CFD non axisymmetric design external pods etc.....
- Thread starter xasltd
- Start date
Help Support The Rocketry Forum:
pic 1 - boundary layer flow - pretty stable
pic 2 - airflow around shuttle causing off axis drag - as expected
pic 3 - airflow over stack top - seems to be just off centre of main tank towards shuttle bottom (which is good in our case because that's where centre of thrust from the motors are......phew!
pic 4 - showing high loadings on wing towards main shuttle body reinforcing the wing with formers and stringer. Wings will come off at around 1000 newton pressure.
pic 5 - shows centre of pressure around the middle of the stack - which we kind of expected......
All in all not to bad
Possible fixes...
moving shuttle as close to the main tank as possible to reduce stagnant air in between the main tank an shuttle rear.....
loads of nose weight!
pic 2 - airflow around shuttle causing off axis drag - as expected
pic 3 - airflow over stack top - seems to be just off centre of main tank towards shuttle bottom (which is good in our case because that's where centre of thrust from the motors are......phew!
pic 4 - showing high loadings on wing towards main shuttle body reinforcing the wing with formers and stringer. Wings will come off at around 1000 newton pressure.
pic 5 - shows centre of pressure around the middle of the stack - which we kind of expected......
All in all not to bad
Possible fixes...
moving shuttle as close to the main tank as possible to reduce stagnant air in between the main tank an shuttle rear.....
loads of nose weight!
TheAviator
Well-Known Member
- Joined
- Jan 18, 2009
- Messages
- 936
- Reaction score
- 123
Don't want to rain on your parade, but Solidwork's CFD code isn't really appropriate for external flows. Even if it does "work" in this case, your tube needs to be MUCH larger or you'll get image effects from the walls. Your "tube" should be at least 5 times the circumscribed diameter of the model at a minimum, and larger than that for better results.
We'll re-run with a bigger wind tunnel. ...ta x
TheAviator
Well-Known Member
- Joined
- Jan 18, 2009
- Messages
- 936
- Reaction score
- 123
What do you want to know? As I said earlier, CFD is probably massive overkill. Fluent has been suggested and would work. Solidworks might give you an acceptable answer, but it might not. Most of the confusion depends on what Reynolds' Stress model the CFD code uses, and I don't know which one the Solidworks CFD uses.
I'll find out.......
Still awaiting a response from Solidworks....had a chat with my friend from down under, the plug in used was just under $13000! Holy cow that's a lot of money......
TheAviator
Well-Known Member
- Joined
- Jan 18, 2009
- Messages
- 936
- Reaction score
- 123
I'm not sure they're going to tell you. Sometimes that's part of the proprietary information. I'm not sure that I could make a suggestion based on that info anyways, as I don't work with CFD enough to tell you. I was more asking what information are you looking for concerning the flight/structural characteristics of the model.
CFD is usually the last step in determining flight characteristics. I suggest that you start with a linear method like AVL and then work your way up to CFD. Linear methods take much less computer time (orders of magnitude) than CFD, so you can tweak more and run more cases in a lot less time.
https://web.mit.edu/drela/Public/web/avl/
We need to find out where the air balances out over the whole stack. without an xyz axis its difficult to explain but unless we get the thrust running through the centre of pressure its going to cartwheel.......
TheAviator
Well-Known Member
- Joined
- Jan 18, 2009
- Messages
- 936
- Reaction score
- 123
Actually, you want to send the thrust through the center of mass, which is where your solid models come in. A lateral offset of the center of pressure is not going to put that much torque on the vehicle. Far far less than the moment caused by the shifting (laterally) CoM. The key is to make sure that the vehicle is aerodynamically stable. If it's unstable, it doesn't matter where you put the thrust line.
The cfd in solid works shows the vehicle to be more stable than was expected. As Long as the cfd is correct that is........there is some static air behind the main tank between the shuttle. which is causing some issues.
the com already runs through the centre of thrust we already did that and the stack will be lined up with lasers from the roof of the warehouse. If we can get the stack to also have it's cp running through that line then all the better. Getting the mass point centred is easy, getting the cp requires cfd....it's the only way to do it.......and I can't afford a wind tunnel.....we can't really move the cp point but we can move the mass point that's still in our control......so it should make sence to try to get the mass point through the cp rather than the other way around. I'll get a pic up showing all the points and see if anyone has any advice.....
