Shields: Physical Impact Issues

Written: 2002-05-04
Last updated: 2003-04-20

Introduction

It is widely assumed that if a sci-fi shield can withstand X joules of energy from a laser, it must be able to withstand X joules of energy from a physical impactor. However, this is not necessarily the case. As attractive as the simplistic numbers game is, if we apply a little bit of physics knowledge to the situation, we can see that if anyone were to build such a beast, the situation would be more complex than that.

So what would make physical impactors more dangerous? The answer to that question comes down to damage mechanisms. To put it simply, a physical impactor inflicts damage upon its target in a variety of ways. While an energy weapon will generally attempt to heat the target, thus permitting specialized one-dimensional defensive strategies, a large, fast-moving physical impactor presents a more complex threat:

Threat type

Damage mechanisms

Energy weapon (laser, phaser, turbolaser bolt, etc.)

Heats the target surface.

Physical impactor (asteroid, high-velocity ramming attack, hyper-velocity railgun, etc.)

Subjects the target to severe structural stresses, usually resulting in penetration. If it fails to penetrate, it pulverizes and/or vapourizes at the point of contact due to internal stresses and work-heating, thus producing a large cloud of high-temperature material at the target surface. This cloud heats the target surface through convection and radiation.

That is why an effective defense strategy would use guns to destroy large physical impactors, forcefields to deflect small physical impactors, and shields to reflect, absorb and retransmit, or scatter energy weapons rather than the "one size fits all" approach that seems to be popular among fanboys.

Collision Physics

When a physical object strikes a shielded vessel, it must be decelerated by the vessel's defensive systems. Most people tend to assume that if the shield holds, the ship is undamaged. However, this is not necessarily the case. Consider the following image (and please, keep in mind that I am not a professional graphic artist):

Impact

Let's assume that the rectangular assembly at left is a shielded starship (yes, I know, it looks cheesy, but please bear with me). The big brown rock at right is hitting the ship's shields, and it is being decelerated (hence the rightward force F being applied to the rock by the forcefield). For every action, there is an equal and opposite reaction, so there must be a counter-balancing force for that forcefield. A forcefield must be coupled to something, and in this case, it would obviously be the shield generator. Therefore, there is a leftward force F being applied to the shield generator (the blue square) in the middle of the ship. But the shield generator cannot move relative to the ship or it would be torn loose from its moorings, so its mounting brackets (the four red blocks) must each apply a rightward force 0.25F in order to hold the shield generator in place. These four reaction forces, in turn, push the entire ship to the left with force F, so the net result is to stop the impactor while accelerating the ship.

Are we clear on that? Now here's where it gets interesting: what if the shield generator's projected forcefield is easily strong enough to decelerate the asteroid to zero before the moment of impact, but the four little red blocks aren't strong enough to hold the generator in place? Guess what: the shield generator will be torn from its moorings, and the rock will slam into the ship. This is where momentum can rule over energy; a low-momentum, high-energy weapon such as a laser might not be as dangerous to a shielded vessel as a high-momentum, low-energy physical impactor. In this scenario, the potential points of failure are the shield generator itself, the points where it is mounted to the vessel, and the structure of the vessel itself. In other words, the mounting brackets, bolts, welds, shield generator internal mechanisms, shield generator forcefield strength, and all other connecting bits are parts of a chain through which reaction forces must go in order to make the end-to-end connection between the ship and the impactor. It can be thought of as a chain, and as in any chain, it is the weakest link that will cause your downfall.

As you can see, even if it was possible to build a deflector shield generator of virtually infinite strength, the overall effectiveness of the system would still be limited by good old-fashioned structural limits. Ultimately, the survivability of a shielded spacecraft against physical impacts could (and would, given sufficient shield strength) conceivably come down to a set of bolts holding a shield generator onto the ship's spaceframe. This example highlights the severe problem with most attempts to rationalize sci-fi technologies, which is that people tend to look for the strongest link in the chain, not the weakest link in the chain.

Shield Collision Physics Summary

Physical impacts and energy weapons should not be treated as functionally identical, particularly in terms of the relationship of energy to structural stress in the target. Collision physics are still ruled by Newton, and all of the deflector shields and fancy tricks in sci-fi will not prevent reaction forces from acting upon the physical structure of a target spacecraft.

Ramming tactics are widely used in sci-fi (click here for an example analysis of a collision event). In Star Trek, Worf called for "ramming speed" in STFC, Jem'Hadar vessels rammed the USS Odyssey and destroyed it in DS9, and Commander Riker prepared to ram the Borg Cube in the TNG two-part episode "Best of Both Worlds". In Babylon 5, we saw a Starfury crash into and through a Minbari war cruiser's dorsal fin in the Battle of the Line as shown in the movie "In the Beginning", and Jeffrey Sinclair tried to ram another war cruiser later in that same battle. We also saw an Earth-force cruiser ramming a Minbari war cruiser in a brief flashback to the events leading up to that battle. Other examples include Battlestar Galactica, where Cylon raiders routinely crashed into the Galactica's flight decks (thus making the viewer wonder why there were no weapon emplacements near these flight deck entrances), Transformers (where the Autobots' stolen Quintesson corkscrew-ship rammed through Unicron's eye), and of course, ROTJ, where an A-wing crashed through the bridge windows of the Executor after its bridge shields were knocked out (which would imply that the Executor's unshielded bridge windows are similar in strength to the dorsal fin of a Minbari war cruiser).

The effectiveness of these popular ramming tactics has often been used as an excuse to downgrade shield estimates against energy weapons. But this implies an equivalency which does not exist. The "real-world" explanation for the effectiveness of ramming in sci-fi is that ramming is a very dramatic event, filled with imagery of martyrs and heroes. But the physics of collisions and reaction forces provide us with an "in-universe" explanation that works just as well.


Conclusion

Do not assume that all energy shields employ the same mechanisms. We can view the behaviour of energy shields in action and see that there is significant diversity in their operation, and this must be considered when attempting to synthesize a consistent model of their operation.



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