribution website - This is a large depository of designs in OR format. List of useful custom expressions - In version 12.09 and later, you are not limited to using just the built-in simulation variables in your plots and analysis. With the custom expression feature, you can write expressions to calculate other values of interest during the simulation. These can then be plotted or exported just like the built in variables. |