You want the center of mass (or center of gravity or c.g.) to be ahead of the center of pressure (c.p.). The line of action for the thrust should go through the c.g.
We have already done 2 smaller shuttle stack launches 1 version about 18'' and another about 7ft. Both worked acceptably well.
Obviously, you have a beautiful and complicated model. For a simple symmetrical model one wants the c.g. ahead of the c.p., so that the aerodynamic forces are biased to create a moment to turn the model about the c.g. and restore the vehicle to a straight flight. This is more difficult for an unsymmetrical model. For your model you have asymmetry coming from the shuttle hanging on one side, which has some lift, if there is an angle of attack, and the boosters, which are distributed unsymmetrically about the vehicle. I found an elaborate German HPR model of the US Space Shuttle on Rocket Reviews here:
https://www.rolfstabroth.de/Hobby/M...lemodell/Shuttle Home/Space_Shuttle_Home.html
This modeler had problems. I tend to think that this model was under-powered and would have done better if the model had gotten higher. It is my understanding that Andy Woerner's US Space Shuttle model flew well. I strongly suspect that Andy did not use a CFD analysis. Andy is the former owner of What's Up Hobbies. I forget the name of the new owner, but I wonder if you contact them, you may get some useful information. I don't think your wings have much airfoil or camber, but they probably produce some lift if there is an angle of attack. Your model is more asymmetric than the US Space Shuttle with regard to the strap-on boosters, but I would tend to think that is not as much problem as the shuttle's wings. I can see that the German model had motors in both the boosters and the orbiter. I think Andy may have done things differently. As I recall I think that the German model may have used clear plastic fins. Andy's model may have used clear plastic fins, too. I would think that you would want to check the load on the shuttle attach points, so that it remains attached during ascent.
https://www.rolfstabroth.de/Hobby/M...lemodell/Shuttle Home/Space_Shuttle_Home.html
This modeler had problems. I tend to think that this model was under-powered and would have done better if the model had gotten higher. It is my understanding that Andy Woerner's US Space Shuttle model flew well. I strongly suspect that Andy did not use a CFD analysis. Andy is the former owner of What's Up Hobbies. I forget the name of the new owner, but I wonder if you contact them, you may get some useful information. I don't think your wings have much airfoil or camber, but they probably produce some lift if there is an angle of attack. Your model is more asymmetric than the US Space Shuttle with regard to the strap-on boosters, but I would tend to think that is not as much problem as the shuttle's wings. I can see that the German model had motors in both the boosters and the orbiter. I think Andy may have done things differently. As I recall I think that the German model may have used clear plastic fins. Andy's model may have used clear plastic fins, too. I would think that you would want to check the load on the shuttle attach points, so that it remains attached during ascent.
Last edited:
This is true, which we learned off the previous 2 launches, however I am really interested in finding out where the cp pressure is in both x and y axis. Its an unsymmetrical design and therefore the CP isn't like assessing a symmetrical rocket where it runs nicely down the centre of a single 'tube. It will be slightly off to one side and that's where the cfd comes in handy.......
I am familiar with the German model flown. There was a British company that built one that was about 12m high. They had a few tricks which helped it stay on course. Shifting the srbs and a massive amount of nose weight.... They had motors in the main tank only. They also had a fairly complex programme to analyse the shifting mass point and had access to wind tunnel testing.....a big budget and some pretty clever brains...we have a home made router, drill and tea and biscuits.......
Here is Andy's flight. Perhaps, you have already seen it.
https://www.youtube.com/watch?v=BQRl37aI1yk
It looks like he has a configuration similar to the German model, i.e., two big booster motors and three smaller orbiter motors. I don't think I see any clear plastic fins on Andy's model. I was browsing around on the internet and I saw Andy discussing his model. For the higher powered flight he used a K-motor in each SRB and 3 J's in the orbiter. He stated that in his commercial model (originally priced $2,999) that he was going to make a lighter model and with a lighter orbiter. It looks like Andy's model is getting off the pad straighter than the German model and going higher, although, it looks like the trajectory may be turning downward a little bit early like the German model. This might mean that an analysis regardless, if it is CFD or not, should look at more than one point of the trajectory.
https://www.youtube.com/watch?v=BQRl37aI1yk
It looks like he has a configuration similar to the German model, i.e., two big booster motors and three smaller orbiter motors. I don't think I see any clear plastic fins on Andy's model. I was browsing around on the internet and I saw Andy discussing his model. For the higher powered flight he used a K-motor in each SRB and 3 J's in the orbiter. He stated that in his commercial model (originally priced $2,999) that he was going to make a lighter model and with a lighter orbiter. It looks like Andy's model is getting off the pad straighter than the German model and going higher, although, it looks like the trajectory may be turning downward a little bit early like the German model. This might mean that an analysis regardless, if it is CFD or not, should look at more than one point of the trajectory.
