Misuse of Phantom Body Tubes in OpenRocket for Motor Mounts

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H. Craig Miller

OpenRocket Development Team
TRF Supporter
Sep 8, 2020
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Placer County, California
While phantom body tubes have a number of uses in OpenRocket, the use of a phantom body tube to enclose motor mounts (centering rings and the motor tube) prevents some of the OpenRocket features from functioning corrently. And, there is simply no reason to use a phantom body tube in this manner.

Moreover, those who use phantom body tubes should be aware of the warnings that may be triggered. The best example of which is the use of a phantom body tube to attach a fin shaped like an Estes motor hook. When this hack is used, a series of warnings is triggered: (1) discontinuity because the phantom tube (an airframe component) is a different diameter than that of the adjacent airframe component; (2) thick fins because because of the ratio between the phantom tube diameter and the width of the fin (motor hook); and (3) jagged-edged fin because of the shape of the motor hook. This use of the phantom tube is currently necessary to achieve the desired effect and does not interfere with other OpenRocket functions, though Issue #1522 proposes a new feature so that the use of a phantom tube for this purpose will no longer be required.

A phantom body tube is an aerodynamic component that may affect flight characteristics, in addition to preventing some of the OpenRocket features from functioning corrently. To the extent that a design can be created without the use of phantom body tubes, normal design techniques should be used; the use of phantom body tubes should serve a purpose that cannot otherwise be achieved.
It appears that the above issue may stem from a misunderstanding of how to incorporate motor mounts created by K'Tesh into a new design.

In my view, K'Tesh has the best of the best when it comes to OpenRocket files. And, K'tesh must use a phantom body tube for attachment purposes to create a motor mount (centering rings and motor tube) because the phantom tube is a placeholder of sorts for the internal components; that phantom tube should not be part of the new model rocket.

Transferring motor mount components from a K'Tesh motor mount file to a new design is a two-step process: (1) copy the internal motor mount components (EXCLUDING the phantom body tube) from the K'Tesh design and paste those internal motor mount components onto the desired body tube in the new design; then (2) copy the other phantom body tube with the engine hook from the K'Tesh design and paste that phantom body tube onto the new design (its placement is after the body tube onto which the internal motor mount components were pasted).

And, all should function as intended.

Motor Mount.png

This should be a one step process when Beta 5 is released; cross your fingers.

Motor Mount.Copy-Paste.png
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