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The Space Transportation System (STS) could bring a fully loaded shuttle orbiter of 109 tons to orbit (or maybe empty+payload 68+25=93 tons). The Space Launch System (SLS) Block I is based on STS, but is said to be able to lift only two thirds to three quarters of that, 70 tons, to LEO. I would have thought that the SLS instead would be more powerful than the shuttle's launch stack.

SLS has an added fifth segment in its solid boosters and four instead of three of the same main engines. Also, it will have an upper stage with 26 tons of fuel compared with the smaller orbital maneuvering system of the shuttle. The thrust is greater for the SLS on each of the stages. So what does all the extra power go to? If the vague mass-to-LEO measure is useless here, what would a good comparative bottom line measure of their capabilities be? Could SLS Block I carry a fully loaded shuttle orbiter if it could be attached to it?

Solid booster (same isp 269 sec):

STS 15,000 kN

SLS 16,000 kN (+6%)

First stage (same isp 452 sec vacuum):

STS 5,250 kN

SLS 7,440 kN (+40%)

Upper stages OMS versus ICPS:

STS 2x27 kN and isp 316 sec

SLS 110 kN and isp 462 sec

From Wikipedia for STS and SLS.

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  • $\begingroup$ The STS had something similar to an upper stage, the OMS. Without using the OMS, the STS could not achieve an orbit. It would follow the external tank back into the atmosphere. The OMS was also used for the deorbit burn. $\endgroup$
    – Uwe
    May 14, 2017 at 15:57
  • $\begingroup$ @Uwe Ah, yes of course. But that too seems to be weaker than the SLS ICPS upper stage, with 2x27 kN and isp 316 seconds. $\endgroup$
    – LocalFluff
    May 14, 2017 at 16:03
  • $\begingroup$ Shuttle is ~100 tons including payload. STS capability is very difficult to make apples-to-apples comparisons with, because it's part launcher and part payload. SLS puts both the payload and the (dry) second stage into orbit -- another 3.5-4 tons there. $\endgroup$
    – Russell Borogove
    May 14, 2017 at 16:03
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    $\begingroup$ Aha - that's EM-1: 70 tons to 1800x93 without using the ICPS. The low perigee ensures reentry and disposal of the core stage, and the ICPS will do a translunar injection burn at 1800km. My launch simulator suggests that SLS + ICPS can get ~77.5 tons of payload to 200km circular orbit, but I could be off by quite a bit. $\endgroup$
    – Russell Borogove
    May 15, 2017 at 16:47
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    $\begingroup$ @LocalFluff Its for EM-1 and similar flight profiles, the 93km perigee ensures safe seperation from the core stage which will re-enter and burn up. The upper stage will hen do the TMI burn or other required maneuvers. $\endgroup$
    – Polygnome
    May 15, 2017 at 17:55

1 Answer 1

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It's always difficult to make apples-to-apples comparisons between the space shuttle and other launchers, because the orbiter is ambiguously part launcher and part payload. This is compounded by the broadness of the term "LEO"; shuttle payloads went to a variety of altitudes and inclinations.

However, since the title of the question specifies "mass to LEO" rather than useful payload, we can start there. At the space shuttle's MECO, it's in an eccentric orbit of perhaps 300km x 70km; this is comparable to a circular orbit of 185km. The total mass on this trajectory, assuming the OMS has not been used for ascent assist, is at most:

  • 109 tons of orbiter including payload and propellant
  • 26.5 tons of external tank (empty SLWT)

or 135.5 tons.

The SLS EM-1 mission intends to put an ICPS and Orion spacecraft into a higher orbit than that, 1800km x 93km; this is the trajectory at core stage main engine cutoff (MECO), without the ICPS having fired yet. The low perigee guarantees that the core stage will re-enter Earth's atmosphere and burn up. At this point, we have a total mass in orbit of:

  • 85 tons empty core stage
  • 31 tons loaded ICPS stage
  • 5 tons interstage
  • 26 tons Orion spacecraft

For a total of 147 tons in this higher orbit.

My reference says the shuttle payload was reduced by 25kg per km altitude, suggesting the shuttle would have to give up about 19 tons of payload to match the SLS EM-1 trajectory, making the comparison 147 tons to 116.5 tons.

So it appears that the larger SRBs and fourth SSME do give greater performance to the Block 1 SLS: roughly 25% more mass to LEO, which seems intuitively reasonable.

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