Spudder's Design Tool (SDT)

[nealius]

I started messing around with the TIme Scaler and noticed that it can have a pretty big effect on the results. Any guidance on how to choose a value? Any reason 0.5 isn't set as the default? Thanks!

[D_Hall]

I started messing around with the TIme Scaler and noticed that it can have a pretty big effect on the results. Any guidance on how to choose a value? Any reason 0.5 isn't set as the default? Thanks!

The smaller the value, the better (more accurate results). However, if you go too small it will cause an error.

What's going on behind the scenes....

I wanted the spreadsheet to be of a reasonable size. Sure, I could make a spreadsheet that was 10,000 rows long, but I didn't want to do that. But if you've only got a small(ish) number of rows, then picking a time step between each state (row) in the spreadsheet is a non-trivial thing. A small gun will (likely) need small time steps, while a large gun (Vera!) might need very large time steps.... Assuming you want the spreadsheet to run to the spud exiting the muzzle that is.

So I try to pick a time step intelligently. I first make a very crude guesstimate of muzzle energy. That gives me a (crude estimate of) muzzle velocity. I then assume constant acceleration and determine how long it would take a projectile to clear the muzzle. If you use a time scaler of 1.0.... That's the nominal length of time the spreadsheet will cover. 0.5 will be half as long. 2.0 will be twice as long. I'm sure you get it by now....

But smaller scalers means smaller time steps. That means more accurate results. Larger scalers means larger time steps. That means less accurate results.

But if the scaler is TOO small, the spreadsheet doesn't "cover" enough time and the projectile will still be somewhere in the barrel when the last calculation is run.

I'd like to pick my timestep a bit more intelligently, but for now... It'll do.

[nealius]

I believe this makes sense. Thanks for your explanation! Is it correct to say that the best time scaler value for a given design is one that puts the projectile at the barrel exit at the 250th node? Overshooting the time scaler by a little bit probably doesn't matter much, but getting it too small is like shortening the barrel if I understand correctly.

[samlevy0515]

Thanks for building this! I've tested the core ballistic calculations in the spreadsheet and they match my old HGDT results perfectly. The main feedback is that the pressure-volume chart doesn't auto-scale the axes when I change barrel length – is that an Excel-specific feature or could a dynamic range be added?

[D_Hall]

I believe this makes sense. Thanks for your explanation! Is it correct to say that the best time scaler value for a given design is one that puts the projectile at the barrel exit at the 250th node? Overshooting the time scaler by a little bit probably doesn't matter much, but getting it too small is like shortening the barrel if I understand correctly.

You've got it.