The three primary missions of the Engineers in combat are mobility, counter-mobility, and force protection. Rather obviously, this means ensuring our freedom of maneuver, by improving roads and reducing obstacles, both natural and man made; emplacing obstacles to slow, channel or turn an enemy force; and digging or building positions for friendly forces.
As you might expect, a large portion of this can be accomplished by earthmoving. As a mechanized Infantryman mounted on a Bradley, my most common interaction with the Engineers was when we had a D7 bulldozer dig fighting positions for our vehicles.
Merely pushing a berm in front of the position does little to offer protection for fighting vehicles. While it might defeat HEAT rounds, kinetic rounds hardly notice a dirt berm before passing through the frontal armor, engine block, turret basket and troop compartment and then exiting the rear ramp armor. So the position is dug deep enough to fully conceal the vehicle. But the vehicle also has to be able to fight from the position, so there is a step on the front half of the position that the Bradley can drip up on, exposing only the turret, giving it a field of fire. Pop up, shoot, scoot back, scan for the next target. In gunnery terms, this is known as a “berm drill.”
While the D7 bulldozer is very, very well suited for digging said positions, it is not without its drawbacks.
First, it is completely unarmored. If the position isn’t completely secure, the operator is at an unacceptable risk. But failing to construct the positions then places the fighting vehicles at a completely unacceptable risk.
Secondly, the D7 is rather slow, with a maximum speed of around 7 miles per hour. That means it has to be transported from location to location on a heavy equipment trailer. That also means the trailer is restricted to relatively good terrain. The truck and trailer also are unarmored, and add an additional logistical, manning, and maintenance burden.
And so, starting in the late 1980s, the Army began fielding a lightweight vehicle known as the M9 ACE or Armored Combat Earthmover. A relatively lightweight tracked vehicle with a bulldozer blade on front, it was proof against small arms fire and artillery fragments. The driver was protected. The hydropnuematic suspension allowed it to travel cross country, and on roads at a respectable 30 miles per hour or so. Maybe not enough to keep up with Bradley’s and M1 Abrams, but enough that the wait for ACE shouldn’t be too long.
Light weight is a disadvantage for a bulldozer, though. The tracks need significant weight on them to increase the dozing ability. So the M9 can actually also act as a grader/scraper, and load a ballast compartment just behind the blade with earth to improve its earthmoving ability. When it is done, it can also eject that earth. In between missions, that space can be used to carry cargo or engineer supplies.
My experience with the M9 is very limited. I have heard that some dozer operators didn’t like it, and felt it was a rather poor earthmover, especially those who had previous experience with the D7. It has also had a long, long history of maintenance issues, primarily associated with its complex suspension system.
What’s especially interesting is the long development time of the M9. As I mentioned, the Army didn’t start buying the M9 until the late 1980s. But that doesn’t mean it was a new design. Its design actually dates back to the early 1960s.
With a few minor changes, the UET would become the M9. So why the 20 year gap between design and fielding? First, just as the Army was finishing development, Vietnam happened. And the money that would have gone for the UET instead went to fighting that war. In the years after Vietnam, the Army’s funding priorities were on the Big Five, the M1, M2/M3, UH-60, AH-64, and Patriot missile. It wasn’t until those programs were well in hand that other priorities could be addressed.