Preliminary OHKLA design | Rocket body discussion | Forum

So what does everyone think?

I think that this is nice work. There are obviously some problems, but just showing this tells me that you have the software and ability to do some real design work. I'm excited to see how this develops.

Can we start designing our OHKLA rocket?

Absolutely! There isn't anyone who is going to stop us from doing this. Have you considered forming a local CSTART chapter? Having a physical group of people who can meet will really help you push this forward.

I know there has been talks that it would be easier to hire other rocket boosters, but I would still like to continue to develop our own. I have been researching more and more about rocket nozzles and such and as a result I have been learning more and more. One of the biggest mistakes I made in my prior cfd attempt was the velocity at the inlet, it was extremely too high. I was, of course, just playing around then, but I believe that to be the reason for the result I was given. I have since modified the model and the setup. My results are much more promising, but still need more work. All I am doing is providing a nozzle shape, inlet fluid velocity (60 m/s) and the fluid (N2O, I know this isn't what we are using, I just needed something). To truly understand what will happen in real life, I will need real life inputs, which I hope we can find in the future.

Here are some resulting pictures:

There are still quite many improvements, but one thing that I like is that if you look, the fluid velocity at the throat of the nozzle is near the speed of sound, which is what it should be. Also, the velocity appears to increase as it exits. I'll continue working and keeping everyone up to date. Instead of an inlet velocity, I will try more for inlet pressure and look for a more appropriate fluid. What are chemical compositions we would be using? Any input would be greatly appreciated.

Rocket-To-The-Moon said:

Very cool, I bet you are having a great time learning about this.

What is the atmospheric pressue in these tests? Is it vaccum?

Yes, it is really nice to learn about what is going wrong and improving on it. There is something great about not having to spend money on materials and manufacturing costs and creating digital prototypes to simulate how it will work.

I was running through the simulations with 1atm pressure. A decision needs to be made about what pressure should be used to do these simulations. Is it at the halfway point in altitude that the combustion will be happening? What pressure is that? I guess we won't know until we understand more about the fuel used.

Post edited 9:06 am – February 18, 2010 by rpulkrabek

I have done some more work towards OHKLA. My knowledge is limited towards hybrid rocket design, and would like some guidance. I have uploaded pictures from the Pro/E model I am working with to the wiki as a host.

http://cstart.org/wiki/images/…..cept_1.jpg

http://cstart.org/wiki/images/…..cept_2.jpg

http://cstart.org/wiki/images/…..cept_3.jpg

http://cstart.org/wiki/images/…..cept_4.jpg

http://cstart.org/wiki/images/…..cept_5.jpg

http://cstart.org/wiki/images/…..cept_6.jpg

http://cstart.org/wiki/images/…..cept_7.jpg

http://cstart.org/wiki/images/…..cept_8.jpg

http://cstart.org/wiki/images/…..cept_9.png

I will edit one of these to include arrows pointing to what each component is. Basically, I used the same/similar geometry as I had with the CFD for the nozzle, and created all new components to form an assembly. There is a lot I want to investigate and optimize. For one, I am a bit unsure of the nozzle shape. Sure it looks good in the cfd, but we may lose some of the performance as a result of the drag. I will investigate further and see if Ansys can perform an optimized shape through design exploration.

The fins I am completely unsure of what is best. Is their only purpose to prevent pitch/yaw? Is there a way to optimize to provide maximum thrust or even lift? I modeled these with a draft hinge that is parallel to the leading edge. I noticed Copenhagen suborbitals had a draft hinge as well. What is its purpose? How many should we use? I remember seeing the Ares I have 12 fins: http://www.nasa.gov/mission_pa…..old_1.html

I am still unsure about what geometry the fuel grain should have. Obviously we may need to do some tests to determine what suites us the most. It may be the geometry that provides the most thrust at the beginning of its flight. Also, the material needs to be determined. The dimensions should be investigated also, such as its height and diameter. This goes for the oxidizer too.

The nose will contain the avionics and other instruments. The dimensions of this will be changed to accommodate  everything.

Feel free to poke holes in this design and I will do my best to modify it.

As I promised earlier, here is a picture with arrows and short descriptions.¨

http://cstart.org/wiki/images/…..ptions.png

• Sorry about the images. I realized the effect after I saved the post. Next time I will make them smaller.
• I have created a clone and I will play around to see if I can upload my files there. I think it's good for me that I learn this. I'll inform you when I am ready to merge.
• Looking at this image, do you think it is a good idea to look more into the multi-fin geometry while still considering the multiple core?
• I'll try to modify the fuel grain plate. I'll model it to be a large hole in the center, just enough to hold the grain in place.
• About the fins being attached to the nozzle. You could be right, but I almost wonder if having fins there will help dissipate the heat from the nozzle since we are increasing the surface area which will increase the convection. I can try a simple thermal FEA of this to get an idea.
• I agree about getting rocketry clubs involved. They would have much more knowledge and experience than I do.

Very good work! I am excited about OHKLA because it is a project that can produce some real results quite soon

I do believe that the image you linked to is specifically referring to solid fuel grains, but if thrust is a function of the exposed surface area then they should also be an approximate for a hybrid motor. I am trying to understand if it is better to have low or high thrust initially while the rocket is operating in the most dense part of the atmosphere. My thought is that it is best to use a fuel grain with a single central port because its thrust profile starts out low and then ramps up. This would cause the motor to burn the least amount of propellant when it is operating in the dense atmosphere down low. Drag increases with the square of velocity which means that it is ideal for the rocket to travel slowly in the densest part of the atmosphere and then accelerate more rapidly once it gets above 10,000-15,000m or so.

Does this make sense or should we have high thrust initially (as in the dual composition grain) so that the rocket's exposure to the lower atmosphere is as brief as possible?