The Ballista

Just ordered this little truck crane.

It’s time Firefly stepped forth from her usual bed.

This is the setup I have been using to induce rotational pre-tension into Firefly’s rope bundles.  The multiple holes in the top of the spanner are to allow it’s lever arm to point in the correct direction independent of where the crossbars are.

This rotational torque is essential to generate power and is applied only after the springs have been stretched linearly.  Keeping the springs as straight as possible with a linear stretch can be likened to building a good foundation for the twisting procedure seen above.  In my experience, both are essential for developing power.  While we will attempt to improve the linear stretching of Phoenix’s bundles by use of a stretcher to address each strand as it is laid down, I have no doubt that the procedure seen here will still be vital to build up the final power of the machine as well as tuning the bundles to have equal thrust.

I would respectfully suggest that if you are a fellow torsion catapult reconstructor, and are having problems of weak performance, it is likely you are skimping on your rotational pre-tension.  Straight springs are great, it is just that they are not the whole story.     ………. And be careful,  this can all get pretty extreme in a hurry…….

The loops and tangs evident in the Orsova artifacts demonstrate the use of modular components in iron framed Roman ballistas.  In designing Firefly, I tried to extend this theme of modularity, figuring that if the artifacts themselves demonstrate this feature, then there is no telling where those sneaky old Romans left off with the strategy.   Firefly’s rope springs are modular, pre-wound bundles that are carefully stitched together to insure they maintain their shape as they are sucked into position in the field frames during the installation process.  Changing out the spring bundle in a field frame takes about an hour.

After proper pretension and tuning with the limb in place, this modular approach seems to work quite well.   The advantages of having spare, pre-made springs to swap out quickly would seem to be a huge advantage to anyone trying to maintain these machines.  It certainly was for this muggins who had to do all the work alone.   It is difficult to imagine that the innovative Romans who came up with as revolutionary a design as the Orsova machine, would not have tumbled to this approach also.

In this next photo we see the set up used to induce linear stretch into a pre-wound bundle that had just been installed into the port field frame.   If you blow the photo up to maximum size it is possible to see the steel spacers that are slipped under the crossbars to lock in the linear stretch.   One might call it an updated version of Philon’s “wedge-machine”.  (See Marsden’s,  Greek and Roman Artillery.)

On the other hand, perhaps the iconoclastic Romans engineers that developed the Orsova machine had enough willing hands to run the spring cord around the crossbars and tension each strand as they went.  If that more traditional approach to pre-tensioning yielded a performance advantage, that would pretty well negate any argument for modular springs because they were merely more convenient for the maintenance department.

…….. And so, if we are to get to the bottom of this, it seems Phoenix will need springs installed the old fashioned way. Tediously and with a stretcher.  Strand by strand by strand…..

Maybe.

I pulled the above scribble out of the swamp that is my picture archive.   It should be entirely possible to reduce the length of our next Orsova reconstruction to something like this.  I wonder if those angled struts would be better attached directly to the kamarion?  Perhaps the struts could attach at the point of those two holes we use to thread our sight string through.  Because the two holes in question are evident on the original kamarion they are a focal point for designs that can explain their purpose.  Perhaps if these holes were used to house a locking pin for a wedge,  and the wedge would induce a yoke to tighten around the kamarion ……..

Phoenix will be about the details.

I am going to try my hand at knifemaking.   This will lead to the destruction testing of lots of trial knives.  These will be made in a variety of carbon steels and with a variety of spring tempers.  This tangential advernture will make for some valuable experience when we set out to build the MK. XI set of limbs.  Those babies will explore a more historically accurate design for Phoenix’s limbs.  Something with high-tensile steel strapping on the back of the limbs. And ending in a hook.  A steel hook.

In the meantime, we have a little gear showing up next week……