Propulsion and Running Friction

[COSTransplant]

So, I have this very little bit of information that I am trying to translate -- well, understand.

Friction Forces in Launch Tube:

Tests run with 1-1/4" PVC pipe with an inner diameter of 1.38 inch showed a running friction of 1400 grams.
With a 1.62 ID Acetate shipping tube, the running friction was 200 grams. Adding in the vehicle weight, the
force required to balance weight and friction varies between 10 and 20 Newtons. This means that without breech
pressure, no motor with an initial thrust less than 20 Newtons would even move the vehicle.

So if I have a BT-58 based model with an OD of 1.540 inches, how do I calculate the optimal ID of an acetate launch tube?
I'm building this to be as robust (and light) as possible.
Best estimates currently put the model at +/-51 inches and 12 oz to 14 oz.
I'm hypothesizing that you want running friction at as close to zero as possible?

Thoughts?

Thanks!

 

[Alan15578]

There is much historical literature on closed breech and piston launchers. Your optimal launch tube diameter may well be less than your model diameter. Use a wiper seal. How are you measuring "running friction"?

 

[lakeroadster]

Are you wanting the exhaust of the motor to pressurize the tube and actively force the rocket out of the tube?

If the tube is just open ended and just guiding the rocket until it becomes stable, then the friction forces are negligible.

​

 

[COSTransplant]

Thanks all for sharing your thoughts. This will be a breach loaded closed bottom tube, where I do expect the gasses to pressurize the tube and actively force the rocket out of the tube. I assume your yellow clamp in the picture above is just holding your launch tube to the launch rail?

 

[COSTransplant]

I even pondered the idea of using something like this somewhere around the top third of the launch tube. ID of the piece would be able to accomodate a 38mm tube and the OD of the piece would be the ID of the ideal launch tube.

 

[lakeroadster]

Thanks all for sharing your thoughts. This will be a breach loaded closed bottom tube, where I do expect the gasses to pressurize the tube and actively force the rocket out of the tube. I assume your yellow clamp in the picture above is just holding your launch tube to the launch rail?

Yep, that's what the clamp does.

I had also designed THUNK! as a closed silo arrangement, with a seal at the bottom designed to self-separate from the rocket at the top of the silo... I haven't pulled the trigger on that. I had visions of it not working and the rocket burning down inside the tube so I tried the open tube first. Low-Tech approach.

​

 

[COSTransplant]

There is much historical literature on closed breech and piston launchers. Your optimal launch tube diameter may well be less than your model diameter. Use a wiper seal. How are you measuring "running friction"?

Thank you for this, and a wiper seal sounds like a great idea -- would you use one at the top? one at the bottom? one top and bottom? And I have not measured, I was just referencing what the builder had stated about running friction. I would have little idea where to start. I know that they made a coating / tape to put on the underside of some race cars that was supposed to help eliminate drag / running friction -- I am going to have to try and look that up.