Prompt: Boston Dynamics is a company that specializes in producing robots that can really do the unimaginable. Take Sand Flea. Sand Flea weighs 11 lbs and can jump to heights of up to 30 ft. Such a feat represents a very attractive design goal across all industries for the coming years: the ability to store massive amounts of energy and release it very quickly. Considering that these high energy-density type systems are in fact the way of the future, using what you know about energy come up with either at least 3 ways of storing/quickly releasing a large amount of usable mechanical energy or come up with one way and describe at least 3 ways to look at your solution.
There are countless applications for these kind of high energy-density storage, quick release type systems; however, in this post, we will simply be looking at a few design considerations that a mechanical engineer would have to account for if he/she wanted to design a robot capable of jumping to great heights by taking advantage of the expansion associated with the release of compressed gases.
As shown above, Sir Isaac Newton’s 3rd Law of Motion essentially states that for every action, there is an equal and opposite reaction. This simple statement has profound complex consequences in the real world. If you have ever seen a spaceship lift off, felt the recoil from a firearm, had a hammer bounce back at you after hitting a nail, or wanted to see what would happen if you were to use a fire extinguisher as a booster while sitting in a rolling chair, you have seen the phenomena that Newton’s 3rd Law of Motion is referring to.
Now in reality, things get much more complicated as there are many inter-dependencies that affect these reactions (i.e. the reaction of air resistance on the skin of the spaceship, the rerouting of gases in automatic weapons to chamber the next round and reduce recoil, the elastic properties of the steel head of the hammer, the rolling friction of the chair wheels or that associated with the bearings of the wheels, and the list goes on…).
So getting back on topic, what would we need to consider if we wanted to design a robot that used compressed fluids to jump?
As a class of fluids, gases are much more compressible than liquids; this fact, in combination with the tendency of gases to fill the shape of their containers (or flow paths) makes them ideal for use in high-compression fluid storage to be used for imparting large amounts of kinetic energy to objects via momentum transfer from their expansion kinetics. By storing a large volume of gas under heightened pressure within a properly suited pressure vessel (PV), large volumetric expansion ratios/rates can be realized when the gas is released. This gas expansion leads to massive amounts of energy being released very quickly from these areas in which massive amounts of fluid can be stored in a very compact space (thus high energy density storage and fast release). The energy and momentum associated with this gas can be used to propel our robot up and into the air, but know what? Did we intend to design our robot to be single use or do we want it to be able to jump again? We need to find a way to refill the system.
The means by which these systems are refilled or “recharged” is most often dependent on the kind of gas being used. In order to replenish your energy stores in an air-based system, you could simply pull the air from the atmosphere using an air compressor (although this becomes significantly harder when you are for example 500m under the ocean’s surface). Another complication associated with this method is the fact that an air compressor may not be ideal for our application on the basis of size, weight, vibration, noise, power requirements (as the air compressor will up the robot’s power requirements and thus decrease its run time for the same amount of power supply or even necessitate the use of all new control circuitry), etc.
This situation is complicated even further when the gas being used is not air, as now you have to store all of your gas reserves unless you can find out some way of extracting the gas you need from the atmosphere, which is simply not feasible, especially on such a small scale. So how do we get around these limitations?
Many air-based systems use air-pistons in order to recharge their compressed gas reserves as anyone who has ever had to pump a BB gun or water gun has experienced. The mechanism for actuating this piston is not always something as obvious as the lever action on your Daisy Red Ryder BB gun (remember to always practice proper gun safety, lest you shoot your eye out…referencing the movie A Christmas Story). Some systems may have a protruding pin that (when depressed) cycles a small pump compressor or reciprocating piston to recharge thy system. In some applications, exhaust gases may be re-routed and used to actuate the compressing mechanism. Technological creativity knows no bounds and, as such, it is more realistically finding a way to recharge a system comes down to design time, costs, and other system-level tradeoffs. Would it not be easier if we simply did not allow the gas to ever leave the system in the first place?
We could create a closed-system by using a highly elastic material to cover the exit nozzles of our robot, so that when we released the gas, it would rapidly balloon out and propel our robot through the elastic collision interaction mechanics between the gas-filled elastic material and the ground (think Newton’s 3rd Law of Motion) without expelling the gases to the atmosphere. To better understand this, visualize taking one of those squeeze toys where the eyes bulge out (see below) placing the toy with the eyes against the ground and then filling the eyes with a ton of gas really rapidly and watching the thing fly up in the air! This is just one example of the open-minded nature of engineering problem solving. The only bummer would be that in reality, this feat would likely be far more difficult than stated here and would likely require the development of a custom material with the proper amount of elasticity or “stretch”, tensile strength, strain-recovery time (how fast the material returns to its original size/shape), and would likely need include stiffer/more rigid pads where contact would be made with the floor so that the material could transfer more of the gases kinetic energy to the ground (rather than simply absorbing all of the gas’s energy through the material’s ductility).
In order to bring this article to end sooner rather than later (sometimes I get carried away and don’t realize how much I’m writing, so my apologies!), I will quickly move through this subject for consideration as well as the 3rd one.
Most gases would experience significant thermal energy generation under large compression ratios and thus the surrounding system would likely require cooling to prevent damage to the electronics or to the user; this cooling could be accomplished by passive controls such as extended surfaces/fins or forced heat transfer by way of a fan – the cheapest active thermal control. You have likely seen both of these types of systems mounted to your computer’s CPU.
Mechanics of Materials
A very noteworthy consideration in this application would be the strength of the pressure vessel (PV). Depending on gas used, compression ratio, allowed temperature rise and transmittance of impulse (high energy transfer over small time period) to the PV, hoop stresses (circumferential) can become significant and therefore require thicker PV sidewalls, use of stronger material for the PV, or rib reinforcement of the PV.
So anyways, there is a long-winded look into just a few of the many, many things a mechanical engineer might consider in their design of a robot capable of jumping to great heights using compressed gas. One of the many beauties of mechanical engineering is the fact that it is such an incredibly open-minded field that requires creativity, intelligence, determination, and the realization that there is never one right answer.