EXECL chat

[Bob B.]

Oct 29, 2010 11:29:48 GMT -4 BertL said:

Talking about Excel: Bob B., is there any chance you could upload or share the Excel files themselves? I'm quite interested in seeing how you format and use the numbers and equations to come to the wonderful diagrams you post on this website.

There really isn’t anything too tricky about making the diagrams – I just use the chart function that comes with Excel. All the tools to make and format the charts are a part of Excel. To display them here or on my web page, I just do a print screen and then paste it into Photoshop, where I can make additional edits, if necessary, and save the file as a GIF or JPEG.

The equations, on the other hand, can be rather complex depending on what it is I’m trying to do. And, of course, it takes some creativity to figure out the logic necessary to achieve my goals. Every problem is different, so I’m not sure where you want to start. Is there something in particular I’ve done that interests you and that you want to learn more about?

If you’re interested, I once made a simulation of a Soyuz launch for a friend at another forum. I sent him a copy of the file and posted a description of how to use it, which you can find here:

www.thespacerace.com/forum/index.php?topic=2570.msg18462#msg18462

And here is a PDF of what the Excel file looks like that I’m describing:

www.braeunig.us/misc/launchsim.pdf

This simulation is way too complex of a file to start with, but it gives you an example of some of the things I’ve done. If you want to study a sample file, I’d recommend something simpler. Perhaps I could throw together a simple simulation from scratch that we could share, discuss and dissect. Depending on how much you want to continue the discussion, we could gradually add more complexity.

[ka9q]

This is really cool stuff. How do you control pitch during first stage flight? Are you doing a straight gravity turn, or do you apply other constraints on max-Q, maximum aerodynamic heating, or maximum bending torque? I know that some if not all launchers will quickly break up if the angle of attack exceeds some small value during much of first stage flight. The Saturns had an "emergency detection system" (EDS) to trigger an automatic abort when this happened because breakup could occur too quickly for the crew to react manually.

[Bob B.]

Nov 1, 2010 17:11:48 GMT -4 ka9q said:

This is really cool stuff. How do you control pitch during first stage flight? Are you doing a straight gravity turn, or do you apply other constraints on max-Q, maximum aerodynamic heating, or maximum bending torque? I know that some if not all launchers will quickly break up if the angle of attack exceeds some small value during much of first stage flight. The Saturns had an "emergency detection system" (EDS) to trigger an automatic abort when this happened because breakup could occur too quickly for the crew to react manually.

I don’t do anything nearly that sophisticated. I just figure out the flight path angle that gets me to where I want to go. When I command the rocket to pitch to a certain angle, I assume the engines gimbal as necessary to achieve that end, but as far as the simulation is concerned, the thrust vector is fixed to the long axis of the rocket.

I calculate drag as if I have a zero angle of attack. Rather than getting into a bunch of messy math, I just alter the drag coefficient to account for it. In reality, I’m probably using too high a drag coefficient and to low an area, but hopefully the answer comes out about right. Part of the problem is that I have never been able to find any solid information regarding drag coefficients (do you have any sources?). I’ve had to just back my way into it using what scant information I’ve been able to find. I’d like to do more, but so far I just haven’t had time to get any more elaborate.

I haven’t worry at all about torque – that’s a level of detail that is beyond what I’ve been able to do so far.

I would like to someday get more detailed and incorporate many of the things we’ve discussed. For instance, I’d like to steer the rocket by gimbaling of the engines rather than just telling the rocket to change pitch from X to Y. In this case I’d have to include a more complex aerodynamic model, taking into account lateral forces and so on. I might also want alter atmospheric conditions (temperature, pressure, humidity) and add wind – perhaps even random gusts.

Adding these extra details is probably more than I can do with Excel because I’d have to write some control logic. It’s been years since I’ve written any computer code, so it will take some study to get caught up. I’m confident I can do it but I just don’t have the time with everything else I’ve got going on. I see this as a project to work on in my retirement.