The ACES and its in orbit refuelling maybe future of ULA.

Two Vulcans (6x SRB) can deliver approx 35t to TLI. One Vulcan launches payload the other is tanker(40t).

A fully fuelled(60t) ACES in LEO could deliver >50t to TLI, >35t to LLO and approx 40t TMI.

Compare this to SLS 1B which can do 39t to TLI (3100ms) and 32t TMI.

I'm trying to tease the details of this out a bit more. This has turned out to be a bit of a long read but I wanted to explain my math and reasoning here. Firm figures on what the Vulcan could do in its various configurations and forms is difficult to find. Bruno stated at the April 12th press conference that the fully upgraded Vulcan with 6 boosters and an ACES stage could lift 30% more than the Delta IV Heavy. The difficulty is that the tanker used in the ACES architecture is both the upper stage and the payload. Just adding 30% more to the Delta's payload doesn't give us the amount of propellant that could be delivered.

Going by the ACES numbers in the paper

"A Commercially Based Lunar Architecture"(PDF) we get an upper and lower bound for how much propellant an ACES 71 tanker could deliver to a depot or another ACES stage and payload. It states that such a tanker could deliver 23.5mt (26 tons) with an Atlas with 5 SRMs or 30.8mt (34 tons) with a Delta Heavy. The ACES/ Delta core stages faster than the Atlas and thus requires the ACES to burn less propellant to reach orbit so more is left over.

The mass of propellant Vulcan could deliver is somewhere between those two numbers. Vulcan with a Centaur can lift more than an Atlas V 551 and less than a Delta IV heavy based on what was said at the press conference. That means for the same initial ACES mass the Vulcan core with 6 SRMs imparts more velocity than the Atlas Core with 5SRMs and less of than the 3 Delta cores. While it is unknown where the Vulcan ACES 71 delivered propellant mass is in that range a dual launch architecture can be examined in relation to SLS.

Unfortunately for the dual Vulcan launch architecture the total payload pushed through TLI is limited by lift capacity of one Vulcan. This is 36.8mt (Delta IV Heavy with RS-68A engines + 30%) while SLS is capable of 39mt through TLI. However we can look at if the maximum single payload could be thrown through TLI and if not how much could a dual Vulcan architecture throw.

From the same paper as above we know that the dry mass of the ACES 41 is 4.5 mt (5 tons) that plus the maximum payload gives us a dry mass of 41.3mt which needs 3.2km/s imparted upon it. The clustering of RL-10s is going to reduce the expansion ratio of the engines and thus ISP. ULA estimates this between 2-7 seconds. I'll go with the middle of the range and use a figure of 455s for ISP. A dual launch Vulcan can impart a Delta V of 2.4 km/s at the high end of what the tanker can carry and 2 km/s at the low end.

What about if the rocket carrying the payload carried less of a payload and had propellant left over? There could be enough propellant to push a payload through TLI. The amount the tanker brings will determine the maximum payload up to a point because there needs to be propellant left over in the ACES that orbited the payload to make up a full propellant load. Assuming the tanker brings up 28mt of propellant the system can shoot a payload of 32.5mt thoguh TLI. Using an ACES 71 as the departure stage wouldn’t limit the payload due to propellant capacity. To get 36.8 mt though TLI would take 42.7mt of propellant Which means two tankers are needed. Also it wouldn’t be as much as 36.8mt because the payload would have to be launched on a more massive upper stage. The increase in mass of the upper stage would linearly reduce payload

SLS can lift at least 6.5mt more through TLI. Without assembly of the payload on orbit the most any number of Vulcans can push anywhere will be 36.8mt. These figure show why one can't just take the payload of two rockets and add them together to compare compare one launch system or architecture to another.