An army marches on its stomach, as somebody (the quote is attributed to both Napoleon and Friedrich II) once said. Accordingly, let’s take a very rough look at the logistics of a planetary invasion.
I’m saying a very rough look because, ultimately, we don’t yet know what kind of technology is used, and this will change the number. But, for now, since we just want a rough estimate, I’ll be using roughly modern supply numbers.
Supplies: Tons per Day
How many supplies does a modern army need? Project Rho gives about 135t per day for a Stryker Brigade Combat Team. We’ll take 150t to be sure.
Note that this is for a motorized infantry brigade. A tank formation will be far more thirsty for supplies; light infantry less. A tank formation, for example, can be assumed to consume maybe five times more fuel (this is a very rough estimate), which would increase that to about 350t.
A division, of which we said last time we’d need about 15 to secure the spacehead, would therefore need about 600t of supplies per day (which surprisingly fits quite well with WW2 numbers, for which I have found a quote by Eisenhower of 600-700t of supplies per day for a reinforced division), which gives us only about 9,000 tons of supplies per day, or about 270,000 tons per month. That’s “only” about five Atlas per week.
As a comparison, WW2’s Normandy invasion (for which one can find numbers) saw similar supply numbers: The two Mulberry harbours deployed were supposed to land about 12,000 tons of supplies (plus vehicles, for a total tonnage of 40,000t) per day; and eventually, 200,000t of of supplies were unloaded at Le Havre alone in December 1944. Similarly, in the Lend-Lease program, an average of about 350,000t per month were shipped.
How can we reduce the supplies? One of the main shipping requirements is fuel, which makes up half of the load. This can be reduced by providing locally-produced fuel. This will not be petrol-based (because it would imply capturing both oil wells and refineries and the existence of oil supplies), but can probably be hydrogen-based. Water can be found mostly everywhere you want to fight, since it’s a requirement for human habitation.
A standard-Spaceships mobile refinery (at 100 tons, six refinery systems, three fusion reactors, armour and fuel tanks), can produce 9t of hydrogen per day (with the Slower Industrial Systems setting switch) and therefore breaks even after only eleven days of fighting. This reduces required supplies to only 5,000 tons per day, or three Atlas per week.
Landing the Equipment
Looking again at a Stryker Brigade, it apparently has a combat mass of at least 15,000 tons, giving us about 60,000t per division. In addition to the 270,000t of supplies per month, we’d therefore have to ship another 900,000 tons in people and equipment!
A single division can be transported using about four to five Atlas, and more if you want them to be protected during transport. There might indeed be specialized transports massing a hundred thousand tons to ship a whole division at once and without splitting it up. That’s far bigger than any sea-going ship today, but we don’t have to deal with the same kind of constraints.
Still, when looking at the required landing capacity to land fifteen divisions in three days, that’s 300,000t per day. That is almost two thousand Cargolifter shuttle flights per day. In a commercial setting, we have seen four hours per flight and two flights in total. We can surely decrease the time between flights by assuming faster loading through brute force and more optimal choice of parking orbit (the advantage of not having to listen to traffic control), maybe to two hours each. Through round-the-clock operation, one should be able to surge about ten flights per day per shuttle - for two hundred shuttles in total, with a combined tonnage of 60,000t. These can, of course, stay deployed in orbit and be used to ship down supplies and reinforcements.
A typical transport might look like the Gallipoli below. It transports 2,400 soldiers plus their equipment (which corresponds to about half a Brigade Combat Team) and includes a hangar for three shuttles. Admittedly, these take a total of 65 flights (so about three days at surge operations) to unload all of their cargo and troops.
|||Light Alloy Armour|
|[2-6]||Cargo Hold (1500t each)|
|||Light Alloy Armour|
|[3-5]||Habitat (200 bunkrooms each, 800 people each)|
|||Fuson Reactor (de-rated, 1PP)|
|[core]||Smaller Systems (Control Room, Habitat, Defensive Battery])|
|||Light Alloy Armour|
|[2-5]||Fuel Tank (4500t, 30km/s total)|
|||Water Fusion Torch (1.5g)|
SM+11, 30,000 tons, unstreamlined, 7,500t tons cargo, 2,400 soldiers
|10^||Gallipoli||200||-3/4||12||1.5g/30km/s||30,000t||13,500t||+11||30ASV + 2400ASV||35d/7d/35d||1|
An alternate configuration, the Mulberry, exchanges all of the cargo holds and habitats with hangars to provide 9,000t of hangar for thirty shuttles. These can transport almost 5,000t of cargo or 7,200 people at once, for about 20,000t/28,000 people per day during non-surge operations.
Invasion means transporting, in addition to all of the combat spacecraft, fifteen divisions for the first wave. Transporting them takes, very roughly, 120 Gallipoli transports. That’s quite a large initial convoy. These arrive in planetary orbit, unload the formations during the first three days and move back again. Every fifteen days (five days to the staging area, two days of loading, five days back, and three days of unloading), these can add another fifteen divisions to the deployed troops, landing them either at the existing spacehead or creating a new one.
The additional supplies, 270,000t per month, are transported on Atlas. For this, 20 Atlas are needed, operating continuously. Of course, that’s 20 Atlas for the first two weeks; using the Gallipolis to continuously ferry new combat formations, 60 will be needed beginning with the end of the first month and 100 with the end of the second month.
If we assume that sixty divisions are all that will be deployed (that’s 1.2 million people), then we have just four transport cycles, i.e. everything will be on planet a month after the initial landing. This month saw four cycles of intense landing operations during which one shuttle touches down every 22 seconds.
In parallel, convoys of Atlas arrive, probably at the same time as the actual landing (since this greatly simplifies escort requirements). They are unloaded by shuttles provided by one of the Mulberries in orbit, which should be able to unload three Atlas every two days. Accordingly, most of the supplies will probably be dumped in orbit before shipped down.
To ship them down, 4,000 Cargolifter flights are needed per month, or about 150 every day. For context, that’s about a quarter of the Berlin Airbridge’s flights during the same timespan. Of course, every cargolifter also has about ten times a typical Berlin Airbridge’s plane’s capacity. I can only hope that the landing site is well-prepared (the equivalent of Seebees is probably quite important), seeing as they have to process an average of one shuttle every ten minutes during supply runs, and one shuttle every thirty seconds during fast embarking.
The latter, by the way, suggests that there are at least two types of shuttles: Heavy-lift ones (the Cargolifter), which require a prepared landing site, and VTOL-shuttles which don’t, and can be used during the initial assault.
Planetary invasion, while certainly expensive and a logistical nightmare, seems surprisingly doable - a total of about 250 transport spacecraft with a combined mass of about 7.5 million tons are needed for a month, but that should be achievable for most nations. Of course, they still need to actually win the ground war against an enemy with home advantage.