I posted about this in the what I did today rocket-wise thread but decided to post a bit more of my process since I had not designed a model by measuring pictures in a while. I could not find a good blueprint, plan, or dimensions for this rocket so needed to measure parts.
I first search extensively for prototype pictures. Looking for one that is square to camera (and has fins square to camera). I prefer to find a clear picture of an actual rocket but the measurements will be distorted if the picture is not square (or if the camera person was too close to the rocket). Given this I sometimes need to rely on pictures of the model or diagrams. Some examples:
I use Inkscape to measure one picture. It has a convenient ruler tool that returns results in pixels. You can also measure multiple pictures if different parts are clear in various pictures but need to anchor your measurements on something common so that you can adjust and compute sizes. I usually anchor on main body diameter.
I enter all my measurements in an Excel spreadsheet. You basically need to set the dimensions to the scale appropriate for the model you are building. In this case I wanted to make the main body tube a BT-80 tube so I enter the OD for that body tube into the table next to all the measurements. I then take a proportion of the measured dimension to the actual BT-80 dimension. 66.04 / 75.3 = .877% So basically I multiply all my measured dimensions by that ratio/percentage. This gives me the numbers in the right column. Note: I measure fins the way I design them (a square with triangle cutouts) but you can also measure fins with points that help you enter them into OR or RockSim.
I then use these dimensions (rounded to reasonable numbers) to mock up the main elements to see how realistic the rocket is to make. I replace any cylinders with standard body tubes (I am willing to make compromise to use a cheap standard body tube rather than custom ordering something that is more exact), design simple 3D parts, replace fins with 2mm elements that represent the plywood fins I will use, etc...
I first search extensively for prototype pictures. Looking for one that is square to camera (and has fins square to camera). I prefer to find a clear picture of an actual rocket but the measurements will be distorted if the picture is not square (or if the camera person was too close to the rocket). Given this I sometimes need to rely on pictures of the model or diagrams. Some examples:
I use Inkscape to measure one picture. It has a convenient ruler tool that returns results in pixels. You can also measure multiple pictures if different parts are clear in various pictures but need to anchor your measurements on something common so that you can adjust and compute sizes. I usually anchor on main body diameter.
I enter all my measurements in an Excel spreadsheet. You basically need to set the dimensions to the scale appropriate for the model you are building. In this case I wanted to make the main body tube a BT-80 tube so I enter the OD for that body tube into the table next to all the measurements. I then take a proportion of the measured dimension to the actual BT-80 dimension. 66.04 / 75.3 = .877% So basically I multiply all my measured dimensions by that ratio/percentage. This gives me the numbers in the right column. Note: I measure fins the way I design them (a square with triangle cutouts) but you can also measure fins with points that help you enter them into OR or RockSim.
I then use these dimensions (rounded to reasonable numbers) to mock up the main elements to see how realistic the rocket is to make. I replace any cylinders with standard body tubes (I am willing to make compromise to use a cheap standard body tube rather than custom ordering something that is more exact), design simple 3D parts, replace fins with 2mm elements that represent the plywood fins I will use, etc...
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