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![Fifteen Years of Ask Aaron [Click Me]](pic2/Ten_2.gif)
Team Run Amok receives a lot of email about designing and building combat robots. In 2003 my son and team member Aaron Joerger (then 12 years old) asked for a question and answer page to document our responses.
We welcome combat robot questions. Check the Ask Aaron Archives first to see if your question has already been answered, then click the blue button.
Click to browse thousands of previously answered questions by category, or search for specific topics. Includes  
Even small combat robots can be dangerous! Learn proper construction and safety techniques before attempting to build and operate a combat robot. Do not operate combat robots without proper safeguards.
Q: How to activate flipper with no sensors [Dublin. Ireland]
A: [Mark J.] I'm not sure what you're asking. SOMETHING has to sense the right moment to fire your flipper mechanism and that something is by definition a 'sensor'. The Hamburger is Bad.
Typically it's the driver who watches and taps a transmitter button to activate the flipper. If you're trying to fire automatically without electronics, it's possible to use an entirely mechanical sensor trigger like Dale Heatherington's Flip-O-Matic.
A professional golfer and a blind musician are in a bar. They chat for a while and the topic of golf comes up.
Q: Adding to the earlier Melty Brain question; do Meltys count as an active weapon? If I'm interpreting the SPARC ruleset correctly, they would classify a Melty Brain as a passive weapon. I'm only asking cause I've been to a couple tournaments before with an active weapon rule and I want to make sure I can still compete. [Yuba City, California]
A: [Mark J.] Although there is no functional difference between a Melty Full-Body Spinner and a conventional FBS, the guidance given by the current SPARC Robot Construction Specifications v1.3 is open to interpretation by Event Organizers. The rule often cited in the SPARC ruleset is in the 'Sportsman Class' section 4.2:
Q: Hi Mark, me again! Working on my antweight wedge bot and I had a clarification on some of the the information on the Rio Botz Combat Tutorial. On page 213 it states:
A suggested value for the side angle α’ to launch spinners would be, for a smooth titanium wedge, equal to α’ = αlaunch = 37 degree. And, for a hardened steel wedge, which has a larger coefficient of friction than smooth titanium, a suggested side angle α’ to launch spinners would be Leonardo Da Vinci's αlaunch = 34 degrees.
In addition to the above section, on page 211 it provides a reference of μb ≅ 0.5 for a battle worn titanium wedge while a new wedge would have a μb ≅ 0.3.
For my antweight, I'm planning to use a 6061 or 7075 aluminum (cost/weight considerations) mounted to a 3D Printed nylon base wedge. I'd like to get some clarification on what would be a good starting points for μb for this calculation for 6061/7075? Is there a good way to figure out the μb for a given material without using approximations? On the same note, lapping and polishing the aluminum to say a mirror polish would help reduce μb right? [Arlington, Virginia]
A: [Mark J.] I'm glad to see that you've been studying the RioBotz tutorial, but there are a few odd things about the RBT you should know:
In this case: "Why didn't they mention aluminum in the wedge section?" They didn't mention aluminum because all of their calculations assume that the surface hardness of the spinner and wedge are somewhat comparable, like steel and titanium. Aluminum - even aircraft grade aluminum - is MUCH softer than steel. A steel spinner weapon will dig down into the surface of the material and grab, making all the calculations moot. The rule they forgot to include is:
The RBT does not mention this because as trained engineers they know this so well that they assume everyone knows it -- like the sun rises in the east.
Q: I have been considering building a Melty Brain spinner for my next design. Before you freak out, I am well capable of managing the electronics, my question is more about the physics. Does a Melty Brain spinner offer a significant increase in kinetic energy out compared to a more traditional full body spinner? My gut says yes however i've seen robots like this hockey puck much more than shell spinners and it looks like more energy ends up wasted on impact.
A: [Mark J.] For the benefit of readers unfamiliar with the concept of the 'Melty Brain': Wikipedia article on Translational Drift.
