ramjet vehicle performance estimation tool

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airrocket
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Post by airrocket » Sat Mar 29, 2008 12:09 pm

Any more progress on your software code? I'm working on a solid fuel ram jet and looking for some assistance with a design code. Was wondering if yours would be good start point. I will need to add a burn rate calculation to replace liquid fuels. I plan to start with thrust augmentation design based on fanno flow (flow through a constant dia duct). I might be able to verify some drag cd numbers at that time.

dgsharp
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Post by dgsharp » Sat Mar 29, 2008 2:35 pm

Unfortunately I haven't been able to touch any of my code in a while. Your project sounds cool, very similar to what I had planned as soon as I had some hobby money again (bloody bills!!!), not to mention free time (heh). Anyway.

I have 2 separate tools, a spreadsheet ("ramjet calcs") and a Java program called DragFinder. Both could be very useful to you but I'd suggest starting with the spreadsheet. If/when you get to a point where you need something a simple spreadsheet isn't good at giving you, let me know and maybe I can add a feature or two to DragFinder to suit your needs.

Right now with the spreadsheet you can enter in all your ramjet-specific information (drag coefficient, inside and outside diameters, number of ramjets, vehicle weight, fuel weight, etc). Put in your thrust curve for your booster (or just give the vehicle a high initial velocity to simulate being fired out of a cannon or boosted by a jet engine or whatever), and it tries to show you what it'll do. It's not perfect because it gets the thrust data from Decker's data, which is kind of poorly defined (for instance he has a graph of thrust vs inlet area, but doesn't show how it changes at different altitudes -- I had to make some assumptions about that, but it's done) and presumably really only applies to gasoline, but it's a start. Then in addition to spitting out what it thinks the vehicle performance would be like, it tries to tell you how much fuel you would burn (total, as well as instantaneous rate at each time sample) in order to maintain stoichiometric combustion. You can add your own fuels if you know the fuel/air ratio for stoichiometric combustion (it currently has entries for gasoline, kerosene, propane, rubbing alcohol, acetone, methanol, ethylene, and a solid propellant I was messing with simulating -- although I wouldn't trust the numbers of the latter for your design). It's also got some stuff in it to help design a pressure-fed liquid fuel system that could be useful to others -- put in your starting pressure, regulated pressure, volume, and nozzle information (number and flow rate) and it tells you how long you could maintain a specific fuel pressure. Cheaper than a pump, but not throttleable.

As for the burn rate calculation, I would just get the simulation (spreadsheet or program) doing what you want, and then design the rocket engine to follow that thrust profile. Btw all my software makes the assumption that the vehicle will be flying straight up the way virtually all high-power rocketry stuff is done right now. Any change in that could make a big difference obviously.

Attaching the current version of the spreadsheet. Let me know if you have any questions (private or public). It's something to start with at least.
Attachments
ramjet calcs.xls
Current version of the performance estimation spreadsheet.
(350.5 KiB) Downloaded 229 times

airrocket
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Post by airrocket » Sat Mar 29, 2008 3:43 pm

Thanks for the advice and information. I will play around with it and see what develops.
Retro Flight
RetroFlight.com

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