quickie

 Couldn't sleep, so I updated my proposed design for the EML (pssst: "rale guhn" don't tell anyone) projectile package.  I added a keyway to hopefully keep armature and cap wedded at least until they exit.  Yes, discarding sabots are nice, but let's get the thing working at all first, shall we?

The lighter, white-ish cap on the right is to be made of Delrin (acetyl).  The armature proper at rear is made of high-conductivity aluminum, 1100 series or the like.  I may be able to find this profile off-the-shelf, saving me a lot of machining.  The gaps creating individual fingers are just saw kerfs.  The key and keyway will be done with a dovetail mill.

I might see if I can talk my friend with the CNC mill to set up and knock out about 100 pieces of each.

Note that the size is not pinned down yet.  The design currently calls for a .75" bore.  If I am to use the rails I have in hand, that will be the case.  If I can get my hands on some bigger rails (at the price of copper today? Hah!) I would like to have more inductance in the launcher, just to increase the ratio of stray inductance (wasted) and launcher inductance (creates force on the armature).

 The fingers are a dodge that's been tried a few times by several labs and which seem promising for square bores having very little pre-load on the armature. At the small scales amateur launchers are working at, and tens of kilojoules input energy, we can't afford to waste much of that energy overcoming stiction from a pressed-in armature.  Conversely, since we're in the kilo-amp regime, we don't have as much difficulty with sliding contact resistance as the bigger devices do.  Some preloading will absolutely be necessary, but I want to keep it to a minimum.

Fortunately, this launcher has a very high pressure pre-accelerator, compared to most amateur-built devices.

PS: the shape of the void behind the armature -- or, if you prefer, the inside shape of the contact fingers -- would be better if the fingers were thicker toward the armature than shown here, but the armature proper was designed with off-the-shelf c-channel stock in mind to save time and money.