It's important to draw a distinction between 'kinetic energy stored' and what you're calling 'kinetic energy out':
Q: Hi Mark, I'm working on my antweight wedge and I didn't find an answer on the archives for this. To give some background, this is a 4WD design that's using gearhead motors with the wheels directly attached to the motors. With the way I have the motor mounts setup, I could angle the two front motor mount in such a way to add a degree or so of toe in/toe out. In cars, toe-in helps improves steering response on a corner exit and stability under acceleration while toe-out will improve steering in corner entry but can cause instability under acceleration. Does it make sense to add 1 degree of toe in for each front wheel on a combat robot like this? Or should I leave it at 0? I saw camber was discussed in relation to Donald Hutson's designs in the archive, but haven't seen anything on toe in/toe out. [Arlington, Virginia]
A: [Mark J.] A crossover question! I know a bit about automotive suspension design from my involvement with vintage British race cars: my hobby car.
Combat robots do not turn like cars.
Q: A joule of kinetic energy is equal to a watt-second of electrical output, so if I have a spinner weapon that stores 1200 joules of energy and I power it with a 400 watt brushless outrunner motor it should take
A: [Mark J.] I see this overly simplified equation popping up on the 'net more and more often. The short answer is no, because a "400 watt" electric motor does not produce full output power over the entire speed range from zero RPM to maximum RPM as it spins up a rotary weapon.
The full answer is too long for an Ask Aaron post, so I've published a new page that provides a detailed explanation of brushless motor power output and the damands of spinning up a rotary weapon: Estimating Weapon Spin-up Time.
Q: Apologies in advance if the hamburger is bad here but how would I go about mounting a brushless outrunner to a P61? I've only used 300 series motors before and the installation was pretty straightforward. Im not looking for specific help but more so a jumping off point in where to begin, thanks. [Los Angeles, California]
You might also find Robert Cowan's video on preparing outrunner motors for robot combat duty useful: Battle Hardening Outrunner Motors
Mounting the brushless motor is the easy part. Setting up the brushless ESCs is where it gets tough. I'm sure glad you didn't ask about that...
Q: I've seen screws drilled directly into the UHMW frame instead of being attached via NutStrips. How can I attach screws directly to the frame like the robot in the picture? Do you need a hole tapper? [Arlington, Virginia] 
Q: Hi Mark,I've been watching combat robotics for about 5 years now and I've finally decided to take the plunge and make my first combat robot. I was thinking of making a featherweight bar spinner, very cliche but in the same sort of vein as 'Suitcase Nuke' but instead of having a solid bar it would have a dense moveable mass probably using a spring to keep it in at low speed and then the centrifugal force would force it further out at higher speeds, theoretically meaning it can spin up faster but still hit hard at full speed. Has this sort of thing been done before or do you think that it would be too complicated for what it's worth? Matt (Different from the other Aussie Matt I've seen on here) [Melbourne, Australia]
A: [Mark J.] Hi Different Aussie Matt. Glad to have a new builder joining the ranks. Ask Aaron was started to answer questions from new builders and help them avoid common mistakes that discourage first-time builders. If your first build does not go well it may be your last build. Three comments:
Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.
Q: Would a bar of 1/2" 6061 work for a Hobbyweight spinner? I would want to use steel however all I have to machine with are power hand tools. [Roseville, California]
A: [Mark J.] That's like asking if a spruce 2X4 to would work to build a house -- the answer depends on how you plan to use it. You're not telling me enough about how you plan to use your bar of 1/2" 6061 aluminum for me to comment on its suitability for your unstated design.
Typically I would direct a question like yours to two resources at Ask Aaron:
Q: The Bristol Bots Builders shop sells an N-20 motor with a built-in ESC. They say it gets rid of a lot of wiring clutter in a 150-gram 'bot, but I don't understand how it wires up. What does the wiring diagram look like for two of these motors? [Reddit]
NOTE: Yes, you can use the same trick robot builders use to run the VEX 29 ESC at higher voltage than the receiver -- but that would ruin the tidy wiring layout and defeat the whole purpose of the compact motor/ESC package.
Q: Hi Mark, it seems like hydraulics are only used in heavyweights. I am wondering if you know of any lower weight class robots that make use of hydraulics? Maybe something that used parts from LESU/RC construction equipment? [Arlington, Virginia]
A: [Mark J.] Hydraulic robots are uncommon in any weight class because of weight, complexity, and fragility. Hobby grade hydraulic systems of suitable size for sub-light combat robots are designed to make scale model constuction equipment operate in convincing and realistic slow motion (LESU Video). A LESU hydraulic actuator provides a maximum 110 pounds of force: control valves are too small, the pumps and actuators have very limited pressure capacity, and the system is both heavy and unsuited to combat shock loads.
