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750psi water rocket

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750psi water rocket

Unread postAuthor: dudeman508 » Sat Feb 14, 2009 10:41 am

This is my uncle's and my project, he has access to a machine shop. The only part we need help with is a way to trigger it, we thought about burst disks but we were scarred that a piece of the disk could get stuck in the nozzle. Thanks if you can help.
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Last edited by dudeman508 on Sat Feb 14, 2009 10:50 am, edited 1 time in total.
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Unread postAuthor: D_Hall » Sat Feb 14, 2009 10:50 am

This is actually a problem I've worked in the past. I had a design all up and ready to go, I just couldn't afford to have the machining done as I don't have free access to a machine shop.

That said... I imagine you're running CO2 as your pressurant. Let me ask you this: Why are you still using water? Use CO2 both as your pressurant and your working fluid.

Advantages...

1) As the pressure drops in your chamber, CO2 will boil off, thus raising the pressure back up. IOW, your pressure won't decay as quickly as it would with water and a gas pressurant.

2) Since liquid CO2 will flash boil, you can actually have an expansion cone on the nozzle for additional thrust.


IIRC.... Isp for water rockets is about 10 s. For the CO2 rocket it's about 40. The design I was running with was based around a 20 oz paintball tank and would generate 100 lbs of thrust for about 0.5 s. IF my math was right. I never did finish it, however... Again, access to a machine shop.

I've got better access now. I've been thinking about a redesign to make it cheaper/easier to build.

For what it's worth... My nozzle screwed into the bottle just like a paintball valve would. The nozzle was built monolithically and had a 1/4" deep groove around the perimeter to provide something for the release mechanism to engage.
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Unread postAuthor: Sticky_Tape » Sat Feb 14, 2009 10:54 am

Quick connects? I have made pop bottle rockets before big ones like 6 liters. The regular campbell hausfelds will work but if you use the 1/4'' ones it will have restricted flow but the thrust phase will be longer slowly excellerating the rocket. How many liters of water do you think the rocket will hold not counting the air space? I think if you use burst disks it will work but the rocket may lauch unexspectedly and cause some dammage to people or toys (cars and such) but tirggering it would be quite easy if you make a spring loaded burst disk rupturer or somthing.
EDIT: Basicly the more water the rocket can throw out the back of it the more thrust you will have (newtons third law). But with 750psi and a burst disk about 1/2'' the thrust phase will be very short and the rocket won't go very high but having restricted flow there will still be alot of water being thrown out the back but the thrust phase will be longer letting the rocket go higher.
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Last edited by Sticky_Tape on Sat Feb 14, 2009 11:03 am, edited 2 times in total.
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Unread postAuthor: dudeman508 » Sat Feb 14, 2009 10:56 am

I just thought of this, you can get to 800psi with dry ice....but i think i would make the pvc brittle
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Unread postAuthor: Sticky_Tape » Sat Feb 14, 2009 11:04 am

dudeman508 wrote:I just thought of this, you can get to 800psi with dry ice....but i think i would make the pvc brittle

You don't know how stupid useing pvc would be with that much pressure can you say pipe bomb? You should use aluminum scince you will be machining. Got to the chat.
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Unread postAuthor: sniper hero » Sat Feb 14, 2009 11:18 am

PVC?! as sticky tape mentioned this is a pipe bomb
what would you be machining?
do you have any idea what amount of power this has when the pvc shatters?
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Unread postAuthor: D_Hall » Sat Feb 14, 2009 11:22 am

Sticky_Tape wrote:EDIT: Basicly the more water the rocket can throw out the back of it the more thrust you will have (newtons third law).

For traditional water rockets.... Not true at all. There's a delicate balance. More water (reaction mass) means less air (energy source). All air and no water means it doesn't go far because there's no reaction mass. All water and no air means it doesn't go at all because there's no energy. Between those two extremes there is a performance curve. IIRC, the ideal for traditional water rockets operating in the 125ish psi range is approximately 1/3 full of water (but I conceed that it's been a long time since I studied that problem).

In any event, the point is that more water isn't necessarily better.
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Last edited by D_Hall on Sat Feb 14, 2009 11:25 am, edited 1 time in total.
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Unread postAuthor: D_Hall » Sat Feb 14, 2009 11:24 am

dudeman508 wrote:I just thought of this, you can get to 800psi with dry ice....but i think i would make the pvc brittle

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Dumbest statement I've heard all year.
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Unread postAuthor: Sticky_Tape » Sat Feb 14, 2009 11:37 am

Well what I mean't is a smaller nozel will sort of act like a delaval correct? But the more water the rocket can throw out the back of it the more thrust there will be I never mentioned anything about putting more water in the rocket. Making 2 identical rockets one with a restricted nozel and one with a regular nozel the restricted nozel rocket will go higher because not all the water comes out right at the start of the launch considering there is the same ammount of water to air and pressure.
EDIT: There is a table on this page-scroll down http://ourworld.compuserve.com/homepages/pagrosse/h2orocketlauncherindex.htm
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Unread postAuthor: dudeman508 » Sat Feb 14, 2009 11:51 am

1/2in. sch120pvc is rated to 1200psi, but i might be wrong

dudeman508 wrote:
I just thought of this, you can get to 800psi with dry ice....but i think i would make the pvc brittle

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Dumbest statement I've heard all year.
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Unread postAuthor: Sticky_Tape » Sat Feb 14, 2009 11:53 am

That rocket will not go anywhere bottom line. Do the math to find out how much volume there will be on the inside of that and you will know what I mean.
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Unread postAuthor: jonnyboy » Sat Feb 14, 2009 11:57 am

dudeman508 wrote:1/2in. sch120pvc is rated to 1200psi, but i might be wrong

dudeman508 wrote:
I just thought of this, you can get to 800psi with dry ice....but i think i would make the pvc brittle

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Dumbest statement I've heard all year.

Dry ice IS frozen co2 gas. :lol:
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Unread postAuthor: D_Hall » Sat Feb 14, 2009 12:04 pm

jonnyboy wrote:Dry ice IS frozen co2 gas. :lol:

I'm well aware. Ask yourself this question: "How smart is it to put dry ice in a PVC pipe?"
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Unread postAuthor: D_Hall » Sat Feb 14, 2009 12:05 pm

dudeman508 wrote:1/2in. sch120pvc is rated to 1200psi, but i might be wrong

It's still incredibly stupid.

PVC is impact intollerant.

You're using it as a rocket.

When it comes back down... Hey, that's an impact!

Unless you're planning on making them single use rockets, you're now proposing filling up a potentially damaged pressure vessel to 800 psi.

I say it again... Dumbest idea I've heard all year.
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Unread postAuthor: Sticky_Tape » Sat Feb 14, 2009 12:11 pm

We don't need to flame him anymore on the subject of pvc and high pressure. I got the point accross to him over chat not to use pvc because of the possibility of rupture and the rocket not being able to go anywhere.
BTW D-hall 2 double posts in one topic? dispicable...
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