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Combat Robotics: Weapons & Armor
Written By Kyle Awner
Weapons and armor put the “battle” in BattleBots. There is a multitude of factors that go into the design of both, the most important consideration of how one affects the other. In this article, the focus will be on the material selection for weapons and armor. As a rule, harder weapons work better, at least until they’re so hard they’re becoming brittle. The larger a weapon is, the less hard (and less brittle) it should be. AR500 has become the preeminent material for heavyweight combat robots, both for weapons and armor. It exists in a comfortable sweet spot of toughness, workability, and availability. The trouble with making a weapon from the same material as much of the armor is it becomes much more difficult to deal out damage. Geometry will almost always be the most important factor in determining whether a weapon gets a bite on or through a section of armor. The angle of the attack as well as the supporting structure of both the attacking weapon and defending armor will come into play. Shallower armor will deflect better, but generally increases the footprint, and therefore the weight of the robot.
BattleBots Weapon Types
Sharp weapons are cool because they can punch holes in the armor but keeping those weapons sharp after multiple hits on similarly hard armor is about impossible. Some teams have taken to welding hard-facing rods to the impacting surfaces of their AR500 weapons to get (and keep!) that offensive edge. Team HyperShock has consistently chosen to use S7 tool steel for their sharp weapons and have found that 57 Rockwell C is a wonderful balance of hardness (offense) and toughness (defense). S7 tool steel is substantially more expensive than AR500. For the cost of S7 disks and looking at their average lifespan, Team HyperShock could almost use a fresh AR500 weapon in every fight for the same cost.
HyperShock runs S7 weapon disks every year, and harden them to 57 Rockwell C, which is about as hard as we can make them before the toughness falls off. Pushing the hardness higher than the surface hardness of AR500 (about 52 HRC) allows HyperShock to bite into the armor of most opponents. The visual effect of chunks being torn out of opponents also has great entertainment value. If a match runs out of time and goes to the judges, the more apparent damage that your weapon deals can sway more damage points in our favor.
Spinning weapons come in two main flavors: horizontal and vertical. Horizontal weapons tend to be much larger than vertical weapons. Due to cost and the likelihood of a fracture over the longer unsupported length, horizontals are almost always made from either AR500 or aluminum with steel impactors. Vertical spinners see more variety in material choices. S7 used to be extremely popular, especially before AR500 became so available from sheet metal service vendors. AR400, 450, and 500 are also extremely common, and there’s a handful of robots using 4130. Drum spinners are less likely to use AR steel since it is not available in a round stock or billet format.
Aluminum weapons that rely on impactors have become less common over the years as builders have seen the impactors fly off or the aluminum sections fail. In smaller weight classes, aluminum drums with steel impactor teeth used to be common. More recently, rectangular “eggbeater” weapons made from aluminum with flat-head screws as impactors have become extremely popular. Less experienced builders have sometimes been lured in by the siren song of Al 7075’s datasheets. Unfortunately, they often learn too late that it is just too brittle for a weapon, particularly large horizontals. Fortunately, 6061 is still an option, and it is more available, cheaper, and as a bonus, it anodizes beautifully.
Non-spinning weapons like lifters and hammers are commonly made from a few pieces of sheet metal, so AR500 is overwhelmingly common for these. In the case of hammers, the prevailing strategies will use the weapon dozens of times in a fight, so the chances of a sharp weapon staying sharp are low. Lifters that use a steel arm need that weapon to survive repeated hits from the spinners, so making it from a durable material like AR500 is an excellent choice. Some teams have taken to making their lifters out of heavy-duty polymers like UHMW so the arm can flex out of the way on a hit.
Armor choices in heavyweight robots are even more varied than the weapon material selections. The two most common materials across the board are AR500 and 6061-T6 aluminum. Bots using aluminum armor will generally rely on thick sections around the outside of the robot that will be deformed or carved into by an opponent’s weapon. Steel armor solutions take advantage of the surface hardness and use shallow angles to deflect weapons away.
BattleBot armor is often discussed in comparison to the thick armor seen on military vehicles, but they’re actually quite different problems to solve. Military vehicles are designed to resist ballistic (instantaneous) impacts, whereas combat robotics engagements have a lot more follow-through to any impact. Builders are also less concerned with their robot surviving the first hit than they are with the second and third hit when a spinner ‘juggles’ them across the arena with a flurry of hits. Those follow-up hits can catch unpredictable parts of the robot since the whole thing may be tumbling through the air. In addition to designing geometry that reduces the chances of another weapon getting a good bite, the construction and fastening methods need to be omnidirectional in their effectiveness.