Hello Mark, I come back once again to, our pneumatic robot is in the test phase but we have a problem with the pressure;
We have a 0.5 litre compressed air cylinder with 4000 psi the cylinder is fitted with a paintball regulator set for 160 psi, but the circuit has only 9 bar of pressure and for only 4 shots (despite this the cylinder remains at 3500 psi) are there more suitable regulators? (air flow and outlet pressure)
A: [Mark J.] Hello, Jean Pierre.
I went into the archives and found your prior questions to refresh my memory on your design. You're building a 20 KG pneumatic flipper, but I know little more about the pneumatic design you've settled on: ram size, valving, buffer tank, etc. You also did not give any information about the paintball regulator you have found to be unsuitable.
The most popular small pneumatic regulators used by US builders are paintball units from Palmer Pursuit. Palmer makes a wide range of high performance regulators adaptable to a variety of fittings and layouts. Leave them a note with your requirements and they will help you select a suitable model.
If low flow rate thru the regulator continues to be a problem, you may want to include a small 'buffer tank' as described on the Team DaVinci Understanding Pneumatics page to accumulate a reserve of low-pressure air.
I am continuing to go through your website and others as part of my research and I'd like to get some input from you on an observation I've made. For ant/beetles that make use of brushless motors, it seems like many people are using ESCs and motors designed for quad copters. I have a background in quad copters as well as 1/10 and 1/8 scale RC racing buggies. From my experience, the brushess motors used in RC cars (and boats) are a lot more durable (i.e typically larger shaft bearings, shafts, etc) and and can take a lot of hard running. My question is why aren't people using brushless motors and ESCs designed for RC cars in their combat robots? These certainly do weigh more, but I'd imagine the durability and power makes up for it no? I feel like I'm missing something here. [Arlington, Virginia]
A: [Mark J.] Have a read thru the Ask Aaron Brushless Motor Selection Guide. Briefly:
Q: I’ve never 3D printed a robot part in my life, but I want to start. Is there a buyer's guide for 3D printers specifically for making robot parts? The general public-facing reviews tend not to cover that topic. [Woburn, Massachusetts]
A: [Mark J.] Talk to the printers on the Facebook Robot Combat group. The group is your best source for up-to-date information on 3D printers for use in combat robotics. Factors like price, bed size, and the ability to print specific filament types are common discussion topics. Printers given high marks by the group include, in no particular order:
Q: Hi, I'm currently designing a 3D printed ant weight (US antweight mind you, 1LB/~450g). I was planning to use NylonX/NylonG for the chassis, however I recently came across Markforged and their continuous Carbon Fiber printing which claims to be approximately equal to 6061-T6. I have been able to find a local rapid prototyping shop which can print with this material for a small price (more expensive than printing it myself with NylonX though). I looked on your website and similar sites to see people's experience with this material in combat robots but I haven't seen any solid details. Most deal with regular chopped CF Nylon Onyx version which is similar to NylonX, not the continuous fiber version. Are you aware of any ant/beetle weight (or even larger) bots that make use of this material? [Arlington, Virginia] A: [Mark J.] The Facebook Robot Combat group has an active and opinionated corps of combat chassis printers. Some members there have used continuous fiber reinforcement in printed insect chassis -- but the preferred fiber material is Kevlar over Carbon for it's better combination of strength and impact resistance. One of the members is a Markforged employee who had this to say about reinforcing a Markforged Onyx beetleweight robot chassis with continuous fiber: Markforged guy here -- Onyx is mostly Nylon 6 so it's pretty impact resistant in general. The chopped fibers don't add much to strength (they're mostly for printability) so if you're looking to add impact resistance/strength past the base Onyx with continuous fiber I'd suggest looking at reinforcing those prints with our Kevlar filament. It's much more energy absorbing than the Carbon Fiber, while still adding a good deal of strength.
If you'd care to join the Facebook group, I'm sure the printers there would like nothing more than to discuss fiber, infill, drying nylon, base temperature, and all the other variables to consider in printing the ultimate insect chassis.