Obviously, you do not want to repeat the final result of the Space Shuttle Reliant. It looks like the Space Shuttle Reliant designers had a major portion of their of motors in the external tank aft end. Thus, the major thrust component is in-line with the one of the major drag components. This should be helpful. It looks like the flight of the Reliant is fairly straight, however, there may be doctoring of the film in some places, slowing down, heightened crash, etc . (We have discussed this before on TRF.)
It looks like both the German model and Andy's model have the SRB's with larger motors and smaller motors in the orbiter. I do not see any plastic fins on Andy's model, but I could be mistaken. It looks like the German website shows an analysis for the line of action for all the motors. Off the top of my head, I would think that the designer would want all these line of actions to go through the c.g.
Normally, a model rocket simulation computer code like RocSim will evaluate the c.p. based on an algebraic formula like the Barrowman method, cut-out method, etc. The idea is that if the model is at an angle to the line of flight there will be a restoring force bringing the model back in line with the flight direction, so that there is no net side-ways force. Thus, even if a designer put their model in a wind tunnel or a CFD simulation, I would think that the designer would want to measure the forces (along with direction) when the model is at a "slight" angle to the direction of flight. From the Reliant video I cannot tell if the experimenter's really did this. It looks to me like they were just looking for obvious bad affects of the winged vehicle. If Andy Woerner did not have any clear plastic fins, then I would think that he is actually relying on the Shuttle wings to give aerodynamic stability.
I just went back and looked at my RocSim9 simulation for my 4x OT, which is "not" symmetrical. I have not paid too much attention to this simulation, because the model was flying fine before the simulation was developed. Nevertheless, the simulation does give a favorable location for the c.p. with respect to the c.g.
It looks like both the German model and Andy's model have the SRB's with larger motors and smaller motors in the orbiter. I do not see any plastic fins on Andy's model, but I could be mistaken. It looks like the German website shows an analysis for the line of action for all the motors. Off the top of my head, I would think that the designer would want all these line of actions to go through the c.g.
Normally, a model rocket simulation computer code like RocSim will evaluate the c.p. based on an algebraic formula like the Barrowman method, cut-out method, etc. The idea is that if the model is at an angle to the line of flight there will be a restoring force bringing the model back in line with the flight direction, so that there is no net side-ways force. Thus, even if a designer put their model in a wind tunnel or a CFD simulation, I would think that the designer would want to measure the forces (along with direction) when the model is at a "slight" angle to the direction of flight. From the Reliant video I cannot tell if the experimenter's really did this. It looks to me like they were just looking for obvious bad affects of the winged vehicle. If Andy Woerner did not have any clear plastic fins, then I would think that he is actually relying on the Shuttle wings to give aerodynamic stability.
I just went back and looked at my RocSim9 simulation for my 4x OT, which is "not" symmetrical. I have not paid too much attention to this simulation, because the model was flying fine before the simulation was developed. Nevertheless, the simulation does give a favorable location for the c.p. with respect to the c.g.
Last edited:
I have spoken to a fella who built the big shuttle, it was designed to fly straight rather than be totally scale and so is ours. It also had fins on the back of the stack which you can't see.....I do have rock sim and will be using it.
I went back and looked carefully at Nadine Kinney's 2007 Calendar for May showing Andy's Space Shuttle. There are two large red fins on the External Tank on the side opposite of the Shuttle. From the front perspective they are hardly visible or in some views not visible. It looks like they at 120 degrees with respect to the Shuttle tail fin. The model is 6 feet seven inches tall with a launch weight of 105 pounds. The outer 2 SRB's were powered each with a K550. The Orbiter had 3 J180's. As I mentioned earlier Andy was talking somewhere on the internet of making a lighter version for sale. He wanted to make the orbiter (R/C) lighter.
dixontj93060
Well-Known Member
- Joined
- Feb 19, 2009
- Messages
- 13,083
- Reaction score
- 45
Sent from my iPhone using Rocketry Forum
Similar threads
