L0k1 Posted October 6, 2023 Share Posted October 6, 2023 Again title says it all. Was wondering what absolute lowest PSI you can use when pressing Whistle? Specifically KBenz whistle. Would like to know BP pressures too but if I had to choose it would be whistle. Whats the absolute highest you can press safely? Again 4 KBenz Is there a perfect amount? Are there noticeable changes when going from low to high or high to low? Link to comment Share on other sites More sharing options...
FrankRizzo Posted October 6, 2023 Share Posted October 6, 2023 It will really depend on how energetic your whistle fuel is. IMO, part of the pressing operation is pre-stressing the walls of the tube, and for lack of a better description, taking out the extra "springiness/elasticity". When the motor is fired, there's a certain pressure that develops in the hollow at the business end of the tube. If the tube still has some elasticity, it may expand with the pressure and allow fire to reach up between the fuel grain and the tube wall, which leads to more pressure, and a CATO. I like 8000-9000 PSI for whistle motors and 5500-6500 for BP. Note that I don't do any special calculations for the lowered amount of surface area around the core pin. I do the calculation based on the flat rammer (full tube ID) and set my press to apply that amount of force for the entire pressing operation on that size motor. 1 Link to comment Share on other sites More sharing options...
L0k1 Posted October 7, 2023 Author Share Posted October 7, 2023 It will really depend on how energetic your whistle fuel is. IMO, part of the pressing operation is pre-stressing the walls of the tube, and for lack of a better description, taking out the extra "springiness/elasticity". When the motor is fired, there's a certain pressure that develops in the hollow at the business end of the tube. If the tube still has some elasticity, it may expand with the pressure and allow fire to reach up between the fuel grain and the tube wall, which leads to more pressure, and a CATO. How do you go about pre-stressing the tubes? I try to stick to only NEPT which seem to hold up well once I have support around them Link to comment Share on other sites More sharing options...
FrankRizzo Posted October 7, 2023 Share Posted October 7, 2023 How do you go about pre-stressing the tubes? I try to stick to only NEPT which seem to hold up well once I have support around themThe 8000-9000 PSI loading pressure accomplishes that. You can press with less loading pressure, but IMO you need that large amount to pre-stress the tube. Link to comment Share on other sites More sharing options...
cmjlab Posted October 8, 2023 Share Posted October 8, 2023 How does pre-stressing the tube help? I'm certainly not trying to discount your theory, I'm just curious what exactly You're referring to and how it helps. Does it prevent the tube from tearing on the inside from over pressing? Are you waxing them to prevent tube compression? Appreciate any insight you can provide, I'm not a dedicated rocket guy myself, but I do enjoy making 1 lb rockets occasionally. Link to comment Share on other sites More sharing options...
FrankRizzo Posted October 10, 2023 Share Posted October 10, 2023 My thought is that the tube walls have some elasticity. By pressing the fuel into the tube with a large amount of force, and restricting the outside dimension from changing (with a support sleeve, etc), you take the elasticity out of the tube wall by pre-loading them and making the walls thinner but more dense and increasing the tube's hoop strength. Later, when the motor is fired, as long as the pressure in the burning motor is less than the pressure used to load the fuel grain, you're less likely to have the tube inflate and allow combustion gasses to reach up along the sides of the fuel grain. I have waxed tubes in the past, and that helps with inhibiting the grain/tube wall boundary as well as facilitating larger increments without causing the tube to get shortened. Link to comment Share on other sites More sharing options...
cmjlab Posted October 11, 2023 Share Posted October 11, 2023 Thanks for the explanation, it makes sense now! Link to comment Share on other sites More sharing options...
Gunter321 Posted October 24, 2023 Share Posted October 24, 2023 Hello i have a question about the whistle Motors i make. My question is has any one here experienced that the whistle fuel slightly expands after pressing because after pressing i notice that my fuel is expanding little and i can notice that the delay is the Form of a very slight Dome and not flat also the motor dosent slip back on to the Spindel so the fuel expanded. Is this normal ? I use potassium benzoate fuel with copper oxychloride and Vaseline. Link to comment Share on other sites More sharing options...
cmjlab Posted October 25, 2023 Share Posted October 25, 2023 (edited) I'll take a crack at an answer since nobody else has - I don't think it's uncommon for whistle, and some other compositions to expand some after being removed from the spindle. Is it causing issues for you? It's not caused an issue for me, but I also don't store rockets for very long (couple weeks at most). As for the domed shaped delay, I'm not sure I follow. Are you saying the delay (whistle fuel above the spindle) you are pressing, is somehow distorting the outside of the tube? If so, are you using a pressing sleeve? And what pressure are you pressing them at? Lastly, what kind of tubes are you using (NEPT or homemade)? Edited October 25, 2023 by cmjlab Link to comment Share on other sites More sharing options...
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