Why did NASA design the SLS do the job of both the Ares I and Ares V?

Question

The design of the Constellation program called for two main launch vehicles, The Ares I and the Ares V. The Ares I was said to be human-rated and was to be used to carry the Orion vehicle and its crew to Low Earth Orbit. On the other hand, the Ares V was said to be purely for lifting cargo. Wikipedia provides an explanation for the reason behind this decision:

Unlike the Saturn V and Space Shuttle, where the crew and cargo were launched together on the same rocket, Project Constellation was planned to use two separate launch vehicles, the Ares I and the Ares V, for crew and cargo respectively. This configuration would have allowed the two launch vehicles to be optimized for their respective missions.

_{(emphasis mine)}

After the cancellation of the Constellation program, the Space Launch System started development. Its goal is to replace both Ares launch vehicles. The Wikipedia article on the Space Launch System notes this change:

The NASA Authorization Act of 2010 envisions the transformation of the Constellation program's Ares I and Ares V vehicle designs into a single launch vehicle usable for both crew and cargo, similar to the Ares IV.

_{(emphasis mine)}

Why did NASA choose to create a single launch vehicle instead of making two separate spacecraft? Why did they choose to go back to the conventional method of launching both crew and cargo in one launch? It seems that there would be many benefits to creating two vehicles. One for crew one for cargo. Was it for cost reasons, or was there more to it?

Answer 1

Keep in mind that the development of a launch vehicle takes considerable time and money, so having to build two new rockets is harder on funding. Also, looking at the proposed vehicles themselves, we see that both launchers would require new tooling and infrastructure, with Ares I requiring 5.5m stages and Ares V needing 10m stages. The massive launch weight of the Ares V would also require extensive strengthening of the launch sites, to support the 8.2 million pound vehicle.

SLS instead keeps the 8.4m and 5m tank tooling used for current launchers, and the solid boosters will be slightly shorter at 5 segments (rather than 5.5 or more), meaning less strain on the crawler. It seems the operation of SLS will be closer to that of the shuttle than either Ares rockets, meaning easier transition of labour.

But there is also a question of usefulness in the launch industry. This was an issue with both Ares I and Ares V. For Ares I, with a LEO payload capacity of 25 tonnes, it may have been hard-pressed to compete with other vehicles such as Atlas V and Delta IV, and in its proposed form it would not compete in BEO performance. Ares V would likely be the opposite, sitting in a class of its own and having the ability to lift any mission payload thrown at it; however it was perhaps too large, and would see little to no use as even a deep space launcher.

SLS loses to Ares V in pure performance but it is a more sensibly proportioned vehicle, having around 1,000 tonnes less liftoff mass. LEO and BEO payload masses are still high, but it is more useful as a super-heavy or deep-space lifter between manned Orion missions. This could also have been a factor.

Answer 2

Not specific to the overall architecture change (manned smaller booster, unmanned larger booster) but specific to the actual design of the Ares I they found that the SRB based booster had a number of problems that were looking hard to solve. If the SRB were to fail, and the Orion aborted, it was looking like the explosion would be big enough to shred the parachute of the capsule trying to escape.

Thus at some level as the new administration decided to cancel Constellation, and resurect SLS, they did not seem to want to bring back a design they knew had a big flaw. (Now it seems likely to be have been a solvable problem, better abort capacity, and it may just have been an excuse).