Dark Shadow - Build Thread

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air.command
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Dark Shadow - Build Thread

Postby air.command » Fri Feb 06, 2015 6:12 pm

One of the projects we are bringing to Thunda this year is our new high pressure water rocket called "Dark Shadow".

It is based on our previous fiberglass rocket called "The Shadow" with a number of improvement goals to hopefully squeeze a little more performance out of it.

- The pressure chamber will be reinforced with carbon fiber for higher pressures.
- The payload/parachute deployment section will be smaller to reduce weight.
- The rocket will have a tail cone for better aerodynamics.
- The fins will be thinner.
- The pressure chamber bulkheads will be integrated into the pressure chamber design.
- The nozzle will be smaller to reduce the overall acceleration to levels similar to The Shadow.
- A new launcher will be built based on our Polaron G2 electronic launcher.
- The rocket will use a tower instead of rail and rail buttons.

Okay some stats on this rocket:

Target launch pressure: 600psi (41 bar)
Capacity: 6.77L
Water amount used: 1.4L
Weight: ~1500 grams dry (exact final weight yet to be measured)
Nozzle: 15.5mm
Diameter: 63mm
Length: 2850mm

And the obligatory open rocket screen shot.
Image

We are not using Open Rocket to simulate the flight, but use it to work out the CG and CP relationships. Open rocket says that we have a fairly high stability margin, but for water rockets this needs to be a little higher because of the heavy water in the tail end of the rocket during the boost phase.

The worst case scenario with 1.4L of water near the tail you get this:
Image

Now mind you it will be in this state for a very brief time, but still I prefer the rocket to be stable over the entire flight profile. If the fins were 2cm shorter, the rocket would be unstable during this time.

Because the rocket is over-stable during the coast phase we'll try to launch during calmer conditions.

We are using a couple of water rocket simulators to estimate the performance, but the estimates can be a little off the mark when dealing with high performance rockets. They are still useful tools though for getting an idea of the expected flight profile.

Without knowing the exact final weight, initial simulations @600psi suggest the following:

Acceleration: ~60G
Top speed: ~430km/h
Peak thrust: 1430 N
Total impulse: ~170 Ns
Altitude: 1400'+

The motor equivalent is just over a full G and something along the lines of a G800. The water will exit the rocket in roughly 100ms and the rest of the high pressure air will follow in another 100ms. So really about 0.2s burn time.

The rocket will make one of 3 sounds:
a) psssssss - not ideal as there is a leak, and we'll have to abort.
b) BOOOM - even less ideal, as there is a BIG leak and noise complaints from Goondiwindi will be ignored.
c) Gun shot type bang - ideal as the rocket made it off the pad.

The first shakedown flight is probably going to be at 500psi and we'll see how things hold up. If that is successful we'll increase the pressure to either 550 or straight to 600psi for the next flight. If that still comes back in one piece we'll try again at 650psi+ etc.

The rocket has been in development for some time so I'll start posting some build pictures next.

-George
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Re: Dark Shadow - Build Thread

Postby cryoscum » Fri Feb 06, 2015 6:22 pm

G800! Impressive, George! REALLY looking forward to seeing it. You and your Dad's work is always top class & certainly worthy of the big event that Thunda is looking to be :)

PS: Have you simulated using XXXX beer as propellant? Just a thought... :)
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Re: Dark Shadow - Build Thread

Postby High Impulse » Fri Feb 06, 2015 6:27 pm

To be fair it would be the only thing XXXX is good for ;)
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Re: Dark Shadow - Build Thread

Postby cryoscum » Fri Feb 06, 2015 6:29 pm

High Impulse wrote:To be fair it would be the only thing XXXX is good for ;)


Hey, don't forget where Thunda is being held! Careful, buddy!!

:lol:
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Total impulse for 2016: 32,458 Ns (thus far)
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Re: Dark Shadow - Build Thread

Postby air.command » Fri Feb 06, 2015 6:39 pm

Thanks Nic. Well in keeping with the spirit of Thunda, we'll have to launch at least one XXXX powered water rocket. Biggest problem will be plying it away from the QRS guys. :D
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Re: Dark Shadow - Build Thread

Postby matthew » Fri Feb 06, 2015 7:07 pm

cryoscum wrote:PS: Have you simulated using XXXX beer as propellant? Just a thought... :)


Yeah, anything would spit that out faster than 100ms. :lol:
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Re: Dark Shadow - Build Thread

Postby High Impulse » Fri Feb 06, 2015 7:25 pm

cryoscum wrote:
High Impulse wrote:To be fair it would be the only thing XXXX is good for ;)


Hey, don't forget where Thunda is being held! Careful, buddy!!

:lol:


Don't worry i know where thunda is :).. I come from the sunshine state but I would need a couple bourbons before i could stomach XXXX, but it would be pretty neat to see some launch a water rocket. If you stood near the launch pad you could inhale vaporized beer.
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Re: Dark Shadow - Build Thread

Postby SpaceManMat » Fri Feb 06, 2015 7:28 pm

Perhaps a beer drag race is in order, then we know which one gets spat out faster :lol:
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Re: Dark Shadow - Build Thread

Postby air.command » Fri Feb 06, 2015 11:06 pm

We actually started on this build back in June last year, but have been working through a series of tests and learning how to build this thing before getting to an important milestone this week with good full size pressure chamber tests.

Since the pressure chamber is the most critical component (during the up part anyway) let's start with some construction details:

Pressure Chamber:

The pressure chamber consists of a lightweight fiberglass inner liner that also forms the shape of the chamber. This is then covered with a carbon fiber sleeve for pressure reinforcement. Before making the full length rocket we decided to make a shorter version to figure out how it would go together and how much pressure it could hold.

