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A recent answer included this graphic of projected SLS versions:
SLS versions

So even the smaller version is capable of lifting 70 tons to orbit. Is there a reason other than politics to use a rocket that powerful, for launching a crew capsule that weighs in the region of 20 tons? At first glance, this seems hugely wasteful, not to mention possibly problematic: they'd have to throttle back the main engines to avoid excessive g-loads.

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The 70ton capacity of the SLS Block 1 is measured for low-earth orbit. But getting people just into LEO isn't the goal of the SLS. It is designed for missions beyond earth orbit to destinations like Moon or Mars. The Orion capsule itself hasn't got enough fuel to get to the moon and back on its own. It has a delta-v capacity of less than 2000 m/s. When you want to get from low earth orbit to the moon, you need about twice as much.

The additional 50 tons of capacity are to be used for a transfer stage to get the SLS into lunar orbit. This is planned for the Exploration Missions 1 and 2. Which upper stage will be used for this hasn't been decided yet. NASA lists the requirements as follows:

Requirements: Human Rated or Human Ratable for Space Systems. Burns (accelerating Orion/MPCV) following placement of ICPS and MPCV at insertion point by SLS. Three engine ignitions to achieve greater than 3050 m/s delta-V. ICPS Lift Capability total weight of 24224 kg (53404 lbs). ICPS Mass less than 71400 lbs.

When the transfer stage weights 32 tons max and the requirements say it needs to push 24 tons, that still leaves 14 tons of unused LEO capacity on the SLS.

I am not sure if the payload fairings and truss structures to connect the transfer stage to the rocket are already accounted for, but when they aren't that would likely be another ton or two.

Other uses for the excess payload capacity could be secondary payloads in form of unmanned satellites. But even when you do make use of this, keep in mind that manned missions, and especially those using a new launch vehicle for the first time, should always have some tolerance.

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Orion, the official current, manned payload for SLS is pretty heavy. Heavy enough that a Delta-4 Heavy is about the only American vehicle currently flying that could otherwise launch it.

But Orion is not meant for missions to LEO, everyone is agreed that would be a stupendous waste of money. It is meant for BEO (Beyond Earth Orbit) missions, like Lagrange points, asteriods, the Moon, and maybe Mars.

When you launch to areas in BEO, you need more propellant to get there. The Service Module of the Orion is quite heavy as well, and clearly can carry more or less propellant, within its capacity.

Thus a third stage of sorts, to propel the vehicle to its destination is needed. Perhaps that is the Service Module, perhaps it is a second (or more) firing of the SLS upper stage. (What do the SRB's count as? Stage 0 usually..)

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Have you heard of any other suggested heavy payload for the SLS other than the Orion? A space telescope or a probe? –  LocalFluff Jun 12 at 7:33
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