Short fibers add max 1.5-2x UTS over virgin nylon 6 or maybe 1.25-1.5x vs ABS, whereas by adding continuous fibers you're looking at 13-15x UTS for FG/Kevlar and 23x for Carbon Fiber. At that point though you need to look into Izod impact test results too. Q: I've been looking into receivers and I'm confused about "supply voltage". My current robot uses a 6CH AFHDS Receiver with a stated supply voltage range of 4v to 6.5v, but I run it on 11.1v and it seems fine. For my next robot I'm thinking about using a Flysky FS2A with a stated supply voltage range is 3.3v to 10v, but I want to run it on 14.8v. Is that gonna work, or am I gonna smoke the receiver? Thanks! [Anonymous]
A: [Mark J.] In most combat robots the receiver is not directly connected to the battery; the battery pack connects only to the Electronic Speed Controllers (ESCs). Most robot ESCs and some aircraft ESCs have a 'Battery Eliminator Circuit' (BEC) that sends a nice, steady 5 volts out to the receiver via the 3-wire receiver leads, while passing the full pack voltage to the motors on demand. I suspect that is how your robot is wired.
If none of your ESCs have a built-in BEC you can purchase a 'stand alone' BEC that connects to your battery and plugs into your receiver to provide that same steady 5 volts. Search for 'BEC' in the Ask Aaron Radio and Electrical archive for more information on battery eliminator circuits.
Q: What's the difference between Banebots' Sport Gearboxes vs their P61's? [Lincoln, California] A: [Mark J.] The BaneBots 'Sport' gearboxes are designed for FIRST Robotics Competition (FRC) builders. FIRST robots may weigh as much as 150 pounds, and the gearboxes may be subjected to destructive stalling at high torque loading -- something uncommon in the drivetrains of combat robots.
Compared to P61 gearboxes of the same reduction ratio, the 'Sport' gearboxes are a little larger, a lot heavier, and can survive much greater output torque loads. The lighter weight and longer shaft of the P61 makes it a better choice for use in combat robot drive trains, but the Sport gearboxes may be useful for combat lifting or clamping weapons. Note: Some ratios of the Sport gearbox are available in a "heavy duty" version that increases max torque output from 110 ft-lb to 140 ft-lb -- see the BaneBots website for additional information. 
Q: What components do I need to consider if I want to connect a motor, a drive belt and a wheel. [Brighton, England]
A: [Mark J.] Another school assignment, Brighton?
== See Next Post Down ==
You're welcome to use the Ask Aaron archives for your homework, but I'm not going to write essays for every schoolboy in southern England.
Something's going on in the UK and I think 'Ask Aaron' is being abused. I'm getting multiple versions of very specific robot design questions about the same layout from the same region of southern England. This sounds to me like a school project assignment, and I'm not interested in doing homework about vaporbots for all of Blighty. See for yourself:
Q: Hi, i have 2 questions to ask
A: [Mark J.] What's going on? Scan down the page and read the last few questions. I'm seeing a theme here...
The short answers to your questions are:
Q: I need to find a motor that will generate 0.2Nm of torque per wheel for a fightbot. The back 2 wheels will be direct driven from 2 motors, and the front 2 will be belt driven. I have a £50 budget so i know gearing will be required, could you help me choose a good ratio and also how I can find a suitable motor? what is the best type of wheels for a small robot and which material is best? [Brighton, England]
Once you have determined your actual torque, speed, and voltage requirements you can look thru gearmotors at on-line robot suppliers in your country -- like 'Robotshop'. A quick search there found a 12V 970RPM Econ Metal Gearmotor that may meet your requirements and budget.
About wheels and tires: There is no single 'best' wheel/tire type. How 'small' is your robot? What type of surface does the arena floor have? How important is traction versus durability? Will the wheels be exposed to direct weapon impacts or are the wheels protected by armor?
There are dozens of posts on wheel and tire selection in the Ask Aaron Materials and Components archive, and for very small robots there are additional posts in the Ants, Beetles, and Fairies archive. Search there for guidance on wheel/tire selection.
Q: i have to produce a combat fighting robot and my part is weapons electrical, and therefore what calculations do i make as my robot consists of two axes on the sides of the robot and a spinner blade in the middle [Edgware, England]
A: [Mark J.] I can't teach you Mechanical Engineering in a few paragraphs, Edgware -- but I do have a collection of on-line tools and Excel spreadsheets here at runamok.tech that can help with your design calculations:
What weapons are most effective in robot combat? The answer may surprise you: What Weapons Win?
