		|==============================|
		|--------HATRA BALLISTA--------|
		|==============================|

		-by Paul Sountsov (SiegeLord)

Table Of Contents:

I: Contents of the ZipFile

II: General Instructions

III: Theoretical Considerations

IV: Some Specific Problems

V: Other Thoughts

VI: Legalese

-------------------------------------------------------------------------------

I: Contents of the ZipFile:

-3 Principal views of the base (BaseFront.png, BaseSide.png, BaseTop.png)
-3 Principal views of the frame (FrameFront.png, FrameSide.png, FrameTop.png)
-Detailed view of the Pouch/Release Mechanism (PouchAndRelease.png)
-Detailed view of the trigger system (TriggerDetail.png)
-3 Reference photographs (dwight gym4.jpg, dwight gym10.jpg, ballista.png)
-3 Reference views (View1.png, View2.png, View3.png)
-3d Rendering (Render.png)
-This file (ReadMe.txt)

II: General Instructions

Congratulations! You have just received the plans of the winning trebuchet
of the Massachussets state science olympiad of 2005, the Hatra Ballista. It has 
also been sighted at MA regionals of 2005 and the National Science Olympiad
of 2005.

This design is based on a recently uncovered wreck in Hatra, Iraq, of a ballista
frame. This frame was unusual for its uncharacteristic width, archaeologists
immediately surmised that this ballista was of the unorthodox "in-swinger"
type. This configuration allowed for greater draw length, and it was blessed
with a more favourable force v. displacement graph.

As the last of the true ballistas designed by me it represented the pinnacle
of SO ballista technology, its efficiency was stellar compared to its MA
brethren, and in the upper 10% of the national trebuchets. It suffered from
material imperfections, that no-doubt a dedicated builder can sort out.

The construction itself is relatively simple. Choice of wood is up to you, I
used pine quite succesfully, but harder woods like poplar will work even
better. Always use screws with predrilled holes. Avoid glue unless you are convinced that this design will be your final, add more screws if you feel 
that it is too wobbly. You will not that on all of the blueprints I have 
neglected to put any locations of the screws. Use your own judgement in screw
placement, aethetics do not matter much - all that matters is that they hold.

The blueprints themselves were regenerated from the supplied reference photographs
and as such do not truly represent the actual Hatra Ballista depicted in them,
I, however, tried my best to duplicate is as much as possible. You will note
that not every possible dimension is made in the blueprints, and that is
deliberate. The final form of the Hatra was made both by the design considerations
and the availability of raw materials, as such some connections may seem wasteful
and arbitrary to you. And they are, but they represent the materials I had to
work with. As such all of the designated measurements are the ones that are
critical for the design's workings. All of the measurements omitted are irrelevant
and will vary depedning on the individuals who are constructing it.

III: Theoretical Considerations

This ballista is a 100% theoretical efficiency device. The force diagram
in the input direction is identical to the force diagram in the release
direction. The physics of this design are relatively simple, they can be
readily approached with simple energy conservation tricks. By design all
of the energy of the counterweight is transfered to the torsion bundles,
and all of that energy is transfered to the projectile. Obviously in real
world there is friction and also the arms and the pouch are not massless,
so any residual motion in them is considered lost and is not transfered to
the projectile. Still given light materials in both this can be reduced
quite a bit.

IV: Some Specific Problems

If constructed well, and if checked thoroughly before every launch this
ballista launches perfectly and consistently. However flukes aside there
are some permanent issues with this design.

First, the friction of the release cart thing and the launch tray is significant
all of the later SiegeLords(IV, V, VI and VII) had wheels installed on it,
and that worked very well, they were not included in these blueprints as they
were absent in the original design.

Second, the internal friction in the torsion bundles is very significant,
I tried to alleviate it by lubricating every strand with grease but a lot
of friction remained. I do not see a way this can be remedied, and I see
that this is responsible for stealing at least 15% of this ballista's efficiency.
This is the sole great failure of this design.

Third, the thing is pretty heavy even compared to some FAT's. Make sure you
have friends(unlike me) who are willing to help you carry it. If not, consider
installing wheels.

Fourth, the plague of all of the ballistas and the SiegeLords is the stretchiness
of thestring. I used ice fishing line for most of my designs and it stretched
quite a bit. Since any stretching in the string between the release cart and the
ring for the counterweight never gets transfered to the projectile it is
especially bad in terms of efficiency. Also the release mechanism does not travel
the same distance with the different counterweights due to this stretching.
If it triggers fine with 1kg it will not trigger at all with 3kg, since the
string will stretch and the release mechanism will never hit the launch bar.
This is what happened at 2005 nationals. I have recently found some fishing
line of the SpiderWire brand that does not stretch as much, but it has low
tensile strength. Also I have found some sources of Kevlar fiber on the internet
but I have never took advantage of them, so I am not sure of their strength etc.

V: Other Thoughts

While I will be very pleased if anyone does indeed build this device, I warn
you that this device is not great and I see little improvement that can be done
with this specific configuration. Other configurations exist, and I shall be
building one more during this summer(2006). So if you feel that you will learn
a significant ammount by buiding it(I assure you that you will) do so, but
there is a significant chance that the newer design will supercede it in all
parameters. They should be similar in general construction, so it will not
be a complete waste of time to build this design anyway.

Also note that the size parameters are critical for this design, as you will
see this design is quite long and is quite wide. It *needs* 75 cm for its width,
it *needs* 80cm for its length. It simply will not work without that ratio.
Saying that if and when I receive the new rules, I will be happy to adapt
this and future designs to the new specifications.

I purposely left out some of the more specific instructions on its construction
as to stimulate your brains, and also I may have forgot some stuff. In any case
if you are stumped and/or curious at the construction feel free to contact me
through a PM at www.scioly.org (SiegeLord), or through email @ psuntsov@hotmail.com.

VI: Legalese

In the Event Rules of the 2006 Coaches Manual for Division C is says on page 3:

"Team members are expected to be the builders of all the devices used in the
events."

Thus it is perfectly legal to use this design verbatim in the competition
since you will be the actual builders of it. The fact that you will have
to design the components I have omitted reinforces that fact even more.

These plans are now considered your property and you may use in any way you
wish for both Science Olympiad and for profit or any other way you may choose.
I will require no mention of my name to be located on its manifestations, but
I will appreciate if the true source of the design is aknowledged during 
the inevitable questioning. I will doubly appreciate if you write its name on its
side - "Hatra Ballista". Regardless of the way you end up using it I wish you
the best of luck with it, and thanks for showing interest in my designs.