Thank you Jon and James :)
Jon, we really don't know what the pressure limit is yet. During development we hydro tested a ~40mm long section of a tube made using the same technique and the same end cap retention as this rocket uses and it was still holding 560psi. We didn't go higher because our hoses aren't rated for much higher pressures. We then cut the tube in half leaving the end caps in place and joined the tubes back together using an internal coupler so that we could test how well the join would hold. We were making the pressure chamber from two sections for easier construction. We then tested this rejoined tube up to 500psi and it was still holding that pressure.
On these launches we are incrementally raising the pressure doing hydro tests along the way to the target launch pressure. With the tube being this long there is always more chance of there being a weak spot and potentially failing at a lower pressure. We'd rather get
in a few launches at a lower pressures and then build up from there. We will do a hydro test again to perhaps 450 or 460psi, and then launch it at that pressure again.
Realistically I think 500psi would be about the limit for this tube's construction technique and still retain a little bit of safety margin. It is only made from plain weave E-glass and West Systems epoxy. Using S-glass, more exotic materials and more optimized weave orientation would probably have to be used from that point on. The other issue at higher pressures (higher accelerations) is the tendency of the rocket to want to bend. CF or a bigger diameter should offer the stiffness needed. One of the reasons we are going with the 19mm nozzle is to keep the peak acceleration down and reduce stress on all the components.
There is really nothing special about the nozzle or launcher. We wanted metal on metal for the contact area to prevent deformation at the
higher loads. The nozzle itself is made from aluminium screwed into the PVC bulkhead:
The release mechanism is based on exactly the same principle as the Clark cable-tie launcher. The cable ties have been replaced with a fiberglass tube cut into sections and the cable tie clips have been replaced with a machined piece of brass that fits exactly onto the nozzle and is bonded to the fiberglass.
Here are some technical drawings with dimensions of the entire arrangement:
--- In firstname.lastname@example.org, Jon Mehlferber <jmehlferber@...> wrote:
> Congratulations! That is an amazing accomplishment. Sorry if I missed you
> saying somewhere before, but what do you think
is the pressure limit for
> this rocket (theoretical plus some safety margin)? Anything special about
> the launcher and/or wrocket nozzle?
> Jon Mehlferber