So a Vega-like, even larger SLV is coming for satellites.
Simorgh evolution to Sarir could have following background: Iran needs a liquid fuel missile for its potential manned program. A liquid fuel SLV can be shut down if a problem occurs, is lower on vibration and most importantly: After 20 years work and production of the original Shahab-3 engine, it may now have reached such reliability levels that it can be safely used in a manned program.
We don't know about the second stage engine but those motors would also need to be very reliable and so much of higher performance that the payload is increased sufficiently to allow for a lightweight single astronaut capsule.
Second stage efficiency defines much of the possible payload and the increase from Simorgh to Sarir would have to be significant.
So best would be the RD-217 the North Koreans use, sure but it could retain the Simorgh first stage engines due to reliability issues. But then it would also need a very high performance second stage that is very reliable too. So I guess the second stage may be a RD-217 variant that is deemed as reliable enough if it is just a single engine and not a cluster of four that must all work flawlessly like in the first stage.
At this point the 9m longer length of the Sarir must be taken into consideration and its uniform diameter: The weight increase is likely to be more than the Simorgh stage-1 cluster can accelerate fast enough. If thats the case, new engines in RD-217 class are needed. Plus at that volume increase even a 3-stage solution becomes a possibility, which would again increase lift capability.
To all of this economy considerations may be made: Those high production numbers of the Shahab-3/Ghadr engine, makes it very competitive.
My speculation on the Sarir:
~100t vehicle weight
~160t lift-off thrust (4 Ghadr cluster)
~40t second stage thrust (Khorramshahr engine optimized for vacuum operation)
~15t third stage thrust (8 instead of 4-cluster Simorgh second stage engine)
~1,5t payload to LEO
In that configuration, proven engines with high serial production numbers that are not too expensive would be employed. Ultimate goal would be to safely deliver a single-man astronaut capsule into LEO.
Heavy equipment haul would be done by the new solid SLV, while liquid only for astronaut transit.
