Advantages of Block 2
#1: Block 2 doesn't have to be so expensive to develop. The current path baselined by NASA is expensive, but it doesn't have to be. The current one has the following order of upgrades:
1. Develop new boosters for SLS, either advanced solid or liquid; increases payload to >105 tons.j
2. Develop an in-space stage for SLS to replace the iCPS.
3. Wait nine years, fly SLS once every one or two years.
4. Develop a new, massive J-2X powered upper stage, redesign the core you've been flying for 15 years by adding an extra engine. Increases payload to >130 tons to LEO.
Now, after some time, new boosters will be required. There are sufficient booster casings for ten SLS flights with standard 5-segment boosters, but if we ever get serious with SLS, new ones would become necessary. However, having already reached 130 tons, this would allow for cheaper, affordability focused boosters instead of the big, performance focused advanced boosters using the F-1 engine proposed by Dynetics. In fact, they could be even cheaper than the ATK advanced boosters because they won't have to be as big.
And other possible development path is by adding a 4x RL-10 upper stage to Block 1, then adding advanced boosters. No core redesign required, and although Advanced Boosters will become necessary, the new upper stage would be significantly cheaper than the J-2X upper stage, both to develop and operate.
Either of these paths would probably allow Block 2 to be ready by ~2025 instead of the current 2032.
#2: Block 2 is actually cheaper than sticking with Block 1 in the long run. While it would cost more to develop, Block 2 would cost pretty much the same on a yearly or per-flight basis as Block 1. The rocket shares tooling for the upper stage, costing very little extra. The engines, either RL-10 or J-2X aren't free but don't cost hundreds of millions either. The boosters, whether they're designed to be powerful or affordable, are supposed to be cheaper than the 5-segment ones on Block 1. The fixed costs would be slightly lower, marginal cost slightly higher, but the difference is very small either way. Assuming identical cost, SLS Block 2 costs significantly less at payloads above 70-90 tons, and anything above 140-180 tons because of requiring less flights. The use of advanced boosters compared to normal boosters also allows for lower fixed costs, though the exact costs of this is hard to estimate because of a lack of info. ATK claims a cost reduction of 40% for the boosters, but I don't know how much a booster costs, so the exact difference is hard to pin down.
There's also something I glanced over previously. For sticking with Block 1, you'll eventually need to develop an in-space cryogenic propulsion stage and new boosters too. The cost saved from sticking with Block 1 aren't very significant at all.
#3: Block 2 frees up more money for actual missions. You might be wondering what I'm smoking right now. Believe me, I'm not smoking, this might actually be true. The current plan is to operate Block 1A for nine years to perform all missions to asteroids/the moon and continue developing Block 2 in the background at the same time. By going to Block 2 directly, you save money afterwards because you don't have to keep on developing upgrades for SLS. You might put back missions compared to sticking with Block 1A, but they can be developed quicker afterwards. Considering NASA's dire lack of funding, stopping funding for development of the launcher and flying the final config as soon as possible seems like a more realistic path to other worlds than sticking with a less capable, just as expensive variant that will have to be replaced later anyway. Unless a new NASA Authorization Act is made dropping the 130 ton requirement, Block 1A would require an upgrade sooner and later, and putting it off might only cost more money in the long run.
#4: Unlike Block 1, Block 2 is actually a big improvement over other launchers. Block 1's 70-90 ton LEO payload is very good compared to current rockets, but it's not something cheaper rockets could reach for less. 130 tons to LEO, however, is a different story. I previously estimated that Falcon Heavy with a Raptor upper stage could get ~65 tons to LEO, and Atlas Phase 2 can get 75 tons into LEO according to ULA. Sticking with Block 1 would neuter the point of even having SLS, since these launchers would give similar LEO performance for less. Block 1A would give a decent increase to 105 tons, but 130 tons is sky high above all other launchers and is the only payload capacity that could somewhat justify an expensive Shuttle Derived Architecture over, for example a 70 ton inline kerolox concept combined with in-space refueling. Whether we need that capacity is a bit of a different story, but there certainly are reasons it could be useful.
