Knozzle Too Tight or Too Much Drag

[Billkuhl53]

I tried my second timer rocket with the side hinge nose. The knozzle of the bottle was really tight on the Pitsco stopper, I should check the bottles before building I guess. This rocket had a lot of things hanging outside but the idea seemed to work well on the bench. Trouble is the rocket never went higher than 40 feet. Of course the rocket was on the ground before the timer went off.

So now I am wondering if there was just too much drag or the tight knozzle was holding it back, or maybe a combination of both. For my next rocket I was thinking of having a deployment unit that could be taken out of the rocket, wound and set and then put back in the rocket. I was also thinking of using the force of inertia at launch to trip the timer.

Welcome any suggestions or comments.

Bill Kuhl
http://www.scienceguy.org

[Billkuhl53]

I have a picture of the rocket, sure is ugly but I thought I could use it as a test bed for some ideas. I am thinking the drag was the main reason it did not get very high.

[RebelRockets]

Hi Bill,
Glad you got the photo posted.

Is that a 2L bottle and a larger / expanded snack can, or a smaller size bottle that you're using? Maybe you don't have enough volume to lift the weight of the rest. If you wish to build using the snack can diameter for your payload, could you possibly make a tapered adapter to fit it to a 2L ? The Pitsco rockets look like they use a tapered section that fits a 2L, and I believe that you mentioned elsewhere that a snack can replaced the Pitsco tube well when it became damaged.

If the stopper was too tight, was the rocket releasing water and pressure before it actually left the launcher? Could one of the launcher's jaws been slower than the others in releasing? My guess if the stopper was extremely tight it would either not come out at all or there would be a delay before then suddenly popping out. Unless the stopper itself was really damaged I don't see how it would have slowed the rocket. Does Pitsco recommend any kind of grease for it?

It probably would be a good idea to test launch any new bottle just by itself to see what you can get out of it. If a particular bottle type performs better than others at the same pressure that would most likely be a good choice to build on.

Good luck!
Dennis

[rockets-in-brighton]

I have a picture of the rocket, sure is ugly but I thought I could use it as a test bed for some ideas. I am thinking the drag was the main reason it did not get very high.

[Billkuhl53]

This bottle was smaller than 2 liter, 24 ounces I believe. The rocket did not seem that heavy, I think there was just too much drag and not a proper design. I plan to work with 2 liter bottles in my timer efforts in the future before attempting the smaller rockets. The idea I had about a removable module for the parachute deployment makes more sense the more I think about it. Would be able to switch it between rockets, work on it easily, or replace new module in the same rocket.

Bill

[Jelo]

go duct tape!!
i think you should put something over the nose cone cos all the tape creates a lot of drag

[rockets-in-brighton]

This bottle was smaller than 2 liter, 24 ounces I believe. The rocket did not seem that heavy, I think there was just too much drag and not a proper design. I plan to work with 2 liter bottles in my timer efforts in the future before attempting the smaller rockets. The idea I had about a removable module for the parachute deployment makes more sense the more I think about it. Would be able to switch it between rockets, work on it easily, or replace new module in the same rocket.

Bill

I failed to post my response correctly

It was meant to ask for more concrete information on the failed launch: dry weight of the rocket, launch pressure, nozzle size and so on, to see if the height you got was in the expected range as calculated by simulator. If the sim says you should get 100 feet then you know that there's a problem with your setup, perhaps a pressure leak. If it says 40 feet, then you'll have to improve something.

[Billkuhl53]

I didn't use a simulator, this was a quick and dirty project. Too much so I am afraid. This was a full nozzle (no restriction) pumped up to 100 psi. It was like the rockets the park recreation kids built last summer, most of them never went very high. The fins could have been fluttering also.

Looking at my previous timer effort, the design wasn't that bad if I could get away from the long rod that pushes the nose off.

Bill Kuhl

[rockets-in-brighton]

I didn't use a simulator, this was a quick and dirty project. Too much so I am afraid. This was a full nozzle (no restriction) pumped up to 100 psi. It was like the rockets the park recreation kids built last summer, most of them never went very high. The fins could have been fluttering also.

