Water Rocket Motor Placement in Rocksim

[Anonymous]

I've been diligently working on building a water rocket in Rocksim for research into optimization and have been closely following the instructions from an Apogee guide (found here: Apogee Rockets Guide). The guide has been immensely helpful, but I've encountered a specific issue that I need some advice on.

In the file example included in the guide, the motor mount is positioned at the threaded portion of the bottle. However, I noticed that both the motor in the example and the one I plan to use have diameters that are significantly larger than this threaded portion, i.e. they have the diameter of the main part of the bottle's body. Given that the water is actually stored in the main body section of the plastic bottle, this placement seems a bit puzzling.

I've made some modifications to the design to better fit the dimensions and nose cone of my bottle, but I'm still uncertain about the correct placement of the motor mount for accurate simulations. The guide doesn't explicitly clarify this, and I want to ensure that my simulations are as realistic as possible.

Any help or clarifications are greatly appreciated

 

[cls]

Whatever fractions of an inch you get so precise on, pales in comparison to the great mass of the water and how it moves and changes the CG during operation. Does your ENG file accurately represent this?

 

[Anonymous]

I made the engine file with TCTracer and the following website, http://www.aircommandrockets.com/sim/simulator.htm#collapseThree, making sure to imitate as closely as possible my final design.

 

[Anonymous]

The doubt was more about where you are supposed to place the motor (even if theoretically it is a water rocket) inside Rocksim 10. Can it be at the main body of the model, even though this part isn't in contact with the outside because it is blocked by a transition imitating a bottle shape? Or does it have to be placed somewhere that has no obstruction to the outside of the model, such as the final threaed portion which imitates the bottle end? I have added a file to clarify below (ignore the mass,, CP, and CG, since the model is missing the engine which adds about 500g).

 

[Anonymous]

Whatever fractions of an inch you get so precise on, pales in comparison to the great mass of the water and how it moves and changes the CG during operation. Does your ENG file accurately represent this?

I am able to make a change to the .eng file to make a CG shift for the engine with Rocksim's engine application, yet I do not know exactly how this varies over time and have found no graphs online detailing it for me to be able to copy.

 

[cls]

I am able to make a change to the .eng file to make a CG shift for the engine with Rocksim's engine application, yet I do not know exactly how this varies over time and have found no graphs online detailing it for me to be able to copy.

It's an interesting question!

I think we could assume that the propellant (water) is all aft, because of 1G on the pad and more than 1G under acceleration.

So if the tank is a cylinder, and the water egress rate is constant, CG of the tank would move aft linearly. I guess?

Just thinking aloud here...

 

[Anonymous]

It's an interesting question!

I think we could assume that the propellant (water) is all aft, because of 1G on the pad and more than 1G under acceleration.

So if the tank is a cylinder, and the water egress rate is constant, CG of the tank would move aft linearly. I guess?

Just thinking aloud here...

In the guide, he shows an example of an aproximate shape of the CG-versus-time for a "typical water rocket", shown below. The thing is I want it appropriate for my design, so I don't know how to adapt the shape. Also, it seems to be the only place in the whole internet where a CG-versus-time graph for water rockets has been shown, making it harder to draw specific conclusions.

 

[Anonymous]

Do you have any ideas on the other question I posed, in relation to where to mount the motor in Rocksim? Thanks

 

[cls]

Do you have any ideas on the other question I posed, in relation to where to mount the motor in Rocksim? Thanks

Rocksim is not a full CFD, so it doesn't model exhaust expansion, Krushnik effect, etc. so I think it doesn't matter where you put the motor, except for CG.

Prove this to yourself: start with a known model, like Alpha III.

Run one simulation with the motor mounted as it would be built.

Then, slide it forward like half an inch, so the motor nozzle is inside the fin can. Rerun the same simulation.

See the difference? No acceleration or altitude difference, slight difference in stability from moving the CG.

 

[Anonymous]

Thanks, I should have tried that from the start.