Build Your Own Combat Robot

Chapter 10: Weapons Systems for Your Robot 235
If a carbon dioxide tank is used for the gas source, consider using buffer tanks
on the low-pressure side of the gas regulator to compensate for the limited conversion
rate of the carbon dioxide from liquid to gas. A high-pressure air or nitrogen
source will provide a greater air flow, at the expense of more room taken up by the
gas tanks. The most powerful spear designs use no regulator at all, instead running
full-pressure carbon dioxide straight from the storage tanks. Although this approach
overcomes the gas flow problems by running at a much higher pressure, it
is difficult and expensive to implement safely.
note In the comparison of carbon dioxide and high-pressure air (HPA) (or nitrogen), it’s
true that HPA has an advantage in flow because there is no phase change from liquid to gas;
but when using HPA, large bore tubing and valves and a downstream accumulator are still
essential elements to achieving high flow in a system. Using HPA with small-diameter tubing
will still have significant flow restrictions and less-than-optimal performance. This discussion
also applies to the air flow discussion in the “Hammer Bots” section earlier in the chapter.
Another approach is to use a powerful spring to accelerate the spear. This approach
has the advantage of the spear doing most of its acceleration in the early
part of its stroke. The disadvantage of this concept is the need for a complex mechanical
re-cocking system to crank the spear back and latch it in place until it is
needed again. A long re-cocking time on a weapon makes that weapon nearly useless,
as the opponent can freely attack while the weapon is re-cocking itself. A
third approach is to use a crankshaft to drive the spear to convert a constant motor
rotation to reciprocating forward and backward motion of the spear. While it is a
less-complex approach to the spear weapon, crankshaft drive spear weapons tend
not to be effective in practice. The spear will reach its maximum speed only at the
middle of its travel, and will actually be decelerating for the second half of its
travel. Furthermore, on striking the opponent, the weapon will either stall and be
unusable or push the other bot away and ensure that the next impact between the
spear and the target bot will be near the end of the spear’s travel—where it will be
traveling slowly.
The best head design for penetrating armor is a three- or four-sided, thin, pyramid-
or diamond-shaped head. Conical points are less effective at penetrating armor;
the head should have sharp edges so it can cut open rather than force open the armor
material. The downside of effective penetration is that the spear head may get
stuck inside the target robot after being fired, jamming the two robots together
and risking damaging the spear mechanism as the target bot struggles to get free.
One possible way to minimize the potential to get stuck is to machine the entire
shaft to slightly increase the diameter of the spear toward the robot’s body. Some
teams use deliberately blunt weapon heads, hoping to knock out the opponent
through impact damage rather than penetrate armor.
Maximize the spear velocity to get the most effect. Mass of the weapon head is
less important than the speed at which it travels.