Q: Hi, if i am building a robot that is 3kg, is there a way of choosing the right wheel radius using calculations or do i just make an assumption. [London, England] A: [Mark J.] There is no single 'right' wheel radius for a robot of a given weight. For a specific motor and weight, the correct drive train will be a function of wheel radius and gear ratio: larger wheels require greater gear reduction, smaller wheels require less gear reduction. See the post immediately below for links to equations and tools to select the correct combination of motor, gearing, and wheel radius. Q: Hi I am doing a project where I am building a fighting robot and was wondering what the best way is to choose a motor with calculations. The only thing I have to work with is that the mass of the robot is 3kg. What equations do I need to consider [Brighton, England]
A: [Mark J.] I've got a whole webpage on that topic, Brighton: Optimizing Combat Robot Drivetrains.
Once you have the theory down you can automate the selection of drive motors and gear ratios with the on-line Tentacle Drivetrain Calculator.
If you have trouble with the Tentacle Calculator, I have a step-by-step Example Drivetrain Analysis.
Q: Hi! My team is building a 15-lb bot. We are going with a kiwi drive holonomic setup and a drum spinner, all powered by brushless motors. 
We have calculated that we can use Propdrive 2836 2200kv outrunners with 20:1 gearboxes for our drivetrain, and the same motor with a 2:1 belt drive for the spinner. Since all 4 motors are the same, could we run them off one of the 4 in 1 quadcopter ESCs? (With the requisite firmware flashing, control board, etc.) Something like this. These ESCs are designed for quadcopter use where there is plenty of airflow, but we figure we can manage heat by mounting it to our aluminum chassis with a thermal pad.
Thanks for your help. [Cambridge Massachusetts]
A: [Mark J.] Massive wheels... Tiny drum... But you only asked about the ESC, so let's talk about that.
Yes, you can run all the motors from a single 4-in-1 quadcopter ESC, and the motors don't even need to all be alike. I know of several small 'bots running two drive motors and a dissimilar weapon motor from a single compact quad ESC.
NOTE Quadcopter ESCs in general do not use the common 'PWM' receiver output protocol with one three-wire connector per radio channel. The specific ESC you are considering uses the 'DShot' serial protocol, so you will need a receiver with that type of output. Check the requirements of any quad ESC before you proceed.
Update - I asked some builders familiar with Quad ESCs about your choice. It seems the APD f-series ESCs may not be programmable for reverse operation. Another builder suggested the Racerstar ReachUP 100A, but I don't have confirmation on usability. Stay tuned.
Q: I give up. How does it work? [Multiple Requests]
A: It's called a Killough Platform -- similar to an omni-wheel in action, but different in structure. The two wheels in each of three cradles are connected by gears to each other and to a drive motor which can rotate the wheels while they remain oriented at 90 degrees to each other. One of the pair of wheels is always in contact with the floor as they rotate and 'walk' the platform along. The wheels remain free to spin on their own axles and roll sideways to comply with motion imparted by the other two cradles. You can see the action clearly in this video.
Q: What do your combat robots think of the current COVID-19 pandemic? [Kansas City, Missouri]
A: [Mark J.] My robots don't care. My robots don't spread, suffer from, or die from Covid-19 -- but you can. Don't be selfish. Follow the science. Stay safe.
Q: how can robots help us deal better with hurricanes and why? [Ontario, California] A: [Aaron] Few people in Nebraska are threatened by hurricanes, so send a swarm of killer robots into low Atlantic and gulf coastal areas to drive the puny human inhabitants toward Nebraska. Problem solved. Robot haiku:
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One advantage that Meltys have is their mechanical simplicity. There is no separate spinner shell support, no weapon motor, no gearboxes -- just one or two direct-drive motors and the exotic electronics. That's worth something!
A: [Mark J.] The process is very similar to mounting a brushed motor. Here's a bare-bones description based on info from builder Emmanuel Carrillo:
Unless you're prepared to design, manufacture, test, and certify your own components -- like Team Whyachi does for heavyweight 'Hydra' -- you are unlikely to piece together an effective hydraulic weapon system.
That 5 volt power feed falls within the 'supply voltage' requirement of all hobby-grade receivers, and that is the voltage your receiver will 'see' as long as your main battery voltage remains in the range accepted by your ESC.