We started of by rolling a simple 3 wrap tube with 85gsm cloth. The kids wanted to get involved too.

Image

Pulled off the mandrel
Image

Trimmed ends
Image

We then made a couple of wooden plugs for the forward closure and tailcone so that the resulting fiberglassed components would nicely fit inside the tube.

Image

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These were spray painted with spray putty and sanded back. We then used Estapol 2 pac clear varnish on them to make them nice and smooth.
Image

We then had to experiment with the number of gores to get just the right fit. We finally settled on 11 gores with a small strip around the base. These were again made of 85gsm cloth. We use 2 balloons over the top of each other on the plug each being lightly coated with silicone mold release. This makes it very easy to pull the fiberglass off.

Image

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Here are the tailcone and forward closure trimmed and sanded. The wall thickness of the tubes is around 0.45mm while the forward closure and tailcone is around 0.55mm. We used West systems to fiberglass the different components and then used super strength Araldite epoxy to join the sections together.

Image

Image

The forward closure needed something sticking out the top of it to attach the shock cord to. We can't simply put a bolt through it as this would weaken the integrity of the pressure chamber. After one failed attempt at machining a PVC cap for the top of the pressure chamber,

Failed attempt at making a PVC top to the pressure chamber.
Image

we decided to use the neck of a PET bottle and heat shrink to fit the forward closure exactly.

Evenly heat shrinking the bottle on the lathe.
Image

It fits snugly on the forward closure
Image

The bottle was then sanded and perforated to make it adhere better. The scaloped edge is used to distribute stress along the join.
This was then glued in place. It gives a very solid connection with minimal weight. It also makes it easy to cover with the sleeve.

Prepared for gluing
Image

Pressing down allows the epoxy to squeeze out of the perforations.
Image

We decided to do the same at the tail end. The difference here is that the tailcone has a hole through it. In order to attach the aluminium nozzle, we made a PVC insert into the tailcone bottle and cut a thread in it. This allows us to swap the nozzles between rockets if needed.

We machined a brass tool that allows us to flare the PVC pipe for the nozzle insert. You basically heat the PVC pipe and push it on.
Again it's all glued together with araldite.

Flaring tool being machined
Image

Final nozzle insert flared and a thread cut in it
Image

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Before gluing
Image

Here is the tube with forward closure and tailcone all glued together. Again note the scaloped edges to help spread the stress.

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Tailcone glued in place and sanded
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Forward closure glued in place and sanded.
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After all the components were glued together we filled all joints with epoxy/microballoon mix to give a smooth finish to the outside of the liner. We want to reduce the possibility of air pockets developing between the sleeve and liner. At the intended pressures any larger air pockets would likely get crushed.

All assembled
Image

We bought the carbon fiber sleeve from Soller Composites as they had very reasonable prices and the right size sleeves. Initially we bought a range of short length sleeves from them so that we could try them on the liner in order to figure out the weave angle once the sleeve was pulled tight. For pressure chambers the ideal fiber winding angle is 54.7 degrees. We found that the 2" heavy sleeve gave us an angle of 60 degrees which was the closest to ideal as we could get. The others were 43 for the 2.5" heavy and 34 degrees for the 3" heavy. The 1.5" was to too small to get over the pressure chamber.

Measuring wrapping angle
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We then put the sleeve onto the liner using a roller and again with West Systems epoxy. We ended up putting peel ply over the top to squeeze out more of the epoxy.

The sleeve this length just fit on the pressure chamber
Image

Getting ready to pull sleeve on
Image

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After a few days cure we pressure tested it to 650psi. We were pretty happy with that, but we don't know what the upper limit is yet. We didn't want to go higher because the hose isn't rated for much higher pressures.

Just prior to hydro pressure test. We test it inside this cut off scuba tank in case any pieces come off.
Image

650psi and still holding
Image

Next time I'll cover making the full length pressure chamber with some tips from Nic.
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Re: Dark Shadow - Build Thread

Postby CATO » Sat Feb 07, 2015 5:25 am

cryoscum wrote:You and your Dad's work is always top class & certainly worthy of the big event that Thunda is looking to be.

x2 George, looks great...

I think, Tooheys New is the go here...
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Re: Dark Shadow - Build Thread

Postby kopius » Sat Feb 07, 2015 6:30 am

That is just incredible work George! With all of the big motors flying at Thunda, this is one of rockets I really want to see :-)
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Re: Dark Shadow - Build Thread

Postby kopius » Sat Feb 07, 2015 6:33 am

On a side note, shame, shame, shame (haven't heard Darren in a while) to those guys who are knocking XXXX. There is no such thing as a bad beer, just some are better than others. Whilst many people have different presences of drink choice, I remember a good mate of mine once saying drinking XXXX Gold was like having angels urinate on your tonsils ;-)
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Re: Dark Shadow - Build Thread

Postby jase » Sat Feb 07, 2015 8:33 am

Ingenuity at its best on display here 8)

Awesome work George!
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Re: Dark Shadow - Build Thread

Postby Sumo310 » Sat Feb 07, 2015 10:44 am

Looks fantastic, I can't wait to see this at Thunda! CG Composites in Brisbane also do CF sleeve if you wanted a local source in the future.
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Re: Dark Shadow - Build Thread

Postby Lamp » Sat Feb 07, 2015 12:36 pm

Excellent stuff George, I am always in awe of your builds!
I'm really looking forward to watching "Dark Shaddow" fly out of the tower!
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