Easy way to make your water rocket stable without fins

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
ckiki lwai
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Easy way to make your water rocket stable without fins

Post by ckiki lwai »

Hi everybody

Half a year ago I was searching a new way to make my water rocket stable because I had some trouble with attaching fins to my bottle.
So I searched the internet and I found the torpedo design (a ring attached to the fins).
I used this design but I used some plastic extensions attached to the side of the bottle instead of the fins.
The funny thing is, I launched my new design a couple of times until the ring accidentally came of and only the plastic extensions were left.
But to my surprise the rocket flew much better without the ring! :shock:

I attached a picture of my rocket without the ring, so you have an idea where I am talking about.
The plastic extensions are made of another plastic bottle: I cut out the middle part, and I cut it again in 2 parts and I taped 1 part on each side.

I am using this configuration for half year now, and since I had some aerodynamics on my university I started to ask myself why it worked.
One reason I can come up with is that I lowered the center of pressure a bit while I raised the center of gravity by adding weight to the nose.
Another reason I can come up with is that the plastic extensions bend during flight and create a more stable flow below rocket.
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Spaceman Spiff
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Post by Spaceman Spiff »

That's an interesting discovery! I wonder if the rocket is stable also because you moved the center of thrust farther forward on the rocket? The thrust would be pushing slightly in front of the back of the rocket.

I maye have to try this and see if there's an optimum size. I have the most problems keeping fins straight on my rockets. This idea would be really easy to mount and not come off or get bent.
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ckiki lwai
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Post by ckiki lwai »

What's the center of thrust? :)
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Spaceman Spiff
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Post by Spaceman Spiff »

ckiki lwai wrote:What's the center of thrust? :)
It's the point at the middle of the nozzle where the force of the water coming out is acting to push the rocket.
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ckiki lwai
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Post by ckiki lwai »

Maybe something important I forgot to tell: my nosecone (a small bottle with some water) sits loosely on top of my rocket and detaches after "burn-out" and flies further because of its inertia.
This way my water rocket stays stable during the propelled phase but starts to spin as soon the noseweight detaches.

Since there was no wind today I took the opportunity to do I some extra tests on my special flaps, and payed extra attention to how straight it flew.
I did 13 flights and only 2 fell further than 5 meters from where they were launched, after flying more than 30 meters into the sky, I even had one rocket falling closer than a meter to its launch site.
I also did a flight with the noseweight taped to my water rocket to see how stable it flew. The result was that is got into a spin that looked like an hourglass.
This probably happened because the water rocket spun around its longitudinal axis,
it centrifuged the water in the small bottle that acted as noseweigth, which resulted in the hourglass like spin of the whole rocket.
I will launch my water rocket again in a couple of days with a small stone taped to the rocket to avoid this effect.

If there is a day with more wind I will do some more launches to see if it starts flying in the wind or if it drifts away.
I think the weathercocking effect will be smaller because the side surface area is smaller.
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Spaceman Spiff
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Post by Spaceman Spiff »

ckiki lwai wrote:Maybe something important I forgot to tell: my nosecone (a small bottle with some water) sits loosely on top of my rocket and detaches after "burn-out" and flies further because of its inertia.
This way my water rocket stays stable during the propelled phase but starts to spin as soon the noseweight detaches.

Since there was no wind today I took the opportunity to do I some extra tests on my special flaps, and payed extra attention to how straight it flew.
I did 13 flights and only 2 fell further than 5 meters from where they were launched, after flying more than 30 meters into the sky, I even had one rocket falling closer than a meter to its launch site.
I also did a flight with the noseweight taped to my water rocket to see how stable it flew. The result was that is got into a spin that looked like an hourglass.
This probably happened because the water rocket spun around its longitudinal axis,
it centrifuged the water in the small bottle that acted as noseweigth, which resulted in the hourglass like spin of the whole rocket.
I will launch my water rocket again in a couple of days with a small stone taped to the rocket to avoid this effect.

If there is a day with more wind I will do some more launches to see if it starts flying in the wind or if it drifts away.
I think the weathercocking effect will be smaller because the side surface area is smaller.
Your special fin design appears to be working well. I'm anxious to try it out, but it's winter now and good launch days are rare now for me. I would like to test your idea on my rockets. I wonder if this fin only works on short rockets or not. I would like to try it on one of my multi-bottle spliced rockets and see how it works.
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Tim Chen
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Post by Tim Chen »

ckiki lwai wrote:Maybe something important I forgot to tell: my nosecone (a small bottle with some water) sits loosely on top of my rocket and detaches after "burn-out" and flies further because of its inertia.
This way my water rocket stays stable during the propelled phase but starts to spin as soon the noseweight detaches.

Since there was no wind today I took the opportunity to do I some extra tests on my special flaps, and payed extra attention to how straight it flew.
I did 13 flights and only 2 fell further than 5 meters from where they were launched, after flying more than 30 meters into the sky, I even had one rocket falling closer than a meter to its launch site.
I also did a flight with the noseweight taped to my water rocket to see how stable it flew. The result was that is got into a spin that looked like an hourglass.
This probably happened because the water rocket spun around its longitudinal axis,
it centrifuged the water in the small bottle that acted as noseweigth, which resulted in the hourglass like spin of the whole rocket.
I will launch my water rocket again in a couple of days with a small stone taped to the rocket to avoid this effect.

If there is a day with more wind I will do some more launches to see if it starts flying in the wind or if it drifts away.
I think the weathercocking effect will be smaller because the side surface area is smaller.
I never considered using water for ballast to keep the rocket stable. It makes me think maybe there is a way to design the nose so that as the rocket slows due to drag that the water is forced out and the ballast reduces until apogee and then the rocket is unstable and comes back using tumble recovery. It would be the ultimate in simple recovery. Your idea has me thinking!
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Post by Drag_Racer408a »

Tim I have seen people use that water idea on sustainers for 2 stagers. It works really well as long as you can get it to fully become unstable. You wouldn't want it to only tumble half way in decent and then lawn dart!
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ckiki lwai
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Post by ckiki lwai »

Drag_Racer408a wrote:Tim I have seen people use that water idea on sustainers for 2 stagers. It works really well as long as you can get it to fully become unstable. You wouldn't want it to only tumble half way in decent and then lawn dart!
Well that's maybe an advantage of my fins, they give the rocket almost no stability unless there is some weight in the nosecone.
But my fins have yet to prove themselves on longer rockets of course.
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Andromeda
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Post by Andromeda »

Tim Chen wrote: It makes me think maybe there is a way to design the nose so that as the rocket slows due to drag that the water is forced out and the ballast reduces until apogee and then the rocket is unstable and comes back using tumble recovery. It would be the ultimate in simple recovery.
That sounds impossible to get right. It would never work the same way twice, I'm afraid.
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