Looking at my previous timer effort, the design wasn't that bad if I could get away from the long rod that pushes the nose off.

Bill Kuhl

OK... here's what we have:

• volume: 24 fl oz = 0.7 L
  launch pressure: 100 PSI
  nozzle diameter: 22 mm

Sound about right? Here are some guesses for other numbers, are they close?

• bottle diameter 70 mm ?
  water fill 0.2 L ?
  dry mass: 100 g ?

Plugging those into CH gives... apogee at 55.48 metres (182.0 feet) after 3.18 seconds. Clearly that did not happen.

Possible adjustments: it is actually heavier than 100 g; the water fill volume was different; it is much draggier than 0.3; the pressure was not what you measured due to gauge inaccuracy, pressure leaks etc.

Adjusting each of those in turn, aiming for height approx 12 m:

• weight: 350 g
  OR
  water: fill 0.65 L
  OR
  coefficient of drag: > 5
  OR
  launch pressure: 25 PSI

Thinking about these to see which is the most likely...

• weight: I can't see it being that much heavier, given the bottle size, (bottle 35-45 g, can etc 30-40 g, tape 10-20 g, chute 10-20 g)
  water: You surely wouldn't have filled the water almost to the brim
  Cd: I think even a hedgehog would be more aerodynamic than a rocket with Cd > 5
  pressure: mmmm

That just leaves the launch pressure. The most plausible answer is that friction on the launcher prevented the rocket from taking off before most of the pressure had leaked away. QED.

[ninja_iga]

Cd: I think even a hedgehog would be more aerodynamic than a rocket with Cd > 5

i like this!


i think it's possibly too much water inside?

[rockets-in-brighton]

Cd: I think even a hedgehog would be more aerodynamic than a rocket with Cd > 5

i like this!

i think it's possibly too much water inside?

No, if you look at the photo carefully you can see the water level. To get the result that Bill reported it would have needed to be filled to the top, but I'd say it's about 50%.

[U.S. Water Rockets1]

Cd: I think even a hedgehog would be more aerodynamic than a rocket with Cd > 5

i like this!

i think it's possibly too much water inside?

No, if you look at the photo carefully you can see the water level. To get the result that Bill reported it would have needed to be filled to the top, but I'd say it's about 50%.

Stanley is correct that this much water is excessive. Perhaps not enough to cause the complete failure, but it's not optimal fill.

[ninja_iga]

and looking at the combos Bill has,
it's definately adds up..

weight, aero, not optimised water fill, dunno about the air pressure (!!)



speaking of this topic, i'm using a Clark cable launcher to great success (relatively lol)
and i came across the 9mm vs 2mm topic, so i decided to give it a go.

i had a machined nylon piece for another WRocket project so i scavenged it and drilled it to 10mm + countersink / fillet to help with the water flow.

this is placed inside a spliced pair whilst gluing, so it's inside the bottle neck, but it wont come out because there's a neck to hold it in place.

anyways, i launched with this restrictor + a 3/8 launch tube (no friction good fit) and with this material i use, it's pretty slippery! (Teflon)

results: good soft launch but the height is not much if at all. 50ft?
i thought it would be weight slowing down the Wrocket due to nozzle's lesser thrust..
but i used a lightweight nose module but results are similar.. hmm!

[Billkuhl53]

I was looking at the picture again of the water level, the Pringles tube cover part of the bottle so level of the water was not as high as it looked.

Working on new ideas with a timer. Using the plunger arrangement with tension springs.

Bill Kuhl - http://www.scienceguy.org

[SaskAlex]

The only way the pressure could have dropped is if a lot of water came out at the seal just before launch. Even if it was half full with water, and all of the water leaked out before launch, you'd still have half of your pressure. So I don't see how it could be a pressure problem.

I also don't think it would have much to do with the tight nozzle. It looked like the launcher didn't have much of launch tube, just enough inside the bottle neck to make a seal, correct? Now think in terms of energy. The energy lost due to friction would be the length of that tube multiplied by the force of friction (which we know is less than the pressure X nozzle area, or the rocket would not leave the launcher), and this is negligible compared to all of the energy going into the rocket (~2PA x the distance over which you have thrust).