Wide-range universal launcher
Posted: Tue Jul 24, 2018 7:37 am
Good day ladies and gents!
I thought I'd put a post together regarding my new launcher which is designed to fit rockets with 5-40mm nozzles, without changing any parts out.
While my previous launcher used a gardena nozzle (fairly common) - this particular system instead opts for utilising a conical filling nozzle (the "launch plate") which fits inside the throat of the rocket's nozzle, and creates a seal using an O-ring: with a launch-clamp preventing the rocket from breaking the seal as it is fueled - launch is accomplished simply by releasing the pressure on the clamp. As you can see from these CAD images, the seal between the nozzle and the launch plate can be accomplished for pretty much any size smaller than the launch plate diameter - though it is yet to be seen if the water stream passing over the O-ring for a few milliseconds during launch is going to be enough to blow it out of the seat! After that short uncertainty period, the jet of water should miss the O-ring entirely.
If it transpires that the O-ring is consistently blown out of the nozzle during launch, I have an alternative sealing method using a separate O-ring carrier ring, which fits between the nozzle and the cone and is deliberately left behind during launch (which may actually serve to reduce the weight of the nozzle as well - but at the same time creates a second path for water to pass around the seal under pressure - which may lower the possible range of pressures that can be used.
I have already tested this on a small-scale using a piece of wood in lieu of a hold-down clamp and results are promising. The test-nozzle has a 5mm diameter and is mounted on a hose which is connected to a regular water rocket (instead of launching, it exhausts through the test-nozzle) and it managed to not blow the 7mm x 1.5mm O-ring out when the launch plate was disengaged.
The launcher itself is fairly simple - consisting of a compressor fitting (on the end of a 30m hose) connecting to either a water reservoir (if the rocket is to be "fuelled" on the pad) or directly to the launcher.
The launcher has a T-junction - with the centre-connection going to the launch plate and the other connection going to a springloaded ballcock valve which serves as a way to depressurise the rocket quickly in the event of an emergency. The exhaust is vented via a 3D printed howell-bunger valve which directs a spray of water directly upwards rather than sidewards, with intent on reducing erosion to the surroundings/launcher/rocket. It is pretty much just a very inefficient nozzle :P)
The water reservoir consists of any number of water bottles - during pressurisation, water is forced out of the reservoir and into the rocket - after which the air simply passes through the reservoir and into the rocket. This obviously leaves the problem that there is now a compressed air vessel on the pad - as such, a number of small vessels are preferable to a single large vessel (to limit riskin the event of a catastrophic failure: a single vessel can dissipate it's contents quickly while a smaller vessel will be forced to vent through the other vessels, slowing the rate that the energy can be released.
I am not happy with the safety involved in such a reservoir system, so I'm going to add that at a later date though once I work out a safe way to do it - it will probably be simpler just to add a tube to the rocket that allows the ullage to vent until the reservoir is empty, at which point we close off the reservoir and compress/charge the rocket as normal - I'll figure that one out once I've got the launcher itself completed - since the reservoir-based fuelling method is more of a luxury than anything else!
The other difficulty with the launcher is the design of the hold-down clamps: which must have the following characteristics;
A) Fit varying nozzle-flange sizes (this launch plate supports up to 50mm: corresponding to a 40mm nozzle diameter - but the body of the rocket can become wider after that if needed)
B) Impede the motion of the rocket as little as possible (the rocket should slide out the clamp rather than having to force its way out)
C) Deform only minimally while providing enough downward force to counteract the pressure at the nozzle (200 PSI applied to a circle 40mm wide gives us around 1700 newtons - though the actual area might be greater as it is a cone which will reduce the equivalent force - but not by much (if anything))
D) Fail safe in the event that the nozzle seal is broken; the rocket should be incapable of launching.
E) Survive multiple launches (obviously useful!)
A) should be fairly simple - there are numerous ways to accomplish this: < shaped brackets are an easy option though there are others too! The lateral movement of the clamp required to allow an unimpeded launch is slightly larger than the depth of the flange: which will typically be 3-5mm (depending on engineering requirements under "C")
B) is accomplished by using a chamfered flange which can be seen along the top/outer rim of the nozzle. This fits into a correspondingly chamfered edge on the clamp - as the rocket moves up, it simply slides out of the clamp - the motion of the clamp can be assisted using springs if needs must.
C) This is probably the hardest part - particularly if I up the pressure even more than 200 PSI (though don't intend to do that anytime soon!). Using 200 PSI as a ballpark figure (worst-case scenario) and assuming a nozzle diameter of 40mm gives us around 1730 newtons - the equivalent of requiring around 173kg sitting on top of it to prevent a launch, if my calculations are correct (please forgive me if I've made a dumb error somewhere!) As such, I will probably be building the launch clamp assembly out of welded steel - which poses a whole other set of problems.
D) Interlocks can be used to prevent clamp movement: clamp arms can be designed so that both must be released before they will move. Clamps can be locked in place allowing redundancy in the locking mechanism.
E) Materials choice, waterproofing, etc.
I peronally think that "C" might be a show-stopper - what do you folks reckon (bearing in mind that 200 PSI is an absolute maximum - a deliberate overestimate: it's more likely to run at 100PSI - maybe occasionally going to 150PSI for smaller rockets (which won't have the force problem owing to a smaller nozzle diameter)
I thought I'd put a post together regarding my new launcher which is designed to fit rockets with 5-40mm nozzles, without changing any parts out.
