I am looking for a parts list and dimensions to the parts of the old CMR Boom tube , aka Piston Launcher. I'd like to clone it. If anybody happens to still have one , could you take the dimensions and provide a parts list?
The plans are over at ye old rocket plans , but the instructions to me, are unclear.
If anybody has one or used one in the past, would you be so kind as to help me with this?

https://plans.rocketshoppe.com/pubs/CMR/Piston.pdf

was this the same piston Launcher that was offered by QCR? Anybody got plns for that one if it was different?

TIA

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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.

Yeah....like that drawing and instructions are kinda hard to follow. Maybe this picture can clear things up a little.....

Basically, the "cylinder" is a body tube that the piston slides into. So the length of the tube and the piston (dowel) is dependent on what size you want to use. The piston should always be longer than the cylinder.

The wires are just an extension of the igniter (starter) clips. Does this help it make since?

Attached Thumbnails

 

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Don
NAR 53455
"Carpe Diem"

Thanks Barone, it that's the internals from a brass head design by the late, great George Gassaway.

After further review the I was confused about the length of the piston tube( shown as a rb50 cylinder). I understand that this butts up against the enclosed rb50/engine block, inside the rb52 ( bt-5/bt-5+)

I assume this rb50 cylinder is what is known today as the piston tube.

What I don't understand is there no piston stop, so this must be a floating head piston?

But it can't be , because the piston is glued to the piston rod(the aluminum tube).

I hope this makes sense.

How does the rb50 piston tube separate from the rocket?

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"Old Rocketeer's don't die; they just go OOP".....unless you 3D print them.

Oh! Oh! After years of lurking, something I can actually help with!

The 'rubber ring' is a tight fit at the bottom of the 'Cylinder RB 50 body tube'. Tighter than the friction fit of the rocket at the top. When the rubber ring meets the top piston stop, it stops the cylinder and the rocket pops off the top.

That rubber ring is like 1/2 inch long (longer than that drawing implies), so it is quite solidly in place.

If someone else does not beat me to it, I'll see if I can put my hands on mine and get you some dimensions.

To clarify, what I'm referencing as 'piston stop' is what's labeled as 'piston' - the wooden part glued to the top of the aluminum tube.

As the RB 50 cylinder moves, I think of that part as the 'piston'

RB50 cylinder length - 11.5 in
RB77 'adapter' at the top end - 2.75 in

Let me know if you need other specifics.

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The rubber ring - approx 0.5 in

Note it has small ribbing to help with the friction fit in into the bottom of the cylinder.

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The CMR piston is a fixed head piston (the cylinder reaches it's limit of travel and the rocket "BOOMs" off the cylinder). The drawing I provided is a floating head piston. The "head" isn't attached to the piston. This allows the cylinder to actually go air borne (with the head retained in the cylinder) with the rocket until the internal pressure in the cylinder is high enough to cause it to detach from the rocket. For NAR FAI NCR events, you can't use the floating head.

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Don
NAR 53455
"Carpe Diem"

Well...for FAI events, period, one can no longer use floating head pistons, whether they are flown as part of NRC or not. This is a change the FAI made in 2018.

Fixed head pistons may be used in FAI flying for duration events, but not for altitude, payload altitude or scale altitude (S1, S2, S5).

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Bernard Cawley
NAR 89040 L1 - Life Member
SAM 0061
AMA 42160
KG7AIE

OHiggins,

Thanks for the great photos.

What is not clear from the instructions is that the "rubber ring" actually slides easily up and down the piston rod and serves as the piston stop. Once the igniter is in place, the piston tube slides down over the piston head and igniter and the rubber ring piston stop is then friction fit into the bottom of the piston tube.

The piston tube is then slid all the way down and the aft end of the rocket is affixed to the upper end of the tube and slid down onto the internally wired igniter. This is very similar to how European pistons work today. They are fixed head, but rather than using the rubber ring piston stop they use fiberglass tubes with screw on rings at the bottom which act as the piston stop (they also include a spring inside the bottom of the tube to absorb some of the impact of the stop hitting the piston head).

The CML arrangement, using the rubber ring, necessitates that the friction fit of the ring into the tube is significantly greater than the friction of the aft end of the rocket motor into the top of the tube. Otherwise the ring will just be pushed out the bottom of the tube which will stay attached to the rocket motor never releasing from the aft end of the rocket. Since the piston tubes must be removed between flights, the removable friction fit rubber ring is a clever way to make the tubes easily removeable.

The notation in the instructions that this type of piston was successfully used by the U.S. team at several World Championships is historically interesting. It strongly suggests that the friction fit of the rocket to the piston tube was quite weak, which implies far less performance than we get out of pistons today. Current U.S. pistons, using 34" long paper body tube piston tubes, with piston rings (short pieces of coupler tube) glued into the bottoms of the tubes, strike the piston heads with such force that if left unreinforced with strapping tape the tubes will simply rip apart, or rip the piston head off the piston rod. The reason for this extreme force is that rockets today are very firmly friction fit into the tops of the piston tubes.