There are also many mission possibilities that could be opened up by Block 2 that would require several launches on other launchers, including other SLS blocks. Assuming 130.0 tons to LEO, a Block 2 with 0.9 PMF CPS optimized for lunar missions and 462 Isp would get the following performances to other orbits:
- 44.6 metric tons into Low Lunar Orbit
- 56.3 tons to Trans Lunar Injection
- 54.2 tons to the nearest Near Earth Asteroids
- 48.9 tons to Mars ( ~7-9 month travel time, 3700 m/s)
Now assuming these values, there are a few options that open up that Block 1A would require multiple launches for.
One is a manned lunar landing with 3-4 astronauts for a week down on the surface. A paper with presentation from Spaceworks Enterprises Inc. includes the estimates for lunar landers from L2 with several different propellants. The lander+in space stage combinations show mass estimates for single stage landers that can land from and return to LLO. The mass of a single stage methane lander, including 4 ton habitat, is 20.9 tons, and with hydrogen it's 16.8 metric tons. According to the Boeing paper I referred to earlier, the empty mass of an Orion spacecraft is 14.9 tons; even with a specific impulse of only 316 (STS OMS) and a worst case 1200 m/s ∆V to return, the Orion has a mass of 21.95 tons in LLO. 20.8+21.95= 42.85 tons into Low Lunar Orbit, which fits within the 44.6 ton capability mentioned earlier. A more optimistic case with a hydrolox lander and 900 m/s return ∆V gives a total mass of only 36.8 tons, which fits very well and gives plenty of margin. This Lander uses a habitat that is actually bigger and with more supplies than the Boeing reusable lander, which carries 3 people for 7 days. While the SEI lander is designed for 2 people for 20 days, it could support a crew of 3 or 4 people for a full week, giving the lander similar capability to the Altair lunar lander for much lower mass. A Constellation class mission in a single launch could be possible with SLS Block 2.
Another option could be a mission to a Near Earth Object mission in a single launch. The mass of the notional habitat baselined by NASA has a mass of 23.9 metric tons. It has 72 m^3 of habitable volume and can support a crew of three for a year. Judging from the diameter, this hab would be based on the ISS MPLM module which is built in Italy. Combined with a fully fueled Orion, worst case mass of 24.2 metric tons, the mass of this spacecraft would be 48.1 tons. SLS Block 2 with in-space stage would be able to get this entire stack to a trajectory of 3760 m/s, enough to visit an asteroid. In fact, this is enough to send the crew to visit Mars, though the hab would require some mods to allow for such a mission time, and it wouldn't be able to do anything useful there.
The last option is a Mars Direct type mission. The 48.9 ton capacity to Mars with a C3 of 11 km^2/s^2. According to the NASA trajectory browser this is sufficient to reach Mars in 200-300 days, depending on launch window. That same browser shows plenty of windows to return from Mars with 800 m/s of delta V, or 2.3 km/s total including escape velocity. Assuming aerocapture to reduce MOI delta V to only 100 m/s and 2300 m/s to return, as well as methane/oxygen propulsion to return, a spacecraft of 48.9 tons would have an empty mass of 25.2 tons. This is enough to hold the propellant tanks and propulsion system (2.7 tons), a crew return capsule (8.94 tons) and an inflatable habitat (13.5 tons). That might seem small for such a habitat, but a study from SEI estimated the mass of an inflatable hab for a crew of three with 60m^3 of habitable volume at just 10 tons. Such an ERV, similar to the Mars Semi Direct approach, would be much roomier than the capsule-sized ERV Zubrin proposed in the original Mars Direct, which had a mass of just 7 tons.
All in all, it seems to me that developing Block 2 immediately instead of sticking with Block 1 or 1A has many advantages and it could very well be the right path forward. Unless the 130 ton requirement is dropped, upgrading to it will be necessary, and getting the upgrading out of the way will certainly help. It will all depend on what missions SLS is supposed to perform though. For any case though, getting as much out of SLS as possible certainly seems like the right way to do so.