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On ‘Grapeshot and Grenadiers’: An Engineering Primer

28 March 2020
Emerging Technologies
Robotics & Autonomous Systems
Shiny steel robot humanoid in leaning forward running stance

Recently the Land Power Forum published an article titled “Grapeshot and Grenadiers: Winning the Counter Robot Battle.” That article rightly raised the prospect of future combat involving a mix of human and humanoid robotic combatants. However, while the sentiment of that piece was topical and timely its main argument was highly speculative, resting on multiple untested assumptions.

This response seeks to explore some of the design considerations facing any engineer tasked with designing the type of humanoid robot described in ‘Grapeshot and Grenadiers’. The aim is to inject some engineering rigour into the discussion and contextualise the design constraints involved in such an undertaking. This response will not discuss potential tactics that these systems might employ.

Physical Design and Layout

It is important to understand from the start that the same trade-offs (in terms of mobility, firepower and protection) that shape armoured vehicle design will apply to any future humanoid robot. There is also likely to be an upper size limit above which extant anti-armour weapons will be able to economically engage these systems thereby negating some of their asymmetric advantages. This implies that any humanoid robot must be roughly human sized if it is to make existing anti-armour weapons uneconomical to employ.

Being roughly human sized will place relatively fixed limits on the total volume of the system (if it is to remain humanoid and not evolve into a more generic armed UGV). Within this volume critical subsystems will need to be housed (power, processors, motors, mechanical actuators, communications equipment, sensors, etc).

In terms of body design, optimal weight distribution generally requires placing as much mass as possible close to the centre of mass. This helps to minimise the mass moment of inertia around the three axes that enable it to spin/pivot quickly and maintain balance more easily.

When considering protection, burying critical systems as deeply as possible within the structure provides a degree of passive protection (a principal employed in combat aircraft design). In a humanoid structure this drives the designer toward burying the most critical systems within the torso section. To do otherwise risks a design that has sub optimal mass distribution, requiring more energy and more sophisticated control systems to achieve the same movements with an increased drain on power. It would also mean critical systems would be more exposed thereby requiring more armour for equivalent levels of protection. This in turn would increase mass, decreasing performance and endurance (for equivalent power systems).


Due to the size and weight constraints explained earlier, the ability to fit a humanoid robot with protective systems is limited. For example, there is unlikely to be the space, power or weight available to support the addition of a defensive aid suite (like those currently found on tanks) without significant advances in miniaturising those systems first. Even an unarmed early warning system using a small radar to track incoming threats will come with significant power, space and weight requirements. While not impossible, significant technological breakthroughs are required before this kind of approach becomes viable. This is on top of the development of the robotic technology itself.

Now consider potential armour materials. While research is ongoing into new armour materials and concepts (such as shear thickening fluids) most candidate materials remain immature with significant cost or technological barriers impeding their development and uptake. Despite this, current armour technologies exist that would likely be suitable in the short to medium term. Materials currently used for body armour systems such as ultra-high molecular weight polyethylene (UHMW PE) and reaction sintered silicon carbide could be used and provide reasonable protection to the critical sections. Fabric armour systems such as Kevlar could then be used to line the extremities, if it was considered necessary to protect mechanical components.

Regardless of what is used, the inherent design compromises and cost constraints will likely drive the designer toward a lower cost/lower technical risk option. This would likely result in existing proven technologies being used.


Given the previous discussion of possible design layout and protection measures, it is worth considering what possible vulnerabilities of humanoid robots exist. Consider how their bodies and systems might respond to blast pressure waves and projectile (bullet or fragmentation) penetration. As these are the common threats currently faced by dismounted soldiers, they are well worth considering.  

Blast waves are a threat to most systems due to the extreme pressures generated and the speed at which they travel. Robots are no different and will be similarly vulnerable as the blast pressure overmatches material yield strengths by an order of magnitude. Unfortunately, though this is only true for objects in close proximity to, a detonation as blast pressure decreases exponentially with radial distance from the detonation, so any lethal effect has minimal range. This precludes robots charging through exploding artillery with total invulnerability.

Despite this, small scale detonating munitions on the scale of anti-personnel mines may become an option, at least technologically, for delivering this effect in the future. However, their use would raise significant ethical and legal concerns given the current legal status of anti-personnel mines. A less controversial option might be to pursue penetrating munitions.

Robotic systems may not be vulnerable to penetrating munitions in the same way as humans but they are still vulnerable, particularly if penetration promotes internal fires or explosions. To that end the development of new ammunition natures for extant small arms systems may provide a viable path to deploying a counter robotic capability relatively quickly while retaining the extant anti-personnel capabilities of those systems. The internal ballistics (in the weapon system) would be little different to firing blank, ball or tracer ammunition.

Examples of possible solutions are incendiary armour piercing rounds designed to penetrate the robot’s structure and then cause electrical fires internally. Another option would be various forms of explosive rounds, although this would require the development of miniaturised fuses. However, this approach also carries legal and ethical concerns,  as the technology to develop these rounds has been subject to legal controls since the St Petersburg Declaration of 1868.


‘Grapeshot and Grenadiers’ was rightly concerned about the prospect of autonomous humanoid robots in combat. However it is important to contextualise this against the likely engineering constraints and drivers that will shape its development. Many of the design and cost constraints that apply to current technologies will continue to apply in the future and the laws of physics are inviolable. By keeping this in mind it is possible to draw reasoned inferences about what future systems might look like and how they might then be countered.

The views expressed in this article and subsequent comments are those of the author(s) and do not necessarily reflect the official policy or position of the Australian Army, the Department of Defence or the Australian Government.


Matthew Struthers on 7 April 2020 - 5:12am

Editor's note: This Land Power Forum post is now open for discussion.