When fighting a horizontal, particularly ones with large diameters, many teams will attach a thick plow to their robot. The design of a plow or wedge will vary, but there are three considerations that every team should have: shallow leading edges to deflect the horizontal weapon upwards initially, outward sloping geometry to deflect the horizontal away, and some kind of shoulder or wing overhanging the plow that prevents the horizontal from deflecting over the top of your robot. When a horizontal spinner becomes destabilized, there is a risk of that large weapon coming down on top of the other team’s robot, where the armor is probably thinner to make weight for the large plow on the front or back. AR500 is a common and excellent choice for defensive features like anti-horizontal wedges.
The armor considerations for fighting vertical spinners and “top-attack” weapons (hammers and overhead saws) are more nuanced. As always, very shallow geometry is recommended, but many of these types of robots will use long, narrow forks which are exceptionally effective at getting under wide, straight features. The risk becomes the vertical spinner catching the underside of the outer edge of that shallow geometry, or the top-attack robot scooping it up and slamming the other robot across the arena before smashing their weapon down on a now-stationary opponent. The primary limitation on armor is weight. As the robot gets bigger, the armor will get thinner. Alternatively, thicker armor can be used if the whole robot stays very small. Each team must find the balance that works for them. HyperShock side-steps part of this problem by relying on agility rather than armor for defense. As a vertical spinner, HyperShock shows up with the option to equip a thick defensive plot if matched up against a horizontal spinner.
Polymers like UHMW are sometimes used as “ablative armor.” This kind of armor is designed to be torn away or damaged while insulating the rest of the robot from the impact. It works as both a crumple zone and tear-away padding. Since they are less dense than aluminum, the available thickness is the primary benefit to the material. Spinners can only get so much bite, so lining the outside of a robot with inches of soft plastic can protect the delicate insides of the robot quite effectively. The material is also soft enough that a sharp spinner is liable to cut through the armor without getting a bite which would send the robot flying. In the judging criteria, damage done to ablative armor is only marginally more valuable than cosmetic damage, which is to say it barely scores as damage points.
Every year, a rookie team tries to use a “ballistically resistant” composite material as armor, like Kevlar-woven fiberglass or carbon fiber. As previously discussed, the impacts in robotic combat are not instantaneous points, they are broad and continuous, meaning the distributive capability of a fiber composite is completely nullified. Robot hits are more analogous to resisting a pick-ax swing than a projectile impact. Projectile energy primarily comes from speed, whereas the energy in a bot’s weapon is as much from the weight as the speed. There’s also continuous force being applied from whatever chain or belt transmission a robot is using, rather than the pure instantaneous force of a projectile colliding with an object.
Titanium is an expensive material to make armor out of, but the strength-to-weight balance sits nicely between aluminum and steel armor. Grade 5 titanium is by far the most common titanium alloy in combat robotics. The steps teams make between equivalent armors are often about half the thickness of titanium and can be swapped in for aluminum. Slightly less than half that thickness again will be the steel option. Aluminum sheet metal armor is minimally effective, so it is more of a cover than it is armor. It will resist some impacts, like a robot being rammed into a wall, but really will not stand up to a weapon until it is at least about a half-inch thick. What titanium lacks in surface hardness, it more than makes up for in flexibility. Teams like to use titanium against hammers since the energy of the hit is not usually enough to break through the material, but steel or aluminum in its place would likely dent. A handful of very well-funded teams use thick sections of titanium as primary armor, but the benefits aren’t staggeringly advantageous. It’s still fairly soft, so harder steel weapons can bite in and find purchase make their own handle with which to throw their opponent. Tossing an opponent through the air is a great spectacle and destabilizes them, creating opportunities for more devastating attacks. Thin sheets of titanium have been used to “skin” over thick sheets of plastic and polymer armor. The logic is the somewhat-hard, but harder-than-plastic titanium can flex into the dense UHMW sheet, absorbing the energy of a hit, kind of like a momentary crumple zone.
The variety of materials in combat robotics isn’t as broad as what would be found on a single airplane, but the selection of materials is extremely nuanced. The reasons a team chooses a given material may be dominated simply by the cost. The prevalence of on-demand manufacturing has opened up a lot of options for small batches of parts without having to buy large sheets or billets of a material. It’s not uncommon for teams to bring small quantities of various materials to events so they can manufacture emergency spares or replacement parts while on-site. The workability of these materials and the availability of machine tools at events vary, so the materials selected at the design phase are more or less what robot teams will be stuck with. Ensuring that these materials are of the highest quality is critical too. Teams have been badly burned by sourcing materials from dubious suppliers or using the lowest bidder manufacturers who use recycled or recast materials instead of fresh stock. The considerations of how a weapon material is going to interact with the various armor materials it will face are endless. The risks of a hard, sharp weapon against thick, tough armor are steep, but when it works, the results can be mind-blowing.
About the Author
Logistics Director, Team HyperShockhypershock.tv
Kyle has been a member of Team HyperShock since 2018. He handles the team's logistics both at home and at BattleBots and acts as a weapon operator during fights. When he's not destroying robots, he works as an engineering automation specialist for an aerospace engineering firm in South Florida.