While my previous launcher used a gardena nozzle (fairly common) - this particular system instead opts for utilising a conical filling nozzle (the "launch plate") which fits inside the throat of the rocket's nozzle, and creates a seal using an O-ring: with a launch-clamp preventing the rocket from breaking the seal as it is fueled - launch is accomplished simply by releasing the pressure on the clamp. As you can see from these CAD images, the seal between the nozzle and the launch plate can be accomplished for pretty much any size smaller than the launch plate diameter - though it is yet to be seen if the water stream passing over the O-ring for a few milliseconds during launch is going to be enough to blow it out of the seat! After that short uncertainty period, the jet of water should miss the O-ring entirely.
If it transpires that the O-ring is consistently blown out of the nozzle during launch, I have an alternative sealing method using a separate O-ring carrier ring, which fits between the nozzle and the cone and is deliberately left behind during launch (which may actually serve to reduce the weight of the nozzle as well - but at the same time creates a second path for water to pass around the seal under pressure - which may lower the possible range of pressures that can be used.
I have already tested this on a small-scale using a piece of wood in lieu of a hold-down clamp and results are promising. The test-nozzle has a 5mm diameter and is mounted on a hose which is connected to a regular water rocket (instead of launching, it exhausts through the test-nozzle) and it managed to not blow the 7mm x 1.5mm O-ring out when the launch plate was disengaged.
The launcher itself is fairly simple - consisting of a compressor fitting (on the end of a 30m hose) connecting to either a water reservoir (if the rocket is to be "fuelled" on the pad) or directly to the launcher.
The launcher has a T-junction - with the centre-connection going to the launch plate and the other connection going to a springloaded ballcock valve which serves as a way to depressurise the rocket quickly in the event of an emergency. The exhaust is vented via a 3D printed howell-bunger valve which directs a spray of water directly upwards rather than sidewards, with intent on reducing erosion to the surroundings/launcher/rocket. It is pretty much just a very inefficient nozzle :P)
The water reservoir consists of any number of water bottles - during pressurisation, water is forced out of the reservoir and into the rocket - after which the air simply passes through the reservoir and into the rocket. This obviously leaves the problem that there is now a compressed air vessel on the pad - as such, a number of small vessels are preferable to a single large vessel (to limit riskin the event of a catastrophic failure: a single vessel can dissipate it's contents quickly while a smaller vessel will be forced to vent through the other vessels, slowing the rate that the energy can be released.
I am not happy with the safety involved in such a reservoir system, so I'm going to add that at a later date though once I work out a safe way to do it - it will probably be simpler just to add a tube to the rocket that allows the ullage to vent until the reservoir is empty, at which point we close off the reservoir and compress/charge the rocket as normal - I'll figure that one out once I've got the launcher itself completed - since the reservoir-based fuelling method is more of a luxury than anything else!
The other difficulty with the launcher is the design of the hold-down clamps: which must have the following characteristics;
A) Fit varying nozzle-flange sizes (this launch plate supports up to 50mm: corresponding to a 40mm nozzle diameter - but the body of the rocket can become wider after that if needed)
B) Impede the motion of the rocket as little as possible (the rocket should slide out the clamp rather than having to force its way out)
C) Deform only minimally while providing enough downward force to counteract the pressure at the nozzle (200 PSI applied to a circle 40mm wide gives us around 1700 newtons - though the actual area might be greater as it is a cone which will reduce the equivalent force - but not by much (if anything))
D) Fail safe in the event that the nozzle seal is broken; the rocket should be incapable of launching.
E) Survive multiple launches (obviously useful!)
A) should be fairly simple - there are numerous ways to accomplish this: < shaped brackets are an easy option though there are others too! The lateral movement of the clamp required to allow an unimpeded launch is slightly larger than the depth of the flange: which will typically be 3-5mm (depending on engineering requirements under "C")
B) is accomplished by using a chamfered flange which can be seen along the top/outer rim of the nozzle. This fits into a correspondingly chamfered edge on the clamp - as the rocket moves up, it simply slides out of the clamp - the motion of the clamp can be assisted using springs if needs must.
C) This is probably the hardest part - particularly if I up the pressure even more than 200 PSI (though don't intend to do that anytime soon!). Using 200 PSI as a ballpark figure (worst-case scenario) and assuming a nozzle diameter of 40mm gives us around 1730 newtons - the equivalent of requiring around 173kg sitting on top of it to prevent a launch, if my calculations are correct (please forgive me if I've made a dumb error somewhere!) As such, I will probably be building the launch clamp assembly out of welded steel - which poses a whole other set of problems.
D) Interlocks can be used to prevent clamp movement: clamp arms can be designed so that both must be released before they will move. Clamps can be locked in place allowing redundancy in the locking mechanism.
E) Materials choice, waterproofing, etc.
I peronally think that "C" might be a show-stopper - what do you folks reckon (bearing in mind that 200 PSI is an absolute maximum - a deliberate overestimate: it's more likely to run at 100PSI - maybe occasionally going to 150PSI for smaller rockets (which won't have the force problem owing to a smaller nozzle diameter)